bionic (8) openvpn.8.gz

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NAME

       openvpn - secure IP tunnel daemon.

SYNOPSIS

       openvpn [ options ... ]

INTRODUCTION

       OpenVPN  is  an  open source VPN daemon by James Yonan.  Because OpenVPN tries to be a universal VPN tool
       offering a great deal of flexibility, there are a lot of options on this manual page.  If you're  new  to
       OpenVPN,  you might want to skip ahead to the examples section where you will see how to construct simple
       VPNs on the command line without even needing a configuration file.

       Also note that there's more documentation and examples on the OpenVPN web site: http://openvpn.net/

       And if you would like to see a shorter version of this manual, see the openvpn usage message which can be
       obtained by running openvpn without any parameters.

DESCRIPTION

       OpenVPN  is  a  robust  and  highly  flexible  VPN  daemon.   OpenVPN supports SSL/TLS security, ethernet
       bridging, TCP or UDP tunnel transport through proxies or NAT, support for dynamic IP addresses and  DHCP,
       scalability to hundreds or thousands of users, and portability to most major OS platforms.

       OpenVPN is tightly bound to the OpenSSL library, and derives much of its crypto capabilities from it.

       OpenVPN  supports  conventional  encryption using a pre-shared secret key (Static Key mode) or public key
       security (SSL/TLS mode) using client & server certificates.  OpenVPN also supports non-encrypted  TCP/UDP
       tunnels.

       OpenVPN is designed to work with the TUN/TAP virtual networking interface that exists on most platforms.

       Overall,  OpenVPN  aims  to  offer  many  of  the key features of IPSec but with a relatively lightweight
       footprint.

OPTIONS

       OpenVPN allows any option to be placed either on the command line or in a configuration file.  Though all
       command  line  options  are  preceded by a double-leading-dash ("--"), this prefix can be removed when an
       option is placed in a configuration file.

       --help Show options.

       --config file
              Load additional config options from file where each line corresponds to one command  line  option,
              but with the leading '--' removed.

              If  --config  file is the only option to the openvpn command, the --config can be removed, and the
              command can be given as openvpn file

              Note that configuration files can be nested to a reasonable depth.

              Double quotation or single quotation characters ("", '') can be used to enclose single  parameters
              containing  whitespace,  and  "#"  or  ";"  characters  in  the first column can be used to denote
              comments.

              Note that OpenVPN 2.0 and higher performs backslash-based shell escaping  for  characters  not  in
              single quotations, so the following mappings should be observed:

                  \\       Maps to a single backslash character (\).
                  \"       Pass a literal doublequote character ("), don't
                           interpret it as enclosing a parameter.
                  \[SPACE] Pass a literal space or tab character, don't
                           interpret it as a parameter delimiter.

              For example on Windows, use double backslashes to represent pathnames:

                  secret "c:\\OpenVPN\\secret.key"

              For examples of configuration files, see http://openvpn.net/examples.html

              Here is an example configuration file:

                  #
                  # Sample OpenVPN configuration file for
                  # using a pre-shared static key.
                  #
                  # '#' or ';' may be used to delimit comments.

                  # Use a dynamic tun device.
                  dev tun

                  # Our remote peer
                  remote mypeer.mydomain

                  # 10.1.0.1 is our local VPN endpoint
                  # 10.1.0.2 is our remote VPN endpoint
                  ifconfig 10.1.0.1 10.1.0.2

                  # Our pre-shared static key
                  secret static.key

   Tunnel Options:
       --mode m
              Set  OpenVPN  major  mode.   By default, OpenVPN runs in point-to-point mode ("p2p").  OpenVPN 2.0
              introduces a new mode ("server") which implements a multi-client server capability.

       --local host
              Local host name or IP address for bind.  If specified, OpenVPN will bind to this address only.  If
              unspecified, OpenVPN will bind to all interfaces.

       --remote host [port] [proto]
              Remote  host  name  or  IP address.  On the client, multiple --remote options may be specified for
              redundancy, each referring to a different OpenVPN server.  Specifying  multiple  --remote  options
              for  this  purpose  is  a  special  case  of the more general connection-profile feature.  See the
              <connection> documentation below.

              The OpenVPN client will try to connect to a server at host:port in the order specified by the list
              of --remote options.

              proto indicates the protocol to use when connecting with the remote, and may be "tcp" or "udp".

              For forcing IPv4 or IPv6 connection suffix tcp or udp with 4/6 like udp4/udp6/tcp4/tcp6.

              The  client  will  move on to the next host in the list, in the event of connection failure.  Note
              that at any given time, the OpenVPN client will at most be connected to one server.

              Note that  since  UDP  is  connectionless,  connection  failure  is  defined  by  the  --ping  and
              --ping-restart options.

              Note  the  following corner case:  If you use multiple --remote options, AND you are dropping root
              privileges on the client with --user and/or --group, AND the client is running a  non-Windows  OS,
              if the client needs to switch to a different server, and that server pushes back different TUN/TAP
              or route settings, the client may lack the necessary privileges to close and  reopen  the  TUN/TAP
              interface.  This could cause the client to exit with a fatal error.

              If  --remote is unspecified, OpenVPN will listen for packets from any IP address, but will not act
              on those packets unless they pass all authentication tests.  This requirement  for  authentication
              is  binding  on all potential peers, even those from known and supposedly trusted IP addresses (it
              is very easy to forge a source IP address on a UDP packet).

              When used in TCP mode, --remote will act as a filter, rejecting connections from  any  host  which
              does not match host.

              If  host is a DNS name which resolves to multiple IP addresses, OpenVPN will try them in the order
              that the system getaddrinfo() presents them, so priorization and DNS randomization is done by  the
              system  library.   Unless  an  IP  version  is  forced by the protocol specification (4/6 suffix),
              OpenVPN will try both IPv4 and IPv6 addresses, in the order getaddrinfo() returns them.

       --remote-random-hostname
              Prepend a random string (6 bytes, 12 hex characters) to hostname  to  prevent  DNS  caching.   For
              example, "foo.bar.gov" would be modified to "<random-chars>.foo.bar.gov".

       <connection>
              Define a client connection profile.  Client connection profiles are groups of OpenVPN options that
              describe how to connect to a given OpenVPN  server.   Client  connection  profiles  are  specified
              within  an  OpenVPN  configuration  file,  and  each  profile  is  bracketed  by  <connection> and
              </connection>.

              An OpenVPN client will try each connection profile sequentially until  it  achieves  a  successful
              connection.

              --remote-random can be used to initially "scramble" the connection list.

              Here is an example of connection profile usage:

                  client
                  dev tun

                  <connection>
                  remote 198.19.34.56 1194 udp
                  </connection>

                  <connection>
                  remote 198.19.34.56 443 tcp
                  </connection>

                  <connection>
                  remote 198.19.34.56 443 tcp
                  http-proxy 192.168.0.8 8080
                  </connection>

                  <connection>
                  remote 198.19.36.99 443 tcp
                  http-proxy 192.168.0.8 8080
                  </connection>

                  persist-key
                  persist-tun
                  pkcs12 client.p12
                  remote-cert-tls server
                  verb 3

              First we try to connect to a server at 198.19.34.56:1194 using UDP.  If that fails, we then try to
              connect to 198.19.34.56:443 using TCP.  If that also fails, then try connecting  through  an  HTTP
              proxy at 192.168.0.8:8080 to 198.19.34.56:443 using TCP.  Finally, try to connect through the same
              proxy to a server at 198.19.36.99:443 using TCP.

              The following OpenVPN options may be used inside of a <connection> block:

              bind, connect-retry, connect-retry-max, connect-timeout,  explicit-exit-notify,  float,  fragment,
              http-proxy,  http-proxy-option,  link-mtu,  local,  lport,  mssfix, mtu-disc, nobind, port, proto,
              remote, rport, socks-proxy, tun-mtu and tun-mtu-extra.

              A defaulting mechanism exists for specifying options to apply to all  <connection>  profiles.   If
              any  of  the above options (with the exception of remote ) appear outside of a <connection> block,
              but in a configuration file which has one or more <connection> blocks, the option setting will  be
              used as a default for <connection> blocks which follow it in the configuration file.

              For  example,  suppose  the nobind option were placed in the sample configuration file above, near
              the top of the file, before the first <connection> block.  The effect would be as if  nobind  were
              declared in all <connection> blocks below it.

       --proto-force p
              When iterating through connection profiles, only consider profiles using protocol p ('tcp'|'udp').

       --remote-random
              When  multiple  --remote  address/ports  are  specified, or if connection profiles are being used,
              initially randomize the order of the list as a kind of basic load-balancing measure.

       --proto p
              Use protocol p for communicating with remote host.  p can be udp, tcp-client, or tcp-server.

              The default protocol is udp when --proto is not specified.

              For UDP operation, --proto udp should be specified on both peers.

              For TCP operation, one peer must use --proto tcp-server and the other must use --proto tcp-client.
              A  peer started with tcp-server will wait indefinitely for an incoming connection.  A peer started
              with tcp-client will attempt to connect, and if that fails, will sleep for 5  seconds  (adjustable
              via  the  --connect-retry  option)  and  try again infinite or up to N retries (adjustable via the
              --connect-retry-max option).  Both TCP client and server will simulate a SIGUSR1 restart signal if
              either side resets the connection.

              OpenVPN  is  designed to operate optimally over UDP, but TCP capability is provided for situations
              where UDP cannot be used.  In comparison with UDP, TCP will usually be somewhat less efficient and
              less robust when used over unreliable or congested networks.

              This article outlines some of problems with tunneling IP over TCP:

              http://sites.inka.de/sites/bigred/devel/tcp-tcp.html

              There  are  certain  cases,  however,  where  using  TCP  may  be advantageous from a security and
              robustness perspective, such as tunneling non-IP or application-level UDP protocols, or  tunneling
              protocols which don't possess a built-in reliability layer.

       --connect-retry n [max]
              Wait n seconds  between connection attempts (default=5). Repeated reconnection attempts are slowed
              down after 5 retries per remote by doubling the wait time after  each  unsuccessful  attempt.  The
              optional  argument max specifies the maximum value of wait time in seconds at which it gets capped
              (default=300).

       --connect-retry-max n
              n specifies the number of times each --remote or <connection> entry is tried. Specifying n as  one
              would   try   each   entry   exactly   once.   A   successful   connection   resets  the  counter.
              (default=unlimited).

       --show-proxy-settings
              Show sensed HTTP or SOCKS proxy settings. Currently, only Windows clients support this option.

       --http-proxy server port [authfile|'auto'|'auto-nct'] [auth-method]
              Connect to remote host  through  an  HTTP  proxy  at  address  server  and  port  port.   If  HTTP
              Proxy-Authenticate  is required, authfile is a file containing a username and password on 2 lines,
              or "stdin" to prompt from console. Its content can also be specified in the config file  with  the
              --http-proxy-user-pass option. (See section on inline files)

              auth-method should be one of "none", "basic", or "ntlm".

              HTTP Digest authentication is supported as well, but only via the auto or auto-nct flags (below).

              The  auto  flag  causes  OpenVPN to automatically determine the auth-method and query stdin or the
              management interface for username/password credentials, if required.  This flag exists on  OpenVPN
              2.1 or higher.

              The  auto-nct  flag  (no  clear-text  auth)  instructs  OpenVPN  to  automatically  determine  the
              authentication  method,  but  to  reject  weak  authentication  protocols  such  as   HTTP   Basic
              Authentication.

       --http-proxy-option type [parm]
              Set extended HTTP proxy options.  Repeat to set multiple options.

              VERSION version -- Set HTTP version number to version (default=1.0).

              AGENT user-agent -- Set HTTP "User-Agent" string to user-agent.

              CUSTOM-HEADER  name content -- Adds the custom Header with name as name and content as the content
              of the custom HTTP header.

       --socks-proxy server [port] [authfile]
              Connect to remote host through a Socks5 proxy at address  server  and  port  port  (default=1080).
              authfile  (optional) is a file containing a username and password on 2 lines, or "stdin" to prompt
              from console.

       --resolv-retry n
              If hostname resolve fails for --remote, retry resolve for n seconds before failing.

              Set n to "infinite" to retry indefinitely.

              By default, --resolv-retry infinite is enabled.  You can disable by setting n=0.

       --float
              Allow remote peer to change its IP address and/or port number, such as due to DHCP  (this  is  the
              default  if --remote is not used).  --float when specified with --remote allows an OpenVPN session
              to initially connect to a peer at a known address, however if packets arrive from  a  new  address
              and  pass  all  authentication  tests,  the new address will take control of the session.  This is
              useful when you are connecting to a peer which holds a dynamic address such as a dial-in  user  or
              DHCP client.

              Essentially,  --float tells OpenVPN to accept authenticated packets from any address, not only the
              address which was specified in the --remote option.

       --ipchange cmd
              Run command cmd when our remote ip-address is initially authenticated or changes.

              cmd consists of a path to script (or executable program), optionally followed  by  arguments.  The
              path and arguments may be single- or double-quoted and/or escaped using a backslash, and should be
              separated by one or more spaces.

              When cmd is executed two arguments are appended after any arguments specified in cmd , as follows:

              cmd ip_address port_number

              Don't use --ipchange in --mode server mode.  Use a --client-connect script instead.

              See the "Environmental Variables" section below for additional parameters passed as  environmental
              variables.

              If you are running in a dynamic IP address environment where the IP addresses of either peer could
              change without notice, you can use this script, for example, to edit the /etc/hosts file with  the
              current  address  of  the  peer.  The script will be run every time the remote peer changes its IP
              address.

              Similarly if our IP address changes due to DHCP, we should configure our IP address change  script
              (see man page for dhcpcd(8) ) to deliver a SIGHUP or SIGUSR1 signal to OpenVPN.  OpenVPN will then
              reestablish a connection with its most recently authenticated peer on its new IP address.

       --port port
              TCP/UDP port number or port name for both local and remote (sets both --lport and --rport  options
              to  given  port).  The current default of 1194 represents the official IANA port number assignment
              for OpenVPN and has been used since version 2.0-beta17.  Previous versions used port 5000  as  the
              default.

       --lport port
              Set local TCP/UDP port number or name.  Cannot be used together with --nobind option.

       --rport port
              Set  TCP/UDP  port  number  or name used by the --remote option. The port can also be set directly
              using the --remote option.

       --bind [ipv6only]
              Bind to local address  and  port.  This  is  the  default  unless  any  of  --proto  tcp-client  ,
              --http-proxy or --socks-proxy are used.

              If  the  ipv6only  keyword  is present OpenVPN will bind only to IPv6 (as oposed to IPv6 and IPv4)
              when a IPv6 socket is opened.

       --nobind
              Do not bind to local address and port.  The IP stack will allocate a dynamic  port  for  returning
              packets.  Since the value of the dynamic port could not be known in advance by a peer, this option
              is only suitable for peers which will be initiating connections by using the --remote option.

       --dev tunX | tapX | null
              TUN/TAP virtual network device ( X can be omitted for a dynamic device.)

              See examples section below for an example on setting up a TUN device.

              You must use either tun devices on both ends of the connection or tap devices on both  ends.   You
              cannot mix them, as they represent different underlying network layers.

              tun  devices  encapsulate  IPv4 or IPv6 (OSI Layer 3) while tap devices encapsulate Ethernet 802.3
              (OSI Layer 2).

       --dev-type device-type
              Which device type are we using?  device-type should be tun (OSI Layer 3) or  tap  (OSI  Layer  2).
              Use this option only if the TUN/TAP device used with --dev does not begin with tun or tap.

       --topology mode
              Configure  virtual  addressing  topology  when  running  in --dev tun mode.  This directive has no
              meaning in --dev tap mode, which always uses a subnet topology.

              If you set this directive  on  the  server,  the  --server  and  --server-bridge  directives  will
              automatically  push  your  chosen topology setting to clients as well.  This directive can also be
              manually pushed to clients.  Like the --dev directive, this directive must  always  be  compatible
              between client and server.

              mode can be one of:

              net30 -- Use a point-to-point topology, by allocating one /30 subnet per client.  This is designed
              to allow point-to-point semantics when some or all of the  connecting  clients  might  be  Windows
              systems.  This is the default on OpenVPN 2.0.

              p2p  --  Use  a  point-to-point  topology  where the remote endpoint of the client's tun interface
              always points to the local endpoint of the server's tun interface.  This mode allocates  a  single
              IP  address  per  connecting  client.   Only  use  when none of the connecting clients are Windows
              systems.  This mode is functionally equivalent to the --ifconfig-pool-linear  directive  which  is
              available in OpenVPN 2.0, is deprecated and will be removed in OpenVPN 2.5

              subnet -- Use a subnet rather than a point-to-point topology by configuring the tun interface with
              a local IP address and subnet mask, similar to  the  topology  used  in  --dev  tap  and  ethernet
              bridging mode.  This mode allocates a single IP address per connecting client and works on Windows
              as well.  Only available when server and clients are OpenVPN 2.1 or higher, or OpenVPN 2.0.x which
              has  been  manually  patched  with  the --topology directive code.  When used on Windows, requires
              version 8.2 or higher of the TAP-Win32 driver.  When used on *nix, requires that  the  tun  driver
              supports an ifconfig(8) command which sets a subnet instead of a remote endpoint IP address.

              This option exists in OpenVPN 2.1 or higher.

              Note:  Using  --topology  subnet changes the interpretation of the arguments of --ifconfig to mean
              "address netmask", no longer "local remote".

       --dev-node node
              Explicitly set the device node rather  than  using  /dev/net/tun,  /dev/tun,  /dev/tap,  etc.   If
              OpenVPN  cannot  figure out whether node is a TUN or TAP device based on the name, you should also
              specify --dev-type tun or --dev-type tap.

              Under Mac OS X this option can be used to specify the default tun implementation. Using --dev-node
              utun  forces  usage  of  the  native  Darwin  tun kernel support. Use --dev-node utunN to select a
              specific utun instance. To force using the tun.kext (/dev/tunX)  use  --dev-node  tun.   When  not
              specifying a --dev-node option openvpn will first try to open utun, and fall back to tun.kext.

              On  Windows  systems,  select the TAP-Win32 adapter which is named node in the Network Connections
              Control Panel or the raw GUID of the adapter enclosed by braces.  The --show-adapters option under
              Windows  can  also  be  used  to enumerate all available TAP-Win32 adapters and will show both the
              network connections control panel name and the GUID for each TAP-Win32 adapter.

       --lladdr address
              Specify the link layer address, more commonly known as the  MAC  address.   Only  applied  to  TAP
              devices.

       --iproute cmd
              Set  alternate  command  to  execute instead of default iproute2 command.  May be used in order to
              execute OpenVPN in unprivileged environment.

       --ifconfig l rn
              Set TUN/TAP adapter parameters.  l is the IP address of the local VPN endpoint.  For  TUN  devices
              in  point-to-point mode, rn is the IP address of the remote VPN endpoint.  For TAP devices, or TUN
              devices used with --topology subnet, rn is the subnet mask of the virtual network segment which is
              being created or connected to.

              For  TUN  devices, which facilitate virtual point-to-point IP connections (when used in --topology
              net30 or p2p mode), the proper usage of --ifconfig is to use two private IP  addresses  which  are
              not  a  member  of  any  existing subnet which is in use.  The IP addresses may be consecutive and
              should have their order reversed on the remote peer.  After the VPN is established, by pinging rn,
              you will be pinging across the VPN.

              For  TAP devices, which provide the ability to create virtual ethernet segments, or TUN devices in
              --topology subnet mode (which create virtual "multipoint networks"), --ifconfig is used to set  an
              IP  address and subnet mask just as a physical ethernet adapter would be similarly configured.  If
              you are attempting to connect to a remote ethernet bridge, the IP address and subnet should be set
              to  values  which  would  be valid on the the bridged ethernet segment (note also that DHCP can be
              used for the same purpose).

              This option, while primarily a proxy for the ifconfig(8) command, is designed to simplify  TUN/TAP
              tunnel  configuration  by providing a standard interface to the different ifconfig implementations
              on different platforms.

              --ifconfig parameters which are IP addresses can also be specified as a  DNS  or  /etc/hosts  file
              resolvable name.

              For  TAP devices, --ifconfig should not be used if the TAP interface will be getting an IP address
              lease from a DHCP server.

       --ifconfig-noexec
              Don't actually execute ifconfig/netsh commands, instead  pass  --ifconfig  parameters  to  scripts
              using environmental variables.

       --ifconfig-nowarn
              Don't  output  an  options  consistency check warning if the --ifconfig option on this side of the
              connection doesn't match the remote side.  This is useful when you  want  to  retain  the  overall
              benefits of the options consistency check (also see --disable-occ option) while only disabling the
              ifconfig component of the check.

              For example, if you have a configuration where the local host uses --ifconfig but the remote  host
              does not, use --ifconfig-nowarn on the local host.

              This  option will also silence warnings about potential address conflicts which occasionally annoy
              more experienced users by triggering "false positive" warnings.

       --route network/IP [netmask] [gateway] [metric]
              Add route to routing table after connection is established.  Multiple  routes  can  be  specified.
              Routes will be automatically torn down in reverse order prior to TUN/TAP device close.

              This  option  is intended as a convenience proxy for the route(8) shell command, while at the same
              time providing portable semantics across OpenVPN's platform space.

              netmask default -- 255.255.255.255

              gateway default -- taken from --route-gateway or the second parameter to --ifconfig when --dev tun
              is specified.

              metric default -- taken from --route-metric otherwise 0.

              The default can be specified by leaving an option blank or setting it to "default".

              The  network  and  gateway parameters can also be specified as a DNS or /etc/hosts file resolvable
              name, or as one of three special keywords:

              vpn_gateway -- The remote VPN endpoint address (derived either from --route-gateway or the  second
              parameter to --ifconfig when --dev tun is specified).

              net_gateway  -- The pre-existing IP default gateway, read from the routing table (not supported on
              all OSes).

              remote_host -- The --remote address if OpenVPN is being run in client mode, and  is  undefined  in
              server mode.

       --route-gateway gw|'dhcp'
              Specify a default gateway gw for use with --route.

              If  dhcp  is  specified  as  the  parameter,  the  gateway  address  will be extracted from a DHCP
              negotiation with the OpenVPN server-side LAN.

       --route-metric m
              Specify a default metric m for use with --route.

       --route-delay [n] [w]
              Delay n seconds (default=0) after connection establishment, before  adding  routes.  If  n  is  0,
              routes  will  be  added  immediately  upon connection establishment.  If --route-delay is omitted,
              routes will be added immediately after TUN/TAP device open and --up script execution,  before  any
              --user or --group privilege downgrade (or --chroot execution.)

              This option is designed to be useful in scenarios where DHCP is used to set tap adapter addresses.
              The delay will give the DHCP handshake time to complete before routes are added.

              On Windows, --route-delay tries to be more intelligent by waiting w seconds (w=30 by default)  for
              the TAP-Win32 adapter to come up before adding routes.

       --route-up cmd
              Run command cmd after routes are added, subject to --route-delay.

              cmd  consists  of  a path to script (or executable program), optionally followed by arguments. The
              path and arguments may be single- or double-quoted and/or escaped using a backslash, and should be
              separated by one or more spaces.

              See  the "Environmental Variables" section below for additional parameters passed as environmental
              variables.

       --route-pre-down cmd
              Run command cmd before routes are removed upon disconnection.

              cmd consists of a path to script (or executable program), optionally followed  by  arguments.  The
              path and arguments may be single- or double-quoted and/or escaped using a backslash, and should be
              separated by one or more spaces.

              See the "Environmental Variables" section below for additional parameters passed as  environmental
              variables.

       --route-noexec
              Don't  add  or  remove  routes  automatically.   Instead  pass  routes  to --route-up script using
              environmental variables.

       --route-nopull
              When  used  with  --client  or  --pull,  accept  options  pushed  by  server  EXCEPT  for  routes,
              block-outside-dns and dhcp options like DNS servers.

              When  used  on  the  client,  this  option  effectively  bars the server from adding routes to the
              client's routing table, however note that this option still allows the server to  set  the  TCP/IP
              properties of the client's TUN/TAP interface.

       --allow-pull-fqdn
              Allow  client  to  pull  DNS  names  from  server  (rather  than  being limited to IP address) for
              --ifconfig, --route, and --route-gateway.

       --client-nat snat|dnat network netmask alias
              This pushable client option sets up a stateless one-to-one  NAT  rule  on  packet  addresses  (not
              ports), and is useful in cases where routes or ifconfig settings pushed to the client would create
              an IP numbering conflict.

              network/netmask (for example 192.168.0.0/255.255.0.0) defines the local view of  a  resource  from
              the client perspective, while alias/netmask (for example 10.64.0.0/255.255.0.0) defines the remote
              view from the server perspective.

              Use snat (source NAT) for resources owned by the client and  dnat  (destination  NAT)  for  remote
              resources.

              Set --verb 6 for debugging info showing the transformation of src/dest addresses in packets.

       --redirect-gateway flags...
              Automatically  execute routing commands to cause all outgoing IP traffic to be redirected over the
              VPN.  This is a client-side option.

              This option performs three steps:

              (1) Create a static route for the --remote address which  forwards  to  the  pre-existing  default
              gateway.  This is done so that (3) will not create a routing loop.

              (2) Delete the default gateway route.

              (3)   Set  the  new  default  gateway  to  be  the  VPN  endpoint  address  (derived  either  from
              --route-gateway or the second parameter to --ifconfig when --dev tun is specified).

              When the tunnel is torn down, all of the above steps are reversed so  that  the  original  default
              route is restored.

              Option flags:

              local  --  Add  the local flag if both OpenVPN servers are directly connected via a common subnet,
              such as with wireless.  The local flag will cause step 1 above to be omitted.

              autolocal -- Try to automatically determine whether to enable local flag above.

              def1 -- Use this flag to override the default gateway by using 0.0.0.0/1  and  128.0.0.0/1  rather
              than  0.0.0.0/0.   This  has  the  benefit  of  overriding but not wiping out the original default
              gateway.

              bypass-dhcp -- Add a direct route to the DHCP server (if  it  is  non-local)  which  bypasses  the
              tunnel (Available on Windows clients, may not be available on non-Windows clients).

              bypass-dns  --  Add a direct route to the DNS server(s) (if they are non-local) which bypasses the
              tunnel (Available on Windows clients, may not be available on non-Windows clients).

              block-local -- Block access to local LAN when the tunnel is active, except  for  the  LAN  gateway
              itself.   This  is accomplished by routing the local LAN (except for the LAN gateway address) into
              the tunnel.

              ipv6 -- Redirect IPv6 routing into the tunnel.  This works similar to the def1 flag, that is, more
              specific IPv6 routes are added (2000::/4, 3000::/4), covering the whole IPv6 unicast space.

              !ipv4  --  Do  not  redirect IPv4 traffic - typically used in the flag pair ipv6 !ipv4 to redirect
              IPv6-only.

       --link-mtu n
              Sets an upper bound on the size of UDP packets which are sent between OpenVPN  peers.   It's  best
              not to set this parameter unless you know what you're doing.

       --redirect-private [flags]
              Like  --redirect-gateway,  but  omit  actually  changing the default gateway.  Useful when pushing
              private subnets.

       --tun-mtu n
              Take the TUN device MTU to be n and derive the link MTU from it (default=1500).   In  most  cases,
              you will probably want to leave this parameter set to its default value.

              The  MTU  (Maximum  Transmission  Units)  is  the  maximum datagram size in bytes that can be sent
              unfragmented over a particular network path.  OpenVPN requires that packets on the control or data
              channels be sent unfragmented.

              MTU problems often manifest themselves as connections which hang during periods of active usage.

              It's best to use the --fragment and/or --mssfix options to deal with MTU sizing issues.

       --tun-mtu-extra n
              Assume  that  the  TUN/TAP  device might return as many as n bytes more than the --tun-mtu size on
              read.  This parameter defaults to 0, which is sufficient for most TUN devices.   TAP  devices  may
              introduce  additional  overhead in excess of the MTU size, and a setting of 32 is the default when
              TAP devices are used.  This parameter only controls internal OpenVPN buffer sizing, so there is no
              transmission overhead associated with using a larger value.

       --mtu-disc type
              Should  we  do  Path  MTU discovery on TCP/UDP channel?  Only supported on OSes such as Linux that
              supports the necessary system call to set.

              'no' -- Never send DF (Don't Fragment) frames
              'maybe' -- Use per-route hints
              'yes' -- Always DF (Don't Fragment)

       --mtu-test
              To empirically measure MTU on connection startup, add the --mtu-test option to your configuration.
              OpenVPN will send ping packets of various sizes to the remote peer and measure the largest packets
              which were successfully received.  The --mtu-test  process  normally  takes  about  3  minutes  to
              complete.

       --fragment max
              Enable internal datagram fragmentation so that no UDP datagrams are sent which are larger than max
              bytes.

              The max parameter is interpreted in the same way as the --link-mtu parameter, i.e. the UDP  packet
              size after encapsulation overhead has been added in, but not including the UDP header itself.

              The --fragment option only makes sense when you are using the UDP protocol ( --proto udp ).

              --fragment adds 4 bytes of overhead per datagram.

              See the --mssfix option below for an important related option to --fragment.

              It should also be noted that this option is not meant to replace UDP fragmentation at the IP stack
              level.  It is only meant as a last resort when path MTU discovery is broken.  Using this option is
              less  efficient  than fixing path MTU discovery for your IP link and using native IP fragmentation
              instead.

              Having said that, there are circumstances where using OpenVPN's internal fragmentation  capability
              may be your only option, such as tunneling a UDP multicast stream which requires fragmentation.

       --mssfix max
              Announce  to  TCP  sessions running over the tunnel that they should limit their send packet sizes
              such that after OpenVPN has encapsulated them, the resulting UDP packet size that OpenVPN sends to
              its peer will not exceed max bytes. The default value is 1450.

              The  max parameter is interpreted in the same way as the --link-mtu parameter, i.e. the UDP packet
              size after encapsulation overhead has been added in, but not  including  the  UDP  header  itself.
              Resulting  packet would be at most 28 bytes larger for IPv4 and 48 bytes for IPv6 (20/40 bytes for
              IP header and 8 bytes for UDP header). Default value of 1450 allows IPv4 packets to be transmitted
              over a link with MTU 1473 or higher without IP level fragmentation.

              The  --mssfix option only makes sense when you are using the UDP protocol for OpenVPN peer-to-peer
              communication, i.e.  --proto udp.

              --mssfix and --fragment can be ideally used together, where --mssfix will try  to  keep  TCP  from
              needing  packet  fragmentation  in  the  first place, and if big packets come through anyhow (from
              protocols other than TCP), --fragment will internally fragment them.

              Both --fragment and --mssfix are designed to work around cases where Path MTU discovery is  broken
              on the network path between OpenVPN peers.

              The usual symptom of such a breakdown is an OpenVPN connection which successfully starts, but then
              stalls during active usage.

              If --fragment and --mssfix are used together, --mssfix will take its default  max  parameter  from
              the --fragment max option.

              Therefore,  one  could  lower  the  maximum  UDP packet size to 1300 (a good first try for solving
              MTU-related connection problems) with the following options:

              --tun-mtu 1500 --fragment 1300 --mssfix

       --sndbuf size
              Set the TCP/UDP socket send buffer size.  Defaults to operation system default.

       --rcvbuf size
              Set the TCP/UDP socket receive buffer size.  Defaults to operation system default.

       --mark value
              Mark encrypted packets being sent with value. The mark value can be matched in policy routing  and
              packetfilter  rules.  This  option  is only supported in Linux and does nothing on other operating
              systems.

       --socket-flags flags...
              Apply the given flags to the OpenVPN transport socket.  Currently, only TCP_NODELAY is supported.

              The TCP_NODELAY socket flag is useful in TCP mode, and causes the kernel to  send  tunnel  packets
              immediately  over the TCP connection without trying to group several smaller packets into a larger
              packet.  This can result in a considerably improvement in latency.

              This option is pushable from server to client, and should be used on both client  and  server  for
              maximum effect.

       --txqueuelen n
              (Linux only) Set the TX queue length on the TUN/TAP interface.  Currently defaults to 100.

       --shaper n
              Limit bandwidth of outgoing tunnel data to n bytes per second on the TCP/UDP port.  Note that this
              will only work if mode is set to p2p.  If you want to limit the bandwidth in both directions,  use
              this option on both peers.

              OpenVPN  uses the following algorithm to implement traffic shaping: Given a shaper rate of n bytes
              per second, after a datagram write of b bytes is queued on the TCP/UDP port, wait a minimum of  (b
              / n) seconds before queuing the next write.

              It should be noted that OpenVPN supports multiple tunnels between the same two peers, allowing you
              to construct full-speed and reduced bandwidth tunnels at the same time, routing low-priority  data
              such  as  off-site  backups  over the reduced bandwidth tunnel, and other data over the full-speed
              tunnel.

              Also note that for low bandwidth tunnels (under 1000 bytes per second), you  should  probably  use
              lower  MTU  values  as  well  (see  above),  otherwise the packet latency will grow so large as to
              trigger timeouts in the TLS layer and TCP connections running over the tunnel.

              OpenVPN allows n to be between 100 bytes/sec and 100 Mbytes/sec.

       --inactive n [bytes]
              Causes OpenVPN to exit after n seconds of inactivity on the TUN/TAP device.  The  time  length  of
              inactivity  is measured since the last incoming or outgoing tunnel packet.  The default value is 0
              seconds, which disables this feature.

              If the optional bytes parameter is included, exit if less than bytes of  combined  in/out  traffic
              are produced on the tun/tap device in n seconds.

              In  any  case, OpenVPN's internal ping packets (which are just keepalives) and TLS control packets
              are not considered "activity", nor are they counted as traffic, as they  are  used  internally  by
              OpenVPN and are not an indication of actual user activity.

       --ping n
              Ping  remote  over the TCP/UDP control channel if no packets have been sent for at least n seconds
              (specify --ping on both peers to cause ping packets to be sent in both  directions  since  OpenVPN
              ping  packets  are  not  echoed like IP ping packets).  When used in one of OpenVPN's secure modes
              (where  --secret,  --tls-server,  or  --tls-client  is  specified),  the  ping  packet   will   be
              cryptographically secure.

              This option has two intended uses:

              (1) Compatibility with stateful firewalls.  The periodic ping will ensure that a stateful firewall
              rule which allows OpenVPN UDP packets to pass will not time out.

              (2) To provide a basis for the remote to test the existence of  its  peer  using  the  --ping-exit
              option.

       --ping-exit n
              Causes  OpenVPN  to  exit  after  n  seconds pass without reception of a ping or other packet from
              remote.  This option can be  combined  with  --inactive,  --ping,  and  --ping-exit  to  create  a
              two-tiered inactivity disconnect.

              For example,

              openvpn [options...] --inactive 3600 --ping 10 --ping-exit 60

              when  used on both peers will cause OpenVPN to exit within 60 seconds if its peer disconnects, but
              will exit after one hour if no actual tunnel data is exchanged.

       --ping-restart n
              Similar to --ping-exit, but trigger a SIGUSR1 restart after n seconds pass without reception of  a
              ping or other packet from remote.

              This  option  is  useful in cases where the remote peer has a dynamic IP address and a low-TTL DNS
              name is used to track the IP address using a service such as http://dyndns.org/ +  a  dynamic  DNS
              client such as ddclient.

              If  the  peer  cannot  be  reached,  a  restart  will be triggered, causing the hostname used with
              --remote to be re-resolved (if --resolv-retry is also specified).

              In server mode, --ping-restart, --inactive, or any other type of internally generated signal  will
              always  be applied to individual client instance objects, never to whole server itself.  Note also
              in server mode that any internally generated signal which would normally  cause  a  restart,  will
              cause the deletion of the client instance object instead.

              In  client mode, the --ping-restart parameter is set to 120 seconds by default.  This default will
              hold until the client pulls a replacement value from the server, based on the --keepalive  setting
              in  the  server  configuration.   To  disable  the 120 second default, set --ping-restart 0 on the
              client.

              See the signals section below for more information on SIGUSR1.

              Note  that  the  behavior  of  SIGUSR1  can  be  modified  by  the  --persist-tun,  --persist-key,
              --persist-local-ip, and --persist-remote-ip options.

              Also note that --ping-exit and --ping-restart are mutually exclusive and cannot be used together.

       --keepalive interval timeout
              A helper directive designed to simplify the expression of --ping and --ping-restart.

              This  option  can  be  used on both client and server side, but it is in enough to add this on the
              server side as it will push appropriate --ping and --ping-restart options to the client.  If  used
              on both server and client, the values pushed from server will override the client local values.

              The  timeout  argument  will  be twice as long on the server side.  This ensures that a timeout is
              detected on client side before the server side drops the connection.

              For example, --keepalive 10 60 expands as follows:

                   if mode server:
                     ping 10                    # Argument: interval
                     ping-restart 120           # Argument: timeout*2
                     push "ping 10"             # Argument: interval
                     push "ping-restart 60"     # Argument: timeout
                   else
                     ping 10                    # Argument: interval
                     ping-restart 60            # Argument: timeout

       --ping-timer-rem
              Run the --ping-exit / --ping-restart timer only if we have a remote address.  Use this  option  if
              you are starting the daemon in listen mode (i.e. without an explicit --remote peer), and you don't
              want to start clocking timeouts until a remote peer connects.

       --persist-tun
              Don't close and reopen TUN/TAP device or run up/down  scripts  across  SIGUSR1  or  --ping-restart
              restarts.

              SIGUSR1  is  a restart signal similar to SIGHUP, but which offers finer-grained control over reset
              options.

       --persist-key
              Don't re-read key files across SIGUSR1 or --ping-restart.

              This option can be combined with --user nobody to allow restarts triggered by the SIGUSR1  signal.
              Normally  if you drop root privileges in OpenVPN, the daemon cannot be restarted since it will now
              be unable to re-read protected key files.

              This option solves the problem by persisting keys across SIGUSR1 resets, so they don't need to  be
              re-read.

       --persist-local-ip
              Preserve  initially  resolved  local  IP  address and port number across SIGUSR1 or --ping-restart
              restarts.

       --persist-remote-ip
              Preserve most recently  authenticated  remote  IP  address  and  port  number  across  SIGUSR1  or
              --ping-restart restarts.

       --mlock
              Disable  paging by calling the POSIX mlockall function.  Requires that OpenVPN be initially run as
              root (though OpenVPN can subsequently downgrade its UID using the --user option).

              Using this option ensures that key material and tunnel data are  never  written  to  disk  due  to
              virtual memory paging operations which occur under most modern operating systems.  It ensures that
              even if an attacker was able to crack the box running OpenVPN, he would not be able  to  scan  the
              system  swap  file  to recover previously used ephemeral keys, which are used for a period of time
              governed by the --reneg options (see below), then are discarded.

              The downside of using --mlock is that it will reduce the amount of physical  memory  available  to
              other applications.

       --up cmd
              Run command cmd after successful TUN/TAP device open (pre --user UID change).

              cmd  consists  of  a path to script (or executable program), optionally followed by arguments. The
              path and arguments may be single- or double-quoted and/or escaped using a backslash, and should be
              separated by one or more spaces.

              The up command is useful for specifying route commands which route IP traffic destined for private
              subnets which exist at the other end of the VPN connection into the tunnel.

              For --dev tun execute as:

              cmd tun_dev tun_mtu link_mtu ifconfig_local_ip ifconfig_remote_ip [ init | restart ]

              For --dev tap execute as:

              cmd tap_dev tap_mtu link_mtu ifconfig_local_ip ifconfig_netmask [ init | restart ]

              See the "Environmental Variables" section below for additional parameters passed as  environmental
              variables.

              Note  that  if cmd includes arguments, all OpenVPN-generated arguments will be appended to them to
              build an argument list with which the executable will be called.

              Typically, cmd will run a script to add routes to the tunnel.

              Normally the up script is called after the TUN/TAP device is opened.  In this  context,  the  last
              command  line  parameter  passed  to  the script will be init.  If the --up-restart option is also
              used, the up script will be called for restarts as well.  A restart is considered to be a  partial
              reinitialization of OpenVPN where the TUN/TAP instance is preserved (the --persist-tun option will
              enable such preservation).  A restart can be generated  by  a  SIGUSR1  signal,  a  --ping-restart
              timeout,  or  a  connection  reset when the TCP protocol is enabled with the --proto option.  If a
              restart occurs, and --up-restart has been specified, the up script will be called with restart  as
              the last parameter.

              NOTE:  on  restart,  OpenVPN  will  not  pass the full set of environment variables to the script.
              Namely, everything related to routing and gateways will not be passed, as nothing needs to be done
              anyway  - all the routing setup is already in place.  Additionally, the up-restart script will run
              with the downgraded UID/GID settings (if configured).

              The following standalone example shows how the --up script can be called in both an initialization
              and restart context.  (NOTE: for security reasons, don't run the following example unless UDP port
              9999 is blocked by your firewall.  Also, the example will run indefinitely, so  you  should  abort
              with control-c).

              openvpn  --dev  tun  --port  9999  --verb  4  --ping-restart  10 --up 'echo up' --down 'echo down'
              --persist-tun --up-restart

              Note that OpenVPN also provides the --ifconfig option to automatically ifconfig  the  TUN  device,
              eliminating  the  need  to  define an --up script, unless you also want to configure routes in the
              --up script.

              If --ifconfig is also specified, OpenVPN will pass the ifconfig local and remote endpoints on  the
              command line to the --up script so that they can be used to configure routes such as:

              route add -net 10.0.0.0 netmask 255.255.255.0 gw $5

       --up-delay
              Delay TUN/TAP open and possible --up script execution until after TCP/UDP connection establishment
              with peer.

              In --proto udp mode, this  option  normally  requires  the  use  of  --ping  to  allow  connection
              initiation to be sensed in the absence of tunnel data, since UDP is a "connectionless" protocol.

              On  Windows,  this  option will delay the TAP-Win32 media state transitioning to "connected" until
              connection establishment, i.e. the receipt of the first authenticated packet from the peer.

       --down cmd
              Run command cmd after TUN/TAP device close  (post  --user  UID  change  and/or  --chroot  ).   cmd
              consists  of  a path to script (or executable program), optionally followed by arguments. The path
              and arguments may be single- or double-quoted and/or escaped using  a  backslash,  and  should  be
              separated by one or more spaces.

              Called with the same parameters and environmental variables as the --up option above.

              Note  that  if  you reduce privileges by using --user and/or --group, your --down script will also
              run at reduced privilege.

       --down-pre
              Call --down cmd/script before, rather than after, TUN/TAP close.

       --up-restart
              Enable the --up and --down scripts to be called for restarts as well  as  initial  program  start.
              This option is described more fully above in the --up option documentation.

       --setenv name value
              Set a custom environmental variable name=value to pass to script.

       --setenv FORWARD_COMPATIBLE 1
              Relax  config  file  syntax  checking  so that unknown directives will trigger a warning but not a
              fatal error, on the assumption that a given unknown directive might be  valid  in  future  OpenVPN
              versions.

              This  option  should  be  used with caution, as there are good security reasons for having OpenVPN
              fail if it detects problems in a config file.  Having said  that,  there  are  valid  reasons  for
              wanting new software features to gracefully degrade when encountered by older software versions.

              It  is also possible to tag a single directive so as not to trigger a fatal error if the directive
              isn't recognized.  To do this, prepend the following before the directive: setenv opt

              Versions prior to OpenVPN 2.3.3 will always ignore options set with the setenv opt directive.

              See also --ignore-unknown-option

       --setenv-safe name value
              Set a custom environmental variable OPENVPN_name=value to pass to script.

              This directive is designed to be pushed by the server to clients, and the prepending of "OPENVPN_"
              to  the  environmental variable is a safety precaution to prevent a LD_PRELOAD style attack from a
              malicious or compromised server.

       --ignore-unknown-option opt1 opt2 opt3 ... optN
              When one of options opt1 ... optN is encountered in the configuration file the configuration  file
              parsing   does   not  fail  if  this  OpenVPN  version  does  not  support  the  option.  Multiple
              --ignore-unknown-option options can be given to support a larger number of options to ignore.

              This option should be used with caution, as there are good security  reasons  for  having  OpenVPN
              fail  if  it  detects  problems  in  a  config file. Having said that, there are valid reasons for
              wanting new software features to gracefully degrade when encountered by older software versions.

              --ignore-unknown-option is available since OpenVPN 2.3.3.

       --script-security level
              This directive offers policy-level control over OpenVPN's usage of external programs and  scripts.
              Lower level values are more restrictive, higher values are more permissive.  Settings for level:

              0 -- Strictly no calling of external programs.
              1 -- (Default) Only call built-in executables such as ifconfig, ip, route, or netsh.
              2 -- Allow calling of built-in executables and user-defined scripts.
              3 -- Allow passwords to be passed to scripts via environmental variables (potentially unsafe).

              OpenVPN  releases before v2.3 also supported a method flag which indicated how OpenVPN should call
              external commands and scripts.  This could be either execve or system.  As of  OpenVPN  2.3,  this
              flag is no longer accepted.  In most *nix environments the execve() approach has been used without
              any issues.

              Some directives such as --up allow options to be passed to the external  script.  In  these  cases
              make  sure  the  script  name  does  not contain any spaces or the configuration parser will choke
              because it can't determine where the script name ends and script options start.

              To run scripts in Windows in earlier OpenVPN versions you needed to either add a full path to  the
              script  interpreter which can parse the script or use the system flag to run these scripts.  As of
              OpenVPN 2.3 it is now a strict requirement to have  full  path  to  the  script  interpreter  when
              running  non-executables files.  This is not needed for executable files, such as .exe, .com, .bat
              or .cmd files.  For example, if you have a Visual Basic script, you must use this syntax now:

                  --up 'C:\\Windows\\System32\\wscript.exe C:\\Program\ Files\\OpenVPN\\config\\my-up-script.vbs'

              Please note the single quote marks  and  the  escaping  of  the  backslashes  (\)  and  the  space
              character.

              The  reason  the  support for the system flag was removed is due to the security implications with
              shell expansions when executing scripts via the system() call.

       --disable-occ
              Don't output a warning message if option inconsistencies are detected between peers.   An  example
              of  an option inconsistency would be where one peer uses --dev tun while the other peer uses --dev
              tap.

              Use of this option is discouraged, but is provided as a temporary fix in situations where a recent
              version of OpenVPN must connect to an old version.

       --user user
              Change the user ID of the OpenVPN process to user after initialization, dropping privileges in the
              process.  This option is useful to protect the system in the event that  some  hostile  party  was
              able  to  gain  control  of  an  OpenVPN  session.   Though  OpenVPN's security features make this
              unlikely, it is provided as a second line of defense.

              By setting user to nobody or somebody similarly unprivileged, the hostile party would  be  limited
              in  what damage they could cause.  Of course once you take away privileges, you cannot return them
              to an OpenVPN session.  This means, for example, that if you want to reset an OpenVPN daemon  with
              a  SIGUSR1 signal (for example in response to a DHCP reset), you should make use of one or more of
              the --persist options to ensure that OpenVPN doesn't need to execute any privileged operations  in
              order to restart (such as re-reading key files or running ifconfig on the TUN device).

       --group group
              Similar  to  the  --user  option, this option changes the group ID of the OpenVPN process to group
              after initialization.

       --cd dir
              Change directory to dir prior to reading  any  files  such  as  configuration  files,  key  files,
              scripts,  etc.   dir should be an absolute path, with a leading "/", and without any references to
              the current directory such as "." or "..".

              This option is useful when you are running OpenVPN in --daemon mode, and you want  to  consolidate
              all of your OpenVPN control files in one location.

       --chroot dir
              Chroot  to  dir  after  initialization.  --chroot essentially redefines dir as being the top level
              directory tree (/).  OpenVPN will therefore be unable to access any files outside this tree.  This
              can be desirable from a security standpoint.

              Since  the  chroot  operation  is  delayed  until  after initialization, most OpenVPN options that
              reference files will operate in a pre-chroot context.

              In many cases, the dir parameter can point to an empty directory, however complications can result
              when scripts or restarts are executed after the chroot operation.

              Note:  The SSL library will probably need /dev/urandom to be available inside the chroot directory
              dir.  This is because SSL libraries occasionally  need  to  collect  fresh  random.   Newer  linux
              kernels  and  some  BSDs implement a getrandom() or getentropy() syscall that removes the need for
              /dev/urandom to be available.

       --setcon context
              Apply SELinux context after initialization. This essentially  provides  the  ability  to  restrict
              OpenVPN's  rights to only network I/O operations, thanks to SELinux. This goes further than --user
              and --chroot in that those two, while being great security features, unfortunately do not  protect
              against  privilege  escalation  by  exploitation  of  a  vulnerable system call. You can of course
              combine all three, but please note that since setcon requires access to /proc  you  will  have  to
              provide it inside the chroot directory (e.g. with mount --bind).

              Since  the  setcon  operation  is delayed until after initialization, OpenVPN can be restricted to
              just network-related system calls, whereas by applying the context before  startup  (such  as  the
              OpenVPN  one  provided  in  the  SELinux  Reference  Policies)  you will have to allow many things
              required only during initialization.

              Like with chroot, complications can result when scripts or restarts are executed after the  setcon
              operation,  which  is  why  you  should  really consider using the --persist-key and --persist-tun
              options.

       --daemon [progname]
              Become a daemon after all initialization functions are completed.   This  option  will  cause  all
              message and error output to be sent to the syslog file (such as /var/log/messages), except for the
              output of scripts and ifconfig commands, which will go to /dev/null unless  otherwise  redirected.
              The syslog redirection occurs immediately at the point that --daemon is parsed on the command line
              even though the daemonization point occurs later.  If one of the --log options is present, it will
              supercede syslog redirection.

              The optional progname parameter will cause OpenVPN to report its program name to the system logger
              as progname.  This can be useful in linking OpenVPN messages in  the  syslog  file  with  specific
              tunnels.  When unspecified, progname defaults to "openvpn".

              When  OpenVPN  is  run  with  the  --daemon  option,  it will try to delay daemonization until the
              majority of initialization functions which are capable of generating fatal  errors  are  complete.
              This  means  that  initialization  scripts can test the return status of the openvpn command for a
              fairly reliable indication of whether the command has correctly initialized and entered the packet
              forwarding event loop.

              In OpenVPN, the vast majority of errors which occur after initialization are non-fatal.

              Note:  as  soon  as  OpenVPN  has daemonized, it can not ask for usernames, passwords, or key pass
              phrases anymore.  This has certain consequences, namely that using  a  password-protected  private
              key will fail unless the --askpass option is used to tell OpenVPN to ask for the pass phrase (this
              requirement is new in v2.3.7, and is a consequence of calling  daemon()  before  initializing  the
              crypto layer).

              Further,  using  --daemon  together  with --auth-user-pass (entered on console) and --auth-nocache
              will fail as soon as key renegotiation (and reauthentication) occurs.

       --syslog [progname]
              Direct log output to system logger, but do not become a daemon.  See --daemon directive above  for
              description of progname parameter.

       --errors-to-stderr
              Output  errors  to  stderr  instead  of stdout unless log output is redirected by one of the --log
              options.

       --passtos
              Set the TOS field of the tunnel packet to what the payload's TOS is.

       --inetd [wait|nowait] [progname]
              Use this option when OpenVPN is being run from the inetd or xinetd(8) server.

              The wait/nowait option must match what is specified in the inetd/xinetd config file.   The  nowait
              mode  can only be used with --proto tcp-server.  The default is wait.  The nowait mode can be used
              to instantiate the OpenVPN daemon as a classic TCP server, where client  connection  requests  are
              serviced  on  a single port number.  For additional information on this kind of configuration, see
              the OpenVPN FAQ: http://openvpn.net/faq.html#oneport

              This option precludes the use of --daemon, --local, or --remote.  Note  that  this  option  causes
              message  and  error  output  to  be  handled in the same way as the --daemon option.  The optional
              progname parameter is also handled exactly as in --daemon.

              Also note that in wait mode, each OpenVPN tunnel requires a separate TCP/UDP port and  a  separate
              inetd  or  xinetd  entry.   See the OpenVPN 1.x HOWTO for an example on using OpenVPN with xinetd:
              http://openvpn.net/1xhowto.html

       --log file
              Output logging messages to file, including output to stdout/stderr which is  generated  by  called
              scripts.   If file already exists it will be truncated.  This option takes effect immediately when
              it is parsed in the command line and will supercede syslog output if --daemon or --inetd  is  also
              specified.   This option is persistent over the entire course of an OpenVPN instantiation and will
              not be reset by SIGHUP, SIGUSR1, or --ping-restart.

              Note that on Windows, when OpenVPN is started as a service, logging occurs by default without  the
              need to specify this option.

       --log-append file
              Append logging messages to file.  If file does not exist, it will be created.  This option behaves
              exactly like --log except that it appends to rather than truncating the log file.

       --suppress-timestamps
              Avoid writing timestamps to log  messages,  even  when  they  otherwise  would  be  prepended.  In
              particular, this applies to log messages sent to stdout.

       --machine-readable-output
              Always  write  timestamps and message flags to log messages, even when they otherwise would not be
              prefixed. In particular, this applies to log messages sent to stdout.

       --writepid file
              Write OpenVPN's main process ID to file.

       --nice n
              Change process priority after initialization ( n greater than 0 is lower  priority,  n  less  than
              zero is higher priority).

       --fast-io
              (Experimental)  Optimize  TUN/TAP/UDP  I/O writes by avoiding a call to poll/epoll/select prior to
              the write operation.  The purpose of such a call would normally be to block until  the  device  or
              socket  is  ready to accept the write.  Such blocking is unnecessary on some platforms which don't
              support write blocking on UDP sockets or TUN/TAP devices.  In such cases,  one  can  optimize  the
              event loop by avoiding the poll/epoll/select call, improving CPU efficiency by 5% to 10%.

              This  option  can  only  be  used  on non-Windows systems, when --proto udp is specified, and when
              --shaper is NOT specified.

       --multihome
              Configure a multi-homed UDP server.  This option needs to be used when a server has more than  one
              IP  address  (e.g.  multiple  interfaces,  or secondary IP addresses), and is not using --local to
              force binding to one specific address only.  This option will add some extra lookups to the packet
              path  to  ensure  that  the  UDP reply packets are always sent from the address that the client is
              talking to. This is not supported on all platforms, and it  adds  more  processing,  so  it's  not
              enabled by default.

              Note: this option is only relevant for UDP servers.

              Note  2:  if  you  do  an IPv6+IPv4 dual-stack bind on a Linux machine with multiple IPv4 address,
              connections to IPv4 addresses will not work right on kernels before 3.15, due  to  missing  kernel
              support  for the IPv4-mapped case (some distributions have ported this to earlier kernel versions,
              though).

       --echo [parms...]
              Echo parms to log output.

              Designed to be used to send messages to a controlling application which is receiving  the  OpenVPN
              log output.

       --remap-usr1 signal
              Control whether internally or externally generated SIGUSR1 signals are remapped to SIGHUP (restart
              without persisting state) or SIGTERM (exit).

              signal can be set to "SIGHUP" or "SIGTERM".  By default, no remapping occurs.

       --verb n
              Set output verbosity to n (default=1).  Each level shows all info from the previous levels.  Level
              3 is recommended if you want a good summary of what's happening without being swamped by output.

              0 -- No output except fatal errors.
              1 to 4 -- Normal usage range.
              5  --  Output  R and W characters to the console for each packet read and write, uppercase is used
              for TCP/UDP packets and lowercase is used for TUN/TAP packets.
              6 to 11 -- Debug info range (see errlevel.h for additional information on debug levels).

       --status file [n]
              Write operational status to file every n seconds.

              Status can also be written to the syslog by sending a SIGUSR2 signal.

       --status-version [n]
              Choose the status file format version number.  Currently n can be 1, 2, or 3 and defaults to 1.

       --mute n
              Log at most n consecutive messages in the same category.   This  is  useful  to  limit  repetitive
              logging of similar message types.

       --compress [algorithm]
              Enable a compression algorithm.

              The  algorithm  parameter  may  be  "lzo", "lz4", or empty.  LZO and LZ4 are different compression
              algorithms, with LZ4 generally offering the best performance with least CPU usage.  For  backwards
              compatibility with OpenVPN versions before v2.4, use "lzo" (which is identical to the older option
              "--comp-lzo yes").

              If the algorithm parameter is empty, compression will be turned off, but the  packet  framing  for
              compression will still be enabled, allowing a different setting to be pushed later.

       --comp-lzo [mode]
              DEPRECATED  This  option  will  be  removed in a future OpenVPN release.  Use the newer --compress
              instead.

              Use LZO compression -- may add up to 1 byte per packet  for  incompressible  data.   mode  may  be
              "yes", "no", or "adaptive" (default).

              In  a  server  mode setup, it is possible to selectively turn compression on or off for individual
              clients.

              First, make sure the client-side config file enables selective compression by having at least  one
              --comp-lzo directive, such as --comp-lzo no.  This will turn off compression by default, but allow
              a future directive push from the server to dynamically change the on/off/adaptive setting.

              Next in a --client-config-dir file, specify the compression setting for the client, for example:

                  comp-lzo yes
                  push "comp-lzo yes"

              The first line sets the comp-lzo setting for the server side of the  link,  the  second  sets  the
              client side.

       --comp-noadapt
              When  used in conjunction with --comp-lzo, this option will disable OpenVPN's adaptive compression
              algorithm.  Normally, adaptive compression is enabled with --comp-lzo.

              Adaptive compression tries to optimize the case where you have compression enabled,  but  you  are
              sending  predominantly  incompressible (or pre-compressed) packets over the tunnel, such as an FTP
              or rsync  transfer  of  a  large,  compressed  file.   With  adaptive  compression,  OpenVPN  will
              periodically  sample  the  compression  process to measure its efficiency.  If the data being sent
              over the tunnel is already compressed, the compression efficiency will  be  very  low,  triggering
              openvpn to disable compression for a period of time until the next re-sample test.

       --management IP port [pw-file]
              Enable a TCP server on IP:port to handle daemon management functions.  pw-file, if specified, is a
              password file (password on first line) or "stdin" to prompt from  standard  input.   The  password
              provided  will  set  the  password  which  TCP  clients  will  need  to provide in order to access
              management functions.

              The management interface can also listen on a unix domain socket, for those platforms that support
              it.   To use a unix domain socket, specify the unix socket pathname in place of IP and set port to
              'unix'.  While the default behavior is to create a unix domain socket that may be connected to  by
              any  process, the --management-client-user and --management-client-group directives can be used to
              restrict access.

              The management interface provides a special mode where the TCP management link  can  operate  over
              the tunnel itself.  To enable this mode, set IP = "tunnel".  Tunnel mode will cause the management
              interface to listen for a TCP connection on the local VPN address of the TUN/TAP interface.

              While the management port is designed for programmatic control of OpenVPN by  other  applications,
              it  is  possible to telnet to the port, using a telnet client in "raw" mode.  Once connected, type
              "help" for a list of commands.

              For detailed documentation on the management interface, see the management-notes.txt file  in  the
              management folder of the OpenVPN source distribution.

              It  is  strongly  recommended that IP be set to 127.0.0.1 (localhost) to restrict accessibility of
              the management server to local clients.

       --management-client
              Management interface will connect as a TCP/unix domain client to IP:port specified by --management
              rather than listen as a TCP server or on a unix domain socket.

              If  the  client connection fails to connect or is disconnected, a SIGTERM signal will be generated
              causing OpenVPN to quit.

       --management-query-passwords
              Query management channel for private key password and  --auth-user-pass  username/password.   Only
              query the management channel for inputs which ordinarily would have been queried from the console.

       --management-query-proxy
              Query management channel for proxy server information for a specific --remote (client-only).

       --management-query-remote
              Allow management interface to override --remote directives (client-only).

       --management-external-key
              Allows usage for external private key file instead of --key option (client-only).

       --management-external-cert certificate-hint
              Allows usage for external certificate instead of --cert option (client-only).  certificate-hint is
              an arbitrary string  which  is  passed  to  a  management  interface  client  as  an  argument  of
              NEED-CERTIFICATE notification.  Requires --management-external-key.

       --management-forget-disconnect
              Make OpenVPN forget passwords when management session disconnects.

              This directive does not affect the --http-proxy username/password.  It is always cached.

       --management-hold
              Start OpenVPN in a hibernating state, until a client of the management interface explicitly starts
              it with the hold release command.

       --management-signal
              Send SIGUSR1 signal to OpenVPN if management session disconnects.  This is useful when you wish to
              disconnect  an  OpenVPN  session on user logoff. For --management-client this option is not needed
              since a disconnect will always generate a SIGTERM.

       --management-log-cache n
              Cache the most recent n lines of log file history for usage by the management channel.

       --management-up-down
              Report tunnel up/down events to management interface.

       --management-client-auth
              Gives management interface client the responsibility to authenticate clients  after  their  client
              certificate  has  been  verified.   See  management-notes.txt in OpenVPN distribution for detailed
              notes.

       --management-client-pf
              Management interface clients must specify a packet filter file for each  connecting  client.   See
              management-notes.txt in OpenVPN distribution for detailed notes.

       --management-client-user u
              When  the  management  interface is listening on a unix domain socket, only allow connections from
              user u.

       --management-client-group g
              When the management interface is listening on a unix domain socket, only  allow  connections  from
              group g.

       --plugin module-pathname [init-string]
              Load  plug-in  module  from  the  file  module-pathname, passing init-string as an argument to the
              module initialization function.  Multiple plugin modules may be loaded into one OpenVPN process.

              The module-pathname argument can be just a filename or a filename  with  a  relative  or  absolute
              path.   The  format  of the filename and path defines if the plug-in will be loaded from a default
              plug-in directory or outside this directory.

                  --plugin path        Effective directory used
                  ====================================================
                   myplug.so            DEFAULT_DIR/myplug.so
                   subdir/myplug.so     DEFAULT_DIR/subdir/myplug.so
                   ./subdir/myplug.so   CWD/subdir/myplug.so
                   /usr/lib/my/plug.so  /usr/lib/my/plug.so

              DEFAULT_DIR is replaced by the default plug-in directory, which is configured at the build time of
              OpenVPN.   CWD  is  the  current directory where OpenVPN was started or the directory OpenVPN have
              swithed into via the --cd option before the --plugin option.

              For more information and examples on how to build OpenVPN plug-in modules, see the README file  in
              the plugin folder of the OpenVPN source distribution.

              If  you  are using an RPM install of OpenVPN, see /usr/share/openvpn/plugin.  The documentation is
              in doc and the actual plugin modules are in lib.

              Multiple plugin modules can be cascaded, and modules can be used  in  tandem  with  scripts.   The
              modules will be called by OpenVPN in the order that they are declared in the config file.  If both
              a plugin and script are configured for the same callback, the script will be called last.  If  the
              return  code  of  the  module/script  controls  an  authentication  function  (such as tls-verify,
              auth-user-pass-verify, or client-connect), then every module and script must return success (0) in
              order for the connection to be authenticated.

              WARNING:
                  Plug-ins  may  do  deferred execution, meaning the plug-in will return the control back to the
                  main OpenVPN process and provide the plug-in  result  later  on  via  a  different  thread  or
                  process.  OpenVPN does NOT support multiple authentication plug-ins where more than one plugin
                  tries to do deferred authentication.  If this behaviour is detected, OpenVPN  will  shut  down
                  upon first authentication.

       --keying-material-exporter label len
              Save  Exported  Keying  Material  [RFC5705] of len bytes (must be between 16 and 4095 bytes) using
              label in environment (exported_keying_material) for use  by  plugins  in  OPENVPN_PLUGIN_TLS_FINAL
              callback.

              Note  that  exporter  labels  have  the potential to collide with existing PRF labels. In order to
              prevent this, labels MUST begin with "EXPORTER".

              This option requires OpenSSL 1.0.1 or newer.

   Server Mode
       Starting with OpenVPN 2.0, a multi-client TCP/UDP server mode is supported, and can be enabled  with  the
       --mode  server  option.   In  server  mode,  OpenVPN  will  listen  on  a single port for incoming client
       connections.  All client connections will be routed through a single tun or tap interface.  This mode  is
       designed  for  scalability  and  should  be  able  to  support  hundreds  or even thousands of clients on
       sufficiently fast hardware.  SSL/TLS authentication must be used in this mode.

       --server network netmask ['nopool']
              A helper directive designed  to  simplify  the  configuration  of  OpenVPN's  server  mode.   This
              directive  will set up an OpenVPN server which will allocate addresses to clients out of the given
              network/netmask.  The server itself will take the ".1" address of the given network for use as the
              server-side endpoint of the local TUN/TAP interface.

              For example, --server 10.8.0.0 255.255.255.0 expands as follows:

                   mode server
                   tls-server
                   push "topology [topology]"

                   if dev tun AND (topology == net30 OR topology == p2p):
                     ifconfig 10.8.0.1 10.8.0.2
                     if !nopool:
                       ifconfig-pool 10.8.0.4 10.8.0.251
                     route 10.8.0.0 255.255.255.0
                     if client-to-client:
                       push "route 10.8.0.0 255.255.255.0"
                     else if topology == net30:
                       push "route 10.8.0.1"

                   if dev tap OR (dev tun AND topology == subnet):
                     ifconfig 10.8.0.1 255.255.255.0
                     if !nopool:
                       ifconfig-pool 10.8.0.2 10.8.0.253 255.255.255.0
                     push "route-gateway 10.8.0.1"
                     if route-gateway unset:
                       route-gateway 10.8.0.2

              Don't use --server if you are ethernet bridging.  Use --server-bridge instead.

       --server-bridge gateway netmask pool-start-IP pool-end-IP

       --server-bridge ['nogw']

              A  helper  directive  similar  to  --server  which  is  designed  to simplify the configuration of
              OpenVPN's server mode in ethernet bridging configurations.

              If --server-bridge is used without any  parameters,  it  will  enable  a  DHCP-proxy  mode,  where
              connecting  OpenVPN  clients will receive an IP address for their TAP adapter from the DHCP server
              running on the OpenVPN server-side LAN.  Note that only clients that support the binding of a DHCP
              client  with  the  TAP  adapter  (such  as Windows) can support this mode.  The optional nogw flag
              (advanced) indicates that gateway information should not be pushed to the client.

              To configure ethernet bridging, you must first use your OS's bridging capability to bridge the TAP
              interface  with  the  ethernet  NIC  interface.  For example, on Linux this is done with the brctl
              tool, and with Windows XP it is done in the Network Connections Panel by  selecting  the  ethernet
              and TAP adapters and right-clicking on "Bridge Connections".

              Next  you  you  must manually set the IP/netmask on the bridge interface.  The gateway and netmask
              parameters to --server-bridge can be set to either the IP/netmask of the bridge interface, or  the
              IP/netmask of the default gateway/router on the bridged subnet.

              Finally, set aside a IP range in the bridged subnet, denoted by pool-start-IP and pool-end-IP, for
              OpenVPN to allocate to connecting clients.

              For example, server-bridge 10.8.0.4 255.255.255.0 10.8.0.128 10.8.0.254 expands as follows:

                  mode server
                  tls-server

                  ifconfig-pool 10.8.0.128 10.8.0.254 255.255.255.0
                  push "route-gateway 10.8.0.4"

              In another example, --server-bridge (without parameters) expands as follows:

                  mode server
                  tls-server

                  push "route-gateway dhcp"

              Or --server-bridge nogw expands as follows:

                  mode server
                  tls-server

       --push option
              Push a config file option back to the client for remote  execution.   Note  that  option  must  be
              enclosed  in  double  quotes ("").  The client must specify --pull in its config file.  The set of
              options which can be pushed is limited by both feasibility and security.   Some  options  such  as
              those  which  would  execute  scripts are banned, since they would effectively allow a compromised
              server to execute arbitrary code on the client.  Other options  such  as  TLS  or  MTU  parameters
              cannot  be pushed because the client needs to know them before the connection to the server can be
              initiated.

              This is a partial list of  options  which  can  currently  be  pushed:  --route,  --route-gateway,
              --route-delay,  --redirect-gateway,  --ip-win32,  --dhcp-option,  --inactive, --ping, --ping-exit,
              --ping-restart,  --setenv,  --auth-token,  --persist-key,   --persist-tun,   --echo,   --comp-lzo,
              --socket-flags, --sndbuf, --rcvbuf

       --push-reset
              Don't  inherit  the  global  push  list  for a specific client instance.  Specify this option in a
              client-specific context such as with a --client-config-dir configuration file.  This  option  will
              ignore --push options at the global config file level.

       --push-remove opt
              selectively  remove all --push options matching "opt" from the option list for a client.  "opt" is
              matched  as  a  substring  against  the  whole  option  string  to-be-pushed  to  the  client,  so
              --push-remove  route  would  remove  all  --push route ...  and --push route-ipv6 ...  statements,
              while --push-remove 'route-ipv6 2001:' would only remove IPv6 routes for 2001:... networks.

              --push-remove can only be used in a client-specific context, like in a  --client-config-dir  file,
              or --client-connect script or plugin -- similar to --push-reset, just more selective.

              NOTE: to change an option, --push-remove can be used to first remove the old value, and then add a
              new --push option with the new value.

       --push-peer-info
              Push additional information about the client to server.  The following data is  always  pushed  to
              the server:

              IV_VER=<version> -- the client OpenVPN version

              IV_PLAT=[linux|solaris|openbsd|mac|netbsd|freebsd|win] -- the client OS platform

              IV_LZO_STUB=1 -- if client was built with LZO stub capability

              IV_LZ4=1 -- if the client supports LZ4 compressions.

              IV_PROTO=2 -- if the client supports peer-id floating mechansim

              IV_NCP=2  --  negotiable  ciphers,  client supports --cipher pushed by the server, a value of 2 or
              greater indicates client supports AES-GCM-128 and AES-GCM-256.

              IV_UI_VER=<gui_id> <version>  --  the  UI  version  of  a  UI  if  one  is  running,  for  example
              "de.blinkt.openvpn 0.5.47" for the Android app.

              When --push-peer-info is enabled the additional information consists of the following data:

              IV_HWADDR=<mac address> -- the MAC address of clients default gateway

              IV_SSL=<version string> -- the ssl version used by the client, e.g. "OpenSSL 1.0.2f 28 Jan 2016".

              IV_PLAT_VER=x.y - the version of the operating system, e.g. 6.1 for Windows 7.

              UV_<name>=<value> -- client environment variables whose names start with "UV_"

       --disable
              Disable  a particular client (based on the common name) from connecting.  Don't use this option to
              disable a client due to key or password compromise.   Use  a  CRL  (certificate  revocation  list)
              instead (see the --crl-verify option).

              This  option  must  be  associated  with  a  specific client instance, which means that it must be
              specified either in a  client  instance  config  file  using  --client-config-dir  or  dynamically
              generated using a --client-connect script.

       --ifconfig-pool start-IP end-IP [netmask]
              Set  aside  a pool of subnets to be dynamically allocated to connecting clients, similar to a DHCP
              server.  For tun-style tunnels, each client will be given a /30 subnet (for interoperability  with
              Windows clients).  For tap-style tunnels, individual addresses will be allocated, and the optional
              netmask parameter will also be pushed to clients.

       --ifconfig-pool-persist file [seconds]
              Persist/unpersist ifconfig-pool data to file, at seconds intervals (default=600), as  well  as  on
              program startup and shutdown.

              The  goal  of  this option is to provide a long-term association between clients (denoted by their
              common name) and the virtual IP address assigned to them from the  ifconfig-pool.   Maintaining  a
              long-term  association  is  good  for  clients  because  it  allows  them  to  effectively use the
              --persist-tun option.

              file is a comma-delimited ASCII file, formatted as <Common-Name>,<IP-address>.

              If seconds = 0, file will be treated as read-only.  This is useful if you would like to treat file
              as a configuration file.

              Note  that  the  entries  in  this  file are treated by OpenVPN as suggestions only, based on past
              associations between a common name and IP address.  They do not guarantee that  the  given  common
              name  will  always  receive  the  given  IP  address.   If  you  want  guaranteed  assignment, use
              --ifconfig-push

       --ifconfig-pool-linear
              DEPRECATED This option will be removed in OpenVPN 2.5

              Modifies the --ifconfig-pool directive to allocate individual TUN interface addresses for  clients
              rather than /30 subnets.  NOTE:  This option is incompatible with Windows clients.

              This  option  is  deprecated,  and  should  be  replaced with --topology p2p which is functionally
              equivalent.

       --ifconfig-push local remote-netmask [alias]
              Push virtual IP endpoints for client tunnel, overriding the --ifconfig-pool dynamic allocation.

              The parameters local and remote-netmask are set according to the --ifconfig  directive  which  you
              want  to  execute  on the client machine to configure the remote end of the tunnel.  Note that the
              parameters local and remote-netmask are from the perspective of the client, not the server.   They
              may  be  DNS  names rather than IP addresses, in which case they will be resolved on the server at
              the time of client connection.

              The optional alias parameter may be used in cases where NAT causes the client view  of  its  local
              endpoint  to  differ  from  the  server view.  In this case local/remote-netmask will refer to the
              server view while alias/remote-netmask will refer to the client view.

              This option must be associated with a specific client  instance,  which  means  that  it  must  be
              specified  either  in  a  client  instance  config  file  using --client-config-dir or dynamically
              generated using a --client-connect script.

              Remember also to include a --route directive in the main OpenVPN config file which encloses local,
              so that the kernel will know to route it to the server's TUN/TAP interface.

              OpenVPN's internal client IP address selection algorithm works as follows:

              1 -- Use --client-connect script generated file for static IP (first choice).
              2 -- Use --client-config-dir file for static IP (next choice).
              3 -- Use --ifconfig-pool allocation for dynamic IP (last choice).

       --iroute network [netmask]
              Generate  an  internal  route to a specific client. The netmask parameter, if omitted, defaults to
              255.255.255.255.

              This directive can be used to route a fixed  subnet  from  the  server  to  a  particular  client,
              regardless  of  where the client is connecting from.  Remember that you must also add the route to
              the system routing table as well (such as by using the --route directive).   The  reason  why  two
              routes  are  needed  is  that  the --route directive routes the packet from the kernel to OpenVPN.
              Once in OpenVPN, the --iroute directive routes to the specific client.

              This option must be specified either in a client instance config file using --client-config-dir or
              dynamically generated using a --client-connect script.

              The  --iroute  directive  also  has  an  important  interaction with --push "route ...".  --iroute
              essentially defines a subnet which is owned by a particular client (we will call this  client  A).
              If  you  would  like  other clients to be able to reach A's subnet, you can use --push "route ..."
              together with --client-to-client to effect this.  In order for all  clients  to  see  A's  subnet,
              OpenVPN  must push this route to all clients EXCEPT for A, since the subnet is already owned by A.
              OpenVPN accomplishes this by not not pushing a route to a client if it matches one of the client's
              iroutes.

       --client-to-client
              Because the OpenVPN server mode handles multiple clients through a single tun or tap interface, it
              is  effectively  a  router.   The  --client-to-client  flag  tells  OpenVPN  to  internally  route
              client-to-client  traffic  rather  than  pushing  all  client-originating  traffic  to the TUN/TAP
              interface.

              When this option is used, each client will "see" the other clients which are currently  connected.
              Otherwise,  each  client  will only see the server.  Don't use this option if you want to firewall
              tunnel traffic using custom, per-client rules.

       --duplicate-cn
              Allow multiple clients with the same common name to concurrently connect.  In the absence of  this
              option,  OpenVPN will disconnect a client instance upon connection of a new client having the same
              common name.

       --client-connect cmd
              Run command cmd on client connection.

              cmd consists of a path to script (or executable program), optionally followed  by  arguments.  The
              path and arguments may be single- or double-quoted and/or escaped using a backslash, and should be
              separated by one or more spaces.

              The command is passed the  common  name  and  IP  address  of  the  just-authenticated  client  as
              environmental  variables  (see  environmental variable section below).  The command is also passed
              the pathname of a freshly created temporary  file  as  the  last  argument  (after  any  arguments
              specified in cmd ), to be used by the command to pass dynamically generated config file directives
              back to OpenVPN.

              If the script wants to generate a dynamic config file to be applied on the server when the  client
              connects, it should write it to the file named by the last argument.

              See  the  --client-config-dir  option below for options which can be legally used in a dynamically
              generated config file.

              Note that the return value of script is significant.  If script returns a non-zero  error  status,
              it will cause the client to be disconnected.

       --client-disconnect cmd
              Like  --client-connect  but  called  on  client  instance shutdown.  Will not be called unless the
              --client-connect script and plugins (if defined) were previously  called  on  this  instance  with
              successful (0) status returns.

              The  exception  to this rule is if the --client-disconnect command or plugins are cascaded, and at
              least one client-connect function succeeded, then  ALL  of  the  client-disconnect  functions  for
              scripts and plugins will be called on client instance object deletion, even in cases where some of
              the related client-connect functions returned an error status.

              The --client-disconnect command is passed the same pathname as the corresponding  --client-connect
              command as its last argument. (after any arguments specified in cmd ).

       --client-config-dir dir
              Specify  a  directory  dir  for  custom  client  config files.  After a connecting client has been
              authenticated, OpenVPN will look in this directory for a file having the same name as the client's
              X509  common  name.   If  a matching file exists, it will be opened and parsed for client-specific
              configuration options.  If no matching file is found, OpenVPN will instead try to open and parse a
              default  file  called  "DEFAULT",  which  may  be  provided  but  is  not  required. Note that the
              configuration files must be readable by the  OpenVPN  process  after  it  has  dropped  it's  root
              privileges.

              This  file  can  specify  a  fixed IP address for a given client using --ifconfig-push, as well as
              fixed subnets owned by the client using --iroute.

              One of the useful properties of this option is that it allows client  configuration  files  to  be
              conveniently  created, edited, or removed while the server is live, without needing to restart the
              server.

              The following options are legal in a client-specific context: --push, --push-reset, --push-remove,
              --iroute, --ifconfig-push, and --config.

       --ccd-exclusive
              Require,  as  a  condition  of  authentication, that a connecting client has a --client-config-dir
              file.

       --tmp-dir dir
              Specify a directory dir for temporary files.  This directory will be used by openvpn processes and
              script  to  communicate  temporary data with openvpn main process. Note that the directory must be
              writable by the OpenVPN process after it has dropped it's root privileges.

              This directory will be used by in the following cases:

              * --client-connect scripts to dynamically generate client-specific configuration files.

              * OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin hook to return success/failure via auth_control_file
              when using deferred auth method

              * OPENVPN_PLUGIN_ENABLE_PF plugin hook to pass filtering rules via pf_file

       --hash-size r v
              Set  the size of the real address hash table to r and the virtual address table to v.  By default,
              both tables are sized at 256 buckets.

       --bcast-buffers n
              Allocate n buffers for broadcast datagrams (default=256).

       --tcp-queue-limit n
              Maximum number of output packets queued before TCP (default=64).

              When OpenVPN is tunneling data from a TUN/TAP device to a remote client over a TCP connection,  it
              is  possible  that  the TUN/TAP device might produce data at a faster rate than the TCP connection
              can support.  When the number of output packets queued before sending to the  TCP  socket  reaches
              this  limit for a given client connection, OpenVPN will start to drop outgoing packets directed at
              this client.

       --tcp-nodelay
              This macro sets the TCP_NODELAY socket flag on the server as  well  as  pushes  it  to  connecting
              clients.   The  TCP_NODELAY flag disables the Nagle algorithm on TCP sockets causing packets to be
              transmitted immediately with low latency, rather than waiting a short period of time in  order  to
              aggregate  several  packets  into  a  larger  containing  packet.   In  VPN applications over TCP,
              TCP_NODELAY is generally a good latency optimization.

              The macro expands as follows:

                   if mode server:
                     socket-flags TCP_NODELAY
                     push "socket-flags TCP_NODELAY"

       --max-clients n
              Limit server to a maximum of n concurrent clients.

       --max-routes-per-client n
              Allow a maximum of n internal routes per client (default=256).  This is designed to  help  contain
              DoS  attacks  where  an authenticated client floods the server with packets appearing to come from
              many unique MAC addresses, forcing the server to deplete virtual memory as  its  internal  routing
              table  expands.   This  directive can be used in a --client-config-dir file or auto-generated by a
              --client-connect script to override the global value for a particular client.

              Note that this directive affects OpenVPN's internal routing table, not the kernel routing table.

       --stale-routes-check n [t]
              Remove routes haven't had activity for n seconds (i.e. the ageing time).

              This check is ran every t seconds (i.e. check interval).

              If t is not present it defaults to n

              This option helps to keep the dynamic routing table small.  See also --max-routes-per-client

       --connect-freq n sec
              Allow a maximum of n new connections per sec seconds from clients.  This is  designed  to  contain
              DoS  attacks  which  flood  the  server  with  connection  requests  using certificates which will
              ultimately fail to authenticate.

              This is an imperfect solution however, because in a  real  DoS  scenario,  legitimate  connections
              might also be refused.

              For  the best protection against DoS attacks in server mode, use --proto udp and either --tls-auth
              or --tls-crypt.

       --learn-address cmd
              Run command cmd to validate client virtual addresses or routes.

              cmd consists of a path to script (or executable program), optionally followed  by  arguments.  The
              path and arguments may be single- or double-quoted and/or escaped using a backslash, and should be
              separated by one or more spaces.

              Three arguments will be appended to any arguments in cmd as follows:

              [1] operation -- "add", "update", or "delete" based on whether or not the address is  being  added
              to, modified, or deleted from OpenVPN's internal routing table.
              [2]  address  --  The  address  being  learned  or unlearned.  This can be an IPv4 address such as
              "198.162.10.14", an IPv4 subnet such as "198.162.10.0/24", or an ethernet MAC address (when  --dev
              tap is being used) such as "00:FF:01:02:03:04".
              [3]  common  name  -- The common name on the certificate associated with the client linked to this
              address.  Only present for "add" or "update" operations, not "delete".

              On "add" or "update" methods, if the script returns a failure code (non-zero), OpenVPN will reject
              the address and will not modify its internal routing table.

              Normally,  the  cmd  script  will  use  the information provided above to set appropriate firewall
              entries on the VPN TUN/TAP interface.  Since OpenVPN provides the association between  virtual  IP
              or  MAC  address  and  the  client's authenticated common name, it allows a user-defined script to
              configure firewall access policies with regard to the client's high-level common name, rather than
              the low level client virtual addresses.

       --auth-user-pass-verify cmd method
              Require  the  client to provide a username/password (possibly in addition to a client certificate)
              for authentication.

              OpenVPN will run command cmd to validate the username/password provided by the client.

              cmd consists of a path to script (or executable program), optionally followed  by  arguments.  The
              path and arguments may be single- or double-quoted and/or escaped using a backslash, and should be
              separated by one or more spaces.

              If method is set to "via-env", OpenVPN will call script with the environmental variables  username
              and  password  set  to  the  username/password strings provided by the client.  Be aware that this
              method is insecure on some platforms which make the environment of a process publicly  visible  to
              other unprivileged processes.

              If  method  is  set  to  "via-file", OpenVPN will write the username and password to the first two
              lines of a temporary file.  The filename will be passed as an argument to  script,  and  the  file
              will  be automatically deleted by OpenVPN after the script returns.  The location of the temporary
              file is controlled by the  --tmp-dir  option,  and  will  default  to  the  current  directory  if
              unspecified.   For  security,  consider  setting  --tmp-dir  to  a volatile storage medium such as
              /dev/shm (if available) to prevent the username/password file from touching the hard drive.

              The script should examine the username and password, returning a success  exit  code  (0)  if  the
              client's authentication request is to be accepted, or a failure code (1) to reject the client.

              This   directive   is  designed  to  enable  a  plugin-style  interface  for  extending  OpenVPN's
              authentication capabilities.

              To protect against a client passing a maliciously formed username or password string, the username
              string must consist only of these characters: alphanumeric, underbar ('_'), dash ('-'), dot ('.'),
              or at ('@').  The password string can consist of any printable characters except  for  CR  or  LF.
              Any  illegal  characters  in  either the username or password string will be converted to underbar
              ('_').

              Care must be taken by any user-defined scripts to avoid creating a security vulnerability  in  the
              way  that  these  strings  are  handled.  Never use these strings in such a way that they might be
              escaped or evaluated by a shell interpreter.

              For a sample script that  performs  PAM  authentication,  see  sample-scripts/auth-pam.pl  in  the
              OpenVPN source distribution.

       --auth-gen-token [lifetime]
              After successful user/password authentication, the OpenVPN server will with this option generate a
              temporary authentication token and push that to client.   On  the  following  renegotiations,  the
              OpenVPN  client will pass this token instead of the users password.  On the server side the server
              will do the token authentication internally and it will  NOT  do  any  additional  authentications
              against configured external user/password authentication mechanisms.

              The  lifetime  argument defines how long the generated token is valid.  The lifetime is defined in
              seconds.  If lifetime is not set or it is set to 0, the token will never expire.

              This feature is useful for environments which is configured to use One  Time  Passwords  (OTP)  as
              part of the user/password authentications and that authentication mechanism does not implement any
              auth-token support.

       --opt-verify
              Clients that connect with options  that  are  incompatible  with  those  of  the  server  will  be
              disconnected.

              Options  that  will  be  compared  for  compatibility  include dev-type, link-mtu, tun-mtu, proto,
              ifconfig, comp-lzo, fragment, keydir, cipher, auth, keysize, secret, no-replay,  no-iv,  tls-auth,
              key-method, tls-server, and tls-client.

              This option requires that --disable-occ NOT be used.

       --auth-user-pass-optional
              Allow   connections   by  clients  that  do  not  specify  a  username/password.   Normally,  when
              --auth-user-pass-verify or --management-client-auth is  specified  (or  an  authentication  plugin
              module),  the  OpenVPN  server  daemon  will  require connecting clients to specify a username and
              password.  This option makes the submission of a username/password by  clients  optional,  passing
              the  responsibility  to the user-defined authentication module/script to accept or deny the client
              based on other factors (such as the setting of X509 certificate  fields).   When  this  option  is
              used, and a connecting client does not submit a username/password, the user-defined authentication
              module/script will see the username and  password  as  being  set  to  empty  strings  ("").   The
              authentication module/script MUST have logic to detect this condition and respond accordingly.

       --client-cert-not-required
              DEPRECATED This option will be removed in OpenVPN 2.5

              Don't require client certificate, client will authenticate using username/password only.  Be aware
              that using this directive is less secure than requiring certificates from all clients.

              Please note: This is replaced by --verify-client-cert  which  allows  for  more  flexibility.  The
              option --verify-client-cert none is functionally equivalent to --client-cert-not-required

       --verify-client-cert none|optional|require
              Specify whether the client is required to supply a valid certificate.

              Possible options are

              none   :   a   client  certificate  is  not  required.  the  client  need  to  authenticate  using
              username/password only.  Be aware  that  using  this  directive  is  less  secure  than  requiring
              certificates from all clients.

              If  you  use  this  directive,  the  entire  responsibility  of  authentication  will rest on your
              --auth-user-pass-verify script, so keep in  mind  that  bugs  in  your  script  could  potentially
              compromise the security of your VPN.

              --verify-client-cert none is functionally equivalent to --client-cert-not-required.

              optional  :  a  client may present a certificate but it is not required to do so.  When using this
              directive, you should also use  a  --auth-user-pass-verify  script  to  ensure  that  clients  are
              authenticated using a certificate, a username and password, or possibly even both.

              Again,  the  entire  responsibility  of  authentication  will rest on your --auth-user-pass-verify
              script, so keep in mind that bugs in your script could potentially compromise the security of your
              VPN.

              require : this is the default option. A client is required to present a certificate, otherwise VPN
              access is refused.

              If you don't use this directive (or use --verify-client-cert require ) but  you  also  specify  an
              --auth-user-pass-verify  script,  then  OpenVPN  will  perform  double authentication.  The client
              certificate verification AND the --auth-user-pass-verify script will need to succeed in order  for
              a client to be authenticated and accepted onto the VPN.

       --username-as-common-name
              For  --auth-user-pass-verify  authentication,  use  the authenticated username as the common name,
              rather than the common name from the client cert.

       --compat-names [no-remapping]
              DEPRECATED This option will be removed in OpenVPN 2.5

              Until OpenVPN v2.3 the format of the X.509 Subject fields was formatted like this:

              /C=US/L=Somewhere/CN=John Doe/emailAddress=john@example.com

              In addition the old behaviour was to remap  any  character  other  than  alphanumeric,  underscore
              ('_'),  dash  ('-'),  dot ('.'), and slash ('/') to underscore ('_').  The X.509 Subject string as
              returned by the tls_id environmental variable, could additionally contain  colon  (':')  or  equal
              ('=').

              When  using the --compat-names option, this old formatting and remapping will be re-enabled again.
              This is purely implemented for compatibility reasons when using older plug-ins  or  scripts  which
              does not handle the new formatting or UTF-8 characters.

              In  OpenVPN  2.3  the  formatting of these fields changed into a more standardised format.  It now
              looks like:

              C=US, L=Somewhere, CN=John Doe, emailAddress=john@example.com

              The new default format in OpenVPN 2.3 also does not do  the  character  remapping  which  happened
              earlier.   This  new  format  enables  proper support for UTF-8 characters in the usernames, X.509
              Subject fields and  Common  Name  variables  and  it  complies  to  the  RFC  2253,  UTF-8  String
              Representation of Distinguished Names.

              The  no-remapping  mode  flag can be used with the --compat-names option to be compatible with the
              now deprecated --no-name-remapping option.  It is only available at the  server.  When  this  mode
              flag  is used, the Common Name, Subject, and username strings are allowed to include any printable
              character  including  space,  but  excluding  control  characters  such  as  tab,   newline,   and
              carriage-return. no-remapping is only available on the server side.

              Please note: This option is immediately deprecated.  It is only implemented to make the transition
              to the new formatting less intrusive.  It will be removed in OpenVPN 2.5.  So please  update  your
              scripts/plug-ins where necessary.

       --no-name-remapping
              DEPRECATED This option will be removed in OpenVPN 2.5

              The   --no-name-remapping   option  is  an  alias  for  --compat-names no-remapping.   It  ensures
              compatibility with server configurations using the --no-name-remapping option.

              Please note: This option is now deprecated.  It will be removed in OpenVPN 2.5.   So  please  make
              sure you support the new X.509 name formatting described with the --compat-names option as soon as
              possible.

       --port-share host port [dir]
              When run in TCP server mode, share the OpenVPN port with another application,  such  as  an  HTTPS
              server.  If OpenVPN senses a connection to its port which is using a non-OpenVPN protocol, it will
              proxy the connection to the server at host:port.  Currently only designed to work with HTTP/HTTPS,
              though it would be theoretically possible to extend to other protocols such as ssh.

              dir  specifies  an optional directory where a temporary file with name N containing content C will
              be dynamically generated for each proxy connection, where N is the source IP:port  of  the  client
              connection  and  C  is the source IP:port of the connection to the proxy receiver.  This directory
              can be used as a dictionary by the proxy receiver to determine the origin of the connection.  Each
              generated file will be automatically deleted when the proxied connection is torn down.

              Not implemented on Windows.

   Client Mode
       Use  client  mode  when  connecting  to  an OpenVPN server which has --server, --server-bridge, or --mode
       server in it's configuration.

       --client
              A helper directive designed  to  simplify  the  configuration  of  OpenVPN's  client  mode.   This
              directive is equivalent to:

                   pull
                   tls-client

       --pull This  option  must be used on a client which is connecting to a multi-client server.  It indicates
              to OpenVPN that it should accept options pushed by the server, provided they are part of the legal
              set of pushable options (note that the --pull option is implied by --client ).

              In particular, --pull allows the server to push routes to the client, so you should not use --pull
              or --client in situations where you don't trust the server  to  have  control  over  the  client's
              routing table.

       --pull-filter accept|ignore|reject text
              Filter  options  received  from  the  server  if the option starts with text.  Runs on client. The
              action flag accept allows the option, ignore removes it and reject flags an error and  triggers  a
              SIGUSR1  restart.   The filters may be specified multiple times, and each filter is applied in the
              order it is specified. The filtering of each option stops as soon as a match is  found.  Unmatched
              options are accepted by default.

              Prefix comparison is used to match text against the received option so that

                  --pull-filter ignore "route"

              would   remove  all  pushed  options  starting  with  route  which  would  include,  for  example,
              route-gateway.  Enclose text in quotes to embed spaces.

                  --pull-filter accept "route 192.168.1."
                  --pull-filter ignore "route "

              would remove all routes that do not start with 192.168.1.

              This option may be used only on clients.  Note that reject may  result  in  a  repeated  cycle  of
              failure  and  reconnect,  unless  multiple remotes are specified and connection to the next remote
              succeeds. To silently ignore an option pushed by the server, use ignore.

       --auth-user-pass [up]
              Authenticate with server using username/password.  up is a file containing username/password on  2
              lines. If the password line is missing, OpenVPN will prompt for one.

              If up is omitted, username/password will be prompted from the console.

              The   server   configuration   must  specify  an  --auth-user-pass-verify  script  to  verify  the
              username/password provided by the client.

       --auth-retry type
              Controls how OpenVPN responds to username/password verification errors  such  as  the  client-side
              response  to  an  AUTH_FAILED  message  from the server or verification failure of the private key
              password.

              Normally used to prevent auth  errors  from  being  fatal  on  the  client  side,  and  to  permit
              username/password requeries in case of error.

              An  AUTH_FAILED  message  is  generated  by  the  server  if  the  client  fails  --auth-user-pass
              authentication, or if the server-side --client-connect script returns an  error  status  when  the
              client tries to connect.

              type can be one of:

              none -- Client will exit with a fatal error (this is the default).
              nointeract  --  Client  will  retry  the  connection  without  requerying  for an --auth-user-pass
              username/password.  Use this option for unattended clients.
              interact -- Client will requery for  an  --auth-user-pass  username/password  and/or  private  key
              password before attempting a reconnection.

              Note that while this option cannot be pushed, it can be controlled from the management interface.

       --static-challenge t e
              Enable static challenge/response protocol using challenge text t, with echo flag given by e (0|1).

              The echo flag indicates whether or not the user's response to the challenge should be echoed.

              See   management-notes.txt   in  the  OpenVPN  distribution  for  a  description  of  the  OpenVPN
              challenge/response protocol.

       --server-poll-timeout n, --connect-timeout n
              When connecting to a remote server do not wait for more than n  seconds  waiting  for  a  response
              before  trying  the  next  server.  The default value is 120s. This timeout includes proxy and TCP
              connect timeouts.

       --explicit-exit-notify [n]
              In UDP client mode or point-to-point mode, send server/peer an  exit  notification  if  tunnel  is
              restarted  or  OpenVPN  process is exited.  In client mode, on exit/restart, this option will tell
              the server to immediately close its client instance object rather than waiting for a timeout.  The
              n parameter (default=1) controls the maximum number of attempts that the client will try to resend
              the exit notification message.

              In UDP server mode, send RESTART control channel command to connected  clients.  The  n  parameter
              (default=1)  controls  client  behavior.  With  n = 1 client will attempt to reconnect to the same
              server, with n = 2 client will advance to the next server.

              OpenVPN will not send any exit notifications unless this option is enabled.

       --allow-recursive-routing
              When this option is set, OpenVPN will not drop incoming tun packets with same destination as host.

   Data Channel Encryption Options:
       These options are meaningful for both Static & TLS-negotiated  key  modes  (must  be  compatible  between
       peers).

       --secret file [direction]
              Enable  Static Key encryption mode (non-TLS).  Use pre-shared secret file which was generated with
              --genkey.

              The optional direction parameter enables the use of 4 distinct  keys  (HMAC-send,  cipher-encrypt,
              HMAC-receive,  cipher-decrypt),  so  that each data flow direction has a different set of HMAC and
              cipher keys.  This has a number of desirable security  properties  including  eliminating  certain
              kinds of DoS and message replay attacks.

              When  the  direction  parameter  is omitted, 2 keys are used bidirectionally, one for HMAC and the
              other for encryption/decryption.

              The direction parameter should always be complementary on either side of the connection, i.e.  one
              side should use "0" and the other should use "1", or both sides should omit it altogether.

              The  direction  parameter  requires  that file contains a 2048 bit key.  While pre-1.5 versions of
              OpenVPN generate 1024 bit  key  files,  any  version  of  OpenVPN  which  supports  the  direction
              parameter, will also support 2048 bit key file generation using the --genkey option.

              Static key encryption mode has certain advantages, the primary being ease of configuration.

              There  are  no  certificates  or certificate authorities or complicated negotiation handshakes and
              protocols.  The only requirement is that you have a pre-existing secure  channel  with  your  peer
              (such  as  ssh  )  to initially copy the key.  This requirement, along with the fact that your key
              never changes unless you manually generate a new one, makes it somewhat less secure than TLS  mode
              (see below).  If an attacker manages to steal your key, everything that was ever encrypted with it
              is compromised.  Contrast that to the perfect forward secrecy features of TLS mode  (using  Diffie
              Hellman key exchange), where even if an attacker was able to steal your private key, he would gain
              no information to help him decrypt past sessions.

              Another advantageous aspect of Static Key encryption mode is that it is a handshake-free  protocol
              without  any distinguishing signature or feature (such as a header or protocol handshake sequence)
              that would mark the ciphertext packets as being generated by OpenVPN.  Anyone eavesdropping on the
              wire would see nothing but random-looking data.

       --key-direction
              Alternative  way  of  specifying  the optional direction parameter for the --tls-auth and --secret
              options. Useful when using inline files (See section on inline files).

       --auth alg
              Authenticate data channel packets and (if enabled) tls-auth  control  channel  packets  with  HMAC
              using  message  digest  algorithm  alg.   (The default is SHA1 ).  HMAC is a commonly used message
              authentication algorithm (MAC) that uses a data string, a secure hash algorithm,  and  a  key,  to
              produce a digital signature.

              The  OpenVPN  data  channel protocol uses encrypt-then-mac (i.e. first encrypt a packet, then HMAC
              the resulting ciphertext), which prevents padding oracle attacks.

              If an AEAD cipher mode (e.g. GCM) is chosen, the specified --auth algorithm  is  ignored  for  the
              data  channel,  and  the  authentication method of the AEAD cipher is used instead.  Note that alg
              still specifies the digest used for tls-auth.

              In static-key encryption mode, the HMAC key is included in the key file generated by --genkey.  In
              TLS  mode,  the  HMAC  key  is  dynamically generated and shared between peers via the TLS control
              channel.  If OpenVPN receives a packet with a bad HMAC it will drop the packet.  HMAC usually adds
              16 or 20 bytes per packet.  Set alg=none to disable authentication.

              For more information on HMAC see http://www.cs.ucsd.edu/users/mihir/papers/hmac.html

       --cipher alg
              Encrypt data channel packets with cipher algorithm alg.

              The  default  is  BF-CBC, an abbreviation for Blowfish in Cipher Block Chaining mode.  When cipher
              negotiation (NCP) is allowed,  OpenVPN  2.4  and  newer  on  both  client  and  server  side  will
              automatically  upgrade  to  AES-256-GCM.   See --ncp-ciphers and --ncp-disable for more details on
              NCP.

              Using BF-CBC is no longer recommended, because of its 64-bit block size.  This  small  block  size
              allows     attacks     based     on    collisions,    as    demonstrated    by    SWEET32.     See
              https://community.openvpn.net/openvpn/wiki/SWEET32 for details.  Due to this, support for  BF-CBC,
              DES, CAST5, IDEA and RC2 ciphers will be removed in OpenVPN 2.6.

              To see other ciphers that are available with OpenVPN, use the --show-ciphers option.

              Set alg=none to disable encryption.

       --ncp-ciphers cipher_list
              Restrict  the  allowed  ciphers  to be negotiated to the ciphers in cipher_list.  cipher_list is a
              colon-separated list of ciphers, and defaults to "AES-256-GCM:AES-128-GCM".

              For servers, the first cipher from cipher_list will be  pushed  to  clients  that  support  cipher
              negotiation.

              Cipher  negotiation  is  enabled  in  client-server  mode only.  I.e. if --mode is set to 'server'
              (server-side, implied by setting --server ), or if --pull is specified  (client-side,  implied  by
              setting --client).

              If  both  peers  support  and  do  not disable NCP, the negotiated cipher will override the cipher
              specified by --cipher.

              Additionally, to allow for more smooth transition, if NCP is enabled,  OpenVPN  will  inherit  the
              cipher  of  the  peer  if  that  cipher is different from the local --cipher setting, but the peer
              cipher is one of the ciphers specified in --ncp-ciphers.  E.g. a non-NCP client (<=v2.3,  or  with
              --ncp-disabled  set)  connecting to a NCP server (v2.4+) with "--cipher BF-CBC" and "--ncp-ciphers
              AES-256-GCM:AES-256-CBC" set can either specify "--cipher BF-CBC" or  "--cipher  AES-256-CBC"  and
              both will work.

       --ncp-disable
              Disable "negotiable crypto parameters".  This completely disables cipher negotiation.

       --keysize n
              DEPRECATED This option will be removed in OpenVPN 2.6.

              Size  of cipher key in bits (optional).  If unspecified, defaults to cipher-specific default.  The
              --show-ciphers option (see below) shows all available OpenSSL ciphers, their  default  key  sizes,
              and  whether the key size can be changed.  Use care in changing a cipher's default key size.  Many
              ciphers have not been extensively cryptanalyzed with non-standard key lengths, and  a  larger  key
              may offer no real guarantee of greater security, or may even reduce security.

       --prng alg [nsl]
              (Advanced) For PRNG (Pseudo-random number generator), use digest algorithm alg (default=sha1), and
              set nsl (default=16) to the size in bytes of the nonce secret length (between 16 and 64).

              Set alg=none to disable the PRNG and use the  OpenSSL  RAND_bytes  function  instead  for  all  of
              OpenVPN's pseudo-random number needs.

       --engine [engine-name]
              Enable OpenSSL hardware-based crypto engine functionality.

              If  engine-name  is  specified,  use  a specific crypto engine.  Use the --show-engines standalone
              option to list the crypto engines which are supported by OpenSSL.

       --no-replay
              DEPRECATED This option will be removed in OpenVPN 2.5.

              (Advanced) Disable OpenVPN's protection against replay attacks.  Don't use this option unless  you
              are prepared to make a tradeoff of greater efficiency in exchange for less security.

              OpenVPN provides datagram replay protection by default.

              Replay  protection  is  accomplished  by tagging each outgoing datagram with an identifier that is
              guaranteed to be unique for the key being used.  The peer that receives the  datagram  will  check
              for  the  uniqueness  of  the  identifier.   If  the identifier was already received in a previous
              datagram, OpenVPN will drop the packet.  Replay protection is important to defeat attacks such  as
              a  SYN  flood  attack,  where  the  attacker  listens  in  the  wire,  intercepts a TCP SYN packet
              (identifying it by the context in which it occurs in relation to other packets), then  floods  the
              receiving peer with copies of this packet.

              OpenVPN's  replay  protection  is  implemented  in  slightly  different ways, depending on the key
              management mode you have selected.

              In Static Key mode or when using an CFB  or  OFB  mode  cipher,  OpenVPN  uses  a  64  bit  unique
              identifier that combines a time stamp with an incrementing sequence number.

              When  using  TLS  mode for key exchange and a CBC cipher mode, OpenVPN uses only a 32 bit sequence
              number without a time stamp, since OpenVPN can guarantee the uniqueness of  this  value  for  each
              key.   As in IPSec, if the sequence number is close to wrapping back to zero, OpenVPN will trigger
              a new key exchange.

              To check for replays, OpenVPN uses the sliding window algorithm used by IPSec.

       --replay-window n [t]
              Use a replay protection sliding-window of size n and a time window of t seconds.

              By default n is 64 (the IPSec default) and t is 15 seconds.

              This option is only relevant in UDP mode, i.e.  when  either  --proto  udp  is  specified,  or  no
              --proto option is specified.

              When  OpenVPN  tunnels IP packets over UDP, there is the possibility that packets might be dropped
              or delivered out of order.  Because OpenVPN, like IPSec, is emulating the physical network  layer,
              it  will  accept  an out-of-order packet sequence, and will deliver such packets in the same order
              they were received to the TCP/IP protocol stack, provided they satisfy several constraints.

              (a) The packet cannot be  a  replay  (unless  --no-replay  is  specified,  which  disables  replay
              protection altogether).

              (b)  If  a  packet  arrives  out  of order, it will only be accepted if the difference between its
              sequence number and the highest sequence number received so far is less than n.

              (c) If a packet arrives out of order, it will only be accepted if  it  arrives  no  later  than  t
              seconds after any packet containing a higher sequence number.

              If  you  are using a network link with a large pipeline (meaning that the product of bandwidth and
              latency is high), you may want to use a larger value for n.  Satellite links in  particular  often
              require this.

              If  you  run  OpenVPN at --verb 4, you will see the message "Replay-window backtrack occurred [x]"
              every time the maximum sequence number backtrack seen thus far increases.  This  can  be  used  to
              calibrate n.

              There  is some controversy on the appropriate method of handling packet reordering at the security
              layer.

              Namely, to what extent should the security layer protect the encapsulated  protocol  from  attacks
              which masquerade as the kinds of normal packet loss and reordering that occur over IP networks?

              The  IPSec  and  OpenVPN  approach  is  to allow packet reordering within a certain fixed sequence
              number window.

              OpenVPN adds to the IPSec model by limiting the window size in time as well as sequence space.

              OpenVPN also adds TCP transport as an option (not offered by IPSec)  in  which  case  OpenVPN  can
              adopt  a very strict attitude towards message deletion and reordering:  Don't allow it.  Since TCP
              guarantees reliability, any packet loss or reordering event can be assumed to be an attack.

              In this sense, it could be argued that TCP tunnel transport is preferred when tunneling non-IP  or
              UDP  application  protocols  which  might be vulnerable to a message deletion or reordering attack
              which falls within the normal operational parameters of IP networks.

              So I would make the statement that one should never tunnel a non-IP protocol  or  UDP  application
              protocol  over UDP, if the protocol might be vulnerable to a message deletion or reordering attack
              that falls within the normal operating parameters of what is to be expected from the  physical  IP
              layer.  The problem is easily fixed by simply using TCP as the VPN transport layer.

       --mute-replay-warnings
              Silence  the  output  of  replay  warnings, which are a common false alarm on WiFi networks.  This
              option preserves the security of the replay protection code without the verbosity associated  with
              warnings about duplicate packets.

       --replay-persist file
              Persist replay-protection state across sessions using file to save and reload the state.

              This  option  will  strengthen  protection  against  replay attacks, especially when you are using
              OpenVPN in a dynamic context (such as with --inetd) when OpenVPN sessions are  frequently  started
              and stopped.

              This  option  will  keep  a disk copy of the current replay protection state (i.e. the most recent
              packet timestamp and sequence number received from the remote peer), so that if an OpenVPN session
              is stopped and restarted, it will reject any replays of packets which were already received by the
              prior session.

              This option only makes sense when replay protection is enabled (the default)  and  you  are  using
              either --secret (shared-secret key mode) or TLS mode with --tls-auth.

       --no-iv
              DEPRECATED This option will be removed in OpenVPN 2.5.

              (Advanced)  Disable  OpenVPN's  use  of  IV (cipher initialization vector).  Don't use this option
              unless you are prepared to make a tradeoff of greater efficiency in exchange for less security.

              OpenVPN uses an IV by default, and requires it for CFB and OFB cipher  modes  (which  are  totally
              insecure  without  it).   Using  an  IV is important for security when multiple messages are being
              encrypted/decrypted with the same key.

              IV is implemented differently depending on the cipher mode used.

              In CBC mode, OpenVPN uses a pseudo-random IV for each packet.

              In CFB/OFB mode, OpenVPN uses a unique sequence number and time stamp as  the  IV.   In  fact,  in
              CFB/OFB  mode,  OpenVPN  uses a datagram space-saving optimization that uses the unique identifier
              for datagram replay protection as the IV.

       --use-prediction-resistance
              Enable prediction resistance on mbed TLS's RNG.

              Enabling prediction resistance causes the RNG to reseed in each call for  random.  Reseeding  this
              often can quickly deplete the kernel entropy pool.

              If you need this option, please consider running a daemon that adds entropy to the kernel pool.

       --test-crypto
              Do  a  self-test  of  OpenVPN's crypto options by encrypting and decrypting test packets using the
              data channel encryption options specified above.  This option does not require a peer to function,
              and therefore can be specified without --dev or --remote.

              The typical usage of --test-crypto would be something like this:

              openvpn --test-crypto --secret key

              or

              openvpn --test-crypto --secret key --verb 9

              This  option  is  very  useful  to  test OpenVPN after it has been ported to a new platform, or to
              isolate problems in the compiler, OpenSSL crypto library, or OpenVPN's crypto code.  Since it is a
              self-test  mode,  problems  with  encryption  and  authentication can be debugged independently of
              network and tunnel issues.

   TLS Mode Options:
       TLS mode is the most powerful crypto mode of OpenVPN in both security and flexibility.  TLS mode works by
       establishing  control  and  data  channels  which  are  multiplexed  over a single TCP/UDP port.  OpenVPN
       initiates a TLS session over the control channel and uses it to exchange cipher and HMAC keys to  protect
       the  data  channel.   TLS  mode  uses  a robust reliability layer over the UDP connection for all control
       channel communication, while the data channel, over which encrypted  tunnel  data  passes,  is  forwarded
       without any mediation.  The result is the best of both worlds: a fast data channel that forwards over UDP
       with only the overhead of encrypt, decrypt, and HMAC functions, and a control channel that  provides  all
       of  the  security  features of TLS, including certificate-based authentication and Diffie Hellman forward
       secrecy.

       To use TLS mode, each peer that runs OpenVPN should have its own local certificate/key pair ( --cert  and
       --key ), signed by the root certificate which is specified in --ca.

       When  two  OpenVPN  peers connect, each presents its local certificate to the other.  Each peer will then
       check that its partner peer presented a certificate which was signed by the master  root  certificate  as
       specified in --ca.

       If  that  check  on  both  peers succeeds, then the TLS negotiation will succeed, both OpenVPN peers will
       exchange temporary session keys, and the tunnel will begin passing data.

       The  OpenVPN  project  provides  a   set   of   scripts   for   managing   RSA   certificates   &   keys:
       https://github.com/OpenVPN/easy-rsa

       --tls-server
              Enable  TLS  and  assume  server  role  during  TLS handshake.  Note that OpenVPN is designed as a
              peer-to-peer application.  The designation of  client  or  server  is  only  for  the  purpose  of
              negotiating the TLS control channel.

       --tls-client
              Enable TLS and assume client role during TLS handshake.

       --ca file
              Certificate  authority  (CA)  file in .pem format, also referred to as the root certificate.  This
              file can have multiple certificates in .pem format, concatenated together.  You can construct your
              own certificate authority certificate and private key by using a command such as:

              openssl req -nodes -new -x509 -keyout ca.key -out ca.crt

              Then  edit  your  openssl.cnf  file  and  edit  the certificate variable to point to your new root
              certificate ca.crt.

              For testing purposes only, the OpenVPN distribution includes a sample CA certificate (ca.crt).  Of
              course  you  should  never  use  the test certificates and test keys distributed with OpenVPN in a
              production environment, since by virtue of the fact that they are distributed with  OpenVPN,  they
              are totally insecure.

       --capath dir
              Directory containing trusted certificates (CAs and CRLs).  Not available with mbed TLS.

              When using the --capath option, you are required to supply valid CRLs for the CAs too.  CAs in the
              capath directory are expected to be named <hash>.<n>.  CRLs are expected to be named  <hash>.r<n>.
              See  the  -CApath  option of openssl verify , and the -hash option of openssl x509 and openssl crl
              for more information.

       --dh file
              File containing Diffie Hellman parameters in .pem format (required for --tls-server only).

              Set file=none to disable Diffie Hellman key exchange (and use ECDH only). Note that this  requires
              peers  to  be  using  an SSL library that supports ECDH TLS cipher suites (e.g. OpenSSL 1.0.1+, or
              mbed TLS 2.0+).

              Use openssl dhparam -out dh2048.pem 2048  to  generate  2048-bit  DH  parameters.  Diffie  Hellman
              parameters may be considered public.

       --ecdh-curve name
              Specify  the  curve  to use for elliptic curve Diffie Hellman. Available curves can be listed with
              --show-curves.  The specified curve will only be used for ECDH TLS-ciphers.

              This option is not supported in mbed TLS builds of OpenVPN.

       --cert file
              Local peer's signed certificate in .pem format -- must be signed by a certificate authority  whose
              certificate is in --ca file.  Each peer in an OpenVPN link running in TLS mode should have its own
              certificate and private key file.  In addition, each certificate should have been  signed  by  the
              key  of  a  certificate authority whose public key resides in the --ca certificate authority file.
              You can easily make your own certificate authority (see above) or pay money to  use  a  commercial
              service  such as thawte.com (in which case you will be helping to finance the world's second space
              tourist :).  To generate a certificate, you can use a command such as:

              openssl req -nodes -new -keyout mycert.key -out mycert.csr

              If your certificate authority private key lives on another machine, copy the  certificate  signing
              request  (mycert.csr)  to  this  other  machine (this can be done over an insecure channel such as
              email).  Now sign the certificate with a command such as:

              openssl ca -out mycert.crt -in mycert.csr

              Now copy the certificate (mycert.crt) back to the peer which initially  generated  the  .csr  file
              (this  can  be  over a public medium).  Note that the openssl ca command reads the location of the
              certificate authority key from its configuration file such as /usr/share/ssl/openssl.cnf  --  note
              also  that for certificate authority functions, you must set up the files index.txt (may be empty)
              and serial (initialize to 01 ).

       --extra-certs file
              Specify a file containing one or more PEM certs (concatenated together) that  complete  the  local
              certificate chain.

              This  option is useful for "split" CAs, where the CA for server certs is different than the CA for
              client certs.  Putting certs in this file allows them to be used to complete the local certificate
              chain  without trusting them to verify the peer-submitted certificate, as would be the case if the
              certs were placed in the ca file.

       --key file
              Local peer's private key in .pem format.  Use the private key which was generated when  you  built
              your peer's certificate (see --cert file above).

       --tls-version-min version ['or-highest']
              Sets  the  minimum  TLS  version  we  will  accept from the peer (default is "1.0").  Examples for
              version include "1.0", "1.1",  or  "1.2".   If  'or-highest'  is  specified  and  version  is  not
              recognized, we will only accept the highest TLS version supported by the local SSL implementation.

       --tls-version-max version
              Set  the maximum TLS version we will use (default is the highest version supported).  Examples for
              version include "1.0", "1.1", or "1.2".

       --pkcs12 file
              Specify a PKCS #12 file containing local private key, local certificate, and root CA  certificate.
              This option can be used instead of --ca, --cert, and --key.  Not available with mbed TLS.

       --verify-hash hash [algo]
              Specify  SHA1 or SHA256 fingerprint for level-1 cert.  The level-1 cert is the CA (or intermediate
              cert) that signs the leaf certificate, and is  one  removed  from  the  leaf  certificate  in  the
              direction of the root.  When accepting a connection from a peer, the level-1 cert fingerprint must
              match hash or certificate verification will fail.  Hash is specified as XX:XX:... For example:

                  AD:B0:95:D8:09:C8:36:45:12:A9:89:C8:90:09:CB:13:72:A6:AD:16

              The algo flag can be either SHA1 or SHA256.  If not provided, it defaults to SHA1.

       --pkcs11-cert-private [0|1]...
              Set if access to certificate object should be performed after login.  Every provider has  its  own
              setting.

       --pkcs11-id name
              Specify  the  serialized  certificate  id  to  be  used.  The  id  can be gotten by the standalone
              --show-pkcs11-ids option.

       --pkcs11-id-management
              Acquire PKCS#11 id  from  management  interface.  In  this  case  a  NEED-STR  'pkcs11-id-request'
              real-time  message  will  be  triggered,  application  may use pkcs11-id-count command to retrieve
              available number of certificates,  and  pkcs11-id-get  command  to  retrieve  certificate  id  and
              certificate body.

       --pkcs11-pin-cache seconds
              Specify how many seconds the PIN can be cached, the default is until the token is removed.

       --pkcs11-protected-authentication [0|1]...
              Use  PKCS#11  protected  authentication  path,  useful  for biometric and external keypad devices.
              Every provider has its own setting.

       --pkcs11-providers provider...
              Specify a RSA Security Inc. PKCS #11 Cryptographic Token Interface (Cryptoki) providers  to  load.
              This option can be used instead of --cert, --key, and --pkcs12.

              If  p11-kit  is  present  on  the system, its p11-kit-proxy.so module will be loaded by default if
              either the --pkcs11-id or --pkcs11-id-management options are specified  without  --pkcs11-provider
              being given.

       --pkcs11-private-mode mode...
              Specify  which  method to use in order to perform private key operations.  A different mode can be
              specified for each provider.  Mode is encoded as hex  number,  and  can  be  a  mask  one  of  the
              following:

              0 (default) -- Try to determine automatically.
              1 -- Use sign.
              2 -- Use sign recover.
              4 -- Use decrypt.
              8 -- Use unwrap.

       --cryptoapicert select-string
              Load  the  certificate  and private key from the Windows Certificate System Store (Windows/OpenSSL
              Only).

              Use this option instead of --cert and --key.

              This makes it possible to use any  smart  card,  supported  by  Windows,  but  also  any  kind  of
              certificate,  residing  in  the Cert Store, where you have access to the private key.  This option
              has been tested with a couple of different smart cards  (GemSAFE,  Cryptoflex,  and  Swedish  Post
              Office  eID)  on  the  client side, and also an imported PKCS12 software certificate on the server
              side.

              To select a certificate, based on a substring search in the certificate's subject:

              cryptoapicert "SUBJ:Peter Runestig"

              To select a certificate, based on certificate's thumbprint:

              cryptoapicert "THUMB:f6 49 24 41 01 b4 ..."

              The thumbprint hex string can easily be copy-and-pasted from the Windows Certificate Store GUI.

       --key-method m
              DEPRECATED This option will be removed in OpenVPN 2.5

              Use data channel key negotiation method m.  The key  method  must  match  on  both  sides  of  the
              connection.

              After  OpenVPN  negotiates a TLS session, a new set of keys for protecting the tunnel data channel
              is generated and exchanged over the TLS session.

              In method 1 (the default for OpenVPN 1.x), both sides generate random encrypt and  HMAC-send  keys
              which  are forwarded to the other host over the TLS channel. Method 1 is deprecated in OpenVPN 2.4
              , and will be removed in OpenVPN 2.5.

              In method 2, (the default for OpenVPN 2.0) the client generates a random  key.   Both  client  and
              server also generate some random seed material.  All key source material is exchanged over the TLS
              channel. The actual keys are generated using the TLS PRF function, taking source entropy from both
              client  and  server.   Method 2 is designed to closely parallel the key generation process used by
              TLS 1.0.

              Note that in TLS mode, two separate levels of keying occur:

              (1) The TLS connection is initially negotiated,  with  both  sides  of  the  connection  producing
              certificates  and  verifying  the certificate (or other authentication info provided) of the other
              side.  The --key-method parameter has no effect on this process.

              (2) After the TLS connection is established, the tunnel session  keys  are  separately  negotiated
              over  the existing secure TLS channel.  Here, --key-method determines the derivation of the tunnel
              session keys.

       --tls-cipher l
              A list l of allowable TLS ciphers delimited by a colon (":").

              This setting can be used to ensure that certain cipher suites are used (or not used) for  the  TLS
              connection.   OpenVPN uses TLS to secure the control channel, over which the keys that are used to
              protect the actual VPN traffic are exchanged.

              The supplied list of ciphers is (after potential OpenSSL/IANA name translation) simply supplied to
              the  crypto  library.   Please  see  the  OpenSSL and/or mbed TLS documentation for details on the
              cipher list interpretation.

              Use --show-tls to see a list of TLS ciphers supported by your crypto library.

              Warning!  --tls-cipher is an expert feature, which - if used correcly - can improve  the  security
              of  your  VPN connection.  But it is also easy to unwittingly use it to carefully align a gun with
              your foot, or just break your connection.  Use with care!

              The default for --tls-cipher is to use mbed TLS's default cipher  list  when  using  mbed  TLS  or
              "DEFAULT:!EXP:!LOW:!MEDIUM:!kDH:!kECDH:!DSS:!PSK:!SRP:!kRSA" when using OpenSSL.

       --tls-timeout n
              Packet retransmit timeout on TLS control channel if no acknowledgment from remote within n seconds
              (default=2).  When OpenVPN sends a control packet to its  peer,  it  will  expect  to  receive  an
              acknowledgement  within  n  seconds  or  it  will  retransmit  the  packet,  subject to a TCP-like
              exponential backoff algorithm.  This parameter only applies  to  control  channel  packets.   Data
              channel  packets  (which  carry  encrypted  tunnel  data)  are  never  acknowledged, sequenced, or
              retransmitted by OpenVPN because the higher level network protocols running on top of  the  tunnel
              such as TCP expect this role to be left to them.

       --reneg-bytes n
              Renegotiate  data  channel  key  after  n  bytes  sent  or  received  (disabled by default with an
              exception, see below).  OpenVPN allows the lifetime of a key to be expressed as a number of  bytes
              encrypted/decrypted,  a  number  of  packets, or a number of seconds.  A key renegotiation will be
              forced if any of these three criteria are met by either peer.

              If using ciphers with cipher block sizes less than 128-bits,  --reneg-bytes  is  set  to  64MB  by
              default,  unless  it  is  explicitly  disabled  by  setting  the  value  to  0, but this is HIGHLY
              DISCOURAGED as this is designed to add some protection against the  SWEET32  attack  vector.   For
              more information see the --cipher option.

       --reneg-pkts n
              Renegotiate data channel key after n packets sent and received (disabled by default).

       --reneg-sec n
              Renegotiate data channel key after n seconds (default=3600).

              When  using  dual-factor authentication, note that this default value may cause the end user to be
              challenged to reauthorize once per hour.

              Also, keep in mind that this option can be used on both the client and server, and whichever  uses
              the  lower  value  will  be  the  one  to  trigger  the renegotiation.  A common mistake is to set
              --reneg-sec to a higher value on either the  client  or  server,  while  the  other  side  of  the
              connection  is  still using the default value of 3600 seconds, meaning that the renegotiation will
              still occur once per 3600 seconds.  The solution is to increase --reneg-sec on both the client and
              server, or set it to 0 on one side of the connection (to disable), and to your chosen value on the
              other side.

       --hand-window n
              Handshake Window -- the TLS-based key  exchange  must  finalize  within  n  seconds  of  handshake
              initiation  by  any  peer (default = 60 seconds).  If the handshake fails we will attempt to reset
              our connection with our peer and try again.  Even in the event of handshake failure we will  still
              use  our  expiring  key  for up to --tran-window seconds to maintain continuity of transmission of
              tunnel data.

       --tran-window n
              Transition window -- our old key can live this many seconds after a new a key renegotiation begins
              (default  = 3600 seconds).  This feature allows for a graceful transition from old to new key, and
              removes the key renegotiation sequence from the critical path of tunnel data forwarding.

       --single-session
              After initially connecting to a remote peer, disallow any  new  connections.   Using  this  option
              means that a remote peer cannot connect, disconnect, and then reconnect.

              If the daemon is reset by a signal or --ping-restart, it will allow one new connection.

              --single-session  can  be  used  with --ping-exit or --inactive to create a single dynamic session
              that will exit when finished.

       --tls-exit
              Exit on TLS negotiation failure.

       --tls-auth file [direction]
              Add an additional layer of HMAC authentication on top of the TLS control channel to  mitigate  DoS
              attacks and attacks on the TLS stack.

              In  a  nutshell, --tls-auth enables a kind of "HMAC firewall" on OpenVPN's TCP/UDP port, where TLS
              control channel packets bearing an incorrect HMAC signature can  be  dropped  immediately  without
              response.

              file (required) is a file in OpenVPN static key format which can be generated by --genkey

              Older  versions  (up  to  OpenVPN  2.3)  supported  a freeform passphrase file.  This is no longer
              supported in newer versions (v2.4+).

              See the --secret option for more information on the optional direction parameter.

              --tls-auth is recommended when you are running OpenVPN in a mode where it is listening for packets
              from  any  IP  address,  such  as  when  --remote  is not specified, or --remote is specified with
              --float.

              The rationale for this feature is as follows.  TLS requires a multi-packet exchange before  it  is
              able  to  authenticate  a  peer.   During  this  time before authentication, OpenVPN is allocating
              resources (memory and CPU) to this potential peer.  The potential peer is also exposing many parts
              of  OpenVPN and the OpenSSL library to the packets it is sending.  Most successful network attacks
              today seek to either exploit bugs in programs (such as buffer overflow attacks) or force a program
              to  consume  so  many  resources that it becomes unusable.  Of course the first line of defense is
              always to produce clean, well-audited code.  OpenVPN has been written with buffer overflow  attack
              prevention  as  a  top  priority.   But as history has shown, many of the most widely used network
              applications have, from time to time, fallen to buffer overflow attacks.

              So as a second line of defense, OpenVPN offers this special layer of authentication on top of  the
              TLS  control  channel  so  that  every  packet  on the control channel is authenticated by an HMAC
              signature and a unique ID for replay protection.  This signature will also  help  protect  against
              DoS  (Denial  of  Service)  attacks.   An important rule of thumb in reducing vulnerability to DoS
              attacks is to minimize the amount of resources a potential, but as yet unauthenticated, client  is
              able to consume.

              --tls-auth does this by signing every TLS control channel packet with an HMAC signature, including
              packets which are sent before the TLS level has had a chance to authenticate the peer.  The result
              is  that  packets  without the correct signature can be dropped immediately upon reception, before
              they have a chance to consume additional system resources such as by initiating a  TLS  handshake.
              --tls-auth  can  be  strengthened  by adding the --replay-persist option which will keep OpenVPN's
              replay protection state in a file so that it is not lost across restarts.

              It should be emphasized that this feature is optional and that the key file used  with  --tls-auth
              gives  a  peer nothing more than the power to initiate a TLS handshake.  It is not used to encrypt
              or authenticate any tunnel data.

              Use --tls-crypt instead if you want to use the key file to not only authenticate, but also encrypt
              the TLS control channel.

       --tls-crypt keyfile

              Encrypt  and  authenticate all control channel packets with the key from keyfile.  (See --tls-auth
              for more background.)

              Encrypting (and authenticating) control channel packets:

              • provides more privacy by hiding the certificate used for the TLS connection,

              • makes it harder to identify OpenVPN traffic as such,

              • provides  "poor-man's"  post-quantum  security,  against  attackers  who  will  never  know  the
                pre-shared key (i.e. no forward secrecy).

              In contrast to --tls-auth, --tls-crypt does *not* require the user to set --key-direction.

              Security Considerations

              All  peers  use  the  same  --tls-crypt  pre-shared  group key to authenticate and encrypt control
              channel messages.  To ensure that IV collisions remain unlikely, this key should not  be  used  to
              encrypt  more  than  2^48  client-to-server  or 2^48 server-to-client control channel messages.  A
              typical initial negotiation is  about  10  packets  in  each  direction.   Assuming  both  initial
              negotiation  and  renegotiations  are  at  most  2^16  (65536)  packets  (to be conservative), and
              (re)negotiations happen each minute for each user (24/7), this limits the tls-crypt  key  lifetime
              to 8171 years divided by the number of users.  So a setup with 1000 users should rotate the key at
              least once each eight years.  (And a setup with 8000 users each year.)

              If IV collisions were to occur, this could result in the security of --tls-crypt degrading to  the
              same  security  as  using  --tls-auth.  That is, the control channel still benefits from the extra
              protection against active man-in-the-middle-attacks and DoS attacks, but may no longer offer extra
              privacy and post-quantum security on top of what TLS itself offers.

       --askpass [file]
              Get certificate password from console or file before we daemonize.

              For  the extremely security conscious, it is possible to protect your private key with a password.
              Of course this means that every time the OpenVPN daemon is started you must be there to  type  the
              password.   The --askpass option allows you to start OpenVPN from the command line.  It will query
              you for a password before it daemonizes.  To protect a private key with a password you should omit
              the  -nodes  option  when you use the openssl command line tool to manage certificates and private
              keys.

              If file is specified, read the password from the first line of file.  Keep in  mind  that  storing
              your  password  in  a file to a certain extent invalidates the extra security provided by using an
              encrypted key.

       --auth-nocache
              Don't cache --askpass or --auth-user-pass username/passwords in virtual memory.

              If specified, this directive will cause OpenVPN to  immediately  forget  username/password  inputs
              after  they  are  used.   As  a result, when OpenVPN needs a username/password, it will prompt for
              input from stdin, which may be multiple times during the duration of an OpenVPN session.

              When using --auth-nocache in combination with a user/password file and --chroot or --daemon,  make
              sure to use an absolute path.

              This directive does not affect the --http-proxy username/password.  It is always cached.

       --auth-token token
              This  is not an option to be used directly in any configuration files, but rather push this option
              from a --client-connect script or a --plugin which hooks into the OPENVPN_PLUGIN_CLIENT_CONNECT or
              OPENVPN_PLUGIN_CLIENT_CONNECT_V2 calls.  This option provides a possibility to replace the clients
              password with an authentication token during the lifetime of the OpenVPN client.

              Whenever the connection is renegotiated and the --auth-user-pass-verify script or --plugin  making
              use of the OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY hook is triggered, it will pass over this token as
              the password instead of the password the user provided.  The  authentication  token  can  only  be
              reset  by  a full reconnect where the server can push new options to the client.  The password the
              user entered is never preserved once an authentication token have been set.  If the OpenVPN server
              side rejects the authentication token, the client will receive an AUTH_FAIL and disconnect.

              The  purpose  of  this is to enable two factor authentication methods, such as HOTP or TOTP, to be
              used without needing to retrieve a new OTP code each time the connection is renegotiated.  Another
              use  case is to cache authentication data on the client without needing to have the users password
              cached in memory during the life time of the session.

              To make use of this feature, the --client-connect script or --plugin needs to put

                  push "auth-token UNIQUE_TOKEN_VALUE"

              into the file/buffer for dynamic configuration data.  This will then make the  OpenVPN  server  to
              push this value to the client, which replaces the local password with the UNIQUE_TOKEN_VALUE.

       --tls-verify cmd
              Run  command cmd to verify the X509 name of a pending TLS connection that has otherwise passed all
              other tests of certification (except for revocation via  --crl-verify  directive;  the  revocation
              test occurs after the --tls-verify test).

              cmd should return 0 to allow the TLS handshake to proceed, or 1 to fail.

              cmd  consists  of  a path to script (or executable program), optionally followed by arguments. The
              path and arguments may be single- or double-quoted and/or escaped using a backslash, and should be
              separated by one or more spaces.

              When cmd is executed two arguments are appended after any arguments specified in cmd , as follows:

              cmd certificate_depth subject

              These  arguments are, respectively, the current certificate depth and the X509 common name (cn) of
              the peer.

              This feature is useful if the peer you want to trust has a  certificate  which  was  signed  by  a
              certificate authority who also signed many other certificates, where you don't necessarily want to
              trust all of them, but rather be selective about which peer certificate  you  will  accept.   This
              feature  allows  you  to  write a script which will test the X509 name on a certificate and decide
              whether or not it should be accepted.  For a simple perl script which will test  the  common  name
              field on the certificate, see the file verify-cn in the OpenVPN distribution.

              See  the "Environmental Variables" section below for additional parameters passed as environmental
              variables.

       --tls-export-cert directory
              Store the certificates the clients uses upon connection to  this  directory.  This  will  be  done
              before  --tls-verify  is  called.   The certificates will use a temporary name and will be deleted
              when the tls-verify script returns.  The file name used for the certificate is available  via  the
              peer_cert environment variable.

       --x509-username-field [ext:]fieldname
              Field  in  the X.509 certificate subject to be used as the username (default=CN).  Typically, this
              option is specified with fieldname as either of the following:

              --x509-username-field emailAddress
              --x509-username-field ext:subjectAltName

              The first example uses the value of the "emailAddress"  attribute  in  the  certificate's  Subject
              field  as  the  username.   The  second  example  uses  the  ext: prefix to signify that the X.509
              extension fieldname "subjectAltName" be searched for an rfc822Name (email) field to be used as the
              username.  In cases where there are multiple email addresses in ext:fieldname, the last occurrence
              is chosen.

              When this option is used, the --verify-x509-name option will match against  the  chosen  fieldname
              instead of the Common Name.

              Only the subjectAltName and issuerAltName X.509 extensions are supported.

              Please  note: This option has a feature which will convert an all-lowercase fieldname to uppercase
              characters, e.g., ou -> OU.  A mixed-case fieldname or one having the ext:  prefix  will  be  left
              as-is.  This automatic upcasing feature is deprecated and will be removed in a future release.

       --verify-x509-name name type
              Accept  connections  only if a host's X.509 name is equal to name.  The remote host must also pass
              all other tests of verification.

              Which X.509 name is compared to name depends on the setting of type.  type  can  be  "subject"  to
              match the complete subject DN (default), "name" to match a subject RDN or "name-prefix" to match a
              subject RDN prefix.  Which RDN is verified as name depends on  the  --x509-username-field  option.
              But  it  defaults  to  the  common  name  (CN), e.g. a certificate with a subject DN "C=KG, ST=NA,
              L=Bishkek, CN=Server-1" would be matched by:

              --verify-x509-name 'C=KG, ST=NA, L=Bishkek, CN=Server-1' and --verify-x509-name Server-1  name  or
              you  could  use  --verify-x509-name  Server-  name-prefix  if  you  want  a  client to only accept
              connections to "Server-1", "Server-2", etc.

              --verify-x509-name is a useful replacement for the --tls-verify option to verify the remote  host,
              because --verify-x509-name works in a --chroot environment without any dependencies.

              Using a name prefix is a useful alternative to managing a CRL (Certificate Revocation List) on the
              client, since it allows the client to refuse all certificates except  for  those  associated  with
              designated servers.

              NOTE: Test against a name prefix only when you are using OpenVPN with a custom CA certificate that
              is under  your  control.   Never  use  this  option  with  type  "name-prefix"  when  your  client
              certificates are signed by a third party, such as a commercial web CA.

       --x509-track attribute
              Save  peer  X509  attribute  value  in  environment  for  use by plugins and management interface.
              Prepend a '+' to attribute to save values from  full  cert  chain.   Values  will  be  encoded  as
              X509_<depth>_<attribute>=<value>.   Multiple --x509-track options can be defined to track multiple
              attributes.

       --ns-cert-type client|server
              DEPRECATED This  option  will  be  removed  in  OpenVPN  2.5.   Use  the  more  modern  equivalent
              --remote-cert-tls instead.  This option will be removed in OpenVPN 2.5.

              Require  that  peer  certificate was signed with an explicit nsCertType designation of "client" or
              "server".

              This is a useful security option for clients, to ensure that the  host  they  connect  with  is  a
              designated server.

              See  the easy-rsa/build-key-server script for an example of how to generate a certificate with the
              nsCertType field set to "server".

              If the server certificate's nsCertType field is set to "server", then the clients can verify  this
              with --ns-cert-type server.

              This  is  an  important security precaution to protect against a man-in-the-middle attack where an
              authorized client attempts to connect to another client by impersonating the server.   The  attack
              is   easily  prevented  by  having  clients  verify  the  server  certificate  using  any  one  of
              --ns-cert-type, --verify-x509-name, or --tls-verify.

       --remote-cert-ku [v...]
              Require that peer certificate was signed with an explicit key usage.

              If present in the certificate, the keyUsage value is validated by the TLS library during  the  TLS
              handshake.  Specifying this option without arguments requires this extension to be present (so the
              TLS library will verify it).

              If the list v...  is also supplied, the keyUsage field must have at least the same bits set as the
              bits in one of the values supplied in the list v...

              The key usage values in the list must be encoded in hex, e.g.  "--remote-cert-ku a0"

       --remote-cert-eku oid
              Require that peer certificate was signed with an explicit extended key usage.

              This  is  a  useful  security  option  for  clients,  to ensure that the host they connect to is a
              designated server.

              The extended key usage should be encoded in oid notation, or OpenSSL symbolic representation.

       --remote-cert-tls client|server
              Require that peer certificate was signed with an explicit key usage and extended key  usage  based
              on RFC3280 TLS rules.

              This  is  a  useful  security  option  for  clients,  to ensure that the host they connect to is a
              designated server.  Or the other way around; for a server to verify that only hosts with a  client
              certificate can connect.

              The  --remote-cert-tls  client option is equivalent to --remote-cert-ku --remote-cert-eku "TLS Web
              Client Authentication"

              The --remote-cert-tls server option is equivalent to --remote-cert-ku --remote-cert-eku  "TLS  Web
              Server Authentication"

              This  is  an  important security precaution to protect against a man-in-the-middle attack where an
              authorized client attempts to connect to another client by impersonating the server.   The  attack
              is   easily  prevented  by  having  clients  verify  the  server  certificate  using  any  one  of
              --remote-cert-tls, --verify-x509-name, or --tls-verify.

       --crl-verify crl ['dir']
              Check peer certificate against the file crl in PEM format.

              A CRL (certificate revocation list) is used when a particular key  is  compromised  but  when  the
              overall PKI is still intact.

              Suppose  you  had a PKI consisting of a CA, root certificate, and a number of client certificates.
              Suppose a laptop computer containing a client key and  certificate  was  stolen.   By  adding  the
              stolen  certificate  to  the  CRL  file, you could reject any connection which attempts to use it,
              while preserving the overall integrity of the PKI.

              The only time when it would be necessary to rebuild the entire PKI from scratch would  be  if  the
              root certificate key itself was compromised.

              If the optional dir flag is specified, enable a different mode where crl is a directory containing
              files named as revoked serial numbers (the files may be empty, the contents are never read).  If a
              client  requests  a connection, where the client certificate serial number (decimal string) is the
              name of a file present in the directory, it will be rejected.

              Note: As the crl file (or directory) is read every time a peer connects, if you are dropping  root
              privileges with --user, make sure that this user has sufficient privileges to read the file.

   SSL Library information:
       --show-ciphers
              (Standalone) Show all cipher algorithms to use with the --cipher option.

       --show-digests
              (Standalone) Show all message digest algorithms to use with the --auth option.

       --show-tls
              (Standalone) Show all TLS ciphers supported by the crypto library.  OpenVPN uses TLS to secure the
              control channel, over which the keys  that  are  used  to  protect  the  actual  VPN  traffic  are
              exchanged.  The TLS ciphers will be sorted from highest preference (most secure) to lowest.

              Be  aware that whether a cipher suite in this list can actually work depends on the specific setup
              of both peers (e.g. both peers must support the cipher, and an ECDSA cipher suite will not work if
              you are using an RSA certificate, etc.).

       --show-engines
              (Standalone)  Show currently available hardware-based crypto acceleration engines supported by the
              OpenSSL library.

       --show-curves
              (Standalone) Show all available elliptic curves to use with the --ecdh-curve option.

   Generate a random key:
       Used only for non-TLS static key encryption mode.

       --genkey
              (Standalone) Generate a random key to be used as a  shared  secret,  for  use  with  the  --secret
              option.  This file must be shared with the peer over a pre-existing secure channel such as scp(1)

       --secret file
              Write key to file.

   TUN/TAP persistent tunnel config mode:
       Available  with  Linux  2.4.7+.  These options comprise a standalone mode of OpenVPN which can be used to
       create and delete persistent tunnels.

       --mktun
              (Standalone) Create a persistent tunnel on platforms which support them such as  Linux.   Normally
              TUN/TAP  tunnels exist only for the period of time that an application has them open.  This option
              takes advantage of the TUN/TAP driver's ability to build  persistent  tunnels  that  live  through
              multiple instantiations of OpenVPN and die only when they are deleted or the machine is rebooted.

              One  of the advantages of persistent tunnels is that they eliminate the need for separate --up and
              --down scripts to run the appropriate ifconfig(8) and route(8) commands.  These  commands  can  be
              placed in the the same shell script which starts or terminates an OpenVPN session.

              Another  advantage  is that open connections through the TUN/TAP-based tunnel will not be reset if
              the OpenVPN peer restarts.  This can be useful to provide uninterrupted connectivity  through  the
              tunnel  in  the  event  of a DHCP reset of the peer's public IP address (see the --ipchange option
              above).

              One disadvantage of persistent tunnels is that it is harder to automatically configure  their  MTU
              value (see --link-mtu and --tun-mtu above).

              On some platforms such as Windows, TAP-Win32 tunnels are persistent by default.

       --rmtun
              (Standalone) Remove a persistent tunnel.

       --dev tunX | tapX
              TUN/TAP device

       --user user
              Optional user to be owner of this tunnel.

       --group group
              Optional group to be owner of this tunnel.

   Windows-Specific Options:
       --win-sys path
              Set  the  Windows  system  directory  pathname  to use when looking for system executables such as
              route.exe and netsh.exe.  By default, if this directive is not specified,  OpenVPN  will  use  the
              SystemRoot environment variable.

              This option have changed behaviour in OpenVPN 2.3.  Earlier you had to define --win-sys env to use
              the SystemRoot environment variable, otherwise it defaulted to C:\WINDOWS.  It is  not  needed  to
              use  the env keyword any more, and it will just be ignored. A warning is logged when this is found
              in the configuration file.

       --ip-win32 method
              When using --ifconfig on Windows, set the TAP-Win32 adapter IP address and netmask  using  method.
              Don't use this option unless you are also using --ifconfig.

              manual  --  Don't  set  the  IP address or netmask automatically.  Instead output a message to the
              console telling the user to configure the adapter manually and  indicating  the  IP/netmask  which
              OpenVPN expects the adapter to be set to.

              dynamic  [offset] [lease-time] -- Automatically set the IP address and netmask by replying to DHCP
              query messages generated by the kernel.  This mode is probably the "cleanest" solution for setting
              the  TCP/IP  properties  since  it  uses  the  well-known  DHCP protocol.  There are, however, two
              prerequisites for using this mode: (1) The TCP/IP properties for the TAP-Win32 adapter must be set
              to  "Obtain  an  IP  address  automatically,"  and (2) OpenVPN needs to claim an IP address in the
              subnet for use as the virtual DHCP server address.  By default in --dev  tap  mode,  OpenVPN  will
              take  the normally unused first address in the subnet.  For example, if your subnet is 192.168.4.0
              netmask 255.255.255.0, then OpenVPN will take the IP address 192.168.4.0 to  use  as  the  virtual
              DHCP server address.  In --dev tun mode, OpenVPN will cause the DHCP server to masquerade as if it
              were coming from the remote endpoint.  The optional offset parameter is an integer which is > -256
              and < 256 and which defaults to -1.  If offset is positive, the DHCP server will masquerade as the
              IP address at network address + offset.  If offset is negative, the DHCP server will masquerade as
              the  IP  address  at broadcast address + offset.  The Windows ipconfig /all command can be used to
              show what Windows thinks the DHCP server address is.  OpenVPN will "claim" this address,  so  make
              sure  to  use  a  free  address.   Having  said  that, different OpenVPN instantiations, including
              different ends of the same connection, can share  the  same  virtual  DHCP  server  address.   The
              lease-time  parameter  controls  the  lease  time  of  the  DHCP assignment given to the TAP-Win32
              adapter, and is denoted in seconds.  Normally a very long  lease  time  is  preferred  because  it
              prevents  routes  involving  the  TAP-Win32 adapter from being lost when the system goes to sleep.
              The default lease time is one year.

              netsh -- Automatically set the IP address and  netmask  using  the  Windows  command-line  "netsh"
              command.  This method appears to work correctly on Windows XP but not Windows 2000.

              ipapi  --  Automatically  set  the  IP  address and netmask using the Windows IP Helper API.  This
              approach does not have ideal semantics, though  testing  has  indicated  that  it  works  okay  in
              practice.   If  you  use  this option, it is best to leave the TCP/IP properties for the TAP-Win32
              adapter in their default state, i.e. "Obtain an IP address automatically."

              adaptive -- (Default) Try dynamic method initially and fail over to netsh if the DHCP  negotiation
              with the TAP-Win32 adapter does not succeed in 20 seconds.  Such failures have been known to occur
              when certain third-party firewall  packages  installed  on  the  client  machine  block  the  DHCP
              negotiation  used by the TAP-Win32 adapter.  Note that if the netsh failover occurs, the TAP-Win32
              adapter TCP/IP properties will be reset from DHCP to static, and this will  cause  future  OpenVPN
              startups  using  the adaptive mode to use netsh immediately, rather than trying dynamic first.  To
              "unstick" the adaptive mode from using netsh, run OpenVPN at least once using the dynamic mode  to
              restore the TAP-Win32 adapter TCP/IP properties to a DHCP configuration.

       --route-method m
              Which method m to use for adding routes on Windows?

              adaptive  (default)  --  Try IP helper API first.  If that fails, fall back to the route.exe shell
              command.
              ipapi -- Use IP helper API.
              exe -- Call the route.exe shell command.

       --dhcp-option type [parm]
              Set extended TAP-Win32 TCP/IP properties, must be  used  with  --ip-win32  dynamic  or  --ip-win32
              adaptive.   This  option can be used to set additional TCP/IP properties on the TAP-Win32 adapter,
              and is particularly useful for configuring an OpenVPN client to access a Samba server  across  the
              VPN.

              DOMAIN name -- Set Connection-specific DNS Suffix.

              DNS  addr -- Set primary domain name server IPv4 address.  Repeat this option to set secondary DNS
              server addresses.

              DNS6 addr -- Set primary domain name server IPv6 address.  Repeat this option to set secondary DNS
              server IPv6 addresses.

              Note:  currently  this  is  handled using netsh (the existing DHCP code can only do IPv4 DHCP, and
              that protocol only permits IPv4 addresses anywhere).  The option will be put into the environment,
              so an --up script could act upon it if needed.

              WINS  addr  --  Set  primary  WINS  server address (NetBIOS over TCP/IP Name Server).  Repeat this
              option to set secondary WINS server addresses.

              NBDD addr -- Set primary NBDD server address (NetBIOS over TCP/IP  Datagram  Distribution  Server)
              Repeat this option to set secondary NBDD server addresses.

              NTP  addr  --  Set  primary NTP server address (Network Time Protocol).  Repeat this option to set
              secondary NTP server addresses.

              NBT type -- Set NetBIOS over TCP/IP Node type.  Possible options: 1 =  b-node  (broadcasts),  2  =
              p-node  (point-to-point  name  queries  to  a  WINS server), 4 = m-node (broadcast then query name
              server), and 8 = h-node (query name server, then broadcast).

              NBS scope-id -- Set NetBIOS over TCP/IP Scope. A NetBIOS Scope  ID  provides  an  extended  naming
              service  for the NetBIOS over TCP/IP (Known as NBT) module. The primary purpose of a NetBIOS scope
              ID is to isolate NetBIOS traffic on a single network to only those nodes  with  the  same  NetBIOS
              scope  ID.   The  NetBIOS scope ID is a character string that is appended to the NetBIOS name. The
              NetBIOS scope ID on two hosts must match, or the two hosts will not be able  to  communicate.  The
              NetBIOS Scope ID also allows computers to use the same computer name, as they have different scope
              IDs. The Scope ID becomes a part of the NetBIOS name, making the name unique.   (This  description
              of NetBIOS scopes courtesy of NeonSurge@abyss.com)

              DISABLE-NBT -- Disable Netbios-over-TCP/IP.

              Note  that if --dhcp-option is pushed via --push to a non-windows client, the option will be saved
              in the client's environment before the up script is called, under the name "foreign_option_{n}".

       --tap-sleep n
              Cause OpenVPN to sleep for n seconds immediately after the  TAP-Win32  adapter  state  is  set  to
              "connected".

              This  option  is  intended  to be used to troubleshoot problems with the --ifconfig and --ip-win32
              options, and is used to give the TAP-Win32 adapter time to come up before Windows  IP  Helper  API
              operations are applied to it.

       --show-net-up
              Output  OpenVPN's  view  of the system routing table and network adapter list to the syslog or log
              file after the TUN/TAP adapter has been brought up and any routes have been added.

       --block-outside-dns
              Block DNS servers on other network adapters  to  prevent  DNS  leaks.  This  option  prevents  any
              application  from  accessing  TCP  or  UDP  port  53 except one inside the tunnel. It uses Windows
              Filtering Platform (WFP) and works on Windows Vista or later.

              This option is considered  unknown  on  non-Windows  platforms  and  unsupported  on  Windows  XP,
              resulting  in  fatal  error.   You  may  want  to use --setenv opt or --ignore-unknown-option (not
              suitable for Windows XP) to ignore said error.  Note that pushing unknown options from server does
              not trigger fatal errors.

       --dhcp-renew
              Ask  Windows  to  renew the TAP adapter lease on startup.  This option is normally unnecessary, as
              Windows automatically triggers a DHCP renegotiation on the TAP adapter when it comes  up,  however
              if  you  set  the TAP-Win32 adapter Media Status property to "Always Connected", you may need this
              flag.

       --dhcp-release
              Ask Windows to release the TAP adapter lease on shutdown.  This option has no effect now, as it is
              enabled by default starting with OpenVPN 2.4.1.

       --register-dns
              Run  ipconfig /flushdns and ipconfig /registerdns on connection initiation.  This is known to kick
              Windows into recognizing pushed DNS servers.

       --pause-exit
              Put up a "press any key to continue" message on the console prior to OpenVPN program  exit.   This
              option  is  automatically used by the Windows explorer when OpenVPN is run on a configuration file
              using the right-click explorer menu.

       --service exit-event [0|1]
              Should be used when OpenVPN is being automatically executed by another program in such  a  context
              that  no  interaction  with  the  user via display or keyboard is possible.  In general, end-users
              should never need to explicitly use this option, as it  is  automatically  added  by  the  OpenVPN
              service wrapper when a given OpenVPN configuration is being run as a service.

              exit-event is the name of a Windows global event object, and OpenVPN will continuously monitor the
              state of this event object and exit when it becomes signaled.

              The second parameter indicates the initial state of exit-event and normally defaults to 0.

              Multiple OpenVPN processes can be simultaneously executed with the same exit-event parameter.   In
              any  case,  the  controlling  process can signal exit-event, causing all such OpenVPN processes to
              exit.

              When executing an OpenVPN process using the --service directive, OpenVPN will probably not have  a
              console  window  to  output  status/error  messages,  therefore  it  is  useful  to  use  --log or
              --log-append to write these messages to a file.

       --show-adapters
              (Standalone) Show available TAP-Win32 adapters which can be selected using the --dev-node  option.
              On non-Windows systems, the ifconfig(8) command provides similar functionality.

       --allow-nonadmin [TAP-adapter]
              (Standalone)  Set TAP-adapter to allow access from non-administrative accounts.  If TAP-adapter is
              omitted, all TAP adapters on the system  will  be  configured  to  allow  non-admin  access.   The
              non-admin access setting will only persist for the length of time that the TAP-Win32 device object
              and driver remain loaded, and will need to be re-enabled after a  reboot,  or  if  the  driver  is
              unloaded and reloaded.  This directive can only be used by an administrator.

       --show-valid-subnets
              (Standalone)  Show  valid  subnets for --dev tun emulation.  Since the TAP-Win32 driver exports an
              ethernet interface to Windows, and since TUN devices are point-to-point in nature, it is necessary
              for the TAP-Win32 driver to impose certain constraints on TUN endpoint address selection.

              Namely, the point-to-point endpoints used in TUN device emulation must be the middle two addresses
              of a /30 subnet (netmask 255.255.255.252).

       --show-net
              (Standalone) Show OpenVPN's view of the system routing table and network adapter list.

   PKCS#11 Standalone Options:
       --show-pkcs11-ids [provider] [cert_private]
              (Standalone) Show PKCS#11 token object list. Specify cert_private as 1 if certificates are  stored
              as private objects.

              If  p11-kit  is  present  on the system, the provider argument is optional; if omitted the default
              p11-kit-proxy.so module will be queried.

              --verb option can be used BEFORE this option to produce debugging information.

   Standalone Debug Options:
       --show-gateway [v6target]
              (Standalone) Show current IPv4 and IPv6 default gateway and interface towards the gateway (if  the
              protocol  in  question  is enabled).  If an IPv6 address is passed as argument, the IPv6 route for
              this host is reported.

   IPv6 Related Options
       The following options exist to support IPv6  tunneling  in  peer-to-peer  and  client-server  mode.   All
       options  are  modeled after their IPv4 counterparts, so more detailed explanations given there apply here
       as well (except for --topology , which has no effect on IPv6).

       --ifconfig-ipv6 ipv6addr/bits ipv6remote
              configure IPv6 address ipv6addr/bits on the ``tun'' device.  The second parameter is used as route
              target for --route-ipv6 if no gateway is specified.

       --route-ipv6 ipv6addr/bits [gateway] [metric]
              setup  IPv6  routing in the system to send the specified IPv6 network into OpenVPN's ``tun''.  The
              gateway parameter is only used for IPv6  routes  across  ``tap''  devices,  and  if  missing,  the
              ``ipv6remote'' field from --ifconfig-ipv6 is used.

       --server-ipv6 ipv6addr/bits
              convenience-function  to  enable a number of IPv6 related options at once, namely --ifconfig-ipv6,
              --ifconfig-ipv6-pool and --push tun-ipv6 Is only accepted if ``--mode server'' or ``--server''  is
              set.  Pushing  of  the  --tun-ipv6  directive  is done for older clients which require an explicit
              ``--tun-ipv6'' in their configuration.

       --ifconfig-ipv6-pool ipv6addr/bits
              Specify an IPv6 address pool for dynamic assignment to clients.  The pool starts at  ipv6addr  and
              matches the offset determined from the start of the IPv4 pool.

       --ifconfig-ipv6-push ipv6addr/bits ipv6remote
              for   ccd/   per-client   static   IPv6   interface  configuration,  see  --client-config-dir  and
              --ifconfig-push for more details.

       --iroute-ipv6 ipv6addr/bits
              for ccd/ per-client static IPv6 route configuration, see --iroute for more details  how  to  setup
              and use this, and how --iroute and --route interact.

SCRIPTING AND ENVIRONMENTAL VARIABLES

       OpenVPN exports a series of environmental variables for use by user-defined scripts.

   Script Order of Execution
       --up   Executed after TCP/UDP socket bind and TUN/TAP open.

       --tls-verify
              Executed when we have a still untrusted remote peer.

       --ipchange
              Executed after connection authentication, or remote IP address change.

       --client-connect
              Executed in --mode server mode immediately after client authentication.

       --route-up
              Executed  after  connection  authentication,  either  immediately after, or some number of seconds
              after as defined by the --route-delay option.

       --route-pre-down
              Executed right before the routes are removed.

       --client-disconnect
              Executed in --mode server mode on client instance shutdown.

       --down Executed after TCP/UDP and TUN/TAP close.

       --learn-address
              Executed in --mode server mode whenever an IPv4 address/route or MAC address is added to OpenVPN's
              internal routing table.

       --auth-user-pass-verify
              Executed in --mode server mode on new client connections, when the client is still untrusted.

   String Types and Remapping
       In  certain  cases, OpenVPN will perform remapping of characters in strings.  Essentially, any characters
       outside the set of permitted characters for each string type will be converted to underbar ('_').

       Q: Why is string remapping necessary?

       A: It's an important security feature to prevent the malicious coding of strings from  untrusted  sources
       to  be  passed as parameters to scripts, saved in the environment, used as a common name, translated to a
       filename, etc.

       Q: Can string remapping be disabled?

       A: Yes, by using the --no-name-remapping option, however this should be considered an advanced option.

       Here is a brief rundown of OpenVPN's current string types and the  permitted  character  class  for  each
       string:

       X509  Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), at ('@'), colon (':'), slash ('/'), and
       equal ('=').  Alphanumeric is defined as a character which will cause the C library isalnum() function to
       return true.

       Common Names: Alphanumeric, underbar ('_'), dash ('-'), dot ('.'), and at ('@').

       --auth-user-pass  username:  Same  as  Common  Name, with one exception: starting with OpenVPN 2.0.1, the
       username is passed to the OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plugin in its  raw  form,  without  string
       remapping.

       --auth-user-pass  password:  Any  "printable"  character  except  CR or LF.  Printable is defined to be a
       character which will cause the C library isprint() function to return true.

       --client-config-dir filename as derived from common name or username: Alphanumeric, underbar ('_'),  dash
       ('-'),  and  dot  ('.')  except  for  "."  or ".." as standalone strings.  As of v2.0.1-rc6, the at ('@')
       character has been added as well for compatibility with the common name character class.

       Environmental variable names: Alphanumeric or underbar ('_').

       Environmental variable values: Any printable character.

       For all cases, characters in a string which are not members of the legal character class for that  string
       type will be remapped to underbar ('_').

   Environmental Variables
       Once set, a variable is persisted indefinitely until it is reset by a new value or a restart,

       As  of OpenVPN 2.0-beta12, in server mode, environmental variables set by OpenVPN are scoped according to
       the client objects they are associated with, so there should not be any issues with scripts having access
       to stale, previously set variables which refer to different client instances.

       bytes_received
              Total  number  of  bytes  received  from client during VPN session.  Set prior to execution of the
              --client-disconnect script.

       bytes_sent
              Total number of bytes sent  to  client  during  VPN  session.   Set  prior  to  execution  of  the
              --client-disconnect script.

       common_name
              The  X509  common  name  of  an authenticated client.  Set prior to execution of --client-connect,
              --client-disconnect, and --auth-user-pass-verify scripts.

       config Name of first --config file.  Set on program initiation and reset on SIGHUP.

       daemon Set to "1" if the --daemon directive is specified, or "0" otherwise.  Set  on  program  initiation
              and reset on SIGHUP.

       daemon_log_redirect
              Set  to  "1"  if  the  --log  or  --log-append directives are specified, or "0" otherwise.  Set on
              program initiation and reset on SIGHUP.

       dev    The actual name of the TUN/TAP device, including a unit number if it exists.  Set prior to --up or
              --down script execution.

       dev_idx
              On Windows, the device index of the TUN/TAP adapter (to be used in netsh.exe calls which sometimes
              just do not work right with interface names).  Set prior to --up or --down script execution.

       foreign_option_{n}
              An option pushed via --push to a client which does not natively support it, such as  --dhcp-option
              on  a  non-Windows  system, will be recorded to this environmental variable sequence prior to --up
              script execution.

       ifconfig_broadcast
              The broadcast address for the virtual ethernet segment which is derived from the --ifconfig option
              when  --dev  tap  is used.  Set prior to OpenVPN calling the ifconfig or netsh (windows version of
              ifconfig) commands which normally occurs prior to --up script execution.

       ifconfig_ipv6_local
              The local VPN endpoint IPv6 address specified in the  --ifconfig-ipv6  option  (first  parameter).
              Set  prior  to  OpenVPN calling the ifconfig or netsh (windows version of ifconfig) commands which
              normally occurs prior to --up script execution.

       ifconfig_ipv6_netbits
              The prefix length of the IPv6 network on the VPN interface.  Derived from the  /nnn  parameter  of
              the  IPv6  address  in the --ifconfig-ipv6 option (first parameter).  Set prior to OpenVPN calling
              the ifconfig or netsh (windows version of ifconfig) commands which normally occurs prior  to  --up
              script execution.

       ifconfig_ipv6_remote
              The  remote  VPN endpoint IPv6 address specified in the --ifconfig-ipv6 option (second parameter).
              Set prior to OpenVPN calling the ifconfig or netsh (windows version of  ifconfig)  commands  which
              normally occurs prior to --up script execution.

       ifconfig_local
              The local VPN endpoint IP address specified in the --ifconfig option (first parameter).  Set prior
              to OpenVPN calling the ifconfig or netsh (windows version of  ifconfig)  commands  which  normally
              occurs prior to --up script execution.

       ifconfig_remote
              The  remote  VPN  endpoint  IP  address specified in the --ifconfig option (second parameter) when
              --dev tun is used.  Set prior to OpenVPN  calling  the  ifconfig  or  netsh  (windows  version  of
              ifconfig) commands which normally occurs prior to --up script execution.

       ifconfig_netmask
              The  subnet  mask  of  the  virtual  ethernet segment that is specified as the second parameter to
              --ifconfig when --dev tap is being used.  Set prior to  OpenVPN  calling  the  ifconfig  or  netsh
              (windows version of ifconfig) commands which normally occurs prior to --up script execution.

       ifconfig_pool_local_ip
              The  local  virtual  IP  address for the TUN/TAP tunnel taken from an --ifconfig-push directive if
              specified, or otherwise from the ifconfig pool (controlled  by  the  --ifconfig-pool  config  file
              directive).   Only set for --dev tun tunnels.  This option is set on the server prior to execution
              of the --client-connect and --client-disconnect scripts.

       ifconfig_pool_netmask
              The virtual IP netmask  for  the  TUN/TAP  tunnel  taken  from  an  --ifconfig-push  directive  if
              specified,  or  otherwise  from  the  ifconfig pool (controlled by the --ifconfig-pool config file
              directive).  Only set for --dev tap tunnels.  This option is set on the server prior to  execution
              of the --client-connect and --client-disconnect scripts.

       ifconfig_pool_remote_ip
              The  remote  virtual  IP address for the TUN/TAP tunnel taken from an --ifconfig-push directive if
              specified, or otherwise from the ifconfig pool (controlled  by  the  --ifconfig-pool  config  file
              directive).   This  option  is  set  on  the server prior to execution of the --client-connect and
              --client-disconnect scripts.

       link_mtu
              The maximum packet size (not including the IP header) of tunnel data in UDP tunnel transport mode.
              Set prior to --up or --down script execution.

       local  The --local parameter.  Set on program initiation and reset on SIGHUP.

       local_port
              The  local  port  number  or  name, specified by --port or --lport.  Set on program initiation and
              reset on SIGHUP.

       password
              The password provided by  a  connecting  client.   Set  prior  to  --auth-user-pass-verify  script
              execution  only when the via-env modifier is specified, and deleted from the environment after the
              script returns.

       proto  The --proto parameter.  Set on program initiation and reset on SIGHUP.

       remote_{n}
              The --remote parameter.  Set on program initiation and reset on SIGHUP.

       remote_port_{n}
              The remote port number, specified by --port or --rport.  Set on program initiation  and  reset  on
              SIGHUP.

       route_net_gateway
              The  pre-existing  default  IP  gateway  in  the  system  routing table.  Set prior to --up script
              execution.

       route_vpn_gateway
              The default gateway used by --route options, as specified in either the --route-gateway option  or
              the  second  parameter  to  --ifconfig  when  --dev  tun  is  specified.  Set prior to --up script
              execution.

       route_{parm}_{n}
              A set of variables which define each route  to  be  added,  and  are  set  prior  to  --up  script
              execution.

              parm will be one of "network", "netmask", "gateway", or "metric".

              n is the OpenVPN route number, starting from 1.

              If the network or gateway are resolvable DNS names, their IP address translations will be recorded
              rather than their names as denoted on the command line or configuration file.

       route_ipv6_{parm}_{n}
              A set of variables which define each IPv6 route to be added, and are  set  prior  to  --up  script
              execution.

              parm   will  be  one  of  "network"  or  "gateway"  ("netmask"  is  contained  as  "/nnn"  in  the
              route_ipv6_network_{n}, unlike IPv4 where it is passed in a separate environment variable).

              n is the OpenVPN route number, starting from 1.

              If the network or gateway are resolvable DNS names, their IP address translations will be recorded
              rather than their names as denoted on the command line or configuration file.

       peer_cert
              Temporary file name containing the client certificate upon connection.  Useful in conjunction with
              --tls-verify

       script_context
              Set to "init" or  "restart"  prior  to  up/down  script  execution.   For  more  information,  see
              documentation for --up.

       script_type
              Prior to execution of any script, this variable is set to the type of script being run.  It can be
              one  of  the  following:  up,  down,  ipchange,   route-up,   tls-verify,   auth-user-pass-verify,
              client-connect, client-disconnect, or learn-address.  Set prior to execution of any script.

       signal The  reason  for  exit  or  restart.   Can  be  one  of sigusr1, sighup, sigterm, sigint, inactive
              (controlled by --inactive option), ping-exit  (controlled  by  --ping-exit  option),  ping-restart
              (controlled  by  --ping-restart  option),  connection-reset  (triggered  on TCP connection reset),
              error, or unknown (unknown signal).  This variable is set just prior to down script execution.

       time_ascii
              Client connection timestamp, formatted as a human-readable time string.  Set prior to execution of
              the --client-connect script.

       time_duration
              The  duration  (in  seconds)  of  the  client  session  which  is now disconnecting.  Set prior to
              execution of the --client-disconnect script.

       time_unix
              Client connection timestamp, formatted as a unix integer date/time value.  Set prior to  execution
              of the --client-connect script.

       tls_digest_{n} / tls_digest_sha256_{n}
              Contains  the  certificate SHA1 / SHA256 fingerprint, where n is the verification level.  Only set
              for TLS connections.  Set prior to execution of --tls-verify script.

       tls_id_{n}
              A series of certificate fields from the remote peer, where n is the verification level.  Only  set
              for TLS connections.  Set prior to execution of --tls-verify script.

       tls_serial_{n}
              The  serial  number  of  the  certificate from the remote peer, where n is the verification level.
              Only set for TLS connections.  Set prior to execution of --tls-verify script. This is in the  form
              of  a decimal string like "933971680", which is suitable for doing serial-based OCSP queries (with
              OpenSSL, do not prepend "0x" to the string) If something goes wrong while reading the  value  from
              the  certificate  it  will  be  an  empty  string,  so  your  code  should  check  that.   See the
              contrib/OCSP_check/OCSP_check.sh script for an example.

       tls_serial_hex_{n}
              Like tls_serial_{n}, but in hex form (e.g. "12:34:56:78:9A").

       tun_mtu
              The MTU of the TUN/TAP device.  Set prior to --up or --down script execution.

       trusted_ip (or trusted_ip6)
              Actual IP address of connecting client or  peer  which  has  been  authenticated.   Set  prior  to
              execution  of  --ipchange,  --client-connect,  and  --client-disconnect  scripts.   If  using ipv6
              endpoints (udp6, tcp6), trusted_ip6 will be set instead.

       trusted_port
              Actual port number of connecting client or peer  which  has  been  authenticated.   Set  prior  to
              execution of --ipchange, --client-connect, and --client-disconnect scripts.

       untrusted_ip (or untrusted_ip6)
              Actual  IP  address  of connecting client or peer which has not been authenticated yet.  Sometimes
              used to nmap the connecting host in a --tls-verify script to ensure  it  is  firewalled  properly.
              Set  prior  to  execution  of  --tls-verify  and  --auth-user-pass-verify  scripts.  If using ipv6
              endpoints (udp6, tcp6), untrusted_ip6 will be set instead.

       untrusted_port
              Actual port number of connecting client or peer which has not been authenticated yet.   Set  prior
              to execution of --tls-verify and --auth-user-pass-verify scripts.

       username
              The  username  provided  by  a  connecting  client.   Set  prior to --auth-user-pass-verify script
              execution only when the via-env modifier is specified.

       X509_{n}_{subject_field}
              An X509 subject field from the remote peer certificate, where n is the verification  level.   Only
              set for TLS connections.  Set prior to execution of --tls-verify script.  This variable is similar
              to tls_id_{n} except the component X509 subject fields are broken out,  and  no  string  remapping
              occurs  on  these  field values (except for remapping of control characters to "_").  For example,
              the following variables would be set on the OpenVPN server using the sample client certificate  in
              sample-keys  (client.crt).  Note that the verification level is 0 for the client certificate and 1
              for the CA certificate.

                  X509_0_emailAddress=me@myhost.mydomain
                  X509_0_CN=Test-Client
                  X509_0_O=OpenVPN-TEST
                  X509_0_ST=NA
                  X509_0_C=KG
                  X509_1_emailAddress=me@myhost.mydomain
                  X509_1_O=OpenVPN-TEST
                  X509_1_L=BISHKEK
                  X509_1_ST=NA
                  X509_1_C=KG

INLINE FILE SUPPORT

       OpenVPN allows including files in the main configuration  for  the  --ca,  --cert,  --dh,  --extra-certs,
       --key, --pkcs12, --secret, --crl-verify, --http-proxy-user-pass, --tls-auth and --tls-crypt options.

       Each inline file started by the line <option> and ended by the line </option>

       Here is an example of an inline file usage

           <cert>
           -----BEGIN CERTIFICATE-----
           [...]
           -----END CERTIFICATE-----
           </cert>

       When  using the inline file feature with --pkcs12 the inline file has to be base64 encoded. Encoding of a
       .p12 file into base64 can be done for example with OpenSSL by running openssl base64 -in input.p12

SIGNALS

       SIGHUP Cause OpenVPN to close all TUN/TAP and network connections,  restart,  re-read  the  configuration
              file (if any), and reopen TUN/TAP and network connections.

       SIGUSR1
              Like  SIGHUP, except don't re-read configuration file, and possibly don't close and reopen TUN/TAP
              device, re-read key files, preserve local IP address/port, or preserve most recently authenticated
              remote   IP   address/port   based   on   --persist-tun,  --persist-key,  --persist-local-ip,  and
              --persist-remote-ip options respectively (see above).

              This  signal  may  also  be  internally  generated  by  a  timeout  condition,  governed  by   the
              --ping-restart option.

              This signal, when combined with --persist-remote-ip, may be sent when the underlying parameters of
              the host's network interface change such as when the host is a DHCP client and is assigned  a  new
              IP address.  See --ipchange above for more information.

       SIGUSR2
              Causes  OpenVPN  to  display  its  current  statistics (to the syslog file if --daemon is used, or
              stdout otherwise).

       SIGINT, SIGTERM
              Causes OpenVPN to exit gracefully.

TUN/TAP DRIVER SETUP

       If you are running Linux 2.4.7 or higher, you probably have the TUN/TAP driver already installed.  If so,
       there are still a few things you need to do:

       Make device: mknod /dev/net/tun c 10 200

       Load driver: modprobe tun

EXAMPLES

       Prior  to  running  these  examples,  you  should  have  OpenVPN  installed  on two machines with network
       connectivity between them.  If you have not yet installed OpenVPN, consult the INSTALL file  included  in
       the OpenVPN distribution.

   TUN/TAP Setup:
       If you are using Linux 2.4 or higher, make the tun device node and load the tun module:

              mknod /dev/net/tun c 10 200

              modprobe tun

       If you installed from RPM, the mknod step may be omitted, because the RPM install does that for you.

       Only Linux 2.4 and newer are supported.

       For other platforms, consult the INSTALL file at http://openvpn.net/install.html for more information.

   Firewall Setup:
       If  firewalls  exist  between  the  two  machines,  they  should  be set to forward UDP port 1194 in both
       directions.  If you do not have control over the firewalls between the two machines,  you  may  still  be
       able  to use OpenVPN by adding --ping 15 to each of the openvpn commands used below in the examples (this
       will cause each peer to send out a UDP ping to its remote peer once every 15  seconds  which  will  cause
       many stateful firewalls to forward packets in both directions without an explicit firewall rule).

       If  you  are  using a Linux iptables-based firewall, you may need to enter the following command to allow
       incoming packets on the TUN device:

              iptables -A INPUT -i tun+ -j ACCEPT

       See the firewalls section below for more information on configuring firewalls for use with OpenVPN.

   VPN Address Setup:
       For purposes of our example, our two machines will be called bob.example.com and  alice.example.com.   If
       you are constructing a VPN over the internet, then replace bob.example.com and alice.example.com with the
       internet hostname or IP address that each machine will use to contact the other over the internet.

       Now we will choose the tunnel endpoints.  Tunnel endpoints  are  private  IP  addresses  that  only  have
       meaning  in  the  context  of the VPN.  Each machine will use the tunnel endpoint of the other machine to
       access it over the VPN.  In our example, the tunnel endpoint for bob.example.com will be 10.4.0.1 and for
       alice.example.com, 10.4.0.2.

       Once  the  VPN is established, you have essentially created a secure alternate path between the two hosts
       which is addressed by using the tunnel endpoints.  You can control which network traffic  passes  between
       the  hosts  (a)  over  the  VPN  or  (b) independently of the VPN, by choosing whether to use (a) the VPN
       endpoint address or (b) the public internet address, to access the remote host. For example if you are on
       bob.example.com and you wish to connect to alice.example.com via ssh without using the VPN (since ssh has
       its own built-in security) you would  use  the  command  ssh  alice.example.com.   However  in  the  same
       scenario,   you   could   also  use  the  command  telnet  10.4.0.2  to  create  a  telnet  session  with
       alice.example.com over the VPN, that would use the VPN to secure the session rather than ssh.

       You can use any address you wish for the tunnel endpoints but make sure that they are  private  addresses
       (such  as  those  that begin with 10 or 192.168) and that they are not part of any existing subnet on the
       networks of either peer, unless you are bridging.  If you use an address  that  is  part  of  your  local
       subnet for either of the tunnel endpoints, you will get a weird feedback loop.

   Example 1: A simple tunnel without security
       On bob:

              openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 9

       On alice:

              openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 9

       Now verify the tunnel is working by pinging across the tunnel.

       On bob:

              ping 10.4.0.2

       On alice:

              ping 10.4.0.1

       The  --verb  9  option will produce verbose output, similar to the tcpdump(8) program.  Omit the --verb 9
       option to have OpenVPN run quietly.

   Example 2: A tunnel with static-key security (i.e. using a pre-shared secret)
       First build a static key on bob.

              openvpn --genkey --secret key

       This command will build a random key file called key (in ascii format).  Now copy key  to  alice  over  a
       secure medium such as by using the scp(1) program.

       On bob:

              openvpn --remote alice.example.com --dev tun1 --ifconfig 10.4.0.1 10.4.0.2 --verb 5 --secret key

       On alice:

              openvpn --remote bob.example.com --dev tun1 --ifconfig 10.4.0.2 10.4.0.1 --verb 5 --secret key

       Now verify the tunnel is working by pinging across the tunnel.

       On bob:

              ping 10.4.0.2

       On alice:

              ping 10.4.0.1

   Example 3: A tunnel with full TLS-based security
       For  this test, we will designate bob as the TLS client and alice as the TLS server.  Note that client or
       server designation only has meaning for the TLS subsystem. It has no bearing on  OpenVPN's  peer-to-peer,
       UDP-based communication model.

       First,  build a separate certificate/key pair for both bob and alice (see above where --cert is discussed
       for more info).  Then construct Diffie Hellman parameters (see above where --dh  is  discussed  for  more
       info).   You  can  also  use  the  included test files client.crt, client.key, server.crt, server.key and
       ca.crt.  The .crt files are certificates/public-keys, the .key files are private keys, and  ca.crt  is  a
       certification authority who has signed both client.crt and server.crt.  For Diffie Hellman parameters you
       can use the  included  file  dh1024.pem.   Note  that  all  client,  server,  and  certificate  authority
       certificates  and  keys  included in the OpenVPN distribution are totally insecure and should be used for
       testing only.

       On bob:

              openvpn --remote alice.example.com --dev  tun1  --ifconfig  10.4.0.1  10.4.0.2  --tls-client  --ca
              ca.crt --cert client.crt --key client.key --reneg-sec 60 --verb 5

       On alice:

              openvpn  --remote  bob.example.com  --dev  tun1  --ifconfig  10.4.0.2  10.4.0.1  --tls-server --dh
              dh1024.pem --ca ca.crt --cert server.crt --key server.key --reneg-sec 60 --verb 5

       Now verify the tunnel is working by pinging across the tunnel.

       On bob:

              ping 10.4.0.2

       On alice:

              ping 10.4.0.1

       Notice the --reneg-sec 60 option we used above.  That tells OpenVPN to renegotiate the data channel  keys
       every minute.  Since we used --verb 5 above, you will see status information on each new key negotiation.

       For production operations, a key renegotiation interval of 60 seconds is probably too frequent.  Omit the
       --reneg-sec 60 option to use OpenVPN's default key renegotiation interval of one hour.

   Routing:
       Assuming you can ping across the tunnel, the next step is to route a real subnet over the secure  tunnel.
       Suppose that bob and alice have two network interfaces each, one connected to the internet, and the other
       to a private network.  Our goal is to securely connect both private networks.  We will assume that  bob's
       private subnet is 10.0.0.0/24 and alice's is 10.0.1.0/24.

       First, ensure that IP forwarding is enabled on both peers.  On Linux, enable routing:

              echo 1 > /proc/sys/net/ipv4/ip_forward

       and enable TUN packet forwarding through the firewall:

              iptables -A FORWARD -i tun+ -j ACCEPT

       On bob:

              route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2

       On alice:

              route add -net 10.0.0.0 netmask 255.255.255.0 gw 10.4.0.1

       Now  any  machine  on  the  10.0.0.0/24  subnet can access any machine on the 10.0.1.0/24 subnet over the
       secure tunnel (or vice versa).

       In a production environment, you could put the route command(s) in a script and  execute  with  the  --up
       option.

FIREWALLS

       OpenVPN's  usage of a single UDP port makes it fairly firewall-friendly.  You should add an entry to your
       firewall rules to allow incoming OpenVPN packets.  On Linux 2.4+:

              iptables -A INPUT -p udp -s 1.2.3.4 --dport 1194 -j ACCEPT

       This will allow incoming packets on UDP port 1194 (OpenVPN's default UDP port) from an  OpenVPN  peer  at
       1.2.3.4.

       If  you are using HMAC-based packet authentication (the default in any of OpenVPN's secure modes), having
       the firewall filter on source address can be considered optional, since HMAC packet authentication  is  a
       much more secure method of verifying the authenticity of a packet source.  In that case:

              iptables -A INPUT -p udp --dport 1194 -j ACCEPT

       would  be  adequate and would not render the host inflexible with respect to its peer having a dynamic IP
       address.

       OpenVPN also works well on stateful firewalls.  In some cases, you may not need to add any  static  rules
       to  the  firewall  list if you are using a stateful firewall that knows how to track UDP connections.  If
       you specify --ping n, OpenVPN will be guaranteed to send a packet to its  peer  at  least  once  every  n
       seconds.   If  n  is  less  than  the  stateful  firewall connection timeout, you can maintain an OpenVPN
       connection indefinitely without explicit firewall rules.

       You should also add firewall rules to allow incoming IP traffic on TUN or TAP devices such as:

              iptables -A INPUT -i tun+ -j ACCEPT

       to allow input packets from tun devices,

              iptables -A FORWARD -i tun+ -j ACCEPT

       to allow input packets from tun devices to be forwarded to other hosts on the local network,

              iptables -A INPUT -i tap+ -j ACCEPT

       to allow input packets from tap devices, and

              iptables -A FORWARD -i tap+ -j ACCEPT

       to allow input packets from tap devices to be forwarded to other hosts on the local network.

       These rules are secure if you use packet authentication, since no incoming packets will arrive on  a  TUN
       or TAP virtual device unless they first pass an HMAC authentication test.

FAQ

       http://openvpn.net/faq.html

HOWTO

       For  a  more  comprehensive guide to setting up OpenVPN in a production setting, see the OpenVPN HOWTO at
       http://openvpn.net/howto.html

PROTOCOL

       For a description of OpenVPN's underlying protocol, see http://openvpn.net/security.html

WEB

       OpenVPN's web site is at http://openvpn.net/

       Go here to download the latest version of OpenVPN, subscribe to the mailing lists, read the mailing  list
       archives, or browse the SVN repository.

BUGS

       Report all bugs to the OpenVPN team <info@openvpn.net>.

SEE ALSO

       dhcpcd(8), ifconfig(8), openssl(1), route(8), scp(1) ssh(1)

NOTES

       This product includes software developed by the OpenSSL Project ( http://www.openssl.org/ )

       For more information on the TLS protocol, see http://www.ietf.org/rfc/rfc2246.txt

       For      more      information      on      the     LZO     real-time     compression     library     see
       http://www.oberhumer.com/opensource/lzo/

       Copyright (C) 2002-2017 OpenVPN Technologies, Inc. This program is free software; you can redistribute it
       and/or  modify  it  under  the terms of the GNU General Public License version 2 as published by the Free
       Software Foundation.

AUTHORS

       James Yonan <jim@yonan.net>

                                                 25 August 2016                                       openvpn(8)