Provided by: openvpn_2.0.6-1_i386 bug

NAME

       openvpn - secure IP tunnel daemon.

SYNOPSIS

       openvpn [ --help ]

       openvpn [ --config file ]

       openvpn [ --genkey ] [ --secret file ]

       openvpn       [ --mktun ]       [ --rmtun ]       [ --dev tunX | tapX ]
           [ --dev-type device-type ] [ --dev-node node ]

       openvpn     [ --test-crypto ]     [ --secret file ]      [ --auth alg ]
           [ --cipher alg ]   [ --engine ]   [ --keysize n ]   [ --no-replay ]
           [ --no-iv ]

       openvpn [ --askpass [file] ]  [ --auth-nocache ]  [ --auth-retry type ]
           [ --auth-user-pass-verify script ]          [ --auth-user-pass up ]
           [ --auth alg ]         [ --bcast-buffers n ]          [ --ca file ]
           [ --ccd-exclusive ]  [ --cd dir ]  [ --cert file ] [ --chroot dir ]
           [ --cipher alg ]                     [ --client-cert-not-required ]
           [ --client-config-dir dir ]             [ --client-connect script ]
           [ --client-disconnect ]     [ --client-to-client ]     [ --client ]
           [ --comp-lzo ]         [ --comp-noadapt ]         [ --config file ]
           [ --connect-freq n sec ] [ --connect-retry n ] [ --crl-verify crl ]
           [ --cryptoapicert select-string ]           [ --daemon [progname] ]
           [ --dev-node node ]                      [ --dev-type device-type ]
           [ --dev tunX | tapX | null ]   [ --dev tunX | tapX ]  [ --dh file ]
           [ --dhcp-option type [parm] ]  [ --dhcp-release ]  [ --dhcp-renew ]
           [ --disable-occ ]   [ --disable ]   [ --down-pre ]   [ --down cmd ]
           [ --duplicate-cn ] [ --echo [parms...] ] [ --engine [engine-name] ]
           [ --explicit-exit-notify [n] ]       [ --fast-io ]      [ --float ]
           [ --fragment max ]          [ --genkey ]          [ --group group ]
           [ --hand-window n ]          [ --hash-size r v ]         [ --help ]
           [ --http-proxy-option type [parm] ]          [ --http-proxy-retry ]
           [ --http-proxy-timeout n ]
           [ --http-proxy server port [authfile] [auth-method] ]
           [ --ifconfig-noexec ]                         [ --ifconfig-nowarn ]
           [ --ifconfig-pool-linear ]
           [ --ifconfig-pool-persist file [seconds] ]
           [ --ifconfig-pool start-IP end-IP [netmask] ]
           [ --ifconfig-push local remote-netmask ]        [ --ifconfig l rn ]
           [ --inactive n ]               [ --inetd [wait|nowait] [progname] ]
           [ --ip-win32 method ]                            [ --ipchange cmd ]
           [ --iroute network [netmask] ]                  [ --keepalive n m ]
           [ --key-method m ]          [ --key file ]          [ --keysize n ]
           [ --learn-address cmd ]      [ --link-mtu n ]      [ --local host ]
           [ --log-append file ]    [ --log file ]   [ --suppress-timestamps ]
           [ --lport port ] [ --management-hold ] [ --management-log-cache n ]
           [ --management-query-passwords ] [ --management IP port [pw-file] ]
           [ --max-clients n ]    [ --max-routes-per-client n ]    [ --mktun ]
           [ --mlock ]   [ --mode m ]   [ --mssfix max ]   [ --mtu-disc type ]
           [ --mtu-test ] [ --mute-replay-warnings ] [ --mute n ] [ --nice n ]
           [ --no-iv ]               [ --no-replay ]              [ --nobind ]
           [ --ns-cert-type client|server ]   [ --passtos ]   [ --pause-exit ]
           [ --persist-key ]   [ --persist-local-ip ]  [ --persist-remote-ip ]
           [ --persist-tun ]      [ --ping-exit n ]       [ --ping-restart n ]
           [ --ping-timer-rem ]         [ --ping n ]         [ --pkcs12 file ]
           [ --plugin module-pathname init-string ]            [ --port port ]
           [ --proto p ]   [ --pull ]   [ --push-reset ]   [ --push "option" ]
           [ --rcvbuf size ]         [ --redirect-gateway ["local"] ["def1"] ]
           [ --remap-usr1 signal ]                         [ --remote-random ]
           [ --remote host [port] ]   [ --reneg-bytes n ]   [ --reneg-pkts n ]
           [ --reneg-sec n ]                         [ --replay-persist file ]
           [ --replay-window n [t] ]     [ --resolv-retry n ]      [ --rmtun ]
           [ --route-delay [n] [w] ]                    [ --route-gateway gw ]
           [ --route-method m ]     [ --route-noexec ]      [ --route-up cmd ]
           [ --route network [netmask] [gateway] [metric] ]   [ --rport port ]
           [ --secret file [direction] ]                     [ --secret file ]
           [ --server-bridge gateway netmask pool-start-IP pool-end-IP ]
           [ --server network netmask ]         [ --service exit-event [0|1] ]
           [ --setenv name value ]      [ --shaper n ]     [ --show-adapters ]
           [ --show-ciphers ]      [ --show-digests ]       [ --show-engines ]
           [ --show-net-up ]           [ --show-net ]           [ --show-tls ]
           [ --show-valid-subnets ]   [ --single-session ]   [ --sndbuf size ]
           [ --socks-proxy-retry ]             [ --socks-proxy server [port] ]
           [ --status file [n] ]                        [ --status-version n ]
           [ --syslog [progname] ]  [ --tap-sleep n ]  [ --tcp-queue-limit n ]
           [ --test-crypto ]                   [ --tls-auth file [direction] ]
           [ --tls-cipher l ]          [ --tls-client ]         [ --tls-exit ]
           [ --tls-remote x509name ]   [ --tls-server ]    [ --tls-timeout n ]
           [ --tls-verify cmd ]      [ --tmp-dir dir ]     [ --tran-window n ]
           [ --tun-ipv6 ]        [ --tun-mtu-extra n ]         [ --tun-mtu n ]
           [ --txqueuelen n ]   [ --up-delay ]  [ --up-restart ]  [ --up cmd ]
           [ --user user ]     [ --username-as-common-name ]      [ --verb n ]
           [ --writepid file ]

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  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, so the following mappings should be observed:

              \\       Maps to a single backslash character (\).
              \"       Pass a literal doublequote character ("), dont
                       interpret it as enclosing a parameter.
              \[SPACE] Pass a literal space or tab character, dont
                       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.  If specified, OpenVPN will bind
              to this address only.  If unspecified, OpenVPN will bind to  all
              interfaces.

       --remote host [port]
              Remote  host  name  or  IP  address.   On  the  client, multiple
              --remote options may be specified for redundancy, each referring
              to a different OpenVPN server.

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

              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,
              one  will  be  randomly  chosen, providing a sort of basic load-
              balancing and failover capability.

       --remote-random
              When multiple --remote address/ports  are  specified,  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.   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
              For --proto tcp-client, take n as the number of seconds to  wait
              between connection retries (default=5).

       --http-proxy server port [authfile] [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.

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

       --http-proxy-retry
              Retry indefinitely on HTTP proxy errors.  If an HTTP proxy error
              occurs, simulate a SIGUSR1 reset.

       --http-proxy-timeout n
              Set proxy timeout to n seconds, default=5.

       --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.

       --socks-proxy server [port]
              Connect to remote host through a Socks5 proxy at address  server
              and port port (default=1080).

       --socks-proxy-retry
              Retry  indefinitely  on  Socks  proxy  errors.  If a Socks proxy
              error occurs, simulate a SIGUSR1 reset.

       --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
              Execute   shell  command  cmd  when  our  remote  ip-address  is
              initially authenticated or changes.

              Execute as:

              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.

              Note that cmd can be a shell command with multiple arguments, in
              which  case  all OpenVPN-generated arguments will be appended to
              cmd to build a command line which will be passed to the  script.

              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  for  both  local and remote.  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
              TCP/UDP port number for local.

       --rport port
              TCP/UDP port number for remote.

       --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 protocols.

              tun devices  encapsulate  IPv4  while  tap  devices  encapsulate
              ethernet 802.3.

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

       --tun-ipv6
              Build  a tun link capable of forwarding IPv6 traffic.  Should be
              used in conjunction with --dev tun or  --dev  tunX.   A  warning
              will  be  displayed  if no specific IPv6 TUN support for your OS
              has been compiled into OpenVPN.

       --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.

              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.

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

              For TUN devices,  which  facilitate  virtual  point-to-point  IP
              connections,  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,  --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.

              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
              Specify a default gateway gw 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
              Execute  shell  command  cmd  after routes are added, subject to
              --route-delay.

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

              Note that cmd can be a shell command with multiple arguments.

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

       --redirect-gateway ["local"] ["def1"]
              (Experimental) Automatically execute routing commands  to  cause
              all outgoing IP traffic to be redirected over the VPN.

              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.

              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.

              Add the def1 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.

              Using  the  def1  flag  is  highly recommended, and is currently
              planned to become the default by OpenVPN 2.1.

       --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.

       --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 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  --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.  Currently defaults  to
              65536 bytes.

       --rcvbuf size
              Set  the TCP/UDP socket receive buffer size.  Currently defaults
              to 65536 bytes.

       --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.  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
              (Experimental)  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 tunnel packet.

       --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 n m
              A helper directive designed to simplify the expression of --ping
              and --ping-restart in server mode configurations.

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

               if mode server:
                 ping 10
                 ping-restart 120
                 push "ping 10"
                 push "ping-restart 60"
               else
                 ping 10
                 ping-restart 60

       --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
              Shell  command  to run after successful TUN/TAP device open (pre
              --user UID change).  The up  script  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 cmd can be a shell command with multiple arguments, in
              which  case  all OpenVPN-generated arguments will be appended to
              cmd to build a command line which will be passed to the shell.

              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.

              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 --upecho up--downecho 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
              Shell command to run after TUN/TAP device close (post --user UID
              change  and/or  --chroot ).  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.

       --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.

       --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 shell 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.

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

       --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.

       --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.

       --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 or 2 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.

       --comp-lzo
              Use fast LZO compression -- may add up to 1 byte per packet  for
              incompressible data.

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

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

       --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.

              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.

   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
              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

               if dev tun:
                 ifconfig 10.8.0.1 10.8.0.2
                 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
                   push "route 10.8.0.1"

               if dev tap:
                 ifconfig 10.8.0.1 255.255.255.0
                 ifconfig-pool 10.8.0.2 10.8.0.254 255.255.255.0
                 push "route-gateway 10.8.0.1"

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

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

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

              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"

       --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, --persist-key, --persist-tun, --echo

       --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.

       --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
              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.

       --ifconfig-push local remote-netmask
              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.

              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 script
              Run  script  on  client  connection.   The  script is passed the
              common name and IP address of the just-authenticated  client  as
              environmental  variables  (see  environmental  variable  section
              below).  The script is also passed the pathname  of  a  not-yet-
              created  temporary  file  as  $1  (i.e.  the  first command line
              argument),  to  be  used  by  the  script  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 $1.

              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
              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 script
              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.

       --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.

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

       --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 queued TCP output packets (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 queued TCP output
              packets reaches  this  limit  for  a  given  client  connection,
              OpenVPN  will  start  to  drop outgoing packets directed at this
              client.

       --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.

       --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 --tls-auth.

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

              cmd will be executed with 3 parameters:

              [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 script method
              Require  the  client to provide a username/password (possibly in
              addition to a client certificate) for authentication.

              OpenVPN will execute script as a shell command to  validate  the
              username/password provided by the client.

              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.

       --client-cert-not-required
              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.

              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.

              If you don’t use this directive, 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.

   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.

       --auth-user-pass [up]
              Authenticate with server using username/password.  up is a  file
              containing username/password on 2 lines (Note: OpenVPN will only
              read passwords from a  file  if  it  has  been  built  with  the
              --enable-password-save   configure  option,  or  on  Windows  by
              defining ENABLE_PASSWORD_SAVE in config-win32.h).

              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.

       --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  retries that the client will
              attempt to resend the exit notification message.

   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.

       --auth alg
              Authenticate 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.

              OpenVPN’s  usage of HMAC is to first encrypt a packet, then HMAC
              the resulting ciphertext.

              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 packets with cipher algorithm alg.  The default  is  BF-
              CBC, an abbreviation for Blowfish in Cipher Block Chaining mode.
              Blowfish has the advantages of  being  fast,  very  secure,  and
              allowing  key  sizes of up to 448 bits.  Blowfish is designed to
              be used in situations where keys are changed infrequently.

              For      more      information      on       blowfish,       see
              http://www.counterpane.com/blowfish.html

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

              OpenVPN supports the CBC, CFB, and OFB cipher modes.

              Set alg=none to disable encryption.

       --keysize n
              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.

       --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
              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 specifed, 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
              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.

       --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  distribution  contains  a set of scripts for managing RSA
       certificates & keys, located in the easy-rsa subdirectory.

       The  easy-rsa  package   is   also   rendered   in   web   form   here:
       http://openvpn.net/easyrsa.html

       --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 tmp-ca.key -out tmp-ca.crt

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

              For testing purposes only, the OpenVPN distribution  includes  a
              sample  CA certificate (tmp-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.

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

              openssl dhparam -out dh1024.pem 1024

              to  generate  your  own,  or  use  the  existing dh1024.pem file
              included  with  the  OpenVPN   distribution.    Diffie   Hellman
              parameters may be considered public.

       --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 ).

       --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).

       --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.

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

              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  (":").
              If  you  require  a  high level of security, you may want to set
              this parameter manually, to prevent a  version  rollback  attack
              where  a  man-in-the-middle attacker tries to force two peers to
              negotiate to the lowest level of  security  they  both  support.
              Use --show-tls to see a list of supported TLS ciphers.

       --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).  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.

       --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).

       --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 protect against DoS attacks.

              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 key  file  which  can  be  in  one  of  two
              formats:

              (1)  An  OpenVPN static key file generated by --genkey (required
              if direction parameter is used).

              (2) A freeform passphrase file.  In this case the HMAC key  will
              be  derived by taking a secure hash of this file, similar to the
              md5sum(1) or sha1sum(1) commands.

              OpenVPN will first try format (1), and  if  the  file  fails  to
              parse as a static key file, format (2) will be used.

              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 passphrase/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.

       --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 (Note: OpenVPN will only read passwords from a
              file if  it  has  been  built  with  the  --enable-password-save
              configure option, or on Windows by defining ENABLE_PASSWORD_SAVE
              in config-win32.h).

       --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.

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

       --tls-verify cmd
              Execute shell 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 is executed as

              cmd certificate_depth X509_NAME_oneline

              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.

              Note that cmd can be a shell command with multiple arguments, in
              which case all OpenVPN-generated arguments will be  appended  to
              cmd  to build a command line which will be passed to the script.

       --tls-remote name
              Accept connections only from a host with  X509  name  or  common
              name  equal  to  name.  The remote host must also pass all other
              tests of verification.

              Name can also be a common name prefix, for example if you want a
              client  to  only  accept  connections to "Server-1", "Server-2",
              etc., you can simply use --tls-remote Server

              Using a common 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.

              --tls-remote is a useful replacement for the --tls-verify option
              to verify the remote  host,  because  --tls-remote  works  in  a
              --chroot environment too.

       --ns-cert-type client|server
              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,  --tls-remote,  or
              --tls-verify.

       --crl-verify crl
              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.

   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 (TLS used only as a control
              channel).   The  TLS  ciphers  will  be  sorted   from   highest
              preference (most secure) to lowest.

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

   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

   Windows-Specific Options:
       --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] -- (Default) 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 0.
              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."

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

              ipapi (default) -- 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.   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 address.  Repeat this
              option to set secondary DNS server addresses.

              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.

       --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 the same caveats as --dhcp-renew above.

       --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.

       --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.

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.

       --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.

       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 2.0.1-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.

       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_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, 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.

       script_context
              Set to "init" or "restart" prior to  up/down  script  execution.
              For more information, see documentation for --up.

       script_type
              One of 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.

       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.

       tun_mtu
              The  MTU  of  the  TUN/TAP  device.  Set prior to --up or --down
              script execution.

       trusted_ip
              Actual IP address of connecting client or peer  which  has  been
              authenticated.   Set prior to execution of --ipchange, --client-
              connect, and --client-disconnect scripts.

       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
              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.

       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.

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

       If you have Linux 2.2 or earlier, you should obtain version 1.1 of  the
       TUN/TAP  driver  from  http://vtun.sourceforge.net/tun/  and follow the
       installation instructions.

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.

       If  you  have  Linux  2.2, you should obtain version 1.1 of the TUN/TAP
       driver   from   http://vtun.sourceforge.net/tun/   and    follow    the
       installation instructions.

       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 may.kg and
       june.kg.  If you are constructing a VPN over the internet, then replace
       may.kg  and  june.kg 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 may.kg will be
       10.4.0.1 and for june.kg, 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 may.kg
       and you wish to connect to june.kg via ssh without using the VPN (since
       ssh  has  its  own  built-in  security)  you  would use the command ssh
       june.kg.  However in the same scenario, you could also use the  command
       telnet  10.4.0.2  to create a telnet session with june.kg 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 may:

              openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2
              --verb 9

       On june:

              openvpn --remote may.kg --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 may:

              ping 10.4.0.2

       On june:

              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 may.

              openvpn --genkey --secret key

       This command will build a random key file called key (in ascii format).
       Now  copy  key to june over a secure medium such as by using the scp(1)
       program.

       On may:

              openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2
              --verb 5 --secret key

       On june:

              openvpn  --remote may.kg --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 may:

              ping 10.4.0.2

       On june:

              ping 10.4.0.1

   Example 3: A tunnel with full TLS-based security
       For this test, we will designate may as the TLS client and june as  the
       TLS  server.   Note  that client or server designation only has meaning
       for the TLS subsystem. It has no  bearing  on  OpenVPNs  peer-to-peer,
       UDP-based communication model.

       First, build a separate certificate/key pair for both may and june (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   tmp-ca.crt.    The   .crt   files  are
       certificates/public-keys, the .key files are  private  keys,  and  tmp-
       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 may:

              openvpn --remote june.kg --dev tun1 --ifconfig 10.4.0.1 10.4.0.2
              --tls-client --ca tmp-ca.crt --cert client.crt --key  client.key
              --reneg-sec 60 --verb 5

       On june:

              openvpn  --remote may.kg --dev tun1 --ifconfig 10.4.0.2 10.4.0.1
              --tls-server --dh dh1024.pem --ca tmp-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 may:

              ping 10.4.0.2

       On june:

              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 may and june 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 may’s private subnet is 10.0.0.0/24 and
       june’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 may:

              route add -net 10.0.1.0 netmask 255.255.255.0 gw 10.4.0.2

       On june:

              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
       shell 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 CVS
       repository.

BUGS

       Report   all   bugs   to   the    OpenVPN    users    list    <openvpn-
       users@lists.sourceforge.net>.   To  subscribe  to  the  list or see the
       archives, go to http://openvpn.net/mail.html

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

       Copyright  (C)  2002-2005  OpenVPN  Solutions LLC. 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>

                                 3 August 2005                      openvpn(8)