Ubuntu Manpage: openvpn - Secure IP tunnel daemon

Provided by: openvpn_2.5.11-0ubuntu0.22.04.1_amd64

NAME

       openvpn - Secure IP tunnel daemon

SYNOPSIS

       openvpn [ options ... ]
       openvpn  --help

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:
       https://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.

Generic Options
This section covers generic options which are accessible regardless of which mode OpenVPN
is configured as.

--help Show options.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

For examples of configuration files, see
https://openvpn.net/community-resources/how-to/

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

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

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

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

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

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

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

--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.

--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.

--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.

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

--ignore-unknown-option args
Valid syntax:

ignore-unknown-options opt1 opt2 opt3 ... optN

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

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

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

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

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

Valid syntax:

keying-material-exporter label len

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

--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.

The limit on how much memory can be locked and how that limit is enforced are
OS-dependent. On Linux the default limit that an unprivileged process may lock
(RLIMIT_MEMLOCK) is low, and if privileges are dropped later, future memory
allocations will very likely fail. The limit can be increased using ulimit or
systemd directives depending on how OpenVPN is started.

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

--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.

--providers providers
Load the list of (OpenSSL) providers. This is mainly useful for using an external
provider for key management like tpm2-openssl or to load the legacy provider with

--providers legacy default

Behaviour of changing this option between SIGHUP might not be well behaving. If
you need to change/add/remove this option, fully restart OpenVPN.

--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.

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

0 Strictly no calling of external programs.

1 (Default) Only call built-in executables such as ifconfig, ip, route, or
netsh.

2 Allow calling of built-in executables and user-defined scripts.

3 Allow passwords to be passed to scripts via environmental variables
(potentially unsafe).

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

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

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

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

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

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

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

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

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

--status args
Write operational status to file every n seconds.

Valid syntaxes:

status file
status file n

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

With multi-client capability enabled on a server, the status file includes a list
of clients and a routing table. The output format can be controlled by the
--status-version option in that case.

For clients or instances running in point-to-point mode, it will contain the
traffic statistics.

--status-version n
Set the status file format version number to n.

This only affects the status file on servers with multi-client capability enabled.
Valid status version values:

1 Traditional format (default). The client list contains the following fields
comma-separated: Common Name, Real Address, Bytes Received, Bytes Sent,
Connected Since.

2 A more reliable format for external processing. Compared to version 1, the
client list contains some additional fields: Virtual Address, Virtual IPv6
Address, Username, Client ID, Peer ID, Data Channel Cipher. Future versions
may extend the number of fields.

3 Identical to 2, but fields are tab-separated.

--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

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.

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

This directory will be used by in the following cases:

• --client-connect scripts and OPENVPN_PLUGIN_CLIENT_CONNECT plug-in hook to
dynamically generate client-specific configuration client_connect_config_file and
return success/failure via client_connect_deferred_file when using deferred
client connect method

• OPENVPN_PLUGIN_AUTH_USER_PASS_VERIFY plug-in hooks returns success/failure via
auth_control_file when using deferred auth method

• OPENVPN_PLUGIN_ENABLE_PF plugin hook to pass filtering rules via pf_file

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

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

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

--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).

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

Log options
--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.

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

--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
supersede 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.

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

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

--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.

--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.

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

--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 Outputs 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 in the source code for additional
information on debug levels).

Protocol options
Options in this section affect features available in the OpenVPN wire protocol. Many of
these options also define the encryption options of the data channel in the OpenVPN wire
protocol. These options must be configured in a compatible way between both the local and
remote side.

--allow-compression mode
As described in the --compress option, compression is a potentially dangerous
option. This option allows controlling the behaviour of OpenVPN when compression
is used and allowed.

Valid syntaxes:

allow-compression
allow-compression mode

The mode argument can be one of the following values:

asym (default)
OpenVPN will only decompress downlink packets but not compress uplink
packets. This also allows migrating to disable compression when changing
both server and client configurations to remove compression at the same time
is not a feasible option.

no OpenVPN will refuse any non-stub compression.

yes OpenVPN will send and receive compressed packets.

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

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

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

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

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

--cipher alg
This option is deprecated for server-client mode. --data-ciphers or possibly
--data-ciphers-fallback` should be used instead.

Encrypt data channel packets with cipher algorithm alg.

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

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

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

Set alg to none to disable encryption.

--compress algorithm
DEPRECATED Enable a compression algorithm. Compression is generally not
recommended. VPN tunnels which use compression are susceptible to the VORALCE
attack vector.

The algorithm parameter may be lzo, lz4, lz4-v2, stub, stub-v2 or empty. LZO and
LZ4 are different compression algorithms, with LZ4 generally offering the best
performance with least CPU usage.

The lz4-v2 and stub-v2 variants implement a better framing that does not add
overhead when packets cannot be compressed. All other variants always add one extra
framing byte compared to no compression framing.

If the algorithm parameter is stub, stub-v2 or empty, compression will be turned
off, but the packet framing for compression will still be enabled, allowing a
different setting to be pushed later. Additionally, stub and stub-v2 wil disable
announcing lzo and lz4 compression support via IV_ variables to the server.

Note: the stub (or empty) option is NOT compatible with the older option --comp-lzo
no.

*Security Considerations*

Compression and encryption is a tricky combination. If an attacker knows or is able
to control (parts of) the plain-text of packets that contain secrets, the attacker
might be able to extract the secret if compression is enabled. See e.g. the CRIME
and BREACH attacks on TLS and VORACLE on VPNs which also leverage to break
encryption. If you are not entirely sure that the above does not apply to your
traffic, you are advised to not enable compression.

--comp-lzo mode
DEPRECATED Enable LZO compression algorithm. Compression is generally not
recommended. VPN tunnels which uses compression are suspectible to the VORALCE
attack vector.

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

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

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

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

comp-lzo yes
push "comp-lzo yes"

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

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

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

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

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

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

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

For servers, the first cipher from cipher-list that is also supported by the client
will be pushed to clients that support cipher negotiation.

Starting with OpenVPN 2.5.9 a cipher can be prefixed with a ? to mark it as
optional. This allows including ciphers in the list that may not be available on
all platforms. E.g. AES-256-GCM:AES-128-GCM:?CHACHA20-POLY1305 would only enable
Chacha20-Poly1305 if the underlying SSL library (and its configuration) supports
it.

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

If no common cipher is found during cipher negotiation, the connection is
terminated. To support old clients/old servers that do not provide any cipher
negotiation support see --data-ciphers-fallback.

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

Note for using NCP with an OpenVPN 2.4 peer: This list must include the AES-256-GCM
and AES-128-GCM ciphers.

This list is restricted to be 127 chars long after conversion to OpenVPN ciphers.

This option was called --ncp-ciphers in OpenVPN 2.4 but has been renamed to
--data-ciphers in OpenVPN 2.5 to more accurately reflect its meaning.

--data-ciphers-fallback alg
Configure a cipher that is used to fall back to if we could not determine which
cipher the peer is willing to use.

This option should only be needed to connect to peers that are running OpenVPN 2.3
and older version, and have been configured with --enable-small (typically used on
routers or other embedded devices).

--ncp-disable
DEPRECATED Disable "Negotiable Crypto Parameters". This completely disables cipher
negotiation.

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

Valid syntaxes:

secret file
secret file direction

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.

--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.

Client Options
The client options are used when connecting to an OpenVPN server configured to use
--server, --server-bridge, or --mode server in its configuration.

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

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

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

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

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

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

push "auth-token UNIQUE_TOKEN_VALUE"

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

Newer clients (2.4.7+) will fall back to the original password method after a
failed auth. Older clients will keep using the token value and react according to
--auth-retry

--auth-token-user base64username
Companion option to --auth-token. This options allows to override the username used
by the client when reauthenticating with the auth-token. It also allows to use
--auth-token in setups that normally do not use username and password.

The username has to be base64 encoded.

--auth-user-pass
Authenticate with server using username/password.

Valid syntaxes:

auth-user-pass
auth-user-pass up

If up is present, it must be a file containing username/password on 2 lines. If the
password line is missing, OpenVPN will prompt for one.

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

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

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

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

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

type can be one of:

none Client will exit with a fatal error (this is the default).

nointeract
Client will retry the connection without requerying for an --auth-user-pass
username/password. Use this option for unattended clients.

interact
Client will requery for an --auth-user-pass username/password and/or private
key password before attempting a reconnection.

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

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

pull
tls-client

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

Examples:

client-nat snat 192.168.0.0/255.255.0.0
client-nat dnat 10.64.0.0/255.255.0.0

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

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

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

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

--connect-retry-max n
n specifies the number of times each --remote or <connection> entry is tried.
Specifying n as 1 would try each entry exactly once. A successful connection resets
the counter. (default unlimited).

--connect-timeout n
See --server-poll-timeout.

--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 if not present) controls the maximum number of attempts
that the client will try to resend the exit notification message.

In UDP server mode, send RESTART control channel command to connected clients. The
n parameter (default 1 if not present) controls client behavior. With n = 1 client
will attempt to reconnect to the same server, with n = 2 client will advance to the
next server.

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

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

Valid syntaxes:

inactive n
inactive n bytes

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

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

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

Note that this specifically only filters by the transport layer protocol, i.e. UDP
or TCP. This does not affect whether IPv4 or IPv6 is used as IP protocol.

For implementation reasons the option accepts the 4 and 6 suffixes when specifying
the protocol (i.e. udp4 / udp6 / tcp4 / tcp6). However, these behave the same as
without the suffix and should be avoided to prevent confusion.

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

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

--pull-filter args
Filter options on the client pushed by the server to the client.

Valid syntaxes:

pull-filter accept text
pull-filter ignore text
pull-filter reject text

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

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

pull-filter ignore "route"

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

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

would remove all routes that do not start with 192.168.1.

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

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

IV_VER=<version>
The client OpenVPN version

IV_LZO_STUB=1
If client was built with LZO stub capability

IV_LZ4=1
If the client supports LZ4 compressions.

IV_PROTO
Details about protocol extensions that the peer supports. The variable is a
bitfield and the bits are defined as follows (starting a bit 0 for the first
(unused) bit:

• bit 1: The peer supports peer-id floating mechanism

• bit 2: The client expects a push-reply and the server may send this reply
without waiting for a push-request first.

• bit 3: The client is capable of doing key derivation using RFC5705 key
material exporter.

• bit 4: The client is capable of accepting additional arguments to the
AUTH_PENDING message.

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

IV_CIPHERS=<ncp-ciphers>
The client announces the list of supported ciphers configured with the
--data-ciphers option to the server.

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

IV_SSO=[crtext,][openurl,][proxy_url]
Additional authentication methods supported by the client. This may be set
by the client UI/GUI using --setenv

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

IV_HWADDR=<string>
This is intended to be a unique and persistent ID of the client. The string
value can be any readable ASCII string up to 64 bytes. OpenVPN 2.x and some
other implementations use the MAC address of the client's interface used to
reach the default gateway. If this string is generated by the client, it
should be consistent and preserved across independent session and preferably
re-installations and upgrades.

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

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

UV_<name>=<value>
Client environment variables whose names start with UV_

--remote args
Remote host name or IP address, port and protocol.

Valid syntaxes:

remote host
remote host port
remote host port proto

The port and proto arguments are optional. The OpenVPN client will try to connect
to a server at host:port. The proto argument indicates the protocol to use when
connecting with the remote, and may be tcp or udp. To enforce IPv4 or IPv6
connections add a 4 or 6 suffix; like udp4 / udp6 / tcp4 / tcp6.

On the client, multiple --remote options may be specified for redundancy, each
referring to a different OpenVPN server, in the order specified by the list of
--remote options. Specifying multiple --remote options for this purpose is a
special case of the more general connection-profile feature. See the <connection>
documentation below.

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.

Examples:

remote server1.example.net
remote server1.example.net 1194
remote server2.example.net 1194 tcp

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

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

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

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

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

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

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

--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.

--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.

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

--static-challenge args
Enable static challenge/response protocol

Valid syntax:

static-challenge text echo

The text challenge text is presented to the user which describes what information
is requested. The echo flag indicates if the user's input should be echoed on the
screen. Valid echo values are 0 or 1.

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

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

--http-proxy args
Connect to remote host through an HTTP proxy. This requires at least an address
server and port argument. If HTTP Proxy-Authenticate is required, a file name to
an authfile file containing a username and password on 2 lines can be given, or
stdin to prompt from console. Its content can also be specified in the config file
with the --http-proxy-user-pass option. (See section on inline files)

The last optional argument is an auth-method which should be one of none, basic, or
ntlm.

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

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

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

Examples:

http-proxy proxy.example.net 3128
http-proxy proxy.example.net 3128 authfile.txt
http-proxy proxy.example.net 3128 stdin
http-proxy proxy.example.net 3128 auto basic
http-proxy proxy.example.net 3128 auto-nct ntlm

--http-proxy-option args
Set extended HTTP proxy options. Requires an option type as argument and an
optional parameter to the type. Repeat to set multiple options.

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

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

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

Examples:

http-proxy-option VERSION 1.1
http-proxy-option AGENT OpenVPN/2.4
http-proxy-option X-Proxy-Flag some-flags

--socks-proxy args
Connect to remote host through a Socks5 proxy. A required server argument is
needed. Optionally a port (default 1080) and authfile can be given. The authfile
is a file containing a username and password on 2 lines, or stdin can be used to
prompt from console.

Server Options
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.

--auth-gen-token args
Returns an authentication token to successfully authenticated clients.

Valid syntax:

auth-gen-token [lifetime] [external-auth]

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

The tokens implemented by this mechanism include an initial timestamp and a renew
timestamp and are secured by HMAC.

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

The token will expire either after the configured lifetime of the token is reached
or after not being renewed for more than 2 * reneg-sec seconds. Clients will be
sent renewed tokens on every TLS renogiation to keep the client's token updated.
This is done to invalidate a token if a client is disconnected for a sufficently
long time, while at the same time permitting much longer token lifetimes for active
clients.

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

When the external-auth keyword is present the normal authentication method will
always be called even if auth-token succeeds. Normally other authentications
method are skipped if auth-token verification suceeds or fails.

This option postpones this decision to the external authentication methods and
checks the validity of the account and do other checks.

In this mode the environment will have a session_id variable that holds the session
id from auth-gen-token. Also an environment variable session_state is present. This
variable indicates whether the auth-token has succeeded or not. It can have the
following values:

Initial
No token from client.

Authenticated
Token is valid and not expired.

Expired
Token is valid but has expired.

Invalid
Token is invalid (failed HMAC or wrong length)

AuthenticatedEmptyUser / ExpiredEmptyUser
The token is not valid with the username sent from the client but would be
valid (or expired) if we assume an empty username was used instead. These
two cases are a workaround for behaviour in OpenVPN 3. If this workaround
is not needed these two cases should be handled in the same way as Invalid.

Warning: Use this feature only if you want your authentication method called on
every verification. Since the external authentication is called it needs to also
indicate a success or failure of the authentication. It is strongly recommended to
return an authentication failure in the case of the Invalid/Expired auth-token with
the external-auth option unless the client could authenticate in another acceptable
way (e.g. client certificate), otherwise returning success will lead to
authentication bypass (as does returning success on a wrong password from a
script).

--auth-gen-token-secret file
Specifies a file that holds a secret for the HMAC used in --auth-gen-token If file
is not present OpenVPN will generate a random secret on startup. This file should
be used if auth-token should validate after restarting a server or if client should
be able to roam between multiple OpenVPN servers with their auth-token.

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

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

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

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

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

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

--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.

--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.

--connect-freq args
Allow a maximum of n new connections per sec seconds from clients.

Valid syntax:

connect-freq n sec

This is designed to contain DoS attacks which flood the server with connection
requests using certificates which will ultimately fail to authenticate.

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

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

--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.

--ifconfig-pool args
Set aside a pool of subnets to be dynamically allocated to connecting clients,
similar to a DHCP server.

Valid syntax:

ifconfig-pool start-IP end-IP [netmask]

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-ipv6-pool args
Specify an IPv6 address pool for dynamic assignment to clients.

Valid args:

ifconfig-ipv6-pool ipv6addr/bits

The pool starts at ipv6addr and matches the offset determined from the start of the
IPv4 pool. If the host part of the given IPv6 address is 0, the pool starts at
ipv6addr +1.

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

Valid syntax:

ifconfig-pool-persist file [seconds]

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-push args
Push virtual IP endpoints for client tunnel, overriding the --ifconfig-pool dynamic
allocation.

Valid syntax:

ifconfig-push local remote-netmask [alias]

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

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

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).

--ifconfig-ipv6-push args
for --client-config-dir per-client static IPv6 interface configuration, see
--client-config-dir and --ifconfig-push for more details.

Valid syntax:

ifconfig-ipv6-push ipv6addr/bits ipv6remote

--inetd args
Valid syntaxes:

inetd
inetd wait
inetd nowait
inetd wait progname

Use this option when OpenVPN is being run from the inetd or xinetd(8) server.

The wait and 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:
https://community.openvpn.net/openvpn/wiki/325-openvpn-as-a--forking-tcp-server-which-can-service-multiple-clients-over-a-single-tcp-port

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: https://openvpn.net/community-resources/1xhowto/

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

Notes:

• This option is only relevant for UDP servers.

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

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

Valid syntax:

iroute network [netmask]

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.

--iroute-ipv6 args
for --client-config-dir per-client static IPv6 route configuration, see --iroute
for more details how to setup and use this, and how --iroute and --route interact.

Valid syntax:

iroute-ipv6 ipv6addr/bits

--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.

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

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

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

--port-share args
Share OpenVPN TCP with another service

Valid syntax:

port-share host port [dir]

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

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

Not implemented on Windows.

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

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

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

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

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

NOTE 2: due to implementation details, 'ifconfig' and 'ifconfig-ipv6' can only be
removed with an exact match on the option ( push-remove ifconfig), no substring
matching and no matching on the IPv4/IPv6 address argument is possible.

--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.

NOTE: --push-reset is very thorough: it will remove almost all options from the
list of to-be-pushed options. In many cases, some of these options will need to be
re-configured afterwards - specifically, --topology subnet and --route-gateway will
get lost and this will break client configs in many cases. Thus, for most
purposes, --push-remove is better suited to selectively remove push options for
individual clients.

--server args
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. If the optional nopool flag is given, no dynamic IP address pool
will prepared for VPN clients.

Valid syntax:

server network netmask [nopool]

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

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

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

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

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

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

Valid syntaxes:

server-bridge gateway netmask pool-start-IP pool-end-IP
server-bridge [nogw]

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

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

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

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

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

mode server
tls-server

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

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

mode server
tls-server

push "route-gateway dhcp"

Or --server-bridge nogw expands as follows:

mode server
tls-server

--server-ipv6 args
Convenience-function to enable a number of IPv6 related options at once, namely
--ifconfig-ipv6, --ifconfig-ipv6-pool and --push tun-ipv6.

Valid syntax:

server-ipv6 ipv6addr/bits

Pushing of the --tun-ipv6 directive is done for older clients which require an
explicit --tun-ipv6 in their configuration.

--stale-routes-check args
Remove routes which haven't had activity for n seconds (i.e. the ageing time).
This check is run every t seconds (i.e. check interval).

Valid syntax:

stale-routes-check n [t]

If t is not present it defaults to n.

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

--username-as-common-name
Use the authenticated username as the common-name, rather than the common-name from
the client certificate. Requires that some form of --auth-user-pass verification is
in effect. As the replacement happens after --auth-user-pass verification, the
verification script or plugin will still receive the common-name from the
certificate.

The common_name environment variable passed to scripts and plugins invoked after
authentication (e.g, client-connect script) and file names parsed in client-config
directory will match the username.

--verify-client-cert mode
Specify whether the client is required to supply a valid certificate.

Possible mode options are:

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

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

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

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

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

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

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

--vlan-tagging
Server-only option. Turns the OpenVPN server instance into a switch that
understands VLAN-tagging, based on IEEE 802.1Q.

The server TAP device and each of the connecting clients is seen as a port of the
switch. All client ports are in untagged mode and the server TAP device is
VLAN-tagged, untagged or accepts both, depending on the --vlan-accept setting.

Ethernet frames with a prepended 802.1Q tag are called "tagged". If the VLAN
Identifier (VID) field in such a tag is non-zero, the frame is called
"VLAN-tagged". If the VID is zero, but the Priority Control Point (PCP) field is
non-zero, the frame is called "prio-tagged". If there is no 802.1Q tag, the frame
is "untagged".

Using the --vlan-pvid v option once per client (see --client-config-dir), each port
can be associated with a certain VID. Packets can only be forwarded between ports
having the same VID. Therefore, clients with differing VIDs are completely
separated from one-another, even if --client-to-client is activated.

The packet filtering takes place in the OpenVPN server. Clients should not have any
VLAN tagging configuration applied.

The --vlan-tagging option is off by default. While turned off, OpenVPN accepts any
Ethernet frame and does not perform any special processing for VLAN-tagged packets.

This option can only be activated in --dev tap mode.

--vlan-accept args
Configure the VLAN tagging policy for the server TAP device.

Valid syntax:

vlan-accept all|tagged|untagged

The following modes are available:

tagged Admit only VLAN-tagged frames. Only VLAN-tagged packets are accepted, while
untagged or priority-tagged packets are dropped when entering the server TAP
device.

untagged
Admit only untagged and prio-tagged frames. VLAN-tagged packets are not
accepted, while untagged or priority-tagged packets entering the server TAP
device are tagged with the value configured for the global --vlan-pvid
setting.

all (default)
Admit all frames. All packets are admitted and then treated like untagged
or tagged mode respectively.

Note: Some vendors refer to switch ports running in tagged mode as "trunk ports"
and switch ports running in untagged mode as "access ports".

Packets forwarded from clients to the server are VLAN-tagged with the originating
client's PVID, unless the VID matches the global --vlan-pvid, in which case the tag
is removed.

If no PVID is configured for a given client (see --vlan-pvid) packets are tagged
with 1 by default.

--vlan-pvid v
Specifies which VLAN identifier a "port" is associated with. Only valid when
--vlan-tagging is speficied.

In the client context, the setting specifies which VLAN ID a client is associated
with. In the global context, the VLAN ID of the server TAP device is set. The
latter only makes sense for --vlan-accept untagged and --vlan-accept all modes.

Valid values for v go from 1 through to 4094. The global value defaults to 1. If no
--vlan-pvid is specified in the client context, the global value is inherited.

In some switch implementations, the PVID is also referred to as "Native VLAN".

ENCRYPTION OPTIONS

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

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

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

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

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

--show-groups
(Standalone) Show all available elliptic curves/groups to use with the --ecdh-curve
and tls-groups options.

Generating key material
--genkey args
(Standalone) Generate a key to be used of the type keytype. if keyfile is left out
or empty the key will be output on stdout. See the following sections for the
different keytypes.

Valid syntax:

--genkey keytype keyfile

Valid keytype arguments are:

secret Standard OpenVPN shared secret keys

tls-crypt Alias for secret

tls-auth Alias for secret

auth-token Key used for --auth-gen-token-key

tls-crypt-v2-server TLS Crypt v2 server key

tls-crypt-v2-client TLS Crypt v2 client key

Examples:

$ openvpn --genkey secret shared.key
$ openvpn --genkey tls-crypt shared.key
$ openvpn --genkey tls-auth shared.key
$ openvpn --genkey tls-crypt-v2-server v2crypt-server.key
$ openvpn --tls-crypt-v2 v2crypt-server.key --genkey tls-crypt-v2-client v2crypt-client-1.key

• Generating Shared Secret Keys Generate a shared secret, for use with the
--secret, --tls-auth or --tls-crypt options.

Syntax:

$ openvpn --genkey secret|tls-crypt|tls-auth keyfile

The key is saved in keyfile. All three variants (--secret, tls-crypt and
tls-auth) generate the same type of key. The aliases are added for convenience.

If using this for --secret, this file must be shared with the peer over a
pre-existing secure channel such as scp(1).

• Generating TLS Crypt v2 Server key Generate a --tls-crypt-v2 key to be used by an
OpenVPN server. The key is stored in keyfile.

Syntax:

--genkey tls-crypt-v2-server keyfile

• Generating TLS Crypt v2 Client key Generate a --tls-crypt-v2 key to be used by
OpenVPN clients. The key is stored in keyfile.

Syntax

--genkey tls-crypt-v2-client keyfile [metadata]

If supplied, include the supplied metadata in the wrapped client key. This
metadata must be supplied in base64-encoded form. The metadata must be at most
735 bytes long (980 bytes in base64).

If no metadata is supplied, OpenVPN will use a 64-bit unix timestamp representing
the current time in UTC, encoded in network order, as metadata for the generated
key.

A tls-crypt-v2 client key is wrapped using a server key. To generate a client
key, the user must therefore supply the server key using the --tls-crypt-v2
option.

Servers can use --tls-crypt-v2-verify to specify a metadata verification command.

• Generate Authentication Token key Generate a new secret that can be used with
--auth-gen-token-secret

Syntax:

--genkey auth-token [keyfile]

Note: This file should be kept secret to the server as anyone that has access to
this file will be able to generate auth tokens that the OpenVPN server
will accept as valid.

Data Channel Renegotiation
When running OpenVPN in client/server mode, the data channel will use a separate ephemeral
encryption key which is rotated at regular intervals.

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

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

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

--reneg-sec args
Renegotiate data channel key after at most max seconds (default 3600) and at least
min seconds (default is 90% of max for servers, and equal to max for clients).

reneg-sec max [min]

The effective --reneg-sec value used is per session pseudo-uniform-randomized
between min and max.

With the default value of 3600 this results in an effective per session value in
the range of 3240 .. 3600 seconds for servers, or just 3600 for clients.

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

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

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

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

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

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

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

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

Valid syntaxes:

askpass
askpass file

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.

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

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

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

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

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

CAs in the capath directory are expected to be named <hash>.<n>. CRLs are expected
to be named <hash>.r<n>. See the -CApath option of openssl verify, and the -hash
option of openssl x509, openssl crl and X509_LOOKUP_hash_dir()(3) for more
information.

Similar to the --crl-verify option, CRLs are not mandatory - OpenVPN will log the
usual warning in the logs if the relevant CRL is missing, but the connection will
be allowed.

--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).

--crl-verify args
Check peer certificate against a Certificate Revocation List.

Valid syntax:

crl-verify file/directory flag

Examples:

crl-verify crl-file.pem
crl-verify /etc/openvpn/crls dir

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.

The option is not mandatory - if the relevant CRL is missing, OpenVPN will log a
warning in the logs - e.g.

VERIFY WARNING: depth=0, unable to get certificate CRL

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

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

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

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

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

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

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

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

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

--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.

--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. Not
available with mbed TLS.

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

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

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

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

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

If key-usage is a list of usage bits, the keyUsage field must have at least the
same bits set as the bits in one of the values supplied in the key-usage list.

The key-usage values in the list must be encoded in hex, e.g.

remote-cert-ku a0

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

Valid syntaxes:

remote-cert-tls server
remote-cert-tls client

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

The --remote-cert-tls client option is equivalent to

remote-cert-ku
remote-cert-eku "TLS Web Client Authentication"

The --remote-cert-tls server option is equivalent to

remote-cert-ku
remote-cert-eku "TLS Web Server Authentication"

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

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

Valid syntaxes:

tls-auth file
tls-auth file 0
tls-auth file 1

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

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

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

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

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

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

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

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

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

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

--tls-groups list
A list of allowable groups/curves in order of preference.

Set the allowed elliptic curves/groups for the TLS session. These groups are
allowed to be used in signatures and key exchange.

mbedTLS currently allows all known curves per default.

OpenSSL 1.1+ restricts the list per default to

"X25519:secp256r1:X448:secp521r1:secp384r1".

If you use certificates that use non-standard curves, you might need to add them
here. If you do not force the ecdh curve by using --ecdh-curve, the groups for ecdh
will also be picked from this list.

OpenVPN maps the curve name secp256r1 to prime256v1 to allow specifying the same
tls-groups option for mbedTLS and OpenSSL.

Warning: this option not only affects elliptic curve certificates but also the key
exchange in TLS 1.3 and using this option improperly will disable TLS 1.3.

--tls-cert-profile profile
Set the allowed cryptographic algorithms for certificates according to profile.

The following profiles are supported:

insecure
Identical for mbed TLS to legacy

legacy (default)
SHA1 and newer, RSA 2048-bit+, any elliptic curve.

preferred
SHA2 and newer, RSA 2048-bit+, any elliptic curve.

suiteb SHA256/SHA384, ECDSA with P-256 or P-384.

This option is only fully supported for mbed TLS builds. OpenSSL builds use the
following approximation:

insecure
sets "security level 0"

legacy (default)
sets "security level 1"

preferred
sets "security level 2"

suiteb sets "security level 3" and --tls-cipher "SUITEB128".

OpenVPN will migrate to 'preferred' as default in the future. Please ensure that
your keys already comply.

WARNING: --tls-ciphers, --tls-ciphersuites and tls-groups
These options are expert features, which - if used correctly - can improve the
security of your VPN connection. But it is also easy to unwittingly use them to
carefully align a gun with your foot, or just break your connection. Use with care!

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

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

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

For OpenSSL, the --tls-cipher is used for TLS 1.2 and below.

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

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

--tls-ciphersuites l
Same as --tls-cipher but for TLS 1.3 and up. mbed TLS has no TLS 1.3 support yet
and only the --tls-cipher setting is used.

The default for --tls-ciphersuites is to use the crypto library's default.

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

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

Encrypting (and authenticating) control channel packets:

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

• makes it harder to identify OpenVPN traffic as such,

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

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

Security Considerations

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

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

For large setups or setups where clients are not trusted, consider using
--tls-crypt-v2 instead. That uses per-client unique keys, and thereby improves the
bounds to 'rotate a client key at least once per 8000 years'.

--tls-crypt-v2 keyfile
Use client-specific tls-crypt keys.

For clients, keyfile is a client-specific tls-crypt key. Such a key can be
generated using the --genkey tls-crypt-v2-client option.

For servers, keyfile is used to unwrap client-specific keys supplied by the client
during connection setup. This key must be the same as the key used to generate the
client-specific key (see --genkey tls-crypt-v2-client).

On servers, this option can be used together with the --tls-auth or --tls-crypt
option. In that case, the server will detect whether the client is using
client-specific keys, and automatically select the right mode.

--tls-crypt-v2-verify cmd
Run command cmd to verify the metadata of the client-specific tls-crypt-v2 key of a
connecting client. This allows server administrators to reject client connections,
before exposing the TLS stack (including the notoriously dangerous X.509 and ASN.1
stacks) to the connecting client.

OpenVPN supplies the following environment variables to the command:

• script_type is set to tls-crypt-v2-verify

• metadata_type is set to 0 if the metadata was user supplied, or 1 if it's a
64-bit unix timestamp representing the key creation time.

• metadata_file contains the filename of a temporary file that contains the client
metadata.

The command can reject the connection by exiting with a non-zero exit code.

--tls-exit
Exit on TLS negotiation failure.

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

--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-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.

--tls-version-min args
Sets the minimum TLS version we will accept from the peer (default is "1.0").

Valid syntax:

tls-version-min version ['or-highest']

Examples for version include 1.0, 1.1, or 1.2. If or-highest is specified and
version is not recognized, we will only accept the highest TLS version supported by
the local SSL implementation.

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

--verify-hash args
Specify SHA1 or SHA256 fingerprint for level-1 cert.

Valid syntax:

verify-hash hash [algo]

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

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

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

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

Valid syntax:

verify-x509 name type

Which X.509 name is compared to name depends on the setting of type. type can be
subject to match the complete subject DN (default), name to match a subject RDN or
name-prefix to match a subject RDN prefix. Which RDN is verified as name depends on
the --x509-username-field option. But it defaults to the common name (CN), e.g. a
certificate with a subject DN

C=KG, ST=NA, L=Bishkek, CN=Server-1

would be matched by:

verify-x509-name 'C=KG, ST=NA, L=Bishkek, CN=Server-1'
verify-x509-name Server-1 name
verify-x509-name Server- name-prefix

The last example is useful if you want a client to only accept connections to
Server-1, Server-2, etc.

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

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

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

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

--x509-username-field args
Field in the X.509 certificate subject to be used as the username (default CN).

Valid syntax:

x509-username-field [ext:]fieldname

Typically, this option is specified with fieldname as either of the following:

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

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

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

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

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

PKCS#11 / SmartCard options
--pkcs11-cert-private args
Set if access to certificate object should be performed after login. Every
provider has its own setting.

Valid syntaxes:

pkcs11-cert-private 0
pkcs11-cert-private 1

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

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

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

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

0 (default) Try to determine automatically.

1 Use sign.

2 Use sign recover.

4 Use decrypt.

8 Use unwrap.

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

Valid syntaxes:

pkcs11-protected-authentication 0
pkcs11-protected-authentication 1

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

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

--show-pkcs11-ids args
(Standalone) Show PKCS#11 token object list.

Valid syntax:

show-pkcs11 [provider] [cert_private]

Specify cert_private as 1 if certificates are stored as private objects.

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

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

DATA CHANNEL CIPHER NEGOTIATION

OpenVPN 2.4 and higher have the capability to negotiate the data cipher that is used to
encrypt data packets. This section describes the mechanism in more detail and the
different backwards compatibility mechanism with older server and clients.

OpenVPN 2.5 and higher behaviour
When both client and server are at least running OpenVPN 2.5, that the order of the
ciphers of the server's --data-ciphers is used to pick the the data cipher. That means
that the first cipher in that list that is also in the client's --data-ciphers list is
chosen. If no common cipher is found the client is rejected with a AUTH_FAILED message (as
seen in client log):
AUTH: Received control message: AUTH_FAILED,Data channel cipher negotiation failed (no
shared cipher)

OpenVPN 2.5 will only allow the ciphers specified in --data-ciphers. To ensure backwards
compatibility also if a cipher is specified using the --cipher option it is automatically
added to this list. If both options are unset the default is AES-256-GCM:AES-128-GCM.

OpenVPN 2.4 clients
The negotiation support in OpenVPN 2.4 was the first iteration of the implementation and
still had some quirks. Its main goal was "upgrade to AES-256-GCM when possible". An
OpenVPN 2.4 client that is built against a crypto library that supports AES in GCM mode
and does not have --ncp-disable will always announce support for AES-256-GCM and
AES-128-GCM to a server by sending IV_NCP=2.

This only causes a problem if --ncp-ciphers option has been changed from the default of
AES-256-GCM:AES-128-GCM to a value that does not include these two ciphers. When a OpenVPN
servers try to use AES-256-GCM or AES-128-GCM the connection will then fail. It is
therefore recommended to always have the AES-256-GCM and AES-128-GCM ciphers to the
--ncp-ciphers options to avoid this behaviour.

OpenVPN 3 clients
Clients based on the OpenVPN 3.x library (https://github.com/openvpn/openvpn3/) do not
have a configurable --ncp-ciphers or --data-ciphers option. Instead these clients will
announce support for all their supported AEAD ciphers (AES-256-GCM, AES-128-GCM and in
newer versions also Chacha20-Poly1305).

To support OpenVPN 3.x based clients at least one of these ciphers needs to be included in
the server's --data-ciphers option.

OpenVPN 2.3 and older clients (and clients with --ncp-disable)
When a client without cipher negotiation support connects to a server the cipher specified
with the --cipher option in the client configuration must be included in the
--data-ciphers option of the server to allow the client to connect. Otherwise the client
will be sent the AUTH_FAILED message that indicates no shared cipher.

If the client is 2.3 or older and has been configured with the --enable-small ./configure
argument, using data-ciphers-fallback cipher in the server config file with the explicit
cipher used by the client is necessary.

OpenVPN 2.4 server
When a client indicates support for AES-128-GCM and AES-256-GCM (with IV_NCP=2) an OpenVPN
2.4 server will send the first cipher of the --ncp-ciphers to the OpenVPN client
regardless of what the cipher is. To emulate the behaviour of an OpenVPN 2.4 client as
close as possible and have compatibility to a setup that depends on this quirk, adding
AES-128-GCM and AES-256-GCM to the client's --data-ciphers option is required. OpenVPN
2.5+ will only announce the IV_NCP=2 flag if those ciphers are present.

OpenVPN 2.3 and older servers (and servers with --ncp-disable)
The cipher used by the server must be included in --data-ciphers to allow the client
connecting to a server without cipher negotiation support. (For compatibility OpenVPN 2.5
will also accept the cipher set with --cipher)

If the server is 2.3 or older and has been configured with the --enable-small ./configure
argument, adding data-ciphers-fallback cipher to the client config with the explicit
cipher used by the server is necessary.

Blowfish in CBC mode (BF-CBC) deprecation
The --cipher option defaulted to BF-CBC in OpenVPN 2.4 and older version. The default was
never changed to ensure backwards compatibility. In OpenVPN 2.5 this behaviour has now
been changed so that if the --cipher is not explicitly set it does not allow the weak
BF-CBC cipher any more and needs to explicitly added as --cipher BFC-CBC or added to
--data-ciphers.

We strongly recommend to switching away from BF-CBC to a more secure cipher as soon as
possible instead.

NETWORK CONFIGURATION

OpenVPN consists of two sides of network configuration. One side is the link between the
local and remote side, the other side is the virtual network adapter (tun/tap device).

Link Options
This link options section covers options related to the connection between the local and
the remote host.

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

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

--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.

--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.

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

Valid syntax:

keepalive interval timeout

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

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

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

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

--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.

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

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

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

--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.

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

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

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

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

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

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

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

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

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

--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.

Valid types:

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.

--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.

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

--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
https://www.nsupdate.info/ + 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.

--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.

--proto p
Use protocol p for communicating with remote host. p can be udp, tcp-client, or
tcp-server. You can also limit OpenVPN to use only IPv4 or only IPv6 by specifying
p as udp4, tcp4-client, tcp4-server or udp6, tcp6-client, tcp6-server,
respectively.

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

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

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

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

This article outlines some of problems with tunneling IP over TCP:
http://sites.inka.de/sites/bigred/devel/tcp-tcp.html

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

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

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

--replay-window args
Modify the replay protection sliding-window size and time window.

Valid syntax:

replay-window n [t]

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

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

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

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

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

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

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

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

If you run OpenVPN at --verb 4, you will see the message "PID_ERR 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.

--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.

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

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

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

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

The macro expands as follows:

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

Virtual Network Adapter (VPN interface)
Options in this section relates to configuration of the virtual tun/tap network interface,
including setting the VPN IP address and network routing.

--bind-dev device
(Linux only) Set device to bind the server socket to a Virtual Routing and
Forwarding device

--block-ipv6
On the client, instead of sending IPv6 packets over the VPN tunnel, all IPv6
packets are answered with an ICMPv6 no route host message. On the server, all IPv6
packets from clients are answered with an ICMPv6 no route to host message. This
options is intended for cases when IPv6 should be blocked and other options are not
available. --block-ipv6 will use the remote IPv6 as source address of the ICMPv6
packets if set, otherwise will use fe80::7 as source address.

For this option to make sense you actually have to route traffic to the tun
interface. The following example config block would send all IPv6 traffic to
OpenVPN and answer all requests with no route to host, effectively blocking IPv6
(to avoid IPv6 connections from dual-stacked clients leaking around IPv4-only VPN
services).

Client config

--ifconfig-ipv6 fd15:53b6:dead::2/64 fd15:53b6:dead::1
--redirect-gateway ipv6
--block-ipv6

Server config
Push a "valid" ipv6 config to the client and block on the server

--push "ifconfig-ipv6 fd15:53b6:dead::2/64 fd15:53b6:dead::1"
--push "redirect-gateway ipv6"
--block-ipv6

Note: this option does not influence traffic sent from the server towards the
client (neither on the server nor on the client side). This is not seen as
necessary, as such traffic can be most easily avoided by not configuring IPv6 on
the server tun, or setting up a server-side firewall rule.

--dev device
TUN/TAP virtual network device which can be tunX, tapX, null or an arbitrary name
string (X can be omitted for a dynamic device.)

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

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

tun devices encapsulate IPv4 or IPv6 (OSI Layer 3)

tap devices encapsulate Ethernet 802.3 (OSI Layer 2).

Valid syntaxes:

dev tun2
dev tap4
dev ovpn

When the device name starts with tun or tap, the device type is extracted
automatically. Otherwise the --dev-type option needs to be added as well.

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

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

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

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

--dhcp-option args
Set additional network parameters on supported platforms. May be specified on the
client or pushed from the server. On Windows these options are handled by the
tap-windows6 driver by default or directly by OpenVPN if dhcp is disabled or the
wintun driver is in use. The OpenVPN for Android client also handles them
internally.

On all other platforms these options are only saved in the client's environment
under the name foreign_option_{n} before the --up script is called. A plugin or an
--up script must be used to pick up and interpret these as required. Many Linux
distributions include such scripts and some third-party user interfaces such as
tunnelblick also come with scripts that process these options.

Valid syntax:

dhcp-option type [parm]

DOMAIN name
Set Connection-specific DNS Suffix to name.

ADAPTER_DOMAIN_SUFFIX name
Alias to DOMAIN. This is a compatibility option, it should not be used in
new deployments.

DOMAIN-SEARCH name
Add name to the domain search list. Repeat this option to add more entries.
Up to 10 domains are supported.

DNS address
Set primary domain name server IPv4 or IPv6 address. Repeat this option to
set secondary DNS server addresses.

Note: DNS IPv6 servers are currently set using netsh (the existing DHCP code
can only do IPv4 DHCP, and that protocol only permits IPv4 addresses
anywhere). The option will be put into the environment, so an --up script
could act upon it if needed.

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

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

NTP address
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)

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.

--ifconfig args
Set TUN/TAP adapter parameters. It requires the IP address of the local VPN
endpoint. For TUN devices in point-to-point mode, the next argument must be the VPN
IP address of the remote VPN endpoint. For TAP devices, or TUN devices used with
--topology subnet, the second argument is the subnet mask of the virtual network
segment which is being created or connected to.

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

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

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

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

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

Examples:

# tun device in net30/p2p mode
ifconfig 10.8.0.2 10.8.0.1

# tun/tap device in subnet mode
ifconfig 10.8.0.2 255.255.255.0

--ifconfig-ipv6 args
Configure an IPv6 address on the tun device.

Valid syntax:

ifconfig-ipv6 ipv6addr/bits [ipv6remote]

The ipv6addr/bits argument is the IPv6 address to use. The second parameter is used
as route target for --route-ipv6 if no gateway is specified.

The --topology option has no influence with --ifconfig-ipv6

--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.

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

--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.

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

This option performs three steps:

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

2. Delete the default gateway route.

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

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

Option flags:

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

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

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

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

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

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

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

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

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

--route args
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.

Valid syntaxes:

route network/IP
route network/IP netmask
route network/IP netmask gateway
route network/IP netmask gateway metric

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
defaults to 255.255.255.255 when not given

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

metric default taken from --route-metric if set, otherwise 0.

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

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

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

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

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

--route-delay args
Valid syntaxes:

route-delay
route-delay n
route-delay n m

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
(default 30 by default) for the TAP-Win32 adapter to come up before adding routes.

--route-ipv6 args
Setup IPv6 routing in the system to send the specified IPv6 network into OpenVPN's
tun.

Valid syntax:

route-ipv6 ipv6addr/bits [gateway] [metric]

The gateway parameter is only used for IPv6 routes across tap devices, and if
missing, the ipv6remote field from --ifconfig-ipv6 or --route-ipv6-gateway is used.

--route-gateway arg
Specify a default gateway for use with --route.

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

Valid syntaxes:

route-gateway gateway
route-gateway dhcp

--route-ipv6-gateway gw
Specify a default gateway gw for use with --route-ipv6.

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

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

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

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

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

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

mode can be one of:

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

p2p Use a point-to-point topology where the remote endpoint of the client's tun
interface always points to the local endpoint of the server's tun interface.
This mode allocates a single IP address per connecting client. Only use when
none of the connecting clients are Windows systems.

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

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

--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 and 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.

TUN/TAP standalone operations
These two standalone operations will require --dev and optionally --user and/or --group.

--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.

Virtual Routing and Forwarding
Options in this section relates to configuration of virtual routing and forwarding in
combination with the underlying operating system.

As of today this is only supported on Linux, a kernel >= 4.9 is recommended.

This could come in handy when for example the external network should be only used as a
means to connect to some VPN endpoints and all regular traffic should only be routed
through any tunnel(s). This could be achieved by setting up a VRF and configuring the
interface connected to the external network to be part of the VRF. The examples below will
cover this setup.

Another option would be to put the tun/tap interface into a VRF. This could be done by an
up-script which uses the ip link set command shown below.

VRF setup with iproute2
Create VRF vrf_external and map it to routing table 1023

ip link add vrf_external type vrf table 1023

Move eth0 into vrf_external

ip link set master vrf_external dev eth0

Any prefixes configured on eth0 will be moved from the :code`main` routing table into
routing table 1023

VRF setup with ifupdown
For Debian based Distributions ifupdown2 provides an almost drop-in replacement for
ifupdown including VRFs and other features. A configuration for an interface eth0 being
part of VRF code:vrf_external could look like this:

auto eth0
iface eth0
address 192.0.2.42/24
address 2001:db8:08:15::42/64
gateway 192.0.2.1
gateway 2001:db8:08:15::1
vrf vrf_external

auto vrf_external
iface vrf_external
vrf-table 1023

OpenVPN configuration
The OpenVPN configuration needs to contain this line:

bind-dev vrf_external

Further reading
Wikipedia has nice page one VRFs:
https://en.wikipedia.org/wiki/Virtual_routing_and_forwarding

This talk from the Network Track of FrOSCon 2018 provides an overview about advanced layer
2 and layer 3 features of Linux

• Slides:
https://www.slideshare.net/BarbarossaTM/l2l3-fr-fortgeschrittene-helle-und-dunkle-magie-im-linuxnetzwerkstack

• Video (german):
https://media.ccc.de/v/froscon2018-2247-l2_l3_fur_fortgeschrittene_-_helle_und_dunkle_magie_im_linux-netzwerkstack

SCRIPTING INTEGRATION

OpenVPN can execute external scripts in various phases of the lifetime of the OpenVPN
process.

Script Order of Execution
1. --up

Executed after TCP/UDP socket bind and TUN/TAP open.

2. --tls-verify

Executed when we have a still untrusted remote peer.

3. --ipchange

Executed after connection authentication, or remote IP address change.

4. --client-connect

Executed in --mode server mode immediately after client authentication.

5. --route-up

Executed after connection authentication, either immediately after, or some number of
seconds after as defined by the --route-delay option.

6. --route-pre-down

Executed right before the routes are removed.

7. --client-disconnect

Executed in --mode server mode on client instance shutdown.

8. --down

Executed after TCP/UDP and TUN/TAP close.

9. --learn-address

Executed in --mode server mode whenever an IPv4 address/route or MAC address is added
to OpenVPN's internal routing table.

10. --auth-user-pass-verify

Executed in --mode server mode on new client connections, when the client is still
untrusted.

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

Valid syntax:

auth-user-pass-verify cmd method

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

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

If method is set to via-env, OpenVPN will call script with the environmental
variables username and password set to the username/password strings provided by
the client. Beware 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-connect cmd
Run command cmd on client connection.

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

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

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

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

If a --client-connect wants to defer the generating of the configuration then the
script needs to use the client_connect_deferred_file and client_connect_config_file
environment variables, and write status accordingly into these files. See the
Environmental Variables section for more details.

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

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

The --client-disconnect command is not passed any extra arguments (only those
arguments specified in cmd, if any).

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

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

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

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

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

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

cmd ip address port number

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

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

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

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

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

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

$1 - [operation]
"add", "update", or "delete" based on whether or not the address is being
added to, modified, or deleted from OpenVPN's internal routing table.

$2 - [address]
The address being learned or unlearned. This can be an IPv4 address such as
"198.162.10.14", an IPv4 subnet such as "198.162.10.0/24", or an ethernet
MAC address (when --dev tap is being used) such as "00:FF:01:02:03:04".

$3 - [common name]
The common name on the certificate associated with the client linked to this
address. Only present for "add" or "update" operations, not "delete".

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

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

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

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

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

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

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

Valid syntaxes:

setenv name value
setenv FORWARD_COMPATIBLE 1
setenv opt config_option

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

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

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

UNSUPPORTED OPTIONS

       Options listed in this section have been removed from OpenVPN and are no longer supported

       --client-cert-not-required
              Removed in OpenVPN 2.5.  This should be replaxed with --verify-client-cert none.

       --ifconfig-pool-linear
              Removed in OpenVPN 2.5.  This should be replaced with --topology p2p.

       --key-method
              Removed in OpenVPN 2.5.   This  option  should  not  be  used,  as  using  the  old
              key-method  weakens  the  VPN  tunnel  security.   The old key-method was also only
              needed when the remote side was older than OpenVPN 2.0.

       --no-iv
              Removed in OpenVPN 2.5.  This option should not be  used  as  it  weakens  the  VPN
              tunnel security.  This has been a NOOP option since OpenVPN 2.4.

       --no-replay
              Removed  in  OpenVPN  2.5.   This  option  should not be used as it weakens the VPN
              tunnel security.

       --ns-cert-type
              Removed in OpenVPN 2.5.  The nsCertType field is  no  longer  supported  in  recent
              SSL/TLS  libraries.   If  your certificates does not include key usage and extended
              key usage fields, they must be upgraded and the --remote-cert-tls option should  be
              used instead.

CONNECTION PROFILES

Client configuration files may contain multiple remote servers which it will attempt to
connect against. But there are some configuration options which are related to specific
--remote options. For these use cases, connection profiles are the solution.

By enacpulating the --remote option and related options within <connection> and
</connection>, these options are handled as a group.

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

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

Here is an example of connection profile usage:

client
dev tun

<connection>
remote 198.19.34.56 1194 udp
</connection>

<connection>
remote 198.19.34.56 443 tcp
</connection>

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

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

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

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

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

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

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

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

INLINE FILE SUPPORT

       OpenVPN  allows  including  files  in  the  main configuration for the --ca, --cert, --dh,
       --extra-certs,   --key,   --pkcs12,   --secret,   --crl-verify,    --http-proxy-user-pass,
       --tls-auth, --auth-gen-token-secret, --tls-crypt and --tls-crypt-v2 options.

       Each inline file started by the line <option> and ended by the line </option>

       Here is an example of an inline file usage

          <cert>
          -----BEGIN CERTIFICATE-----
          [...]
          -----END CERTIFICATE-----
          </cert>

       When using the inline file feature with --pkcs12 the inline file has to be base64 encoded.
       Encoding of a .p12 file into base64 can be  done  for  example  with  OpenSSL  by  running
       openssl base64 -in input.p12

SIGNALS

       SIGHUP Cause  OpenVPN  to  close all TUN/TAP and network connections, restart, re-read the
              configuration file (if any), and reopen TUN/TAP and network connections.

       SIGUSR1
              Like SIGHUP`, except don't re-read configuration file, and possibly don't close and
              reopen  TUN/TAP  device,  re-read  key  files,  preserve  local IP address/port, or
              preserve most recently authenticated remote IP address/port based on --persist-tun,
              --persist-key, --persist-local-ip and --persist-remote-ip options respectively (see
              above).

              This signal may also be internally generated by a timeout  condition,  governed  by
              the --ping-restart option.

              This  signal,  when  combined  with  --persist-remote-ip,  may  be  sent  when  the
              underlying parameters of the host's network interface change such as when the  host
              is  a  DHCP  client  and  is  assigned  a  new IP address.  See --ipchange 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.

FAQ

       https://community.openvpn.net/openvpn/wiki/FAQ

HOWTO

       For  a  more  comprehensive  guide  to setting up OpenVPN in a production setting, see the
       OpenVPN HOWTO at https://openvpn.net/community-resources/how-to/

PROTOCOL

       For      a      description      of      OpenVPN's      underlying      protocol,      see
       https://openvpn.net/community-resources/openvpn-protocol/

WEB

       OpenVPN's web site is at https://openvpn.net/

       Go  here  to  download the latest version of OpenVPN, subscribe to the mailing lists, read
       the mailing list archives, or browse the SVN repository.

BUGS

       Report all bugs to the OpenVPN team info@openvpn.net

NOTES

       This product includes software developed by the OpenSSL Project (https://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
       https://www.oberhumer.com/opensource/lzo/

COPYRIGHT

       Copyright (C) 2002-2020 OpenVPN Inc This program is free software; you can redistribute it
       and/or modify it under the terms of the GNU General Public License version 2 as  published
       by the Free Software Foundation.

AUTHORS

       James Yonan james@openvpn.net

                                                                                       OPENVPN(8)

NAME

SYNOPSIS

INTRODUCTION

DESCRIPTION

OPTIONS

ENCRYPTION OPTIONS

DATA CHANNEL CIPHER NEGOTIATION

NETWORK CONFIGURATION

SCRIPTING INTEGRATION

UNSUPPORTED OPTIONS

CONNECTION PROFILES

INLINE FILE SUPPORT

SIGNALS

FAQ

HOWTO

PROTOCOL

WEB

BUGS

SEE ALSO

NOTES

COPYRIGHT

AUTHORS