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