Как узнать версию openvpn linux

How to check server.conf error for OpenVPN

After configuration I run sudo service openvpn start , but failed to start.

Is there a some kind of «grammar check» option for server.conf ?

I put the config file in /etc/openvpn/server.conf , and I’m using CentOS 6.6 .

And this is a result of openvpn —version :

What should I do to determine the cause of error?

1 Answer 1

I don’t believe that there’s any specific command to do a syntax check, but you can run openvpn in the foreground, which should show the specific error:

Как узнать версию openvpn в linux

Для удобства настройки заходим под суперпользователем:

Подготовка Ubuntu

Обновляем установленные пакеты:

apt-get update && apt-get upgrade

Устанавливаем утилиту для синхронизации времени:

apt-get install chrony

Разрешаем ее автозапуск и стартуем сервис:

systemctl enable chrony —now

Установим правильную временную зону:

cp /usr/share/zoneinfo/Europe/Moscow /etc/localtime

* в данном примере московское время.

Установка, настройка и запуск VPN-сервера

Установка OpenVPN

Устанавливаем необходимые пакеты следующей командой:

apt-get install openvpn easy-rsa

Создание сертификатов

Создаем каталог, в котором будем хранить сертификаты:

Переходим в созданный нами каталог:

Скопируем в текущий каталог шаблоны скриптов для формирования сертификатов:

cp -r /usr/share/easy-rsa/* .

Чтобы ускорить процесс создания ключей, откроем на редактирование следующий файл:

и приведем его к следующему виду:

export KEY_COUNTRY=»RU»
export KEY_PROVINCE=»Sankt-Petersburg»
export KEY_CITY=»Sankt-Petersburg»
export KEY_ORG=»DMOSK COMPANY»
export KEY_EMAIL=»master@dmosk.ru»
export KEY_CN=»DMOSK»
export KEY_OU=»DMOSK»
export KEY_NAME=»name-openvpn-server.dmosk.ru»
export KEY_ALTNAMES=»name-openvpn-server»

* где KEY_CN и KEY_OU: рабочие подразделения (например, можно указать название отдела); KEY_NAME: адрес, по которому будет выполняться подключение (можно указать полное наименование сервера); KEY_ALTNAMES — альтернативный адрес.

Запускаем отредактированный файл на исполнение:

Если скрипт выполнится без ошибок, мы увидим следующее:

NOTE: If you run ./clean-all, I will be doing a rm -rf on /etc/openvpn/easy-rsa/keys

Почистим каталог от старых сертификатов:

Для генерирования сертификатов необходим конфигурационный файл openssl.cnf — по умолчанию, он отсутствует, но есть файл openssl-1.0.0.cnf. Создаем на него симлинк:

ln -s openssl-1.0.0.cnf openssl.cnf

* в каталоге /etc/openvpn/easy-rsa может быть несколько разных версий конфигурационного файла openssl-x.x.x.cnf. Чтобы узнать точное имя файла, вводим команду ls /etc/openvpn/easy-rsa.

Генерируем сертификат корневого центра:

на все запросы нажимаем Enter, так как всю верную информацию мы ввели в файле vars.

Теперь последовательно вводим следующие команды для формирования остальных сертификатов.

openvpn —genkey —secret keys/ta.key

Копируем каталог с созданными сертификатами и ключами в /etc/openvpn:

cp -r keys /etc/openvpn/

Настройка OpenVPN-сервера

Создаем конфигурационный файл:

И вставляем в него следующее:

local 192.168.0.15
port 443
proto udp
dev tun
ca keys/ca.crt
cert keys/server.crt
key keys/server.key
dh keys/dh2048.pem
tls-auth keys/ta.key 0
server 172.16.10.0 255.255.255.0
ifconfig-pool-persist ipp.txt
keepalive 10 120
max-clients 32
persist-key
persist-tun
status /var/log/openvpn/openvpn-status.log
log-append /var/log/openvpn/openvpn.log
verb 4
mute 20
daemon
mode server
tls-server
comp-lzo

• local — IP-адрес, на котором будет обрабатывать запросы OpenVPN;
• port — сетевой порт (443 позволит избежать проблем при использовании Интернета в общественных местах, но может быть любым из свободных, например 1723).
• proto — используемый транспортный протокол.
• dev — виртуальный сетевой адаптер, который будет создан для работы OpenVPN.
• ca — путь до сертификата корневого центра сертификации.
• cert — путь до открытого сертификата сервера.
• key — путь до закрытого сертификата сервера.
• dh — путь до ключа Диффи — Хеллмана.
• tls-auth — путь до tls-ключа.
• server — задаем IP-адрес сервера в сети VPN.
• ifconfig-pool-persist — путь к файлу для хранения клиентских IP-адресов.
• keepalive X Y — каждые X секунд отправляется ping-запрос на удаленный узел. Если за Y секунд не получено ответа — перезапускать туннель.
• max-clients — максимум одновременных подключений.
• persist-key — не перезагружать ключи при повторной загрузки из-за разрыва соединения.
• persist-tun — не изменять устройства tun/tap при перезапуске сервера.
• status — путь до журнала статусов.
• log-append — путь до файла лога с дополнительным выводом информации.
• verb — уровень логирования событий. От 0 до 9.
• mute — ограничение количества подряд отправляемых в лог событий.
• daemon — работа в режиме демона.
• mode — в каком режиме работает openvpn (сервер или клиент).
• tls-server — указывает, что данный сервер работает с использованием TLS.
• comp-lzo — использовать сжатие.

Создадим каталог для логов:

Разрешаем автоматический старт сервиса vpn и запускаем его:

Простая настройка OpenVPN Linux

OpenVPN часто используется для создания виртуальных безопасных сетей между компьютерами, которые находятся на очень большом расстоянии, но при этом между ними нужно получить шифрованное безопасное соединение, которое невозможно перехватить и прослушать.

Другое применение для OpenVPN — это обход блокировок различных интернет ресурсов. Существуют различные серверы OpenVPN серверы в интернете, но будет намного надежнее если развернуть OpenVPN на своем сервере. Мы рассматривали процесс в статье установка OpenVPN на Ubuntu, но для новичков это достаточно сложно. В этой инструкции мы рассмотрим как выполняется настройка OpenVPN linux с помощью скрипта openvpn-install, который упрощает процесс установки и настройки в несколько раз. С его помощью вы получите работающий сервер в течение нескольких минут.

Настройка OpenVPN сервера в Linux

В этой статье, в качестве системы для примера мы будем использовать операционную систему CentOS, но операции не будут отличаться в других дистрибутивах. Сначала скачайте сам скрипт:

wget https://git.io/vpn -O openvpn-install.sh

Затем запустите скрипт от имени суперпользователя с помощью такой команды:

Дальше скрипт задаст несколько вопросов по настройкам OpenVPN. Сначала нужно указать точный ip адрес VPS, на котором будет запущен OpenVPN, именно на этом адресе программа будет слушать соединения:

Затем выберите протокол, который вы хотите использовать. Например, если я хочу использовать TCP то нужно выбрать 2:

Затем введите номер порта, на котором будет слушать программа, можно оставить значение по умолчанию:

Выберите сервера DNS, которые нужно использовать, мне, например, нравятся сервера от Google:

Введите имя клиента, для которого будут подписаны сертификаты на доступ:

Дальше нажмите Enter, и подтвердите установку OpenVPN. Программа сама установит нужные пакеты в вашем дистрибутиве, а затем подпишет все необходимые сертификаты, запустит сервер openvpn и даже добавит его в автозагрузку systemd:

Последний шаг, это утилита спросит есть ли у вашего сервера дополнительный внешний ip адрес. Это может быть полезно, если вы используете NAT, и для доступа к серверу применяете какой-либо туннель. В других случаях этот пункт можно пропустить, для VPS точно пропускаем.

Теперь установка и настройка OpenVPN сервера Linux завершена. Конфигурационный файл для клиента сохранен по адресу /root/sergiy-pc.ovpn, может иметь и другое имя, в зависимости от того, какое имя вы дали клиенту. Самое интересное, что для того чтобы добавить новых клиентов, вам просто достаточно запустить скрипт еще раз. Осталось только скопировать полученный файл клиенту, для этого можно на клиентском компьютере выполнить scp:

scp root@адрес_сервера:/root/sergiy-pc.ovpn /etc/openvpn/

Настройка клиента OpenVPN

Вы можете настроить OpenVPN на клиентской машине как захотите, например, с помощью NetworkManager. Но мы рассмотрим ручной вариант. Сначала установите саму программу. Установка OpenVPN Linux Ubuntu выполняется командой:

sudo apt install openvpn

sudo yum install openvpn

Теперь для подключения к нашему серверу используем полученный файл:

openvpn —config /etc/openvpn/client.ovpn

Вы можете использовать команду ip addr чтобы проверить ip адрес интерфейса OpenVPN — tun0:

Как видите, OpenVPN автоматически выдал IP вашему компьютеру, теперь можете проверить подключение к этому серверу с помощью ping:

Вот и все, ваша сеть готова к работе.

Выводы

Пять минут и настройка OpenVPN Linux завершена, а не подключение следующего клиента уйдет еще меньше. Этот метод не специфичен для RPM систем и может использоваться также в DEB системах, таких как Ubuntu или Debian. Если у вас остались вопросы, спрашивайте в комментариях!

Introduction

This page contains instructions for using OpenVPN project’s own software repositories. For a list of unofficial repositories (e.g. EPEL) please refer to the Unofficial OpenVPN software repositories page.

Latest OpenVPN releases are available in the OpenVPN project’s apt repositories. This allow you to use more up-to-date version of OpenVPN than what is typically available in your distribution’s repositories. Please note that all commands listed below have to be run as root, e.g. using sudo or su.

Pre-built Linux binaries are only available for Debian and Ubuntu. This is so for two reasons:

• Official Debian and Ubuntu repositories tend to have fairly old OpenVPN versions available
• The Fedora and Fedora EPEL provides fairly up-to-date OpenVPN releases for supported Fedora and Red Hat Enterprise Linux (including clones such as CentOS, Scientific Linux) releases.

All packages are available in i386 and amd64/x86_64 flavours. Even if a package is built on a particular OS, it does not mean it won’t work on older and/or newer versions of the same distro, or even on a different operating system. If you encountered any issues with the package, please file a new bug report.

Debian / Ubuntu: Using OpenVPN apt repositories

We maintain several OpenVPN (OSS) software repositories. To setup the repositories you need to change to the root user. Typically this is done using sudo:

Then import the public GPG key that is used to sign the packages:

Next you need to create a sources.list fragment (as root) so that apt can find the new OpenVPN packages. One way to do it is this:

Where <version> can be one of

1. stable: stable releases only – no alphas, betas or RCs
2. testing: latest releases, including alphas/betas/RCs
3. release/2.3: OpenvPN 2.3 releases
4. release/2.4: OpenVPN 2.4 releases, including alphas/betas/RCs

and <osrelease> depends your distribution:

• wheezy (Debian 7.x)
• jessie (Debian 8.x)
• precise (Ubuntu 12.04)
• trusty (Ubuntu 14.04)
• xenial (Ubuntu 16.04)

Now you’re set for installing OpenVPN. Note that packages built for older operating system releases might work just fine on newer release of the same operating system.

Installing OpenVPN

On Debian/Ubuntu use

Notes on expired keys

If the apt signing key expires, apt will complain when refreshing the package cache (e.g. apt-get update). To fix this remove the expired key from apt keychain:

Then add the new key using wget and apt-key as described above. Then verify that the new key is in the keychain:

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.

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 randomness. Newer linux kernels and some BSDs implement a getrandom() or getentropy() syscall that removes the need for /dev/urandom to be available.

—compat-mode version This option provides a convenient way to alter the defaults of OpenVPN to be more compatible with the version version specified. All of the changes this option applies can also be achieved using individual configuration options.

The version specified with this option is the version of OpenVPN peer OpenVPN should try to be compatible with. In general OpenVPN should be compatible with the last two previous version without this option. E.g. OpenVPN 2.6.0 should be compatible with 2.5.x and 2.4.x without this option. However, there might be some edge cases that still require this option even in these cases.

Note: Using this option reverts defaults to no longer recommended values and should be avoided if possible.

The following table details what defaults are changed depending on the version specified.

• 2.5.x or lower: —allow-compression asym is automatically added to the configuration if no other compression options are present.
• 2.4.x or lower: The cipher in —cipher is appended to —data-ciphers.
• 2.3.x or lower: —data-cipher-fallback is automatically added with the same cipher as —cipher.
• 2.3.6 or lower: —tls-version-min 1.0 is added to the configuration when —tls-version-min is not explicitly set.

If not required, this is option should be avoided. Setting this option can lower security or disable features like data-channel offloading.

—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:

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

Here is an example configuration file:

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-dco Disable "data channel offload" (DCO).

On Linux don’t use the ovpn-dco device driver, but rather rely on the legacy tun module.

You may want to use this option if your server needs to allow clients older than version 2.4 to connect.

—disable-occ DEPRECATED Disable "options consistency check" (OCC) in configurations that do not use TLS.

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:

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.

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.

If the platform has the getrlimit(2) system call, OpenVPN will check for the amount of mlock-able memory before calling mlockall(2), and tries to increase the limit to 100 MB if less than this is configured. 100 Mb is somewhat arbitrary — it is enough for a moderately-sized OpenVPN deployment, but the memory usage might go beyond that if the number of concurrent clients is high.

—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 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

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:

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:

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. n defaults to 60 if not specified.

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:

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

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 and pending authentification via pending_auth_file.

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 an unprivileged user dedicated to run openvpn, 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).

NOTE: Previous versions of openvpn used nobody as the example unpriviledged user. It is not recommended to actually use that user since it is usually used by other system services already. Always create a dedicated user for openvpn.

—writepid file Write OpenVPN’s main process ID to file.

Log options

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 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.

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.

The mode argument can be one of the following values:

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.

—cipher alg This option should not be used any longer in TLS mode and still exists for two reasons:

• compatibility with old configurations still carrying it around;
• allow users connecting to OpenVPN peers older than 2.6.0 to have —cipher configured the same way as the remote counterpart. This can avoid MTU/frame size warnings.

Before 2.4.0, this option was used to select the cipher to be configured on the data channel, however, later versions usually ignored this directive in favour of a negotiated cipher. Starting with 2.6.0, this option is always ignored in TLS mode when it comes to configuring the cipher and will only control the cipher for —secret pre-shared-key mode (note: this mode is deprecated and strictly not recommended).

If you wish to specify the cipher to use on the data channel, please see —data-ciphers (for regular negotiation) and —data-ciphers-fallback (for a fallback option when the negotiation cannot take place because the other peer is old or has negotiation disabled).

To see 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. See also the migrate parameter below.

The algorithm parameter may be lzo, lz4, lz4-v2, stub, stub-v2, migrate 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.

Especially stub-v2 is essentially identical to no compression and no compression framing as its header indicates IP version 5 in a tun setup and can (ab)used to complete disable compression to clients. (See the migrate option below)

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.

Using migrate as compression algorithm enables a special migration mode. It allows migration away from the —compress/—comp-lzo options to no compression. This option sets the server to no compression mode and the server behaves identical to a server without a compression option for all clients without a compression in their config. However, if a client is detected that indicates that compression is used (via OCC), the server will automatically add —push compress stub-v2 to the client specific configuration if supported by the client and otherwise switch to comp-lzo no and add —push comp-lzo to the client specific configuration.

*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:

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). —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:CHACHA20-POLY1305 when Chacha20-Poly1305 is available and otherwise 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.

For more details see the chapter on Data channel cipher negotiation. Especially if you need to support clients with OpenVPN versions older than 2.4!

Starting with OpenVPN 2.6 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.

If —compat-mode is set to a version older than 2.5.0 the cipher specified by —cipher will be appended to —data-ciphers if not already present.

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 or older versions, and have been configured with —enable-small (typically used on routers or other embedded devices).

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

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

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

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

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

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

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

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

—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.

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

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.

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:

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:

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 args Wait n seconds between connection attempts (default 1). Repeated reconnection attempts are slowed down after 5 retries per remote by doubling the wait time after each unsuccessful attempt.

If the optional argument max is specified, the maximum wait time in seconds gets capped at that value (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. —dns args Client DNS configuration to be used with the connection.

The —dns search-domains directive takes one or more domain names to be added as DNS domain suffixes. If it is repeated multiple times within a configuration the domains are appended, thus e.g. domain names pushed by a server will amend locally defined ones.

The —dns server directive is used to configure DNS server n. The server id n must be a value between -128 and 127. For pushed DNS server options it must be between 0 and 127. The server id is used to group options and also for ordering the list of configured DNS servers; lower numbers come first. DNS servers being pushed to a client replace already configured DNS servers with the same server id.

The address option configures the IPv4 and / or IPv6 address(es) of the DNS server. Up to eight addresses can be specified per DNS server. Optionally a port can be appended after a colon. IPv6 addresses need to be enclosed in brackets if a port is appended.

The resolve-domains and exclude-domains options take one or more DNS domains which are explicitly resolved or explicitly not resolved by a server. Only one of the options can be configured for a server. resolve-domains is used to define a split-dns setup, where only given domains are resolved by a server. exclude-domains is used to define domains which will never be resolved by a server (e.g. domains which can only be resolved locally). Systems which do not support fine grained DNS domain configuration, will ignore these settings.

The dnssec option is used to configure validation of DNSSEC records. While the exact semantics may differ for resolvers on different systems, yes likely makes validation mandatory, no disables it, and optional uses it opportunistically.

The transport option enables DNS-over-HTTPS (DoH) or DNS-over-TLS (DoT) for a DNS server. The sni option can be used with them to specify the server-name for TLS server name indication.

Each server has to have at least one address configured for a configuration to be valid. All the other options can be omitted.

Note that not all options may be supported on all platforms. As soon support for different systems is implemented, information will be added here how unsupported options are treated.

The —dns option will eventually obsolete the —dhcp-option directive. Until then it will replace configuration at the places —dhcp-option puts it, so that —dns overrides —dhcp-option. Thus, —dns can be used today to migrate from —dhcp-option.

—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.

If both server and client support sending this message using the control channel, the message will be sent as control-channel message. Otherwise the message is sent as data-channel message, which will be ignored by data-channel offloaded peers.

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 if messages are sent in data-channel mode.

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.

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.

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

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

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:

• 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.

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

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.

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 120. This timeout includes proxy and TCP connect timeouts. —static-challenge args Enable static challenge/response protocol

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.

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.

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.

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.

If renewal-time is not set it defaults to reneg-sec.

The token will expire either after the configured lifetime of the token is reached or after not being renewed for more than 2 * renewal-time seconds. Clients will be sent renewed tokens on every TLS renegotiation. If renewal-time is lower than reneg-sec the server will push an updated temporary authentication token every reneweal-time seconds. This is done to invalidate a token if a client is disconnected for a sufficiently 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 succeeds 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:

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.

Please note that when using data channel offload this option has no effect. Packets are always sent to the tunnel interface and then routed based on the system routing table.

—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.

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

This limit applies after —connect-freq-initial and only applies to client that have completed the three-way handshake or client that use —tls-crypt-v2 without cookie support (allow-noncookie argument to —tls-crypt-v2).

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.

—connect-freq-initial args (UDP only) Allow a maximum of n initial connection packet responses per sec seconds from the OpenVPN server to clients.

OpenVPN starting at 2.6 is very efficient in responding to initial connection packets. When not limiting the initial responses an OpenVPN daemon can be abused in reflection attacks. This option is designed to limit the rate OpenVPN will respond to initial attacks.

Connection attempts that complete the initial three-way handshake will not be counted against the limit. The default is to allow 100 initial connection per 10s.

—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.

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.

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.

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.

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

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

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

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

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

• 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).

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.

However, when using DCO, the —iroute directive is usually enough for DCO to fully configure the routing table. The extra —route directive is required only if the expected behaviour is to route the traffic for a specific network to the VPN interface also when the responsible client is not connected (traffic will then be dropped).

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.

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

—opt-verify DEPRECATED 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

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, —dns, —inactive, —ping, —ping-exit, —ping-restart, —setenv, —auth-token, —persist-key, —persist-tun, —echo, —comp-lzo, —socket-flags, —sndbuf, —rcvbuf, —session-timeout

—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.

For example, —server 10.8.0.0 255.255.255.0 expands as follows:

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.

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:

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

Or —server-bridge nogw expands as follows:

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).

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:

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.

The following modes are available:

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

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

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

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.

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 733 bytes long (980 characters in base64, though note that 980 base64 characters can encode more than 733 bytes).

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

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.

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).

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

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:

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:

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:

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.

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.

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.

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.

The —hand-window parameter also controls the amount of time that the OpenVPN client repeats the pull request until it times out.

—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.

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

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

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, —peer-fingerprint 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.

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

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:

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

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

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’.

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.

The optional parameters force-cookie allows only tls-crypt-v2 clients that support a cookie based stateless three way handshake that avoids replay attacks and state exhaustion on the server side (OpenVPN 2.6 and later). The option allow-noncookie explicitly allows older tls-crypt-v2 clients. The default is (currently) allow-noncookie.

—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 in 2.6.0 and later is "1.2").

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 DEPRECATED Specify SHA1 or SHA256 fingerprint for level-1 cert.

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

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

This option can also be inlined

If the option is inlined, algo is always SHA256.

When the —peer-fingerprint option is used, specifying a CA with —ca or —capath is optional. This allows the he —peer-fingerprint to be used as alternative to a PKI with self-signed certificates for small setups. See the examples section for such a setup.

—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.

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

would be matched by:

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.

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

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. The last example uses the value of the CN attribute in the Subject field, combined with the _ separator and the hexadecimal representation of the certificate’s serialNumber.

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 and serialNumber X.509 attribute 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.

Non-compliant symbols are being replaced with the _ symbol, same as the field separator, so concatenating multiple fields with such or _ symbols can potentially lead to username collisions.

PKCS#11 / SmartCard options

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.

If p11-kit is present on the system and was enabled during build, its p11-kit-proxy.so module will be loaded by default if either the —pkcs11-id or —pkcs11-id-management options is present without —pkcs11-providers. If default loading is not enabled in the build and no providers are specified, the former options will be ignored.

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 later 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):

OpenVPN 2.5 and later will only allow the ciphers specified in —data-ciphers. If —data-ciphers is not set the default is AES-256-GCM:AES-128-GCM. In 2.6 and later the default is changed to AES-256-GCM:AES-128-GCM:CHACHA20-POLY1305 when Chacha20-Poly1305 is available.

For backwards compatibility OpenVPN 2.6 and later with —compat-mode 2.4.x (or lower) and OpenVPN 2.5 will automatically add a cipher specified using the —cipher option to this list.

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 an OpenVPN server tries 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. Newer versions by default disable legacy AES-CBC, BF-CBC, and DES-CBC ciphers. These clients will always 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.

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.

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

If the mtu parameter is present the max parameter is interpreted to include IP and UDP encapsulation overhead. The mtu parameter is introduced in OpenVPN version 2.6.0.

If the mtu parameter is absent, 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.

Send ping once every interval seconds, restart if ping is not received for timeout seconds.

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:

Due to variable header size of IP header (20 bytes for IPv4 and 40 bytes for IPv6) and dynamically negotiated data channel cipher, this option is not reliable. It is recommended to set tun-mtu with enough headroom instead.

—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 args Valid syntax:

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 1492 mtu. Use 0 as max to disable mssfix.

If the mtu parameter is specified the max value is interpreted as the resulting packet size of VPN packets including the IP and UDP header. Support for the mtu parameter was added with OpenVPN version 2.6.0.

If the mtu parameter is not specified, 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 OpenVPN packets to be transmitted over IPv4 on a link with MTU 1478 or higher without IP level fragmentation (and 1498 for IPv6).

If the fixed parameter is specified, OpenVPN will make no attempt to calculate the VPN encapsulation overhead but instead will set the MSS to limit the size of the payload IP packets to the specified number. IPv4 packets will have the MSS value lowered to mssfix — 40 and IPv6 packets to mssfix — 60.

if —mssfix is specified is specified without any parameter it inherits the parameters of —fragment if specified or uses the default for —mssfix otherwise.

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.

—max-packet-size, —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:

If the max-packet-size size option is used in the configuration it will also act as if mssfix size mtu was specified in the configuration.

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

no Never send DF (Don’t Fragment) frames

maybe Use per-route hints

yes Always DF (Don’t Fragment)

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

When using OpenVPN in server mode see also —keepalive.

—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.

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.

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.

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 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.

—session-timeout n Raises SIGTERM for the client instance after n seconds since the beginning of the session, forcing OpenVPN to disconnect. In client mode, OpenVPN will disconnect and exit, while in server mode all client sessions are terminated.

This option can also be specified in a client instance config file using —client-config-dir or dynamically generated using a —client-connect script. In these cases, only the related client session is terminated.

—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:

OpenVPN will try to keep its control channel messages below this size but due to some constraints in the protocol this is not always possible. If the option is not set, the control packet maximum size defaults to 1250. The control channel packet size will be restricted to values between 154 and 2048. The maximum packet size includes encapsulation overhead like UDP and IP.

In terms of —mssfix it will expand to:

If you need to set —mssfix for data channel and control channel maximum packet size independently, use —max-packet-size first, followed by a —mssfix in the configuration.

In general the default size of 1250 should work almost universally apart from specific corner cases, especially since IPv6 requires a MTU of 1280 or larger.

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.

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).

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:

What happens if the device name is not tun or tap is platform dependent.

On most platforms, tunN (e.g. tun2, tun30) and tapN (e.g. tap3) will create a numbered tun/tap interface with the number specified — this is useful if multiple OpenVPN instances are active, and the instance-to-device mapping needs to be known. Some platforms do not support "numbered tap", so trying —dev tap3 will fail.

Arbitrary device names (e.g. —dev tun-home) will only work on FreeBSD (with the DCO kernel driver for tun devices) and Linux (for both tun and tap devices, DCO and tun/tap driver).

If such a device name starts with tun or tap (e.g. tun-home), OpenVPN will choose the right device type automatically. Otherwise the desired device type needs to be specified with —dev-type tun or —dev-type tap.

On Windows, only the names tun and tap are supported. Selection among multiple installed drivers or driver instances is done with —dev-node and —windows-driver.

—dev-node node This is a highly system dependent option to influence tun/tap driver selection.

On Linux, tun/tap devices are created by accessing /dev/net/tun, and this device name can be changed using —dev-node . .

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.

On other platforms, —dev-node node will influence the naming of the created tun/tap device, if supported on that platform. 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.

—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_ 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.

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:

This option currently only works on OpenVPN for Android and requires Android 10 or later.

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.

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:

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

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.

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:

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.

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.

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:

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

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

The default for tun-mtu is 1500.

The OCC MTU can be used to avoid warnings about mismatched MTU from clients. If occ-mtu is not specified, it will to default to the tun-mtu.

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.

Note: Depending on the platform, the operating system allows to receive packets larger than tun-mtu (e.g. Linux and FreeBSD) but other platforms (like macOS) limit received packets to the same size as the MTU.

—tun-max-mtu maxmtu This configures the maximum MTU size that a server can push to maxmtu, by configuring the internal buffers to allow at least this packet size. The default for maxmtu is 1600. Currently, only increasing beyond 1600 is possible, and attempting to reduce max-mtu below 1600 will be ignored. —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.

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

Move eth0 into vrf_external

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:

OpenVPN configuration

The OpenVPN configuration needs to contain this line:

Further reading

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

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.

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.

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.

11. —client-crresponse

SCRIPT HOOKS

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 cmd 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 cmd, 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, a failure code (1) to reject the client, or a that the authentication is deferred (2). If the authentication is deferred, the script must fork/start a background or another non-blocking operation to continue the authentication in the background. When finshing the authentication, a 1 or 0 must be written to the file specified by the auth_control_file.

If the file specified by auth_failed_reason_file exists and has non-empty content, the content of this file will be used as AUTH_FAILED message. To avoid race conditions, this file should be written before auth_control_file.

This auth fail reason can be something simple like "User has been permanently disabled" but there are also some special auth failed messages.

The TEMP message indicates that the authentication temporarily failed and that the client should continue to retry to connect. The server can optionally give a user readable message and hint the client a behavior how to proceed. The keywords of the AUTH_FAILED,TEMP message are comma separated keys/values and provide a hint to the client how to proceed. Currently defined keywords are:

For example, the message TEMP[backoff 42,advance no]: No free IP addresses indicates that the VPN connection can currently not succeed and instructs the client to retry in 42 seconds again.

When deferred authentication is in use, the script can also request pending authentication by writing to the file specified by the auth_pending_file. The first line must be the timeout in seconds, the required method on the second line (e.g. crtext) and third line must be the EXTRA as documented in the client-pending-auth section of doc/management.txt.

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-crresponse

OpenVPN will write the response of the client into a temporary file. The filename will be passed as an argument to cmd, and the file will be automatically deleted by OpenVPN after the script returns.

The response is passed as is from the client. The script needs to check itself if the input is valid, e.g. if the input is valid base64 encoding.

The script can either directly write the result of the verification to auth_control_file or further defer it. See `—auth-user-pass-verify« for details.

For a sample script that implement TOTP (RFC 6238) based two-factor authentication, see sample-scripts/totpauth.py.

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

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.

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.

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.

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

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.

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.

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

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.

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.

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

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

cmd consists of a path to 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.

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.

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.

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

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

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

See also —ignore-unknown-option

—setenv-safe args Set a custom environmental variable OPENVPN_name to value to pass to scripts.

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

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

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

cmd consists of a path to 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.

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

These arguments are, respectively, the current certificate depth and the X509 subject distinguished name (dn) of the peer.

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

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

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

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.

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

For —dev tun execute as:

For —dev tap execute as:

See the Environmental Variables section below for additional parameters passed as environmental variables. The 0 argument used to be link_mtu which is no longer passed to scripts — to keep the argument order, it was replaced with 0.

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

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

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

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

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

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

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

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

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

String Types and Remapping

In certain cases, OpenVPN will perform remapping of characters in strings. Essentially, any characters outside the set of permitted characters for each string type will be converted to underbar (‘_’).

Here is a brief rundown of OpenVPN’s current string types and the permitted character class for each string:

For all cases, characters in a string which are not members of the legal character class for that string type will be remapped to underbar (‘_’).

Environmental Variables

Once set, a variable is persisted indefinitely until it is reset by a new value or a restart,

As of OpenVPN 2.0-beta12, in server mode, environmental variables set by OpenVPN are scoped according to the client objects they are associated with, so there should not be any issues with scripts having access to stale, previously set variables which refer to different client instances.

For deferred (background) handling, the script or plugin MUST write 2 to the file to indicate the deferral and then return with exit code 0 to signal deferred handler started OK.

A background process or similar must then take care of writing the configuration to the file indicated by the client_connect_config_file environment variable and when finished, write the a 1 to this file (or 0 in case of an error).

The absence of any character in the file when the script finishes executing is interpreted the same as 1. This allows scripts that are not written to support the defer mechanism to be used unmodified.

common_name The X509 common name of an authenticated client. Set prior to execution of —client-connect, —client-disconnect and —auth-user-pass-verify scripts. config Name of first —config file. Set on program initiation and reset on SIGHUP. daemon Set to "1" if the —daemon directive is specified, or "0" otherwise. Set on program initiation and reset on SIGHUP. daemon_log_redirect Set to "1" if the —log or —log-append directives are specified, or "0" otherwise. Set on program initiation and reset on SIGHUP. dev The actual name of the TUN/TAP device, including a unit number if it exists. Set prior to —up or —down script execution. dev_idx On Windows, the device index of the TUN/TAP adapter (to be used in netsh.exe calls which sometimes just do not work right with interface names). Set prior to —up or —down script execution. dns_* The —dns configuration options will be made available to script execution through this set of environment variables. Variables appear only if the corresponding option has a value assigned. For the semantics of each individual variable, please refer to the documentation for —dns.

parm will be one of network, netmask", gateway, or metric.

n is the OpenVPN route number, starting from 1.

If the network or gateway are resolvable DNS names, their IP address translations will be recorded rather than their names as denoted on the command line or configuration file.

route_ipv6__ A set of variables which define each IPv6 route to be added, and are set prior to —up script execution.

parm will be one of network, gateway or metric. route_ipv6_network_ contains netmask as /nnn, unlike IPv4 where it is passed in a separate environment variable.

n is the OpenVPN route number, starting from 1.

If the network or gateway are resolvable DNS names, their IP address translations will be recorded rather than their names as denoted on the command line or configuration file.

peer_cert Temporary file name containing the client certificate upon connection. Useful in conjunction with —tls-verify. script_context Set to "init" or "restart" prior to up/down script execution. For more information, see documentation for —up. script_type Prior to execution of any script, this variable is set to the type of script being run. It can be one of the following: up, down, ipchange, route-up, tls-verify, auth-user-pass-verify, client-connect, client-disconnect or learn-address. Set prior to execution of any script. signal The reason for exit or restart. Can be one of sigusr1, sighup, sigterm, sigint, inactive (controlled by —inactive option), ping-exit (controlled by —ping-exit option), ping-restart (controlled by —ping-restart option), connection-reset (triggered on TCP connection reset), error or unknown (unknown signal). This variable is set just prior to down script execution. time_ascii Client connection timestamp, formatted as a human-readable time string. Set prior to execution of the —client-connect script. time_duration The duration (in seconds) of the client session which is now disconnecting. Set prior to execution of the —client-disconnect script. time_unix Client connection timestamp, formatted as a unix integer date/time value. Set prior to execution of the —client-connect script. tls_digest_ / tls_digest_sha256_ Contains the certificate SHA1 / SHA256 fingerprint, where n is the verification level. Only set for TLS connections. Set prior to execution of —tls-verify script. tls_id_ A series of certificate fields from the remote peer, where n is the verification level. Only set for TLS connections. Set prior to execution of —tls-verify script. tls_serial_ The serial number of the certificate from the remote peer, where n is the verification level. Only set for TLS connections. Set prior to execution of —tls-verify script. This is in the form of a decimal string like "933971680", which is suitable for doing serial-based OCSP queries (with OpenSSL, do not prepend "0x" to the string) If something goes wrong while reading the value from the certificate it will be an empty string, so your code should check that. See the contrib/OCSP_check/OCSP_check.sh script for an example. tls_serial_hex_ Like tls_serial_ , but in hex form (e.g. 12:34:56:78:9A). tun_mtu The MTU of the TUN/TAP device. Set prior to —up or —down script execution. trusted_ip / trusted_ip6) Actual IP address of connecting client or peer which has been authenticated. Set prior to execution of —ipchange, —client-connect and —client-disconnect scripts. If using ipv6 endpoints (udp6, tcp6), trusted_ip6 will be set instead. trusted_port Actual port number of connecting client or peer which has been authenticated. Set prior to execution of —ipchange, —client-connect and —client-disconnect scripts. untrusted_ip / untrusted_ip6 Actual IP address of connecting client or peer which has not been authenticated yet. Sometimes used to nmap the connecting host in a —tls-verify script to ensure it is firewalled properly. Set prior to execution of —tls-verify and —auth-user-pass-verify scripts. If using ipv6 endpoints (udp6, tcp6), untrusted_ip6 will be set instead. untrusted_port Actual port number of connecting client or peer which has not been authenticated yet. Set prior to execution of —tls-verify and —auth-user-pass-verify scripts. username The username provided by a connecting client. Set prior to —auth-user-pass-verify script execution only when the via-env modifier is specified. X509__ An X509 subject field from the remote peer certificate, where n is the verification level. Only set for TLS connections. Set prior to execution of —tls-verify script. This variable is similar to tls_id_ except the component X509 subject fields are broken out, and no string remapping occurs on these field values (except for remapping of control characters to "_"). For example, the following variables would be set on the OpenVPN server using the sample client certificate in sample-keys (client.crt). Note that the verification level is 0 for the client certificate and 1 for the CA certificate.

Management Interface Options

OpenVPN provides a feature rich socket based management interface for both server and client mode operations.

Похожие публикации:

1. Ald pro astra linux что это
2. Digital crown в apple watch как перевернуть
3. Ls l linux что выводит
4. Как вызвать командную строку в linux