OpenVPN 3 Linux

OpenVPN 3 Linux is an OpenVPN platform which builds on capabilities available on newer Linux distributions. Compared to the more classic OpenVPN 2.x generation, OpenVPN 3 Linux covers many more aspects of the VPN configuration and session life-cycle than prior OpenVPN generations did.

To quickly compare them, OpenVPN 2.x provides a single executable which is responsible for a single VPN session. There are no configuration or session management in OpenVPN 2.x itself, it depends on the systemd [email protected] and [email protected] unit files, the Network Manager OpenVPN plug-in or other third-party management tools.

OpenVPN 3 Linux provides full configuration and session management in addition to providing the VPN tunnel itself. For example, it has built in privilege separation and execution models, for improved process security. This allows unprivileged users to start their own VPN sessions and manage them themselves. VPN configuration profiles can be shared with other users on the system or kept private. All without installing anything additionally.

Through this privilege separation model, the network configuration aspect of the VPN tunnel is split out into its own process which runs with as few privileges as possible. In practice that means it can only do network configuration changes. This process knows nothing about the connection to the VPN server, it just facilitates creating the virtual network adapter and configuring it with network routes. This network configuration service is also capable of setting up the DNS resolver out-of-the-box. For OpenVPN 2.x to do that, it would need to run additional scripts or use specific plug-ins to trigger such updates on the system.

The same OpenVPN 3 Core library which is used in the OpenVPN Connect clients is also used in this project. This implementation does not support all options OpenVPN 2.x does, but if you have a functional configuration with OpenVPN Connect (typically on Android or iOS devices) it should work with this client. In general OpenVPN 3 supports routed TUN configurations; TAP and bridged setups are not supported and will not work.

The OpenVPN 3 Linux architecture is based on splitting up the functionality into several independently running services. They are referred to as backend services. The interaction with these services happens through what is referred to as a user front-end. This project also ships with a Python 3 module which can be used to implement your own OpenVPN front-ends.

On a more technical level, the integration between the user front-end and the backend services is built on top of D-Bus. Any programming language supporting D-Bus can also be used to extend and implement a richer functionality.

Pre-built binaries

See the instructions on https://community.openvpn.net/openvpn/wiki/OpenVPN3Linux how to install pre-built OpenVPN 3 Linux packages on Debian, Ubuntu, Fedora, Red Hat Enterprise Linux, CentOS and Scientific Linux.

Getting started using OpenVPN 3 Linux

See the QUICK-START document to get started using OpenVPN 3 Linux.

Introduction to the OpenVPN 3 Linux architecture

To interact with the various OpenVPN 3 services running in the background, three different utilities are provided.

• openvpn2 (man page)

  This is an interface which tries to look and behave more like the classic OpenVPN 2.x versions. It does only allow options which are supported by the OpenVPN 3 Core Library, plus there are a handful options which are ignored as it is possible to establish connections without those options active. Only client side options are supported.

  When running openvpn2 with --daemon it will return a D-Bus session path to the VPN session. This path can be used by the openvpn3 utility to further manage this session.

• openvpn3 (man page)

  This is a brand new command line interface which does not look like OpenVPN 2.x at all. It can be used to start, stop, pause, resume tunnels and retrieve tunnel statistics. It can also be used as import, retrieve and manage configurations stored in the configuration manager, as well as handling access control lists for VPN configuration profiles and running VPN sessions.

• openvpn3-admin (man page)

  This will mostly only work when run as root. This is used to adjust some settings or retrieve information for some of the backend services and related system administration tasks.

As mentioned earlier, the OpenVPN 3 Linux project is built on top of D-Bus. This provides an API which can be used to further interact with the OpenVPN 3 Linux stack. It can be used to create a new user front-end or it can be used to trigger other operations on the host when certain events happens.

The OpenVPN 3 Linux stack consists of several D-Bus services running in the background. There are six services which is good to beware of. All of these services will normally start automatically. And when they are idle for a while with no data to maintain, they will shut-down automatically.

• openvpn3-service-configmgr (man page | D-Bus documentation)

  This is the configuration manager. All configuration profiles will be uploaded to and managed by this service before a tunnel is started. This service also ensures only users granted access to imported VPN profiles has the proper access to them. By default this process is started as the openvpn user.

• openvpn3-service-sessionmgr (man page | D-Bus documentation)

  This manages all VPN tunnels which are about to start or has started. It takes care of communicating with the VPN backend processes and ensures only users with the right access levels can manage the various tunnels. This service is started as the openvpn user.

• openvpn3-service-backendstart (man page | D-Bus documentation)

  This is a helper service and is only availble for the session manager. The only task this service has is to start a new VPN client backend processes (the VPN tunnel instances). By default this is also started as the openvpn user.

• openvpn3-service-client (man page | D-Bus documentation)

  This must be started by the openvpn3-service-backendstart service. One such process is started per VPN client. Once it has started, it registers itself with the session manager and the session manager provides it with the needed details so it can retrieve the proper configuration profile from the configuration manager. This service will depend on the openvpn3-service-netcfg to manage the tun interface and related configuration. This service is started as the openvpn users.

• openvpn3-service-netcfg (man page | D-Bus documentation)

  This provides a service similar to a VPN API on some platforms. It is responsible for creating, managing and destroying of virtual tunnel interfaces, such as the tun or ovpn Data Channel Offload interfaces. It will also configure them in addition to handle the DNS configuration provided by the VPN server. This is the most privileged process which only have a few capabilities enabled (such as CAP_NET_ADMIN and possibly CAP_DAC_OVERRIDE or CAP_NET_RAW). With these capabilities, the service can run as the openvpn user.

  Currently DNS configuration is done by manipulating /etc/resolv.conf directly. Support for systemd-resolved has been added. On Linux distrubutions expected to be pre-configured with systemd-resolved, OpenVPN 3 Linux will use this service. On other distributions this need to be enabled manually by running the following command as root:

  # openvpn3-admin netcfg-service --config-set systemd-resolved true
  

  Next time the openvpn3-service-netcfg service restarts, systemd-resolved support will be used instead. Note, this requires at least systemd v243 or newer (or a distribution which has back-ported a newer version). This works now with CentOS 8, Fedora 31 and newer, Red Hat Enterprise Linux 8 or Ubuntu 20.04 and newer.

  To disable the systemd-resovled integration and use /etc/resolv.conf instead, run these commands as root:

  # openvpn3-admin netcfg-service --config-unset systemd-resolved
  # openvpn3-admin netcfg-service --config-set resolv-conf /etc/resolv.conf
  

• openvpn3-service-logger (man page | D-Bus documentation)

  This service will listen for log events happening from all the various services in the OpenVPN 3 Linux stack. It supports writing these events to the console (stdout), files or redirect to syslog or the systemd-journald. This is also automatically started when needed, if it isn't already running.

More information can be found in the openvpn3-linux(7) man page and OpenVPN 3 D-Bus overview.

TECH PREVIEW: Kernel based Data Channel Offload (DCO) support

BEWARE - UNDER HEAVY DEVELOPMENT

 This feature is under heavy development.  It is NOT production
 ready and the API between the kernel module and OpenVPN 3 Linux may
 change in incompatible ways for the time being until the API is
 considered stable.

The Data Channel Offload support moves the processing of the OpenVPN data channel operations from the client process to the kernel, via the ovpn-dco kernel module. This means the encryption and decryption of the tunnelled network traffic is kept entirely in kernel space instead of being send back and forth between the kernel and the OpenVPN client process. This has the potential to improve the overall VPN throughput. This module must be installed before OpenVPN 3 Linux can make use of this feature. This is shipped in the OpenVPN 3 Linux package repositories or can be built from the source code.

The ovpn-dco kernel module currently only support Linux kernel 5.4 and newer. Currently supported distributions with DCO support:

• CentOS 8
• Fedora 36 and newer
• Red Hat Enterprise Linux 8 and newer
• Ubuntu 20.04 and newer

The ovpn-dco kernel module is currently not functional on RHEL/CentOS due to the kernel version is older than 4.18. OpenVPN 3 Linux will build with the --enable-dco feature but requires a functional ovpn-dco kernel module to be fully functional.

To build OpenVPN 3 Linux with this support, add --enable-dco to the ./configure command.

SELinux support

The openvpn3-service-netcfg service depends on being able to pass a file descriptor to the tun device it has created on behalf of the openvpn3-service-client service (where each of these processes represents a single VPN session). This is done via D-Bus. But on systems with SELinux, the D-Bus daemon is not allowed to pass file descriptors related to /dev/net/tun.

The OpenVPN 3 Linux project ships two SELinux policy modules, which will be installed in /usr/share/selinux/packages.

The openvpn3.pp policy package adds a SELinux boolean, dbus_access_tuntap_device, which grants processes, such as dbus-daemon or dbus-broker daemon (running under the system_dbusd_t process context) access to files labelled as tun_tap_device_t; which matches the label of /dev/net/tun. Without this policy enabled, the openvpn3-service-netcfg service will not be able to create or manage TUN devices.

To install and activate this SELinux security module, as root run:

     # semodule -i /etc/openvpn3/selinux/openvpn3.pp
     # semanage boolean --m --on dbus_access_tuntap_device

For users installing the pre-built RPM binaries, this is handled by the RPM scriptlet during package install.

The second policy module, openvpn3_service.pp, will confine both the openvpn3-service-netcfg and openvpn3-service-client processes into their own SELinux process contexts (openvpn3_netcfg_t and openvpn3_client_t). See the src/selinux/openvpn3_service.te source for more details.

For the RPM builds, both SELinux policies are provided in the openvpn3-selinux package.

Logging

Logging happens via openvpn3-service-logger. If not started manually, it will automatically be started by the backend processes needing it. The default configuration sends log data to syslog or systemd-journald, depending on the Linux distribution. Unless --syslog, --journald or --log-file is provided, it will log to the console (stdout).

Real-time log events can be received on a per-session level, by using the openvpn3 log command.

This log service is managed via openvpn3-admin log-service. For systems using systemd-journald, the openvpn3-admin journal command provides a convenient approach to retrive only OpenVPN 3 Linux related log entries from the systemd journal.

For more information about logging, see the openvpn3-service-logger(8), man page, D-Bus Logging and net.openvpn.v3.log D-Bus service documentation.

Debugging

For information about debugging, please see docs/debugging.md

Building from source

For information about building OpenVPN 3 Linux from source, please see BUILD.md.

Contribution

• Code contributions Code contributions are most welcome. Please submit patches for review to the [email protected] mailing list. All patches must carry a Signed-off-by line and must be reviewed publicly before acceptance. Pull requests are not acceptable unless it is for early reviews and patch discussions. Final patches MUST go to the mailing list.

• Testing This code is quite new, but has been used a lot in various setups. Please reach out on libera.chat @ #openvpn for help and discussing issues you encounter, or subscribe to and ask on the [email protected] mailing list.

• Packagers We are beginning to targeting packaging in Linux distributions. The Fedora Copr repository is one which is currently available. We are looking for people willing to package this in other Linux distributions as well.