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OpenVPN data channel offload

2.6.0+ implements support for data-channel offloading where the data packets are directly processed and forwarded in kernel space thanks to the ovpn-dco kernel module. The userspace openvpn program acts purely as a control plane application.

Overview of current release

  • See the "Limitations by design" and "Current limitations" sections for features that are not and/or will not be supported by OpenVPN + ovpn-dco.

Getting started (Linux)

  • Use a recent Linux kernel. Linux 5.4.0 and newer are known to work with ovpn-dco.

Get the ovpn-dco module from one these urls and build it:

e.g.

git clone https://github.com/OpenVPN/ovpn-dco
cd ovpn-dco
make
sudo make install

If you want to report bugs please ensure to compile ovpn-dco with make DEBUG=1 and include any debug message being printed by the kernel (you can view those messages with dmesg).

Clone and build OpenVPN (or use OpenVPN 2.6+). For example:

git clone https://github.com/openvpn/openvpn.git
cd openvpn
autoreconf -vi
./configure --enable-dco
make
sudo make install # Or just run src/openvpn/openvpn

When starting openvpn it will automatically detect DCO support and use the kernel module. Add the option --disable-dco to disable data channel offload support. If the configuration contains an option that is incompatible with data channel offloading, OpenVPN will automatically disable DCO support and warn the user.

Should OpenVPN be configured to use a feature that is not supported by ovpn-dco or should the ovpn-dco kernel module not be available on the system, you will see a message like

Note: Kernel support for ovpn-dco missing, disabling data channel offload.

in your log.

Getting started (Windows)

Official releases published at https://openvpn.net/community-downloads/ include ovpn-dco-win driver since 2.6.0.

There are also snapshot releases available at https://build.openvpn.net/downloads/snapshots/github-actions/openvpn2/ . This installer contains the latest OpenVPN code and the ovpn-dco-win driver.

DCO and P2P mode

DCO is also available when running OpenVPN in P2P mode without --pull / --client option. P2P mode is useful for scenarios when the OpenVPN tunnel should not interfere with overall routing and behave more like a "dumb" tunnel, like GRE.

However, DCO requires DATA_V2 to be enabled, which is available for P2P mode only in OpenVPN 2.6 and later.

OpenVPN prints a diagnostic message for the P2P NCP result when running in P2P mode:

P2P mode NCP negotiation result: TLS_export=1, DATA_v2=1, peer-id 9484735, cipher=AES-256-GCM

Double check that you have DATA_v2=1 in your output and a supported AEAD cipher (AES-XXX-GCM or CHACHA20POLY1305).

Routing with ovpn-dco

The ovpn-dco kernel module implements a more transparent approach to configuring routes to clients (aka "iroutes") and consults the main kernel routing tables for forwarding decisions.

  • Each client has a VPN IPv4 and/or a VPN IPv6 assigned to it;
  • additional IP ranges can be routed to a client by adding a route with a client VPN IP as the gateway/nexthop (i.e. ip route add a.b.c.d/24 via $VPNIP);
  • due to the point above, there is no real need to add a companion --route for each --iroute directive, unless you want to blackhole traffic when the specific client is not connected;
  • no internal routing is available. If you need truly internal routes, this can be achieved either with filtering using iptables or using ip rule;
  • client-to-client behaviour, as implemented in userspace, does not exist: packets always reach the tunnel interface and are then re-routed to the destination peer based on the system routing table.

Limitations by design

  • Layer 3 (dev tun) only;
  • only the following AEAD ciphers are currently supported: Chacha20-Poly1305 and AES-GCM-128/192/256;
  • no support for compression or compression framing:
    • see also the --compress migrate option to move to a setup without compression;
  • various features not implemented since they have better replacements:
    • --shaper, use tc instead;
    • packet manipulation, use nftables/iptables instead;
  • OpenVPN 2.4.0 is the minimum version required for peers to connect:
    • older versions are missing support for the AEAD ciphers;
  • topology subnet is the only supported --topology for servers;
  • iroute directives install routes on the host operating system, see also Routing with ovpn-dco;
  • (ovpn-dco-win) client and p2p mode only;
  • (ovpn-dco-win) Chacha20-Poly1305 support available starting with Windows 11.

Current implementation limitations

  • --persist-tun not tested;
  • IPv6 mapped IPv4 addresses need Linux 5.4.189+/5.10.110+/5.12+ to work;
  • some incompatible options may not properly fallback to non-dco;
  • no per client statistics. Only total statistics available on the interface.