Data Channel Offload: the Linux Userspace API

This page describes the API used by OpenVPN in userspace to control the ovpn-dco Linux kernel module.

A similar API exists for Windows and FreeBSD, but due to technical differences they need to be documented separately. This being said, their abstraction remain the same so the code in userspace needs very little adjustments in order to properly operate on all platforms.

Please also note that this API is currently in a development stage, meaning that it may still change and (possibly) break compatibility with userspace. The goal is to freeze it and make it stable by the time ovpn-dco will be accepted upstream in the Linux kernel.

Netlink

The ovpn-dco userspace API is based on the Netlink protocol. Netlink allows userspace and kernel modules to communicate by exchanging family specific messages normally crafted using the TLV (Type Length Value) format. This peculiarity allows developers to not break the API upon adding new messages or new attributes.
You can read more about Netlink ​here.

ovpn-dco has identified itself using the 'ovpn-dco' netlink family until version v0.1.20230206 and has switched to the new 'ovpn-dco-v2' family since version v0.2.20230313.
This change was required because the two versions are not compatible with each other as a breaking change was implemented.

NOTE: this breaking events are not expected to happen often, and actually we have planned for another similar change once ovpn-dco is (hopefully) merged with the upstream linux kernel codebase.

API

The ovpn-dco Netlink API is composed by a set of commands aimed at managing the main objects living in kernel space: peers and keys.
Since the whole data channel processing happens in kernel space, ovpn-dco needs to be aware of all the needed details so that it can operate independently from userspace.

Peer handling

The following commands are used to create, manage and destroy a peer in kernel space. Creating a peer is an essential step in order to enable sending and receiving data packets to/from it.

OVPN_CMD_NEW_PEER

Inform ovpn-dco about a new peer.

OVPN_CMD_SET_PEER

Configure peer parameters.

OVPN_CMD_GET_PEER

Retrieve the whole list of connected peers with their status. It is possible to limit the retrieval to one peer only by specifying its ID.

OVPN_CMD_DEL_PEER

Delete peer from ovpn-dco data structures.

Key handling

The following commands are used to create, swap and delete primary and secondary keys for a specific peer. This means that a peer must be created before adding a new key.
A key comes with its own cipher, therefore, it is possible to use different ciphers for each peer and, possibly, switch cipher for a certain peer at runtime (not tested).

OVPN_CMD_NEW_KEY

Add a new encryption/decryption key pair for a specific peer

OVPN_CMD_SWAP_KEYS

Swap primary and secondary keys for a specific peer.

OVPN_CMD_DEL_KEY

Erase the key from the given slot socket for a specific peer.

Events (from kernel to userspace)

OVPN_CMD_DEL_PEER

Inform userspace that a peer has been deleted.