Pushing IETF standard

[lauri]

Hello, I've been a long time OpenVPN user and trainer. I've been digging around and I haven't found any confirmation about whether there are any plans for standardize the OpenVPN protocol that runs inside the TLS tunnel? If no, could you also list some concrete reasons? If you're part of the company behind OpenVPN please also clearly indicate so

I am also curious about CLA, do you have one and where could I find it? OpenVPN is licensed under GPL license but who actually owns the copyright?

[jamesyonan]

We are planning to create an RFC detailing the OpenVPN protocol. For now, the protocol details are spelled out in the OpenVPN package documentation.

Re: CLA, contributing to OpenVPN 2.x does not require a CLA. With OpenVPN 3 however, we are introducing a CLA.

Protocol details are below.

Code: Select all

/*
 * OpenVPN Protocol, taken from ssl.h in OpenVPN source code.
 *
 * TCP/UDP Packet:  This represents the top-level encapsulation.
 *
 * TCP/UDP packet format:
 *
 *   Packet length (16 bits, unsigned) -- TCP only, always sent as
 *       plaintext.  Since TCP is a stream protocol, the packet
 *       length words define the packetization of the stream.
 *
 *   Packet opcode/key_id (8 bits) -- TLS only, not used in
 *       pre-shared secret mode.
 *            packet message type, a P_* constant (high 5 bits)
 *            key_id (low 3 bits, see key_id in struct tls_session
 *              below for comment).  The key_id refers to an
 *              already negotiated TLS session.  OpenVPN seamlessly
 *              renegotiates the TLS session by using a new key_id
 *              for the new session.  Overlap (controlled by
 *              user definable parameters) between old and new TLS
 *              sessions is allowed, providing a seamless transition
 *              during tunnel operation.
 *
 *   Payload (n bytes), which may be a P_CONTROL, P_ACK, or P_DATA
 *       message.
 *
 * Message types:
 *
 *  P_CONTROL_HARD_RESET_CLIENT_V1 -- Key method 1, initial key from
 *    client, forget previous state.
 *
 *  P_CONTROL_HARD_RESET_SERVER_V1 -- Key method 2, initial key
 *    from server, forget previous state.
 *
 *  P_CONTROL_SOFT_RESET_V1 -- New key, with a graceful transition
 *    from old to new key in the sense that a transition window
 *    exists where both the old or new key_id can be used.  OpenVPN
 *    uses two different forms of key_id.  The first form is 64 bits
 *    and is used for all P_CONTROL messages.  P_DATA messages on the
 *    other hand use a shortened key_id of 3 bits for efficiency
 *    reasons since the vast majority of OpenVPN packets in an
 *    active tunnel will be P_DATA messages.  The 64 bit form
 *    is referred to as a session_id, while the 3 bit form is
 *    referred to as a key_id.
 *
 *  P_CONTROL_V1 -- Control channel packet (usually TLS ciphertext).
 *
 *  P_ACK_V1 -- Acknowledgement for P_CONTROL packets received.
 *
 *  P_DATA_V1 -- Data channel packet containing actual tunnel data
 *    ciphertext.
 *
 *  P_CONTROL_HARD_RESET_CLIENT_V2 -- Key method 2, initial key from
 *   client, forget previous state.
 *
 *  P_CONTROL_HARD_RESET_SERVER_V2 -- Key method 2, initial key from
 *   server, forget previous state.
 *
 * P_CONTROL* and P_ACK Payload:  The P_CONTROL message type
 * indicates a TLS ciphertext packet which has been encapsulated
 * inside of a reliability layer.  The reliability layer is
 * implemented as a straightforward ACK and retransmit model.
 *
 * P_CONTROL message format:
 *
 *   local session_id (random 64 bit value to identify TLS session).
 *   HMAC signature of entire encapsulation header for integrity
 *       check if --tls-auth is specified (usually 16 or 20 bytes).
 *   packet-id for replay protection (4 or 8 bytes, includes
 *       sequence number and optional time_t timestamp).
 *   P_ACK packet_id array length (1 byte).
 *   P_ACK packet-id array (if length > 0).
 *   P_ACK remote session_id (if length > 0).
 *   message packet-id (4 bytes).
 *   TLS payload ciphertext (n bytes) (only for P_CONTROL).
 *
 * Once the TLS session has been initialized and authenticated,
 * the TLS channel is used to exchange random key material for
 * bidirectional cipher and HMAC keys which will be
 * used to secure actual tunnel packets.  OpenVPN currently
 * implements two key methods.  Key method 1 directly
 * derives keys using random bits obtained from the RAND_bytes
 * OpenSSL function.  Key method 2 mixes random key material
 * from both sides of the connection using the TLS PRF mixing
 * function.  Key method 2 is the preferred method and is the default
 * for OpenVPN 2.0.
 * 
 * TLS plaintext content:
 *
 * TLS plaintext packet (if key_method == 1):
 *
 *   Cipher key length in bytes (1 byte).
 *   Cipher key (n bytes).
 *   HMAC key length in bytes (1 byte).
 *   HMAC key (n bytes).
 *   Options string (n bytes, null terminated, client/server options
 *       string should match).
 *
 * TLS plaintext packet (if key_method == 2):
 *
 *   Literal 0 (4 bytes).
 *   key_method type (1 byte).
 *   key_source structure (pre_master only defined for client ->
 *       server).
 *   options_string_length, including null (2 bytes).
 *   Options string (n bytes, null terminated, client/server options
 *       string must match).
 *   [The username/password data below is optional, record can end
 *       at this point.]
 *   username_string_length, including null (2 bytes).
 *   Username string (n bytes, null terminated).
 *   password_string_length, including null (2 bytes).
 *   Password string (n bytes, null terminated).
 *
 * The P_DATA payload represents encrypted, encapsulated tunnel
 * packets which tend to be either IP packets or Ethernet frames.
 * This is essentially the "payload" of the VPN.
 *
 * P_DATA message content:
 *   HMAC of ciphertext IV + ciphertext (if not disabled by
 *       --auth none).
 *   Ciphertext IV (size is cipher-dependent, if not disabled by
 *       --no-iv).
 *   Tunnel packet ciphertext.
 *
 * P_DATA plaintext
 *   packet_id (4 or 8 bytes, if not disabled by --no-replay).
 *       In SSL/TLS mode, 4 bytes are used because the implementation
 *       can force a TLS renegotation before 2^32 packets are sent.
 *       In pre-shared key mode, 8 bytes are used (sequence number
 *       and time_t value) to allow long-term key usage without
 *       packet_id collisions.
 *   User plaintext (n bytes).
 *
 * Notes:
 *   (1) ACK messages can be encoded in either the dedicated
 *       P_ACK record or they can be prepended to a P_CONTROL message.
 *   (2) P_DATA and P_CONTROL/P_ACK use independent packet-id
 *       sequences because P_DATA is an unreliable channel while
 *       P_CONTROL/P_ACK is a reliable channel.  Each use their
 *       own independent HMAC keys.
 *   (3) Note that when --tls-auth is used, all message types are
 *       protected with an HMAC signature, even the initial packets
 *       of the TLS handshake.  This makes it easy for OpenVPN to
 *       throw away bogus packets quickly, without wasting resources
 *       on attempting a TLS handshake which will ultimately fail.
 */