SYNCTHING-RELAY(7)

NAME
syncthing-relay - Relay Protocol v1

WHAT IS A RELAY?

Relay is a service which relays data between two devices which are not able to connect to each other directly otherwise. This is usually due to both devices being behind a NAT and neither side being able to open a port which would be directly accessible from the internet.

A relay was designed to relay BEP protocol, hence the reliance on device ID’s in the protocol spec, but at the same time it is general enough that could be reused by other protocols or applications, as the data transferred between two devices which use a relay is completely obscure and does not affect the relaying.

OPERATION MODES

Relay listens on a single TCP socket, but has two different connection modes, where a connection mode is a predefined set of messages which the relay and the device are expecting to exchange.

The first mode is the protocol mode which allows a client to interact with the relay, for example join the relay, or request to connect to a device, given it is available on the relay. Similarly to BEP, protocol mode requires the device to connect via TLS using a strong suite of ciphers (same as BEP), which allows the relay to verify and derive the identity (Device ID) of the device.

The second mode is the session mode which after a few initial messages connects two devices directly to each other via the relay, and is a plain-text protocol, which for every byte written by one device, sends the same set of bytes to the other device and vica versa.

IDENTIFYING THE CONNECTION MODE

Because both connection modes operate over the same single socket, a method of detecting the connection mode is required.

When a new client connects to the relay, the relay checks the first byte that the client has sent, and if that matches 0x16, that implies to us that the connection is a protocol mode connection, due to 0x16 being the first byte in the TLS handshake, and only protocol mode connections use TLS.

If the first byte is not 0x16, then we assume that the connection is a session mode connection.

PROTOCOL MODE

Protocol mode uses TLS and protocol name defined by the TLS header should be bep-relay.

Protocol mode has two submodes: 1. Permanent protocol submode - Joining the relay, and waiting for messages from the relay asking to connect to some device which is interested in having a session with you. 2. Temporary protocol submode - Only used to request a session with a device which is connected to the relay using the permanent protocol submode.

  Permanent protocol submode

A permanent protocol submode begins with the client sending a JoinRelayRequest message, which the relay responds to with either a ResponseSuccess or ResponseAlreadyConnected message if a client with the same device ID already exists.

After the client has joined, no more messages are exchanged apart from Ping/Pong messages for general connection keep alive checking.

From this point onwards, the client stand-by’s and waits for SessionInvitation messages from the relay, which implies that some other device is trying to connect with you. SessionInvitation message contains the unique session key which then can be used to establish a connection in session mode.

If the client fails to send a JoinSessionRequest message within the first ping interval, the connection is terminated. If the client fails to send a message (even if it’s a ping message) every minute (by default), the connection is terminated.

  Temporary protocol submode

A temporary protocol submode begins with ConnectRequest message, to which the relay responds with ResponseNotFound if the device the client it is after is not available, with a RelayFull if the relay has reached its limits, or with a SessionInvitation, which contains the unique session key which then can be used to establish a connection in session mode.

The connection is terminated immediately after that.

  Example Exchange

Client A - Permanent protocol submode Client B - Temporary protocol submode
# | Client (A) | Relay | Client (B)
1 | JoinRelayRequest-> |  |
2 |  | <-ResponseSuccess |
3 | Ping-> |  |
4 |  | <-Pong |
5 |  |  | <-ConnectRequest(A)
6 |  | SessionInvitation(A)-> |
7 |  | <-SessionInvitation(B) |
8 |  |  | (Disconnects)
9 | Ping-> |  |
10 |  | <-Pong |
11 | Ping-> |  |
12 |  | <-Pong |

SESSION MODE

The first and only message the client sends in the session mode is the JoinSessionRequest message which contains the session key identifying which session you are trying to join. The relay responds with one of the following messages:

1.  ResponseNotFound - Session key is invalid

2.  ResponseAlreadyConnected - Session is full (both sides already connected)

3.  ResponseSuccess - You have successfully joined the session

4.  RelayFull - Relay limits are too strict for you to be able to join the session. The relay immediately terminates the connection after sending this.

    After the successful response, all the bytes written and received will be relayed between the two devices in the session directly.

  Example Exchange

Client A is the first to join the session.

Client B is the second to join the session.

Both are in session mode.
# | Client (A) | Relay | Client (B)
1 | JoinSessionRequest(A)-> |  |
2 |  | <-ResponseSuccess |
3 | Data-> | (Buffers data) |
4 | Data-> | (Buffers data) |
5 |  |  | <-JoinSessionRequest(B)
6 |  | ResponseSuccess-> |
7 |  | Relays data -> |
8 |  | Relays data -> |
9 |  | <-Relays data | <-Data

MESSAGES

All messages are preceded by a header message. Header message contains the magic value 0x9E79BC40, message type integer, and message length.

WARNING: Some messages have no content, apart from the implied header which allows us to identify what type of message it is.

  Header structure

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             Magic                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Message Type                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Message Length                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    struct Header {
            unsigned int Magic;
            int MessageType;
            int MessageLength;
    }

  Ping message (Type = 0)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    struct Ping {
    }

  Pong message (Type = 1)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    struct Pong {
    }

  JoinRelayRequest message (Type = 2)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    struct JoinRelayRequest {
    }

  JoinSessionRequest message (Type = 3)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Length of Key                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                                               /
    \                     Key (variable length)                     \
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    struct JoinSessionRequest {
            opaque Key<32>;
    }

: Key
    This is a unique random session key generated by the relay server. It is used to identify which session you are trying to connect to.

  Response message (Type = 4)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             Code                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Length of Message                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                                               /
    \                   Message (variable length)                   \
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    struct Response {
            int Code;
            string Message<>;
    }

: Code
    An integer representing the status code.

: Message
    Message associated with the code.

        ResponseSuccess           = Response{0, "success"}
        ResponseNotFound          = Response{1, "not found"}
        ResponseAlreadyConnected  = Response{2, "already connected"}
        ResponseInternalError     = Response{99, "internal error"}
        ResponseUnexpectedMessage = Response{100, "unexpected message"}

  ConnectRequest message (Type = 5)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Length of ID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                                               /
    \                     ID (variable length)                      \
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    struct ConnectRequest {
            opaque ID<32>;
    }

: ID
    Device ID to which the client would like to connect.

  SessionInvitation message (Type = 6)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Length of From                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                                               /
    \                    From (variable length)                     \
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Length of Key                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                                               /
    \                     Key (variable length)                     \
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Length of Address                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    /                                                               /
    \                   Address (variable length)                   \
    /                                                               /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            0x0000             |             Port              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                  Server Socket (V=0 or 1)                   |V|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    struct SessionInvitation {
            opaque From<32>;
            opaque Key<32>;
            opaque Address<32>;
            unsigned int Port;
            bool ServerSocket;
    }

: From
    Device ID identifying who you will be connecting with.

: Key
    A unique random session key generated by the relay server. It is used to identify which session you are trying to connect to.

: Address
    An optional IP address on which the relay server is expecting you to connect, in order to start a connection in session mode. Empty/all zero IP should be replaced with the relay’s public IP address that was used when establishing the protocol mode connection.

: Port
    The port on which the relay server is expecting you to connect, in order to start a connection in session mode.

: Server Socket
    Because both sides connecting to the relay use the client side of the socket, and some protocols behave differently depending if the connection starts on the server side or the client side, this boolean indicates which side of the connection this client should assume it’s getting. The value is inverted in the invitation which is sent to the other device, so that there is always one client socket, and one server socket.

  RelayFull message (Type = 7)

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    struct RelayFull {
    }

HOW SYNCTHING USES RELAYS, AND GENERAL SECURITY

In the case of Syncthing and BEP, when two devices connect via relay, they start their standard TLS connection encapsulated within the relay’s plain-text session connection, effectively upgrading the plain-text connection to a TLS connection.

Even though the relay could be used for man-in-the-middle attack, using TLS at the application/BEP level ensures that all the traffic is safely encrypted, and is completely meaningless to the relay. Furthermore, the secure suite of ciphers used by BEP provides forward secrecy, meaning that even if the relay did capture all the traffic, and even if the attacker did get their hands on the device keys, they would still not be able to recover/decrypt any traffic which was transported via the relay.

After establishing a relay session, Syncthing looks at the SessionInvitation message, and depending which side it has received, wraps the raw socket in either a TLS client socket or a TLS server socket depending on the ServerSocket boolean value in the SessionInvitation, and starts the TLS handshake.

From that point onwards it functions exactly the same way as if Syncthing was establishing a direct connection with the other device over the internet, performing device ID validation, and full TLS encryption, and provides the same security properties as it would provide when connecting over the internet.

EXAMPLES OF STRONG CIPHER SUITES
ID | Name | Description
0x009F | DHE-RSA-AES256-GCM-SHA384 | TLSv1.2 DH RSA AESGCM(256) AEAD
0x006B | DHE-RSA-AES256-SHA256 | TLSv1.2 DH RSA AES(256) SHA256
0xC030 | ECDHE-RSA-AES256-GCM-SHA384 | TLSv1.2 ECDH RSA AESGCM(256) AEAD
0xC028 | ECDHE-RSA-AES256-SHA384 | TLSv1.2 ECDH RSA AES(256) SHA384
0x009E | DHE-RSA-AES128-GCM-SHA256 | TLSv1.2 DH RSA AESGCM(128) AEAD
0x0067 | DHE-RSA-AES128-SHA256 | TLSv1.2 DH RSA AES(128) SHA256
0xC02F | ECDHE-RSA-AES128-GCM-SHA256 | TLSv1.2 ECDH RSA AESGCM(128) AEAD
0xC027 | ECDHE-RSA-AES128-SHA256 | TLSv1.2 ECDH RSA AES(128) SHA256

AUTHOR
The Syncthing Authors

COPYRIGHT
2014-2026, The Syncthing Authors