Provided by: syncthing_1.27.7~ds1-5_amd64 
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 either ResponseNotFound if the device the client it is after is not
available, 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 Response messages:
1. ResponseNotFound - Session key is invalid
2. ResponseAlreadyConnected - Session is full (both sides already connected)
3. ResponseSuccess - You have successfully joined the session
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 - Permanent protocol mode Client B - Temporary protocol 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.
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.
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
2014-2019, The Syncthing Authors