Local Discovery Protocol v4

Mode of Operation

Each participating device periodically sends an Announcement packet. It also keeps a table of the announcements it has seen. There is no way to solicit a reply; the only message type is Announcement.

On multihomed hosts the announcement packets should be sent on each interface on which Syncthing will accept connections.

The announcement packet is sent over UDP.

For IPv4, the Announcement packet is broadcast either to the link-specific broadcast address, or to the generic link-local broadcast address 255.255.255.255, with destination port 21027.

For IPv6, the Announcement packet is multicast to the transient link-local multicast address ff12::8384, with destination port 21027.

It is recommended that local discovery Announcement packets be sent on a 30 to 60 second interval, possibly with immediate transmissions when a previously unknown device is discovered or a device has restarted (see the instance_id field).

Device ID

The device ID is the SHA-256 (32 bytes) of the device X.509 certificate. See Understanding Device IDs in the Syncthing documentation.

Announcement packet

The Announcement packet has the following structure:

  1.  0                   1                   2                   3
  2.  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
  3. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  4. |                             Magic                             |
  5. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  6. /                                                               /
  7. \                       Announce Message                        \
  8. /                                                               /
  9. +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

There is no explicit length field as the length is given by the length of the discovery announcement packet itself.

The Magic field is a 32 bit word representing 0x2EA7D90B in network (big endian) byte order. It identifies the packet as being a Syncthing discovery protocol packet.

The Announce Message contents are in protocol buffer format using the following schema:

  1. message Announce {
  2.     bytes           id          = 1;
  3.     repeated string addresses   = 2;
  4.     int64           instance_id = 3;
  5. }

The id field contains the Device ID of the sending device.

The addresses field contains a list of addresses where the device can be contacted. Direct connections will typically have the tcp:// scheme. Relay connections will typically use the relay:// scheme.

When interpreting addresses with an unspecified address, e.g., tcp://0.0.0.0:22000 or tcp://:42424, the source address of the discovery announcement is to be used.

The instance_id field is set to a randomly generated ID at client startup. Other devices on the network can detect a change in instance ID between two announces and conclude that the announcing device has restarted.