NAME

       syncthing-localdisco - 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 device-ids in
       the Syncthing documentation.

ANNOUNCEMENT PACKET

       The Announcement packet has the following 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                             |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          /                                                               /
          \                       Announce Message                        \
          /                                                               /
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       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:

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

       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.

AUTHOR

       The Syncthing Authors

COPYRIGHT

       2014-2019, The Syncthing Authors