bionic (7) syncthing-localdisco.7.gz

Provided by: syncthing_0.14.43+ds1-6_amd64 bug

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


       2015, The Syncthing Authors