1SYNCTHING-LOCALDISCO(7)            Syncthing           SYNCTHING-LOCALDISCO(7)
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NAME

6       syncthing-localdisco - Local Discovery Protocol v4
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MODE OF OPERATION

9       Each participating device periodically sends an Announcement packet. It
10       also keeps a table of the announcements it has seen. There is no way to
11       solicit a reply; the only message type is Announcement.
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13       On multihomed hosts the announcement packets should be sent on each in‐
14       terface on which Syncthing will accept connections.
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16       The announcement packet is sent over UDP.
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18       For IPv4, the Announcement packet is broadcast either to the  link-spe‐
19       cific broadcast address, or to the generic link-local broadcast address
20       255.255.255.255, with destination port 21027.
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22       For IPv6,  the  Announcement  packet  is  multicast  to  the  transient
23       link-local multicast address ff12::8384, with destination port 21027.
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25       It  is recommended that local discovery Announcement packets be sent on
26       a 30 to 60 second interval, possibly with immediate transmissions  when
27       a  previously  unknown  device  is discovered or a device has restarted
28       (see the instance_id field).
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DEVICE ID

31       The device ID is the SHA-256 (32 bytes) of the  device  X.509  certifi‐
32       cate. See device-ids in the Syncthing documentation.
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ANNOUNCEMENT PACKET

35       The Announcement packet has the following structure:
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37           0                   1                   2                   3
38           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
39          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
40          |                             Magic                             |
41          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
42          /                                                               /
43          \                       Announce Message                        \
44          /                                                               /
45          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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47       There  is no explicit length field as the length is given by the length
48       of the discovery announcement packet itself.
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50       The Magic field is a 32 bit word  representing  0x2EA7D90B  in  network
51       (big  endian) byte order. It identifies the packet as being a Syncthing
52       discovery protocol packet.
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54       The Announce Message contents are in protocol buffer format  using  the
55       following schema:
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57          message Announce {
58              bytes           id          = 1;
59              repeated string addresses   = 2;
60              int64           instance_id = 3;
61          }
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63       The id field contains the Device ID of the sending device.
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65       The  addresses  field contains a list of addresses where the device can
66       be contacted. Direct connections will typically have the tcp:// scheme.
67       Relay connections will typically use the relay:// scheme.
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69       When   interpreting   addresses  with  an  unspecified  address,  e.g.,
70       tcp://0.0.0.0:22000 or tcp://:42424, the source address of the  discov‐
71       ery announcement is to be used.
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73       The  instance_id  field  is  set  to  a randomly generated ID at client
74       startup. Other devices on the network can detect a change  in  instance
75       ID  between  two  announces and conclude that the announcing device has
76       restarted.
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AUTHOR

79       The Syncthing Authors
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82       2014-2019, The Syncthing Authors
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87v1.22.2                          Dec 29, 2022          SYNCTHING-LOCALDISCO(7)
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