syncthing/man/syncthing-localdisco.7

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.TH "SYNCTHING-LOCALDISCO" "7" "September 08, 2017" "v0.14" "Syncthing"
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.SH NAME
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syncthing-localdisco \- Local Discovery Protocol v4
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.SH MODE OF OPERATION
.sp
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.
.sp
On multihomed hosts the announcement packets should be sent on each interface
on which Syncthing will accept connections.
.sp
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The announcement packet is sent over UDP.
.sp
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For IPv4, the Announcement packet is broadcast either to the link\-specific
broadcast address, or to the generic link\-local broadcast address
\fB255.255.255.255\fP, with destination port 21027.
.sp
For IPv6, the Announcement packet is multicast to the transient link\-local
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multicast address \fBff12::8384\fP, with destination port 21027.
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.sp
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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
\fBinstance_id\fP field).
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.SH DEVICE ID
.sp
The device ID is the SHA\-256 (32 bytes) of the device X.509 certificate. See
device\-ids in the Syncthing documentation.
.SH ANNOUNCEMENT PACKET
.sp
The Announcement packet has the following structure:
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
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 |
+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+
/ /
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\e Announce Message \e
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/ /
+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+\-+
.ft P
.fi
.UNINDENT
.UNINDENT
.sp
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There is no explicit length field as the length is given by the length of
the discovery announcement packet itself.
.sp
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.
.sp
The Announce Message contents are in protocol buffer format using the
following schema:
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.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
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message Announce {
bytes id = 1;
repeated string addresses = 2;
int64 instance_id = 3;
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}
.ft P
.fi
.UNINDENT
.UNINDENT
.sp
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The \fBid\fP field contains the Device ID of the sending device.
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.sp
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The \fBaddresses\fP field contains a list of addresses where the device can be
contacted. Direct connections will typically have the \fBtcp://\fP scheme.
Relay connections will typically use the \fBrelay://\fP scheme.
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.sp
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When interpreting addresses with an unspecified address, e.g.,
\fBtcp://0.0.0.0:22000\fP or \fBtcp://:42424\fP, the source address of the
discovery announcement is to be used.
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.sp
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The \fBinstance_id\fP 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.
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.SH AUTHOR
The Syncthing Authors
.SH COPYRIGHT
2015, The Syncthing Authors
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