Syncthing Relay Server

Synopsis

strelaysrv [-debug] [-ext-address=<address>] [-global-rate=<bytes/s>] [-keys=<dir>] [-listen=<listen addr>]
           [-message-timeout=<duration>] [-nat] [-nat-lease=<duration> [-nat-renewal=<duration>]
           [-nat-timeout=<duration>] [-network-timeout=<duration>] [-per-session-rate=<bytes/s>]
           [-ping-interval=<duration>] [-pools=<pool addresses>] [-protocol=<string>] [-provided-by=<string>]
           [-status-srv=<listen addr>]

Description

Syncthing relies on a network of community-contributed relay servers. Anyone can run a relay server, and it will automatically join the relay pool and be available to Syncthing users. The current list of relays can be found at http://relays.syncthing.net/.

Options

-debug

Enable debug output.

-ext-address=<address>

An optional address to advertising as being available on. Allows listening on an unprivileged port with port forwarding from e.g. 443, and be connected to on port 443.

-global-rate=<bytes/s>

Global rate limit, in bytes/s.

-keys=<dir>

Directory where cert.pem and key.pem is stored (default ”.”).

-listen=<listen addr>

Protocol listen address (default ”:22067”).

-message-timeout=<duration>

Maximum amount of time we wait for relevant messages to arrive (default 1m0s).

-nat

Use UPnP/NAT-PMP to acquire external port mapping

-nat-lease=<duration>

NAT lease length in minutes (default 60)

-nat-renewal=<duration>

NAT renewal frequency in minutes (default 30)

-nat-timeout=<duration>

NAT discovery timeout in seconds (default 10)

-network-timeout=<duration>

Timeout for network operations between the client and the relay. If no data is received between the client and the relay in this period of time, the connection is terminated. Furthermore, if no data is sent between either clients being relayed within this period of time, the session is also terminated. (default 2m0s)

-per-session-rate=<bytes/s>

Per session rate limit, in bytes/s.

-ping-interval=<duration>

How often pings are sent (default 1m0s).

-pools=<pool addresses>

Comma separated list of relay pool addresses to join (default “http://relays.syncthing.net/endpoint”). Blank to disable announcement to a pool, thereby remaining a private relay.

-protocol=<string>

Protocol used for listening. ‘tcp’ for IPv4 and IPv6, ‘tcp4’ for IPv4, ‘tcp6’ for IPv6 (default “tcp”).

-provided-by=<string>

An optional description about who provides the relay.

-status-srv=<listen addr>

Listen address for status service (blank to disable) (default ”:22070”).

Setting Up

Primarily, you need to decide on a directory to store the TLS key and certificate and a listen port. The default listen port of 22067 works, but for optimal compatibility a well known port for encrypted traffic such as 443 is recommended. This may require additional setup to work without running as root or a privileged user, see Running on port 443 as an unprivileged user below. In principle something similar to this should work on a Linux/Unix system:

$ sudo useradd relaysrv
$ sudo mkdir /etc/relaysrv
$ sudo chown relaysrv /etc/relaysrv
$ sudo -u relaysrv /usr/local/bin/relaysrv -keys /etc/relaysrv

This creates a user relaysrv and a directory /etc/relaysrv to store the keys. The keys are generated on first startup. The relay will join the global relay pool, unless a -pools="" argument is given.

To make the relay server start automatically at boot, use the recommended procedure for your operating system.

Client configuration

Syncthing can be configured to use specific relay servers (exclusively of the public pool) by adding the required servers to the Sync Protocol Listen Address field, under Actions and Settings. The format is as follows:

relay://<host name|IP>[:port]/?id=<relay device ID>

For example:

relay://private-relay-1.example.com:443/?id=ITZRNXE-YNROGBZ-HXTH5P7-VK5NYE5-QHRQGE2-7JQ6VNJ-KZUEDIU-5PPR5AM

The relay’s device ID is output on start-up.

Running on port 443 as an unprivileged user

It is recommended that you run the relay on port 443 (or another port which is commonly allowed through corporate firewalls), in order to maximise the chances that people are able to connect. However, binding to ports below 1024 requires root privileges, and running a relay as root is not recommended. Thankfully there are a couple of approaches available to you.

One option is to run the relay on port 22067, and use an iptables rule to forward traffic from port 443 to port 22067, for example:

iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 443 -j REDIRECT --to-port 22067

Or, if you’re using ufw, add the following to /etc/ufw/before.rules:

*nat
:PREROUTING ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]

-A PREROUTING -i eth0 -p tcp --dport 443 -j REDIRECT --to-port 22067

COMMIT

You will need to start relaysrv with -ext-address ":443". This tells relaysrv that it can be contacted on port 443, even though it is listening on port 22067. You will also need to let both port 443 and 22067 through your firewall.

Another option is described here, although your milage may vary.

Firewall Considerations

The relay server listens on two ports by default. One for data connections and the other for providing public statistics at http://relays.syncthing.net/. The firewall, such as iptables, must permit incoming TCP connections to the following ports:

  • Data port: 22067/tcp overridden with -listen and advertised with -ext-address
  • Status port: 22070/tcp overridden with -status-srv

Runtime iptables rules to allow access to the default ports:

iptables -I INPUT -p tcp --dport 22067 -j ACCEPT
iptables -I INPUT -p tcp --dport 22070 -j ACCEPT

Please consult Linux distribution documentation to persist firewall rules.

See Also

syncthing-relay(7), syncthing-faq(7), syncthing-networking(7)