magic-wormhole-transit-relay/docs/running.md

Running the Transit Relay

First off, you probably don't need to run a relay. The wormhole command, as shipped from magic-wormhole.io, is configured to use a default Transit Relay operated by the author of Magic-Wormhole. This can be changed with the --transit-helper= argument, and other applications that import the Wormhole library might point elsewhere.

The only reasons to run a separate relay are:

• You are a kind-hearted server admin who wishes to support the project by paying the bandwidth costs incurred by your friends, who you instruct in the use of --transit-helper=.
• You publish a different application, and want to provide your users with a relay that fails at different times than the official one

Installation

To run a transit relay, first you need an environment to install it.

• create a virtualenv
• pip install magic-wormhole-transit-relay into this virtualenv

% virtualenv tr-venv
...
% tr-venv/bin/pip install magic-wormhole-transit-relay
...

Running

The transit relay is not a standalone program: rather it is a plugin for the Twisted application-running tools named twist (which only runs in the foreground) and twistd (which daemonizes). To run the relay for testing, use something like this:

% tr-venv/bin/twist transitrelay [ARGS]
2017-11-09T17:07:28-0800 [-] not blurring access times
2017-11-09T17:07:28-0800 [-] Transit starting on 4001
2017-11-09T17:07:28-0800 [wormhole_transit_relay.transit_server.Transit#info] Starting factory <wormhole_transit_relay.transit_server.Transit object at 0x7f01164b4550>
...

The relevant arguments are:

• --port=: the endpoint to listen on, like tcp:4001
• --log-fd=: writes JSON lines to the given file descriptor for each connection
• --usage-db=: maintains a SQLite database with current and historical usage data
• --blur-usage=: if provided, logs are rounded to the given number of seconds, and data sizes are rounded too

When you use twist, the relay runs in the foreground, so it will generally exit as soon as the controlling terminal exits. For persistent environments, you should daemonize the server.

Daemonization

A production installation will want to daemonize the server somehow. One option is to use twistd (the daemonizing version of twist). This takes the same plugin name and arguments as twist, but forks into the background, detaches from the controlling terminal, and writes all output into a logfile:

% tr-venv/bin/twistd transitrelay [ARGS]
% cat twistd.log
2017-11-09T17:07:28-0800 [-] not blurring access times
2017-11-09T17:07:28-0800 [-] Transit starting on 4001
2017-11-09T17:07:28-0800 [wormhole_transit_relay.transit_server.Transit#info] Starting factory <wormhole_transit_relay.transit_server.Transit object at 0x7f01164b4550>
...
% cat twistd.pid; echo
18985

To shut down a twistd-based server, you'll need to look in the twistd.pid file for the process id, and kill it:

% kill `cat twistd.pid`

To start the server each time the host reboots, you might use a crontab "@reboot" job, or a systemd unit.

Another option is to run twist underneath a daemonization tool like daemontools or start-stop-daemon. Since twist is just a regular program, this leaves the daemonization tool in charge of issues like restarting a process that exits unexpectedly, limiting the rate of respawning, and switching to the correct user-id and base directory.

Packagers who create an installable transit-relay server package should choose a suitable daemonization tool that matches the practices of the target operating system. For example, Debian/Ubuntu packages should probably include a systemd unit that runs twist transitrelay in some /var/run/magic-wormhole-transit-relay/ directory.

Production environments that want to monitor the server for capacity management can use the --log-fd= option to emit logs, then route those logs into a suitable analysis tool. Other environments might be content to use --usage-db= and run the included Munin plugins to monitor usage.

Configuring Clients

The transit relay will listen on an "endpoint" (usually a TCP port, but it could be a unix-domain socket or any other Endpoint that Twisted knows how to listen on). By default this is tcp:4001. The relay does not know what hostname or IP address might point at it.

Clients are configured with a "Transit Helper" setting that includes both the hostname and the port number, like the default tcp:transit.magic-wormhole.io:4001. The standard wormhole tool takes a --transit-helper= argument to override this. Other applications that use wormhole as a library will have internal means to configure which transit relay they use.

If you run your own transit relay, you will need to provide the new settings to your clients for it to be used.

The standard wormhole tool is used by two sides: the sender and the receiver. Both sides exchange their configured transit relay with their partner. So if the sender overrides --transit-helper= but the receiver does not, they might wind up using either relay server, depending upon which one gets an established connection first.