ephemeral-port-reserve

Sometimes you need a networked program to bind to a port that can't be hard-coded. Generally this is when you want to run several of them in parallel; if they all bind to port 8080, only one of them can succeed.

The usual solution is the "port 0 trick". If you bind to port 0, your kernel will find some arbitrary high-numbered port that's unused and bind to that. Afterward you can query the actual port that was bound to if you need to use the port number elsewhere. However, there are cases where the port 0 trick won't work. For example, mysqld takes port 0 to mean "the port configured in my.cnf". Docker can bind your containers to port 0, but uses its own implementation to find a free port which races and fails in the face of parallelism.

ephemeral-port-reserve provides an implementation of the port 0 trick which is reliable and race-free. You can use it like so:

PORT="$(ephemeral-port-reserve)"
docker run -p 127.0.0.1:$PORT:5000 registry:2

ephemeral-port-reserve is a utility to bind to an ephemeral port, force it into the TIME_WAIT state, and unbind it.

This means that further ephemeral port alloctions won't pick this "reserved" port, but subprocesses can still bind to it explicitly, given that they use SO_REUSEADDR. By default on linux you have a grace period of 60 seconds to reuse this port. To check your own particular value:

$ cat /proc/sys/net/ipv4/tcp_fin_timeout
60

NOTE: By default, the port returned is specifically for localhost, aka 127.0.0.1. If you bind instead to 0.0.0.0, you may encounter a port conflict. If you need to bind to a non-localhost IP, you can pass it as the first argument.