Ephemeral Ports

Binding to port 0 asks the OS to pick a free port from its ephemeral range, which removes hard-coded ports and the race of “find a free port, then hope it stays free” — the kernel reserves it atomically during bind.

Read the assigned port back with getsockname() after binding, then hand it to whoever needs to connect.

Python

import socket
 
with socket.socket() as sock:
    sock.bind(("127.0.0.1", 0))
    port = sock.getsockname()[1]

Closing the socket before reusing the port reintroduces the race — another process can grab it in between. Keep the socket open and pass it (or its file descriptor) to the server whenever possible.

Many servers accept 0 directly:

python3 -m http.server 0 --bind 127.0.0.1 # prints the chosen port on startup

ZMQ

Use bind_to_random_port or the * wildcard, then read LAST_ENDPOINT (see zmq).

port = socket.bind_to_random_port("tcp://127.0.0.1")
 
socket.bind("tcp://127.0.0.1:*")
endpoint = socket.getsockopt_string(zmq.LAST_ENDPOINT)

Ephemeral range

The range is OS-specific; check it before firewalling or when you hit exhaustion.

sysctl net.inet.ip.portrange.first net.inet.ip.portrange.last # macOS: 49152-65535
cat /proc/sys/net/ipv4/ip_local_port_range # Linux: 32768-60999 by default

Find which process got the port with lsof.

Cheatsheet

sock.bind(("127.0.0.1", 0)); sock.getsockname()[1] # plain socket
socket.bind_to_random_port("tcp://127.0.0.1") # ZMQ

python3 -m http.server 0 # let the OS choose
lsof -nP -i tcp:<port> # who owns the port