The Linux ss command shows socket statistics and active network connections. It is one of the fastest command-line tools for checking which ports are listening, which TCP or UDP connections exist, and which processes are using network sockets.

Basic command

ss

Running ss with no options displays open non-listening sockets, including established connections.

Show listening ports

ss -l

The -l option limits the output to listening sockets. This is useful when checking whether a server or service is actually waiting for connections.

TCP and UDP

ss -t
ss -u
ss -t -a
ss -u -a

-t selects TCP sockets, -u selects UDP sockets, and -a includes both listening and non-listening sockets.

A practical technician command

ss -tulpn

This combination is useful during Linux and data-center troubleshooting. It can show TCP and UDP sockets, listening ports, numeric addresses and ports, and process information. Process details may require elevated permissions depending on the system.

Why ss matters

If a web server, monitoring agent, database, mining service, or other network application appears unreachable, ss helps answer a basic question: is the service actually listening on the expected port? It can also help identify established connections and unexpected network listeners before deeper troubleshooting begins.

Quick practice

ss
ss -l
ss -t -a
ss -u -a
ss -tulpn

Compare the output from each command. Look for the socket type, state, local address and port, peer address and port, and process information when available.

Reference

The Linux ss(8) manual describes the command as a utility for investigating sockets and documents its TCP, UDP, listening, process, numeric-output, filtering, and socket-statistics options.

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