ss — Check sockets and ports

💬 "ss command"

ss quickly shows open ports and connection state. It replaces netstat.

Use it to check whether a service is holding a port and who is holding it.

• Concept: Queries the state and addresses of TCP/UDP sockets.
• LISTEN: A server socket waiting for connections.
• Local address: 0.0.0.0:80 means all interfaces, and 127.0.0.1:80 means local only.
• Process: Use -p to show the process holding the port.

Commonly used options

3 essential command combinations

• ss -lntp — Check TCP LISTEN ports and processes
• ss -lntp | grep ':80' — Check which process is holding port 80
• ss -s — Socket state summary statistics

💬 "Tell me a useful way to use this when digging through logs"

Here are common situations on live servers, organized in order.

Step 1 · Check LISTEN

Check whether the service is listening on the expected port.

ss -lntp

Step 2 · Check a specific port

Check for conflicts.

ss -lntp | grep -E ':(80|443|8080)\b'

Step 3 · Check connection state

Check the distribution of states such as TIME_WAIT.

ss -s

In practice, use it like this

1. Port conflict

Situation — A new service fails to bind to 8080.

ss -lntp | grep ':8080'

Result — If another process is already holding it, its PID and name are shown.

Point — Identify the conflicting process and stop it or change the port.

2. Diagnosing blocked external access

Situation — The firewall is open but connections still fail.

ss -lntp | grep ':443'

Result — If it is only listening on 127.0.0.1:443, external clients cannot connect.

Point — Always check the bind address (0.0.0.0 vs 127.0.0.1).

Things to keep in mind

• -p may require sudo because it needs privileges to view other users' socket information.
• Name resolution (without -n) can cause DNS delays, so -n is recommended.
• Checking only LISTEN status does not guarantee that traffic passes through the firewall.

Actual execution output

This is the output after checking LISTEN ports and processes.

!ss command output: LISTEN ports and processes

Live production server

Source (HTML): https://games.aidebatehub.com/programs/linux/network/ss.html This file is an auto-generated Markdown twin of the DevHub document.