Check Open Ports on Linux

What an open port is

A port is a numbered door for network traffic. A service listens on a port and waits for connections. A web server uses port 80 and 443, SSH uses 22.

• TCP checks that every packet arrives, in the right order.
• UDP is fast but does not check delivery. DNS and streaming use it.

Check your open ports from time to time. Close the ones you do not need.

Method 1: ss

ss is the modern replacement of netstat.

List all listening ports

ss -tuln

-t is TCP, -u is UDP, -l is listening, -n shows numbers and not names.

Show the program for each port

sudo ss -tulnp

Only TCP or only UDP

ss -tln
ss -uln

Method 2: lsof

All listening ports

sudo lsof -i -P -n

One port

sudo lsof -i :80

Only TCP listeners

sudo lsof -iTCP -sTCP:LISTEN -P -n

Only UDP

sudo lsof -iUDP -P -n

Method 3: netstat

If netstat is missing, install the net-tools package.

netstat -tuln

Add -p to see the program names (needs sudo):

sudo netstat -tulnp

Method 4: nmap

nmap scans a host from the outside view. Install it:

sudo apt install nmap

Scan your own machine:

nmap localhost

Check one port:

nmap -p 80 localhost

Quick recap

• sudo ss -tulnp is the best first command.
• lsof -i :PORT shows who uses one port.
• nmap shows what an outside scan sees.