How to Quickly Identify and Resolve “Address Already in Use” Errors on Linux

When you encounter the "Address already in use" message, you need to find out which service is listening on the conflicting port. This article explains how to list listening ports on Linux/macOS using netstat, ss, and lsof.

What is a listening port

A network port is identified by its number, associated IP address, and protocol (TCP or UDP). A listening port is a port on which an application or process waits for incoming connections, acting as a communication endpoint. Firewalls can open or close ports, and a port cannot be used by two services simultaneously.

Using netstat to check listening ports

netstat

is a command‑line tool that provides information about network connections. To list all listening TCP and UDP ports with the owning processes, run: sudo netstat -tunlp The options mean: -t – show TCP ports -u – show UDP ports -n – display numeric addresses -l – show only listening sockets -p – show PID and program name (requires root)

Sample output:

Proto Recv-Q Send-Q Local Address   Foreign Address     State       PID/Program name
tcp        0      0 0:22               0:*               LISTEN       445/sshd
tcp        0      0 0:25               0:*               LISTEN       929/master
tcp6       0      0 :::3306            :::*               LISTEN       534/mysqld
tcp6       0      0 :::80              :::*               LISTEN       515/apache2

Key columns are Proto (protocol), Local Address (IP and port), and PID/Program name (process identifier).

You can filter the result with grep. For example, to find the process listening on TCP port 22: sudo netstat -tnlp | grep :22 If the output is empty, no process is listening on that port. Note that netstat is considered deprecated and has been superseded by ss and ip, though it remains widely used.

Using ss

ss

is a newer alternative to netstat. It provides similar information with a faster execution speed. To list all listening ports, run: sudo ss -tunlp The output format is almost identical to netstat:

State    Recv-Q Send-Q Local Address:Port Peer Address:Port
LISTEN   0      128    0:22               0:*   users:("sshd",pid=445,fd=3)
LISTEN   0      128    *:3306             *:*   users:("mysqld",pid=534,fd=30)
LISTEN   0      128    *:80               *:*   users:("apache2",pid=515,fd=4)

Using lsof

lsof

lists open files, and because sockets are treated as files in Linux, it can show listening ports. To list all listening TCP ports:

sudo lsof -nP -iTCP -sTCP:LISTEN

-n

– do not resolve hostnames -P – do not convert port numbers to names -iTCP – select TCP sockets -sTCP:LISTEN – show only sockets in LISTEN state

Sample output:

COMMAND   PID USER   FD TYPE DEVICE SIZE/OFF NODE NAME
sshd      445 root   3u  IPv4 16434      0t0  TCP *:22 (LISTEN)
apache2   515 root   4u  IPv6 16590      0t0  TCP *:80 (LISTEN)
mysqld    534 mysql  30u IPv6 17636      0t0  TCP *:3306 (LISTEN)

To find the process using a specific port, e.g., 3306: sudo lsof -nP -iTCP:3306 -sTCP:LISTEN Output shows that the MySQL server is listening on port 3306.