Who's Using My Port? Quickly Find & Fix 'Address Already in Use' on Linux

What Is a Listening Port

A network port is identified by its number, associated IP address, and communication protocol (TCP or UDP). A listening port is a network port on which an application or process listens, acting as a communication endpoint. Each listening port can be opened or closed via a firewall. You cannot have two services listening on the same port on the same IP address. For example, if Apache is already using ports 80 and 443, installing Nginx will fail because those ports are already in use.

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 along with the service using each port and the socket state, run: sudo netstat -tunlp The options mean: -t – Show TCP ports. -u – Show UDP ports. -n – Show numerical addresses instead of resolving hosts. -l – Show only listening ports. -p – Show the PID and name of the listener process (requires root/sudo).

Example output:

Proto Recv-Q Send-Q Local Address   Foreign Address   State    PID/Program name
 tcp        0      0 0.0.0.0:22      0.0.0.0:*         LISTEN   445/sshd
 tcp        0      0 0.0.0.0:25      0.0.0.0:*         LISTEN   929/master
 tcp6       0      0 :::3306         :::*              LISTEN   534/mysqld
 tcp6       0      0 :::80           :::*              LISTEN   515/apache2
 tcp6       0      0 :::22           :::*              LISTEN   445/sshd
 tcp6       0      0 :::25           :::*              LISTEN   929/master
 tcp6       0      0 :::33060        :::*              LISTEN   534/mysqld
 udp        0      0 0.0.0.0:68      0.0.0.0:*                 966/dhclient

Key columns: Proto – Protocol used by the socket. Local Address – IP address and port number the process is listening on. PID/Program name – PID and process name.

To filter results, pipe to grep. For example, to find the process listening on TCP port 22: sudo netstat -tnlp | grep :22 Output shows port 22 is used by the SSH server:

tcp        0      0 0.0.0.0:22      0.0.0.0:*         LISTEN   445/sshd
 tcp6       0      0 :::22           :::*              LISTEN   445/sshd

If output is empty, nothing is listening on that port. You can also filter by PID, protocol, state, etc. Note: netstat is deprecated and replaced by ss and ip, but remains widely used.

Using ss to Check Listening Ports

ss

is the modern replacement for netstat. It lacks some netstat features but exposes more TCP states and is slightly faster. Command options are nearly identical, making migration easy. To get a list of all listening ports: sudo ss -tunlp Output is almost identical to netstat:

State   Recv-Q  Send-Q  Local Address:Port  Peer Address:Port
 LISTEN  0       128     0.0.0.0:22         0.0.0.0:*        users:(("sshd",pid=445,fd=3))
 LISTEN  0       100     0.0.0.0:25         0.0.0.0:*        users:(("master",pid=929,fd=13))
 LISTEN  0       128     *:3306             *:*              users:(("mysqld",pid=534,fd=30))
 LISTEN  0       128     *:80               *:*              users:(("apache2",pid=765,fd=4),("apache2",pid=764,fd=4),("apache2",pid=515,fd=4))
 LISTEN  0       128     [::]:22            [::]:*           users:(("sshd",pid=445,fd=4))
 LISTEN  0       100     [::]:25            [::]:*           users:(("master",pid=929,fd=14))
 LISTEN  0       70      *:33060            *:*              users:(("mysqld",pid=534,fd=33))

Using lsof to Check Listening Ports

lsof

(list open files) is a powerful utility that shows information about files opened by processes. In Linux, everything is a file; sockets can be thought of as files written to the network. To list all listening TCP ports: sudo lsof -nP -iTCP -sTCP:LISTEN Options used: -n – Do not convert port numbers to port names. -P – Do not resolve hostnames; show numerical addresses. -iTCP -sTCP:LISTEN – Show only network files with TCP state LISTEN.

Example output:

COMMAND  PID   USER   FD  TYPE DEVICE SIZE/OFF NODE NAME
 sshd     445  root   3u  IPv4  16434  0t0    TCP *:22 (LISTEN)
 sshd     445  root   4u  IPv6  16445  0t0    TCP *:22 (LISTEN)
 apache2  515  root   4u  IPv6  16590  0t0    TCP *:80 (LISTEN)
 mysqld   534  mysql  30u IPv6  17636  0t0    TCP *:3306 (LISTEN)
 mysqld   534  mysql  33u IPv6  19973  0t0    TCP *:33060 (LISTEN)
 apache2  764  www-data 4u IPv6  16590  0t0    TCP *:80 (LISTEN)
 apache2  765  www-data 4u IPv6  16590  0t0    TCP *:80 (LISTEN)
 master   929  root   13u IPv4  19637  0t0    TCP *:25 (LISTEN)
 master   929  root   14u IPv6  19638  0t0    TCP *:25 (LISTEN)

Most columns are self-explanatory: COMMAND, PID, USER – Name, PID, and user of the program associated with the port. NAME – Port number.

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

COMMAND PID USER  FD  TYPE DEVICE SIZE/OFF NODE NAME
 mysqld  534 mysql 30u IPv6  17636  0t0    TCP *:3306 (LISTEN)