Posted on March 23, 2023 by nexonhost
How To Check For Listening Ports
This article explains how to use the netstat, ss and lsof commands to find out which services are listening on which ports. The instructions are applicable for all Linux and Unix-based operating systems like macOS.
When troubleshooting network connectivity or application-specific issues, one of the first things to check should be what ports are actually in use on your system and which application is listening on a specific port.
What is Listening Port
Network port is identified by its number, the associated IP address, and type of the communication protocol, such as TCP or UDP.
Listening port is a network port on which an application or process listens on, acting as a communication endpoint.
Each listening port can be open or closed (filtered) using a firewall. In general terms, an open port is a network port that accepts incoming packets from remote locations.
You can’t have two services listening to the same port on the same IP address.
For example, if you are running an Apache web server that listens on ports 80 and 443 and you try to install Nginx , the later will fail to start because the HTTP and HTTPS ports are already in use.
Check Listening Ports with netstat
netstat is a command-line tool that can provide information about network connections.
To list all TCP or UDP ports that are being listened on, including the services using the ports and the socket status use the following command:
sudo netstat -tunlp
The options used in this command have the following meaning:
-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’s process. This information is shown only if you run the command as root or sudo user.
The output will look something like this:
Active Internet connections (only servers) 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 903/sshd tcp 0 0 0.0.0.0:6081 0.0.0.0:* LISTEN 1381/varnishd tcp 0 0 127.0.0.1:6082 0.0.0.0:* LISTEN 1320/varnishd tcp6 0 0 :::22 :::* LISTEN 903/sshd tcp6 0 0 ::1:25 :::* LISTEN 1290/master tcp6 0 0 :::6081 :::* LISTEN 1381/varnishd udp 0 0 127.0.0.1:323 0.0.0.0:* 566/chronyd udp6 0 0 ::1:323 :::* 566/chronyd
The important columns in our case are:
Proto – The protocol used by the socket.
Local Address – The IP Address and port number on which the process listen to.
PID/Program name – The PID and the name of the process.
If you want to filter the results, use the grep command . For example, to find what process listens on TCP port 22 you would type:
sudo netstat -tnlp | grep :22
The output shows that on this machine port 22 is used by the SSH server:
tcp 0 0 0.0.0.0:22 0.0.0.0:* LISTEN 903/sshd tcp6 0 0 :::22 :::* LISTEN 903/sshd
If the output is empty it means that nothing is listening on the port.
You can also filter the list based on criteria, for example, PID, protocol, state, and so on.
netstat is obsolete and replaced with ss and ip , but still it is of the most used commands to check network connections.
Check Listening Ports with ss
ss is the new netstat. It lacks some of the netstat features, but exposes more TCP states and it is slightly faster. The command options are mostly the same, so the transition from netstat to ss is not difficult.
To get a list of all listening ports with ss you would type:
sudo ss -tunlp
The output is almost the same as the one reported by netstat:
Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port
udp UNCONN 0 0 127.0.0.1:323 *:* users:(("chronyd",pid=566,fd=5))
udp UNCONN 0 0 [::1]:323 [::]:* users:(("chronyd",pid=566,fd=6))
tcp LISTEN 0 128 *:22 *:* users:(("sshd",pid=903,fd=3))
tcp LISTEN 0 128 *:6081 *:* users:(("varnishd",pid=1381,fd=6))
tcp LISTEN 0 10 127.0.0.1:6082 *:* users:(("varnishd",pid=1320,fd=5))
tcp LISTEN 0 128 [::]:22 [::]:* users:(("sshd",pid=903,fd=4))
tcp LISTEN 0 100 [::1]:25 [::]:* users:(("master",pid=1290,fd=13))
tcp LISTEN 0 128 [::]:6081 [::]:* users:(("varnishd",pid=1381,fd=7))
Check Listening Ports with lsof
lsof is a powerful command-line utility that provides information about files opened by processes.
In Linux, everything is a file. You can think of a socket as a file that writes to the network.
To get a list of all listening TCP ports with lsof type:
sudo lsof -nP -iTCP -sTCP:LISTEN
The options used are as follows:
-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.
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 of the output columns names are self-explanatory:
COMMAND, PID, USER – The name, the pid and the user running the program associated with the port.
NAME – The port number.
To find what process is listening on a particular port, for example, port 3306 you would use:
sudo lsof -nP -iTCP:3306 -sTCP:LISTEN
The output shows that MySQL server uses port 3306:
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME mysqld 534 mysql 30u IPv6 17636 0t0 TCP *:3306 (LISTEN)
For more information, visit the lsof man page and read about all other powerful options of this tool.
Conclusion
We have shown you several commands that you can use to check what ports are in use on your system, and how to find what process listens on a specific port.