How to Check Listening Ports with lsof

In Linux, the lsof command (List Open Files) is a powerful utility that provides information about files opened by processes, including listening network ports. Monitoring listening ports is crucial for network security and management. This tutorial will guide you through using lsof to check active listening ports.

1. Installing lsof

Most Linux distributions come with lsof pre-installed. You can verify its availability by running:

lsof -v

If it is not installed, you can install it using:

• For Ubuntu:

  sudo apt update
  sudo apt install lsof

• For CentOS:

  sudo yum install lsof

2. Checking Listening Ports

To view all processes that are listening on network ports, use the following command:

sudo lsof -i -n -P | grep LISTEN

Here’s a breakdown of the options used:

• -i: Shows network connections.
• -n: Displays numerical addresses instead of resolving hostnames.
• -P: Displays port numbers instead of service names.

The output will look similar to this:

COMMAND    PID   USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
sshd      1234   root    3u  IPv4  123456      0t0  TCP *:22 (LISTEN)

3. Filtering by Specific Ports

If you want to check if a specific port is listening, you can use:

sudo lsof -i :80

This displays any processes listening on port 80.

4. Checking All Network Connections

To view all current network connections, whether listening or established, run:

sudo lsof -i -n -P

This will show a complete list of all open network connections on the system.

5. Conclusion

By following this tutorial, you have learned how to use the lsof command to check listening ports in Linux. Monitoring active listening ports is essential for network security and troubleshooting. Continue to explore other options and capabilities of lsof to further enhance your Linux networking skills!

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Dorian Kane

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