How to Check Which Ports Are in Use on Your Linux System
One step in securing a Linux computer system is identifying which ports are active. Your system’s active ports give you information about which outside applications may be connected to your system.

This guide introduces three common tools you can use find the active ports on your Linux server or computer with links to guides that dive deeper into each tool.
https://www.linode.com/docs/guides/check-linux-ports-in-use/