Checking the system's open port and listening services are mainly implemented through command line tools. 1. Use netstat -tulnp to view all listening status TCP/UDP ports and corresponding processes; 2. It is recommended to use the more modern ss -tulnp command, which has stronger functions and clearer output; 3. Use lsof -i : to check the services corresponding to the specific port, and the reverse checking of the services can be used lsof -i -n | grep ; 4. You can check the port through netstat -ano | findstr :<port> under Windows, and check the service name according to the PID in combination with task manager or tasklist. Mastering these commands can complete the port and service inspection work.

Checking all open ports and listening services on the system is actually done with several command line tools. Different operating systems are slightly different, but the ideas are basically the same: by checking the network connection status and system service information, we can determine which ports are open and which services are listening.

Use netstat to view open ports and listening services

Although many systems have gradually deprecated netstat and used more modern ss commands now, it is still very common in some older versions of the system. You can use the following command to list all the TCP and UDP ports that are listening:

 sudo netstat -tulnp

• -t means TCP protocol
• -u means UDP protocol
• -l Only display ports that listen (listening) status
• -n Displays the address and port number in digital form, and does not perform DNS resolution
• -p Displays the process ID and name (sudo permission is required)

In the output result you will see something like this:

 Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp6 0 0 :::80 :::* LISTEN 1234/apache2

This means that port 80 is being monitored by the Apache service.

Use ss instead of netstat

If you are using a newer Linux distribution, it is recommended to use ss (Socket Statistics), which is more powerful and faster. The following is a common command:

 sudo ss -tulnp

The meaning of the parameters is similar to netstat , and the output format is similar, but the structure is clearer. for example:

 Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port
tcp LISTEN 0 128 0.0.0.0:22 0.0.0.0:* users:(("sshd",pid=567,fd=3))

You can see that the SSH service is listening on port 22.

Use lsof to view the corresponding ports of specific services

If you want to know more intuitively which port a particular service uses, you can use the lsof command:

 sudo lsof -i :<port>

For example, if you want to check who is using port 80:

 sudo lsof -i :80

You can also check the ports occupied by a service, for example:

 sudo lsof -i -n | grep sshd

This can quickly locate the relationship between the service and the port.

How to check on Windows?

If you are operating on Windows, you can implement similar functions through netstat :

 netstat -ano | findstr :<port>

For example, find the listening situation of port 80:

 netstat -ano | findstr :80

Then you can find the corresponding service name based on the PID through the task manager or tasklist command.

In general, the method of checking open ports and listening services is not complicated. The key is to be familiar with several commonly used commands and parameters. Whether it is troubleshooting problems or doing security audits, these skills are very practical. Basically that's it, it's enough to master these commands.

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