How will verify application listening on a port? Example: 80 port listening with web server.

Go through below steps to verify & troubleshoot listening port :

Initially execute below command (telnet) to verify listening port :

#telnet localhost 80

To check it network side :

##netstat -tunlp | grep :80

To check running open files :##lsof -i :80 (OR#lsof -i tcp:80)

To debug & troubleshoot :##tcpdump -vv port 80

We can use telnet command to know the ports status whether it is in opening state or not. As well netstat command will help you to identify a port is listening or not for a particular service like http,https,ssh etc...

telnet the server IP using port80, e.g. open cmd and telnet192.168.1.180, it connection is successfull means port is open else its blocked somewhere in your network or on system itself.2ndly you can run port scanner softwares.

On the server end, we can verify this using

#  netstat -plunt | grep nginx

Replace nignx with apache2 if running apache server.

From client end

#telnet  < Server IP Address> <port no>

you may check it in /etc/services.

open cmd prompt and run the below command

telnet <server IP address> 80

you should see a blank screen with blinking dash

Use Built-In Tools to See What is Listening on a Port

The first step is to use a command-line tool to see what ports are in use, and use a special flag that tells us which port is assigned to each Windows process identifier number. Then we can use that number to look up exactly which process it is.

Open up a command prompt and type in the following—you may have to open in Administrator mode to see all processes:

This will immediately show you a list, although it’s maybe a little complicated. You’ll see the process name in the list, and you can search for it.

You can also use this other method, which takes an extra step, but makes it easier to locate the actual process:

netstat -aon | more

If you look on the right-hand side, you’ll see where I’ve highlighted the list of PIDs, or Process Identifiers. Find the one that’s bound to the port that you’re trying to troubleshoot—for this example, you’ll see that 0.0.0.0:80, or port 80, is in use by PID 4708.