How to check if a certain port is open and unused?

Question

I want to install webserver-apache on a Linux platform which uses port no.80 but I am not sure whether is port is open or not, and is being used by some other application or not.

1. Output of grep 80 /etc/services is:

http  80/tcp  www www-http   #World Wide web Http
http  80/udp  www www-http   #Hypertext transfer protocol

2. netstat -an | grep 80 | more:

It gives some IP's one of which is

IP:80 TIME_WAIT

Could you please help and tell how can i find out if port 80 is open and unused so that I can start installation.

Answer 1

sudo netstat -anp | grep ':80 '

That should give you pid & name of the process that holds port 80

Answer 2

This can be achieved using the nc command as follows:

# nc -z IP PORT

It will return TRUE if the port is already in use, or FALSE is it (i.e, available not listening currently).

I don't recommend lsof or netstat method as it first try to scan all running PIDs to get all bounded ports:

# time lsof -i:8888
real    0m1.194s
user    0m0.137s
sys 0m1.056s```

# time nc -z 127.0.0.1 8888
real    0m0.014s
user    0m0.011s
sys 0m0.004s

Here 8888 is an unused port. The nc command is ~85 times faster in the above example.

Eg 1:

$ nc -z 127.0.0.1 80 && echo "IN USE" || echo "FREE"
IN USE

$ nc -z 127.0.0.1 81 && echo "IN USE" || echo "FREE"
FREE

Eg 2:

If you are trying with a remote IP, it is better to add a timeout to auto-exit if it is not accepting connection for the specified time.

$ nc -w 2 -z 216.58.196.174 81

Its Google's IP which is not used, so it will timeout after trying for 2 seconds.