Way to detect if tcp port is open

Introduction

Laymen explanation

If you are network admin, you faced the situation where the server is not accessible, however server IP is pingable. There can be various possibility including server crash and in this case, port will not be accessible. However, server may be overloaded. We need to find what is the actual reason. This document try to explain how to check if port is accessible or not.

Technical explanation

There are few tools which try to connect with the port and check if it is open or not. In below approaches, It is assumed that there is no firewall rules to block access to the port.

Detect using telnet

Example if port is open

In this example, admin is testing if port 5001 is open in 192.0.0.1 IP

root@linux:/# telnet 192.0.0.1 5001

Trying 192.0.0.1...

Connected to 192.0.0.1.

Escape character is '^]'.

Example if port is not open

In this example, admin is testing if port 5001 is open in 192.0.0.1 IP

root@ubuntu:/# telnet 192.0.0.1 5001

Trying 192.0.0.1...

^C

root@ubuntu:/#

Detect using Linux nc command

This command should be executed in the linux machine hosting server in test. Note that if network namespace is configured in the box, then you need to execute nc command in the namespace context.

Example for running nc command in network namespace

Find your network namespace

root@ubuntu:/# ip netns

mynamespace

Execute nc command within namespace

root@ubuntu:/# ip netns exec mynamespace nc -v -z -w2 192.0.0.1 5001

Connection to 192.0.0.1 5001 port [tcp/*] succeeded!

Example output If port is open

In this example, admin is testing if port 5001 is open in 192.0.0.1 IP

root@18ebd69ccb09:/# nc -v -z -w2 192.0.0.1 5001

Connection to 192.0.0.1 5001 port [tcp/*] succeeded!

Example output If port is not open

In this example, admin is testing if port 5001 is open in 192.0.0.1 IP

root@ubuntu:/# nc -v -z -w2 192.0.0.1 5001

nc: connect to 192.0.0.1 port 5001 (tcp) timed out: Operation now in progress

Reference

http://superuser.com/questions/621870/test-if-a-port-on-a-remote-system-is-reachable-without-telnet

https://kb.acronis.com/content/7503