I heard too many times a work colleague, or a third-party vendor telling me “there is something wrong on the network here”, or “This port isn’t open!”.

You can use websites to try to test for open ports, a quick Google search should yield you a couple good results, but sometimes that is not enough.

# The problem

Let’s suppose you followed my other post about restricting source address with iptables, in this case those random web tools won’t help you, so you use nmap!