Tip: How to test remote opened ports

At the beginning of times when computers had to be connected among them, some questions arose in the mind of system administrators: Can I check this before the roll-up at night? :)

But, what happens when the systems are so minimal without the possibility to install new packages? Answer; Use nc command.

Telnet

Surely is the most used tool for this task, but inherently it has an enormous disadvantage: has to be explicitly installed by the administrator.

$ telnet site.com port

NC

NC is installed by default in almost all GNU/Linux distros, so, you can use it to test the opened ports of other hosts.

┌[lyonn@almanegra] [/dev/pts/1] 
└[~]> nc -zv google.com 443
Connection to google.com (142.251.33.238) 443 port [tcp/https] succeeded!

Netcat

I do not know anybody that keeps netcat in production servers, but it’s ok for demonstration purposes.

┌[lyonn@almanegra] [/dev/pts/1] 
└[~]> nmap -sT google.com  
Starting Nmap 7.80 ( https://nmap.org ) at 2023-04-03 20:00 CST
Nmap scan report for google.com (142.251.33.238)
Host is up (0.0072s latency).
Other addresses for google.com (not scanned): 2607:f8b0:4012:812::200e
rDNS record for 142.251.33.238: qro01s26-in-f14.1e100.net
Not shown: 997 filtered ports
PORT     STATE SERVICE
80/tcp   open  http
443/tcp  open  https
1723/tcp open  pptp

Nmap done: 1 IP address (1 host up) scanned in 4.44 seconds

Note: NC and netcat are the same thing but with some variations in behavior and compilation time, among others.

Check this post for more information!

What is the difference between telnet and netcat?