nc

Recently, we needed to perform some tests in a network. Specifically, we wanted to check the configuration of a firewall and see what IP are blocked and/or which ports are allowed to go through. To do so, we used NetCat to setup a small web-server to perform our tests.

Netcat (often abbreviated to nc) is a computer networking utility for reading from and writing to network connections using TCP or UDP. Netcat is designed to be a dependable back-end that can be used directly or easily driven by other programs and scripts. At the same time, it is a feature-rich network debugging and investigation tool, since it can produce almost any kind of connection its user could need and has a number of built-in capabilities.

From: https://en.wikipedia.org/wiki/Netcat

while true;
do
  echo -e "HTTP/1.1 200 OK\r\n\r\n<h1>$(hostname) is live</h1>$(date)" | nc -vl -p 5555;
done

or in one line

while true; do echo -e "HTTP/1.1 200 OK\r\n\r\n<h1>$(hostname) is live</h1>$(date)" | nc -vl -p 5555; done

Explanation of code:

• The above code creates an infinite loop that calls nc in listening mode, we had to do this as nc will terminate as soon as it serves one client.
• Using echo we create an html 200 response along with a small “webpage”.
• While generating the webpage, echo -e will execute the commands hostname and date to get the current system values adding them to the resulted text.
• The resulted text is then piped to nc to be served as a response to any incoming clients.
• The date and time that nc will show to the client is not the current date and time when visiting the webpage but the one that was when echo was executed.

nc parameters:

• -v, --verbose Sets the verbosity level and it can be used several times to increase it even further
• -l, --listen Instructs nc to bind and listen for incoming connections (just like a web-server)
• -p, --source-portwith port parameter specifies the source port to be used by nc