What is a reverse shell?

Question

...and how do i do it?

Answer 1

Since this was posted to the computer networking topic, I think you are referring to "reverse telnet" which means that you are telnetting out of an asynchronous line on the router to connect to a modem or to a console port of another device. It's basically called reverse telnet because the router is initiating a telnet session, instead of accepting one.

This is commonly used to aggregate many serial console connections into a single router which then acts as a terminal server. Suppose you have dozens of switches and routers in a data center, each with their own console ports... and imagine having to walk around the data center with a laptop and console cable and hook into every one of them... now imagine connecting all of their console ports into a bunch of async ports on a low-end router and then just using reverse telnet, now you have console access to every device, conveniently and from anywhere you can access the network.

To do it, on a Cisco router for example, you would follow your telnet command with the number 2000+n where n is the line number of the device you want to reverse telnet to. For some example documentation on this, see the link below.

If, on the other hand, you are referring to "reverse shell" this is referring to a normal remote shell session, but over an encrypted tunnel. For more info on this, see http://www.plenz.com/reverseshell

Answer 2

reverse shell is used along a tunnel created with a secure/remote shell program. After a tunnel is created, commands and/or interactive shells can be launched back from the tunnel destination machine to the tunnel originating machine with the tunnels Creator's credentials.