Cfrtjryk

I was experimenting with linux ssh. And in terminal I tried to connect with the same linux machine which I was using and it got connected! Its like calling your same phone from your phone! But how this is possible?

Below is sample terminal commands I entered:

alpha@alpha:~$ ssh alpha@<my IP address>

alpha@<my IP address>'s password: <my password>



Welcome to Ubuntu 18.04.1 LTS (GNU/Linux 4.15.0-42-generic x86_64)



* Documentation:  https://help.ubuntu.com

* Management:     https://landscape.canonical.com

* Support:        https://ubuntu.com/advantage





* Canonical Livepatch is available for installation.

 - Reduce system reboots and improve kernel security. Activate at:

   https://ubuntu.com/livepatch



0 packages can be updated.

0 updates are security updates.



Last login: Mon Dec 10 15:31:35 2018 from <my IP address>

In above terminal is the same IP address of the linux machine by which I used ssh and the above terminal output. And I have also noticed that I can connect with the same machine using ssh in nested connections and I have tried 5 level deep!

Out of curiosity I am asking this question to know is this correct? Can't the linux detect its talking to itself?

In Linux every connection that you are doing normally runs on bash at end, for other unixes other shells are used (and other initialization scripts)

So you can use the current shell initialization scripts to configure the connection behavior of a ssh client by reading the environment variables generated by the ssh client session.

For example in /etc/bash.bashrc something like this can block you from connecting to yourself.

CNIP=$(echo $SSH_CONNECTION | cut -d' ' -f1)

RNIP=$(echo $SSH_CONNECTION | cut -d' ' -f3)



if [ "$CNIP" == "$RNIP" ] && [ "$CNIP" != "" ] ; then

    echo "We are connecting to ourself, exiting.." 

    exit 1

fi

So in linux, with this you probably will prevent ssh from allowing a connection for the server ip itself.

Linux/Unix are very customizable, it will probably allows you to do everything, including broking your system with a infinite loop if you really wanted it. So if you want to prevent some kind of things you need to be more explicity.

In the other hand I didn't see any troble with this behavior and I am unable to determine why you are concerned about it, but if you really want to prevent for any reason, here are probably a solution.

Your system is like a building, not like a phone.

You can make many calls to a building. You can call another person within the building.

So when you ssh to the same machine, it makes the call. Then receives the call. The receiving end is not aware that the call is coming from the same machine, and it handles it correctly.

It does not connect it to the same shell/command-line. Unix is a multi-tasking system. If it could only do one task, then there would be a problem. But it can do many. There can be many users connected to a machine, all doing there own thing. If one of these is the self, it does not matter.