What is ssh?

What is ssh?
How do I use ssh?
What does the warning message mean when I ssh to seas?

What is ssh?

Ssh is a secure remote login program that is similar to rlogin and rsh. The major difference between ssh and other remote login programs is that ssh encrypts the password and other information so that it can't be "sniffed" by others as you type it. Ssh also sets up X11 connections, so the DISPLAY variable does not have to be set on remote machines. Scp is another program used to securely copy files from one host to another.  

How do I use ssh?

The use of ssh is similar to rlogin, rsh, and rcp in syntax.

This: Is Equivalent To:
% ssh eniac.seas.upenn.edu  
% ssh eniac.seas.upenn.edu -l chip 
% scp foo eniac.seas.upenn.edu:~chip/foo
% rsh eniac.seas.upenn.edu  
% rlogin eniac.seas.upenn.edu -l chip 
% rcp foo eniac.seas.upenn.edu:~chip/foo

Your .rhosts file must be configured in the same manner as if you were using rlogin or rsh.

What does the warning message mean when I ssh to seas?

Ssh creates a random key when you initially log onto a machine. In addition to checking your .rhosts file, it checks this key to make sure that you are connected to the host you think your are, and not to an imposter. This prevents someone from getting your password by pretending to be the host that you are trying to access.

If you ssh to seas or eniac.seas, it will try to log into either Red or Blue. Since the keys are specific for each machine, you only have a 50% chance of them matching up. If the keys have not matched up, the following warning message will appear:

Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the host key has just been changed.
Please contact your system administrator.
Add correct host key in /home1r/c/chip/.ssh/known_hosts to get rid of this message.
Password authentication is disabled to avoid trojan horses.
To ensure that you have a secure connection, please ssh to red or blue instead of seas or eniac.


For additional information about ssh, please check the man page:

% man ssh


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