What is secure shell (SSH) software?

Introduction

SSH provides a secure means to access a UNIX command-line shell on a remote computer. It also provides a way of transfering files and securing otherwise unsafe protocols.

The term 'SSH' may be used to name the secure connection protocol and the name of the primary program that implements the protocol.

Basic Usage

The most common way of logging into a remote site is with 'ssh username@remotehost'. An alternative is 'ssh -l username remotehost'. If your local username is the same as your remote username, you needn't specify it on the command line, e.g. 'ssh remotehost'.

Here is an example login:

$ ssh eml_user@emily1.berkeley.edu
The authenticity of host 'emily1.berkeley.edu' can't be established.
RSA key fingerprint is xx:yy...
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'emily1.berkeley.edu' (RSA) to the list of known hosts.
eml_user@emily1.berkeley.edu's password:  
You have mail.
...

Whenever SSH connects to another computer, it receives a digital fingerprint of that computer. If you are connecting to a computer for the first time, it asks whether you want to continue, and then saves the fingerprint for the next time you want to connect. If you have connected to that computer before, it checks to make sure that the fingerprint is the same as it was the first time you connected to that machine. If the fingerprints are different, it will warn you that someone might have installed nefarious ssh software on the remote host. This is useful because bad people to break into computer seldom know the passphrase that the remote administrator used to generate the fingerprint with.