How does ssh remote login work?

[mahela007]

Hi... After reading up about ssh, I understand that it's a protocol used for sending encrypted data over a network. However, I don't understand what's going on when I login to my Ubuntu PC using ssh [over the network]. How does the ssh client application 'know' that it has to take input from one computer and pass it to the other?

EDIT; To put it another way, one could send a mp3 file over ssh right? So instead of the file, what tell the ssh client that the input and output of the computers is what has to be sent?

[GNU-Fan]

The core concept is that standard input and output are kept flexible in Unix systems.

Instead of expecting input from the from a physical keyboard, applications read input from the more abstract STDIN 'file'. Likewise, the output goes to STDOUT with no concern as to whether this is the screen, a line printer or another program.

http://en.wikipedia.org/wiki/Redirection_(computing)

[mahela007]

hm.. interesting. So the server must be able to redirect STDOUT to the ssh client.. right?

[GNU-Fan]

Not directly to the client.
The SSH server reads the output of the program, then sends it over network to the client.

[mahela007]

Thanks.. But how does ssh enable one to loggin to a virtual text based terminal? is the ssh server running in the background?

[GNU-Fan]

Yes, it's running in the background and listens for incoming SSH connections.
If a new connection is established and authenticated correctly, the server program spawns a twin of itself (so the server can keep listening for new connections) and starts a shell (like bash) whose output is send to the client.

[mahela007]

Thanks for your help.