hmm....

Here is a short How-To for ssh:

ssh (secure shell) is a replacement for remote terminal programs like "telnet", "rlogin", etc.. The difference to telnet is that the whole communication is encrypted so even if it is intercepted an intruder cannot gain anything from it.

Like telnet too it consists of two parts: a server- and a client-part. The client part initiates the communication. It is the program you call when you issue

Code:

ssh user@host

The server part is named "sshd". You can find it in the process list by issuing

Code:

# ps -fe | grep sshd
root       987     1  0 Jan02 ?        00:00:00 /usr/sbin/sshd -D
bakunin  16163 16153  0 13:24 pts/0    00:00:00 grep sshd

The server part will take the incoming communication requests and handle them.

The first thing you need to do is to configure the server part on a machine you want to connect to. This is done in the file "/etc/sshd_config" and you can find numerous examples for a simple yet working configuration. You need a startup routine so that "sshd" is started automatically at system startup (depends on the system), but for a first test it suffices to issue

Code:

# /usr/sbin/sshd -D

as user root, which starts it in background.

Now let us consider we have two systems with a running server process and we want to connect from one to the other. We simply issue

Code:

user1@host1 # ssh user2@host2

and if everything is running correctly we are greeted with a login prompt and asked for a password ("user2" has to exist of course).

Because we would not want to enter the password every time we connect we can "create" and "exchange" keys: instead of passwords you can generate key sequences and put these on the other server. These keys reside in the directory "~/.ssh" (per default), which will be created when you generate such a key. Switch to the user you want to use and issue

Code:

user1@host # ssh-keygen

and follow the prompts leaving everything at default. Use "RSA" as encryption mechanism. DO NOT enter a password when asked but instead press "ENTER". You will see some files in this directory now:

Code:

user1@host1 # ls -l .ssh 
total 24
-rw-------. 1 user1 staff 1679 Dec 17  2008 id_rsa
-rw-r--r--. 1 user1 staff  408 Dec 17  2008 id_rsa.pub
-rw-r--r--. 1 user1 staff 2112 Nov 22 14:15 known_hosts

The file "id_rsa.pub" is your public key. Suppose you want to connect from host "host2" as user "user2" to this host as user "user1". Open another window in which you connect to "host2" as "user2". Then create a file "~/.ssh/authorized_keys" into which you paste the contents of the aforementioned file "id_rsa.pub"

Code:

# ssh user2@host2
user2@host2 # scp user1@host1:~/.ssh/id_rsa.pub /tmp/key
user2@host2 # cat /tmp/key >> ~/.ssh/authorized_keys
user2@host2 # chmod 600 ~/.ssh/authorized_keys

You are now able to remotely log in to "host1" as "user1" from here. If you want to be able to connect to another host as another user (or even the same host as another user) you have to create a keyfile with this other user and put it also into the file "authorized_keys". This file can hold as many keys as you want, you just put one after the other.

Note that if user1@host1 is allowed to connect to "host2" as "user2", this does not mean that the other direction is allowed to. "user2@host2" will still have to enter the password if he wants to connect to "host1" as "user1" - you will again have to create the key file and put it in the file "authorized_keys" of the other user to enable passwordless login in the other direction.

I hope this helps.

bakunin