root@server:~# aptitude install openssh-server openssh-client

The SSH server configuration is stored in /etc/ssh/sshd_config.

The netword address and port where ssh server accepts connections can be configured. Our server will only accept connections connections to address 192.168.1.100 and port22 and honoring SSH protocol version 2:

/etc/ssh/sshd_config

# Package generated configuration file
# See the sshd_config(5) manpage for details

# What ports, IPs and protocols we listen for
Port 22
# Use these options to restrict which interfaces/protocols sshd will bind to
#ListenAddress ::
#ListenAddress 0.0.0.0
ListenAddress 192.168.1.100
Protocol 2

#[...]

For safety reasons, root logins will be disabled. This will prevent brute force attacks to root password:

/etc/ssh/sshd_config

#[...]

# Authentication:
LoginGraceTime 120
PermitRootLogin no
StrictModes yes

#[...]

Also, no logins with empty passwords are allowed:

/etc/ssh/sshd_config

#[...]

# To enable empty passwords, change to yes (NOT RECOMMENDED)
PermitEmptyPasswords no

#[...]

Restart the service:

root@server:~# /etc/init.d/ssh restart

It should now be possible to establish a ssh connection to our server:

The first time you login into a remote server you will be warned that the identity of the host could not be established:

fribeiro@laptop:~$ ssh 192.168.1.100
The authenticity of host '192.168.1.100 (192.168.1.100)' can't be established.
RSA key fingerprint is ee:16:b0:c9:1b:ef:b4:64:e1:86:80:f4:36:9f:08:03.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '192.168.1.100' (RSA) to the list of known hosts.
[email protected]'s password:
Linux server 2.6.32-5-amd64 #1 SMP Fri Oct 15 00:56:30 UTC 2010 x86_64

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.

fribeiro@server:~$ logout
Connection to 192.168.1.100 closed.
fribeiro@laptop:~$

root logins are not accepted:

fribeiro@laptop:~$ ssh -l root 192.168.1.100
[email protected]'s password:
Permission denied, please try again.
[email protected]'s password:
Permission denied, please try again.
[email protected]'s password:
Permission denied (publickey,password).

An excellent choice as a windows based SH client is PuTTY:

Because root logins are disabled, to become root in a ssh session, you must login as a 'regular' unprivileged user and then use su command to change your identity to root:

fribeiro@laptop:~$ ssh 192.168.1.100
[email protected]'s password:
Linux server 2.6.32-5-amd64 #1 SMP Fri Oct 15 00:56:30 UTC 2010 x86_64

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Wed Oct 27 11:01:21 2010 from laptop.home.lan
fribeiro@server:~$ su - root
Password:
root@server:~#

• Debian Reference, 6.9. The remote access server and utility (SSH) (http://www.debian.org/doc/manuals/reference/ch06.en.html#_the_remote_access_server_and_utility_ssh
• OpenSSH (http://www.openssh.com/)