Debian Linux Install OpenSSH SSHD Server

How do I install OpenSSH sshd server under Debian GNU/Linux operating systems?

You need to use the apt-get or aptitude command to install OpenSSH server under Debian Linux. Open a terminal and type the following command to update package database as root user:
# apt-get update

Task: Install OpenSSH server Under Debian Linux

Type the following command:
# apt-get install openssh-server
Sample outputs:

Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
  ssh-askpass rssh molly-guard ufw
The following NEW packages will be installed:
  openssh-server
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 318 kB of archives.
After this operation, 717 kB of additional disk space will be used.
Get:1 http://mirror.anl.gov/debian/ squeeze/main openssh-server amd64 1:5.5p1-6+squeeze1 [318 kB]
Fetched 318 kB in 3s (89.0 kB/s)
Preconfiguring packages ...
Selecting previously deselected package openssh-server.
(Reading database ... 153448 files and directories currently installed.)
Unpacking openssh-server (from .../openssh-server_1%3a5.5p1-6+squeeze1_amd64.deb) ...
Processing triggers for man-db ...
Setting up openssh-server (1:5.5p1-6+squeeze1) ...
Creating SSH2 RSA key; this may take some time ...
Creating SSH2 DSA key; this may take some time ...
Restarting OpenBSD Secure Shell server: sshd.

By default openssh will run on the TCP port 22. You can verify the same with the following command:
# netstat -tulpn | grep :22
Sample outputs:

tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      3946/sshd
tcp6       0      0 :::22                   :::*                    LISTEN      3946/sshd

You can connect to the Openssh server, type:
$ ssh user@localhost
$ ssh user@sever-ip-here

How Do I Start / Stop / Restart OpenSSH Server Under Debian Linux?

Type the following commands as root user:
# service ssh stop
# service ssh start
# service ssh restart
# service ssh status
OR
# /etc/init.d/ssh stop
# /etc/init.d/ssh start
# /etc/init.d/ssh restart
# /etc/init.d/ssh status

Sample session:

Click to enlarge the image

How Do I Open Port 22 At The Firewall Level?

Edit your firewall script and append the following rule to restrict access to 192.168.1.0/24:

/sbin/iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 22 -j ACCEPT

Save and close the file. OR, you can type the command as follows and save it to your firewall config file:

# /sbin/iptables -A INPUT -s 192.168.1.0/24 -m state --state NEW -p tcp --dport 22 -j ACCEPT
# iptables-save > /path/to/your.firewall.conf

How Do I Configure and Secure OpenSSH Server under Debian Linux?

You need to edit the /etc/ssh/sshd_config file using the text editor such as vi, run:
# vi /etc/ssh/sshd_config