Well, You need to some Prerequisites for this process,
Open a terminal window by click on Applications -> Accessories -> Terminal, and then elevate your privileges to root.
sudo su -l
We're going to install a required package and a couple useful ones. Openssh-server is a requirement for the FreeNX server and fail2ban will lock out those attempting to log on with incorrect credentials, which helps mitigate brute force attacks. I'm using firehol to help write the iptables firewall.
aptitude update && aptitude install openssh-server fail2ban firehol
Let's test that our ssh server is running:
You should get a listing of an RSA key fingerprint and a query on whether you want to continue to connect. You might jot the fingerprint down. We'll be comparing it in a later step. Hit Control-C because we don't want to connect here, we just want to know that our ssh server is running.
Then, Go ahead and leave that terminal window open.