WARNING: It's easy to lock yourself out of a system implementing these changes so make sure you have physical access to the console of the system you are securing.
To display all processes listening
netstat -anp | grep -i listen
Deny all connections to any port from any external IP address
sshd:192.168.1.54 # My other pc
IPTables Tutorial: http://iptables-tutorial.frozentux.net/
A good starting point to block all except ssh: http://www.cyberciti.biz/tips/linux-iptables-4-block-all-incoming-traffic-but-allow-ssh.html
Disable root login via ssh: http://www.howtogeek.com/howto/linux/security-tip-disable-root-ssh-login-on-linux/
Setting up ssh keys and disabling password logins.
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