In this article, we'll go over how you can use SSH to mitigate the risk of data lose that comes from using the same private key for all the servers you manage.
TL;DR: Installation images for many of the most popular Linux distributions are difficult or impossible to obtain securely via download. Linux security is a very complex topic. Modern Linux distributions boast intricate security features in both kernel and userspace, designed to guard against anything from memory corruption exploits...
http://superuser.com/a/529844 (net-stat is deprecated)
http://www.cyberciti.biz/faq/how-do-i-find-out-what-ports-are-listeningopen-on-my-linuxfreebsd-server/
# nmap -sS -O 127.0.0.1
# lsof -i
# netstat -lptu
# netstat -tulpn