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 -tulpnmail code decode codeDeCode email obfuscator obfuscated