nmap is a fairly common used tool for more than testing. So is dig netstat sockstat nc tcpdump qmail djbdns pf s(sh|cp) traceroute also read your security and t

nmap is a fairly common used tool for more than testing. So is dig netstat sockstat nc tcpdump qmail djbdns pf s(sh|cp) traceroute also read your security and tuning man pages as a great place to start.