The Internet Ports Database

This is pretty slick - they’ve added a dns interface. For example:

~> nslookup -q=txt 80.tcp.portsdb.org
Server:  localhost
Address:  127.0.0.1

80.tcp.portsdb.org      text = “www”
80.tcp.portsdb.org      text = “http”
80.tcp.portsdb.org      text = “www-http”

http.tcp.portsdb.org    text = “80”