We've upgraded some of our servers to 12.04 recently, and i've noticed that when I run nmap from some other server to check open ports on these 12.04 servers it take WAY longer than before. is this a known issue?
Nmap can adjust its timing based on round-trip times and expected responses. A port is considered filtered if no response is received within some time period. If Nmap finds a port open early in the scan, it can reduce the timeout for considering a port filtered, speeding up the rest of the scan. This is why a host with 1 open (or 1 closed) and 999 filtered ports can take significantly less time to scan than one with 1000 filtered ports.