Wow! Craig...you are the MAN! This explains a number of other questions I had too. Thank you very much!
jc Craig McPherson, 2001-Oct-21 10:45 -0500: > I can't believe nobody has answered this correctly yet. UDP is > different than TCP in that it is a stateless protocol, and that means > you have to understand a few things to interpret UDP port scan results > correctly. With TCP scans, you get one of three results: OPEN > (meaning that the TCP handshake sequence to open a connection > completed), CLOSED (meaning that the target sent a "port closed" ICMP > message), or FILTERED (meaning that no response was received at all: > this is also called "stealthed" by so-called "security experts" like > Steve Gibson but it's a good idea to ignore him just on general > principles). > > UDP is a completely stateless protocol, though. Even if you send a UDP > packet to a port that a valid daemon is listening on, the system isn't > obligated to send anything back to you at all: there is no handshake > sequence to establish a connection. So making a determination is > harder than with TCP. If you receive a "port closed" ICMP message, the > port can be safely listed as CLOSED, but if you receive nothing at all, > that could mean that the port is either OPEN or FILTERED -- it's pretty > much impossible to tell the difference. > > NMAP assumes that every UDP port that it doesn't receive a responsee > from is OPEN, which means that if you have your firewall DROP all UDP > connections, every UDP port will appear as open. If you want to fix > this, have your firewall REJECT instead of DROP, and the ports will > appear correctly as CLOSED to a port scan. DROPing connections without > a response is in violation of the RFCs, too, if you care about that > sort of thing. Having the local machine portscan itself will also tell > you which UDP ports are *actually* open, because I assume you don't > have your firewall set to DROP packets from itself. > > Also, did you know that by default, nmap only scans ports listed in its > services file? So although it scans commonly-used ports, it's not > scanning the entire system. If you have enough time (this will make > the scan very slow, especially over a slow network link), use the "-p 1- > " argument to every scan to force nmap to scan every port from 1 to > 65535 instead of just the maybe 400 or 500 ports that it has listed in > its services file. That's the only way you can get a complete picture > of what your box looks like from the outside. -- Jeff Coppock Nortel Networks Systems Engineer http://nortelnetworks.com
