Re: Port Scan for UDP
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
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