From: Franck Veysset (franck.veysset@intranode.com)
Date: Fri Apr 26 2002 - 03:09:31 EDT
Another problem I can see is the time needed to perform such a scan!
If you read RFC 1812 section 4.3.2.8:
"A router which sends ICMP Source Quench messages MUST be able to
limit the rate at which the messages can be generated. A router
SHOULD also be able to limit the rate at which it sends other sorts
of ICMP error messages (Destination Unreachable, Redirect, Time
Exceeded, Parameter Problem). The rate limit parameters SHOULD be
settable as part of the configuration of the router. How the limits
are applied (e.g., per router or per interface) is left to the
implementor's discretion."
You will see that a lot of RFC compliant implementation will implement
rate limitating in term of ICMP generation, which means, UDP scan
limitation. (ICMP port unreachable)
If you read NMAP man page on UDP scan: (thanks Fyodor)
"For example, the Linux kernel (in net/ipv4/icmp.h) limits destination
unreachable message generation to 80 per 4 seconds, with a 1/4 second
penalty if that is exceeded."
I think that some Solaris implementations are even worse.
So, let's assume you want to scan 64K port, 10 packets a port to try
different handshakes... It's gonna take more than a week on a Solaris!
-Franck
"Dawes, Rogan (ZA - Johannesburg)" a écrit :
>
> I think nmap has an explanation of how it determines whether a UDP port is
> listening or not.
>
> Essentially, if a UDP port has a listener, the packet will be accepted, most
> times silently (i.e. if it is not the correct format that the listener would
> normally respond to). If there is no listener there, the machine will return
> an ICMP port unreachable message, containing the port number in question.
>
> Hence, a port scanner can assume, if it gets no response, that there is
> something listening, i.e. the port is "open".
>
> However, this behaviour is easily mimicked (?sp) with a firewall in front of
> the target server. If the firewall is configured to silently drop
> unauthorised packets, the scanner will receive no response to its packets,
> and assume that ALL ports are open.
>
> If there is a screening router in front of the target, and it is configured
> to send ICMP unreachables (fairly standard Cisco filter result), the scanner
> can report that the port is filtered, since the unreachable is coming from a
> different IP address to that of the target.
>
> So, to answer your question eventually, it would be possible to write a port
> scanner that interrogated EVERY port, and only highlighted those that
> responded, however, that would require the following conditions:
>
> The scanner author knows every possible UDP protocol, enough to build a
> first handshake packet, that would cause a response packet. (I would think
> this is prohibitive to start with)
>
> The scanner would have to try EVERY UDP protocol it knows about against
> every port, in order to discern between "not there", and "I'm ignoring
> invalid packets" on non-standard ports. An example might be a TFTP server
> running on the SNMP well-known port. It wouldn't answer to a SNMP handshake,
> but would likely respond to a TFTP handshake . . . .
>
> To your [1], I recommend this, because otherwise, you are providing accurate
> info, rather than the 65535 "positive" results they'd get otherwise.
>
> Hope this was useful.
>
> Rogan
>
--
Franck Veysset -- http://www.INTRANODE.com
Intranode Software Technologies
It is always possible to aglutenate multiple separate
problems into a single complex interdependent solution.
In most cases this is a bad idea. (RFC 1925)
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