Re: nmap and icmp-replies

[Tobias Glemser <tglemser () tele-consulting com>]

Meier,

ICMP Dest-Unreachable should occur when scanning UDP. Since UDP is 
stateless, it's the only way a stack can "tell" a port is 
closed/filtered. Even if you send a packet to an open UDP Port, 
depending on the protocol your scanner has to send a valid payload to 
get an answer. If your target sends ICMP Dest-Unrecheachables, you 
should feel lucky.
If an UDP Port is open your scanner will recognize this, because it 
doesn't matter which payload it sends: If there is no service, it will 
receive the Dest-Unreachable Packet, if there is any service, it won't. 
Conclusion: This port is open.

So don't filter these messages, they make your scan results much more 
reliable.

To answer your question: There is no way to tell nmap to ignore ICMP 
Answers. A iptabled gateway between your system and the target will help 
reaching your questionable goal.

Cheers,

Toby

Meier Zwei wrote on 19.04.2006 14:26:
> Hi list,
>
> while scanning somes hosts I figured out that a scanned packet filter 
> sending back icmp destination-unreachable rather slow forces nmap to 
> slow down scanning. A packet filter just dropping the scan probes has no 
> effect on nmap, so does a host without any packet filter, just sending RST.
> My problem is: filtering out the icmp answers with iptables doesn't 
> work, as nmap seems to hook into the kernel before iptable does. Is 
> there any to tell nmap to ignore icmp replies and just to determine a 
> port status via TCP/UDP?
>
> Thanks in advance
>
> Meier
>
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This List Sponsored by: Cenzic

Concerned about Web Application Security? 
Why not go with the #1 solution - Cenzic, the only one to win the Analyst's 
Choice Award from eWeek. As attacks through web applications continue to rise, 
you need to proactively protect your applications from hackers. Cenzic has the 
most comprehensive solutions to meet your application security penetration 
testing and vulnerability management needs. You have an option to go with a 
managed service (Cenzic ClickToSecure) or an enterprise software 
(Cenzic Hailstorm). Download FREE whitepaper on how a managed service can 
help you: http://www.cenzic.com/news_events/wpappsec.php 
And, now for a limited time we can do a FREE audit for you to confirm your 
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