Re: nmap and icmp-replies

[Meier Zwei <meier_zwei () gmx net>]

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

As far as UDP is concerned I agree. But a box sending those icmp-replies 
 to rate-limit TCP-connects obviously brings nmap to slow down 
dramatically. nmap should ignore icmp-replies when scanning only TCP, as 
TCP doesn't need icmp to rate traffic (AFAIU).


Meier


Tobias Glemser schrieb:
> 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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Choice Award from eWeek. As attacks through web applications continue to rise, 
you need to proactively protect your applications from hackers. Cenzic has the 
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managed service (Cenzic ClickToSecure) or an enterprise software 
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And, now for a limited time we can do a FREE audit for you to confirm your 
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