[Snort-users] How does a reject rule work?

Todd Wease twease at ...1935...
Tue Apr 29 11:21:58 EDT 2008


His rule was an ICMP rule.  He was sending ICMP packets to trigger the 
rule.  The rule was also a reject rule.  ICMP ping packets will not ever 
get an ICMP Destination port unreachable and the inline code (correctly) 
only sends ICMP Destination port unreachable for UDP and TCP packets 
(actually for TCP packets it sends a reset).  The ICMP packet will be 
dropped, but sending a reset or ICMP Destination port unreachable for an 
ICMP packet makes no sense.  I suppose the inline.c code could have 
another case for ICMP packets and send a code 0, 1 or 2 Destination 
unreachable packet, but it doesn't.

By the way, thanks for the RFC tip.

Phil Wood wrote:
> Read RFC 792 to understand the ICMP Destination Unreachable Message.
> "bahamin" not talking about ICMP ports its talking about the messages
> sent by hosts using ICMP to tell the sender of something that is not
> "reachable" on the recipients computer (such as a port).
> 
> One place to find the RFC:
> 
> ftp://ftp.rfc-editor.org/in-notes/rfc792.txt
> 
> On Tue, 2008-04-29 at 09:02 -0400, Todd Wease wrote:
>> You're not going to get 'Destination port unreachable' with ICMP packets
>> (they address no port).  Try with UDP or TCP packets and rule.
>>
>>
>> bahamin takhtaei wrote:
>>> Hi,
>>>
>>> I wrote an icmp rule with "reject" action and
>>> test it by echo request packets (ping), but
>>> I didn't catch any "Destination port unreachable"
>>> message on the test system. It seems that my
>>> "reject" rule works as a "drop" rule only! Now
>>> please tell me What do I to fix this problem?
>>>
>>>
>>> Thanks,
>>> Bahamin
>>>
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