[Snort-users] DOS rules for Nimda

Richard Ellerbrock richarde at ...6992...
Thu Sep 26 08:19:03 EDT 2002

Ah, I think you understand what I am after, probably because you wrote
the book on this kinda stuff - thanks for a great tool!

No, SEQ number not the same. Only thing that is constant is that for
each SYN to a destination there are EXACTLY two of the EXACT same
packets (Flags, SEQ, Source Port and WIN). So here is an example trace: -> D=80 S=4147 SYN SEQ=2132994334 LEN=0
WIN=16384 -> D=80 S=4147 SYN SEQ=2132994334 LEN=0
WIN=16384 -> D=80 S=4148 SYN SEQ=2147466592 LEN=0
WIN=16384 -> D=80 S=4148 SYN SEQ=2147466592 LEN=0
WIN=16384 -> D=80 S=4149 SYN SEQ=2147500852 LEN=0
WIN=16384 -> D=80 S=4149 SYN SEQ=2147500852 LEN=0

(copied from sniffer trace, these packets are 0.002 seconds apart!)

For some hosts the WIN size is 8192, but I think this depends on the MS
version and nothing else.

What snort really needs to be able to do is detect a constant source
address with a rapidly changing destination address doing a port scan,
something along the lines of a TCP verion of a ICMP smurf detector.

I will also check out 1.9 for the threshold option.


>>> Martin Roesch <roesch at ...1935...> 2002/09/26 04:27:38 >>>
Is the seq number constant for every connection?  If so you could do a
simple detection rule looking for the seq + window with the SYN flag
If you use version 1.9 you can even use the new threshold keyword to
get one event notification for every X alerts....


On 9/26/02 7:11 AM, "Richard Ellerbrock" <richarde at ...6992...> wrote:

> I am trying to help a very large site that is being killed by denial
> service due to a large number of MS type workstations infected by
> The standard snort rules are no good as no connection is actually
> just a HUGE SYN flood looking for open Web servers to infect.
> looks like this:
> Each host sends 2x SYN packets exactly the same (same source port,
> and WIN size) to a remote host on port 80. Obviously never gets a
> Within a couple of milliseconds, tries another randon destination.
> Now my understanding of snort points to the stream4 processor to
> this stuff, but how to configure. The docs are a little unclear to
> snort newbie. I do get TTL evasion on stream4, but this does not
> indicate much.
> Any help with rules/setup for this would be great.
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Martin Roesch - Founder/CTO Sourcefire Inc. - (410) 290-1616
Sourcefire: Professional Snort Sensor and Management Console
roesch at ...1935... - http://www.sourcefire.com 
Snort: Open Source Network IDS - http://www.snort.org 

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