[Snort-sigs] Snort not inspecting all traffic sourcing from itself?

L0rd Ch0de1m0rt l0rdch0de1m0rt at ...2420...
Thu Jun 10 09:52:37 EDT 2010


Hello.  That did it.  I guess the issue was checksum offloading.  Thanks a lot!

Cheers,

-L0rd Ch0de1m0rt

On 6/9/10, Leon Ward <lward at ...435...> wrote:
> TCP Checksum offloading?
>
> Do a sniff on the interface and take a look if the checksums are
> correct. Snort's -k none could help you out.
>
> Just the first thing that jumps to mind.
>
> -Leon
>
> On 9 June 2010 21:45, L0rd Ch0de1m0rt <l0rdch0de1m0rt at ...2420...> wrote:
>> Hello.  I'm curious.  I am running Snort 2.8.5.1 on an Ubuntu 8.04
>> web server and I have the following two very simple rules enabled (and
>> nothing else):
>>
>> alert tcp any any -> any any (msg:"TCP SYN packet seen!"; flags:S;
>> classtype:bad-unknown; sid:54002346; rev:1;)
>>
>> alert tcp any any -> any any (msg:"HTTP GET detected"; content:"GET";
>> nocase; classtype:bad-unknown; sid:68000036; rev:1;)
>>
>> Now before people start complaining, this is for testing only, not
>> real rules so no need to criticise them.  Anyway if I make a HTTP GET
>> request from a different machine to the Snort machine (which is
>> running a web server, remember), both rules fire.  If the Snort
>> machine serves up a web page that has the word "GET" in it, the rules
>> fire again (different HTTP session).  However, if I log on to the
>> Snort machine, fire up a web browser, and start surfing, only the SYN
>> rule fires despite the fact that "GET" is clearly in traffic when I do
>> a tcpdump on it.  So for some reason, Snort is not inspecting all
>> traffic (although some since the SYN rule fired) sourcing from itself.
>>  Here are my Streams5 options in snort.conf:
>>
>> preprocessor stream5_global: max_tcp 8192, track_tcp yes, track_udp
>> yes, track_icmp no
>> preprocessor stream5_tcp: policy windows, detect_anomalies, require_3whs
>> 180, \
>>   overlap_limit 10, small_segments 3 bytes 150, timeout 180, \
>>    ports client 21 22 23 25 42 53 79 109 110 111 113 119 135 136 137 139
>> 143 \
>>        161 445 513 514 587 593 691 1433 1521 2100 3306 6665 6666 6667
>> 6668 6669 \
>>        7000 32770 32771 32772 32773 32774 32775 32776 32777 32778 32779, \
>>    ports both 80 443 465 563 636 989 992 993 994 995 1220 2301 3128
>> 6907 7702 7777 7779 7801 7900 7901 7902 7903 7904 7905 \
>>        7906 7908 7909 7910 7911 7912 7913 7914 7915 7916 7917 7918
>> 7919 7920 8000 8008 8028 8080 8180 8888 9999
>> preprocessor stream5_udp: timeout 180
>>
>> Cheers,
>>
>> -L0rd Ch0de1m0rt
>>
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