[Snort-sigs] Advanced DNS rules

Curt Shaffer cshaffer at ...2420...
Mon Feb 20 20:17:17 EST 2012


I guess another question would be does the DNS preprocessor enabled
for experimental and deprecated options catch the Z bit anyway?


On Sun, Feb 19, 2012 at 3:12 PM, Geoffrey Sanders
<gtsanders_70 at ...144...> wrote:
> I don't think you'll be able to accomplish your use case without bit masking. Think of it as using snort as a tcpdump filter.
>
> http://vrt-blog.snort.org/2008/08/checking-multiple-bits-in-flag-field_29.html?m=1
>
> - Geoff
>
> On Feb 19, 2012, at 1:51 PM, Curt Shaffer <cshaffer at ...2420...> wrote:
>
>> I'm looking for some information on way to look for malformed DNS
>> packets. Mainly looking for large UDP requests (dsize:>512) that are
>> NOT DNSSEC related, and a rule looking for the reserved flag (Z),
>> reference here: http://www.networksorcery.com/enp/protocol/dns.htm#Z,
>> in the DNS Flags field. I'm having trouble finding decent
>> documentation. I have the following:
>>
>> Detects large packets, but want this to alert only if we are not using DNSSEC:
>>
>> alert udp $EXTERNAL_NET any -> $HOME_NET 53 (msg:" Inbound Large DNS
>> Packet Detected NOT DNSSEC";  dsize:> 512; classtype:dns;  sid:xxxxx;
>> rev:1; )
>>
>> The following I thought would work for the reserved bit (Z), but I am
>> getting alerts even when the bit is not set:
>>
>> alert udp $EXTERNAL_NET any -> $HOME_NET 53 (msg:"DNS Reserved Bit
>> Set"; content:!"|00|"; offset:25; classtype:dns; sid:9000246; rev:1;)
>>
>> Can anyone point me at some documentation for Snort on these topics or
>> lend a hand to help see what I'm missing?
>>
>> Thanks
>>
>> Curt
>>
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