[Snort-sigs] pcre...what am I doing wrong?

Jason Wallace jason.r.wallace at ...2420...
Tue Mar 31 16:33:02 EDT 2009


Well how about that... Looking back at my packet captures I see it
now. Was expecting to see a dot between them so just assumed (yea I
know) the dot in the pcap was actually a dot.

Guess I should have checked the RFC.

Thx for the help!

On Tue, Mar 31, 2009 at 4:21 PM, Dale Handy <dhandy at ...3370...> wrote:
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> Well, the problem is, in a DNS request, there is no dot "." in the
> request. Where you would normally see a dot character is actually a
> length byte. Thus, the pcre to detect such a DNS request would actually
> be written like this:
>
>        pcre:"/\x09subdomain\x06domain\x03net/i";
>
> Where \x09 represents the length of the "subdomain" component, \x06
> represents the length of the "domain" component, and \x03 is the length
> of the "net" piece. Also, in this case, you would not need the s or m
> modifiers.
>
>
> Jason Wallace wrote:
>> I'm trying to write a rule using a pcre that looks for DNS requests to
>> a large list of domains. I know pcre is compiled in because I see this
>> during the ./configure
>>
>> checking pcre.h usability... yes
>> checking pcre.h presence... yes
>> checking for pcre.h... yes
>> checking for pcre_compile in -lpcre... yes
>> checking for libpcre version 6.0 or greater... yes
>>
>> Here is the simple beginning of the rule...
>>
>> alert udp $HOME_NET any -> $EXTERNAL_NET 53 (msg:"My Message";
>> pcre:"/subdomain\.domain\.net/smi"; classtype:trojan-activity;
>> sid:500000001; rev:1;)
>>
>> This is just a simple example. There will be a large list of domains
>> similar to the large list of file extensions in the "VIRUS OUTBOUND
>> bad file attachment" sid:721 rule. The problem is the the pcre doesn't
>> seem to be working. Using \ to escape the . is correct right? Here are
>> some things I have tried...
>>
>> pcre:"/subdomain\.domain\.net/smi"; does NOT work
>> pcre:"/subdomain\\.domain\\.net/smi"; does NOT work
>> pcre:"/subdomain.domain.net/smi"; DOES work (but not exactly what I'm
>> looking for, because the . could be anything not just a .)
>> pcre:"/domain/smi"; DOES work
>>
>> This not working makes me a little nervous since there are a lot of
>> rules using \ to escape a . and now I'm wondering if any of them are
>> working...
>>
>> Why wouldn't \ work to escape a . ??
>>
>> Thx,
>> Wally
>>
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>
> - --
> Everyone talks about apathy, but no one does anything about it.
>
> - -- Dale L. Handy, P.E.
>   dhandy at ...3370...
>   208-552-8707
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