Thanks everyone for useful responses. I almost used script mentioned by Stephane (http://www.bortzmeyer.org/files/generate-netfilter-u32-dns-rule.py) but I realized that for a rule for "domain.com" it blocks "domain.com" only and their was no easy way out to block subdomains as well. In last few days after my post, I noticed traffic in pattern of sub1.sub2.domain.com where sub1 and sub2 are randomly generated strings. I tried creating .domain.com and other rules in u32 but didn't help for subdomain. Also since there were very high number of subdomains (but limited domains), possibility to generate u32 rule for each sub didn't made sense. I re-visited Hexadecimal string with 03 and 00 for dot was actually able to help. RPZ and some other option I am still exploring. Thanks. On Sat, Feb 8, 2014 at 11:17 PM, David Miller <dmiller@tiggee.com> wrote:
On 02/08/2014 09:40 AM, William Herrin wrote:
On Sat, Feb 8, 2014 at 3:34 AM, Jonathan Lassoff <jof@thejof.com> wrote:
This is going to be tricky to do, as DNS packets don't necessarily contain entire query values or FQDNs as complete strings due to packet label compression (remember, original DNS only has 512 bytes to work with).
Howdy,
The DNS query essentially always contains the full string in a sequence. It doesn't *have* to per the protocol but you'll be hard pressed to find a real-world example where it doesn't.
The catch is, the dots aren't encoded. The components of the name being queried are separated by a byte indicating the length of the next piece. So, instead of www.google.com the query packet contains www 0x06 google 0x03 com.
For the completeness of the archives, the length of the first token is also encoded and final terminator is 0.
0x03 www 0x06 google 0x03 com 0x00
-DMM
You can implement this with --hex-string instead of --string but you'll have to convert the entire thing to hex first
Regards, Bill Herrin
-- Anurag Bhatia anuragbhatia.com Linkedin <http://in.linkedin.com/in/anuragbhatia21> | Twitter<https://twitter.com/anurag_bhatia> Skype: anuragbhatia.com PGP Key Fingerprint: 3115 677D 2E94 B696 651B 870C C06D D524 245E 58E2