4.11. DNS Keywords

There are some more content modifiers (If you are unfamiliar with content modifiers, please visit the page Payload Keywords These ones make sure the signature checks a specific part of the network-traffic.

4.11.1. dns_query

With dns_query the DNS request queries are inspected. The dns_query keyword works a bit different from the normal content modifiers. When used in a rule all contents following it are affected by it. Example:

alert dns any any -> any any (msg:”Test dns_query option”; dns_query; content:”google”; nocase; sid:1;)
../_images/dns_query.png

The dns_query keyword affects all following contents, until pkt_data is used or it reaches the end of the rule.

4.11.1.1. Normalized Buffer

Buffer contains literal domain name

  • <length> values (as seen in a raw DNS request) are literal ‘.’ characters
  • no leading <length> value
  • No terminating NULL (0x00) byte (use a negated relative isdataat to match the end)

Example DNS request for “mail.google.com” (for readability, hex values are encoded between pipes):

DNS query on the wire (snippet):

|04|mail|06|google|03|com|00|

dns_query buffer:

mail.google.com