6.14. 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.
This keyword matches on the opcode found in the DNS header flags.
Match on DNS requests and responses with opcode 4:
Match on DNS requests whre the opcode is NOT 0:
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;)
The dns.query keyword affects all following contents, until pkt_data is used or it reaches the end of the rule.
dns.query is equivalent to the older dns_query.
22.214.171.124. 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
isdataatto match the end)
Example DNS request for “mail.google.com” (for readability, hex values are encoded between pipes):
DNS query on the wire (snippet):