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I currently have the following DNS Query Alert rule set up in Suricata (for test purposes):

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

Which is triggered when it captures DNS events which contain the word "google", such as in this packet:

{"timestamp":"2017-06-08T15:58:59.907085+0000","flow_id":1798294020028434,"in_iface":"ens33","event_type":"dns","src_ip":"172.16.10.132","src_port":53,"dest_ip":"192.168.160.140","dest_port":52385,"proto":"UDP","dns":{"type":"answer","id":57334,"rcode":"NOERROR","rrname":"www.google.com","rrtype":"A","ttl":300,"rdata":"172.217.12.164"}}

However, instead of searching for resource record names that contain "google", I want to use this same kind of alert to trigger on IP addresses that resolve to loopback, as is the case with the following packet (Notice the rdata field):

{"timestamp":"2017-06-08T15:59:37.120927+0000","flow_id":36683121284050,"in_iface":"ens33","event_type":"dns","src_ip":"172.16.10.132","src_port":53,"dest_ip":"192.168.160.140","dest_port":62260,"proto":"UDP","dns":{"type":"answer","id":53553,"rcode":"NOERROR","rrname":"outlook1.us","rrtype":"A","ttl":120,"rdata":"127.0.0.1"}}

As I have noticed, the contentsection of a Suricata rule searches only for a string. My current rule triggers on a text match with the rrname/domain, how would I make it so that the rule triggers on rdata/IP address?

p.s. Just out of curiosity I tried replacing the "google" in the content section of my alert with "127.0.0.1" and that didn't work either, as expected.

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The ip address is just a 32 bit number. In the rule the IP should be represented as a hex value and not a string, for purposes of efficiency and saving bandwidth (a string will be 8+ bytes as opposed to 4 bytes).

Here is my final Suricata rule to alert whenever somebody gets sent to loopback on my network:

alert dns any any -> any any (msg:"BLACKLISTED DOMAIN"; content:"|7F 00 00 01|"; sid:1;)

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