Implement Reverse DNS Annotation

[phillip-stephens]

Adds:

• reverse DNS lookup annotations to ZAnnotate
• README instructions on Input/Output formats to expect/use with zannotate

Example

$ echo "1.1.1.1" | ./zannotate --rdns
{"ip":"1.1.1.1","rdns":{"domain_names":["one.one.one.one"]}}

Notes/Caveats

I noted 2 implementation choices to make here

• CNAME Handling

  • Some IPs like 12.8.130.35 have CNAMEs returned.

    • Opt. 1 - Return both CNAMEs and PTRs. Would need to return a data structure that contains qname, dns record type, and answer.
    • Opt 2 - Return only PTR records. I went with this option. It follows what Go's native net.LookupAddr does, and I figure if people wanted to study the DNS ecosystem, they'd use the more verbose output from ZDNS.

$  dig -x 12.8.130.35
...
;; ANSWER SECTION:
35.130.8.12.in-addr.arpa. 83000 IN      CNAME   35.0/25.130.8.12.in-addr.arpa.
35.0/25.130.8.12.in-addr.arpa. 86400 IN PTR     pm2-26.btconline.net.

• Error handling
  - **Opt 1 - Quiet errors - We return an empty list for errors

echo "127.0.0.1" | ./zannotate --rdns                                                                                                                                                                                                                                                                                         15:54:05
{"ip":"127.0.0.1","rdns":{}}

• Opt 2 - Verbose error in-output- ZDNS will always return the error encountered into the output. This felt too chatty, especially since a lot of IPs don't have a PTR record. I ended up going with Option 1 even if that means that some sort of network issue will just result in a bunch of null entries BUT I do log errors at a DEBUG level for observability.

Performance

As a naive comparison, I replaced the annotation with ZDNS with net.LookupAddr, Go's std. lib. function for reverse PTR lookups. On a set of 10k IPs and a VM with plenty of compute/network bandwidth, ZDNS was 5x faster at 30 sec. for all lookups vs. 2 min, 30 sec.

Related Issues

Resolves #4