DNSGrep — Quickly Searching Large DNS Datasets

The Rapid7 Project Sonar datasets are amazing resources. They represent scans across the internet, compressed and easy to download. This blog post will focus on two of these datasets:

https://opendata.rapid7.com/sonar.rdns_v2/ (rdns)
https://opendata.rapid7.com/sonar.fdns_v2/ (fdns_a)

Unfortunately, working with these datasets can be a bit slow as the rdns and fdns_a datasets each contain over 10GB of compressed text. My old workflow for using these datasets was not efficient:

ubuntu@client:~$ time gunzip -c fdns_a.json.gz | grep "erbbysam.com"
real 11m31.393s
user 12m29.212s
sys 1m37.672s

I suspected there had to be a faster way of searching these two datasets.

(TLDR, reverse and sort domains then binary search)

DNS Structure

A defining features of the DNS system is its tree-like structure. Visiting this page, you are three levels below the root domain:


The grep query above looks for a domain name tied to the root domain, not an arbitrary string in the file. If we could shape our dataset into a DNS tree, an equivalent lookup would just require a quick traversal of this tree.

Binary Search

The task of transforming a large dataset into a tree on disk and traversing this tree can be simplified further using a binary search algorithm.

The first step in using a binary search algorithm is to sort the data. One option, matching for format above, is the form “com.erbbysam.blog”. This would require a slightly more complex DNS reversal algorithm than neccessary. To simplify, reverse each line instead:


There are no one-command solutions to sort a dataset that does not fit into memory (that I am aware of). To sort these large files, split the data into sorted chunks and then merge the results together:

# fetch the fdns_a file
wget -O fdns_a.gz https://opendata.rapid7.com/sonar.fdns_v2/2019-01-25-1548417890-fdns_a.json.gz

# extract and format our data
gunzip -c fdns_a.gz | jq -r '.value + ","+ .name' | tr '[:upper:]' '[:lower:]' | rev > fdns_a.rev.lowercase.txt

# split the data into chunks to sort
# via https://unix.stackexchange.com/a/350068 -- split and merge code
split -b100M fdns_a.rev.lowercase.txt fileChunk

# remove the old files
rm fdns_a.gz
rm fdns_a.rev.lowercase.txt

# Sort each of the pieces and delete the unsorted one
# via https://unix.stackexchange.com/a/35472 -- use LC_COLLATE=C to sort ., chars
for f in fileChunk*; do LC_COLLATE=C sort "$f" > "$f".sorted && rm "$f"; done

# merge the sorted files with local tmp directory
mkdir -p sorttmp
LC_COLLATE=C sort -T sorttmp/ -muo fdns_a.sort.txt fileChunk*.sorted

# clean up
rm fileChunk*

More detailed instructions for running this script and the rdns equivalent can be found here:


Now we can search the data! To accomplish this, I built a simple golang utility that can be found here:

ubuntu@client:~$ ls -lath fdns_a.sort.txt
-rw-rw-r-- 1 ubuntu ubuntu 68G Feb  3 09:11 fdns_a.sort.txt
ubuntu@client:~$ time ./dnsgrep -f fdns_a.sort.txt -i "erbbysam.com",erbbysam.com,blog.erbbysam.com

real    0m0.002s
user    0m0.000s
sys    0m0.000s

That is significantly faster!

The algorithm is pretty simple:

  1. Use a binary search algorithm to seek through the file, looking for a substring match against the query.
  2. Once a match is found, the file is scanned backwards in 10KB increments looking for a non-matching substring.
  3. Once a non-matching substring is found, the file is scanned forwards until all exact matches are returned.


PoC disclaimer: There is no uptime/performance guarantee of this service and I likely will take this offline at some point in the future. Keep in mind that the datasets here are from a scan on 1/25/19 — DNS records may have changed by the time you read this.

As these queries are so quick, I set up an AWS EC2 t2.micro instance with a spinning disk (Cold HDD sc1) and hosted a server that allows queries into these datasets:


ubuntu@client:~$ curl 'https://dns.bufferover.run/dns?q=erbbysam.com' 
	"Meta": {
		"Runtime": "0.000361 seconds",
		"Errors": [
			"rdns error: failed to find exact match via binary search"
		"FileNames": [
		"TOS": "The source of this data is Rapid7 Labs. Please review the Terms of Service: https://opendata.rapid7.com/about/"
	"FDNS_A": [
	"RDNS": null

Having a bit of fun with this, I queried every North Korean domain name, grepping for the IPs not in North Korean IP space:

 ubuntu@client:~$ curl 'https://dns.bufferover.run/dns?q=.kp' 2> /dev/null | grep -v "\"175\.45\.17"
	"Meta": {
		"Runtime": "0.000534 seconds",
		"Errors": null,
		"FileNames": [
		"TOS": "The source of this data is Rapid7 Labs. Please review the Terms of Service: https://opendata.rapid7.com/about/"
	"FDNS_A": [
	"RDNS": [

That’s it! Hopefully this was useful! Give it a try: https://dns.bufferover.run/dns?q=<hostname>