Basic script for finding a list of dns resolvers that are "good". Usage is resolvers.sh or resolvers.sh domain-name.com

resolvers.sh

#!/usr/bin/bash

if [ $# -eq 0 ]; then
  test_domain=example.com
else
  test_domain=$1
fi

function rand(){
  cat /dev/urandom | tr -dc 'a-z0-9' | fold -w 32 | head -n 1
}

tmp_path=/tmp/$(rand)
curl -s https://public-dns.info/nameservers-all.txt | grep -v ":" > $tmp_path

lookups=5
let "end = $lookups * $(wc -l < $tmp_path)"

massdns_out=/tmp/$(rand)

for i in $(seq 1 $end); do echo $(rand).$test_domain ; done | \
  massdns -s 1000 -t A --predictable -r $tmp_path -o Sqrm | \
  grep -E "NXDOMAIN|NOERROR" |  \
  cut -d" " -f1,3 > $massdns_out

grep "NXDOMAIN" $massdns_out | cut -d':' -f1 | sort -u > $massdns_out.NXDOMAIN
grep "NOERROR" $massdns_out | cut -d':' -f1 | sort -u > $massdns_out.NOERROR

correct=$(dig $test_domain A +short)
result=/tmp/resolvers.$test_domain.txt

grep -vf $massdns_out.NOERROR $massdns_out.NXDOMAIN | \
  xargs -n1 -P100 -I{} bash -c 'echo {} $(dig @{} $0 A +short)' $test_domain | \
  grep "$correct" | \
  cut -d' ' -f1 > $result

rm -f $tmp_path $massdns_out.*

echo "===========================" 1>&2
echo "Resolvers saved in: $result" 1>&2
echo "===========================" 1>&2