script for DNS Resolver

Question

How could make bash script to filter list of DNS resolver because it out with false positive results

suggested code

!/bin/sh
for IP in `cat ./resolvers.txt`; do
    printf "%-4s", $IP
    dig @$IP test.com
done

test.com is my domain which I already know what the output would be, then any different result would be from bad resolver.

See [Why you don't read lines with `for`](https://mywiki.wooledge.org/DontReadLinesWithFor) — Charles Duffy, Mar 06 '19 at 05:53
@GenoChen because it could be made in python BUT i don`t have experience in python programming — Emad, Mar 06 '19 at 05:54
@Emad, tag only for topics your question is actually *about*, not for tools you merely speculate might be used. — Charles Duffy, Mar 06 '19 at 05:57
@Emad Then it has nothing to do at this question. I removed it. — Geno Chen, Mar 06 '19 at 05:58
The shebang needs to start with exactly the two characters `#!`. Putting `sh` makes this a Bourne [tag:sh] script, not a [tag:bash] script. See also [Difference between sh and bash](/questions/5725296/difference-between-sh-and-bash). — tripleee, Mar 06 '19 at 05:59

Charles Duffy · Answer 1 · 2019-03-06T06:03:58.083

#!/usr/bin/env bash
case $BASH_VERSION in '') echo "ERROR: Use bash, not sh" >&2; exit 1;; esac

test_domain="test.com"
correct_ip=1.2.3.4

while IFS= read -r ip; do
  if [[ $(dig +short "@$ip" "$test_domain" 2>/dev/null) = "$correct_ip" ]]; then
    printf '%s\n' "$ip"
  fi
done <all-resolvers.txt >good-resolvers.txt

...will read all-resolvers.txt, and write a list of only the DNS servers that correctly function (resolve test_domain to correct_ip) to good-resolvers.txt.

See:

BashFAQ #1 - Best practices for reading an input file in bash line-by-line
the conditional expression, aka [[ ]], on the bash-hackers' wiki.

script for DNS Resolver

1 Answers1