Check DNS resolver reachability for all local IP addresses

README.md

A shell stub to check if all configured DNS resolver are reachable from all local IP addresses. It tries to search CNAME or A for an example query by binding the query to all local IPs. Remember: NXDOMAIN is a valid result. It's like a DNS ping...

cknsc.sh [www.example.com]

Config

Set CACHE to the IPs you like to test.

Some TODOs

• better error checking
• more things configurable
• reliable output
• IPv6
• reverse lookup??
• ...

cknsc.sh

#!/bin/bash

which uname ip awk dig tr >/dev/null 2>&1
if [ $? -gt 0 ]; then
  exit -1
fi

CACHE="8.8.4.4 8.8.8.8"
QUERY=${1:-www.google.com}

host=`uname -n`

ipv4=`ip -4 -o a | awk '!/^[0-9]*: ?lo|link\/ether/ {gsub(/\/.*/, " ",$4); print $2" "$4}'`

# IPv6 not implemented yet
# ipv6=`ip -6 -o a | awk '!/^[0-9]*: ?lo|link\/ether/ {gsub(/\/.*/, " ",$4); print $2" "$4}'`

# your resolver from resolv.conf
for nsc in $CACHE
do
  echo "$ipv4" | while read dev ip
  do
    # man dig(1)
    result=`dig -4 @$nsc -b $ip +short $QUERY`
    if [ $? != 9 ]; then
      echo -n "OK [$nsc]: $ip@$dev@$host"
      if [ -n "$result" ]; then
        echo -n " | $QUERY -> $result" | tr '\n' ' '
      fi
      echo
    else
      # no response from cache
      echo "FAILED [$nsc]: $ip@$dev@$host"
    fi
  done
done

resolv.conf

domain example.com
search foo.example.com bar.example.com
options rotate timeout:2 attempts:2 single-request
nameserver 8.8.4.4
nameserver 8.8.8.8