Test responsiveness of all TLD nameservers

dnstest.sh

#!/bin/bash

set -m
test "$1" = "-n" && { opt=+norecurse; shift; }
DOMAIN=${1:-'apiary.io'}
TLD=${DOMAIN##*.}

dig -t NS +noadditional ${TLD}. @a.root-servers.net \
  | grep "^${TLD}." \
  | while read one two three four five
do
  addr=$five; str="Testing ${addr}... "
  if
    dig +time=1 $opt ${DOMAIN}. @$addr >/dev/null 2>&1
  then
    echo $str OK
  else
    echo $str TIME OUT
  fi
done