nsfail.sh

#!/bin/bash
#
# adapted from https://github.com/kvz/nsfailover/blob/master/nsfailover.sh

[ -z "${NS_FILE}" ] && NS_FILE="/etc/resolv.conf" # Where to write resolving conf

nameservers=""

function ns_healthy() {
  local nserver="${1}"

  result="$(dig @${nserver} +time=3 +tries=1 +short qrux.nl)"
  exitcode="${?}"
  if [ -z "${result}" ] || [ "${exitcode}" -ne 0 ]; then
    echo "no"
    logger -t nsfail -p local0.warn "can't reach nameserver: " ${ns}
  else
    echo "yes"
  fi
}

function nameserver() {
  local ns="${1}"
  if [[ $(ns_healthy ${ns}) = "yes" ]]; then
    resolvconf+=$'\n'"nameserver ${ns}"
    nameservers+=${ns}\ 
  else
    resolvconf+=$'\n'"#nameserver ${ns}"
  fi
}
  
set -ue

resolvconf="options timeout:1 attempts:2 rotate"
nameserver 1.1.1.1
nameserver 8.8.8.8
#nameserver 127.0.0.1
nameserver 8.8.4.4

# Load current config
current="$(cat ${NS_FILE})" || true

# Is the config updated?
if [ "${resolvconf}" != "${current}" ]; then
  curdate="$(date -u +"%Y%m%d%_H%M%S")"
  cp "${NS_FILE}" ${NS_FILE}.bak-${curdate}
  tmpfile="${NS_FILE}.tmp"
  echo "$resolvconf" > $tmpfile
  mv $tmpfile $NS_FILE
  logger -t nsfail -p local0.warn "set nameservers to: " $nameservers
fi