#!/bin/sh resolve() { local domain="$1" local server="$2" if [ -x "$(command -v dig)" ]; then if [ -z "$server" ]; then dig +short "$domain" else dig +short "@$server" "$domain" fi return $? fi if [ -x "$(command -v drill)" ]; then if [ -z "$server" ]; then drill "$domain" | grep "^$domain." | cut -d$'\t' -f5 else drill "@$server" "$domain" | grep "^$domain." |\ cut -d$'\t' -f5 fi return $? fi exit 1 } try_update() { while read ip; do if ntpdate -bu "$ip"; then return 0 fi done return 1 } pool="pool.ntp.org" dns="1.1.1.1" # try default DNS server first resolve "$pool" "" | try_update [ $? -eq 0 ] && exit 0 # try fallback public dns server ping -q -c 1 "$dns" || exit 1 resolve "$pool" "$dns" | try_update exit $?