#!/bin/bash
# Simple script to Monitor DNS set resolvers hosts for availability and trigger alarm  via preset email if any of the nameservers on the host cannot resolve
# Use a configured RESOLVE_HOST to try to resolve it via available configured nameservers in /etc/resolv.conf
# if machines are not reachable send notification email to a preconfigured email
# script returns OK 1 if working correctly or 0 if there is issue with resolving $RESOLVE_HOST on $SELF_HOSTNAME and mail on $ALERT_EMAIL
# output of script is to be kept inside DNS_status.log

ALERT_EMAIL='your.email.address@email-fqdn.com';
log=/var/log/dns_status.log;
TIMEOUT=3; DNS=($(grep -R nameserver /etc/resolv.conf | cut -d ' ' -f2));  

SELF_HOSTNAME=$(hostname --fqdn);
RESOLVE_HOST=$(hostname --fqdn);

for i in ${DNS[@]}; do dns_status=$(timeout $TIMEOUT nslookup $RESOLVE_HOST  $i); 

if [[ "$?" == '0' ]]; then echo "$(date "+%y.%m.%d %T") $RESOLVE_HOST $i on host $SELF_HOST OK 1" | tee -a $log; 
else 
echo "$(date "+%y.%m.%d %T")$RESOLVE_HOST $i on host $SELF_HOST NOT_OK 0" | tee -a $log; 

echo "$(date "+%y.%m.%d %T") $RESOLVE_HOST $i DNS on host $SELF_HOST resolve ERROR" | mail -s "$RESOLVE_HOST /etc/resolv.conf $i DNS on host $SELF_HOST resolve ERROR";

fi

 done