check_dns_sync.sh

#!/bin/bash

set -eu

declare -a STATES=(OK WARNING CRITICAL UNKNOWN)
declare -i STATE_OK=0
declare -i STATE_WARNING=1
declare -i STATE_CRITICAL=2
declare -i STATE_UNKNOWN=3

declare -i RET=$STATE_OK
MSG=""

MASTER="$1"
ZONE="$2"

read _ _ REF_SERIAL _ < <(dig +short "@$MASTER" "$ZONE" SOA)

while read _ _ _ SERIAL _ _ _ _ _ _ IP _; do
    if [[ "$SERIAL" = "out;" ]]; then
        MSG="${MSG}Not all servers responded, "
        if [[ $RET = $STATE_OK ]]; then
            RET=$STATE_UNKNOWN
        fi
    elif [[ "$SERIAL" != "$REF_SERIAL" ]]; then
        MSG="${MSG}$IP: $SERIAL != $REF_SERIAL, "
        RET=$STATE_CRITICAL
    fi
done < <(dig -4 +time=1 +nssearch "$ZONE")

if [[ -z "$MSG" ]]; then
    MSG="All servers have zone serial ${REF_SERIAL}"
fi

echo "${STATES[$RET]}: ${MSG%, }"
exit $RET