Compare Domain Serials of hidden primary server with public servers

domain_serial

#!/usr/bin/env bash
#
# Compare Domain Serials of hidden primary server with public servers
#

# Our hidden primary server
_PRIMARY_SERVER="dns0.example.net."

# The domain to query
_domain="$1"

# List of public "secondary" servers
_public_servers="$( dig +short NS "$_domain" |sort )"

# The current Year (start of the domain serial number)
_cur_year="$( date +%Y )"

# Regular expression to extract serial number from SOA record
_expr="${_cur_year}[0-9]{6}(?=[^0-9])"

# SOA record from the hidden primary server
_primary_soa=$( dig +short +nosplit "@${_PRIMARY_SERVER}" -t SOA -q "$_domain" )

# Extract serial from hidden primary SOA record
primary_serial=$( echo "$_primary_soa" | grep -Po "$_expr" )

# Print out the result
printf 'Primary: %18s \t %s \n' "*** hidden ***" "$primary_serial"

# Loop trough public "secondary" servers
for _server in ${_public_servers}; do

    # SOA record from the public server
    _public_soa=$( dig +short +nosplit "@${_server}" -t SOA -q "$_domain" )

    # Extract serial from the public servers SOA record
    _public_serial=$( echo "$_public_soa" | grep -Po "$_expr" )

    # Print out the result
    printf 'Public: %18s \t %s \n' "$_server" "$_public_serial"
done