A bash script to Reverse IP Lookup (a way to identify hostnames that have DNS (A) records associated with an IP address)

reverse-ip-lookup.sh

#!/bin/bash

stop=$#
inputIP=""
inputFile=""
knownIPFile=""
showUsage=0
verbose=0

for ((i=0; i<$stop; i++))
do
	if [ "${1}" == "-f" ]; then
		if [ $((i+1)) -eq $stop ]; then
			showUsage=1
		else
			i=$((i+1))
			inputFile="${1}"
		fi
	elif [ "${1}" == "-k" ]; then
		if [ $((i+1)) -eq $stop ]; then
			showUsage=1
		else
			i=$((i+1))
			knownIPFile="${1}"
		fi
	elif [ "${1}" == "-verbose" ]; then
		verbose=1
	else
		if [ $((i+1)) -eq $stop ]; then
			inputIP="${1}"
		else
			showUsage=1
		fi
	fi
	shift
done

if [ $stop -eq 0 ]; then
	showUsage=1
fi

if [ $showUsage -eq 1 ]; then
	echo "Usage: ip_lookup.sh [-multiline] [-k <KNOWN_IP_FILE] -f <FILENAME>"
	echo "       ip_lookup.sh [-multiline] [-k <KNOWN_IP_FILE] <IP_ADDRESS>"
	echo "Bad Input" >&2
	exit 1
fi

declare -A knownIPs
function LoadKnownIPs() {
	# File should be IP<TAB>Description
	while IFS=$'\t' read -r ip desc; do
		knownIPs["$ip"]="$desc"
	done < "$1"
}

if [ -n "$knownIPFile" ]; then
	LoadKnownIPs "$knownIPFile"
fi

function LookupIP() {
	result=$(nslookup "$1" 2> /dev/null | grep "Name:")
	if [ -z "$result" ]; then
		result="NOT FOUND"
	else
		result=$(echo "$result" | awk '{print $NF}')
	fi
	knownMatch=""
	if [ -n "${knownIPs[$1]}" ]; then
		knownMatch="${knownIPs[$1]}"
	fi
	if [ $verbose -eq 1 ]; then
		echo "$1"
		echo "$result"
		echo "$knownMatch"
	else
		echo -e "$1\t$result\t$knownMatch"
	fi
}

if [ -n "$inputFile" ]; then
	while read -r line; do
		LookupIP "${line}"
	done < "$inputFile"
else
	LookupIP "$inputIP"
fi