#!/usr/bin/env bash

invalid_resolvers1=$1
invalid_resolvers2=$2
valid_resolvers1=$3
valid_resolvers2=$4

if [[ -z "$invalid_resolvers1" ]] || [[ -z "$invalid_resolvers2" ]] || [[ -z "$valid_resolvers1" ]] || [[ -z "$valid_resolvers2" ]]
then
    echo "usage: ./update_dns_centos.sh INVALID_RESOLVERS_1 INVALID_RESOLVERS_2 VALID_RESOLVERS_1 VALID_RESOLVERS_2"
    exit 2
fi

check_net_mgr="$(ps -e | grep -i networkmanager)"
if [[ ! -z "$check_net_mgr" ]]
then
    echo "Network Manager is running on the box, skipping changes" >/dev/stderr
    exit 1
fi

check_dns1="$(grep "$invalid_resolvers1" /etc/resolv.conf)"
check_dns2="$(grep "$invalid_resolvers2" /etc/resolv.conf)"
err_msg="no servers could be reached"
if [[ ! -z "$check_dns1" ]] || [[ ! -z "$check_dns2" ]]
then
	echo "resolv.conf has invalid resolvers"
	chk_domain_resolv="$(dig google.com +short)"
	if [[ ! -z "$chk_domain_resolv" ]] && [[ "$chk_domain_resolv" != *"$err_msg"* ]]
	then
		echo "Domain getting resolved successfully with invalid resolvers"
		cp /etc/resolv.conf /var/tmp
		echo "resolv.conf backup created at /var/tmp"
		echo "Updating invalid resolvers with valid ones"
		sed -i "s/$invalid_resolvers1/$valid_resolvers1/g" /etc/resolv.conf
		success_check1=$?
		sed -i "s/$invalid_resolvers2/$valid_resolvers2/g" /etc/resolv.conf
		success_check2=$?
		if [[ "$success_check1" -ne 0 ]] || [[ "$success_check2" -ne 0 ]]
		then
		    echo "DNS resolvers could not be replaced, restoring backup" >/dev/stderr
		    cp /var/tmp/resolv.conf /etc/resolv.conf
			exit_code=2
		else
            check_domain="$(dig google.com +short)"
            if [[ -z "$check_domain" ]] || [[ "$check_domain" == *"$err_msg"* ]]
            then
                echo "Valid resolvers could not resolve domain, restoring backup" >/dev/stderr
                cp /var/tmp/resolv.conf /etc/resolv.conf
                exit_code=2
            else
                echo "Valid resolvers resolved the domain successfully"
                exit_code=0
            fi
        fi
		echo "Deleting backup"
		rm /var/tmp/resolv.conf
		exit "$exit_code"
    else
		echo "Invalid resolvers did not resolve domain, skipping changes" >/dev/stderr
		exit 2
    fi
else
	echo "resolv.conf does not have invalid resolvers, skipping changes"
	exit 0
fi