Tested to work on CentOS. But probably will also run on any Linux distro.

Description: This script will ping it's Internet gateway and it will alert the administrator via SMS message and email.

Prerequisites :

Create the monitoring script:

	# vi /usr/sbin/netmonitor

	#!/bin/bash
	# Created by: Christian Foronda
	# Ref: http://onlamp.com/pub/a/onlamp/2003/04/03/linuxhacks.html

	PATH=/bin:/usr/bin:/sbin:/usr/sbin:/usr/local/sbin:/home/chr1x2/bin

	GW="215.46.234.1"

	PAUSE=3
	MISSED=0

	while true; do 
	  if ! ping -c 1 -w 1 $GW > /dev/null; then 
		((MISSED++))
	  else
		if [ $MISSED -gt 2 ]; then
		  netmonitor-email-alert && sendsms 01234567890+12345678900 "Automated Alert: Internet seems now UP. Now I can ping the gateway." 
		fi
		MISSED=0
	  fi; 

	  if [ $MISSED -eq 2 ]; then
		netmonitor-email-alert && sendsms 01234567890+12345678900 "Automated Alert: Internet seems DOWN. I cannot ping the gateway. Please check." 
	  fi
	  sleep $PAUSE; 
	done

Create the init script that will let netmonitor to run as daemon:

	# vi /etc/init.d/netmonitor

	#! /bin/sh 
	# created by: Christian Foronda
	#
	# netmonitor    netmonitor
	#
	#
	# chkconfig:    2345 11 02
	# description:  netmonitor daemon
	# processname:  netmonitor
	# pidfile:      /var/run/netmonitor.pid

	# Source function library.
	. /etc/rc.d/init.d/functions

	# Source networking configuration.
	. /etc/sysconfig/network

	NETMONITOR="/usr/sbin/netmonitor &"

	[ -f /usr/sbin/netmonitor ] || exit 0

	RETVAL=0

	# See how we were called.
	case "$1" in
	  start)
			echo -n "Starting Internet connection monitor: "
			daemon $NICELEVEL $NETMONITOR
			RETVAL=$?
			echo
			[ $RETVAL = 0 ] && touch /var/lock/subsys/netmonitor
			;;
	  stop)
			echo -n "Stopping Internet connection monitor: "
			killproc netmonitor
			RETVAL=$?
			echo
			[ $RETVAL = 0 ] && rm -f /var/lock/subsys/netmonitor
			;;
	  restart)
			$0 stop
			$0 start
			RETVAL=$?
			;;
	  status)
			status netmonitor
			RETVAL=$?
			;;
	  *)
			echo "Usage: $0 {start|stop|restart|status}"
			exit 1
	esac

	exit $RETVAL

	# chmod +x /usr/sbin/netmonitor /etc/init.d/netmonitor

Create the email script:

	# vi /usr/sbin/netmonitor-email-alert

	#!/usr/bin/perl

	use Net::SMTP;

	my $message = 'Automated Alert: Internet connection seems down. I cannot ping my gateway. Please check.';
	my $date = `date +%m/%d/%Y`;
	@emails = ("administrator\@mydomain.com","webmaster\@mydomain.com");

	$smtp = Net::SMTP->new("ipofsmtpserver");

	$smtp->mail("administrator\@mydomain.com");
	$smtp->to(@emails);
	$smtp->data();
	$smtp->datasend("From: SERVER ALERT\n");
	$smtp->datasend("Subject: SERVER Internet Connection ALERT: $date \n");
	$smtp->datasend("Internet Connection ALERT! $date\n","$message\n","~" x 40,"~" x 40,"\n","THIS IS AUTO-GENERATED. DO NOT REPLY.");
	$smtp->dataend();
	$smtp->quit();

	# chmod +x /usr/sbin/netmonitor-email-alert

To create the sendsms script. You may follow the guide here.

Add to start on boot:

	# chkconfig --add netmonitor

Start the daemon:

	# /etc/init.d/netmonitor start