hostipcheck.php

<?php

/**** **** **** **** **** **** **** **** **** **** ****
 * Description:
 *   Script that checks the ip address of a specified 
 *   host and sends an email alert if the host's
 *   ip address has changed since the last check
 * 
 * Usage:
 *   Run as a cron job to monitor an IP address, generates
 *   output so pipe to null or set $debug to false
 *
 * Version history:
 *
 *    2015-05-25  First commit
 *    2015-05-28  added check to catch host lookup failure 
 * 
 *
 * License:
 *    GNU GPL2+
 **** **** **** **** **** **** **** **** **** **** ****/


if ($_GET['host'] && $_GET['email']) { 
	$host = $_GET['host'];
	$email = $_GET['email'];
} else {
	// hard code if not passing to script via http
	$host = "www.example.com";
	$email = "email@example.com";
}

// set to false to suppress output
$debug = true;

// resolve host
$current_ip_address = gethostbyname($host);

// gethostbyname will return the host string if the lookup fails, so check string for valid ip format
if (inet_pton($current_ip_address )) {
	$old_ip_address = file_get_contents($host.".txt");
	if ($debug) echo "</p>Host: ".$host."</p>Current IP address: ".$current_ip_address."</p>Old IP address: ".$old_ip_address;
	if ($current_ip_address != $old_ip_address){
		// ip has changed, update file and send notice
		$foo = file_put_contents ($host.".txt",$current_ip_address);
		if ($debug) echo "</p>New IP address - sending email alert to: ".$email;
		$foo = mail ($email,"$host Dynamic IP Has Changed","New Dynamic IP for:\r\n Host: $host\r\n New IP: $current_ip_address");
	} else {
		if ($debug) echo "</p>IP address unchanged, no alert";
	}
} else {
	if ($debug) echo "Host lookup failure";
}