Get the IP of a FQDN

This is a simple utility that is used for obtaining the IP address of a FQDN(fully qualified domain name). There is a shell script file that can be run as a CRON job.

Use Case

Let's say you have a PHP script that requires the IP address of a known host. One scenario might be an API endpoint that needs an IP address. For reasons not necessary to discuss here that endpoint will only work with an IP address and not a FQDN.

The Problem

One solution for our use case might be to resolve the FQDN every time the endpoint is called. In order to obtain the IP address we're interested in we'll call the PHP function gethostbyname().

Overall we would be calling gethostbyname() every time we needed the IP address. And it's possible that gethostbyname() could take longer than expected and affect the performance of our application.

The Solution

Is to utilize a CRON to run our utility. The getfqdnip.sh file is the command we will give to CRON. That command will in turn run a PHP script that finds the IP address and saves it in a JSON file for later retreival.

Set Up

The four primary files are:

• getfqdnip.sh - the shell script ran by CRON
• fqdnvars.php - used by getfqdnip.php and readfqdndata.php, it contains the host to look up, and the path+filename of the output JSON file
• getfqdnip.php - calls gethostbyname() to obtain the IP of the host and it writes the found IP to the JSON file.
• readfqdndata.php - intended to be used by an PHP application, it reads the JSON file and saves an object in the $fqdndata variable

There is also indextest.php. It can be requested by a browser, and it will display the host name and host IP.

File Locations

You may have to create a folder in your documents root named php. For example, the new folder would be at /home/$USER/public_html/php.

You may also have to create a folder in your documents root named data. This folder would be at /home/$USER/public_html/data.

The public_html portion of the path is your server's document root folder, and may be named differently for your server.

The indextest.php file can be placed anywhere in your document root.

NOTE: Where the files are placed depends on your platform, and how it's been set up. For example, if your running on a typical Linux hosting server with cPanel then it's likely that the paths and file locations will closely match the examples here.

Editing

The first file you should look at is getfqdnip.sh:

#!/bin/sh
/usr/local/bin/php /home/$USER/public_html/php/getfqdnip.php

Depending on where you have placed the files on your server you may have to edit the /home/$USER/public_html/php/getfqdnip.php path.

Don't forget to make getfqdnip.sh executable. Use the following command:

chmod +x getfqdnip.sh

The next file to edit is fqdnvars.php:

<?php
// See gethomeip.sh and gethomeip.php for details
define('FQDNHOST', 'your.fqdn-hostnamehere.com');
define('FQDNFILE', '/home/'.get_current_user().'/public_html/data/'.str_replace('.','_',FQDNHOST).'.json');
?>

Typically the line below would be edited to suit your particular platform. Leave 'FQDNFILE' as is, but edit the path as necessary. This includes the str_replace(...) portion but for now it uses the value of 'FQDNHOST' and converts all file name separators ('.') with underscores ('_'). Using the example, 'your.fqdn-hostnamehere.com' becomes 'your_fqdn-hostnamehere_com'.

define('FQDNFILE', '/home/'.get_current_user().'/public_html/data/'.str_replace('.','_',FQDNHOST).'.json');

NOTE: the path in getfqdnip.sh and in fqdnvars.php do not have to be the same location.

CRON

I will assume that you already know how to setup a CRON job on your system.

Expected Output

Using the 'FQDNHOST' example in fqdnvars.php a file named your_fqdn-hostnamehere_com.json should be located in the path specified by FQDNFILE. The file will contain:

{"hostip":"123.123.123.123","hostname":"your.fqdn-hostnamehere.com"}

Reading the Results

The readfqdndata.php file can be used by your application to read the JSON file. When it's run an object variable named $fqdndata will be created. You can access that to get the IP and host:

echo $fqdndata->hostip ."\n";
echo $fqdndata->hostname ."\n";

Example Usage

The indextest.php is provided for testing. It will retrieve the host information and display it in a bulleted list.

Here is the file:

<?php
require_once $_SERVER['DOCUMENT_ROOT'] . '/php/readfqdndata.php';
?>
<!DOCTYPE html>
<html>
  <head>
    <meta charset="UTF-8">
    <title>Test - readfqdndata.php</title>
  </head>
  <body>
    <p>
        <ul>
            <li>FQDNHOST = <?php echo FQDNHOST; ?></li>
            <li>FQDNFILE = <?php echo FQDNFILE; ?></li>
        </ul>
        <ul>
            <li>hostip = <?php echo $fqdndata->hostip; ?></li>
            <li>hostname = <?php echo $fqdndata->hostname; ?></li>
        </ul>
    </p>
  </body>
</html>

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