I am working on a web site that will be taking on many Individual Business Owners. The web site is designed to create sub domains created dynamically according to the input of users as they sign up for the service. Since the web site is expected to accommodate up to 30,000 users it would not be practical to edit the httpd.conf configuration file in Apache for each one. The file would simply grow to be enormous. The solution? Create wild-card sub domains.

Wild-card sub domains can be a great way to handle multiple sub domains that need to be created dynamically by making a simple edit to Apache via the terminal and some creative PHP programming.

The first step is alter Apache. There are a few ways to do this depending on how your server is set up.
Here are some links I have found on how to do this in various server environments;

• PLesk
• cPanel
• In Apache

NOTE: In the case of the project I am working on, we have a dedicated server by RackSpace. It was necessary to register the wild-card sub-domain ( *.you-domain.com ) with their name servers as well in order for this to work.

Step One: Set Up a Wild Card DNS Record

The first step is to create a wildcard DNS record. Your DNS server is already resolving visitors to domain.tld, but it doesn’t know where to resolve them to find subdomain1.domain.tld.

You’ll need to create what is called an “A record,” which is short for “address record.” As the name implies, “A records” tell what IP address a host is pointing to.

The way to do this will vary based on your DNS server and what control panel (or command line) you are using, most are somewhat similar. When you create a name record of type “A” pointing from *.domain.tld to your web server’s IP address.

If you are using a control panel, then likely you can set this using a web form. Sometimes have to get your web host to do this.

Your web server’s DNS service may need to be restarted. You can expext it to take a few hours or even up to a few days sometimes to propagate throughout the Internet.

Step Two: Set Up a Wild Card DNS Record

Test and make sure it working by typing in a random sub-domain url to your site ( ie. Http://random-name.your-domain.com  ). It should resolve to your site’s home.

Now that any sub-domain will point to your domain, you can use some PHP to determine what URL brought your user to your site. Once you have that knowledge you can manipulate the functionality of  your site accordingly. Pretty cool!

Recognize Which Sub Domain Brought Your Visitor With PHP

One way to “recognize” the subdomain from the URL that brought you visitor is to use the a supper global ( which means they are available in all scopes throughout a script. There is no need to do global $variable; to access them within functions or methods) server variable called $_SERVER['HTTP_HOST'].

This super global variable will return the host name.
ie.   sub-domain.your-domain.com
( if there is no sub- domain then it would just be the domain.com)

If you use PHP’s explode with “.” as the delimiter, you can isolate the sub-domain by separating out the first element of the resulting array of URL parts like this…

$url_sections = explode(“.”,$_SERVER['HTTP_HOST']);
$subdomain =$url_sections[0];

Knowing what sub-domain you’re dealing with gives you the PHP power to make you scripts act accordingly. In my case, I used the extracted information for the “virtual” sub-domain to query the MySQL database.

Wild card sub domains can useful for content management platforms like Drupal. With wild card sub domains and a little cleverness you can handle multiple sub domains within a single installation of Drupal.

A Special Note Regarding  SEO

It is very important that you do not have more than one URL (including the sub domain) point to identical content. Google penalizes for “duplicate content” so be sure not to carelessly point various unknown sub domains at your sites home page. With wild card sub domains, if a user makes a mistake and types in a misspelling then use PHP to redirect their page in some way that corrects them and then points them to the right page.

If you are dealing with this already and have any comments or suggestions or corrections feel free to post a comment.