Now, once you do this, all traffic for all subdomains of adamhoffman.net is routed to wildcardsubdomain.cloudapp.net. This is true not only of 3rd level subdomains, but also of 4th level (hello.world.adamhoffman.net) and all levels beyond it (this.is.really.cool.adamhoffman.net). Once we have this, it's really just a matter of inspecting the request to make multi-tenant decisions in our application code using Request.Url.Host which will contain the full domain name that was originally asked for in the request (not the cloudapp.net name), like this:

protected void Page_Load(object sender, EventArgs e)
{
	string[] domainparts = Request.Url.Host.Split(new char[] { '.' });
	if (domainparts.Length > 2)
	{
		// there must be a subdomain in front of the two last parts 
		// (i.e. xxx.adamhoffman.net or xxx.yyy.adamhoffman.net, etc.)
		System.Text.StringBuilder sbOut = new System.Text.StringBuilder("Subdomain Segments: <br/>");
		for (int i = 0; i < domainparts.Length - 2; i++)
		{
			sbOut.Append(domainparts[i] + "<br/>");
		}

		this.output.Text = sbOut.ToString();
	}
}

Now, some of you are saying, hey, can't I use the page routing features of ASP.NET or the MVC framework to add routes to this party as well? Again, yes and no. The out of the box routing functionality does not appear to support subdomain routing, but Maarten Balliauw has done some work to extend it and make this possible. While this is a potential nicety, it's not actually necessary to figure out what subdomain was the source of an original request, and therefore we won't cover that here.

To be sure, there are many DNS providers out there that support wildcard CNAMEs. Route 53 is one of them, and Go Daddy is not. If you've successfully done this with a different DNS provider, drop me a line and I'll add the details here.

Source code is here.

Interesting side note - it appears that you can actually use A records and rely on the IP of an Azure deployment as long as you don't delete and redeploy your, um, deployment... I still wouldn't recommend it, since you'll undoubtedly forget to update your DNS servers the one time you do actually delete and redeploy. Nonetheless, good reading on the subject is here.

Ready to try this yourself? Get a FREE 90 Day trial of Windows Azure here.

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