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Create Dynamic Sub-domains in Windows IIS

Tutorial for creating sub-domains using IIS and ISAPI_Rewrite

6:39 pm on Dec 20, 2006 (gmt 0)

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Create Dynamic Sub-domains in (Windows/IIS) using ISAPI_Rewrite


Do you have a web site that is hosted on a Windows Server that has been growing and now it makes more sense to redefine the architecture and separate that web site into subdomains? In this article, we will be discussing two methods of setting this up. And later in the article, we'll provide you with a comparison analysis and our conclusions.


  1. Ability to update DNS records
  2. IIS web server admin access
  3. ISAPI_Rewrite component (for Solution 2)

Solution 1: Create Multiple IIS Web Sites

Setup DNS Server

You have two options. Since you probably have a limited number of subdomains to manage, you can either list them out individually or just use a wildcard.

Wildcard method: Add the following entry into your DNS server and change the domain and IP address accordingly.

*.example.com IN A

Declare manually: Add an entry for each of the subdomains.

sub1.example.com IN A
sub2.example.com IN A
sub3.example.com IN A

Setup the Web Server

First make sure you have created a root directory for each of the subdomains, as if you are working with a new web site. Your directory might look like the following:


Next, we create a web site for each of the subdomains. Let's create the first one and you can repeat this for all others.

  • Open IIS Management Console
  • Click on the Web Sites folder and select New : Web Site
  • Click on Next to continue.
  • Enter the description for your site then click Next. An example would be: sub1.example.com.
  • On the IP Address and Port Setting, enter sub1.example.com into the Host header for this Web site field.
  • On the next page, enter the path d:\inetpub\wwwroot\example.com\sub1\
  • On the next page, select your options click Next and you're done.

Now here's the fun part. Since you have created a new site for each subdomain, but there are files that are shared across them, such as javascript, style sheets, and images, what you could do is to create a virtual directory to link them to each of the web sites. Here's how you do it.

  • Right click on the subdomain you have just created in IIS Management Console and select New : Virtual Directory
  • As an example we'll share the style sheet folder (css). Click on Next and enter css in the Alias field.
  • Enter the path d:\inetpub\wwwroot\example.com\css\ into the Path field and click on Next.
  • Specify the permission on the next page and you're done.

Solution 1: Summary

What we have accomplished here is we have separated your web site into multiple subdomains, while still keeping only one copy of the shared files across them.

Solution 2: Using ISAPI_Rewrite

Whoa! That's a lot of steps to create all the subdomains I want. Is there an easier way? Well there is an alternative way. Let's get started first then we'll run the comparisons.

Setup DNS Server

Add the following entry into your DNS server and change the domain and IP address accordingly.

*.example.com IN A

Setup the Web Server

We are assuming that you already have a web site created for your main site: www.example.com. So let's just double check to make sure it will be able to accept all variations of the subdomains.

  • Open IIS Management Console and select your web site.
  • Right click on it and select Properties.
  • Click on Web Site tab.
  • Click on Advanced button.
  • Make sure there is one entry under the Multiple identities for this Web Site with Host Header Name field blank. This entry will intercept all requests that comes to this IP address.
  • Make sure the IP address is only used by this web site.

Setup httpd.ini for ISAPI_Rewrite

Add the following code to your httpd.ini in the web root. Make sure they are in the right order.

# Convert http://example.com to http://www.example.com/
RewriteCond Host: ^example.com
RewriteRule (.*) http\://www\.example.com$1 [I,RP]

# Assuming we have limited number of shared folders.
# We will execute them accordingly regardless of the subdomain.
# Example: http://sub1.example.com/img/logo.jpg -> /img/logo.jpg
# Example: http://www.example.com/img/logo.jpg -> /img/logo.jpg
RewriteRule (/css/.*) $1 [I,O,L]
RewriteRule (/js/.*) $1 [I,O,L]
RewriteRule (/img/.*) $1 [I,O,L]

#Redirect all other subdirectories not matching
#to the list above as subdomains
#example: www.example.com\sub1 -> sub1.example.com
RewriteCond Host: www\.highspeed\.com
RewriteRule /(\w*)/(.*) http\://$1\.example\.com$2 [I,RP]

# If the web site starts with www then point the file to the root folder
# If you specifically created a folder /www/ then you can comment out this section.
RewriteCond Host: (?:www\.)example.com
RewriteRule (.*) $1 [I,O,L]

# Any web site starts other than www will be re-mapped to /<subdomain>/
# Example: http://sub1.example.com/default.asp -> /sub1/default.asp
# Note: if the folder does not exists, then the user will get a 404 error automatically.
RewriteCond Host: (.*)\.example.com
RewriteRule (.*) /$1$2 [I,O,L]

#Fix missing slash char on folders
#This has to be at the end because if invalid dir exists,
#we should show 404 first
RewriteCond Host: (.*)
RewriteRule ([^.?]+[^.?/]) http\://$1$2/ [I,RP]

Test the Subdomains

Assuming we have the company website and two subdomains created: sub1, sub2.

URI Location on Server

http://www.example.com /
[sub1.example.com...] /img/logo.jpg
[sub2.example.com...] /sub2/
[abc.example.com...] /abc/ -> 404 Not Found

Solution 2: Summary

The second solution is easier to implement and only requires one instance of the website, the user has to be careful about creating the folders. For example, the user can not create a folder d:\inetpub\wwwroot\example.com\sub1\img\ because it would conflict with the ISAPI_Rewrite Rule of (/img/.*). Therefore the files in that folder will not be accessible.


Multiple IIS Webs - Pros

  • No conflicts to worry about between folders.
  • Separate log files to track them individually.
  • Can be setup for server farms and load balancing easily.

Multiple IIS Webs - Cons

  • Need to create new site per subdomain.
  • Requires access to IIS server.

ISAPI_Rewrite - Pros

  • Easy to setup.
  • One server instance and log file.
  • To add a subdomain, just add a new folder.

ISAPI_Rewrite - Cons

  • Need to keep track of directories to avoid conflicts.
  • Need dedicated IP address.
  • Additional resource for processing each file.
  • Need to find server that supports ISAPI_Rewrite.


While ISAPI_Rewrite solution is much easier to implement, it is recommended for smaller sites with one development team. This is because of added resource and exceptions that the team has to keep track of. For a larger site, even though the admin has to setup multiple instances of the servers, these only need to be done once. Plus, the subdomains are probably developed by different teams and are distinct enough to warrant their own web instance.

So which one should you choose? Give the ISAPI_Rewrite a try first. When you think your sites are big enough you can always switch to the other method anytime.

Happy coding!

6:49 pm on Dec 20, 2006 (gmt 0)

Senior Member from US 

WebmasterWorld Senior Member pageoneresults is a WebmasterWorld Top Contributor of All Time 10+ Year Member Top Contributors Of The Month

joined:Apr 27, 2001
votes: 51

Whew! I know the above is a doosey but it had to be done. Us Windows folks have to work a bit harder to make our servers function like Apache with mod_rewrite. ISAPI_Rewrite is the Windows version of mod_rewrite.

There are other ISAPI products out there too. We've found that the ISAPI_Rewrite product offers the best bang for the buck and has so many advanced features that it amazes me that Windows hosting companies don't have these installed by default.

My programming team and I have been working on setting up our Windows Servers so they have all the bells and whistles that you Apache folks have right out of the box.

If you have specific questions about the above tutorial, please do ask them. I'll do my best to have those questions answered by my team and then post them here for your review. Heck, I might even get my lead programmer to come in and participate. That is a costly proposition. ;).

7:31 pm on Dec 20, 2006 (gmt 0)

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joined:Apr 12, 2002
votes: 0

Thanks pageoneresults, that is great info.
1:35 am on Dec 21, 2006 (gmt 0)

Administrator from JP 

WebmasterWorld Administrator bill is a WebmasterWorld Top Contributor of All Time 10+ Year Member Top Contributors Of The Month Best Post Of The Month

joined:Oct 12, 2000
votes: 49

Outstanding work! Thanks for sharing that. This gives ISAPI_Rewrite a whole new level of usefulness for many of us.
8:23 am on Dec 21, 2006 (gmt 0)

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joined:Apr 30, 2003
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Another MAJOR flag for reference, many thanks page, this is a great Christmas gift!
1:06 am on Dec 23, 2006 (gmt 0)

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