Here is part of the docs
ISAPI_Rewrite is a powerful regular expressions-based URL manipulation engine. It acts mostly like Apache's mod_Rewrite, but it is designed especially for Microsoft Internet Information Server and Microsoft Security and Acceleration Server 2004. If you ever wanted to change your web site's URL scheme, this product is for you!
Some key benefits of ISAPI_Rewrite:
ISAPI_Rewrite is extremely fast and highly scalable solution. It is written by using only pure C/C++ code, Win32 API and ISAPI. It uses intelligent configuration cache mechanism. All work is done just in one stage and there are no recursively requests or any other operations that may take a long time.
ISAPI_Rewrite is designed for operation in a shared environment. It can serve as many sites as you have. ISP and hosting providers can safely permit their users to configure ISAPI_Rewrite and be sure that any configuration changes will affect only local user's environment. ISAPI_Rewrite can even solve many security problems, for example, block an access to some folders or file extensions or create more complex rules.
Flexibility and power of ISAPI_Rewrite come from its regular expression nature. With regular expressions you don't need to write a thousands check strings. The comparison and replace of URLs can be done with a few string patterns. So, ISAPI_Rewrite can do many things that cannot be done using other solutions available for IIS. See examples section for more information.
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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 220.127.116.11
Declare manually: Add an entry for each of the subdomains.
sub1.example.com IN A 18.104.22.168
sub2.example.com IN A 22.214.171.124
sub3.example.com IN A 126.96.36.199
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.
* 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 188.8.131.52
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 /
# 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 ｦ Location on Server
http://www.example.com ｦ /
http://sub1.example.com/img/logo.jpg ｦ /img/logo.jpg
http://sub2.example.com ｦ /sub2/
http://abc.example.com ｦ /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.