ISAPI Rewrite
Pam Dean
September 6, 2004

Have you ever wanted to manage multiple domain names under a single account in IIS (Internet Information Services)? A common way to do this is to handle it from code. There are some disadvantages when handling this directly from code. Static pages aren't redirected, non-developers will need to purchase or dust off their programming books, and evidence remains that a redirection was done.

Along came ISAPI Rewrite www.isapirewrite.com to solve this issue. Running at the ISAPI level in IIS, this is done before any page or code is accessed so it works equally well on static and dynamic content. It overcomes all the objections mentioned above.

With common redirection code, the user would see something like this:
http://www.domain1.com
http://www.domain2.com/domain2
http://www.domain3.com/domain3

With the ISAPI Rewrite component, the user will see this:
http://www.domain1.com
http://www.domain2.com
http://www.domain3.com

Even though behind the scenes the sites can be in completely different sub-folders, the person visiting the website won't have any concept of this. They will simply see the clean domain names.

ISAPI Rewrite will rewrite (kind of like a redirect) the user to a sub-folder even though the web browser doesn't realize this.

This is done very fast and efficiently and managed by a file called httpd.ini which should be placed in the root of the site. It controls which domain names are handled by which sub-folder by utilizing different regular expression rules.

Some tips to remember when creating the rules.

# is used to comment out a line

The following characters must be preceded by a "\" to be recognized as a literal character ( meaning a character that represents itself)

".", "*", "?", "+", "(", ")", "{", "}", "[", "]", "^" and "$".
This becomes important when typing (www.domain.com) it would have to look like (www\.domain\.com)

Parentheses are used to group items together to be marked as what is to be matched, such as "(apple)*" would match all of the string "appleappleapple".

You can see more helpful expression tips here, http://www.isapirewrite.com/docs/examples

Some examples of the rules you will see in the httpd.ini file look like this:

#manual redirect of one domain
RewriteCond  Host:  (www\.)?domain1\.com
RewriteRule  (.*)   /domain1$2

#redirecting to another URL
RewriteCond  Host:  (www\.)?domain1\.com
RewriteRule  (.+)  http\://www\.domain2\.com$1 [R]

#Rewrite just one domain but keep the old link working
RewriteCond  Host:  (www\.)?domain1\.com
RewriteRule  (?!/domain1)(.*)   /domain1$2 [I]

Many tricks come with a catch, and this is no exception. One disadvantage that I have come across is that the response.redirect from ASP or ASP.NET does not work well in conjunction with this component. Absolute paths are required since relative paths are done after ISAPI Rewrite handles the request and aren't redirected as would be expected. The workaround is to use absolute paths with the full domain name in the redirect. If Server.Transfer is to be used, it should be tested as well. It runs into similar issues.

Additionally, when using ASP or ASP.NET, the new virtual root folder should be marked as an application so that the new virtual root's folder is used for global.asa(x), web.config and /bin.

Pam Dean is Web Support Specialist with ORCS Web, Inc. - a company that provides managed hosting solutions for clients who develop and deploy their applications on Microsoft Windows platforms. Services include shared hosting, dedicated hosting, and webfarm hosting, with specialty in .Net, SQL Server, and architecting highly scalable solutions.