ISAPI Filters - Rewriting the URL (or commonly called URL Mapping)

Wade Hilmo called it courageous, I called it useful ...

Last week I delivered a presentation on CGI vs ISAPI and I “learned while on the job” ...In front of a webcast audience of about 50 people, I was showing what the capabilities of ISAPI filters are.  I said that a ISAPI filter could take the following HTTP Get request:

www.mysite.com/foo/default.asp ...and re-write it to:

www.foosite.com ...and I was wrong.  I didn't know.  Wade jumped up from his chair with his head shaking and I was certainly not sure what I had done -- he told me.  In short, filters can ReMap anything BUT the hostname.  So, I corrected AND learned (after 4 years of supporting this product) that Filters can do this:

www.mysite.com/foo/default.asp is re-wrote to www.mysite.com/avoidfoo/default.asp -- This is DOABLE via an ISAPI filter.

I started at Microsoft as a support engineer and I quickly learned that ISAPI filters are difficult for many to write correctly and often sources of troubleshooting messes (I have been known to use stronger adjectives in the past.)

After 4 months of taking phone calls, I spent several hours on the phone troubleshooting why a customer could not enter more than 253 characters in the URI (http://foo/default.asp?id=aaaaaaaaaaaaaaaaa) up to 253.  The production server worked, but he told me that his development server was identical.  3 hours later and again another lesson learned - ISAPI Filters can and do change the way a web server works.  On the development box, they had a ISAPI filter that limited the URI to 253 characters or less.

Hence, I learn something new, often from Wade, regarding ISAPI and often in the world of filters.  I leave you with this...I used to tell my engineers when I was a Technical Lead for the IIS group in WA for Microsoft that one rule existed on troubleshooting and filters - “If isn't on this list, it doesn't exist”

IIS 4.0:

sspifilt.dll:  SSL Filter, fpexedll:  FP Back-Compat filter

IIS 5.0:

sspifilt.dll:  SSL Filter, fpexedll:  FP Back-Compat filter, Compression:  Compression filter, md5filt:  Digest filter

IIS 6.0:

aspnet_filter.dll:  ASP.Net ISAPI

Have a great day,

~Chris