Mod_Rewrite: A Deeper Look

I have used mod_rewrite to great extent in the past with great success. It is great to handle search engine friendly urls. This article won’t be covering how to do that sort of thing as that has been beat to death, rather this article will show you some of the quirks of using mod_rewrite with apache and apache’s different configuration options.

I have used mod_rewrite a great deal and I consider myself to be well versed in how to set it up and use it. That being said, I learned a couple of little quirks about using it in the apache config file and how these might surprise you.

Prior to today I have always used mod_rewrite in a .htaccess file. I feel safe in saying that 100% of my usage of mod_rewrite to date has been via .htaccess. So today when mod_rewrite wasn’t working I tried to use my normal methods of debugging and was stumped as to why it wasn’t working.

First off a little background: We were running mod_rewrite from within the httpd.conf file. Further more we were actually calling RewriteEngine On from inside a virtualhost apache directive. In the past people have told me this is a faster way of using mod_rewrite so we decided to use it as this was a some what speed sensative server. What I discovered is that running mod_rewrite within this directive changes the way mod_rewrite directives work.

Previously when using mod_rewrite via .htaccess I was always able to turn on logging using the directives in the “root” level of the httpd.conf file:

RewriteLog “/var/log/apache/modrewrite.log”
RewriteLogLevel 10

Lesson 1 learned: This works great if you are running mod_rewrite from a .htaccess, however it has zero effect if you are running mod_rewrite from inside a virtualhost directive. For this to work properly in the virtualhost you must place the RewriteLog directives inside the virtualhost directive where you are using RewriteEngine On. Good to know…

Once I turned on the logging I was able to determine that my regex pattern matching for mod_rewrite was missing my target.

Lesson 2 learned: When you are running mod_rewrite in virtualhost you can not use RewriteBase. This means you must manually correct for this in your regex to match the leading / and the trailing / match.

Hopefully those will help out others who aren’t getting the results the expect.

And now for those of you that have made it this far down, here are some fun things you can do with mod_rewrite.

Have all files that don’t have an extension be passed to the PHP parsing engine. This is particularly usefuly for creating scripts that look like directories. So you have and first part of the url (foobar) is actually a PHP script that the rest of the url gets passed to. Sneaky huh? Here it is:

ForceType application/x-httpd-php

I hope this posting helps others in using the powerful apache module mod_rewrite and maybe prevent some lost hairs in the process.

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