Technology Tales

The wonders of mod_rewrite

24th June 2007

When I wrote about tidying dynamic URL’s a little while back, I had no inkling that that would be a second part to the tale. My discovery of mod_rewrite, an Apache module that facilitates URL translation. The effect is that one URL is presented to the user in the browser address bar, and the exact same URL is also seen by search engines, while another is passed to the server for processing. It might sound like subterfuge but it works very well once you manage to get it set up properly. The web host for my hillwalking blog/photo gallery has everything configured such it is ready to go but the same did not apply to the offline Apache 2.2.x server that I have going on my own Windows XP box. There were two parts to getting it working there:

1. Activating mod-rewrite on the server: this is as easy as uncommenting a line in the httpd.conf file for the site (the line in question is: LoadModule rewrite_module modules/mod_rewrite.so).
2. Ensuring that the .htaccess file in the root of the web server directory is active. You need to set the values of the AllowOverride directives for the server root and CGI directories to All so that .htaccess is active. Not doing it for the latter will result in the an error beginning with the following: Options FollowSymLinks or SymLinksIfOwnerMatch is off which implies that RewriteRule directive is forbidden. Having Allow from All set for the required directories is another option to consider when you see errors like that.

Once you have got the above sorted, adding this line to .htaccess: RewriteEngine On. Preceding it with an Options directive to ensure that FollowSymLinks and SymLinksIfOwnerMatch are switched on does no harm at all and may even be needed to get things running. That done, you can set about putting mod_write to work with lines like this:

RewriteRule ^pages/(.*)/?$ pages.php?query=$1

The effect of this is to take http://www.website.com/pages/input and convert it into a form for action by the server; in this case, that is http://www.website.com/pages.php?query=input. Anything contained by a bracket is assigned to the value of a system-named variable. If you have several bracketed sections, they are assigned to sequentially numbered variables as follows: $1 for the first, $2 for the second and so on. It’s all good stuff when you get it going and not only does it make things look much neater but it also possesses an advantage when it comes to future-proofing too. Web addresses can be kept constant over time, even if things change behind the scenes. It means that returning visitors will find what they saw the last time that they visited and surely must ensure good karma in eyes of those all important search engines.