Fix old broken URLs with mod_rewrite

Despite the tons of examples and docs, mod_rewrite is voodoo. Damned cool voodoo, but still voodoo.

I’ve been using mod_rewrite for some time now for various things, but just recently I’ve really started diving into it in order to make cool URIs for skillet.com. The document root is presently a mess, and I want to clean that up for so many reasons, but at the same time I can’t risk breaking existing URLs. It took a good little bit of reading and experimenting, but I finally got mod_rewrite to do what I want.

What I was trying to do was pretty simple… I simply wanted to provide a list of old URLs and have them be redirected to a list of new URLs. This is precisely what RewriteMaps are for, but it took me a little while to really get the hang of them. To save anyone else who is working on this a little time, allow me to explain what I did.

I first created a map file that contained a list of file replacements – first the old file, then some whitespace, then the new file…

#/home/skillet/domains/skillet.com/rewrite-map.txt

/news2/westwood_jan_05.html     /news2/westwood
/news2/grammy-nomination.html   /news2/grammy
/skilletnews.gif                /images/titles/news.gif
/skillet-chat.gif               /images/titles/chat.gif
/skillet-photos.gif             /images/titles/photos.gif
/skillet-pics.gif               /images/titles/pics.gif
/skillet-press.gif              /images/titles/press.gif
/skillet-promo.gif              /images/titles/promo.gif
/skillet-tour.gif               /images/titles/tour.gif
/skillet-video.gif              /images/titles/video.gif

I then added a new RewriteMap, which I chose to name “rewrite-map” (but you can name it anything you want), inside of the skillet VirtualHost. This is the only thing you might have trouble with if you use a shared hosting provider, most of the time you don’t have access to the httpd.conf file, just your .htaccess. (RewriteMaps have to be defined inside the server config or virtual host, though they can actually be used elsewhere)

<virtualhost>
    ...

    RewriteEngine On
    RewriteMap rewrite-map txt:/home/skillet/domains/skillet.com/rewrite-map.txt

    ...
</virtualhost>

Finally, I added the RewriteRule to the .htaccess.

#/home/skillet/domains/skillet.com/public_html/.htaccess

RewriteEngine On

RewriteCond %{REQUEST_FILENAME} -f
RewriteRule .* - [L]

RewriteCond ${rewrite-map:%{REQUEST_URI}} (.+)
RewriteRule .* %1 [L,R]

Now this doesn’t make quite as much sense, so let’s analyze what we’re doing here. The first rewrite condition checks to see if the requested file exists on the file system. If it does, then we simply exit with no changes (that’s what the RewriteRule does) – no sense in looking through the rewrite-map if we don’t have to right? The next RewriteCond takes the requested URI and looks it up in the rewrite-map. If the result is non-null, we continue on; without this RewriteCond, we’d go into a nasty infinite loop of rewrites. Finally, the last RewriteRule replaces the entire URI with the result of the lookup we just performed in the RewriteCond and redirects the user. If the requested file doesn’t exist on the filesystem and there is no entry for it in the rewrite-map, a 404 Error is returned.

If you have a lot of entries in your map, you might consider using a hash file. It provides quicker lookups, but the downside is that you have to rebuild the hash anytime you add new entries. With the plain text map, you can add entries at any time and they’ll automatically be grabbed by apache. If you’re interested in hash files, search here for “hash file”.

So does it work? Try going to http://www.skillet.com/skilletnews.gif and you should be redirected to http://www.skillet.com/images/titles/news.gif. [Perhaps ironically, in the years since this post was written, the site has changed and neither link works any longer.]