Mod Rewrite

The structure of a RewriteRule

RewriteRule Pattern Substitution [OptionalFlags]
example: RewriteEngine on RewriteRule ^old.html$ new.html
The general structure of a RewriteRule is fairly simple if you already understand regular expressions. This article isn’t intended to be a tutorial about regular expressions though - there are already plenty of those available. RewriteRules are broken up as follows:

RewriteRule
This is just the name of the command.
Pattern
A regular expression which will be applied to the “current” URL. If any RewriteRules have already been performed on the requested URL, then that changed URL will be the current URL.
Substitution
Substitution occurs in the same way as it does in Perl, PHP, etc.You can include backreferences and server variable names (%{VARNAME}) in the substitution. Backreferences to this RewriteRule should be written as $N, whereas backreferences to the previous RewriteCond should be written as %N.A special substitution is -. This substitution tells Apache to not perform any substitution. I personally find that this is useful when using the F or G flags (see below), but there are other uses as well.
OptionalFlags
This is the only part of the RewriteRule which isn’t mandatory. Any flags which you use should be surrounded in square brackets, and comma separated. The flags which I find to be most useful are:
  • F - Forbidden. The user will receive a 403 error.
  • L - Last Rule. No more rules will be proccessed if this one was successful.
  • R[=code] - Redirect. The user’s web browser will be visibly redirected to the substituted URL. If you use this flag, you must prefix the substitution with http://www.somesite.com/, thus making it into a true URL. If no code is given, then a HTTP reponse of 302 (temporarily moved) is sent.
A full list of flags is given in the Apache mod_rewrite manual.

A slightly more complicated mod_rewrite example

Let’s try a slightly more meaty example now. Suppose you have a web page which takes a parameter. This parameter tells the page how to be displayed, and what content to pull into it. Humans don’t tend to like remembering the additional syntax of query strings for URLs, and neither do search engines. Both sets of people seem to much prefer a straight URL, with no extra bits tacked onto the end.

In our example, you’ve created a main index page with takes a page parameter. So, a link like index.php?page=software would take you to a software page, while a link to index.php?page=interests would take you to an interests page. What we’ll do with mod_rewrite is to silently redirect users from page/software/ to index.php?page=software etc.

The following is what needs to go into your .htaccess file to accomplish that:
RewriteEngine on
RewriteRule ^page/([^/\.]+)/?$ index.php?page=$1 [L]
Let’s walk through that RewriteRule, and work out exactly what’s going on:

^page/
Sees whether the requested page starts with page/. If it doesn’t, this rule will be ignored.
([^/.]+)
Here, the enclosing brackets signify that anything that is matched will be remembered by the RewriteRule. Inside the brackets, it says “I’d like one or more characters that aren’t a forward slash or a period, please”. Whatever is found here will be captured and remembered.
/?$
Makes sure that the only thing that is found after what was just matched is a possible forward slash, and nothing else. If anything else is found, then this RewriteRule will be ignored.
index.php?page=$1
The actual page which will be loaded by Apache. $1 is magically replaced with the text which was captured previously.
[L]
Tells Apache to not process any more RewriteRules if this one was successful.
Let’s write a quick page to test that this is working. The following test script will simply echo the name of the page you asked for to the screen, so that you can check that the RewriteRule is working.
<html>
<head>
<title>Second mod_rewrite example</title>
</head>
<body>
<p>
The requested page was:
<?php echo $_GET['page']; ?>
</p>
</body>
</html>
Again, upload both the index.php page, and the .htaccess file to the same directory. Then, test it! If you put the page in http://www.somesite.com/mime_test/, then try requesting http://www.somesite.com/mime_test/page/software. The URL in your browser window will show the name of the page which you requested, but the content of the page will be created by the index.php script! This technique can obviously be extended to pass multiple query strings to a page - all you’re limited by is your imagination.

Conditional Statements and mod_rewrite

But what happens when you start getting people hotlinking to your images (or other files)? Hot linking is the act of including an image, media file, etc from someone else’s server in one of your own pages as if it were your own. Obviously, as a webmaster, there are plenty of times when you don’t want people doing that. You’ll almost certainly have seen examples where someone has linked to one image on a website, only for a completely different, “nasty” one to be shown instead. So, how is this done?

It’s pretty simple really. All it takes are a couple of RewriteCond statements in your .htaccess file.

RewriteCond statements are as they sound - conditional statements for RewriteRules. The basic format for a RewriteCond is RewriteCond test_string cond_pattern. For our purpose, we will set the test_string to be the HTTP_REFERER. If the test string is neither empty nor our own server, then we will serve an alternative (low bandwidth) image, which tells the person who is hotlinking off for stealing our bandwidth.

Here’s how we do that:
RewriteEngine on
RewriteCond %{HTTP_REFERER} !^$
RewriteCond %{HTTP_REFERER} !^http://(www\.)?somesite.com/.*$ [NC]
RewriteRule \.(gif|jpg|png)$ http://www.somesite.com/nasty.gif [R,L]
Here, the RewriteRule will only be performed if all the preceeding RewriteConds are fulfilled. In the second RewriteCond, [NC] simply means “No Case”, so it doesn’t matter whether the domain name was written in upper case, lower case or a mixture of the two. So, any requests for gif, jpg or png files from referers other than somesite.com will result in your “nasty” image being shown instead.

The [R,L] in the RewriteRule simply means “Redirect, Last”. So, the RewriteRule will visibly redirect output to “nasty.gif” and no more RewriteRules will be performed on this URL.

If you simply don’t want the hot linkers to see any image at all when they hot link to your images, then simply change the final line to RewriteRule \.(gif|jpg|png)$ - [F]. The - means “don’t rewrite the requested URL”, and the [F] means “Forbidden”. So, the hot linker will get a “403 Forbidden message”, and you don’t end up wasting your bandwidth.

Conclusion

mod_rewrite is an incredibly handy tool to have in your arsenal. This article only scratched the surface of what is possible with mod_rewrite, but should have given you enough information to go out and start mod_rewriting history yourself!

References

Apache module mod_rewrite - Apache’s big long document about the mod_rewrite module.

The Definitive Guide to Apache mod_rewrite - If you’re serious about learning how to use mod_rewrite and need more detail than you got in this article, then I can sincerely recommend buying The Definitive Guide to Apache mod_rewrite.
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