When not to use mod_rewrite

When not to use mod_rewrite

This document supplements the mod_rewrite reference documentation. It describes perhaps one of the most important concepts about mod_rewrite - namely, when to avoid using it.

mod_rewrite should be considered a last resort, when other alternatives are found wanting. Using it when there are simpler alternatives leads to configurations which are confusing, fragile, and hard to maintain. Understanding what other alternatives are available is a very important step towards mod_rewrite mastery.

Note that many of these examples won't work unchanged in your particular server configuration, so it's important that you understand them, rather than merely cutting and pasting the examples into your configuration.

The most common situation in which mod_rewrite is the right tool is when the very best solution requires access to the server configuration files, and you don't have that access. Some configuration directives are only available in the server configuration file. So if you are in a hosting situation where you only have .htaccess files to work with, you may need to resort to mod_rewrite.

Simple Redirection

mod_alias provides the Redirect and RedirectMatch directives, which provide a means to redirect one URL to another. This kind of simple redirection of one URL, or a class of URLs, to somewhere else, should be accomplished using these directives rather than RewriteRule. RedirectMatch allows you to include a regular expression in your redirection criteria, providing many of the benefits of using RewriteRule.

A common use for RewriteRule is to redirect an entire class of URLs. For example, all URLs in the /one directory must be redirected to http://one.example.com/, or perhaps all http requests must be redirected to https.

These situations are better handled by the Redirect directive. Remember that Redirect preserves path information. That is to say, a redirect for a URL /one will also redirect all URLs under that, such as /one/two.html and /one/three/four.html.

To redirect URLs under /one to http://one.example.com, do the following:

Redirect "/one/" "http://one.example.com/"

To redirect one hostname to another, for example example.com to www.example.com, see the Canonical Hostnames recipe.

To redirect http URLs to https, do the following:

<VirtualHost *:80>
    ServerName www.example.com
    Redirect "/" "https://www.example.com/"
</VirtualHost>

<VirtualHost *:443>
    ServerName www.example.com
    # ... SSL configuration goes here
</VirtualHost>

The use of RewriteRule to perform this task may be appropriate if there are other RewriteRule directives in the same scope. This is because, when there are Redirect and RewriteRule directives in the same scope, the RewriteRule directives will run first, regardless of the order of appearance in the configuration file.

In the case of the http-to-https redirection, the use of RewriteRule would be appropriate if you don't have access to the main server configuration file, and are obliged to perform this task in a .htaccess file instead.

URL Aliasing

The Alias directive provides mapping from a URI to a directory - usually a directory outside of your DocumentRoot. Although it is possible to perform this mapping with mod_rewrite, Alias is the preferred method, for reasons of simplicity and performance.

Alias "/cats" "/var/www/virtualhosts/felines/htdocs"

The use of mod_rewrite to perform this mapping may be appropriate when you do not have access to the server configuration files. Alias may only be used in server or virtualhost context, and not in a .htaccess file.

Symbolic links would be another way to accomplish the same thing, if you have Options FollowSymLinks enabled on your server.

Virtual Hosting

Although it is possible to handle virtual hosts with mod_rewrite, it is seldom the right way. Creating individual <VirtualHost> blocks is almost always the right way to go. In the event that you have an enormous number of virtual hosts, consider using mod_vhost_alias to create these hosts automatically.

Modules such as mod_macro are also useful for creating a large number of virtual hosts dynamically.

Using mod_rewrite for vitualhost creation may be appropriate if you are using a hosting service that does not provide you access to the server configuration files, and you are therefore restricted to configuration using .htaccess files.

See the virtual hosts with mod_rewrite document for more details on how you might accomplish this if it still seems like the right approach.

Simple Proxying

RewriteRule provides the [P] flag to pass rewritten URIs through mod_proxy.

RewriteRule "^/?images(.*)" "http://imageserver.local/images$1" [P]

However, in many cases, when there is no actual pattern matching needed, as in the example shown above, the ProxyPass directive is a better choice. The example here could be rendered as:

ProxyPass "/images/" "http://imageserver.local/images/"

Note that whether you use RewriteRule or ProxyPass, you'll still need to use the ProxyPassReverse directive to catch redirects issued from the back-end server:

ProxyPassReverse "/images/" "http://imageserver.local/images/"

You may need to use RewriteRule instead when there are other RewriteRules in effect in the same scope, as a RewriteRule will usually take effect before a ProxyPass, and so may preempt what you're trying to accomplish.

Environment Variable Testing

mod_rewrite is frequently used to take a particular action based on the presence or absence of a particular environment variable or request header. This can be done more efficiently using the <If>.

Consider, for example, the common scenario where RewriteRule is used to enforce a canonical hostname, such as www.example.com instead of example.com. This can be done using the <If> directive, as shown here:

<If "req('Host') != 'www.example.com'">
    Redirect "/" "http://www.example.com/"
</If>

This technique can be used to take actions based on any request header, response header, or environment variable, replacing mod_rewrite in many common scenarios.

See especially the expression evaluation documentation for a overview of what types of expressions you can use in <If> sections, and in certain other directives.