Пункт 42. 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.
Using Alias
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 RewriteRule s 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.
