Apache Mod_Rewrite provides the ability to rewrite incoming requests to different destinations. It can be a bit complicated to wrap your head around the syntax, but once you get used to it, it becomes very powerful.
Rewrite rules can be setup in 2 places, the Apache VirtualHost configuration, or within the website’s .htaccess file. Modifying the .htaccess file allows you to change the rewrite rules on the fly without restarting Apache. However it comes at a steep cost. When using an .htaccess file, each time someone visits your page, the .htaccess file must be processed. So for large rulesets, this could add a substantial CPU load on your web server.
Taken directly from Apache’s website:
You should avoid using .htaccess files completely if you have access to httpd main server config file. Using .htaccess files slows down your Apache http server. Any directive that you can include in a .htaccess file is better set in a Directory block, as it will have the same effect with better performance.
To avoid the performance penalty, I prefer to keep the rules within the Apache VirtualHost configuration. When Apache is restarted, the rules are read into memory, which allows for faster processing.
Rewrite rule example – Step by step
I prefer to learn by example. So lets take an example that is going to be slightly more complicated then it needs to be on purpose to illustrate the syntax.
All requests to http://www.domain.com should be redirected to http://192.168.1.100. Concurrently, https should also redirect to https://192.168.1.100.
A possible solution to put into the Apache VirtualHost configuration would be:
RewriteEngine On
RewriteCond %{SERVER_PORT} ^80$
RewriteCond %{HTTP_HOST} ^domain.com$ [NC,OR]
RewriteCond %{HTTP_HOST} ^www.domain.com$ [NC]
RewriteRule ^/(.*) http://192.168.1.100/$1 [R,L]
RewriteCond %{SERVER_PORT} ^443$
RewriteCond %{HTTP_HOST} ^domain.com$ [NC,OR]
RewriteCond %{HTTP_HOST} ^www.domain.com$ [NC]
RewriteRule ^/(.*) https://192.168.1.100/$1 [R,L]
To explain some of the common regex used:
1. ^ --> String starts with 2. !^ --> String does not start with 3. % --> server variable 4. [NC,OR] --> This means 'NC' Not case sensitive 'OR' It can either match this or the line below. If you want it to match both rules, then just use 'NC' Using an 'AND' statement would break the rule, as the policy already defaults to 'AND'. 5. [R,L] This means: rewrite, last. So basically it is doing the rewrite and it saying this is the last part of the condition, so end the loop. 6. ^/(.*) --> This means anything and everything matching the conditions above (used in this example in the rewriterule)
Easy right? Yea, I know it is not. But it makes more sense the more you have to use it. So let me explain the solution above line by line:
First, we need to enable the rewrite engine:
RewriteEngine On
Now put in the first test condition. For the purposes of this article, we are going to simplify this request by testing for anything coming in over port 80:
RewriteCond %{SERVER_PORT} ^80$
Then we need to test a condition to actually match the URL we are looking to do the rewrite on. In this case, it is domain.com. Now, you should be sure to set the ‘NC’ (non case) so we don’t break the rule if someone types in the request in capital letters. You want to test for both domain.com and www.domain.com. This is done by:
RewriteCond %{HTTP_HOST} ^domain.com$ [NC,OR]
RewriteCond %{HTTP_HOST} ^www.domain.com$ [NC]
Next we write the actual rewrite statement. This is saying, take the result of the above and point it to what we specify here. So to talk this out, it is rewriting anything that matches whats above ^/(.*) and redirecting it to the specified address. The [R,L] rewrite the url ‘R’, and end the statement as it is the last rule ‘L’.
RewriteRule ^/(.*) http://192.168.1.100/$1 [R,L]
Finally, don’t forget to apply all this to https on port 443! You use the same explanations as above for figuring this out.
Common rewrite examples
Enough of the obscene examples and explanations. Below is a bunch of common (and not so common) examples for quick reference:
Rewrite domain.com to www.domain.com:
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www [NC]
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}$1 [R=301,L]
Rewrite www.domain.com to domain.com:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^www.domain.com [NC]
RewriteRule ^(.*)$ http://domain.com$1 [L,R=301]
Force SSL on your domain, meaning take any http requests and redirect them to https:
RewriteEngine On
RewriteCond %{SERVER_PORT} !^443$
RewriteRule ^(.*)$ https://%{HTTP_HOST}$1 [R=301,L]
Force SSL on your domain, meaning take any http requests and redirect them to https when using an SSL terminated load balancer:
RewriteEngine On
RewriteCond %{HTTP:X-Forwarded-Proto} !https
RewriteRule ^(.*)$ https://www.domain.com$1 [R=301,L]
Rewrite one domain to another:
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?domain1.com [NC]
RewriteRule ^(.*)$ http://domain2.com$1 [R=301,L]
Redirect a path to a new domain:
Redirect 301 /path http://new.domain.com Redirect 301 /otherpath/somepage.php http://other.domain.com
Rewrite page with query string to a different page and include query string. Please note, this must be in the Apache VirtualHost config, it will not work in the .htaccess.
# This will redirect www.example.com/products?sku=xxx over to www.example.com/products/sku/xxx
RewriteEngine On
RewriteCond %{REQUEST_URI} ^/products [NC]
RewriteCond %{QUERY_STRING} ^sku=(.*)
RewriteRule (.*) https://www.example.com/products/sku/%1? [R=301,L]
Rewrite page with query string, and strip query string:
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-l
RewriteCond %{REQUEST_URI} ^/pages/pages\.php$
RewriteCond %{QUERY_STRING} ^page=[0-9]*$
RewriteRule ^.*$ http://www.domain.com/path/? [R=301,L]
Force all URL’s to be lowercase. Please note, this must be in the Apache VirtualHost config, it will not work in the .htaccess.
RewriteEngine On
RewriteMap lc int:tolower
RewriteCond %{REQUEST_URI} [A-Z]
RewriteRule (.*) ${lc:$1} [R=301,L]
Exclude phpmyadmin from being rewritten by existing rules (Place this above the problem rule):
RewriteCond %{REQUEST_URI} !=/phpmyadmin
Disable TRACE and TRACK methods:
RewriteEngine On
RewriteCond %{REQUEST_METHOD} ^(TRACE|TRACK)
RewriteRule .* - [F]
Rewrite images to Cloud Files. Please note, this must be in the Apache VirtualHost config, it will not work in the .htaccess.
* Note: The RewriteRule and URL belong on the same line.
<Directory /var/www/vhosts/domain.com/content/images> RewriteEngine On RewriteRule ^(.*)$ http://c0000.cdn00.cloudfiles.rackspacecloud.com/$1 [R=302,L] </Directory>
Rewrite all pages to a maintenance page
Options +FollowSymlinks
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} !/maintenance.html$
RewriteRule .* /maintenance.html [L]