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How To Rewrite URLs with mod_rewrite for Apache on Ubuntu 16.04

PostedJanuary 25, 2017 91.6k views Apache Ubuntu Ubuntu 16.04

Introduction

In this tutorial, we will activate and learn how to manage URL rewrites using Apache 2's mod_rewrite module. This module allows us to rewrite URLs in a cleaner fashion, translating human-readable paths into code-friendly query strings or redirecting URLs based on additional conditions.

This guide is split into two parts. The first sets up an example website and covers a simple rewrite example. The second part contains two more in-depth examples of commonly-used rewrite rules.

Prerequisites

To follow this tutorial, you will need:

Step 1 — Enabling mod_rewrite

First, we need to activate mod_rewrite. It's available but not enabled with a clean Apache 2 installation.

  • sudo a2enmod rewrite

This will activate the module or alert you that the module is already enabled. To put these changes into effect, restart Apache.

  • sudo systemctl restart apache2

mod_rewrite is now fully enabled. In the next step we will set up an .htaccess file that we'll use to define rewrite rules for redirects.

Step 2 — Setting Up .htaccess

An .htaccess file allows us to modify our rewrite rules without accessing server configuration files. For this reason, .htaccess is critical to your web application's security. The period that precedes the filename ensures that the file is hidden.

Note: Any rules that you can put in an .htaccess file can be also put directly into server configuration files. In fact, the official Apache documentation recommends using server configuration files instead of .htaccess because Apache processes it faster that way.

However, in this simple example, the performance increase will be negligible. Additionally, setting rules in .htaccess is convenient, especially with multiple websites on the same server. It does not require a server restart for changes to take effect and it does not require root privileges to edit those rules, simplifying maintenance and and making changes possible with unprivileged account. Some popular open-source software, like Wordpress and Joomla, often relies on an .htaccess file for the software to modify and create additional rules on demand.

We will need to set up and secure a few more settings before we can begin.

By default, Apache prohibits using an .htaccess file to apply rewrite rules, so first you need to allow changes to the file. Open the default Apache configuration file using nano or your favorite text editor.

  • sudo nano /etc/apache2/sites-available/000-default.conf

Inside that file, you will find a <VirtualHost *:80> block starting on the first line. Inside of that block, add the following new block so your configuration file looks like the following. Make sure that all blocks are properly indented.

/etc/apache2/sites-available/000-default.conf
<VirtualHost *:80>
    <Directory /var/www/html>
        Options Indexes FollowSymLinks MultiViews
        AllowOverride All
        Require all granted
    </Directory>

    . . .
</VirtualHost>

Save and close the file. To put these changes into effect, restart Apache.

  • sudo systemctl restart apache2

Now, create the .htaccess file in the web root.

  • sudo nano /var/www/html/.htaccess

Add this line at the top of the new file to activate the rewrite engine.

/var/www/html/.htaccess
RewriteEngine on

Save the file and exit.

You now have an operational .htaccess file which you can use to govern your web application's routing rules. In the next step, we will create sample website files that we'll use to demonstrate rewrite rules.

Step 3 — Configuring URL Rewrites

Here, we will set up a basic URL rewrite, which converts pretty URLs into actual paths to code. Specifically, we will allow users to access http://your_server_ip/about.

Begin by creating a file named about.html in the web root.

  • sudo nano /var/www/html/about.html

Copy the following HTML code into the file, then save and close it.

/var/www/html/about.html
<html>
    <head>
        <title>About Us</title>
    </head>
    <body>
        <h1>About Us</h1>
    </body>
</html>

You can access this page at http://your_server_ip/about.html, but notice that if you try to access http://your_server_ip/about, you will see a 404 Not Found error. If you would users to access the page using simply about instead, rewrite rules will allow this very functionality.

All RewriteRules abide by the following format:

General RewriteRule structure
RewriteRule pattern substitution [flags]
  • RewriteRule specifies the directive.
  • pattern is a regular expression that matches the desired string from the URL, which is what the viewer types in the browser.
  • substitution is the path to the actual URL, i.e. the path of the file Apache servers.
  • flags are optional parameters that can modify how the rule works.

Open up the .htaccess file.

  • sudo nano /var/www/html/.htaccess

After the first line, add the RewriteRule marked in red and save the file.

/var/www/html/.htaccess
RewriteEngine on
RewriteRule ^about$ about.html [NC]

In this case, ^about$ is the pattern, about.html is the substitution, and [NC] is a flag. Our example uses a few characters with special meaning:

  • ^ indicates the start of the URL, after your_server_ip/.
  • $ indicates the end of the URL.
  • about matches the string "about".
  • about.html is the actual file that the user accesses.
  • [NC] is a flag that makes the rule case insensitive.

Now, you should be now able access http://your_server_ip/about in your browser. In fact, with the rule shown above, the following URLs will point to about.html:

  • http://your_server_ip/about, because of the rule definition.
  • http://your_server_ip/About, because the rule is case insensitive.
  • http://your_server_ip/about.html, because original proper filename will always work.

The following will not:

  • http://your_server_ip/about/, because the rule explicitly states that there may be nothing after about using the $ character.
  • http://your_server_ip/contact, because it won't match the about string in the rule.

You now have an operational .htaccess file with a simple rule that you can modify and extend to your needs. In the following sections, we will show two additional examples of commonly used directives.

Example 1 — Simplifying Query Strings with RewriteRule

Web applications often make use of query strings, which are appended to a URL using a question mark (?) after the address. Separate parameters are delimited using an ampersand (&). Query strings may be used for passing additional data between individual application pages.

For example, a search result page written in PHP may use a URL like http://example.com/results.php?item=shirt&season=summer. In this example, two additional parameters are passed to the imaginary result.php application script: item, with the value shirt, and season with the value summer. The application may use the query string information to build the right page for the visitor.

Apache rewrite rules are often employed to simplify such long and unpleasent links as the above into friendly URLs that are easier to type and interpret visually. In this example, we would like to simplify the above link to become http://example.com/shirt/summer. The shirt and summer parameter values are still in the address, but without the query string and script name.

Here's one rule to implement this:

Simple substition
RewriteRule ^shirt/summer$ results.php?item=shirt&season=summer [QSA]

The shirt/summer is explicitly matched in the requested address and Apache is told to serve results.php?item=shirt&season=summer instead.

The [QSA] flags are commonly used in rewrite rules. They tell Apache to append any additional query string to the served URL, so if the visitor types http://example.com/shirt/summer?page=2 the server will respond with results.php?item=shirt&season=summer&page=2. Without it, the additional query string would get discarded.

While this method achieves the desired effect, both the item name and season are hardcoded into the rule. This means the rule will not work for any other items, like pants, or seasons, like winter.

To make the rule more generic, we can use regular expressions to match parts of the original address and use those parts in a substitution pattern. The modified rule will then look as follows:

Simple substition
RewriteRule ^([A-Za-z0-9]+)/(summer|winter|fall|spring) results.php?item=$1&season=$2 [QSA]

The first regular expression group in parenthesis matches a string containing alphanumeric characters and numbers like shirt or pants and saves the matched fragment as the $1 variable. The second regular rexpression group in parenthesis matches exactly summer, winter, fall, or spring, and similarly saves the matched fragment as $2.

The matched fragments are then used in the resulting URL in item and season variables instead of hardcoded shirt and summer values we used before.

The above will convert, for example, http://example.com/pants/summer into http://example.com/results.php?item=pants&season=summer. This example is also future proof, allowing mutliple items and seasons to be correctly rewritten using a single rule.

Example 2 — Adding Conditions with Logic Using RewriteConds

Rewrite rules are not necessarily always evaluated one by one without any limitations. The RewriteCond directive lets us add conditions to our rewrite rules to control when the rules will be processed. All RewriteConds abide by the following format:

General RewriteCond structure
RewriteCond TestString Condition [Flags]
  • RewriteCond specifies the RewriteCond directive.
  • TestString is the string to test against.
  • Condition is the pattern or condition to match.
  • Flags are optional parameters that may modify the condition and evaluation rules.

If a RewriteCond evaluates to true, the RewriteRule immediately following will be considered. If it won't, the rule will be discarded. Multiple RewriteCond may be used one after another and, with default behaviour, all must evaluate to true for the following rule to be considered.

As an example, let's assume you would like to redirect all requests to non-existent files or directories on your site back to the home page instead of showing the standard 404 Not Found error page. This can be achieved with following conditions rules:

Redirect all requests to non-existent files and directories to home page
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /

With the above:

  • %{REQUEST_FILENAME} is the string to check. In this case, it's the requested filename, which is a system variable available for every request.
  • -f is a built-in condition which verifies if the requested name exists on disk and is a file. The ! is a negation operator. Combined, !-f evaluates to true only if a specified name does not exist or is not a file.
  • Similarly, !-d evaluates to true only if a specified name does not exist or is not a directory.

The RewriteRule on the final line will come into effect only for requests to non-existent files or directories. The RewriteRule itself is very simple and redirects every request to the / website root.

Conclusion

mod_rewrite is a useful Apache module that can be used effectively to ensure human-readable URLs. In this tutorial, you learned how to use the RewriteRule directive to redirect URLs, including ones with query strings. You also learned how to conditionally redirect URLs using the RewriteCond directive.

If you'd like to learn more about mod_rewrite, take a look at Apache's mod_rewrite Introduction and Apache's official documentation for mod_rewrite.

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