Not using Ubuntu 20.04?
Choose a different version or distribution.
The author selected the Diversity in Tech Fund to receive a donation as part of the Write for DOnations program.
Introduction
Apache’s mod_rewrite module lets you rewrite URLs more cleanly, translating human-readable paths into code-friendly query strings. It also enables you to rewrite URLs based on conditions.
An .htaccess file lets you create and apply rewrite rules without accessing server configuration files. By placing the .htaccess file in the root of your web site, you can manage rewrites on a per-site or per-directory basis.
In this tutorial, you’ll enable mod_rewrite and use .htaccess files to create a basic URL redirection, and then explore a couple of advanced use cases.
Prerequisites
To follow this tutorial, you will need:
Step 1 — Enabling mod_rewrite
In order for Apache to understand rewrite rules, we first need to activate mod_rewrite. It’s already installed, but it’s disabled on a default Apache installation. Use the a2enmod command to enable the module:
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 making changes possible with an 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.
Before you start using .htaccess files, you’ll need to set up and secure a few more settings.
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
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 an .htaccess file in the webroot.
- 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.
We now have an operational .htaccess file that we can use to govern our 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 that converts pretty URLs into actual paths to pages. Specifically, we will allow users to access http://your_server_ip/about, but display a page called about.html.
Begin by creating a file named about.html in the webroot.
- 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. To access the page using /about instead, we’ll create a rewrite rule.
All RewriteRules follow this format:
General RewriteRule structure
RewriteRule pattern substitution [flags]
RewriteRulespecifies the directive.patternis a regular expression that matches the desired string from the URL, which is what the viewer types in the browser.substitutionis the path to the actual URL, i.e., the path of the file Apache servers.flagsare optional parameters that can modify how the rule works.
Let’s create our URL rewrite rule. Open up the .htaccess file.
- sudo nano /var/www/html/.htaccess
After the first line, add the highlighted RewriteRule 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 afteryour_server_ip/.$indicates the end of the URL.aboutmatches the string “about”.about.htmlis the actual file that the user accesses.[NC]is a flag that makes the rule case insensitive.
You can now access http://your_server_ip/about in your browser. In fact, with the rule shown above, the following URLs will point to about.html:
http://your_server_ip/about, because of the rule definition.http://your_server_ip/About, because the rule is case insensitive.http://your_server_ip/about.html, because the original proper filename will always work.
However, the following will not work:
http://your_server_ip/about/, because the rule explicitly states that there may be nothing afterabout, since the$character appears afterabout.http://your_server_ip/contact, because it won’t match theaboutstring in the rule.
You now have an operational .htaccess file with a basic 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 unpleasant 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 substitution
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 substitution
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 expression 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 multiple items and seasons to be rewritten correctly 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 are processed. All RewriteConds abide by the following format:
General RewriteCond structure
RewriteCond TestString Condition [Flags]
RewriteCondspecifies theRewriteConddirective.TestStringis the string to test against.Conditionis the pattern or condition to match.Flagsare 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 behavior, 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 . / [R=301]
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.-fis a built-in condition that verifies if the requested name exists on disk and is a file. The!is a negation operator. Combined,!-fevaluates to true only if a specified name does not exist or is not a file.- Similarly,
!-devaluates 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. The dot . in the pattern matches anything and the substitution directs every request to the / website root.
Additionally, [R=301] flag tells Apache to return a 301 Moved Permanently redirect HTTP response code to the browser, resulting in a browser knowing the redirect happened and explicitly fetching the website root instead of the requested URL, with the change being reflected on the browser address bar.
Without this flag, Apache would return the website root contents, but the browser would still think the requested page URL exists and would show the originally requested address on the address bar.
mod_rewrite lets you create human-readable URLs. In this tutorial, you used the RewriteRule directive to redirect URLs, including ones with query strings. You also wrote conditionally redirecting 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.