How To Set Up mod_rewrite for Apache on CentOS 7

Introduction

Apache is a modular web server that allows you to customize its capabilities by enabling and disabling modules. This provides administrators the ability to tailor the functionality of Apache to meet the needs of their web application.

In this tutorial, we will install Apache on a CentOS 7 server, confirm that the mod_rewrite module is enabled, and explore some essential functions.

Prerequisite

Before following this tutorial, make sure you have a regular, non-root user with sudo privileges. You can learn more about how to set up a user with these privileges from our guide, How To Create a Sudo User on CentOS.

Step 1 – Installing Apache

We will install Apache using yum, the default package management utility for CentOS.

sudo yum install httpd

When prompted with Is this ok [y/d/N]: message, type Y and press the ENTER key to authorize the installation.

Next, start the Apache daemon, a standalone process that creates a pool of child processes or threads to handle requests, with the systemctl utility:

sudo systemctl start httpd

To make sure Apache successfully started, check its state with the status command:

sudo systemctl status httpd

Output
. . .
systemd[1]: Starting The Apache HTTP Server...
systemd[1]: Started The Apache HTTP Server.

With Apache up and running, let’s turn our attention to its modules.

Step 2 – Verifying mod_rewrite

As of CentOS version 7, the mod_rewrite Apache module is enabled by default. We will verify this is the case with the httpd command and -M flag, which prints a list of all loaded modules:

httpd -M

Output
 . . .
 remoteip_module (shared)
 reqtimeout_module (shared)
 rewrite_module (shared)
 setenvif_module (shared)
 slotmem_plain_module (shared)
 . . .

If the rewrite_module does not appear in the output, enable it by editing the 00-base.conf file with the vi editor:

sudo vi /etc/httpd/conf.modules.d/00-base.conf

Once the text file opens type i to enter insert mode and then add or uncomment the highlighted line below:

#
# This file loads most of the modules included with the Apache HTTP
# Server itself.
#
. . .
LoadModule rewrite_module modules/mod_rewrite.so
. . .

Now press ESC to leave insert mode. Then, type :x then press the ENTER key to save and exit the file.

Next, apply the configuration change by restarting Apache:

sudo systemctl restart httpd

With Apache installed and the mod_rewrite module enabled, we’re ready to configure the use of a .htaccess file.

Step 3 – Setting up a .htaccess File

A .htaccess file allows the defining of directives for Apache, including a RewriteRule, on a per domain basis without altering server configuration files. In Linux, files preceded with a dot (.) are treated as hidden.

Before using a .htaccess file, we need to update the AllowOverride setting to be able to overwrite Apache directives.

sudo vi /etc/httpd/conf/httpd.conf

Locate the <Directory /var/www/html> section and change the AllowOverride directive from None to All:

. . .
<Directory /var/www/html>
. . .
 # 
 # AllowOverride controls what directives may be placed in .htaccess files.
 # It can be "All", "None", or any combination of the keywords:
 # Options FileInfo AuthConfig Limit
 #
 AllowOverride All
. . .
</Directory>
. . .

Save and exit the file and then restart Apache to apply the change:

sudo systemctl restart httpd

Next, create a .htaccess file in the default document root, /var/www/html, for Apache.

sudo vi /var/www/html/.htaccess

Add the following line to the top of the file to activate the RewriteEngine, which instructs Apache to process any rules that follow:

RewriteEngine On

Save and exit the file.

You now have a .htaccess file that will let you define rules to manipulate URLs as needed. Before we get into writing actual rules, let’s take a moment to review the basic mod_rewrite syntax.

Step 4 – Exploring the RewriteRule Syntax

The RewriteRule directive allows us to remap request to Apache based off of the URL. A .htaccess file can house more than one rewrite rule, but at run-time Apache applies the rules in their defined order. A rewrite rule consists of the following structure:

RewriteRule Pattern Substitution [Flags]

• RewriteRule: specifies the RewriteRule directive
• Pattern: a PCRE (Perl Compatible Regular Expression) that matches the desired string. You can learn more about regular expressions here.
• Substitution: where should the matching requests be sent
• [Flags]: optional parameters to modify the rule. For more information on the available flags and their meanings, see Apache’s documentation on Rewrite Flags.

The RewriteRule is the workhorse of the mod_rewrite directives, which is why we predominately focus on it in this tutorial.

Step 5 – Exploring the RewriteCond Syntax

The RewriteCond directive allows us to add conditions to a rewrite rule. A rewrite condition consists of the following structure:

RewriteCond TestString Condition [Flags]

• RewriteCond: specifies the RewriteCond directive
• TestString: a string to test against
• Condition: a pattern to match
• [Flags]: optional parameter to modify the condition.

The RewriteCond directive does not allow Apache to consider any rewrite rules that follow it unless the particular condition evaluates to true.

Step 6 – Setting up Files

We will set up a basic rewrite rule to allow users to visit an about.html page without typing the file extension (.html) in the address bar of a web browser. Start by creating an about.html file in the document root directory:

sudo vi /var/www/html/about.html

Copy the following HTML code into the file:

<!DOCTYPE html>
<html>
    <head>
        <title>About Us</title>
    </head>
    <body>
        <h1>About Us</h1>
    </body>
</html>

Save and exit the file.

In a web browser, navigate to the following address:

http://server_domain_or_IP/about.html

You should see a white page with About Us on it. If you remove the .html from the address bar and reload the page, you’ll receive a 404 Not Found error. Apache can only access components by their full filename, but we can alter that with a rewrite rule.

Step 7 – Setting up a RewriteRule

We would like visitors to the About Us page to access it without having to type .html. To accomplish this, we’ll create a rule.

Open the .htaccess file:

sudo vi /var/www/html/.htaccess

After the RewriteEngine On line, add the following:

RewriteRule ^about$ about.html [NC]

Save and exit the file.

Visitors can now access the About Us page with the http://server_domain_or_IP/about URL.

Let’s examine the rewrite rule:

^about$ serves as the pattern that gets matched from the URL, and what the user types into their browser. Our example uses a couple metacharacters to ensure that the term only exists in a particular location in the URL:

• ^ indicates the start of the URL, after server_domain_or_IP/ is stripped away.
• & means the end of the URL

about.html shows the path to the file that Apache serves when it encounters a matching pattern.

[NC] is a flag that instructs the rewrite rule to be case-insensitive so that a user can enter lower and upper case letters in the URL. For example, the following URLs point to the about.html file:

• server_domain_or_IP/about
• server_domain_or_IP/About
• server_domain_or_IP/ABOUT

With a simple rewrite rule, we’ve added a dynamic aspect to how users can access the About Us page.

Common Patterns

Now that we have a basic understanding of rewrite rules, we will explore two additional examples in this section.

Example files can be set up, but this tutorial does not include creating them; just the rewrite rules themselves.

Example 1: Simplifying Query Strings with a RewriteRule

Web applications often make use of query strings, which are appended to a URL using the question mark character (?) and delimited by the ampersand character (&). Apache ignores these two characters when matching rewrite rules. However, sometimes query strings may be required for passing data between pages. For example, the URL for a search result page written in PHP may look like this:

http://example.com/results.php?item=shoes&type=women

Instead, we would like our visitors to be able to use the following cleaner URL:

http://example.com/shoes/women

We can achieve these results in one of two ways — through a simple replacement or matching options.

Example 1A: Simple Replacement

We’ll create a rewrite rule that performs a simple replacement, simplifying a long query URL:

RewriteRule ^shoes/women$ results.php?item=shoes&type=women

The rule maps shoes/women to results.php?item=shoes&type=women.

Example 1B: Matching Options

In some cases, we might want to generalize the query string to include different types of shoes. We can accomplish this by doing the following:

• Specify a series of options using the vertical pipe |, the Boolean “OR” operator
• Group the match using (), then reference the group using the $1 variable, with 1 for the first matched group

The rewrite rule now becomes:

RewriteRule ^shoes/(men|women|youth) results.php?item=shoes&type=$1

The rule shown above matches a URL of shoes/ followed by a specified type. This will modify the original URL so that:

http://example.com/shoes/men

becomes:

http://example.com/results.php?item=shoes&type=men

This matching option allows Apache to evaluate several patterns without having to create a separate rewrite rule for each one.

Example 1C: Matching Character Sets

However, we would also like to specify any item, not limit it to just /shoes. So, we will do the following:

• Write a regular expression that matches all alphanumeric characters. The bracket expression [ ] matches any character inside of it, and the + matches any number of characters specified in the brackets
• Group the match, and reference it with $2 as the second variable in the file

RewriteRule ^([A-Za-z0-9]+)/(men|women|youth) results.php?item=$1&type=$2

The above example will convert:

http://example.com/pants/men

to:

http://example.com/results.php?item=pants&type=men

We successfully expanded the matching ability to include multiple aspects of a URL.

Example 1D: Passing Query Strings

This section doesn’t introduce any new concepts but addresses an issue that may come up. Using the above example, say we would like to redirect http://example.com/pants/men but will pass an additional query string ?page=2. We would like to map the following URL:

http://example.com/pants/men?page=2

to:

http://example.com/results.php?item=pants&type=men&page=2

If you were to attempt to access the above URL with our current settings, you would find that the query string page=2 gets lost. This is easily fixed using an additional QSA flag, which causes the query strings to be combined. Modifying the rewrite rule to match the following will achieve the desired behavior.

RewriteRule ^([A-Za-z0-9]+)/(men|women|youth) results.php?item=$1&type=$2 [QSA]

Example 2: Adding Conditions with Logic

Now we’re going to look at the use of the RewriteCond directive. If a rewrite condition evaluates to true, then Apache considers the RewriteRule that follows it.

Example 2A: Default Page

Previously, we saw Apache handle a request for an invalid URL by delivering a 404 Not Found page. However, instead of an error page, we would like all malformed URLs redirected back to the homepage. Using a condition, we can check if the requested file exists.

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^admin/(.*)$ /admin/home

This will redirect something like /admin/random_text to /admin/home.

Let’s dissect the above rule:

• %{REQUEST_FILENAME} checks the requested string
• !-f the ! or not operator states that if the requested filename does not exist, then execute the following rewrite rule.
• RewriteRule redirects the requests back to /admin/home

Defining the 404 ErrorDocument would follow best practices. To do that, we’ll create an ErrorDocument rule to point 404 errors to an error.html page:

ErrorDocument 404 /error.html

This redirects any request that results in an HTTP 404 response to the error.html page.

Example 2B: IP Address Restriction

A RewriteCond can be used to allow access to a site by a specific IP address.

This example blocks traffic from everywhere except 198.51.100.24.

RewriteCond %{REMOTE_ADDR} !^(198\.51\.100\.24)$
RewriteRule (.*) - [F,L]

The entire rule states that if the IP address requesting resources is not 198.51.100.24, then do not allow access.

In short:

• %{REMOTE_ADDR} is the address string
• !^(198\.51\.100\.24)$ negates the IP address. The \ backslashes escape the . dot, because otherwise, they serve as metacharacters used to match any character.
• The F flag forbids access, and the L flag indicates that this is the last rule to run, if executed.

If you’d rather block access from the specific address, use the following instead:

RewriteCond %{REMOTE_ADDR} ^(198\.51\.100\.24)$
RewriteRule (.*) - [F,L]

Though you can use other methods to block or allow traffic to your site, setting up the restriction in a .htaccess file is the easiest way to achieve these results.

Conclusion

In this tutorial, we used a .htaccess file to work with the RewriteRule and RewriteCond directives. There are many reasons to use rewrite rules and the following resources detail the capabilities of the mod_rewrite module:

• Apache mod_rewrite Introduction
• Apache Documentation for mod_rewrite
• mod_rewrite Cheat Sheet

The mod_rewrite module is a crucial component of the Apache web server, and you can do a lot with it. However, things do not always go according to plan and when that happens you might find yourself with a redirect loop or an ambiguous 500 forbidden error. For tips on debugging these kinds of situations, review this StackOverflow post.