Скачать или смотреть Matching All Subdomains Except Certain Ones in .htaccess Using mod_rewrite

Learn how to effectively match all subdomains of your domain while excluding specific ones in your `.htaccess` file using negative lookahead and mod_rewrite rules.
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This video is based on the question https://stackoverflow.com/q/74521870/ asked by the user 'Alex Niem' ( https://stackoverflow.com/u/18453424/ ) and on the answer https://stackoverflow.com/a/74522487/ provided by the user 'MrWhite' ( https://stackoverflow.com/u/369434/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Mastering Subdomain Matching in .htaccess

Managing subdomains is a crucial part of web development, especially when you want to create distinct functionalities without affecting the entire domain. One common requirement is to match all subdomains, excluding specific ones. In this guide, we will tackle the challenge of matching all subdomains for example.com while excluding certain subdomains (main1, main2, and main3) using the .htaccess file.

The Problem

Suppose you have several subdomains like 123.example.com, abc.example.com, etc., and you want to ensure that your website serves a special robots.txt file only for subdomains that are not on your exclusion list. Essentially, you want requests to robots.txt to redirect to robots_sub.txt unless the subdomain is any one of the following:

main1.example.com

main2.example.com

main3.example.com

This is where the .htaccess file's mod_rewrite comes into play.

The Solution

To achieve this, we can utilize Apache's mod_rewrite module along with appropriate regular expressions. The key lies in configuring the rewrite conditions correctly to handle exclusions. Let’s break this down step by step.

Step 1: Crafting Your Rewrite Condition

Here is the code that you can add to your .htaccess file:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Understanding the Rewrite Condition

RewriteCond: This line checks if the HTTP host doesn't start with main1, main2, or main3 followed by a dot. The NC flag indicates that the match is case-insensitive.

Negative Lookahead: The use of (?!(main1|main2|main3).) ensures that only the exact subdomain names are excluded. For instance, abcmain1 or main1xyz would still be processed.

Step 3: Using Separate Conditions

Alternatively, you can implement the exclusion using separate conditions:

[[See Video to Reveal this Text or Code Snippet]]

Step 4: How It Works

The ! Prefix: The ! in the first condition negates the expression, meaning the condition is successful when the host header does not start with main1., main2., or main3..

Confirming Subdomain: The second condition ensures that the request is indeed for a subdomain and not the root domain.

Final Results

After implementing the above rewrite rules, here’s the expected behavior:

123.example.com - Match: Redirects to robots_sub.txt

abc.example.com - Match: Redirects to robots_sub.txt

main1.example.com - Exclusion: Normal behavior

zyx.example.com - Match: Redirects to robots_sub.txt

main2.example.com - Exclusion: Normal behavior

main3.example.com - Exclusion: Normal behavior

sub.example.com - Match: Redirects to robots_sub.txt

Conclusion

In this post, we learned how to craft a specific redirect for robots.txt that matches all subdomains of example.com while excluding certain specified subdomains using .htaccess and mod_rewrite rules. This technique allows you to manage content better across your subdomains for tailored user experiences.

Implementing these rules in your .htaccess file not only keeps your website organized but also ensures that search engines crawl and index the appropriate subdomains correctly.

Remember, managing subdomains can be a powerful tool in web development. Keep experimenting with mod_rewrite to fully harness its capabilities!