Subdomain
A subdomain is a subdivision of a main domain that exists under and is linked to the main domain. For example, if "example.com" is the main domain, "blog.example.com" would be a subdomain. Subdomains are generally used to provide additional features, services, or content to the main website.

Uses of Subdomains:

• Different Sections: Used to separate different parts of a website. For example, "shop.example.com" could be an e-commerce section, and "support.example.com" could be a support page.

• Internationalization: Subdomains can be used to serve content for different languages or regions. For example, "en.example.com" could offer English content, and "fr.example.com" could offer French content.

• Development and Testing: Different subdomains can be used for development, testing, or staging, such as "dev.example.com" or "staging.example.com".

• SEO and Brand Management: Subdomains can be strategically used for search engine optimization (SEO) and brand management.

DNS and Subdomains:
A subdomain is configured through DNS (Domain Name System) settings. Each subdomain can be redirected to a separate IP address or can use the same server, depending on the DNS settings of the main domain.
In light of this information, subdomains are highly useful for optimizing the organization and structure of a website.

Dynamic Subdomains
Dynamic subdomains refer to user-specific or dynamically generated subdomains. This type of subdomain is commonly used in web applications to allow users or customers to access their specifically defined content or services.

Features and Use Cases of Dynamic Subdomains:

• User-Specific Content: Dynamic subdomains are often automatically created based on usernames or customer IDs. For example, when a user named "john" creates an account, a subdomain like "john.example.com" can be dynamically generated, allowing users to easily access their personal spaces.

• SaaS Applications: Widely used in Software-as-a-Service (SaaS) platforms. For instance, a project management application can dynamically create subdomains like "companyname.example.com" for each customer.

• SEO Advantage: Dynamic subdomains can be indexed by search engines as separate sites, meaning each subdomain can have its own SEO strategy.

• Advanced Routing and Load Balancing: Dynamic subdomains can be used to manage user traffic more efficiently, such as redirecting to different servers or data centers.

How Do Dynamic Subdomains Work?
Dynamic subdomains typically work in conjunction with web server and DNS configurations. When a request arrives, the server analyzes the subdomain and serves the appropriate content or service. In a dynamic subdomain setup, you do not need to predefine subdomains on your server. After configuring DNS and server settings, all incoming requests will be directed to your project's entry file. The next step is to serve user-specific information based on the subdomain, depending on your project and creativity. Example dynamic subdomains: customer1.example.com, customer2.example.com, customer3.example.com, customer...n.example.com.

Techniques to Use Dynamic Subdomains:

• Wildcard DNS Record: Configure a wildcard DNS record (*).example.com in DNS, so any subdomain request is directed to the server.

• Server Routing Logic: The web server analyzes the subdomain in the incoming request and loads the corresponding content or page. This is typically implemented through backend code or routing logic.

• SSL Certificates: If HTTPS is used, wildcard SSL certificates or services like Let's Encrypt can be used to automatically generate certificates for dynamic subdomains.

In summary, dynamic subdomains are an effective method for delivering personalized experiences to users or for building flexible and scalable structures in web applications.

How to Implement?

1. Installing URL Rewrite Module in IIS
   First, ensure that the URL Rewrite module is installed in IIS. If not, you can install it using the Microsoft Web Platform Installer.

2. Editing the web.config File
   Create or edit a web.config file in the root directory of your site. Add the following URL Rewrite rule to your web.config file:

   xml

   <configuration>
       <system.webServer>
           <rewrite>
               <rules>
                   <rule name="Subdomain to Index">
                       <match url=".*" />
                       <conditions>
                           <add input="{HTTP_HOST}" pattern="^(.*)\.yourdomain\.com$" />
                       </conditions>
                       <action type="Rewrite" url="/index.php" />
                   </rule>
               </rules>
           </rewrite>
       </system.webServer>
   </configuration>

   The pattern="^(.*)\.yourdomain\.com$" part captures the subdomain and redirects all subdomain requests to index.php. Replace yourdomain.com with your own domain name.

   • The .* expression matches any part of the URL, ensuring all incoming requests are redirected to index.php.

   • The action of type Rewrite redirects the request to index.php in the background without changing the URL content.

   Save the file and restart IIS. Now, all subdomain requests will be automatically redirected to index.php.

3. Wildcard Subdomain Configuration on IIS
   To properly redirect all wildcard subdomains from Cloudflare on IIS, generally no additional configuration is required on the IIS server. However, to dynamically route all subdomains, you can follow these steps:

   • Site Binding Settings:
     In IIS, set up a site for your domain and add wildcard subdomain support to this site:

     • In IIS Manager, right-click on the site you created for your domain and select Edit Bindings....

     • Click Add... to add a new binding.

     • In the Hostname field, enter *.example.com. This opens unlimited subdomains for your site. This covers all your subdomains.

4. DNS Redirect Settings on Cloudflare

   • Access Cloudflare Dashboard: Log in to your Cloudflare account and select the domain you want to redirect.

   • Go to DNS Settings: Click the DNS tab in the left menu.

   • Create an A Record: Create an A record using your IIS server's IP address:

     • Name: @ (This represents the root domain for yourdomain.com)

     • IPv4 Address: Your IIS server's IP address

     • TTL: Auto

     • Proxy Status: Proxied (This allows Cloudflare to handle and optimize traffic)

   • Additionally, if you want to redirect wildcard subdomains:

     • Name: * (This covers all subdomains)

     • IPv4 Address: Your IIS server's IP address

     • TTL: Auto

     • Proxy Status: Proxied

   • Create CNAME Records (Optional): If you want to redirect specific subdomains, you can add CNAME records:

     • Name: subdomain (The subdomain name you want to redirect)

     • Target: yourdomain.com (or another appropriate domain)

     • Proxy Status: Proxied

SSL/TLS Configuration
To ensure Cloudflare uses SSL/TLS certificates when redirecting traffic to your IIS server, you need to configure the correct SSL/TLS settings.

• Go to the SSL/TLS Tab: Click the SSL/TLS tab in the Cloudflare Dashboard.

• Select SSL Mode: There are different SSL options here. If you have an SSL certificate on your IIS server, you can select Full or Full (Strict) mode.

  • Flexible: Cloudflare uses SSL between the visitor and itself, but the connection to your server is HTTP (not recommended because the connection to the server is not encrypted).

  • Full: SSL is used both between Cloudflare and the visitor and between Cloudflare and your server.

  • Full (Strict): Similar to Full mode, but the SSL certificate on your server must be valid and issued by a trusted certificate authority.

• Always Use HTTPS: If you enable this option, Cloudflare automatically redirects all HTTP requests to HTTPS.

NGINX Configuration
To redirect dynamic subdomains to a single PHP file (index.php) on Nginx, you need to configure Nginx appropriately. This configuration will redirect all incoming subdomain requests to index.php and make subdomain information available in $_SERVER variables.

Steps:

• Server Block: Create a server block to capture dynamic subdomains.

• Redirect Setting: Redirect all requests to index.php.

• Subdomain Parameters: Pass subdomain information to PHP.

Nginx Configuration Example:
Add the following example to your Nginx configuration file. This example will redirect all subdomains to index.php:

nginx

server {
    listen 80;
    server_name *.example.com;
    root /var/www/html;

    index index.php;

    location / {
        try_files $uri $uri/ /index.php?$args;
    }

    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_pass unix:/var/run/php/php7.4-fpm.sock; # Update according to your PHP version and configuration
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        fastcgi_param SUBDOMAIN $subdomain; # Pass subdomain information
        include fastcgi_params;
    }
}

• server_name *.example.com: This configuration captures all subdomains for example.com. For example, all subdomains like sub1.example.com, sub2.example.com will be directed to this server block.

• try_files $uri $uri/ /index.php?$args;: This line redirects all incoming requests to index.php. If the requested file or directory is not found, it redirects to index.php along with incoming parameters.

• fastcgi_param SUBDOMAIN $subdomain;: This line passes subdomain information to PHP as $_SERVER['SUBDOMAIN']. This information can be processed in PHP using window.location.hostname.

• fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;: This line specifies the PHP-FPM socket file. It may need to be updated according to your PHP version and configuration.

Using Subdomain Information in PHP:
You can retrieve subdomain information in PHP as follows:

php

$subdomain = $_SERVER['SUBDOMAIN'] ?? '';
echo "Subdomain: " . $subdomain;

With this configuration, all subdomain requests are redirected to index.php, and subdomain information becomes available for processing in PHP.

The same configuration can also be used for ASP.NET and ASP.NET Core.