Apache server response headers are an essential part of HTTP communication between the server and client. They provide information about the server, the resources being accessed, and additional metadata that can influence the behavior of web browsers and other clients.

Modifying response headers can be useful for various purposes such as security hardening, performance optimization, or compliance with specific standards or requirements. For example, adding security-related headers like Content-Security-Policy can enhance the security of a web application.

In Apache, response headers can be modified using the mod_headers module. This module provides directives to add, modify, or remove HTTP response headers. On top of all the standard response headers, you can also add your own custom headers if necessary.

1. Launch terminal.
2. Enable headers module for Apache.

   $ sudo a2enmod headers # Ubuntu, Debian and SUSE variants
   Enabling module headers.
   To activate the new configuration, you need to run:
     systemctl restart apache2

   • Distribution with a2enmod support can simply run the command above without having to manually enable the required modules.
   • CentOS and Red Hat enables the module by default so requires no manual action to enable the modules.

   Related: How to enable or disable Apache modules

   Options	Debian, Ubuntu	openSUSE and SLES	Fedora Core, CentOS, RHEL	macOS	homebrew	xampp
   a2enmod support	yes	yes	no	no	no	no
   Modules to install	none
   Module name	n/a			headers
   Loadmodule directive	n/a			LoadModule headers_module <module_locations>/mod_headers.so

3. Open the configuration file for your Apache website using your preferred text editor.

   $ sudo vi /etc/apache2/sites-available/000-mysite.conf

   Related: Location for Apache VirtualHost configuration

4. Locate the section where you want to apply the custom header, such as within a specific VirtualHost or Directory directive.

   <VirtualHost *:80>
           #ServerName www.example.com
    
           ServerAdmin webmaster@localhost
           DocumentRoot /var/www/html
    
           ErrorLog ${APACHE_LOG_DIR}/error.log
           CustomLog ${APACHE_LOG_DIR}/access.log combined
   </VirtualHost>

5. Add or modify headers within the configuration file using the Header directive with your own custom name and value.

   <VirtualHost *:80>
           #ServerName www.example.com
    
           ServerAdmin webmaster@localhost
           DocumentRoot /var/www/html
    
           Header set MyCustomHeader "Set any values here"
    
           ErrorLog ${APACHE_LOG_DIR}/error.log
           CustomLog ${APACHE_LOG_DIR}/access.log combined
   </VirtualHost>

6. Save and exit the text editor.
7. Reload or restart the Apache service to apply the changes.

   $ sudo systemctl restart apache2 # Ubuntu and Debian
   $ sudo systemctl restart httpd # CentOS and Red Hat

   Related: How to manage the Apache web server service

8. Verify Apache returns the configured custom header tools like curl.

   $ curl -I 127.0.0.1
   HTTP/1.1 200 OK
   Date: Sun, 03 Sep 2023 03:41:54 GMT
   Server: Apache/2.4.55 (Ubuntu)
   Last-Modified: Fri, 25 Aug 2023 12:12:15 GMT
   ETag: "29af-603be4163c6a4"
   Accept-Ranges: bytes
   Content-Length: 10671
   Vary: Accept-Encoding
   MyCustomHeader: Set any values here
   Content-Type: text/html

   Look for your custom header in the response to confirm that it's being sent.