How to configure Apache reverse proxy

Apache can be used as a gateway to backend services by configuring it to act as a reverse proxy server. Requests received by Apache will be passed to and processed by a backend server or service which the response will then be returned to the Apache server before finally being passed back to the requesting client.

This allows an Apache server to listen to a single IP address or a DNS name and serve multiple apps and services. This among other things provide centralised control of the backend services while being more secure by not directly exposing the backend servers which are normally hosted in private networks.

Steps to configure Apache as a reverse proxy server:

1. Launch terminal application.
2. Enable proxy and proxy_http module for Apache.

   $ sudo a2enmod proxy_http # Ubuntu and Debian
   Considering dependency proxy for proxy_http:
   Enabling module proxy.
   Enabling module proxy_http.

   How to enable or disable Apache modules

   Options	apache.org source package	Debian, Ubuntu	Fedora Core, CentOS, RHEL	macOS	Windows	openSUSE and SLES
   Modules to install	none
   Module name	proxy, proxy_http
   Loadmodule directive	Loadmodule xxx Loadmodule yyy

3. Add proxy configuration to desired VirtualHost setting.

   <VirtualHost *:80>
           ProxyRequests Off
   
           ProxyPass "/backend-service-01" "http://backend-service-01.local/"
           ProxyPassReverse "/backend-service-01"  "http:///backend-service-01.local"
   
           ProxyPass "/backend-service-02" "http://backend-service-02.local/"
           ProxyPassReverse "/backend-service-02"  "http:///backend-service-02.local"
   </VirtualHost>

   Apache configuration files

4. Restart Apache service.

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

   How to start, restart and stop Apache service

5. Test direct access to backend service.

   $ curl http://backend-service-01.local
   I am backend-service-01.local

   The request will fail if the backend service is hosted in a private network under a NAT.

   This test could also be done by browsing the URL using a web browser.

6. Test accessing backend service via configured Apache reverse proxy.

   $ curl http://proxy-server/backend-service-01
   I am backend-service-01.local