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, Debian and SUSE variants
   Considering dependency proxy for proxy_http:
   Enabling module proxy.
   Enabling module proxy_http.

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

   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			proxy, proxy_http
   Loadmodule directive	n/a			LoadModule proxy_module <module_locations>/mod_proxy.so LoadModule proxy_http_module <module_locations>/mod_proxy_http.so

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>

   Location for Apache VirtualHost configuration

4. Restart Apache service.

   $ sudo systemctl restart apache2 # Ubuntu, Debian, openSUSE and SLES
   $ 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