What is the default frontend used by Zabbix on RHEL?

The default frontend used by Zabbix on RHEL is Apache HTTP Server. Zabbix typically utilizes Apache as the web server for serving its frontend interface. The integration with Apache allows for handling PHP scripts efficiently, which is essential for running Zabbix’s web interface.

Php-fpm is a PHP FastCGI implementation that can serve PHP applications, but it acts as a backend processor for PHP requests rather than a full-fledged web server. While it's commonly used in conjunction with servers like Nginx or Apache, it does not serve content directly as a frontend.

Nginx and Lighttpd are alternative web servers that can also run Zabbix but are not the default on RHEL installations. Thus, the correct choice in this context is the Apache HTTP Server, which serves as the web interface's frontend in the standard installation on RHEL.