Apache Reverse Proxy Setup

Overview

This lecture explains how to configure Apache HTTP Server (httpd) as a reverse proxy, enabling features like load balancing, failover, and centralized control of backend servers.

Introduction to Reverse Proxy

• Apache httpd can function as a reverse proxy, forwarding client requests to backend servers.
• The reverse proxy hides backend infrastructure from clients for better security and control.
• Common reasons for reverse proxy use include security, high availability, load balancing, and centralized authentication.

Simple Reverse Proxying

• Use ProxyPass to map incoming requests to a backend server.
• ProxyPassReverse rewrites Location: headers in backend responses to refer to the proxy server.
• You can proxy all or only specific URIs using these directives.

Clusters and Balancers

• A group of backend servers, or "balancer", provides load balancing and failover.
• Define a balancer with <Proxy> and BalancerMember directives.
• ProxySet sets the load balancing method, e.g., balancing by I/O bytes.

Balancer and BalancerMember Configuration

• Balancer members' behavior can be adjusted using parameters like loadfactor (traffic distribution) and timeout.
• Example: Assign more traffic or set timeouts per worker (backend server).

Failover Strategies

• Configure hot spares (replacements) and hot standbys for advanced failover.
• Spares take over if a worker fails; standbys are used only if all others are unavailable.
• Balancer sets are tried in order for failover.

Balancer Manager

• The embedded balancer-manager tool displays and allows live adjustment of balancer and worker settings.
• Dynamic changes, including adding/removing workers, can be made via a web interface.
• Restrict access to balancer-manager for security.
• Use BalancerPersist to keep changes after restarts.

Dynamic Health Checks

• The ping parameter and mod_proxy_hcheck module support automatic and dynamic health checks of workers before proxying requests.

BalancerMember Status Flags

• Workers have status flags (e.g., Dis, Stop, Err) indicating availability and behavior.
• Flags help manage load, failover, and error handling in the proxy setup.

Key Terms & Definitions

• Reverse Proxy — Server that forwards client requests to backend servers and returns responses.
• Balancer — A group of backend servers for load balancing and failover.
• BalancerMember/Worker — Individual backend server in a balancer group.
• ProxyPass — Directive mapping requests to backend.
• ProxyPassReverse — Rewrites backend response headers for the client.
• Hot Spare — Backup worker used if another fails.
• Hot Standby — Worker activated only if no main or spare workers are available.
• Balancer Manager — Web interface for managing proxy balancer settings.

Action Items / Next Steps

• Secure the balancer-manager interface before enabling it.
• Enable BalancerPersist to save configuration changes across restarts.
• Review the mod_proxy_hcheck documentation for detailed health check configuration.