Proxy Server

• Network intermediary that sits between clients and servers, acting as a gateway for requests from clients seeking resources from other servers
• Client connects to proxy server and requests service (web page, file, connection) from different server
• Proxy evaluates request and performs network transaction on behalf of client
• Key principle: Client thinks it’s talking directly to the server, but proxy handles all communication

Types of Proxy Servers

• Forward Proxy - Traditional proxy between client and internet (what most people mean by “proxy”)
• Reverse Proxy - Sits in front of web servers, intercepting requests from clients (acts on behalf of server, not client)
• Transparent Proxy - Intercepts communication without requiring client configuration (client doesn’t know it exists)
• Anonymous Proxy - Hides client’s IP address from destination server

Forward vs Reverse Proxy Comparison

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FORWARD PROXY:
Client → [Proxy] → Internet → Server
         (hides client)

REVERSE PROXY:
Client → Internet → [Proxy] → Server
                    (protects server)

TRANSPARENT PROXY:
Client → [Hidden Proxy] → Server
         (client unaware)

PROXY REQUEST FLOW:
    Client
      ↓
  [Proxy Server]
      ↓
  Evaluate Request
      ↓
  Forward to Server
      ↓
  Return Response
      ↓
    Client

Common Proxy Functions

• Content Filtering - Block access to specific websites or content types (corporate firewall policies)
• Caching - Store frequently requested content locally to reduce bandwidth and improve response times
• Load Balancing - Distribute incoming requests across multiple backend servers
• SSL Termination - Handle encryption/decryption to offload processing from backend servers
• Authentication - Centralized user authentication before allowing internet access
• Logging/Monitoring - Track user activity and bandwidth usage for compliance

Real-World Use Cases

• Corporate Networks - Squid proxy for web filtering and bandwidth control
• Content Delivery Networks (CDNs) - Reverse proxies cache content closer to users
• API Gateways - Reverse proxy managing API requests, rate limiting, authentication
• School Networks - Transparent proxy blocking social media during class hours

Proxy Server Ports and Protocols

Vocabulary

• Proxy Server - Intermediary server that forwards client requests to other servers
• Forward Proxy - Proxy that acts on behalf of clients (hides client from server)
• Reverse Proxy - Proxy that acts on behalf of servers (hides server from client)
• Transparent Proxy - Proxy that intercepts traffic without client configuration
• SOCKS - Socket Secure protocol for routing network packets between client and server
• SSL Termination - Process where proxy handles SSL encryption/decryption instead of backend server
• Cache Hit - When requested content is found in proxy cache (no need to contact origin server)
• Cache Miss - When requested content not in cache (must retrieve from origin server)

Notes

• Critical distinction: Forward proxy protects client identity, reverse proxy protects server infrastructure
• Proxy authentication typically uses NTLM or Kerberos in Windows environments
• Transparent proxies can cause issues with applications that don’t expect proxy behavior
• Modern browsers support Proxy Auto-Configuration (PAC) files for automatic proxy settings
• Security caveat: Proxies can inspect all traffic, including sensitive data (unless using HTTPS tunneling)
• Load balancer is essentially a specialized reverse proxy focused on distributing traffic
• Performance tip: Proxy caching most effective for static content (images, CSS, JavaScript)
• SOCKS proxies work at lower network level, can handle any protocol (not just HTTP/HTTPS)
• Troubleshooting: Check proxy logs first when users report connectivity issues - often reveals blocked content or authentication failures