Proxy Servers Cheat Sheet

Updated 2026-04-30

Next Topic: Quality of Service - QoS Cheat Sheet

Proxy servers are intermediary systems that sit between clients and destination servers, forwarding requests and responses while enabling features like anonymity, caching, security, load balancing, and content filtering. They operate at various layers of the network stack — from Layer 4 (TCP/UDP) transport proxies to Layer 7 (HTTP/HTTPS) application proxies — and serve critical roles in enterprise networks, CDNs, web scraping, privacy protection, and modern microservices architectures. Understanding proxy types, protocols (HTTP, SOCKS, CONNECT), configuration patterns (forward vs. reverse), and operational concerns (authentication, caching, SSL termination, load balancing algorithms) is essential for anyone building, securing, or scaling networked applications.

What This Cheat Sheet Covers

This topic spans 14 focused tables and 123 indexed concepts. Below is a complete table-by-table outline of this topic, spanning foundational concepts through advanced details.

Table 1: Proxy Server TypesTable 2: Proxy Protocols and MethodsTable 3: Proxy Configuration and DirectivesTable 4: Load Balancing AlgorithmsTable 5: Proxy Headers and Client InformationTable 6: Proxy Authentication and SecurityTable 7: Caching and PerformanceTable 8: Reverse Proxy Tools and SoftwareTable 9: Proxy Anonymity LevelsTable 10: Proxy Security and RisksTable 11: Load Balancing Features and PatternsTable 12: Advanced Proxy ConfigurationsTable 13: Proxy Monitoring and ObservabilityTable 14: Proxy Protocol Ports (Common Defaults)

Table 1: Proxy Server Types

The first thing to get straight is what kind of proxy you're dealing with, because the same word covers wildly different jobs. A forward proxy works on behalf of the client, a reverse proxy works on behalf of the server, and everything else here — transparent, caching, filtering, SOCKS, SSL-terminating, MITM, API gateway — is a specialization built on one of those two directions. Knowing which category a tool falls into tells you immediately who it protects and what it's allowed to see.

Type	Example	Description
Forward Proxy	`Client → Proxy → Internet` `(user anonymity, content filtering)`	• Acts on behalf of clients to access external resources • commonly used for corporate web filtering, anonymity, and bypassing geo-restrictions. Clients explicitly configure or are transparently redirected through it
Reverse Proxy	`Internet → Reverse Proxy → Backend Servers` `(load balancing, SSL termination)`	• Acts on behalf of servers and sits in front of backend infrastructure • handles SSL/TLS termination, load balancing, caching, and DDoS protection. Clients are unaware of backend topology
Transparent Proxy	`Traffic intercepted at network layer` `(no client configuration)`	• Intercepts traffic without requiring client configuration; typically deployed at the network gateway using routing rules or WCCP. • Client sees no proxy settings • Often used for caching or content filtering • Can be detected via HTTP headers
Open Proxy	`Publicly accessible proxy 203.0.113.5:8080` `(anonymous browsing)`	• Proxy server accessible to any Internet user • frequently used for anonymity but poses security risks (abuse for spamming, bot traffic, credential theft). Often appears on public proxy lists.
Caching Proxy	`GET /image.jpg → Cache hit (304 Not Modified)`	• Stores copies of frequently accessed content to reduce bandwidth and improve response times • implements HTTP caching headers (Cache-Control, ETag) and cache invalidation strategies • Examples: Squid, Varnish.

Table 1: Proxy Server Types

Type	Example	Description
Forward Proxy	`Client → Proxy → Internet` `(user anonymity, content filtering)`	• Acts on behalf of clients to access external resources • commonly used for corporate web filtering, anonymity, and bypassing geo-restrictions. Clients explicitly configure or are transparently redirected through it
Reverse Proxy	`Internet → Reverse Proxy → Backend Servers` `(load balancing, SSL termination)`	• Acts on behalf of servers and sits in front of backend infrastructure • handles SSL/TLS termination, load balancing, caching, and DDoS protection. Clients are unaware of backend topology
Transparent Proxy	`Traffic intercepted at network layer` `(no client configuration)`	• Intercepts traffic without requiring client configuration; typically deployed at the network gateway using routing rules or WCCP. • Client sees no proxy settings • Often used for caching or content filtering • Can be detected via HTTP headers
Open Proxy	`Publicly accessible proxy 203.0.113.5:8080` `(anonymous browsing)`	• Proxy server accessible to any Internet user • frequently used for anonymity but poses security risks (abuse for spamming, bot traffic, credential theft). Often appears on public proxy lists.
Caching Proxy	`GET /image.jpg → Cache hit (304 Not Modified)`	• Stores copies of frequently accessed content to reduce bandwidth and improve response times • implements HTTP caching headers (Cache-Control, ETag) and cache invalidation strategies • Examples: Squid, Varnish.