HTTP Caching Proxy Service

A robust HTTP/1.1 forward proxy server written in C. It acts as an intermediary between clients and origin servers, implementing a custom LRU caching engine and manual TCP stream parsing.

This project demonstrates low-level systems programming with BSD sockets, POSIX threads, custom data structures, and strict resource management.

Key Features

• Multithreaded producer-consumer model: Uses a fixed pthreads worker pool to process accepted client sockets concurrently and remove single-threaded request handling bottlenecks.
• Bounded TCP stream parser: Parses request headers with a state machine over the byte stream (\r\n\r\n detection), handling sticky/half-packet cases with explicit buffer bounds.
• Thread-safe LRU cache: Implements Hash Table + Doubly Linked List for average O(1) lookup/update, including stale-entry eviction and cache replacement under a global cache lock.
• HTTP cache semantics: Respects Cache-Control directives (private, no-store, no-cache, must-revalidate) and max-age expiration behavior.
• Dual-stack networking: Built on raw BSD sockets with support for both IPv4 and IPv6 traffic (struct sockaddr_in6), handling address resolution via getaddrinfo.
• Reliability hardening: Masks SIGPIPE, performs explicit error-path cleanup (malloc/free, fd close), and only caches complete origin responses. No leaks were observed in Valgrind test runs.

Tech Stack

• Language: C (Standard C99)
• Concurrency: POSIX Threads (pthreads), condition variables, lock-based cache synchronization
• Networking: BSD Sockets, TCP/IP
• Tools: Valgrind, GDB, cURL, hey
• Containerization: Docker

System Architecture

graph TD
    Client[Client / Browser] -->|HTTP Request| Proxy[HTTP Proxy Server]
    Proxy -->|accept()| Queue[(Work Queue)]
    Queue -->|dispatch| Workers[Worker Pool (pthreads)]
    Workers -->|Lookup Key| Cache{LRU Cache (Hash+DLL)}
    
    Cache -->|Hit| Workers
    Workers -->|Cached Response| Client
    
    Cache -->|Miss| Origin[Origin Server]
    Origin -->|HTTP Response| Workers
    Workers -->|Update| Cache
    Workers -->|Forward Response| Client
    
    style Cache fill:#f9f,stroke:#333,stroke-width:2px
    style Workers fill:#bbf,stroke:#333,stroke-width:2px

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Project Structure

• main.c: Entry point, work queue + worker pool, request/response stream handling, and cache operations.
• proxy.h: Header file defining proxy constants, cache structures, and interfaces.
• Makefile: Build script to compile the htproxy executable.
• Dockerfile: Minimal environment for reproducible builds and testing.

Usage

1) Build Locally

make

This generates the htproxy executable.

2) Run the Proxy

The proxy accepts a port number (-p) and an optional flag (-c) to enable caching:

# Syntax: ./htproxy -p <port> [-c]
./htproxy -p 8080 -c

3) Test with cURL

You can configure your browser to use the proxy, or test directly via command line:

# Send a request through the proxy (-x)
curl -v -x http://localhost:8080 http://www.example.com

4) Run with Docker

Build and run the proxy in an isolated container:

docker build -t http-proxy .
docker run -p 8080:8080 http-proxy