Simple HTTPS Server in C

A lightweight, educational HTTPS server implementation demonstrating systems programming, network engineering, and security concepts using C and OpenSSL.

🎯 Project Overview

This project showcases:

• Systems Programming: BSD Sockets API for low-level network communication
• Security & Cryptography: TLS/SSL handshake implementation using OpenSSL
• Network Engineering: TCP/IP and HTTPS protocol handling
• Memory Management: Proper allocation, deallocation, and error handling

🏗️ Project Structure

simple-https-server/
├── main.c              # Main server implementation
├── Makefile           # Build configuration and automation
├── README.md          # Project documentation
├── server.crt         # SSL certificate (generated automatically)
├── server.key         # SSL private key (generated automatically)
└── https_server       # Compiled executable (after build)

🔧 Prerequisites

Required Dependencies

Ubuntu/Debian:

sudo apt-get update
sudo apt-get install build-essential libssl-dev openssl

CentOS/RHEL/Fedora:

sudo yum install gcc openssl-devel openssl

macOS:

# Install Xcode command line tools
xcode-select --install

# Install OpenSSL via Homebrew
brew install openssl

# Set environment variables for compilation
export CPPFLAGS="-I$(brew --prefix openssl)/include"
export LDFLAGS="-L$(brew --prefix openssl)/lib"

Quick Dependency Installation

The Makefile includes convenience targets:

make install-deps-debian    # For Ubuntu/Debian
make install-deps-redhat    # For CentOS/RHEL/Fedora
make install-deps-macos     # For macOS

🚀 Quick Start

1. Build and Run

# Clone or download the project files
# Navigate to the project directory

# Build everything (server + certificates)
make all

# Run the server
./https_server

2. Test the Server

Open your web browser and navigate to:

https://localhost:8443

Note: You'll see a security warning because we're using a self-signed certificate. Click "Advanced" and "Proceed to localhost" to continue.

3. Alternative Testing with curl

curl -k https://localhost:8443

🔨 Build System

Makefile Targets

Target	Description
make all	Build server and generate certificates
make https_server	Build only the server executable
make certificates	Generate self-signed SSL certificates
make clean	Remove build files
make clean-all	Remove build files and certificates
make test	Build and test the server automatically
make help	Show available targets

Compilation Details

The server is compiled with these flags:

• -lssl -lcrypto: Link against OpenSSL libraries
• -Wall -Wextra: Enable comprehensive warnings
• -std=c99: Use C99 standard
• -O2: Optimize for performance
• -g: Include debugging information

🔐 SSL/TLS Configuration

Certificate Generation

The Makefile automatically generates a self-signed certificate with:

• Algorithm: RSA 2048-bit
• Validity: 365 days
• Subject: CN=localhost, O=Test Server, C=US

Security Features

• Minimum TLS Version: TLS 1.2
• Cipher Suites: Modern secure ciphers (handled by OpenSSL)
• Certificate Validation: Automatic certificate/key matching

🌐 Network Architecture

Server Configuration

• Port: 8443 (standard HTTPS alternative port)
• Protocol: TCP/IP with TLS encryption
• Binding: All network interfaces (0.0.0.0)
• Backlog: 10 pending connections

Connection Flow

1. Socket Creation: Create TCP socket using BSD Sockets API
2. SSL Context: Initialize OpenSSL context with TLS configuration
3. Certificate Loading: Load server certificate and private key
4. Client Accept: Accept incoming TCP connection
5. SSL Handshake: Perform TLS handshake to establish encryption
6. Data Exchange: Read HTTP request and send HTML response
7. Connection Cleanup: Properly close SSL and socket connections

💡 Key Learning Points

Systems Programming Concepts

• File Descriptors: Managing socket file descriptors
• Process Signals: Graceful shutdown handling
• Memory Management: Dynamic allocation with proper cleanup
• Error Handling: Comprehensive error checking and reporting

Network Programming Concepts

• Socket Programming: Creating, binding, listening, and accepting connections
• TCP Protocol: Understanding connection-oriented communication
• HTTP Protocol: Parsing requests and formatting responses
• Client/Server Architecture: Request-response communication pattern

Security Concepts

• Public Key Infrastructure: Certificate and private key usage
• TLS Handshake: Establishing secure, encrypted connections
• Cryptographic Libraries: Integrating OpenSSL for security
• Certificate Management: Loading and validating certificates

🔄 Extending to Multi-threading

The current implementation handles one connection at a time. To extend for multiple concurrent connections:

Thread Pool Approach

#include <pthread.h>

// Thread function for handling clients
void* client_handler(void* arg) {
    SSL* ssl = (SSL*)arg;
    handle_client(ssl);
    SSL_shutdown(ssl);
    SSL_free(ssl);
    return NULL;
}

// In main loop, create thread for each connection
pthread_t thread;
pthread_create(&thread, NULL, client_handler, ssl);
pthread_detach(thread);

Process Forking Approach

#include <sys/wait.h>

// In main loop, fork for each connection
pid_t pid = fork();
if (pid == 0) {
    // Child process handles client
    close(server_socket);
    handle_client(ssl);
    exit(0);
} else if (pid > 0) {
    // Parent process continues listening
    SSL_free(ssl);
    close(client_socket);
}

🐛 Troubleshooting

Common Issues

OpenSSL not found:

# Install development headers
sudo apt-get install libssl-dev  # Ubuntu/Debian

Certificate errors:

# Regenerate certificates
make clean-all
make certificates

Permission denied on port 8443:

# Use unprivileged port or run with sudo
./https_server
# or modify SERVER_PORT in main.c to use port > 1024

Connection refused:

• Check if server is running: ps aux | grep https_server
• Verify port is not in use: netstat -an | grep 8443
• Check firewall settings

Debugging

Compile with debug symbols and run with GDB:

make clean
make CFLAGS="-Wall -Wextra -std=c99 -g -DDEBUG"
gdb ./https_server

📚 Additional Resources

• OpenSSL Documentation
• Beej's Guide to Network Programming
• TCP/IP Illustrated
• TLS 1.3 RFC 8446

🤝 Contributing

This is an educational project. Feel free to:

• Add more HTTP methods (POST, PUT, DELETE)
• Implement HTTP/2 support
• Add configuration file support
• Enhance error handling and logging
• Add unit tests