SOCKS4 Proxy Connector

A complete C implementation of a SOCKS4 proxy client that connects through Tor's SOCKS proxy.

Features

• SOCKS4 Protocol Implementation: Full SOCKS4 client with proper request/response handling
• Tor Integration: Connects to Tor's SOCKS proxy (127.0.0.1:9050)
• Error Handling: Comprehensive error handling for network and protocol issues
• Response Parsing: Interprets all SOCKS4 response codes (90, 91, 92, 93)

Project Structure

connector/
├── connector.c      # Main implementation
├── connector.h      # Headers and SOCKS4 protocol structures
├── Makefile        # Build configuration
└── README.md       # This file

SOCKS4 Protocol

The implementation follows the SOCKS4 protocol specification:

Request Format

+----+----+----+----+----+----+----+----+----+----+....+----+
| VN | CD | DSTPORT |      DSTIP        | USERID       |NULL|
+----+----+----+----+----+----+----+----+----+----+....+----+
   1    1      2              4           variable       1

Response Format

+----+----+----+----+----+----+----+----+
| VN | CD | DSTPORT |      DSTIP        |
+----+----+----+----+----+----+----+----+
   1    1      2              4

Setup and Installation

1. Install Tor (macOS)

brew install tor

2. Install Tor (Linux)

sudo apt install tor

3. Start Tor Service

# macOS
brew services start tor

# Linux
sudo systemctl start tor

4. Verify Tor is Running

lsof -i :9050

This should show Tor listening on port 9050.

Building

make clean && make

Usage

./connector <destination_host> <destination_port>

Examples

# Connect to Google's HTTP server through Tor
./connector google.com 80

# Connect to a web server
./connector example.com 443

Expected Output

Success (with Tor running):

Connected to SOCKS proxy at 127.0.0.1:9050
Sent SOCKS4 request for google.com:80
SOCKS4 connection successful! Ready to relay traffic.

Error (Tor not running):

connect: Connection refused

Response Codes

• 90: Request granted (success)
• 91: Request rejected or failed
• 92: Request rejected - cannot connect to identd on client
• 93: Request rejected - client and identd report different user-ids

Troubleshooting

Connection Refused

• Cause: Tor is not running
• Solution: Start Tor service as shown above

Invalid Response

• Cause: Non-SOCKS4 server or protocol mismatch
• Solution: Verify you're connecting to a SOCKS4 proxy

Implementation Details

• Memory Management: Proper allocation/deallocation of request structures
• Network Byte Order: Correct handling of port and IP address byte ordering
• Error Handling: Comprehensive error checking for all network operations
• Protocol Compliance: Full SOCKS4 specification implementation

Next Steps

• Add SOCKS5 support
• Implement proxy authentication
• Add connection timeout handling
• Support for SOCKS4A (domain name resolution through proxy)