TCP Tunnel – Event-Driven Relay Proxy in C++

This project implements a TCP Tunnel (Layer 4 relay proxy) in C++ using the Windows Sockets API (Winsock 2.2). It establishes a bidirectional communication channel between a local client and a remote service, transparently forwarding traffic while allowing real-time payload inspection.

The system was designed as a network protocol study artifact, focusing on:

• TCP connection lifecycle
• Socket multiplexing with select
• Buffer-level traffic inspection
• Controlled relay architecture
• Hex-level payload analysis

Flow Description:

• Tunnel listens on a local port.
• Client establishes TCP connection with the tunnel.
• Tunnel establishes a second TCP connection to the remote service.
• A multiplexed event loop relays traffic in both directions.
• Payload is optionally logged in hexadecimal representation.

This project is relevant for:

• Understanding TCP state transitions
• Studying transparent proxy mechanisms
• Implementing controlled MITM laboratory environments
• Inspecting raw HTTP traffic
• Observing TLS handshake behavior (when paired with SSL tools)
• Experimental traffic analysis

It provides a foundation for:

• TLS interception research
• Protocol fingerprinting
• Deep Packet Inspection (DPI) experimentation
• Traffic manipulation studies
• Intrusion detection prototypes

SETUP:

Remote server:

127.0.0.1:8080

Tunnel:

127.0.0.1:9909

Client test:

curl http://127.0.0.1:9909

Output:

47 45 54 20 2F 20 48 54 54 50 2F 31 2E 31 ...

Which corresponds to: GET / HTTP/1.1 you can see in powershell.

BUILD ENVIRONMENT: Windows 10+ Visual Studio C++17 Winsock2

ws2_32.lib

Decision	Rationale
select() instead of threads	Deterministic behavior and easier state inspection
Hex dump logging	Protocol-level visibility
Blocking sockets	Controlled experimental environment
No external dependencies	Full low-level understanding

Possible Extensions:

• Non-blocking I/O
• epoll / IOCP adaptation
• TLS wrapping via OpenSSL
• Thread pool model
• Traffic filtering rules
• Rate limiting
• Packet modification engine
• Logging to PCAP format
• Performance benchmarking

Educational Purpose:

• This project was developed as part of ongoing studies in:
• Network protocols
• Socket programming
• TCP/IP internals
• Secure communication systems
• Applied cryptography environments
• It serves as a foundational building block for advanced security research projects.

Information Systems student Focus: Network Security & Systems Programming UFSC