Simple TCP Proxy/Datapipe — Small, Reliable TCP Stream Relay

Simple TCP Proxy/Datapipe — Lightweight TCP Forwarder### Overview

A Simple TCP Proxy/Datapipe is a small, focused tool designed to relay TCP traffic between a client and a target server with minimal overhead. Unlike feature-rich proxies or full TCP stacks, a lightweight TCP forwarder emphasizes simplicity, low resource usage, and ease of deployment. It’s useful for port forwarding, testing services, creating ad-hoc tunnels, and handling basic traffic shaping or logging tasks.

Key features

• Minimal configuration: typically started with a command-line specifying listen address/port and destination address/port.
• Low memory and CPU footprint: suitable for small servers, containers, or embedded devices.
• Bidirectional streaming: forwards bytes in both directions until either side closes the connection.
• Optional logging and metrics: ability to log connections, bytes transferred, and basic errors.
• TLS support (optional): can accept or connect over TLS if required, otherwise plain TCP.
• Connection limits and timeouts: simple controls to prevent resource exhaustion.

Typical use cases

• Port forwarding (local and remote)
• Exposing internal services during development without changing firewall rules
• Creating simple tunnels when VPNs are overkill
• Testing and debugging networked applications by injecting delays or logging traffic
• Scripting or chaining small proxies to perform lightweight transformations (e.g., add/remove headers for plain TCP protocols)

How it works (internals)

At its core, a TCP forwarder performs three main tasks per connection:

1. Accept an incoming client connection on a listening socket.
2. Establish an outbound connection to the configured destination.
3. Relay data between client and destination using two independent copy loops (client→destination and destination→client) until EOF or error.

Efficient implementations use non-blocking I/O and event loops (epoll/kqueue/IOCP) or lightweight thread pools to scale. For simplest implementations, blocking sockets and threads per connection are acceptable for low-traffic scenarios.

Example implementations

• Shell / nc: Quick one-liners using netcat (nc) can forward a port but lack robustness.
• Go: The Go standard library makes building a simple TCP forwarder straightforward using goroutines and io.Copy for bi-directional piping.
• Rust: Tokio provides async primitives for building performant forwarders.
• Python: asyncio or socketserver for small scripts; Twisted for more advanced needs.
• C: Low-level control with sockets for embedded or highly optimized use.

Example Go snippet (core relay logic):

// Accept conn and dial dstConn, then: go io.Copy(dstConn, conn) // client -> destination io.Copy(conn, dstConn)    // destination -> client 

Security considerations

• Authentication: a simple forwarder usually lacks authentication; expose it only to trusted networks or add an auth layer.
• Encryption: use TLS if the data is sensitive, or run inside a secure tunnel/VPN.
• Access controls: restrict listen interfaces or use firewall rules.
• Input validation: while TCP is agnostic, if proxying application-layer protocols, avoid accidentally relaying harmful payloads.
• Rate limiting and connection caps: prevent abuse and DoS scenarios by limiting concurrent connections and per-connection bandwidth or timeouts.

Performance tips

• Use asynchronous I/O or a language runtime with lightweight concurrency (Go, Rust/Tokio).
• Reuse buffers for copying to reduce GC/alloc pressure.
• Tune OS socket options: TCP_NODELAY, SO_REUSEADDR, larger receive/send buffers if necessary.
• Monitor and profile under expected load; optimize the hot path where bytes are forwarded.

Example configuration patterns

• Single port forward: listen 0.0.0.0:8080 -> dest.example.com:80
• Local testing: bind 127.0.0.1:9000 -> localhost:9001 to intercept traffic
• Remote relay: run forwarder on a public host to expose an internal service securely (combine with SSH key-based access)

Troubleshooting

• Connection refused: verify destination is reachable and listening.
• Half-closed connections: ensure both copy loops handle EOF and close the peer properly.
• High latency or stalls: check for buffer sizes, blocking calls, or thread exhaustion.
• Resource leaks: ensure sockets are closed on errors and goroutines/threads exit.

When not to use a lightweight forwarder

• Complex routing, HTTP-level manipulations, authentication, content inspection, or caching — use a full proxy (nginx, HAProxy, Envoy).
• Requirements for fine-grained security policies or observability — use dedicated ingress controllers or service meshes.

Conclusion

Simple TCP Proxy/Datapipe — a Lightweight TCP Forwarder — is an efficient, pragmatic tool for straightforward TCP relaying needs. Its strength is in doing one job well: reliably moving bytes between endpoints with minimal configuration and overhead. For production deployments, augment it with TLS, access controls, and monitoring to reduce operational risk.