TCP Proxy

TcpProxy is a Layer-4 reverse proxy. It accepts raw TCP connections and tunnels bytes bidirectionally to a backend server. Because it operates below the HTTP layer, it is protocol-agnostic: database wire formats, raw TLS passthrough, custom binary protocols, and any other TCP-based traffic all flow through unchanged.

Basic usage

use rust_web_server::tcp_proxy::TcpProxy;

// Proxy raw TCP on port 5432 across two PostgreSQL backends
TcpProxy::new(["backend-1:5432", "backend-2:5432"])
    .connect_timeout_ms(3_000)
    .bind("0.0.0.0:5432")
    .unwrap();

.bind() blocks the calling thread indefinitely. Run it in a dedicated thread or process.

Round-robin selection

Backend selection uses an AtomicUsize counter that increments on every accepted connection. The counter wraps around the backend list, distributing connections evenly with no locking:

TcpProxy::new([
    "db-primary:5432",
    "db-replica-1:5432",
    "db-replica-2:5432",
])

Relay model

Each accepted connection spawns a new OS thread that calls relay(). Inside relay(), two more threads are spawned — one per direction — each running std::io::copy in a loop:

client ──► thread A: io::copy(client → backend) ──► backend
client ◄── thread B: io::copy(backend → client) ◄── backend

When either side closes its half of the connection (shutdown(Write)), the other direction drains and the relay thread exits cleanly.

Timeouts and options

TcpProxy::new(backends)
    .connect_timeout_ms(5_000)  // TCP connect to backend (default: 5 000 ms)

There is no read timeout at the L4 level — idle connections are limited only by the OS TCP keepalive settings. Configure net.ipv4.tcp_keepalive_* on the host if you need to reclaim stuck connections.

No TLS termination

TcpProxy forwards raw bytes. If the client sends a TLS ClientHello, that handshake is relayed to the backend as-is. The backend must speak TLS directly. For TLS termination before proxying, use an rws instance configured with --tls-cert-file in front of TcpProxy.

Config-file usage

Activate TcpProxy from rws.config.toml using a [[tcp_proxy]] section:

[[tcp_proxy]]
bind = "0.0.0.0:5432"
backends = ["db-primary:5432", "db-replica:5432"]
connect_timeout_ms = 3000

Multiple [[tcp_proxy]] sections are supported — each starts a separate listener on its own thread.

Use cases

Protocol	Typical port
PostgreSQL	5432
MySQL / MariaDB	3306
Redis	6379
SMTP	25 / 587
IMAP	143 / 993
Raw TLS passthrough	any
Custom binary RPC	any