tls: native bidirectional TLS bridge for kernel-adjacent forwarding (TUN/VPN/proxy workloads)

What is the problem this feature will solve?

Node’s TLSSocket is designed for application-level stream I/O: cleartext is delivered to JavaScript via the streams API ('data', read(), backpressure via pause()/drain). For high-throughput, full-duplex, kernel-adjacent forwarding (TUN/TAP tunnels, VPN bridges, L3 proxies), users must implement a JS pump between a native fd and a TLSSocket.

That pattern degrades badly in practice:

• Per-chunk V8 boundary — TLSWrap::ClearOut() reads via SSL_read, copies into an allocated buffer, and EmitRead() into JS (src/crypto/crypto_tls.cc, kClearOutChunkSize = 16384).
• Extra event-loop turns — sync underlying writes are deferred with SetImmediate before WriteWrap::Done() (EncOut() / empty DoWrite() paths).
• Dual backpressure — the app must coordinate pause/resume between two independent stream endpoints (e.g. TUN poll + TLS socket) in JavaScript.
• No framing help — TLS is a byte stream; L3 frames (e.g. IPv6 packets) often require reassembly across multiple reads in userland.
• Real-world impact: projects doing iOS CoreDevice / CDTunnel-style forwarding (e.g. Appium appium-ios-tuntap) had to leave Node TLS entirely and implement OpenSSL forwarding in a native addon with dedicated blocking I/O threads — duplicating logic that conceptually belongs next to TLSWrap.

The stream API remains correct for HTTP, RPC, etc.; this gap is specifically for payload forwarding where JS should never see the bytes.

What is the feature you are proposing to solve the problem?

Add a first-class native TLS bridge API that pumps cleartext between an encrypted TCP (or pipe) stream and another native I/O endpoint without surfacing payload data to JavaScript.

Proposed API (sketch):

import tls from 'node:tls';
import net from 'node:net';
const tcp = net.connect({ port });
const bridge = tls.createBridge({
  socket: tcp,              // existing net.Socket / fd
  sink: tunFd,              // numeric fd or native handle (TUN, pipe, etc.)
  credentials: { cert, key }, // or secureContext / PSK options
  direction: 'duplex',      // 'duplex' | 'encrypt-only' | 'decrypt-only'
  onError(err) { /* lifecycle */ },
  onClose() { /* cleanup */ },
});
await bridge.handshake();   // or auto on first I/O
bridge.start();
bridge.stop();

Implementation outline (in core, built on existing TLSWrap):

• Reuse TLSWrap + OpenSSL session setup (lockdown cert, TLS-PSK, etc.) — same crypto as tls.connect().
• Run the hot loop in native code (dedicated per-connection thread or integrated read/write cycle in TLSWrap), analogous to ClearIn → ClearOut → EncOut but wired directly to the sink fd instead of EmitRead/DoWrite.
• JavaScript receives only: handshake result, errors, close — not per-packet callbacks.
• Support dup()/fd ownership semantics documented clearly (who closes what).

Success criteria:

1. Sustained bidirectional TLS 1.2 forwarding at path-MTU record sizes without per-chunk JS allocation.
   p99 latency and CPU use materially better than an equivalent socket.on('data') ↔ tun.write() pump.
2. Add-on authors (TUN, userspace VPN, transparent proxies) can delete custom OpenSSL pthread forwarders.

What alternatives have you considered?

• Stay on TLSSocket + streams — Correct for apps, insufficient for tunnel workloads; requires JS pump and suffers the costs above. This is what forced native-addon workarounds today.
• socket.on('data') + onread static buffer — Helps TCP; TLSWrap::ClearOut() still copies before JS (test-tls-onread-static-buffer.js covers TCP onread, not a TLS zero-copy path). Does not remove the V8 boundary for cleartext payload.
• Third-party native addons (custom OpenSSL in N-API) — Works (proven in production) but duplicates TLSWrap session management, cert/PSK handling, and security updates. Every VPN/tunnel project reinvents the same forwarder.
• node:quic patterns — QUIC has native stream handling oriented toward protocol I/O; CDTunnel / lockdown TLS 1.2 over TCP is a different stack and not a drop-in substitute.
• Document “don’t use TLS for this” only — Honest but pushes ecosystem fragmentation; a small, targeted tls.createBridge() (or similar) keeps advanced use cases on supported Node APIs.

Start by reading src/crypto/crypto_tls.cc around TLSWrap::ClearOut(), EncOut(), and DoWrite(), then inspect test-tls-onread-static-buffer.js. This proposal needs maintainer agreement on the API, fd ownership, threading, and lifecycle behavior. Done means agreed tests and benchmarks demonstrate bidirectional TLS forwarding without per-chunk JavaScript payload handling.