ttun

Minimal point-to-point TCP TUN tunnel. One server, one client, PSK authentication, AEAD obfuscation.

  ┌─────────────────┐    encrypted TCP    ┌─────────────────┐
  │     client      │ ──────────────────> │     server      │
  │  10.1.1.2/30    │ chacha20-poly1305 / │  10.1.1.1/30    │
  │  fc11::2/126    │ <─── aes-256-gcm ── │  fc11::1/126    │
  └────────┬────────┘                     └────────┬────────┘
           │                                       │
          TUN                                     TUN

Design

• Transport: TCP only. TCP_NODELAY + 15s keepalive on both ends.
• Topology: strictly 1:1. The server accepts one peer; after handshake, the listener is idle until the peer disconnects. Other connection attempts block in the TCP backlog.
• PSK auth: K = blake2s-256("ttun/psk/v1" || psk). Each side sends a 16-byte random nonce; both derive SK = blake2s-256("ttun/sk/v1" || K || Nc || Ns). Each side then encrypts a fixed magic with SK; tag verification on the other side fails iff the PSK differs.
• Wire frame: [u16 BE length][AEAD ciphertext+tag]. Length covers ciphertext+tag (max 65535 → plaintext max 65519). Nonce = dir(1B) || 0(3B) || counter(8B BE); directions differ between client→server and server→client so the two flows never share a nonce.
• Cipher: chacha20-poly1305 (default) or aes-256-gcm. No Noise, no key exchange — just the PSK.
• TUN device: wireguard-go's tun package. Auto-named (utunN on darwin, first free ttunN on linux, ttun/ttun<pid> on windows).
• Addresses are hard-coded (dual-stack): server 10.1.1.1/30 + fc11::1/126, client 10.1.1.2/30 + fc11::2/126. No routes added — application traffic must target the tunnel IP directly, or you add your own routes.

Build

./build.sh        # cross-compiles for darwin/linux/windows into ./build
go build ./...    # just the host

Windows runtime needs wintun.dll (download from https://www.wintun.net matching the EXE architecture) placed next to the binary.

Usage

# server (needs root/admin for TUN)
sudo ./ttun server -k <psk>

# client
sudo ./ttun client -k <psk> -s <server-host>:20203

# AES-GCM instead of ChaCha20-Poly1305
sudo ./ttun server -k <psk> -m aes-256-gcm
sudo ./ttun client -k <psk> -s host:20203 -m aes-256-gcm

Flags:

Flag	Default	Notes
-m	chacha20-poly1305	or aes-256-gcm
-k	(required)	preshared key (arbitrary string)
-l	:20203	server listen address
-s	—	client server host:port
-mtu	1280	TUN MTU

Smoke test (after both endpoints are up, from the client):

ping  10.1.1.1
ping6 fc11::1

Limitations

• No UDP transport, no multiple clients, no IP allocation, no routes, no rate limit. By design.
• TCP-over-TCP: under packet loss expect head-of-line blocking; pick a conservative MTU.
• No replay window: each session derives a fresh SK from the handshake nonces, so cross-session replay is impossible. Within a session, monotonic AEAD counters prevent in-session replay.