relay_server
=====

An OTP application with an esockd-based UDP server.

Build
-----

    $ rebar3 compile

Run
---

    $ rebar3 shell

The UDP listener is configured in `config/sys.config`; the checked-in config
listens on port `1443`, while the code fallback default is `16380`. It accepts
RelayKit UDP relay frames.

Protocol
--------

Each UDP datagram is encrypted with ChaCha20-Poly1305. The encrypted datagram
uses the same combined format as CryptoKit `ChaChaPoly`:

    nonce(12), ciphertext, tag(16)

The decrypted plaintext is one `RKP1` frame:

    magic(4) = "RKP1", version(1) = 1, type(1), reserved(2),
    stream_id(8), payload_len(4), payload(payload_len)

Frame types are `open = 1`, `open_ack = 2`, `data = 3`, `close = 4`, and
`error = 5`.
`open` payload is binary encoded as:

    host_len(2), host(host_len), port(2),
    username_len(2), username(username_len),
    password_len(2), password(password_len)

Integer fields are unsigned big-endian values. `host`, `username`, and
`password` are binary strings. The server is a transparent TCP relay, so TLS
negotiation, HTTP methods, and request bytes are carried inside subsequent
`data` frames without being interpreted by the server.

`username` and `password` are checked against the `relay_server` application
`users` config:

    {users, [
      {admin, "password"}
    ]}