SniTun

End-to-End encryption with SNI proxy on top of a TCP multiplexer

Connection flow

                   [ CLIENT ] --AUTH/CONFIG--> [ SESSION MASTER ] (Trusted connection)
                   [ CLIENT ] <--FERNET-TOKEN- [ SESSION MASTER ]
                   [ CLIENT ] --------FERNET-TOKEN---------------------> [ SNITUN ] (Insecure connection)
                   [ CLIENT ] <-------CHALLENGE-RESPONSE-(AES/CBC)-----> [ SNITUN ]


             <--->                                                                  <------------------------------>
[ ENDPOINT ] <---> [ CLIENT ] <---------MULTIPLEXER---(AES/CBC)--------> [ SNITUN ] <------EXTERNAL-CONECTIONS-----> [ DEVICE ]
    |        <--->                                                                  <------------------------------>     |
    |                                                                                                                    |
    | <--------------------------------------------------END-TO-END-SSL------------------------------------------------->|
                                                      (Trusted connection)

Fernet token

The session master creates a Fernet token from the client's configuration (AES/whitelist) and attaches the hostname and a UTC timestamp until which the token is valid.

{
  "valid": 1923841,
  "hostname": "myname.ui.nabu.casa",
  "aes_key": "hexstring",
  "aes_iv": "hexstring"
}

The SniTun server must be able to decrypt this token to validate the client's authenticity. SniTun then initiates a challenge-response handling to validate the AES key and ensure that it is the same client that requested the Fernet token from the session master.

Note: SniTun server does not perform any user authentication!

Challenge/Response

The SniTun server creates a SHA256 hash from a random 40-bit value. This value is encrypted and sent to the client, who then decrypts the value and performs another SHA256 hash with the value and sends it encrypted back to SniTun. If it is valid, the client enters the Multiplexer mode.

Multiplexer Protocol

The header is encrypted using AES/CBC. The payload should be SSL. The ID changes for every TCP connection and is unique for each connection. The size is for the data payload.

The extra information could include the caller IP address for a new message. Otherwise, it is random bits.

|________________________________________________________|
|-----------------HEADER---------------------------------|______________________________________________|
|------ID-----|--FLAG--|--SIZE--|---------EXTRA ---------|--------------------DATA----------------------|
|   16 bytes  | 1 byte | 4 bytes|       11 bytes         |                  variable                    |
|--------------------------------------------------------|----------------------------------------------|

Message Flags/Types:

• 0x01: New | The extra data includes the first byte as an ASCII value of 4 or 6, followed by the caller IP in bytes.
• 0x02: DATA
• 0x04: Close
• 0x08: Ping | The extra data is a ping or pong response to a ping.
• 0x16: Pause the remote reader (added in protocol version 1)
• 0x32: Resume the remote reader (added in protocol version 1)

Configuration via environment variables

The following environment variables, which, to be effective, must be set before importing this package, are available to override internal defaults:

• MULTIPLEXER_INCOMING_QUEUE_MAX_BYTES_CHANNEL - The maximum number of bytes allowed in the incoming queue for each multiplexer channel.
• MULTIPLEXER_INCOMING_QUEUE_MAX_BYTES_CHANNEL_V0 - The maximum number of bytes allowed in the incoming queue for protocol version 0 channels (default: 256MB, larger than standard channels since v0 lacks flow control).
• MULTIPLEXER_INCOMING_QUEUE_LOW_WATERMARK - The low watermark threshold, in bytes, for the incoming queue for each multiplexer channel.
• MULTIPLEXER_INCOMING_QUEUE_HIGH_WATERMARK - The high watermark threshold, in bytes, for the incoming queue for each multiplexer channel.
• MULTIPLEXER_OUTGOING_QUEUE_MAX_BYTES_CHANNEL - The maximum number of bytes allowed in the outgoing queue for the multiplexer channel.
• MULTIPLEXER_OUTGOING_QUEUE_LOW_WATERMARK - The low watermark threshold, in bytes, for the outgoing queue for each multiplexer channel.
• MULTIPLEXER_OUTGOING_QUEUE_HIGH_WATERMARK - The high watermark threshold, in bytes, for the outgoing queue for each multiplexer channel.

Protocol versioning considerations

• The client is responsible for setting the protocol_version key in the token. If no protocol_version is provided, the server must assume protocol version 0.
• The server side must always be updated first when incrementing the protocol version as the client assumes that the server is always running a protocol version that it supports.
• When new message types are added to the Multiplexer, the protocol version must be incremented.