teleport/rfd/0037-desktop-access-protocol.md

authors	state
Andrew Lytvynov (andrew@goteleport.com)	implemented

RFD 37 - Desktop Access - Desktop protocol

What

RFD 33 defines the high-level goals and architecture for Teleport Desktop Access.

This RFD specifies the Desktop protocol - wire protocol used between the OS-specific Teleport desktop service (like windows_desktop_service) and the web client. OS-specific Teleport desktop services are responsible for translating native protocols or APIs (like RDP or X11) into the protocol described here.

Details

Goals

This custom protocol is created with several goals in mind:

• performance - the messages should be compact and fast to encode/decode
• portability - should map easily to standard protocols like RDP
• decoding simplicity - the parsing code should be simple enough for auditing, especially when written in dynamic languages like JavaScript
• extendability - new capabilities can be added in the future

Overview

The protocol consists of if discrete messages, sent between a client and a server. These messages are passed over a secure, authenticated and reliable transport layer, like TLS or a websocket. The protocol leaves all security concerns (authentication, integrity, etc) to the transport layer.

Messages are bi-directional and asynchronous - client and server can send any message at any time to the other end. The expected sequence of messages for a typical desktop connection is described below.

Message sequence

Typical sequence of messages in a desktop session:

+--------+                                +--------+
| client |                                | server |
+--------+                                +--------+
     |           7 - client username          |
     |--------------------------------------->|
     |           1 - client screen spec       |
     |--------------------------------------->|
     |             2 - PNG frame              |
     |<---------------------------------------|
     |             2 - PNG frame              |
     |<---------------------------------------|
     |             3 - mouse move             |
     |--------------------------------------->|
     |             4 - mouse button           |
     |--------------------------------------->|
     |             2 - PNG frame              |
     |<---------------------------------------|
     |             5 - keyboard input         |
     |--------------------------------------->|
     |             2 - PNG frame              |
     |<---------------------------------------|
     |                ....                    |

Note that client username and client screen spec must be the first two messages sent by the client, in that order. Any other incoming messages will be discarded until those two are received.

Message encoding

Each message consists of a sequence of fields. Each field is either fixed size or variable size. The first byte in each message is the message type and defines what fields are expected after it.

Field types

Fields are all numbers, using Go-inspired names. Numbers are encoded in big endian order. For example:

• byte is a single byte
• uint32 is an unsigned 32-bit integer
• int64 is a signed 64-bit integer

Message definitions use the syntax []type to declare a variable size field with elements of type type. The length should be deducted from nearby fields.

Strings are encoded as UTF-8 in a []byte field, with a uint32 length prefix.

Message types

1 - client screen spec

| message type (1) | width uint32 | height uint32 |

This message contains the dimensions of the client view - the dimensions used for drawing the remote desktop image. Sent from client to server.

This message can be sent more than once per session, for example when client resizes their window.

2 - PNG frame

| message type (2) | left uint32 | top uint32 | right uint32 | bottom uint32 | data []byte |

This message contains new bitmap data for a region of the desktop screen. Sent from server to client.

left, top and right, bottom contain the top-left and bottom-right coordinates of the region, in pixels. data contains the PNG-encoded bitmap.

27 - PNG frame 2

| message type (27) | png_length uint32 | left uint32 | top uint32 | right uint32 | bottom uint32 | data []byte |

This is a newer version of the PNG frame message, which includes the length of the PNG data after the message type. This allows for efficiently skipping over the PNG data without performing a PNG decode.

3 - mouse move

| message type (3) | x uint32 | y uint32 |

This message contains new mouse coordinates. Sent from client to server.

4 - mouse button

| message type (4) | button byte | state byte |

This message contains a mouse button update. Sent from client to server.

button identifies which button was used:

• 0 is left button
• 1 is middle button
• 2 is right button

state identifies the button state:

• 0 is not pressed
• 1 is pressed

5 - keyboard input

| message type (5) | key_code uint32 | state byte |

This message contains a keyboard update. Sent from client to server.

key_code is the keyboard code from https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/code/code_values#code_values_on_windows

state identifies the key state:

• 0 is not pressed
• 1 is pressed

Key combinations show up as a sequence of keys going into state=1 and then back to state=0.

6 - clipboard data

| message type (6) | length uint32 | data []byte |

This message contains clipboard data. Sent in either direction. When this message is sent from server to client, it's a "copy" action. When this message is sent from client to server, it's a "paste" action.

7 - client username

| message type (7) | username_length uint32 | username []byte |

This is the first message of the protocol and contains the username to login as on the remote desktop.

8 - mouse wheel scroll

| message type (8) | axis byte | delta int16 |

This message contains a mouse wheel update. Sent from client to server.

axis identifies which axis the scroll happened on:

• 0 is vertical scroll
• 1 is horizontal scroll

delta is the signed scroll distance in pixels.

• on vertical axis, positive delta is up, negative delta is down
• on horizontal axis, positive delta is left, negative delta is right

9 - error

| message type (9) | message_length uint32 | message []byte |

This message indicates an error has occurred.

28 - notification

| message type (28) | message_length uint32 | message []byte | severity byte |

This message sends a notification message with a severity level. Sent from server to client.

message_length denotes the length of the message byte array. It doesn't include the severity byte.

severity defines the severity of the message:

• 0 is for info
• 1 is for a warning
• 2 is for an error

An error (2) means that some fatal problem was encountered and the TDP connection is ending imminently. A notification with severity == 2 should be preferred to the error message above.

A warning (1) means some non-fatal problem was encountered but the TDP connection can still continue.

Info (0) can be used to communicate an arbitrary message back to the client without error semantics.

10 - MFA

| message type (10) | mfa_type byte | length uint32 | JSON []byte |

This message is used to send the MFA challenge to the user when per-session MFA is enabled. It is a container for a JSON-encoded MFA payload.

mfa_type is one of:

• n for Webauthn
• u for U2F

Per-session MFA for desktop access works the same way as it does for SSH sessions. A JSON-encoded challenge is sent over websocket to the user's browser. The only difference is that SSH sessions wrap the MFA JSON in a protobuf encoding, where desktop sessions wrap the MFA JSON in a TDP message.