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https://github.com/wayland-transpositor/wprs


https://github.com/wayland-transpositor/wprs

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README 

        
# wprs



Like [xpra](https://en.wikipedia.org/wiki/Xpra), but for Wayland, and written in

Rust.



wprs implements rootless remote desktop access for remote Wayland (and X11, via

XWayland) applications.



## Building



### Source



`cargo build --profile=release-lto  # or release, but debug is unusably slow`



The following dependencies are required for `wprsc`, `wprsd`, `xwayland-xdg-shell`:

* libxkbcommon (-dev on debian)

* libwayland (-dev on debian)



The launcher (`wprs`) requires:

* python3

* psutil (python3-psutil on debian)

* ssh client



### deb



`dpkg-buildpackage --sanitize-env -us -uc -b -d -rfakeroot`



This requires cargo and a rustc matching the one in rust-toolchain.toml to be

installed. The debian rustc package is not used due to being too old.



## Usage



On the remote host, enable wprsd:

```bash

loginctl enable-linger

systemctl --user enable wprsd.service

systemctl --user start wprsd.service

```



On the local host:

```bash

# starts application on the remote host (starts ssh connection, forwards sockets, starts wprsc, runs application)

wprs  run 



# stops local wprs connections, leaving remote session running (tear down ssh connection and forwarded sockets, stops wprsc)

wprs  detach



# attaches to remote wprs session (starts ssh connection, forwards sockets, starts wprsc)

wprs  attach

```



## System Tuning



Increasing linux's socket buffer limits as described in



will result in improved performance.



TODO: test ssh socket forwarding performance with different values of

wmem_default. wprs uses setsockopt to increase its buffer size, but it doesn't

seem that ssh does.



## Configuration Files



You can create configuration files for `wprsc` and `wprsd` instead of passing additional

arguments to `wprs`. To see what options are available, run `wprsc --help` and

`wprsd --help`.



To generate the default configs, run:

```bash

# on your local machine

wprsc --print-default-config-and-exit=true > ~/.config/wprs/wprsc.ron

```

and

```bash

# on your remote machine

wprsd --print-default-config-and-exit=true > ~/.config/wprs/wprsd.ron

```



Then update the `wprsc.ron` and `wprsd.ron` files with your desired settings.



## Current Limitations



Currently only the the Core and XDG shell protocols are implemented. In

particular, hardware rendering/dmabuf support is not yet implemented.



- Touch event support is not yet implemented.

- Drag-and-drop may be wonky in some cases.

- XWayland drag-and-drop is not (yet?) implemented.

- webauthn security keys don't yet work in browsers



Generally, wprs will aim to support as many protocols as feasible, it's a

question of time and prioritization.



## Architecture



On the remote (server) side, `wprsd` implements a wayland compositor using

[Smithay](https://github.com/Smithay/smithay). Instead of compositing and

rendering though, wprsd serializes the state of the wayland session and sends it

to the connected wprsc client using a custom protocol.



On the local (client) side, `wprsc` implements a wayland client (using the

[Smithay Client Toolkit](https://github.com/Smithay/client-toolkit) that creates

local wayland objects that correspond to remote wayland objects. For example, if

a remote application running against wprsd creates a surface and an

xdg-toplevel, wprsc will create a surface with the same contents, an

xdg-toplevel with the same metadata, etc.. From the local compositor's point of

view, wprsc is just a normal application with a bunch of windows. Input and

other events from the local compositor that wprsc are serialized and sent to

wprsd, which forwards them to the appropriate application (the owner of the

surface which the wprsc surface which received the events corresponds to).



wprs supports session resumption (wprsc disconnection and later reconnection and

wprsc restarts). The wayland protocol is not natively resumable in this way

because it relies on shared state between the compositor and client

applications. By implementing a wayland compositor locally relative to the

application, wprsd stores all state necessary for wayland applications and is

also able to store sufficient state (e.g., the buffer contents for each surface

as of the last commit) for a newly-connected wprsc to correctly set up all

necessary wayland objects. wprsc is stateless, but wprsd is not, so a wprsd

restart will still terminate all wayland applications running against it, like

with any other wayland compositor.



Communication between wprsd and wprsc happens over unix domain sockets; wprsd

creates a socket and wprsc connects to it. The default mode of operation is to,

on the client side, use ssh to forward a local socket to the remote wprsd

socket, but a different transport could be used with, for example, socat or a

custom proxy application. A launcher script (`wprs`) is provided which sets up

the ssh socket forwarding.



### Protocol



The custom protocol used to serialize and transmit wayland state between wprsc

and wprsd is a simplified version of the wayland protocol. Wayland objects are

represented as rust types and serialized using

[rkyv](https://github.com/rkyv/rkyv). Unlike the wayland protocol, the wprs

protocol tries to be idempotent when possible. For example, instead of the

repeated back-and-forth involved in created a surface, creating an xdg-surface,

creating an xdg-toplevel, waiting for it to be configured, creating a buffer,

attaching the buffer, and comitting it, wprsd will send a single commit message

to wprsc with the complete state of the surface (surface's attached buffer

contents (if any), its role (if any) and any associated metadata, etc.) and

wprsc will execute the appropriate dance with the local compositor.



Frame callbacks are scheduled locally by wprsd at the configured framerate, they

are not forwarded from wprsc as that would introduce an unacceptable amount of

frame latency due to network round-trips. When no wprsc is connected, wprsd

pauses sending frame callbacks to wayland applications.



Buffer compression is handled using a custom multithreaded and SIMD-accelerated

lossless image compression algorithm:



1. Transpose the image from an [array of structures to a struct of

   arrays](https://en.wikipedia.org/wiki/AoS_and_SoA). This makes the subequent

   steps significantly faster by letting them be implemented with SIMD

   instructions and additionally improves the compression ratio because each

   color channel is more closely spatially correlated with itself than with the

   other

   channels.

2. Apply an adjacent (wrapping) difference to each color channel (differential

   pulse-code modulation). This improves the compression ratio by taking

   advantage of spatial correlation and transforms (for example) a solid-colored

   line into a single color byte and then a sequence of 0-bytes, or a gradient

   into a sequence of 1-bytes, etc.

3. Transform each color channel into a

   [YUV](https://en.wikipedia.org/wiki/Y%E2%80%B2UV)-like color space: `y := g,

   u := b - g, v := r - g, a := a`. This improves the compression ratio in a

   similar way as the previous step but by taking advantage of cross-color

   correlation.

4. Compress the data with zstd.

This algorithm was designed for reasonably good compression ratios while being

extremely fast: single-digit milliseconds per frame. Decompression is done by

inverting those steps.



This protocol is *not stable*: there is no guarantee that different versions of

wprsc and wprsd, or wprsc and wprsd built with different versions of

dependencies or even rustc will be compatible. This may change in the future,

but it will not happen soon.



### Comparison to Waypipe



[Waypipe](https://gitlab.freedesktop.org/mstoeckl/waypipe)'s model is analogous

to X forwarding, while wprs's model is analgous to Xpra. Waypipe ~transparently

forwards messages between the local compositor and the remote application, so

the client ends up being stateful and sessions can only be resumed through

network reconnections, not client restarts. There are tradeoffs to the two

approaches. Waypipe's approach is partially forward-compatible: it can support

new wayland protocols automatically, however those protocols may be broken if

they use shared resources in a way that waypipe doesn't know how to handle.

wprs, on the other hand, requires explicit implementation for every wayland

protocol.



### XWayland



XWayland support is implemented as a separate binary, `xwayland-xdg-shell`. The

binary implements a wayland compositor (but only for the protocol features used

by xwayland) and client, just like wprsd and wprsc, but in a single binary (so

skipping the serialization/deserialization). This is the same model as

[xwayland-proxy-virtwl](https://github.com/talex5/wayland-proxy-virtwl#xwayland-support),

which is itself inspired by

[sommelier](https://chromium.googlesource.com/chromiumos/platform2/+/main/vm_tools/sommelier/).

xwayland-xdg-shell was primarily written (instead of just using

xwayland-proxy-virtwl) so as to share a common design/codebase with wprs and to

make use of common wayland development in the form of Smithay and its wayland

crates. Additionally, xwayland-xdg-shell is more narrowly focused and its sole

purpose is xwayland support, not virtio-gpu or virtwl.



Like xwayland-proxy-virtwl, xwayland-xdg-proxy can be used to implement external

xwayland support for any wayland compositor instead of re-implementing it inside

the compositor. Aside from eliminating the need to implement xwayland support in

every compositor, this approach has been reported to result in better xwayland

scaling than native xwayland support in some compositor, and it allows xwayland

applications to be treated more like regular wayland applications instead of

getting special access.



### Security



wprsd is a wayland compositor, so it has access to all surfaces displayed by

applications running against it and it can inject input into them. Any process

which implements the wprs protocol and connects to the wprs socket will have the

same access. For that reason, the wprs socket is created in a directory which

only the user has access to ($XDG_RUNTIME_DIR) and the socket itself is only

readable/writable by the user. Malicious applications running as the same user

as wprsd can still access this socket, but at that point you have bigger

problems.



wprs does not do any auth of its own, it relies entirely on whatever transport

is being used (ssh, in the default case).



## Thanks



Huge thanks to the following excellent projects for making this project

significantly easier than it otherwise would have been:



- [Smithay](https://github.com/Smithay)

- [rkyv](https://github.com/rkyv/rkyv)

- [tracing](https://github.com/tokio-rs/tracing)

- [Tracy](https://github.com/wolfpld/tracy)



Thanks to [Waypipe](https://gitlab.freedesktop.org/mstoeckl/waypipe) and

[xwayland-proxy-virtwl](https://github.com/talex5/wayland-proxy-virtwl#xwayland-support)

for paving the way in this problem space.