Re: [Libguestfs] Provide NBD via Browser over Websockets

[Richard W.M. Jones]

On Fri, May 29, 2020 at 09:08:29PM +0000, Eric Wheeler wrote:
> On Fri, 29 May 2020, Richard W.M. Jones wrote:
> > On Fri, May 29, 2020 at 08:58:06AM -0500, Eric Blake wrote:
> > > On 5/29/20 8:50 AM, Daniel P. Berrangé wrote:
> > > 
> > > >>>(2) You need to persuade qemu's NBD client to read from a WebSocket.
> > > >>>I didn't really know anything about WebSockets until today but it
> > > >>>seems as if they are a full-duplex protocol layered on top of HTTP [a].
> > > >>>Is there a WebSocket proxy that turns WS into plain TCP (a bit like
> > > >>>stunnel)?  Google suggests [b].
> > > >>>
> > > >>>[a] https://en.wikipedia.org/wiki/WebSocket#Protocol_handshake
> > > >>>[b] https://github.com/novnc/websockify
> > > >>
> > > >>qemu already knows how to connect as a client to websockets; Dan 
> > > >>Berrange
> > > >>knows more about that setup.  I suspect it would not be too difficult to
> > > >>teach the qemu NBD client code to use a WebSocket instead of a Unix or 
> > > >>TCP
> > > >>socket as its data source.
> > > >
> > > >Actually the inverse. The QIOChannelWebsocket impl is only the server
> > > >side of the problem, as used by QEMU's VNC server. We've never 
> > > >implemented
> > > >the client side. There is nothing especially stopping us doing that - 
> > > >just
> > > >needs someone motivated with time to work on it.
> > > 
> > > In the meantime, you may still be able to set up something like:
> > > 
> > > local machine:
> > > iso -> NBD server -> Unix socket -> websockify -> WebSocket
> > 
> > I guess the idea is to have a zero-install solution for the browser.
> > As I said in the email earlier this is very common for IPMI-type
> > remote access to blade servers and in my experience is implemented
> > using a Java applet and a proprietary protocol terminated at the BMC
> > (which then emulates a virtual CDROM to the server).  There are some
> > HP blade servers on Red Hat's internal Beaker instance where you can
> > play with this.  For qemu we wouldn't need to invent a new protocol
> > when NBD is available and already implemented (albeit not yet on top
> > of WebSockets).
> > 
> > The NBD server must run inside the browser and therefore be either
> > written from scratch in Javascript, or an existing server
> > cross-compiled to WASM (if that is possible - I don't really know).
> 
> Interesting idea about WASM.  I'll see if I can build one of the simple 
> nbd servers that are around.  Not sure how to link it to the JS file IO, 
> however.

After reading a bit about compiling to WebSockets it sounds like you
can cross-compile a C program, but there's no library support at all.
IOW to port an existing server you'd have to implement enough of POSIX
to make it work.  nbdkit has a liberal license deliberately to make it
possible to chop it up and incorporate it into completely forked
codebases (nbdkit is a plot to make NBD more popular).

But since NBD is pretty simple, a fresh Javascript server might be
easier, especially if you stick to only implementing reads.

Rich.

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