[Libguestfs] Provide NBD via Browser over Websockets

[Eric Blake]

[adding qemu list]

On 5/29/20 4:37 AM, Richard W.M. Jones wrote:
> Going back to the original email from 2018:
> 
>> It might be neat to attach ISOs to KVM guests via websockets.  Basically
>> the  browser would be the NBD "server" and an NBD client would run on the
>> hypervisor, then use `virsh change-media vm1 hdc --insert /dev/nbd0` could
>> use an ISO from my desk to boot from.
>>
>> Here's an HTML5 open file example:
>> https://stackoverflow.com/questions/3582671/how-to-open-a-local-disk-file-with-javascript
>>
>> and the NBD protocol looks simple enough to implement in javascript:
>> https://stackoverflow.com/questions/17295140/where-is-the-network-block-device-format-describled
> 
> So I think what you mean here is that in a browser you'd open a local
> (eg) ISO, and then that ISO could be shared with a remote VM.  The
> browser runs a Javascript NBD server.  The remote VM is qemu.  Between
> the two is a WebSocket.
> 
> I've seen this being done with an HP blade server of some kind and
> IIRC the client side used a Java applet.  No idea what the protocol
> was but likely something proprietary.  It was nevertheless a useful
> feature, eg to boot the server from an install CD that you have
> locally.
> 
> I guess the problem is two-fold:
> 
> (1) You need to write an NBD server in Javascript.  Not especially
> difficult, particularly if you avoid any complicated features, and I
> guess you only need read support.

Or use an existing NBD server over a Unix socket paired to a WebSocket 
proxy that forwards all traffic from the Unix socket over a WebSocket. 
That may be easier than writing the NBD server itself in Javascript.

> 
> (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.

> 
> ...
> 
>> When qemu is running headless using a VNC or Spice display we can access
>> the display of https+websockets using things like noVNC---which is out of
>> scope to this converstation---but I'm just saying that such an existing
>> web front-end for the display could be extended to have an "Insert CDROM"
>> button and use the javascript file IO for the user to reference a local
>> file and pass it to qemu over nbd, perhaps via nbdkit.
> 
> I'm not sure how nbdkit would be involved, unless it was compiled
> to WASM or something like that.
> 
> But the plan above sounds feasible, albeit a chunk of work.
> 
> Rich.
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org