[Spice-devel] Graceful failure when VNC client connects to SPICE server ?

[Daniel P. Berrange]

On Wed, Jan 25, 2017 at 12:23:35PM -0500, Frediano Ziglio wrote:
> > 
> > When used with QEMU at least, both SPICE and VNC live in the same port
> > range (5900+). It is thus not entirely uncommon for a user to mistakenly
> > connect to a SPICE server with a VNC client, or vica-verca.
> > 
> > When connecting to VNC server with a SPICE client, you quickly get an
> > error. This is because the VNC server sends its short greeting and then
> > sees the RedLinkMess from the SPICE client, realizes its garbage and
> > drops the connection.
> > 
> > When connecting to a SPICE server with a VNC client though, you get an
> > indefinite hang. The VNC client is waiting for the VNC greeting, which
> > the SPICE server will never send. The SPICE server is waiting for the
> > RedLinkMess which the VNC client will never send.
> > 
> > Since VNC is a server sends first protocol and SPICE is a client sends
> > first protocol, it would seem this problem is impossible to fix, but
> > I think it is possible to tweak things so it can be more gracefully
> > handled.
> > 
> > In VNC the protocol starts with the follow data sent:
> > 
> >  Server: "RFB 003.008\n"
> >  Client: "RFB 003.008\n"
> > 
> > What I'm thinking is that we could easily change the VNC client so that
> > it will send some bytes first. eg
> > 
> >  Client: "RFB "
> >  Server: "RFB 003.008\n"
> >  Client: "003.008\n"
> > 
> > From the VNC server POV, it'll still be receiving the same 12 bytes from
> > the client - it just happens that 4 of them might arrive a tiny bit
> > earlier than the other 8 of them. IOW nothing should break in the VNC
> > server from this change.
> > 
> > I tried this, but we still get a hang in the SPICE server. The problem
> > is that the SPICE server code is waiting for the full RedLinkMess
> > message to be received before checking its content.
> > 
> > Can we change the SPICE server so that it reads the 'uint32 magic' first,
> > validates that, and then reads the remainder of RedLinkMess ?
> > 
> > This would solve the problem, as the SPICE server would see 'RFB ' as the
> > magic, realize it was garbage and so close the connection. The VNC client
> > which is waiting for the VNC greeting will thus terminate rather than hanging
> > forever.
> > 
> > Regards,
> > Daniel
> 
> Perhaps another solution would be to add a timeout to spice server so
> after, say, 2/3 seconds it would close the connection.
> 
> This would work even without changing the VNC client.

I'm not a fan of timeouts because this could negatively impact on
genuine spice clients which hit a latency spike on the network
for whatever reason. The incremental read of RedLinkMess by constrast
is guaranteed to not have any chance of negative impact on spice clients.

Regards,
Daniel
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