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

[Frediano Ziglio]

> 
> 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.

Frediano