[Nottingham] VNC black screen (XDM declaring seesion dead)

Jim Moore jmthelostpacket at googlemail.com
Mon Jan 19 23:01:25 UTC 2009

Michael Quaintance wrote:
> Hi guys
> I know I'm not in Nottingham anymore

Don't worry, you're not missing anything.
>  but I'm hoping someone here can 
> still hit me with a clue stick.
> I'm trying to set up VNC servers so we can persist our xterms when we 
> take our laptops home and connect to the work VPN to continue where we 
> left off.
> I've got a server with RHEL (4 or 5, not sure which) and I've set up 
> some VNC sessions the RedHat way using 
> /etc/sysconfig/vncservers 
> and then 
> service vncserver start
> This works, but if anyone wants to change their resolution, color depth, 
> add a new screen or remove a screen, all the screens have to be killed 
> and restarted. This is no use.
> So I'm trying it the command-line way. I start a server using 
> vncserver -query <machinename> <other options like geometry>

So the server geometry is fixed? The client will usually connect and 
display the window according to the parameters passed here, hence fixed 
on the server process. At least that's how I understand what's happening 
here. Is there such a thing as a dynamic geometry parameter that can be 
passed here that would in theory, negate the need for a client restart?
> This gives me a new screen and I can connect to it and use it fine for 
> the first two minutes.
> After 2 minutes (DNS resolver timeout maybe??) the VNC log file gets an 
> entry
> XDM: too many retransmissions, declaring session dead
> The vnc session then goes black, I still have a cursor that responds but 
> no windowmanager.
sounds like an xserver death to me...
> I'm not even using xdm. The box is configured for gdm but I'm not using 
> that either. The vnc xstartup is calling twm by default (which is fine 
> for our purpose.
> Any ideas why xdm is declaring the session dead? Why does xdm care? How 
> can I stop it caring?
> The sessions started the RedHat way are still running fine. If I change 
> the vnc xstartup to use gnome, the same 2 minute limit is seen.
> -Penfold
> _______________________________________________
> Nottingham mailing list
> Nottingham at mailman.lug.org.uk
> https://mailman.lug.org.uk/mailman/listinfo/nottingham

Yes, weird one that. I've never seen a timeout like that - I've had 
intersite and crossplatform VNC sessions stay up all weekend. Dunno what 
to suggest there except maybe spoofing XDM (don't ask me how, local 
keepalive ping/pong event maybe? That's a kluge but it might work...) 
and fool it into thinking it's receiving the expected data so it doesn't 
time out. I did notice, however, that changing the screen resolution on 
the server did require a client restart (for the same reasons you gave - 
blank screen which I assumed was something to do with the fixed 
resolution of the client window according to the parameters given 
serverside to start the server process in the first place, and the 
change occuring at the server end), which didn't bother me any because I 
had my setup going so that it automagically reconnected if it dropped 
for any reason - at the new screen resolution.


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