shutdown gotcha with Win32 host

Alex Angelopoulos alex "at" bittnet.com
Wed, 06 Mar 2002 17:13:27 +0000


That explains something I've seen, I think.  Comments on the shutdown process
would be appreciated...

I have noted a couple of systems which have had historical problems with
shutdown; after  VNC is installed on them, I've had some confusing experiences
where I attempt to connect after a (failed) remote reboot - and get the NT
default background, sans logon box.  In one extreme case where someone could
not get to a box to reboot it for 3 days, you could connect remotely via VNC,
get the screen, and of course do absolutely nothing.

A few test port scans revealed absolutely nothing except for port 5900
responding (I don't recall whether I checked 5800).

----- Original Message -----
From: "Andrew van der Stock" <ajv "at" greebo.net>
To: <vnc-list "at" uk.research.att.com>
Sent: Wednesday/2002 March 06 08.15
Subject: RE: shutdown gotcha with Win32 host


: It's not the way VNC hooks itself to the video driver, it's the way VNC
: responds to the Service Control Manager's SERVICE_CONTROL_SHUTDOWN
: message - in that we don't. VNC (correctly) dies as soon as the service
: shutdown message is given. But there's a difference between stopping and
: shutting down, and we can leverage that to try and remedy this issue.
:
: Here's the relevant bits from the VNCService.cpp file:
:
: 1019 case SERVICE_CONTROL_STOP:
: 1020 // STOP : The service must stop
: 1021 g_srvstatus.dwCurrentState = SERVICE_STOP_PENDING;
: 1022 ServiceStop();
: 1023 break;
:
: And we do not assert that we are interested in SERVICE_ACCEPT_SHUTDOWN
: in the next function:
:
: 1052 g_srvstatus.dwControlsAccepted = SERVICE_ACCEPT_STOP;
:
: Remedies?
:
: VNC must tell the SCM that it is interested in SERVICE_ACCEPT_SHUTDOWN.
: This will postpone VNC's exit until after the services that do not
: assert this flag, but since VNC has no other service dependencies, it
: will be asked to go down as soon as the other services handlers have
: been called.
:
: We simply ignore the message and set a dwWaitHint of five seconds and
: keep the dwCheckpoint flag incrementing to ensure that we are not
: killed. To ensure that the box goes down fairly promptly, VNC must watch
: if another process (best bet: csrss.exe) from the current desktop dies,
: or hook into the desktop logoff sequence. Once the trigger has been
: given, it's time to go down.
:
: And we need a watchdog - if the shutdown hasn't occurred in say 20
: seconds (roughly the time the NT lets services live for without killing
: them), VNC re-enters the land of the living by completely ignoring the
: SERVICE_CONTROL_SHUTDOWN message. This makes the VNC process unkillable
: if it survives the SCM trying to kill it, and it's the bane of all
: system administrators everywhere.
:
: A potential problem is if the solution gets too complex. The more
: complex it is to cover off all the corner situations, the harder it'll
: be to make it reliable and robust.
:
: Andrew
:
: -----Original Message-----
: From: owner-vnc-list "at" uk.research.att.com
: [mailto:owner-vnc-list "at" uk.research.att.com] On Behalf Of Keith Hall
: Sent: Wednesday, 6 March 2002 8:14 PM
: To: 'vnc-list "at" uk.research.att.com'
: Subject: RE: shutdown gotcha with Win32 host
:
: [snip]
:
: Naturally it would be nice for someone to have a real good think about
: the
: way VNC hooks into the display driver (reason I've been told that it
: can't
: be done easily) and ensure that it cannot be shut down until almost the
: last
: thing before reboot.
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