PPTP VPN lost when DHCP lease renewed

We are using some laptops with Windows XP and UMTS wireless wan cards.
On these laptops a PPTP VPN is setup to the head office.

Depending on the particular UMTS card driver, the card often implements
a virtual ethernet card which is in DHCP mode for getting the address.
The driver runs some DHCP service. The Windows XP OS requests an
address for the ethernet card, and the DHCP service in the driver answers
with the IP address and other details of the UMTS connection.
This works OK.

However, on some drivers the lease is very short: 5 minutes.
(this happens with Ericsson cards, with Option cards the lease is 8 days).

Our problem is: whenever the DHCP lease needs to be renewed, Windows XP
disconnects the PPTP VPN that is running via this card.
Apparently the renewal of the DHCP release resets soms things.

It is not a problem when normally surfing the web, but for the PPTP VPN
it is unusable because the VPN connection has to be made again every
5 minutes.
(for a card with an 8 days lease it is not a real problem)

Is there some way to tell Windows not to reset the PPTP VPN whenever the
lease of the underlying network card is being renewed?

In message <(E-Mail Removed)> Rob
<(E-Mail Removed)> was claimed to have wrote:

>We are using some laptops with Windows XP and UMTS wireless wan cards.
>On these laptops a PPTP VPN is setup to the head office.
>
>Depending on the particular UMTS card driver, the card often implements
>a virtual ethernet card which is in DHCP mode for getting the address.
>The driver runs some DHCP service. The Windows XP OS requests an
>address for the ethernet card, and the DHCP service in the driver answers
>with the IP address and other details of the UMTS connection.
>This works OK.
>
>However, on some drivers the lease is very short: 5 minutes.
>(this happens with Ericsson cards, with Option cards the lease is 8 days).
>
>Our problem is: whenever the DHCP lease needs to be renewed, Windows XP
>disconnects the PPTP VPN that is running via this card.
>Apparently the renewal of the DHCP release resets soms things.
>
>It is not a problem when normally surfing the web, but for the PPTP VPN
>it is unusable because the VPN connection has to be made again every
>5 minutes.
>(for a card with an 8 days lease it is not a real problem)
>
>Is there some way to tell Windows not to reset the PPTP VPN whenever the
>lease of the underlying network card is being renewed?

This isn't normal behaviour at all. Normally leases are renewed
starting at 50% of the leases expiry, so I'm curious if you're seeing
problems at 50% or near 100% of the expiry?

If you're getting close to the expiry then it's likely the DHCP server
isn't responding to renewal requests. There are a couple possibilities,
but first try renewing a lease, check the expiry time with ipconfig/all,
then renew with ipconfig/renew and check again with ipconfig/all, see if
the expiry was extended or not.

If this does get a successful renewal, try again with the VPN up. If
you're not able to renew with the VPN up, but can renew with the VPN
down, it would explain this problem.

My guess is that a routing table change might help, to avoid routing the
DHCP renewal request over the VPN (although in most cases it shouldn't
be routed over the VPN anyway)

Dave Warren <dave-(E-Mail Removed)> wrote:
> This isn't normal behaviour at all. Normally leases are renewed
> starting at 50% of the leases expiry, so I'm curious if you're seeing
> problems at 50% or near 100% of the expiry?
>
> If you're getting close to the expiry then it's likely the DHCP server
> isn't responding to renewal requests. There are a couple possibilities,
> but first try renewing a lease, check the expiry time with ipconfig/all,
> then renew with ipconfig/renew and check again with ipconfig/all, see if
> the expiry was extended or not.

I will test this further when I am back at work tuesday, but what I
know now is that the VPN is being disconnected at the end of the lease
time, and that I can reconnect it at that moment.
It could well be that the DHCP lease cannot be renewed when the VPN
is up, and when the VPN goes down because the lease expires it can then
be renewed and the VPN can again be connected.
I think you may have a good point there...

> My guess is that a routing table change might help, to avoid routing the
> DHCP renewal request over the VPN (although in most cases it shouldn't
> be routed over the VPN anyway)

The VPN gets the default route. It may be that the fake "DHCP server"
that is inside the driver has an address that is outside the area
reachable via the entries in the routing table (apart from the default).

Unfortunately I have not yet found a way to tweak the lease time the
UMTS card driver DHCP server supplies. With a lease longer than one
day this issue would not exist. I have asked the supplier of the card
for this info, but they don't understand what I am asking.

Thanks to your hint I have found the underlying problem.
The system has a group policy defined firewall (in IP security policies) that
did not allow the traffic between the emulated DHCP server and the local system.
Apparently this prevents the DHCP lease from renewing, but it does not prevent
the new lease to be obtained. When time elapses and the lease cannot be renewed,
the interface goes down and the PPTP tunnel is closed, then the new lease is
obtained and the Internet connection is alive again, but the PPTP tunnel remains
down.
(I don't know if a new lease works because this is done using broadcasts as
communication with the DHCP server, or of the security policies are not in
effect when the interface is coming up)

Anyway, thanks for your response as it has been making me think about the
problem in a different way!