dhclient does not inform NetworkManager on renews

[Christoph Mathys]

We have buildservers running 12.04. They use a custom NetworkManager
script in /etc/NetworkManager/dispatcher.d/ to create and maintain
their dns entries. The dns server will delete old entries (I presume
after they have been there for 20+ days or so). The lease-time of dhcp
addresses is about 5 days.

I checked the lease-file of dhclient, and it seems to renew the lease
every couple of days, shifting the date for renew/rebind/expire
forward as expected. However, I don't see anything in syslog that
hints that NetworkManager has received any event. That means that my
custom scripts will never run and the dns entry vanishes eventually.

Does dhclient inform NetworkManager when it renews an interface? How?
How can I know when dhclient renews a dhcp lease? I assume I still
can't use NetworkManager and dhclient hook scripts?
(https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/293139)

Thanks!
Christoph