Comment 149 for bug 548992

The bug seems related to various errors never reported by NetworkManager:

For WLAN:
authentication is perfect. Communication too, but the connection is terminated because of Reason 3 (whatever this means).
In between the connection is assigned an IP address. Doing:
dhclient wlan0
shortly later the connection is functional and up for hours! But: dhclient throws two error messages.

For USB:
All working, IP is assigned, then deassigned again. No error messages logged. Doing:
dhclient usb0
shortly later the connection is up and running, but dhclient throws two error messages.

I had a look at NetworkManager to find out why no errors are reported --- quite simple: NetworkManager just ignores them, but deconfigures the interface.

The error messages thrown by dhclient:
# dhclient wlan0
Rather than invoking init scripts through /etc/init.d, use the service(8)
utility, e.g. service smbd reload

Since the script you are attempting to invoke has been converted to an
Upstart job, you may also use the reload(8) utility, e.g. reload smbd
RTNETLINK answers: File exists
/etc/dhcp/dhclient-script: line 40: local: `new_resolv.dhcp': not a valid identifier
/etc/dhcp/dhclient-script: line 46: new_resolv.dhcp=/etc/resolv.dhcp.dhclient-new: No such file or directory

I did not look deep enough to find out what "dhclient" is calling while initializing the interface. At least: an IP-address is assigned and the connection is working.