Comment 13 for bug 343898

Actually, you gave enough information to know what the problem is. The apparmor profile in 9.04 is specific to the 2.6.28 kernel as included in Ubuntu. There was a bug in the kernel that is worked around in the apparmor profile. This is bug #400349. For people who are using a non-Ubuntu kernel on Ubuntu 9.04 and are seeing this bug, I recommend either using the apparmor profile from dhcp3-client 3.1.2-1ubuntu5 (in Ubuntu 9.10) or disabling the dhclient3 profile (sudo ln -s /etc/apparmor.d/sbin.dhclient3 /etc/apparmor.d/disable) until you can upgrade to Ubuntu 9.10.