Comment 16 for bug 105251

Just a comment: 'exit' is not the only way to terminate a shell script prematurely; if any of the scripts call 'set -e' and then any of the commands executed after that fails (i.e. returns a non-zero exit code) the script will terminate.

I would suggest the same debugging technique: adding

  exec >> /tmp/dhcp.debug 2>&1
  set -x

somewhere near the top of the first dhclient exit hook, and then looking at the execution log in /tmp/dhcp.debug to see whether the dbus-send command from /etc/dhcp3/dhclient-exit-hooks.d/zzzz_dhcdbd got executed or not.