If you want to reserve ip (query only) from dhcp server by dhclient in Ubuntu,
you can use the following command to do so.
# dhclient -q -sf /dev/null -lf /tmp/dhclient.lease -pf /tmp/dhclient.pid
Then you can see the lease file (/tmp/dhclient.lease) to get more information.
PS. In my test, dhclient 3.1 ( in Ubuntu 10.04 ) will have an error with no permission.
Therefore, I download the latest version of dhclient (4.1) from isc.
you can get more information from here.
Wish this helps.