Instead of killing the dhclient process in order to run dhclient again to get a new lease, just release the lease.
# /sbin/dhclient -r eth0
DHCPRELEASE on eth0 to 192.168.1.3 port 67
No need to kill it with pkill or killall. It’s already dead.

Now you can ask for another lease.
# /sbin/dhclient eth0
DHCPDISCOVER on eth0 to 255.255.255.255 port 67 interval 8
DHCPOFFER from 192.168.1.1
DHCPREQUEST on eth0 to 255.255.255.255 port 67
DHCPACK FROM 192.168.1.1
bound to 192.168.1.2 — renewal in 601411 seconds.
Posted by admica @ 14 June 2010