In CentOS, Just add
DHCP_HOSTNAME=foobar
in /etc/sysconfig/network-scripts/ifcfg-[devname]
In Debian Just Define "hostname" in interfases which will be used as the Hostname to be requested (pump, dhcpcd, udhcpc) .This Option could only be used under The dhcp Method
PS:edit the dhclient.conf and change this setting also works
send host-name "foobar";