To configure a DHCP client manually, modify the /etc/sysconfig/network file to enable networking and the configuration file for each network device in the /etc/sysconfig/network-scripts directory. In this directory, each device should have a configuration file named ifcfg-eth0, where eth0 is the network device name. The /etc/sysconfig/network-scripts/ifcfg-eth0 file should contain the following lines:
DEVICE="eth0" BOOTPROTO="dhcp" ONBOOT="yes"
After you change the values in the configuration files for the NIC you have to deactivate and activate it for the modifications to take effect. The ifdown and ifup commands can be used to do this:
ifdown eth0 ifup eth0