On most modern Linux systems, you should be able to configure the network settings in some config file (Manuel gave some good locations) and the system will take care of executing the relevant commands (like
dhclient) for you.
If this is CentOS, you shoud probably set the following in
Just to be sure, check also that
chkconfig --list network returns something like this:
network 0:off 1:off 2:on 3:on 4:on 5:on 6:off
To start, stop and restart you networking, you can use