Where do they belong? "ifconfig eth0 up" & "dhclient"

Solution 1:

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 ifconfig and dhclient) for you.

If this is CentOS, you shoud probably set the following in /etc/sysconfig/networking/devices/ifcfg-eth0:

BOOTPROTO=dhcp
ONBOOT=yes

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 /etc/init.d/network [start|stop|restart].

Solution 2:

How to configure networking depends on the distribution you use. On a Red Hat based system you will find those configs in /etc/sysconfig/networking/, Debian based systems store them in /etc/network/interfaces, Gentoo stores it in /etc/conf.d/net and Arch Linux like BSD under /etc/rc.conf. Which distribution of GNU/Linux do you use?

The cannonical path to ifconfig is /sbin/ifconfig, but this won't solve your problem, I think.