During boot my script will search through the NICs looking for connected networks, and will run dhclient to attempt to obtain a TCP/IP configuration.
Possible causes for not having a valid network connection after boot:
a. Not physically connecting the LAN before booting
b. Not having a DHCP server on the LAN
c. Not having a supported NIC.
You should know if a. or b. are the reason for the trouble. As for c. ...
Check the output of "ifconfig" - the pseudo NICs will be lo0, pflog0, and enc0. Do you have any other NICs shown?
Check your dmesg. Look for devices which are recognized, but for which there is no driver. They are shown as "not configured". Any NICs without drivers?
$ dmesg | grep 'not configured'
|