My host “foobar” used to only exist within my LAN. The Ethernet interface, em1, is manually statically configured, everything worked fine. However, I recently added an external internet interface p2p1, and that NIC is configured with DHCP. The problem is that now many programs use p2p1 for things like ip-address, hostname and nameservice, instead of the LAN interface, em1.

For example my build system is messed up because ant sets the properties to:
ADDR4=76.102.69.112
DOMAIN=hsd1.ca.comcast.net
NAME=c-76-102-69-112

Instead of

ADDR4=192.168.0.3
DOMAIN=hymerfania.com
NAME=foobar

How do I fix this?
Thanks!

The programs aren't using an interface to obtain the local hostname, they use information from /etc/hosts and possibly /etc/HOSTNAME, and the OS performs lookups using the DNS server(s) specified in /etc/resolv.conf.

The problem is probably that the DHCP client modifies these files. You can probably tell it not to with a command line switch or a check mark in the right box. Exactly how you do this, depends on the distribution you're using.