Add the following chunk to your existing ISC dhcpd.conf
file.
if exists user-class and ( option user-class = "iPXE" ) {
filename "http://boot.smidsrod.lan/boot.ipxe";
}
else {
filename "undionly.kpxe";
}
(or see https://gist.github.com/4008017 for a more elaborate setup