This information is based on Linux Centos 5.5, other versions of Linux may vary.
Edit dhcpd.conf with the following lines:
class "UEFI-64-1" {
match if substring(option vendor-class-identifier, 0, 20) = "PXEClient:Arch:00007";
filename "ipxe.efi";
}
class "UEFI-64-2" {
match if substring(option vendor-class-identifier, 0, 20) = "PXEClient:Arch:00009";
filename "ipxe.efi";
}
class "Legacy" {
match if substring(option vendor-class-identifier, 0, 20) = "PXEClient:Arch:00000";
filename "undionly.kpxe";
}