How to specify a configuration file for pxelinux from ISC DHCP server 3

option space pxelinux;
option pxelinux.magic code 208 = string;
option pxelinux.configfile code 209 = text;
site-option-space "pxelinux";
option pxelinux.magic f1:00:74:7e;
if exists dhcp-parameter-request-list {
# Always send the PXELINUX options (specified in hexadecimal)
option dhcp-parameter-request-list = concat(option dhcp-parameter-request-list,d0,d1,d2,d3);
}
option pxelinux.configfile "pxelinux.cfg";