I suddenly found myself needing a separate TFTP boot system for various devices on my network. We utilize a TFTP / PXEBoot installation system for deploying computers.

We also use Vo/IP phones. Mostly the Vo/IP phones are on their own physical network, but there's a few places that isn't practical, so those phones are connected to the regular network. For easy deployment of phones, we provide all of their settings with a combination of a web service and a TFTP configuration pointer from DHCP. That conflicts with the other TFTP settings for desktops.

After a bit of research, I finally figured out how to provide a separate DHCP group within our subnet based on a partial MAC address match - allowing me to choose the phone vendor's MAC prefix as a filter for those devices.

Here's an example file:

ddns-update-style none;
default-lease-time 432000;
max-lease-time 432000;
authoritative;
log-facility local7;

class "phones" {
        match if binary-to-ascii(16,8,":",substring(hardware,0,4)) = "1:0:4:13";
}

class "other" {
        match if not(binary-to-ascii(16,8,":",substring(hardware,0,4)) = "1:0:4:13");
}

# subnet
subnet 192.168.1.0 netmask 255.255.255.0 {
        option routers 192.168.1.254;
        option domain-name-servers 192.168.1.8, 192.168.1.9;
        pool {
                range dynamic-bootp 192.168.1.150 192.168.1.199;
                deny members of "phones";
                allow members of "other";
                next-server 192.168.1.251;
                filename "tftpboot/pxelinux.0";
        }
        pool {
                range 192.168.1.120 192.168.1.129;
                allow members of "phones";
                deny members of "other";
                next-server 192.168.1.250;
                option tftp-server-name "http://192.168.1.250/phset.php?mac={mac}";
                server-name "http://192.168.1.250";
                filename "phset.php?mac={mac}";
        }
}

The "1" as the first octet of the MAC address for the phones designates Ethernet as the medium, so it's not truly part of the MAC. The 0:4:13 matches to the 00:04:13 that SNOM uses for their phones, for instance.