dhcpv6 host-id based on a substring of dhcp6.client-id

[Patrick Warichet]

Hello,

I am running isc-dhcpd 4.2.3 on Ubuntu Trusty, I want to use it to provide
ipv6 addresses based on the serial number of the system.
The serial number of the system in in dhcp6.client-id and read like this
00:02:00:00:00:09:46:4f:43:31:39:34:37:52:31:34:33:00 with the serial
number being FOC1947R143
In order to provide the iPXE bootfile I configure the host section like this

host ncs-5001-b {
       host-identifier option dhcp6.client-id
00:02:00:00:00:09:46:4f:43:31:39:34:37:52:31:34:33:00;
       if exists dhcp6.user-class and substring(option dhcp6.user-class, 2,
4) = "iPXE" {
           option dhcp6.bootfile-url = "http://
[fd:30::172:30:0:22]/ncs5k-mini-2.iso";
       }
       fixed-address6 fd:30:12::172.30.12.52;
}

This works perfectly but I would like to have the serial number as a string
so I did this

option dhcp6.serial-number code 9999 = string;
option dhcp6.serial-number = substring(option dhcp6.client-id, 6, 11);

host ncs-5001-b {
            host-identifier option dhcp6.serial-number "FOC1947R143";
            if exists dhcp6.user-class and substring(option
dhcp6.user-class, 2, 4) = "iPXE" {
                option dhcp6.bootfile-url = "http://
[fd:30::172:30:0:22]/ncs5k-mini-2";
            }
            fixed-address6 fd:30:12::172.30.12.52;
}

But this does not work because my understanding is that only options passed
by the client can be used for the host-identifier.
What would be the correct way to have a host id based only on the string of
the serial number ?

Thank you for your time

/Patrick
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