Using DHCPv6 in ipv6 you don't assign setting per MAC but per DUID.
On many linux flavours you will find the DUID in the firtst line of /var/lib/dhcp/dhclient6.XXXX.leases
as default-duid
where XXXX
is the name of the interface.
e.g. default-duid "\000\001\000\001'\305H\356\016\265<\373[\372";
a string with octal escaped bytes if it is not a printable ACSI charakter.
Almost all DHCPv6 servers expect the DUID in hex notation. Following python3 script will convert it:
duid = "\000\001\000\001'\305H\356\016\265<\373[\372"