allow to specify the DUID to be used int the DHCPv6 client identifier
option: the dhcp-duid property accepts either a hex string or the
special values "lease", "llt", "ll", "stable-llt", "stable-ll" and
"stable-uuid".
"lease": give priority to the DUID available in the lease file if any,
otherwise fallback to a global default dependant on the dhcp
client used. This is the default and reflects how the DUID
was managed previously.
"ll": enforce generation and use of LL type DUID based on the current
hardware address.
"llt": enforce generation and use of LLT type DUID based on the current
hardware address and a stable time field.
"stable-ll": enforce generation and use of LL type DUID based on a
link layer address derived from the stable id.
"stable-llt": enforce generation and use of LLT type DUID based on
a link layer address and a timestamp both derived from the
stable id.
"stable-uuid": enforce generation and use of a UUID type DUID based on a
uuid generated from the stable id.
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