DHCPv6 client classification based on DUID.

Simon Hobson dhcp1 at thehobsons.co.uk
Thu Sep 20 07:11:45 UTC 2012

êþÁýÌ–Â’ ÄÌ”•ÂÈ wrote:

>Well thats confusing a little bit. I was hoping 
>that most of the clients should obey dhcpv6 
>protocol as stated in RFC 3315. And that 
>document says that DUID types 0x1 and 0x3 are 
>containing link-layer address.

Yes, *A* MAC address, not necessarily *THE* MAC 
address of the interface you are configuring, nor 
even of a MAC address still installed in the 
device. As long as the device has persistent 
(which is most systems), it should generate a 
DUID the first time one is needed (or at system 
startup) - and then store it 'forever' even if 
the interface(s) change later. If it has multiple 
interfaces, the DUID (note the singular) will 
only use the MAC address from one of them, and 
the DUID should not change if the interface it 
took the MAC address from is removed from the 

Thus you a) have no guarantee that a device 
encodes a MAC address in the DUID, and b) where 
it does, you have no guarantee that it's from the 
interface you are interested in.

If you've followed this list for a while, there 
has been some "intense" discussion regarding the 
lack of MAC address from the interface being 
configured and how this impacts on various 
operating setups (work flows) already in place. 
Mainly, for many, the MAC address is considered 
the least bad persistent identifier we have 
available - and is usually known before even 
plugging in the system, where any DUID generally 
won't be.

Simon Hobson

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