MAC vs DUID issue

Simon Hobson dhcp1 at
Wed Apr 2 19:11:42 UTC 2014

Valdinei Rodrigues dos Reis <valmatrix at> wrote:
> We are using DHCP to all of our IPv6 clients, fixing the IPv6 address using the MAC address.
> We do not want to use DUID mapping due to a lot of reasons.
> Some clients report duplicated IPv6 address, and we strive to find whats wrong.
> All the clients have a host declaration, we have checked all the MAC addresses for any typos, and everything is Ok.
> After a lot of work, we discovered that the clients had duplicated DUID, due to our support staff use of clonezilla to deliver the machines to the employees.

There have been "vigorous discussions" before about MAC vs GUID ! My understanding was that the MAC is not delivered in the DHCPv6 packets and are not available to the server to use. The reason for this is that those who designed the spec felt that MAC addresses have several flaws - there can be more than one per device, and they can change due to hardware changes. The counter point (to which I subscribe) is that while MACs have issues, they are for many the least bad option since they are fairly invariant, remain across multiple OSs and OS reinstalls (and in your case, 'careless' cloning).

IIRC there was talk of defining an option to carry the MAC address, but I can't remember what the outcome was, and in any case it could be "some time" (if ever) before all devices send it.

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