issues with ipv6 and DUID

[Simon Hobson]

Cory Coager wrote:
>I have a new setup of DHCP with ipv6.  I'm attempting to use fixed
>addresses so I can do static reservations.  I understand that ipv6
>doesn't use the MAC address but instead uses the DUID.  I also
>understand that DUID has different implementations such as LL, LLT and
>EN as covered in rfc 3315.
>
>What I haven't been able to figure out is how to discover a client's
>DUID from the client.  Is there a tool I can run on a Linux server to
>tell me what the DUID is?  I've only been able to discover it from the
>dhclient in verbose mode.

The MAC address is not used - and there's been "considerable discussion" on this list about it. In short, the MAC address isn't a good identifier for a host since it can a) change, and b) the host may have more than one these days. However, for many situations it's the "least bad" identifier we have, and some (I'm one of them) think that the DUID has it's own share of problems - it's liable to change (eg on reinstallation of an OS), the host may have more than one (eg booting multiple OSs), and it's not generally known in advance (unlike the MAC address which is often on the outside of the box in a barcode lable).
There was some talk of implementing a MAC option, which would be filled in either by the server (on a locally connected network) or the relay agent (on a remote network) - but I don't know how far this went, or whether it's made it into any available relay agents.

AFAIK, the only reliable way of getting it is to wait for the client to boot, and get the DUID from the lease created (or the logfile ?). On a busy network, I'm not sure how you'd identify that specific client.