host-identifier with IPv6

Ted Lemon Ted.Lemon at
Mon Mar 2 18:18:28 UTC 2009

On Mar 2, 2009, at 11:02 AM, Frank Sweetser wrote:
> Right now, we frequently use this idiom in a number of different ways:
> MAC address -> DHCP -> IP -> hostname -> software policies
> where "software policies" is something like Active Directory, which  
> does all
> of the heavy lifting required to turn the machine into a lab  
> machine, desktop,
> or whatever else.
> In other cases, DHCP is configured to point clients to any one of a  
> number of
> different TFTP servers, depending on which administrative group owns  
> them.
> If, contrary to my understanding of RFC 3315, the DHCP server is  
> allowed to
> extract the HW address from the DUID, and use that to give matching  
> behavior
> to DHCPv4, then that gets 90% of what I'm looking for.

Right, there's no prohibition in any of the RFCs for doing stuff like  
this, because the RFCs are completely agnostic to "software  
policies."   The reason for the restriction in the RFC (at least, this  
is why I wrote that restriction in the text originally) is that we  
don't want anything other than the full DUID to be used for IP address  

> The other 10% is selection of which HW address the DUID gets  
> generated from.
> Given a host with two interfaces, the DHCP administrator has no way  
> of knowing
> which one will be used to generate the DUID.  Unless the client  
> behavior is
> changed to either a) use a deterministic algorithm to pick which  
> interface the
> DUID is generated from, such as always using the lowest MAC address,  
> or b)
> send a complete list of all interface addresses, I don't see a way  
> that I can
> reliably recreate this workflow in IPv6.

I agree that this is a problem in theory, but I would be willing to  
bet that in practice, it's not a problem at all - the behavior of the  
client is almost certainly deterministic.

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