Help with DHCPv6 client-identifiers

[Simon Hobson]

<scott_stone at trendmicro.com> wrote:

>It seems that the dhcp client identifiers for DHCPv6 are 
>*arbitrary*.  Is there no way to know ahead of time what they would 
>be, since they're not mapped to anything uniquely identifiable on 
>the machine (ie, MAC address)?  Do I need to have someone manually 
>investigate all of these hosts to find their client identifiers or 
>is there something I'm missing here...?

No, you aren't missing anything !
This is a subject that's been flogged to death in the past, but such 
quaint ideas as being able to identify a device by something (more or 
less) immutable* and determinable by looking at a label, such as MAC 
address, isn't supported in the IPv6 DHCP protocol.

AIUI from previous threads (not got into it myself yet), there are 3 
forms for the IPv6 ClientID. One of these is derived from the MAC, 
but is generally only used on devices without persistent storage. 
Other devices generate a unique ID on first boot and then use that - 
until you switch OS or reinstall the OS.

To top it off, the MAC address (hardware) is not included.

I also understand that there is no option for specifying routers via 
DHCP, though that's being reviewed ? The argument for lack of a 
routers option is that it's done by router discovery protocols - but 
that removes a lot of control from network admins.

In short, it would appear that no-one using these sorts of 
assignments was involved in the standards setting process - or if 
they were, then their input was overruled.


* Much debate about this detail.
For most modern systems, the MAC address is pretty well immutable - 
it's not common to be swapping NICs, the majority are built in to the 
motherboard.
The other issue is about devices with multiple NICs - eg a laptop 
with wired and wireless interfaces. The argument for not using the 
MAC address is that by using a Client ID it works the same whichever 
interface(s) is used.

-- 
Simon Hobson

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