On 03/06/2018 22:20, Simon Kelley wrote:
I agree that this is an annoying problem. In DHCPv6 even determining the
MAC address of a client is a slightly dodgy operation - there are
circumstances where it's not possible. That notwithstanding, dnsmasq
does it's best, and allows you to configure an address to allocated by
MAC address.

The problem here is that the client changes DUID - the desired address
gets allocated by MAC address once, but when the DUID changes, the
address is already in use by a first DUID/IAID combination, so it can't
be allocated, even if the MAC address is the same.

The problem is how the DUID is generated, not the DUID itself.
DUID-LL is not the default (and shouldn't be either).
DUID-LLT is a good default, but comes with the aforementioned problems.

These problems are very nicely solved with RFC 6355 which adds DUID-UUID where UUID is taken from the hosts firmware. The UUID can then be displayed on the node alongside the MAC address for provisioning.
https://tools.ietf.org/html/rfc6355

The downside is that no client I know of supports this and I keep meaning to add support to dhcpcd for it. The other downside is that not all hosts have a retrievable UUID as it depends on both the OS and host itself - for example some OS's present a UUID based on the CPUID. Of course this only works if all OS's generate the same UUID from the base data.

TL;DR - this isn't a dnsmasq issue and I agree with Simon that it should not allow nor encourage RFC violations.

Roy

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