To expand a bit on Lukas' brief description :P

Current Situation¶

Currently we take the record we're proposing to create (a mac/ip pair) can then request all the records the proxy can find either by mac or ip. This returns an array, and we remove entries matching the proposed record. If there's anything left, we report it as a conflict. This is executed with this code:

conflicts = [proxy.record(network, mac), proxy.record(network, ip)].delete_if { |c| c  self }.compact

This causes an issue in the case of multiple-ip leases. If a host boots with a PXE UUID, and then gets a new lease from dhcpcd within the OS, we now have:

[mac1, ip1] - PXE lease
[mac1, ip2] - dhcpcd lease

Given the above code, if we provision this system and change nothing, we will still have an entry for [mac1, ip1] in the conflicts array. This is potentially reported to the user via an overwrite confirmation, but that wouldn't be available via the API (or any plugin which is hiding the host edit form). The same happens if you try to change the IP of a pre-existing host - you have to confirm the Overwrite when you save the host.

Analysis¶

We're struggling to come up with a scenario where we would care about leases with the same mac. Macs are a hardware identifier, and as such, if I see a lease for my mac with a different IP, I would interpret that as "My host is releasing the old IP and requesting a lease for the new IP". Since ISC's lease file is a last-lease-wins system, we don't even need to delete the old leases for this mac (although we should, ideally).

It's still true that we care about an IP conflict. In the opposite case to above, where we vary the mac and keep the same IP, we now have a record like [mac2, ip1]. I would interpret that as "My host (mac2) is requesting an IP that belongs to another host (mac1)". In this scenario, we do indeed need to flag this as a conflict, and raise it to the user for investigation.

Proposal¶

What we're proposing is to change the logic to (note that this can be written much more neatly, the below code is for illustration of the logic):

- conflicts = [proxy.record(network, mac), proxy.record(network, ip)].delete_if { |c| c  self }.compact
+ conflicts = [proxy.record(network, ip)].delete_if { |c| c == self }.compact

In other words, do not request leases-by-mac from the proxy during conflict detection, since we do not actually care about them. We might request leases-by-mac during the orchestration, since we would want to find and remove any leases not matching the current mac/ip pair.

Have we missed a usecase for mac conflicts, or does this sound sensible?