Comment 9 for bug 2041276

Some notes on the issue:

This stems from two possible issues: MAAS allowing overlapping subnets (i.e we can create 10.0.0.0/22 and 10.0.1.0/24), and the fact that glue zones (reverse zones for subnets with a prefix length < /24 or < /124 for IPv6) can also generate a zonefile for a /24 (or /124 for IPv6).

The former is also an issue for our DHCP, as it turns out, you can configure MAAS to have two overlapping subnets and attempt to allocate the same IP twice, we only guard against this by the uniqueness on StaticIPAddress.

It is also worth noting BIND does not crash in 3.3 or newer in this case, however the newer subnet's DNS config will overwrite the older subnet's.

We could solve this by validating against overlapping subnets, which also solves the DHCP issue, but this could potentially break existing deployments. We also can't allow overlapping subnets in separate VLANs due to the way we track IPs in the database, though from a networking perspective, that would work.

Alternatively, we could merge the DNS changes on full reloads (this is already a non-issue for dynamic updates), but the potential DHCP issue would persist.