Comment 7 for bug 1664806

There are two DNS concepts in MAAS that are relevant here:

 - Default DNS server
 - Per-subnet DNS server

If MAAS has information on DNS servers that can/should be used when an IP address is configured on a particular subnet, then you are correct: MAAS can associate the DNS server with the YAML for that specific subnet (and ultimately its interface definition in the YAML).

If MAAS only knows what the default DNS server is and doesn't have any per-subnet information, such as the common case of using a public DNS server, or upstream corporate DNS server, then you are correct: MAAS would need to compute which interface(s) should have that DNS server configured based on [potentially overlapping] static routing via particular interfaces, and/or and the interface being used as the default gateway.

If the interface is configured with DHCP, right now I'm not worried about trying to override the default DNS server(s). That should be left up to the DHCP server to decide.