Setting DNS rewrote the connection profile and stopped there, so the
running device kept resolving through whatever it had before. Nothing
noticed while something else always activated the device afterwards: the
profile was written first and NetworkManager picked the resolvers up
when it got round to the interface on its own.
Turning a DHCP client off now applies the profile when the device is not
activated, which moved that activation ahead of the DNS write. A caller
that sets an address, disables DHCP, and then sets resolvers -- the
order a static configuration is naturally applied in -- ends with the
device activated on a profile that had no DNS in it yet, and the write
that follows never reaches the system. The profile carries the
resolvers, resolv.conf carries none, and ipv4.ignore-auto-dns has by
then ruled out the lease's resolvers as well: an interface holding an
address, a route, and no way to resolve a name.
Reapply the device after the change. Unlike turning a client off, this
risks nothing for a caller connected over the interface -- reapply
changes the device in place and does not tear the link down -- so it is
not conditioned on the device being unactivated, which is exactly the
case that needs it.
James Coleman
b3dd8e8118