[dns-wg] rev delegation robot and selection of NS to pull zone from

Another question regarding v6 reverse delegation, but possibly also
applicable to v4 reverse...

One of our customers has a somewhat complex DNS setup which makes us
face the situation that in the SOA record the name of the NS where
the zone originates is not in the set of responses to NS queries.

While this is not the common case, I presume, it seems to NOT be "illegal".

The domain has to on-site name servers and is configured to have a slave
at the NCC.

For this zone we received an alert that the ZXFR has failed from the
machine with the name given in the SOA. That box will never be serving
external zone transfer requests.

So - this may just be a glitch in the alerting script, but I am still
left with the question: how does the robot at the NCC's end determine
the "appropriate" host to try zone transfers from?

Any recommendations?

Thanks,
Wilfreid.