Minimial responses missing glue record.
I have just raised an issue on the NSD mailing list[1] which appears to be a bug in the way nsd handles glue records and it appears that knot suffers from a similar. The issue is slightly different to the one with nsd but the basic problem is that it is possible to receive a referral response that contains no glue records. I believe that a referral should contain at least one glue record and if this is not possible it should then a response with TC=1 should be returned. you can see this issue with the following query
dig ns sub1.example.com. @5.28.63.78 +bufsize=1480
dig ns sub.example.com. @5.28.63.78 +bufsize=1444
The following queries also work for the benefit of testing
dig axfr example.com. @5.28.63.78
dig ch txt version.server @5.28.63.78
[1]https://open.nlnetlabs.nl/pipermail/nsd-users/2016-May/002302.html