There are still some TODOs, however, because I need some guidance on how to deal with thread safety in the TSIG.

So, don't merge this without commenting.

I've created: https://github.com/frankarinnet/axfr

Which might be helpful for testing this. It was the simplest program I could write to demonstrate this bug for manual testing.

But you will still need to:

    set up a DNS server
    with TSIG
    that allows full zone transfers (AXFR)
    configured with a few different-sized zones

For zone sizes, I've been using:

    a small zone (with 8 lines)
    a medium zone (with 8k lines)
    a large zone (with 160k lines)