Looks like dnsextd(8)
was removed from macOS in 10.15 Catalina, and building from source isn't easy.
I used the attached patch to build mDNSResponder 1310.140.1, which removes unused functions except for setTrafficClass
which is unused by the target (it passes mDNSFalse for useBackgroundTraffic).
We're planning to use this as a frontend to BIND 9.18.3 with an external daemon to verify TSIG for dynamic updates. With MDM we should be able to easily provision new TSIG keys remotely on servers, then communicate them out-of-band to the daemon and (hopefully) have the process work almost automatically.
I have heard only RC4 is supported in released versions of macOS, and the implementation is broken in macOS Monterey.
Comments welcome