State "old-serial" for nsd-control

[wtoorop]

As requested in Issue #419
"old-serial" is printed as state with nsd-control when the served serial is older than the one received by transfer. Also, state "future-serial" is printed if the served serial is newer than the one received by transfer.

[pettai]

It worked for us 👍 , I've added this patch to our own build

[wtoorop]

It worked for us 👍 , I've added this patch to our own build

Ah! That's good to know! I guess it's ready for review from Wouter and Jannik then :)

[wcawijngaards]

Maybe this output helps debugging. The code looks good to have.

[pettai]

Not sure if the future-serial state is possible, though.
Seems NSD ignores notifies or transfers of older serials prematurely

Jan  9 10:05:34 sunic nsd[2978647]: xfrd: zone example.net ignoring old serial (2025010900/2024123101) from 130.242.125.XXX

And the state becomes refreshing. I've also tried to update serial on disk to a higher and lower value, but that doesn't seem to have any effect either. Perhaps a huge zone is needed for this to be noticed...

[pettai]

Apparently, the catz zone can also end up in a non-ok state, yet reported as state: ok (ref #419 (comment))
Perhaps that could be fixed too before this pull request gets merged?