IXFR fallback to AXFR if diff is bigger than zone

[Tony Finch]

Klaus Darilion <klaus.mailinglists at pernau.at> wrote:
>
> I wonder how Bind as master handles IXFR when the requested IXFR would
> be much than the AXFR. (For example: if you change the NSEC3 salt).
>
> Are there some mechanisms to detect such a situation and trigger a
> fallback to AXFR or will Bind always perform IXFR?

No. It's only relatively recently that BIND has been able to keep track of
the zone size (which is used for the max-records safety limit) and the
IXFR code has not been taught how to use this information to fall back to
AXFR when that would make sense.

> PS: AFAIK the max journal size is 2 GB.

Not since 9.12 :-) I fixed it so that the default journal size limit is 2x
the zone size. The journal is reduced to half size when it is compacted,
so this limit is enough to support IXFR in any case where it might be
beneficial, but it also supports oversize IXFRs.

> In my case, the IXFR due to NSEC3 salt changes is around 2.5GB. So
> actually I am surprised that such big updates can be handled as IXFR.

Yes, that is curious. Are you sure it isn't actually doing an
IXFR-flavoured AXFR of the whole zone, rather than a delta?

Tony.
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