Assumption about TLSA records

[Viktor Dukhovni]

On Thu, Apr 20, 2017 at 09:27:49AM -0400, John Allen wrote:
On Thu, Apr 20, 2017 at 09:34:27AM -0400, John Allen wrote:
On Thu, Apr 20, 2017 at 09:36:34AM -0400, John Allen wrote:

> Is the following assumption reasonable?
> 
> if there are multiple TLSA dane-ee (type 3) records for a particular
> service, none of which match the current generated record, they can
> (maybe should) be deleted.
> 
> The same "rule" can be could be applied to dane type 2 records.

At all times, TLSA *RRsets* whose TTL has not yet expired (vended
by either the primary or a secondary nameserver) need to contain
at least one RR which matches the *current* certificate chain of
the SMTP server.

To achieve this, the TLSA RRset stored in the master database
needs the both the *current* certificate chain and any *new*
certificate chain planned for deployment within the cache
lifetime of recently served DNS responses.

There is no logical or de jure requirement to serve TLSA records
that match *old* no longer deployed certificate chains.  As soon
as the certificate chain is replaced the old records should go.

In other words, the TLSA records reflect current and near-future
certificate chain state, they need not and should not retain past
state.

-- 
	Viktor.

P.S.  I deliberately said nothing about the certificate usage value,
      it should be clear why.