I (by accident) discovered that glibc has introduced a new resolver
option in resolv.h:

#define RES_TRUSTAD     0x04000000 /* Request AD bit, keep it in
responses.  */

I've done some testing with this, and it resolves the issue with the AD
flag not being returned.

So based on this I think this bug needs to be changed back to postfix,
and postfix needs to be updated to include this flag? Ideally the
behaviour require should be:

* If RES_TRUSTAD is defined, then postfix should use that instead of 
RES_USE_DNSSEC and RES_USE_EDNS0.
* If RES_TRUSTAD is not defined, then postfix should maintain current behaviour 
of using RES_USE_DNSSEC and RES_USE_EDNS0.

If the above is implemented it would reduce the size of the DNS queries,
because they won't include the RRSIG records that "come for free" when
the DO bit is set (based on RES_USE_DNSSEC).

Thanks,
Nick.

** Package changed: glibc (Ubuntu) => postfix (Ubuntu)

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Title:
  after upgrade to 20.04: posttls cannot connect to private/tlsmgr

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