On 12/31/18 2:21 PM, Viktor Dukhovni wrote:

On Dec 31, 2018, at 2:01 PM, zorion <zorion@autistici.org> wrote:

Ok, I had a hard time finding out what exactly the *trust-anchor* was
supposed to be. I took a guess that it was the combined cert chain, but
obviously that is not it.

A trust-anchor is any issuing CA you designate as trusted, and it does
not have to be a root CA, it can also be any intermediate CA.

Thank you for the explanation!

What exactly is the trust-anchor? Is it the top level cert from the CA
in the chain (#4 in your danecheck below)?

It is any of 2, 3 or 4.  The important constraint with DANE-TA(2), as
explained in https://tools.ietf.org/html/rfc7671#section-5.2.2, is
that if you do choose a root CA as your trust-anchor, unlike the
case in non-DANE PKIX protocols, it MUST be sent to the client along
with the intermediate issuer certificates.

Would that be the smtp_tls_CAfile option in postfix? I've got an
intermediate bundle that I provide to that option in main.cf

To compute the digest of a CA certificate, create a PEM file containing
just that certificate.  Or use my "chaingen" script (attached), which
can process a complete chain of certificates, but DO NOT then publish
all the TLSA records it outputs.  Publish no more than one TLSA record
per certificate in the chain, typically just the "3 1 1" for the EE
cert, and "2 1 1" for the TA certs.  And no need to match at every
level.  At most two trust anchors (typically just one) are enough.

Thanks! Is there a benefit for also publishing the "2 1 1" TA certs if
I'm already publishing the "3 1 1" EE cert?


ps. accidentally only sent this directly to Viktor, sending it to the list.