Wildcard certificate and DANE/TLSA records

[Viktor Dukhovni]

On Mon, Dec 31, 2018 at 05:33:16PM -0500, zorion wrote:

> > 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

No, you just put all the requisite certificates along with the
private key in a mode 0600, root-owned smtpd_tls_cert_file.  The
server certificate first, then its issuer CA, then any parent issuer
CA, ... up to possibly the root CA, if that's the DANE trust-anchor
matching the TLSA record.  If the "2 1 1" TLSA record is for an
intermediate CA, then the root CA can be left out, but still list
any intermediates above that, for non-DANE clients.

> 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?

Yes, it can simplify rollover and make your deployment less likely
to fail, provided you only make changes that affect one of the
values at a time.  See the slides from my various talks.

That said, I recommend the "3 1 1 + 3 1 1" approach as more secure.
Whatever you do, have monitoring in place and avoid key rollover
processes that temporarily leave your server with TLSA records
that don't match its cert chain, keeping in mind remote DNS caches.

Also don't forget to bump the SOA when chaning DNS records, otherwise
secondary servers may keep serving stale data.  For example, the
below domains only provide the right TLSA record 2/3 of the time:

    mtexx.email. IN MX 10 mx1.mtexx.email.
    niklas.pro. IN MX 10 mx1.mtexx.email.

    ; Correct
    @ns1.totaldns.de.[212.224.81.8]
    @ns3.totaldns.be.[93.190.68.228]
    _25._tcp.mx1.mtexx.email. TLSA  3 1 2 1FDBE5993E99EFAB8B7DF0EF254AF72FA512697787EEA622591000141F8B9836154D600812B306C842524F6B5B747D024A3EBA8337C71538C158AAD4BCDDDCFA

    ; Stale
    @ns2.totaldns.eu.[78.47.63.220]
    _25._tcp.mx1.mtexx.email. TLSA  3 1 1 21DF011460CF75244534EC7E04F7EFB2A980106C7B44EE0997FC5F86A23D7DD1

Which can result in stochastic email delays.  At least the TTL is
only 5 minutes, so given enough retries the mail will likely get
through...

-- 
	Viktor.