[ This assumes you are willing to trust your issuer CA to not
misissue certificates for your domain, and include the CA
certificate in your server chain file. ]
One approach to making sure that DANE TLSA records are less likely
to fail that should work well for sites using CA-issued certificates
is to publish both "3 1 1" and "2 1 1" TLSA records:
mx.example. IN TLSA 3 1 1 <digest of server public key>
mx.example. IN TLSA 2 1 1 <digest of immediate issuer public key>
* The "3 1 1" record protects against "expiration" accidents, and
unexpected changes in the issuer's public key (if new certificate
chain deployment is automated).
* The "2 1 1" record protects against key rotation errors should a
a new server private key be deployed without updating the TLSA
RRs. Provided the new certificate is issued by the same CA
is unexpired, ... the "2 1 1" record will match.
With a bit of monitoring to ensure that both records match,
simultaneous failure of both is much less likely.
This even makes it possible to avoid pre-deployment DNS TLSA records
updates when rotating certificates, provided at least one of the
issuer public key or the server public key is unchanged in the new
chain.
In particular, this is the best practice with Let's Encrypt
issued SMTP server certificates, as explained in:
https://www.internetsociety.org/deploy360/blog/2016/03/lets-encrypt-certifi…
--
Viktor.
[ Bcc'd to a contact at another large provider, which I hope will
be the next one to top the list... No pressure Christian ... :-) ]
Just recently domeneshop.no have published DANE TLSA records for
the MX hosts that support many of their hosted domains.
So today I count 102638 domains with correct DANE TLSA records for
SMTP. As expected the bulk of the DANE domains are hosted the
handful of DNS/hosting providers who've enabled DANE support in
bulk for the domains they host. The top 10 MX host providers
by domain count are:
42231 domeneshop.no
31928 transip.nl
15092 udmedia.de
1792 bhosted.nl
1262 nederhost.net
905 ec-elements.com
377 core-networks.de
290 uvt.nl
205 omc-mail.com
181 hot-chilli.net
The real numbers are surely larger, because I don't have access to
the full zone data for any ccTLDs, and in particular .de and .nl.
There are 2113 unique zones in which the underlying MX hosts are
found, this counts each of the above registrars as just one zone,
so is a measure of the breadth of adoption in terms of servers
deployed. Alternatively, a similar number is seen in the count
(2219) of distinct MX host server certificates that support the
same ~102000 domains.
Of the ~102000 domains, 539 have "partial" TLSA records, that cover
only a subset of the MX hosts. While this protects traffic to some
of the MX hosts, such domains are still vulnerable to the usual
active attacks via the remaining MX hosts.
The number of domains with incorrect TLSA records or failure to
advertise STARTTLS (even though TLSA records are published) stands
at 85 (~30 are recent additions that will likely be resolved soon,
the remaining ~50 are the long-term stable population of broken
domains).
The number of domains with bad DNSSEC support is 414. The top 10
DNS providers (by broken domain count) are:
50 axc.nl
39 infracom.nl
24 registrar-servers.com
20 loopia.se
19 active24.cz
18 jsr-it.nl
16 forpsi.net
12 cas-com.net
8 is.nl
8 ignum.com
The number of domains that at some point were listed in Gmail's
transparency report is 81 (this is my ad-hoc criterion for a domain
being a large-enough actively used email domain). Of these 43 are
in the most recent report:
gmx.at mail.de otvi.nl
conjur.com.br posteo.de overheid.nl
nic.br ruhr-uni-bochum.de xs4all.nl
registro.br tum.de domeneshop.no
gmx.ch uni-erlangen.de webcruitermail.no
open.ch web.de debian.orggmx.com octopuce.fr freebsd.orgmail.comcomcast.netgentoo.orgxfinity.comdd24.netietf.org
bund.de gmx.netnetbsd.org
fau.de hr-manager.netopenssl.org
gmx.de t-2.netsamba.org
jpberlin.de xs4all.nettorproject.org
kabelmail.de asp4all.nl
lrz.de bhosted.nl
--
Viktor.
