[dns-operations] At least 3 CloudFlare DNS-hosted domains with oddball TLSA lookup ServFail

Andrew Tunnell-Jones andrew at tj.id.au
Wed May 27 23:21:00 UTC 2020


On Thu, May 28, 2020 at 3:18 AM Christian Elmerot <christian at elmerot.se> wrote:
>
>
> On 26/05/2020 12:00, Viktor Dukhovni wrote:
> > On Thu, Apr 23, 2020 at 08:46:02AM -0400, Shumon Huque wrote:
> >
> >>> Great, thanks.  Not yet resolved FWIW:
> >>>
> >>>      http://dnssec-stats.ant.isi.edu/~viktor/dnsviz/cloudflare.com.html
> >> I didn't see the reason for the SERVFAIL in the dnsviz output. So I ran
> >> my own debugging tool on these domains. All the CF servers for the zone
> >> are unresponsive to DNS queries for the TLSA record at those names. I
> >> assume that's why we get SERVFAIL. They respond to other queries fine
> >> such as apex SOA, A, etc):
> > I've rescanned the three domains, still broken (same URL, updated
> > content), and yes silence.
> >
> >      @alla.ns.cloudflare.com.[173.245.58.62]
> >      ; <<>> DiG 9.16.2 <<>> +noidnout +nosearch +dnssec +noall +cmd +comment +qu +ans +auth +nocl +nottl +nosplit +norecur -t tlsa _25._tcp.mx01.mx-hosting.ch @173.245.58.62
> >      ;; connection timed out; no servers could be reached
> >
> >      @guss.ns.cloudflare.com.[173.245.59.172]
> >      ; <<>> DiG 9.16.2 <<>> +noidnout +nosearch +dnssec +noall +cmd +comment +qu +ans +auth +nocl +nottl +nosplit +norecur -t tlsa _25._tcp.mx01.mx-hosting.ch @173.245.59.172
> >      ;; connection timed out; no servers could be reached
> >
> > Unclear why the TLSA queries are dropped, and by whom (is Cloudflare
> > just proxying breakage at the customer's DNS?)
>
> I've looked into the error on our side and the reason for those
> SERVFAILs are due to malformed record content. This is likely due to an
> older version of our API not performing the correct validations for TLSA
> records and it is unfortunate the zone owners never checked the output.
>

The web interface still allows creation of TLSA records with invalid
data. As well a "Certificate" field containing whitespace and hex is
accepted by the web interface but results in the same failure. This is
likely to catch people out as whitespace is part of the presentation
format of that field. The placeholder text for that field is also PEM
rather than hex.


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