Re: [Acme] dns challenges and dynamic dns services (security considerations for domain holders)

Andrew Ayer <> Fri, 29 January 2016 17:38 UTC

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Date: Fri, 29 Jan 2016 09:38:06 -0800
From: Andrew Ayer <>
To: James Cloos <>
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Cc:, Frederik Braun <>
Subject: Re: [Acme] dns challenges and dynamic dns services (security considerations for domain holders)
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On Fri, 29 Jan 2016 11:52:11 -0500
James Cloos <> wrote:

> >>>>> "FB" == Frederik Braun <> writes:
> FB> I'm concerned that everyone having to update their blacklists[1]
> FB> will lead to more trouble.
> They really all ought to forbid any label which start with an
> underscore.

Agreed.  Underscores are not allowed in hostnames, so dynamic DNS
services should not allow registration of such names (I don't even
understand why they would allow TXT records).  If they do allow
registration of arbitrary TXT records for names starting with
underscores, then they are also allowing attackers to set arbitrary
DKIM and DMARC records for their domain, and probably inflict other

The dynamic DNS service in question[1] also claims support for SRV
records, so there's a good chance they're also allowing attackers to
hijack various services for their domain.

I consider this similar to (but less likely than) web hosts allowing
user uploads to the /.well-known directory.  Operators ought to know
better, and while there's a risk that some won't, it's a risk that has
to be tolerated if DV is to exist at all.