Re: [dane] AD bit handling in stub-resolvers: conclusions and compromises

Nico Williams <nico@cryptonector.com> Wed, 09 April 2014 01:35 UTC

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Date: Tue, 8 Apr 2014 20:35:23 -0500
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From: Nico Williams <nico@cryptonector.com>
To: Petr Spacek <pspacek@redhat.com>
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Subject: Re: [dane] AD bit handling in stub-resolvers: conclusions and compromises
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On Fri, Apr 4, 2014 at 8:31 AM, Petr Spacek <pspacek@redhat.com>; wrote:
> Hello,
>
> Let me sum up the discussion and document the compromise for archaeologists:

Thanks.

> Approach A
> ==========
> One approach is to do nothing in stub resolvers and rely on validating
> resolvers or application logic.
> This approach was mentioned by Paul Wouters <pwouters@redhat.com>;, Prasad
> Pandit <ppandit@redhat.com>;, Carlos O'Donell <carlos@redhat.com>;, Olafur
> Gudmundsson <ogud@ogud.com>;.

Also, Michael Richardson had some comments about uninformative
error/timeout handling when the application doesn't do its own DNSSEC
validation.

There are solutions to Michael's problem, such as running an in-app
resolver when the user wants detailed error information.

> It seems that the main concern about AD bit stripping in stub resolvers is
> compatibility with existing applications which rely on AD bit:

We should want fail-closed semantics.  I very much prefer having a
caching validating local server.  I don't mind making people (and
configuration apps) explicitly set a global in /etc/resolv.conf to
disable AD stripping.

I'd provide two settings, one to disable AD stripping IFF the only
nameserver is ::1, and another to disable AD stripping in all other
cases.

> After further discussion, it seems that pwouters is okay with AD bit
> stripping in stub resolver if it is explicitly requested by a calling
> application. (E.g. by special resolver initialization.)

Again, we need fail-closed semantics.

> Approach B
> ==========
> The other approach is to do AD bit stripping in stub resolvers by default.
> It was proposed to add system-wide setting for this (e.g. to resolv.conf)
> and default to "strip AD bit unless admin configured something else".
>
> This approach was mentioned by Petr Spacek <pspacek@redhat.com>;, Simo Sorce
> <ssorce@redhat.com>;, Viktor Dukhovni <viktor1dane@dukhovni.org>;, Tony Finch
> <dot@dotat.at>;, James Cloos <cloos@jhcloos.com>;.
>
> Naturally, application has to be able to do run-time check that it is using
> a library with this new behavior to avoid running in unsafe environment.

This might be done at install time, or first-run time if we commit to
never removing this feature once it's added and/or if we provide an
interface for notifying applications of removal of this feature.  This
approach means not having to extend APIs.  However extending APIs is
probably OK enough I think, given that we have relatively few
DNSSEC-capable apps today.

> Next Steps
> ==========
> Define what is yet not defined:
> - System-wide configuration format (e.g. modify resolv.conf vs. add a new
> file)

The former.

> - Propose specific API changes for major DNS libraries (glibc, maybe ldns
> and co.)

See below.

> - Prepare recommendation for application developers how and when to use a
> new API

Sure.

> Further discussion about implementation details will happen on mailing lists
> related to the particular DNS libraries.

For the res_init() API it'd be nice to have an agreed-upon extension.
Where is that to be discussed, if at all?

Nico
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