Re: [Doh] re original_transport indicator

Martin Thomson <martin.thomson@gmail.com> Fri, 06 April 2018 01:12 UTC

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From: Martin Thomson <martin.thomson@gmail.com>
Date: Fri, 6 Apr 2018 11:12:07 +1000
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To: Patrick McManus <pmcmanus@mozilla.com>
Cc: Paul Vixie <paul@redbarn.org>, dnsop <dnsop@ietf.org>, DoH WG <doh@ietf.org>, Ted Lemon <mellon@fugue.com>, Ray Bellis <ray@bellis.me.uk>
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Subject: Re: [Doh] re original_transport indicator
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+1 to this.

And maybe there is an outcome that doesn't need this parameter.  I
probably misunderstood some of the expectations people have for the
parameter.  With the benefit of time and sleep, it's possible that I
now understand the disconnect.

My model of content-type - and by extension its parameters - is of a
description of the content.  However, it appears as though at least
some people had different uses for the parameter: for requests, it
would be an instruction/suggestion to the server to make a DNS request
using the identified transport; for responses, it would be a
description of where the DNS request came from.

I never considered that interpretation, but - assuming that this is
what it was - and the intent was to provide a means for the client to
control how the server makes requests (and to see how they were made).
In that case, I have a possible alternative design for the use case in
the dnsop draft.

Right now, DOH doesn't really say how a DNS API Server gets its
responses.  It could be that the API server is part of a resolver, or
it could be that the DNS API Server makes a request to another
resolver.  But specific uses of the DOH protocol could come with
additional constraints that would enable the use cases in the dnsop
draft, at least as I understand them.

If the goal of the dnsop draft is to extend the reach of a DNS client
(note: not a DNS API client) across a network that is in some way less
hostile to HTTP than it is to DNS, then I think that it can still use
DOH.  A DNS API server could be configured to operate in a very simple
mode with no caching, just direct transposition of a request from HTTP
to a request on UDP or TCP.  A DNS client could be configured with two
DNS API clients.  Each client uses a different DNS API Server
endpoint.  Those endpoints would be specifically configured to use
only UDP or TCP.

For instance:
https://use.only/tcp{?dns} would cause the server to make a request using TCP.
https://use.only/udp{?dns} would cause the server to make a request using UDP.

At this point, this is simple DOH, but with extra contextual
information about server operation.

The concerns about the requirement for HTTPS is a relevant concern.
If there are cases where an unprotected HTTP connection carrying DNS
is expected to traverse a network without modification where DNS
encounters difficulty, maybe the dnsop draft can concentrate on that
particular aspect of this.  I don't think that the suggestion in DOH
to use HTTP/2 is a problem; DOH permits the use of any version of
HTTP.

On Fri, Apr 6, 2018 at 6:53 AM, Patrick McManus <pmcmanus@mozilla.com>; wrote:
> Hi All,
>
> We've had quite a thread re the -05 optional parameter to the
> dns-udpwireformat registration.
>
> The parameter is defined as having no meaning for DoH, but was included to
> accommodate a use case the dnsop wg is considering. Future proofing, if you
> like.
>
> Upon consideration (and a read of 6838), I think including this in doh is
> premature because Media Type registrations can be updated by mechanisms laid
> out in RFC6838 and in this case such an update could occur without impacting
> existing DoH deployments. (i.e. it does not need to be future proofed).
>
> Therefore the definition of the parameter should accompany the work that
> makes use of it if a future standards document chooses to go down that path.
> As a bonus we avoid unused clutter if it doesn't happen. I also get the
> feeling that there isn't yet strong consensus on the anticipated use case or
> the exact form it needs to take - we should let that process work itself out
> separately before registration.
>
> I've chatted with Paul, and our recommendation is to remove the
> original_transport parameter from DoH and encourage dnsop to update the
> registration if/when a different standard needs to make use of it.
>
> thoughts?
>
> -Patrick
>