Re: [homenet] Updating DNS [was: How many people have installed the homenet code?]

Ted Lemon <mellon@fugue.com> Mon, 25 April 2016 02:40 UTC

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From: Ted Lemon <mellon@fugue.com>
Date: Sun, 24 Apr 2016 22:39:27 -0400
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To: Juliusz Chroboczek <jch@pps.univ-paris-diderot.fr>
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Cc: homenet@ietf.org, Markus Stenberg <markus.stenberg@iki.fi>
Subject: Re: [homenet] Updating DNS [was: How many people have installed the homenet code?]
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On Sun, Apr 24, 2016 at 12:29 PM, Juliusz Chroboczek <
jch@pps.univ-paris-diderot.fr>; wrote:

> > Juliusz, the problem is that existing home network devices that do
> > DNS-based service discovery do not support DNS update. They could, but
> > they don't, because we didn't define an easy way for them to do it.
>
> I'd be grateful if you could expand on that.  Why can't we define a way
> for clients to do DDNS?


We can and should.   The problem is that we won't see that code ship in new
devices anytime soon, so we still have to make mDNS work.


> > Just 2136 isn't enfough, because there's no authentication scheme,
>
> I don't understand this argument.  How is non-secured DDNS any less secure
> than mDNS?  What am I missing?
>

This is an implementation issue, not a security issue--sorry for not making
that clear.   In order to preserve the same security characteristics that
mDNS has, we have to ensure that the update actually originated on the
local link, which requires a different sort of listener than is present in
a typical DNS server.   And existing DNS servers typically don't have any
way to support unauthenticated updates on a first-come, first-served basis,
so if you allow unauthenticated updates, you don't have any way to avoid
collisions.   Otherwise you are correct.   The answer is to write a
document that describes how to do that, and if you read the homenet naming
arch document, you can see that I actually sketched out a solution there,
which I expect to go in a different document, likely in a different working
group.


> Oh, sure, we Poles are not quite as pessimistic as the Finns.  I'm
> actually of a divided mind here -- I rather like distributed solutions
> (hence prefer mDNS to DDNS) but dislike proxying.  Part of me just wishes
> we'd mandate site-local multicast and do mDNS over that


The problem with site-local multicast for mDNS is that multicast isn't a
great solution even on the local wire when that wire is wireless.    And,
you have to do modify the client anyway.

Furthermore, if you consider the mdns hybrid proxy stateless, then you can
have a DNS server that is roughly that stateless too.   I think it provides
better service continuity if you are willing to retain some state, but
everything will still work even if you don't, just as the hybrid proxy does.