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[dnssd] When does MDNS/DNS-SD use IPv6?

"Christian Huitema" <huitema@huitema.net> Wed, 18 January 2017 03:59 UTC

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From: Christian Huitema <huitema@huitema.net>
To: dnssd@ietf.org
Date: Tue, 17 Jan 2017 19:58:57 -0800
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Subject: [dnssd] When does MDNS/DNS-SD use IPv6?
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I am writing a small implementation of MDNS and DNS-SD to test the privacy
and pairing drafts, and I am puzzled by the IPv4 vs. IPv6 issues. The MDNS
spec mentions that queries can be sent to "multicast address 224.0.0.251 or
its IPv6 equivalent FF02::FB". That's nice, but it is wonderfully
underspecified. Or maybe I am just a bit slow these days, and I just need
help... Given a query for some record type in the ".local" namespace, should
the implementation:

1) Send queries to both 224.0.0.251 and FF02::FB and merge the results?
2) Send queries only to 224.0.0.251?
3) Send queries only to FF02::FB?
4) Send to 224.0.0.251 and then try FF02::FB if no response come after a
timeout?
5) Or vice versa?
6) Perform some guesswork as to whether the local network prefers IPv4 or
IPv6?
7) Do different things if the query is for an A record versus AAAA?

Also, is there a different recommendation for one-shot queries and
continuous queries? 

And did we decide on purpose to not document that in the RFC?

-- Christian Huitema

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