[dns-operations] Signaling client protocol to authority

Mark Andrews marka at isc.org
Mon Jan 17 02:26:36 UTC 2011

In message <37672.1295206707 at nsa.vix.com>, Paul Vixie writes:
> everybody just please add AAAA RR's to their web properties, and client
> side vendors please just stop asking for AAAA RR's you cannot reach.

Unfortunately there is no way to do the second part.  The application
can't know that it can't reach the address, if it has a local IPv6
address, without first asking for it and trying.

Even it there is not reachability there should not be a problem as
the routers (or the node itself) will report that the destination
is not reachable.

Where you get a problem is when traffic is sent down a black hole.
* broken 6to4.
* ULA sourced packets past a misconfigured border router.

Basically this proposal is attempting to optimise for local
misconfigurations rather than letting them be exposed and corrected.

Note applications don't need to wait for the first connection attempt
to fail.  They can attempt additional connections without waiting
for the first ones to comlete then just use the the first one to
complete.  It just requires a silghtly more complicated loop over
the addresses returned from gethostbyname()/getaddrinfo().

Note many web browers actually support parallel connections
so in many cases these additional connections would not be made in
vain but would actually be used.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org

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