Address resolution and selection (was: [revisited] DHCPv6 support needed by others as well...)

Am 24.10.2009 um 17:06 schrieb Ron Broersma:

Update on the MacOSX situation: In 10.6 (Snow Leopard) all DNS queries now go via mDNSResponder. mDNSResponder queries for both the "A" and "AAAA" record, but after the first response it cancels the queries and shuts down the socket so that all other responses are rejected (port unreachable). Generally, the response to the "A" query will be first, so you will connect with IPv4, but sometimes the "AAAA" response will win and you will connect with IPv6. The first response wins and all others are lost.

So, this does not appear to be a case where Apple changed the preference, but rather introduced a serious bug in mDNSResponder or related code. Unless they were in a dual stack environment, or were trying to reach dual-stack web sites, they would have never noticed. The net effect is that most IPv6 interactions now shift back to IPv4 transport, and in some other scenarios you lose connectivity altogether.

Almost a month has gone by, we are now at 10.6.2, and this obvious bug is still present.

So I filed another Radar on this. Given that it is quite random, this problem is somewhat difficult to show or force. But the following will help:

1. Run sudo tcpdump -ns 1500 -v '(udp port 53) or icmp' in a window, to observe resolution of the names

2. Go to http://sixy.ch/ to find dual-stacked targets which are not yet cached.

3. Use curl -Iv to see if IPv4 or IPv6 is used as a default transport.

And in the end, you might file a radar as well, since this carpet-bombing with reports seems to be the only way to get things going.

And yes, support for RFC3484 would be nice too...

Cheers,

-daniel