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OS trying to use IPv6 even though it's unavailable
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OS trying to use IPv6 even though it's unavailable


  • Subject: OS trying to use IPv6 even though it's unavailable
  • From: Jens Alfke <email@hidden>
  • Date: Tue, 17 Feb 2009 09:37:02 -0800

(Apologies if this isn't sufficiently programming-y for this list. This issue didn't bite me in the context of writing code, but it definitely might some time in the future.)

Last weekend I tracked down a problem I'd been having for a while where some websites (including apache.org) were extremely slow to load in Safari. I was seeing delays of about 15 seconds before it would load any content at all.

The problem turned out to be that DNS is finding IPv6 addresses for those hosts, and they're being used preferentially to the IPv6 ones, but they're not reachable. (I did a traceroute6 to apache.org, and none of the packets ever came back.) So I watched telnet try to connect via IPv6, time out, and then try IPv6, which connected instantly. Presumably Safari is doing the same thing.

I changed my IPv6 config from "Automatic" to "Off" in my network prefs, and that fixed the problem. AFAIK, Auto is the default setting; I've never knowingly changed it.

Now I'd like to know what caused this.
Should I be getting IPv6 responses from DNS queries at all?
What causes the kernel to decide that IPv6 addresses are reachable, since in this case it's wrong?
And why are the packets being dropped?


I have an AirPort Extreme base station, connected to a cheap-o Trendnet router, connected to Sonic.net DSL. I'd be inclined to blame the router; I looked at its config interface but there are no settings for IPv6 at all.

—Jens _______________________________________________
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