Department of Defense
High Performance Computing Modernization Program

To be certain that the IPv6 protocol is being used to access a website, substitute an IPv6 literal address surrounded by square brackets in the address bar in place of a domain name. For example:

http://ipv6.test-ipv6.com

becomes

http://[2001:470:1:18::119].

For additional information, refer to Request For Comments (RFC) 3986 “Uniform Resource Identifier (URI): Generic Syntax".

(Caution: This substitution can sometimes fail. Explanation of possible reasons for this are available. If you encounter problems, review the Broken User FAQ article found on that website for several possible explanations. For even more possible explanations, review this article on the ARIN IPv6 wiki.)

Many non-Apple browsers and IPv6 enabled applications available today attempt to use IPv6 before IPv4 if the local system and the remote server have v6 addresses. Here’s why debugging IPv6 web connectivity with Safari has been and continues to be difficult.

In 2015: It was reported that changes made in OS X 10.11 El Capitan resulted in improved behavior when accessing IPv6-enabled websites.

In 2011: The Mac OS X 10.7 Lion subsection of this article on the Réseaux IP Européens Network Coordination Center (RIPE NCC) Labs website describes the complications resulting from changes to DNS resolution in OS X 10.7. ("Happy Eyeballs") That article also provides suggestions for debugging IPv6 web connectivity on OS X systems.

In 2009, Apple switched to mDNSResponder for DNS resolution in OS X 10.6 Snow Leopard, but that broke IPv6 address selection because mDNSResponder takes whatever is the first response (A or AAAA) and drops (rejects) all other answers. So, depending on how you ask, and depending on what address comes back first, there is no way for the rest of the system or applications to see all the responses and make appropriate address choices. This seriously impacts IPv6 interactions, because it no longer deterministically prefers AAAA over A, but rather uses only the response that arrives first, ignoring any later responses.

A detailed bug report (#733104) was filed in October, 2009.

Circa 2004, Safari used IPv4 by default for websites with both IPv4 and IPv6 addresses. However, Safari attempted to connect to servers with an IPv6 address first when accessing secure sites using https: (SSL).


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