Ok so we're running an IPv6 tunnel from HE.net on our office LAN. It works but some sites are dead-slow to load or don't load at all. So I've asked everyone to configure their workstations to prefer v4 over v6 when both are available. Unfortunately a few people are using Mac OSX and so far as we've been able to establish it doesn't look like OSX provides a way to set a preference (the way Windows/Linux/*BSD do). I was hopeful some-one might be able to tell us that there is a way. ;)
Which revision OSX? With Lion anything which uses the higher level Cocoa APIs (NSStreamSocket and friends) should effectively run with 'happy eyeballs', trying both v4/v6 and using whichever one has better connection metrics.
Not sure what version of OSX they're using. Ideally I'd like to configure them to always favour IPv4 when available like possible on other OS. ???
Sorry, Apple doesn't think you need to be able to set a preference. At least under OSX Lion, the system tries to obtain both IPv4 and IPv6 addresses and maintains a cache of what protocol works better and prefers that.
See, for example, http://lists.apple.com/archives/Ipv6-dev/2011/Jul/msg00009.html (http://lists.apple.com/archives/Ipv6-dev/2011/Jul/msg00009.html)
(This thread starts here if you want to read the whole thing: http://lists.apple.com/archives/Ipv6-dev/2011/Jul/msg00007.html (http://lists.apple.com/archives/Ipv6-dev/2011/Jul/msg00007.html))