Parece que é "bug" com o Yosemite. Veja esta postagem, link :
you could also try this.. https://github.com/joedj/yosemite_dnsfix (didn't appear to work for me though)
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This has been escalated to Apple engineers by myself and I am awaiting a response, a workaround that I've confirmed works (but am not electing to use myself) is available at: http://arstechnica.com/apple/2015/01/why-dns-in-os-x-10-10-is-broken-and-what-you-can-do-to-fix-it
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Response from applecare is that engineers said it's "Expected behaviour" no idea if they will ever fix it..
E a partir do link :
So the response from Apple is that is expected behavior, and the applecare contact suggested I use the hack proposed at http://arstechnica.com/apple/2015/01/why-dns-in-os-x-10-10-i... (which I sent to them)
At this stage he doesn't know if it will ever get fixed.. no indication from engineering..