I had network issues recently and I had to kill the “discoveryd” process from activity monitor to get DNS resolution working again.
I would, in the past, execute:
sudo killall mDNSResponder
There is a replacement now for Yosemite, called discoveryd. On Yosemite, you would do this instead:
sudo killall discoveryd
Just as I was about to post this I learned that Apple will revert back to mDNSResponder. It seems I’m not the only one with this problem.
I hope this helps.