I’m not going to sugar coat it: Networking has been a mess in Yosemite. If you’ve been noticing wifi drops, slow DNS, failing DNS, mystery devices in the network browser, and more, all I can say is, its not your fault.
It all comes down to Apple’s ditching the long-lived and hard-working mDNSresponder and replacing it with a brand-new creature called discoveryd. Unfortunately, it seems Apple didnt follow the high-altitude baking instructions on the box, because discoveryd is SO not done.
That’s the tl;dr. here’s the situation is a LOT more detail than I care to type out:
Why DNS in OS X 10.10 is broken, and what you can do to fix it
Fortunately, it’s not all bad news. In fact, after a year of putting up with this, there’s some good news at last: Apple has gone back to mDNSresponder in the beta of 10.10.4!
Let’s hope they stick with this plan when 10.10.4 is finally released. Better yet, would be a mea culpa and news on a permanent fix. Say, maybe, next week at WWDC? Seems like a good place for it. Just sayin.