For 12 years, the mDNSResponder service managed a surprisingly large part of our Mac’s networking, and it managed this task well. But as of OS X 10.10, the mDNSResponder has been replaced with discoveryd, which does the same thing. Mostly. Here are some strange networking problems we’ve observed since installing 10.10:

Full article here:
Why DNS in OS X 10.10 is broken, and what you can do to fix it (Ars Technica)