My wife’s macbook pro started behaving strangely after upgrading to Yosemite from Mavericks. The initial upgrade went smoothly but over time certain applications began to quit working. The Pandora desktop client suddenly could never connect. Tunnelbick completely broke no matter what I did. Dnslookups all were fine but pings hung forever, eventually saying they couldn’t resolve a hostname.

It turns out that Yosemite changed the way Mac OSX resolves DNS records from mDNSresponder to dnsdiscoveryd. The issue I had only happened from and upgrade – clean installs didn’t have the issue

The fix for this madness, taken from here, is to remove a few network configuration files and reboot.

First, disconnect from any networks you’re connected to. Then, go to /Library/Preferences/SystemConfiguration/ and remove the following files (if they exist)

com.apple.airport.preferences.plist

com.apple.network.identification.plist
com.apple.wifi.message-tracer.plist

NetworkInterfaces.plist

preferences.plist

After removing those files, reboot. That should fix your problem! (At least, it did for me.)