Macbook can’t resolve any domain names

It turns out the solution was to bounce mDNSResponder:

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.mDNSResponder.plist

This was obtained by a different coworker from this Server Fault question.

OS X 10.10.0 – 10.10.3, Yosemite

Apparently, mDNSResponder doesn't exist in Yosemite (OS X 10.10). You can restart descoveryd instead to fix these issues.

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.discoveryd.plist
sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.discoveryd.plist

OS X 10.10.4+, Yosemite

In OSX 10.10.4 the mDNSResponder has been reintroduced. So use the first one will work again.

I experienced that problem on my MBP as well. I suppose it's a bug in Yosemite. Here's a related question: Yosemite cannot resolve domain names maybe it gives you some insight

My work around is flushing the DNS cache:

sudo discoveryutil mdnsflushcache

if that command doesn't work try this one instead:

sudo discoveryutil udnsflushcaches

I am not sure if this helped fixing it, but I did a PRAM and SMC reset as well.