I have an Ubuntu 14.04 droplet.

A couple of days ago I started noticing a long delay between entering a username and being prompted for password when ssh'ing in. I suspected it was related to dns lookup.

I can ping 8.8.8.8, but ping google.com results in "ping: unknown host google.com"
whois, ssh & msmtp give similar results "host unknown"

I assume I've messed up the dns nameservers somehow. But don't know how to fix.
Appreciate any help.