Resolver issue, not sure how to diagnose

[Jay2]

Hello,

I have a PfSense 2.3.2 deployed in an office, and all is good up until now.

This morning, the resolver stopped resolving for a lot of domains. In this list are:
kijiji.ca
the-drive.com
They just go "DNS Request Timed-out" on my nslookups.
I switched the configs to the Forwarder (to Google's and Level3's DNS) and all works well now.
There are no resolver-related issues in the System Logs.

My question s: Is there an issue with the resolver/top name servers in effect? And also, where should I look to diagnose what went wrong here?

Thanks

[KOM]

This perhaps?

http://arstechnica.com/security/2016/10/dos-attack-on-major-dns-provider-brings-internet-to-morning-crawl/

[Jay2]

@KOM:

This perhaps?

http://arstechnica.com/security/2016/10/dos-attack-on-major-dns-provider-brings-internet-to-morning-crawl/

Perhaps, yes!
It makes a lot of sense.