insane: Cloudflare had an interesting article on something similar a few days ago:
mattie47: This is probably a pretty grey area, and probably something ISPs normally wouldn't have to deal with, other than customers ringing up complaining about why their data usage is through the roof when they haven't been doing anything.
mattie47:mattie47: I also have an internal DNS server, but port 53 isn't forwarded from the firewall.
Wow I take that back. I was looking at my port forwarding rules which didn't show 53 anywhere so presumed was closed. I just did a capture on the internal NIC which showed the same traffic. This got me worried. A quick online port check showed 53 as open (what?).
Having a look again around PFSense showed there's a DNS forwarder page I must have skimmed over. Turns out I had "Enable DNS forwarder" ticked. Okay, that's port 53 traffic going to internal now dropped...