I was just going to post about this. Just going back to the OP post:
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: 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...
Now, a couple of comments, and please don't take it personally...
When running a network service the admin should not "skim over". Any open port with forward traffic may attract unsolicited "visitors". If the service is somehow valuable for them they will use it.
In this situation the service on port 53 can be easily used for a DDoS attack. Even if you don't have an infected computer in your network your devices are still active participants in attacks by simply offering the services and acting on requests from unauthorised users. That's the point above with the link to the DNS amplification attack as described by CloudFlare.
In this case I have to say Slingshot was REALLY NICE to you. The usage was not an error in their systems and they should have every right to charge your account for that. I'm pretty sure in their T&Cs somewhere there'll be a clause saying you're responsible for keeping your connection safe - that means not only safe from malware, but safe from unauthorised access and usage. Running a service and leaving it open it's not their problem, really.
That blog you link in the OP... That guy didn't get the "amplification" part of the attack. He seems to think the attack is directed at him, not at the response from his DNS to the spoofed IP.
Go read the Cloudflare blog again. Secure your network. Make sure to use an external scanner to identify any open ports/active services still lurking (Try GRC ShieldsUP!.
Changing IP addresses without making sure your vulnerabilities aren't protect won't save your bandwidth, as those idiots will find your network again pretty soon - port scanners are capable of going around entire IP blocks very quickly.
Hi Mattie, Looking at your PFsense rules you have an allow all on the WAN. The way firewall rules work on PFsense is that the first rule it find that matches is applied. So the rules you have underneath that don't count at all. It definitely sounds like you are thus being used as an open relay.
What you should do is delete that first rule for allow all. Put your allow rules in place e.g. RDP, HTTP etc. then put a deny all underneath those.
Also, I believe because you are using private address space for your LAN, you should set your destination for the allow rules to be your WAN interface rather than 192.168.XX.XX.