I have this issue also.

Looking after 2 sites that use the 2820 connected to BT ADSL 2+.

Every so often, I will get a call from the sites to tell me the broadband connection is down, however I can see the router is still online and can log into the network via vpn to the IP address. If I then remote onto a server and try an nslookup, all external DNS requests fail but internal ones resolve just fine.

Only way to resolve this is a reboot of the router.

This is a fairly major issue, having external DNS access just fall over like that.

FYI - I was also able to trigger the condition by running namebench dns benchmarking tool.

Posted before I googled fully...

The culprit is the udp flood defense option in the firewall settings. If this is turned off, it seems to prevent lockups. Hopefully that will be it sorted.

Tried that, and a great many other combination of things to no avail!

Sorry, that was quiet rude of me! Lets try that again...

Thanks for the suggestion. I saw this suggestion after some searching. And excitedly tried it. Unfortunately out made no difference.

Thanks though.

Draytek have not come up with a solution to this yet, and I couldn't continue with internet access limping along like it was.

So I bought a Mikrotik 450G, RouterOS device as a temporary solution..... Wow! what an incredible piece of kit!

Not only is the DNS issue no where to be seen, but performance and throughput have noticeably and measurably improved. For £95 delivered Ive got a gigabit router with 5 assignable Gigabit ports that does so much more than the Draytek for half the price!

You can find details at LinITX.

Im still learning all its capabilities but I may well just start buying these now, since I really feel let down by Draytek.

Nice find.

Might get one myself just to have a play.