We have a 2920 (f/w 3.6.3) which randomly (sometimes once a month) seems to cause internet connection problems.

When it gets into this state pinging WAN IPs works fine but requesting a DNS lookup for an external URL times-out. Stuff with pre-established connections will continue to work (and stuff where the IP has presumably already been retrieved and cached works fine too) but anything that requires a DNS lookup to get started will fail.

Rebooting the router immediately resolves the issue for some random amount of time.

I've tried:

* Factory Reset
* Swapping for another identical router
* Changing the internet connection its connected to (we've tried ADSL modem, cable modem & we are now on fibre leased line - all the same)
* Setting router to reboot early in the morning, once a day

No joy! Can't figure out what is going on here. Any suggestions?

I am not sure if will help but can try

Lan > General setup > Lan 1, details page > add dns servers manually if helps
On exit Lan>General setup tick the small box "force router ... to Lan 1,2,3,4 etc .. then ok again

Also if you have enable DoS disable if will make difference

It's a wild shot, but do you use web content filtering? If yes, try switching that off. I've posted my problem, which produces symptoms not entirely unlike the ones you are describing, and for me it's definitely the web content filter (which is not a DNS problem of course, but it feels similar - it's the initial load of a page where the URL gets sent to the filter server, and they don't reply in time it seems).

Did you have any luck with this? I am having exactly the same problem with a 2820n. Factory reset worked for a few weeks, but the problem has returned, oddly after fixing another problem where I had all 5 WAN IP addresses ticked to "Join NAT IP Pool", and only left the router address ticked. I cannot see why this would cause a problem with DNS though?

For anyone else that has this problem, I think I have found the problem, although I haven't tested it yet, but it does seem very plausible. It seems to be the UDP flood defense kicking in. You can either disable or change the setting as per the link below. Firewall >> DoS Defense >> Enable UDP flood defense

See http://community.spiceworks.com/topic/393025-dns-drops-roughly-once-a-week?page=1

Thanks for the suggestion. I actually had this problem with my Vigor 2925, which produced this dns problem every once per month or so.
I had to schedule a weekly reboot at night time to avoid this happening again but I will try it again with this UDP block setting adjusted.

By the way, do you really think enabling DoS defence actually helps? Does this also "slow down" the router?
Thanks again