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We're using a Bell 2Wire Gateway modem/router as our only hub (with some switches attached).

Intermittently, but frequently when a large download is initiated, all computers on our LAN lose the ability to lookup websites (ie: DNS lookup fails). Pinging or accessing the IP address directly works as usual, but any name lookups fail. A reset to the modem usually fixes the problem. This happens daily at least once a day, though I haven't observed a pattern to when it occurs, other than large downloads often trigger it.

We are not running our own nameservers. Our ISP is Bell Sympatico in Canada, if that's of any help.

What could be the problem and what steps should I take to resolve it?

share|improve this question
When hosts are in the condition where DNS is failing, can they still reach Internet sites by IP address? Do their traceroutes by IP pass the router? Can they still ping the router? You need to do some troubleshooting to narrow the range of possible problems. Your problem may be with DNS, or it may be DNS failing because that's what happens first. – David Schwartz Apr 10 '12 at 19:37
I refuse to call Bell tech support, for obvious reasons! - the world is a very large place and parts of it have no idea what the "obvious reasons" are because we don't have "bell". Your router is unlikely to reserve bandwidth for critical traffic out of the box, if at all, as has been said already. Does this issue occur with just one switch with several clients, one or two of which are using up lots of bandwidth, or does the whole wagon train need to be plugged in? This could be a bad internal switch, could be the router, could be the line itself. I've seen all those cause these symptoms. – RobM Apr 10 '12 at 21:42
Fair nuff, re the Bell comment – Tom Auger Apr 11 '12 at 0:48
The issue occurs across the entire LAN of 8 or so computers (some wireless, others wired). When one person is downloading or streaming every one else iscompletely shut down. The strange part is that even on my home network I have never experienced this phenomenon. If one computer is massively downloading, things slow down, sure, but the DNS lookup never fails, and other devices on the lab can still interact with the internet, just not as rapidly. That is the expected behavior. – Tom Auger Apr 11 '12 at 0:52
@David Schwartz excellent questions. I will try to verify done if these tomorrow, and will report back. – Tom Auger Apr 11 '12 at 0:55

I don't think your router or firewall would reserve a certain amount of bandwidth for critical traffic unless you configure it to do so (if it even has these features).

I think you're better off setting up one or two nameservers on your local network and configure the local computers to use that instead. In addition you may install caching nameservers on user's local computers so they have to rely even less on sending queries to remote nameservers. On linux for example you have the "name server cache daemon" or NSCD to do that:

share|improve this answer
Good lord that just sounds complicated! – Tom Auger Apr 11 '12 at 0:48
You asked a question on serverfault. Which "is for Information Technology Professionals needing expert answers related to managing computer systems in a professional capacity." What do you expect? – aseq Apr 11 '12 at 1:37
Good point +1. What I meant to say was: are you really sure that setting up a nameserver (let alone two) is really necessasry? We're talking about a small LAN - max 5 computers - and it feels a little like bringing a blowtorch to a barbecue. I mean, I'm running a home network that doesn't have these problems, so should there be some early diagnostics first before getting into deploying larger-scale solutions? – Tom Auger Apr 11 '12 at 13:22
Setting up a nameserver isn't trivial but also not rocket science, however once you have done it it will just work, and uses up minimal resources. The difference with your home network likely is that your office network sees more bandwidth usage by multiple people, so you get congestion much quicker. A local nameserver can mitigate the problem you're having. I even run a nameserver on my home network (and nscd). – aseq Apr 11 '12 at 18:49
By the way, not saying there may not be something wrong with your modem. It's just that this problem points out there is room to optimize things such as how you use nameservers. – aseq Apr 11 '12 at 18:52

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