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I'm hoping one of the many network gurus here can help me with a problem that just started to happen on our office network. We have a Linksys RV082 router; short description of the problem is that clients on the internal office network are intermittently unable to resolve host names via DNS.

Here's the troubleshooting info I've gathered so far:

  • External connections to servers inside our office are fine.
  • Outbound connections from the office network are fine if I use the IP address (e.g. browse to http://myhostname.com fails with "host not found", but browse to http:[IP address of myhostname.com] works). This is why I'm assuming it's a DNS problem.
  • Problem happens from multiple machines on the office network (it doesn't seem to be restricted to any particular machine).
  • Restarting the router fixed the problem for a short time, but the problem came back after about 15 minutes.
  • Changing the DNS servers defined in the router from our ISP's DNS servers to OpenDNS made things better for a couple of hours, but now we're back to the same problem again. (Note; we don't have a local DNS proxy, the router is set up to go direct to a pair of external DNS servers).

Thanks in advance for your help !

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Can you take a network traffic sample using Network Monitor or Wireshark on one of the client PCs when the problem persists? I'd be willing to bet that you'll see "Standard query response, Server failure". –  Wesley Nov 2 '09 at 23:47
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Furthermore, I'd like to be absolutely certain I understand your set-up. Are the clients looking to the router for DNS or an internal DNS server like an SBS 2008 machine which is in turn looking to the RV082 as a forwarder? I think it's the former and not the latter, but I wan to be certain. –  Wesley Nov 2 '09 at 23:50
    
The clients all look to the router for DNS. Most of them are Linux servers or Macs, so I'll see if I can find an equivalent to Network Monitor or Wireshark and post the traffic sample here - thanks for the help. –  gareth_bowles Nov 2 '09 at 23:57
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2 Answers

up vote 3 down vote accepted

Router DNS proxies are notoriously poorly implemented (see my RFC 5625).

Usually if you tell a Linksys router to use alternate DNS servers it will then put those in any subsequent DHCP offers. However I've not tested that particular model. Please check whether your unit is doing that (i.e. run ipconfig /all on a Windows client or cat /etc/resolv.conf on Unix, and check what DNS settings it has received from the DHCP server).

However since you said that putting OpenDNS servers in the router didn't fix the problem, that suggests that the Linksys settings only change the settings for the proxy, and not the DHCP settings.

If that's the case then your best option would be to turn off the DHCP server in the router and run one yourself.

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Thanks, your answer convinced me to go with our own DHCP server. –  gareth_bowles Nov 3 '09 at 15:54
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Set one of the internal machines to use external DNS directly instead of proxy through the router. This should tell you if the router proxy isn't working correctly or if possibly your provider isn't passing DNS queries correctly.

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Bypassing the router and using external DNS directly works fine - so is that telling me there is something wrong with the router ? What do you suggest for the next troubleshooting step ? –  gareth_bowles Nov 3 '09 at 1:59
    
I'd try reloading/upgrading the firmware in the router. –  Keith Stokes Nov 3 '09 at 11:24
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