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DNS stops working on PCs in LAN

We have about 16 PCs in our office LAN that use a Netopia 3347-02 DSL modem/router for internet access (AT&T Business DSL). All PCs obtain their IP address from this router via DHCP and the router acts as the DNS server for the LAN as well. The results of the nslookup command shows the IP address of the Netopia as the DNS server. Throughout the day, all DNS lookups will suddenly fail, but the PCs can still ping the IP address of hosts on the internet. We can resolve the issue only by cycling power on the router. Once the router is powered up the name resolution begins to work again for a while.
1 Solution
do you have access to the router?
first thing to check would be DNS resolution on the router itself. if it can't resolve there is a local problem and you should contact your vendor (or update the router, test again and then contact the vendor)
sound like the routing is good since you do get ping replies to outside IPs
Try enabling some sort of logging on the router,
for example, make the router send logging messages in Syslog format to a PC on the LAN,
Set it to the highest level on the DNS section,
Open a syslog application on that PC, and inspect the messages.

Generate a DNS resolution, and watch the log, it may help you find the problem.
craws01Author Commented:
I do have access to the router. The router can resolve no problem at all even when the PCs can't.

Next time you have this problem, use wireshark to capture the DNS query and the subsequent http request
from a PC that is having problems, save the pcap to a file, rename it as a jpg file and post it here please.

Please provide some text, where did you try to access, and the source IP of your PC - for the pcap file.

With DNS mutiple problems can arise. I had too many similar problems with many such boxes because of their crappy built in DHCP servers and DNS caches. Only after I switched to dhcpd and named on a dedicated machines all those problems magically disappeared.

Best solution is to disable everything on the box, if possible, even PPPoE, and switch it to bridge mode. Then let the dedicated *nix machine handle stuff like dns, firewalling, nat, PPPoE etc. It can be older retired machine. You will have some problems till you set it up right way, depending on your skill, but in the end you will be grateful.

To pinpoint the location of the problem you might use Wireshark, but if powercycling solves the problem I would bet it's the dns modem/router.

Next time the outage occurs try to point one pc's dns manually through control panel or command line to some well known dns server like OpenDNS (, ). If the resolution starts working for that pc but not for others in the network, the modem/router is definitely the culprit.

This might be caused by buggy dns cache service of the box or by crappy dns server provided by ISP up in the chain.

More over Windows DNS cache caches also negative resolutions for some time, that means although the resolution is already working, it might not show up (in my experience it does affect nslookup). Restarting DNS cache service through service.msc helps. That negative cache behavior can be disabled by registry hack.


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