Comments on: DNS Oddities

Question: DNS Oddities

Lokheed — Fri, 01 Jul 2005 16:48:53 -0800

Home DSL connection. DSL router goes to Netgear router, and three computers connect to the Netgear router. All three computers run XP Pro. On two of the computers, DNS works fine. On the third, some websites don't resolve. Why?

I have checked the .hosts file on the odd computer, and it's just the standard default template with no additional entries. I have checked the TCP/IP properties and made sure that "Obtain DNS Server address automatically" is selected. I have checked the advanced properties of both the DSL router and the Netgear router to be sure no filtering is being done there. So why can I reach, for example, www.monsters.fm on one computer but have it come up as an unknown domain on the other?

By: willnot

willnot — Fri, 01 Jul 2005 17:00:10 -0800

Bring up a command prompt on the bad computer, and try to do a tracert to one of the domains you can't resolve. Can you determine from that where it's getting stuck?

Also, have you tried resolving the bad domains in other browsers to eliminate the browser as a source of the problem?

By: evariste

evariste — Fri, 01 Jul 2005 17:01:40 -0800

Can you ping it on the command line? Get a Command Prompt and type:
ping www.monsters.fm
tell us whether it times out or not.

By: Lokheed

Lokheed — Fri, 01 Jul 2005 17:13:54 -0800

Also, have you tried resolving the bad domains in other browsers to eliminate the browser as a source of the problem?

The same problem happens both with IE and Firefox, so it does not appear to be a browser issue.

By: bemis

bemis — Fri, 01 Jul 2005 18:00:23 -0800

It's possible (and even probable, with a good provider) that you have more than one DNS server being given out by your Netgear DHCP server. If one in the list is responding, but with invalid answers (due to bad configuration or some such), you could see a situation like this.

You could use nslookup to perhaps see if this is the case- check the Netgear config to see what DNS servers it has, and then run nslookup from a command shell, and then use the 'server x.x.x.x' command to go through the list of servers, trying the problematic hostname on each one.

By: Lokheed

Lokheed — Fri, 01 Jul 2005 19:15:27 -0800

Results of ping:
good computer - ping times out, but the domain is translated to an IP address
bad computer - could not resolve host name

Results of tracert:
good computer - makes 16 hops before ending up in "request timed out"
bad computer - could not resolve host name

Results of nslookup:
good computer - DNS request times out with both primary or secondary DNS server specified
bad computer - same

Entering the URL in a web browser:
good computer - page loads fine
bad computer - could not resolve host name

As a hacky fix, I added the IP address to the hosts file on the bad computer, and now the page loads fine. I don't like hacky fixes....

If it was happening on all computers on the network, I would think it was something in the interaction between the Sprint DSL router and the Netgear router. But this whole "it works on two computers but not on the third" just has me baffled.

By: evariste

evariste — Fri, 01 Jul 2005 21:54:06 -0800

I can't figure out what else to tell you to try, but I don't use my ISP's automatically assigned nameservers. Instead I use 4.2.2.1-4.2.2.6 inclusive. Try setting it to use those and see what happens. They're Level1/Verizon's nameservers and generally give me very fast, up-to-date response, unlike my ISP's which aren't very stable.

By: Ryvar

Ryvar — Fri, 01 Jul 2005 22:15:07 -0800

On both a good computer and the bad computer type "ipconfig /all" into a command prompt and at the very bottom of the output should be a list of DNS servers used by both computers. Make sure those are the same.

Also, Start->Run->"services.msc" and check the status of the DNS Client service on both computers, make sure both are the same.

By: RichardP

RichardP — Sat, 02 Jul 2005 01:35:20 -0800

Does the TCP/IP MTU setting on the "odd" computer differ from that on the other two computers? Perhaps the servers that are unreachable have broken path MTU discovery. Pay particular attention to the Example Path MTU Discovery Failure Scenario section on that page.

By: Lokheed

Lokheed — Sat, 02 Jul 2005 05:07:58 -0800

Found the difference, and it should have occured to me sooner. Two of the computers are my work systems (a desktop and a laptop), and I have both VPN'd into my company's network. The third (bad) computer is my personal machine, and is not using the same VPN. As soon as I disconnect from the VPN, the same problem appears on my work computers. (It was looking at ipconfig /all that tipped me off, showing a completely different DNS server).

So either something in one of the two routers is not handling DNS requests properly, or else the two DNS servers provided by Sprint aren't doing squat.

By: Lokheed

Lokheed — Sat, 02 Jul 2005 05:14:06 -0800

...and sure enough, using 4.2.2.1 and 4.2.2.2 as the primary and secondary DNS servers for the personal machine resolved the issues completely.