Friday, November 09, 2007

Identifying A DNS Problem

DNS, which lets you translate a host name or URL into an IP address, is a key process in Internet use. Sometimes, though, it doesn't work. You try to browse to http://www.example.com, and you get a cryptic
Firefox can't find the server at www.example.com.

or
We can't find "www.example.com"

or worse, sometimes the classical
404 Not Found

Or even, the white screen of death - no error - no response.

Now the above example symptoms could have been caused by any of several scenarios.

  1. Host www.example.com doesn't exist.
  2. Host www.example.com isn't operational today.
  3. Your DNS (that translates www.example.com into an IP address) isn't working.
  4. Your MTU setting is causing a problem with accessing www.example.com.
  5. (This is not fiction, folks) - Your ISP, or a government agency, is intentionally blocking your access to www.example.com.
  6. You don't have Internet connectivity.


What to do now? Well, if your Internet connectivity is down, you've got different work to do. But, if you can access any other web sites, or if you're otherwise certain that your service is not the problem, then make sure that your DNS is working. To do this:

  • Find out the IP address of the web site. There are various web sites all over the Internet that will let you use their DNS servers, thru your browser. I use 2 web sites, consistently, and keep their URL and IP addresses available.

    1. All Net Tools, by IP address: http://216.92.207.177/toolbox .
    2. All Net Tools, by name: http://www.all-nettools.com/toolbox .
    3. DNS Stuff, by IP address: http://66.36.247.82/ .
    4. DNS Stuff, by name: http://www.dnsstuff.com/ .

    I use either of those two web sites; in case one goes down I use the other. And, if I'm researching a DNS problem, I access either one by its IP address. Finally, given the possibility that one or the other might change its IP address, I can hopefully resolve its name, using the other website. So, I keep all 4 addresses handy.

    • For All Net Tools, I enter the web site URL into the "SmartWhois" window, and hit Enter or Go!.
    • For DNS Stuff, I enter the web site URL into the "DNS lookup" window, and hit Enter or Lookup.

  • Conduct a simple 4 step test. In this example, I'll target www.yahoo.com, which uses (among many others) 66.94.230.33. Feel free to use whatever web site, for your testing, that pleases you.
    • Clear all caches, to ensure consistency.
    • From a command window:
      1. Ping www.yahoo.com.
      2. Ping 66.94.230.33.
      3. Note success / exact text of error messages.
    • From your browser:
      1. Browse www.yahoo.com.
      2. Browse 66.94.230.33.
      3. Note success / exact text of error messages.

  • Now, consider the results of the tests.
    • If you see a difference between both IP address accesses, as compared to both named accesses, you very likely have a DNS problem.
    • If you can ping (with a successful return), but not browse, with identical results for IP address and name, you may well have an MTU setting problem.
    • If you see a combination of results, you may need to research BOTH a DNS and MTU problem.
    • If the tests aren't conclusive, consider the ubiqitous LSP / Winsock corruption problem.


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