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A vulnerability has been discovered in the DNS servers providing a critical part of internet infrastructure. It's easy to test and easy to work around.

There's been a lot of press around some kind of "big, bad" vulnerability in DNS. I don't need the details, I just want to know what it means to me, and what, if anything, I need to do to be safe.

DNS is one of those critical internet infrastructure things that we just don't think of all that often. But it is, indeed, critical. And when a vulnerability is discovered, it's a big deal.

A very big deal.

In short, DNS is the service that your computer uses to turn names you and I can read and recognize, like "", into IP addresses like that are used by the internet to actually transfer data.

It's good that you don't care about the exact details, because at this writing they haven't actually been divulged yet, and the various theories are fairly complex.

"Even after a very large push ... that vulnerability still exists on too many DNS servers."

The effect of the vulnerability is that if it is successfully exploited, a DNS request for a specific name can be forced to return the wrong IP address. So imagine that you're going to and the DNS request that asks "what's the IP address for" returns an IP address of a hacker's server instead. A hacker's server that is crafted to look like Paypal, but is most definitely not Paypal. How would you know?

That's why it's a big deal. Even after a very large push to get all the DNS servers patched before the vulnerability became public, the fact is that even today that vulnerability still exists on too many DNS servers.

So, what can you do?

The good news is that this is easy to detect, and easy to work around, even though it's not your problem.

That's correct, it's not your problem. This is not something that's present on your computer. (Unless, that is, you're a geek running your own DNS server, like I am.) DNS servers are provided by your ISP, and it's there that the vulnerability may lie.

Test your DNS. Visit this link:

You'll note that's an IP address - if it were a normal name it would require a DNS look up using the very DNS server that you don't yet trust. (Thanks Michael Horowitz for that tidbit. And yes, in theory it could still be spoofed; more on that below.)

You will be presented with two charts. The key is that you want both "Randomness" results to be "Great", and that each time you run the test the graphed dots and the list of "Values Seen" are different. That's all. If you get "Great" for both tests, you're done. (If you travel, or use a hotspot, you'll need to run this test at each location before you can feel safe.)

If you didn't get "Great" for both, there are two things I believe you must do:

  • Complain to your ISP. They are vulnerable, meaning all of their customers are vulnerable. Patches and updates are readily available, so there's simply no excuse not be up to date.

  • Switch to OpenDNS. OpenDNS is a free DNS alternative that is known not to be vulnerable. Whether you stick with it long term is up to you, but as a short term way to avoid your ISP's vulnerable DNS servers, it's a perfect and quick solution. Instructions are here.

Now, I mentioned above that the test could be spoofed. Even when you go to the main page of the test by IP address rather than by name, the test itself still has to use DNS to perform the test. The danger scenario looks like this: your ISP has a vulnerable DNS server, that has been exploited. As part of the exploit the DNS names for the test servers are redirected to IP addresses of servers that always return "Great", no matter what. I honestly don't think this is very likely, but I include it for completeness.

If there's any question at all, you'll be safe switching to OpenDNS.

You'll likely hear more about this vulnerability in the coming weeks, but as long as things are "Great" you'll know you're safe.

Article C3464 - August 2, 2008 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

August 2, 2008 6:29 PM

Great! I was just about to ask you that. :)

My current ISP's DNS servers are not vulnerable but the ISP I am about to switch to is vulnerable (I made a friend run that test). So I would inform them.

But my question is how difficult a thing it is for them to apply the patch? Is it just like software updates (or maybe a little more complex) or is it really truly complex thing that an ISP might not have the expertise to implement in reasonable time?

It'll vary based on exactly which DNS server they happen to be running, but the bottom line is that it should be very easy, and even if it's not this is exactly what we expect our ISPs to be capable of doing and doing well.

FWIW: I updated my DNS server in less than a minute with two, maybe three clicks of a mouse. (Ubuntu Linux) Not all are that easy, but many are.

As I said there's no excuse.


Eldon Gaw (Orwell)
August 5, 2008 8:44 AM

one little difficulty I just experienced, and I'm sure I'm not alone... " Address not available" reply...Hum...Just too many people testing their DNS or has someone disabled the link...or, has my ISP (rogers) blacklisted the address...
George Orwell Lives!

Eldon Gaw (Orwell)
August 5, 2008 9:01 AM

Sorry, I supplied the wrong text on my last email, it should have read:

The page cannot be displayed
The page you are looking for is currently unavailable. The Web site might be experiencing technical difficulties, or you may need to adjust your browser settings. "

Not that it could not find the server. My apologies.


Also, I did look at the OPEN DNS info and realized I have not heard why I might not want to stay with it if I choose to switch, and why I might want to switch, DNS issue or not. It looks pretty inviting, but how do they make their money?


Eld. (Orwell Lives!)

Teresa Smith
August 5, 2008 9:01 AM

Am I to assume this DNS test will not work on a Mac?

DNS, and the test, is platform independant. It should work.


Bob Ruttske
August 5, 2008 9:35 AM

My ISP showed good at first test then on the 2nd try showed poor (just a straight line) on the first graph. Called them and they were unable to resovle the address on your e mail.

August 5, 2008 9:47 AM

Ran the test and received a result of Nominum. Is this company proven to be safe or is more time needed to tell? Better to use OpenDNS in the meantime or just sit tight? Thanks!

Michael Horowitz
August 5, 2008 10:44 AM

For Teresa: The DNS problem does affect Macs. The problem is an Internet networking thing, so it affects ALL computers on the Internet.

To those that asked about OpenDNS see

OpenDNS provides added safety for free
More about OpenDNS, including adult site filtering

Linda Claycomb
August 5, 2008 11:15 AM

I tried to use the test; however, all I got was "Failure to connect to Web Server". Didn't matter what time of day. Would this be a Hughesnet thing?

Bill Nelson
August 5, 2008 6:23 PM

I tried using the test link and received the message, "The requested URL could not be retrieved
While trying to retrieve the URL:

The following error was encountered:

Unable to determine IP address from host name for
The dnsserver returned:

No Address records
This means that:

The cache was not able to resolve the hostname presented in the URL.
Check if the address is correct.
Your cache administrator is support.
Generated Wed, 06 Aug 2008 01:17:19 GMT by (squid/2.5.STABLE11)"
From the preceding notes, I guess I'm not the only one having problems with the link, but the message seems different than others reported.

Gin Fonte
August 6, 2008 11:54 AM

For the record, I ran the test and had no difficulties: it worked GREAT! ;-)

August 7, 2008 3:48 PM

I had no idea what a DNS server was. Now I know and mine are working "GREAT".

August 9, 2008 2:11 AM

My ISP's DNS server is Great, but now I'm wondering about where my website is hosted. Is this an issue? Is there a way to test it?

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