dwmzmm wrote:
> Yesterday, I noticed that I can't open certain websites I visit often, such
> as Facebook and the Ye Old Rocket Forum. When I click the address and/or
> type the address, I'm always directed to a generic page that says "Oops, the
> link is down" or something of that nature. I am able to open many other
> websites, but certain ones just will not display properly without the
> annoying error messages. I tried chatting with an ATT rep last night (I have
> ATT U-verse), but he wasn't able to assist with his trouble shooting efforts,
> saying the problem is with my Microsoft Windows and not ATT. Can any one
> help?
When you type in a symbolic address into a web browser
http://www.sun.com
the computer consults DNS to convert the name to an IP address.
Then it can use the IP address, and send a packet to that node.
http://72.5.124.61
There are various ways to defeat that. For example, if the
DNS servers at your ISP are not working, then new addresses
typed into the browser, wouldn't resolve at all. No symbolic
address would work.
Various OSes, have a "hosts" file, which is a local translation
from symbolic to numeric address. That is typically used to
override what DNS would normally do. For example, if you did
something like this
sun.com 127.0.0.1
that mapping would cause a request to see the Sun Microsystems
website, into a local loopback request. The result is, you
cannot reach the site. This trick is typically used, for
some advertising sites. Some adware tools, might populate
the hosts file, to defeat advertisers.
doubleclick.net 127.0.0.1
There are also DNS exploits that can be carried out on a computer.
http://en.wikipedia.org/wiki/Cache_poisoning
To observe your networking session, and the packets on your
Ethernet, you can use a packet sniffer like Wireshark. In the
"View" menu, set name resolution for the Network layer, in order
to make the trace a bit easier to read. Or you can leave it at
the default value, so the trace consists of IP addresses. Using
the Capture:Interfaces and then clicking "start" next to the most
likely functional network interface, starts your trace. Then,
use your web browser, and watch what happens in the Wireshark window.
Each packet sent and received, should be logged in the window.
http://en.wikipedia.org/wiki/Wireshark
Now, none of this would be very enlightening, without independent
verification of DNS translation. For example, if you went to a
command (DOS) window, and did
nslookup
www.sun.com
that performs the same kind of DNS query, as was done by your web browser.
But, there are a few web sites, that make DNS translation available,
to check them manually. For example
http://www.zoneedit.com/lookup.html
A translation performed there, uses the local DNS available to that
zoneedit computer, so it has a different view of the world than your
computer. If the translations agree, then there is a good chance
everything is OK. If the translations diverge, there could be
good reasons or bad reasons for that.
You can watch Wireshark, to see where your browser is going.
Or, you can use nslookup to emulate what the browser would
do, using the same local DNS. Then, compare to an external website
offering DNS translation, to see if it agrees. Sometimes, a
DNS mapping is being changed, so you may catch the Internet
with an inconsistent mapping. But if everything ends up going
to some poker, porn, or casino site, you know it cannot be that broken
by accident.
HTH,
Paul
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