A common distress call I get is something like "Help! Our Internet is down!"
See also Can't connect to Internet
Sometimes it really is. There's not much I can do if a router in Connecticut went up in flames or a backhoe in Cambridge just cut through a major
data line. If it is a dead or malfunctioning router the problem will
probably correct itself very quickly, but a cable break can sometimes
cause fairly long outages.
Surprisingly often the Internet itself really isn't down at all. My
client has access; what they don't have is name resolution. To test
that, I might ask them to try some IP address that I know is up. If that works, then name resolution is the issue, not internet
Of course the name resolution problem can be caused by someone else
having a real connectivity problem. For example, my DNS comes from
NS23.WORLDNIC.COM and NS24.WORLDNIC.COM. If a physical or routing
problem prevents access to those servers, sooner or later you won't
be able to resolve this site (and quite a few others, though I'm sure you
are most concerned about this, right?). In that case, your symptoms
are that some sites resolve but others will not.
A traceroute might show you where the breakdown is, particularly
if you can do it from different starting points. You might be able to
reach a DNS box from one location but not another just because
of the route packets travel.
(Article continues after the break)
By the way, do the traceroute with an ip address, not a name. That
doesn't matter if DNs is working, but it makes sure of it. The first thing you have to be able to get to is your ISP's default gateway - if you can't get to that, you won't get anywhere.
There can be more subtle problems. I had email yesterday from a customer
who asked if www.aplawrence.com was down. I knew it wasn't, but
I happened to be logged into his machine at that moment, so I tried
pinging www.aplawrence.com. It timed out, unable to resolve.. I
tried just aplawrence.com and that responded. I then logged off
his machine and tried the same
two pings from my home: again the wwww failed to resolve but aplawrence.com
did. Obviously worldnic's servers were doing something incorrectly
at that time, but within half an hour it had cleared up.
The dig and host commands can help you dig into the truth of these access problems. Usually
simple patience is the answer: no doubt many other people are affected
by whatever the problem is, and alarms are probably going off at many
places. In most cases, the issue will be fixed before you can even
make a phone call to complain about it.
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