Loosely related: There are weird times that I can't access my webmail.
The error I get is that the site does not exist. Wait a few minutes,
and all is well. Happens a couple of times a month. Is this related
to DNS changes?
John McKee
It can be. And that can be because you cannot reach your DNS (more likely)
or because of upstream problems. Or it can be that the server has gone
offline for whatever reason. The browser messages are usually somewhat
helpful in identifying the correct cause, unless they are set to "friendly
(meaning uninformative) messages."
Next time it happens, go to a command line and enter:
nslookup server.domain.name
You should be presented with the DNS, or with something informative.
Here's a good result from one of my linux servers:
nslookup www.google.com
Server: 65.32.5.111
Address: 65.32.5.111#53
Non-authoritative answer:
www.google.com canonical name = www.l.google.com.
Name: www.l.google.com
Address: 74.125.47.147
Name: www.l.google.com
Address: 74.125.47.99
Name: www.l.google.com
Address: 74.125.47.103
Name: www.l.google.com
Address: 74.125.47.104
Name: www.l.google.com
Address: 74.125.47.105
Name: www.l.google.com
Address: 74.125.47.106
The first line of response identifies the Name Server that returned the rest
of the information. Now here's what happens if the DNS cannot be reached
(which I simulated by setting false DNS information) :
nslookup www.google.com
;; connection timed out; no servers could be reached
Tying this back to what Ken was saying... Let's say one of the above
servers (. . .106) establishes connection with a mail server, and claims to
be www.google.com (via EHLO www.google.com)
Reverse DNS on the IP address 74.125.47.106 reveals www.l.google.com.
Different, but no matter. Now let's take www.l.google.com and get its list
of addresses (which we did above). Is . . .106 in the list? Yup, so we're
good to go. If .106 had been missing, then some systems (AOL, COX, Comcast,
...) would reject the mail.
Dennis Lovelady
http://www.linkedin.com/in/dennislovelady
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