>You have two name servers - both in your class c - serving sites
>in your class c.
>
>Who cares if you have one outside? If access to your class c
>fails - it doesn't matter that you have a name server outside
>pointing to hosts that can't be reached.
There are a few reasons.
The main reason is so that other sites trying to reach you know that you
are down! If all your servers (DNS, web, mail) are at the same physical
location, suppose something goes wrong at the site (power outage, a backhoe
cutting a major line somewhere nearby, etc.). If anyone tries getting to
your website, they won't be able to get to your DNS servers, so they won't
know where your website it, or whether or not it is reachable. By having a
"good" secondary, they will know where your website is and that it is
currently unreachable.
Then, there's the issue of caching responses. If your secondary DNS server
is reachable, it will normally continue to response for at least several
weeks. So during the time of the outage, DNS servers will be getting good
data and caching it. But what happens if they can't reach any DNS
servers? That's something that is designed to be an extremely rare
case. RFC2308 7.2 says that DNS servers should only cache this information
for 5 minutes, but I'm guessing there are lots of DNS servers/resolvers out
there that would cache the information for at least a day or so (perhaps
only after knowing the server was unreachable for at least a few hours).
There are also other issues, such as the extra bandwidth used.
So having geographically disperse DNS servers is desired, and the "proper"
thing to do, and although it isn't critical, not having geographically
disperse DNS servers does have some drawbacks.
-Scott
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