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9.5 Planning for Disasters



It's a fact of life
on a network that things go wrong. Hardware fails, software has bugs,
and people occasionally make mistakes. Sometimes this results in
minor inconveniences, like having a few users lose connections.
Sometimes the results are catastrophic and involve the loss of
important data and valuable jobs.

Because the Domain Name System relies so heavily on the network, it
is vulnerable to network outages. Thankfully, the design of DNS takes
into account the imperfection of networks: it allows for multiple,
redundant name servers, retransmission of queries, retrying zone
transfers, and so on.

DNS doesn't protect itself from every conceivable
calamity, though. DNS doesn't or
can't protect against certain types of network
failuressome of them quite common. But with a small investment
of time and money, you can minimize the threat of these problems.


9.5.1 Outages


Power
outages, for example, are relatively common in many parts of the
world. In some parts of the U.S., thunderstorms or tornadoes may
cause a site to lose power or have only intermittent power for an
extended period. Elsewhere, typhoons, volcanoes, or construction work
may interrupt electrical service. And you never know when those of
you in California might lose power in a rolling blackout from a lack
of electrical capacity.

If all your hosts are down, of course, you don't
need name service. Quite often, however, sites have problems when
power is restored. Following our
recommendations, they run their name servers on file servers and big,
multiuser machines. And when the power comes up, those machines are
naturally the last to bootbecause all those disks need to be
checked and fixed first! Which means that all the on-site hosts that
are quick to boot do so without the benefit of name service.

This can cause all sorts of wonderful problems, depending on what
services your hosts access when they boot. For example, your PCs may
mount your servers' drives (via net
use
) when they boot. If they do, they almost certainly
specify the servers' domain names or NetBIOS names.

Using hostnames in commands is admirable because it allows
administrators to change the servers' IP addresses
without changing all the startup files on-site. However, if name
service isn't available when your PCs boot, the
net use command will fail, which may cause
successive commands to fail, too. This will certainly not help your
users' productivity.


9.5.2 Recommendations


Our recommendation is to add the names and IP addresses of critical
hosts to your PCs' HOSTS files.
Any host whose name is referenced during the boot process should
appear in this file. You can synchronize the file by copying it from
share to share. On Windows Server 2003, the default location for the
file is %SystemRoot%\System32\Drivers\Etc,
usually C:\Windows\System32\Drivers\Etc. The
format of the file is just like the format of the Unix
/etc/hosts file: each line consists of an IP
address (in dotted-octet notation), which starts in the first column,
followed by whitespace and the canonical name of the host.
Optionally, one or more aliases may follow the canonical name. For
example:

192.249.249.1 wormhole.movie.edu wormhole
192.249.249.3 terminator.movie.edu terminator

Now, if a PC needs to look up wormhole or
wormhole.movie.edu when it boots, it will be
able to resolve the name.

However, using HOSTS files poses some danger:
unless you take care to keep the files up-to-date, their information
may become stale. And since the Windows Server 2003 resolver uses
HOSTS before querying a name server, a stale
entry can cause resolution failures that are hard to diagnose.

The best solution to this problem
is to run a name server on a host with uninterruptible
power. If you rarely experience extended power loss, battery backup
might be enough. If your outages are longer and name service is
critical to you, you should consider an uninterruptible power system
(UPS) with a generator of some kind.

If you can't afford luxuries like these, you might
just try to track down the fastest booting host around and run a name
server on it. Hosts with small filesystems should boot quickly since
they don't have many disks to check.

Once you've located the right host,
you'll need to make sure the host's
IP address appears in the resolver configurations of all of your
hosts that need full-time name service. You'll
probably want to list the backed-up host last since, during normal
operation, hosts should use the name server closest to them. Then,
after a power failure, your critical applications will still have
name service, albeit at a small sacrifice in performance.


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