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Running a Caching-Only
Name Server
One DNS configuration that's common on small
networks is a caching-only system. Such a name server is configured to not be
authoritative for any domains (except possibly reverse DNS on the localhost
network). Instead, the server's sole purpose is to cache external DNS requests.
The idea is to provide quicker DNS lookups by maintaining a cache that's local,
rather than relying on a remote DNS server. This configuration is particularly
likely to improve perceived network speeds in Web browsers and the like when
the link to the outside world is slow, such as a satellite-based broadband
connection, which must cope with high latenciesclose to half a second for a
two-way satellite system. Conventional telephone dial-up links also typically
have latencies of around 200 ms, which is not as bad as a satellite setup, but
can still produce a noticeable lag on lookups.It's important to note that a local DNS cache
won't improve lookup times if the address being looked up isn't in the local
server's cache. Thus, this approach is most useful on a network with a fair
number of users who tend to access many of the same sites, thus building up a
cache that's likely to be exploited on a substantial fraction of DNS lookups.A basic configuration for a caching-only name
server is similar to that shown in
would be missing most or all of the zone definitions. The only zones you might
consider defining are the localhost reverse DNS zone ( 0.0.127.in-addr.arpa ) and the root zone ( . ). Even these zones aren't strictly
necessary.The most important feature of a caching-only
BIND configuration is the forwarders and forward entries in the options section. The forwarders entry must list your ISP's DNS servers; BIND will use those systems
to do its duties, and cache the results. Rather than forward first ,
as shown in
may want to use forward
only . This configuration will cause the server to
stop trying to resolve names if the forwarder systems don't respond.WARNING
align=left border=0> If you include the root zone and use forward first in the options section, BIND may attempt to look up addresses recursively if the
forwarder systems fail. This is normally desirable, but it may slow down certain
types of failure messages in a caching-only configuration, particularly if
your Internet connection has high latencies.
As noted earlier in this chapter, slimmer
packages than BIND can function as caching-only name servers, and may be
superior choices for this role. Nonetheless, the caching-only BIND
configuration is one of the simplest possible for BIND, and because BIND ships
with all major distributions, it may be the simplest to set up. If you prefer
something slimmer for this role, though, dnscache or pdnsd may be
worth investigating.When you configure a caching-only name server (or a more
full-featured local name server), you should specify that system's IP address
as the DNS server address for all your network's local computers. If you add a
DNS server but fail to change existing local systems' DNS configurations, the
local computers will continue to use whatever outside DNS servers they had been
using.
