
Configuring an Authoritative DNS Server
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each time you change the information it offers; the secondary server compares the
primary value to its own value to determine whether it should perform a zone transfer.
The primary configuration file also specifies a retry value, which the secondary server
uses instead of the refresh value if it can’t reach the primary server. This can happen
if the master server or the network fails. In that case, the secondary server masquer-
ades as the master for a little while.
A secondary server can’t masquerade forever, though. Eventually, its information
could become so outdated that it would be preferable for it to stop answering que-
ries altogether. Hence, the configuration file also specifies an expiry time. If this time
passes without a successful update, the secondary server continues trying to contact
the primary server but refuses to answer queries.
There’s one more value that you should be aware of before tackling the configura-
tion files: the minimum time to live (TTL). When a remote DNS server receives an
answer to a query from you, it caches that information and reuses it during the time
specified in the TTL value. Caching is critical to the performance of DNS. Thanks to
caching, if somebody spends an hour visiting various web pages at your site (each of
which may involve multiple downloads), a server near the user will need to ask you
for the domain ...