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DNS Name Server Types and Roles: Primary/Master, Secondary/Slave and Caching-Only Servers
(Page 2 of 3)
Reasons for Using Slave Name Servers
The master name server is obviously
the most essential server. It is on this name server that the master
files for the zone's resource records
are maintained, so the master name server is the final word
for information on the zone. However, there are several reasons why
slave servers are also important:
- Redundancy: We've already discussed how
important DNS is, especially on the Internet; with only one name server,
if it ever failed nobody would be able to resolve names such as www.xyzindustries.com
into IP addresses, which would be a Bad Thing [tm]. J
Slave name servers act as a backup for the masters they support.
- Maintenance: With more than one server,
we can easily take the primary server down for maintenance when needed
without name resolution service being disrupted.
- Load Handling: Busy zones can use multiple
servers to spread the load of name resolution requests to improve performance.
- Efficiency: There are many cases where
there is an advantage to positioning a name server in a particular geographical
location for the sake of efficiency. For example, a company may have
an office in a distant location connected using a low-speed WAN link.
To reduce name resolution traffic across that link, it makes sense to
have that zone's information available in a name server on both sides
of the connection, so one would require two physical servers.
Redundancy is the most important
consideration in setting up master and slave name servers. Sticking
two machines side by side in a server room, plugged into the same electrical
service, both connected to the Internet with the same Internet Service
Provider, and making one your master DNS server and the other your slave
is not a smart move. Ideally, the primary and secondary servers should
be as independent as possible; they should be physically distant and
have separate connections to the Internet.
Propagating Information From the Master Server to Slaves
Just as the names master
and slave suggest, the secondary name servers are not the
original source of information about a zone. They normally obtain their
resource records not from human-edited master files, but from updates
obtained from the master server. This is accomplished using a process
called a zone
transfer. These transfers are performed
on a regular basis to ensure that the slave servers are kept up to date.
The slaves can then respond to name resolution requests with current
information. Both the master and the slave are considered authoritative
for the zone.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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