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DNS Name Server Types and Roles: Primary/Master, Secondary/Slave and Caching-Only Servers
(Page 3 of 3)
Name Server Roles
The “master” and “slave”
roles for a zone are logical and do not always correspond to individual
physical hardware devices. A single physical name server can play multiple
roles in the following cases:
- It can be the master name server for more than
one zone. Each zone in this case has a distinct set of resource records
maintained in separate master files.
- It can be a slave name server for more than one
zone.
- It can be a slave name server for certain zones
as well as a primary for others.
Note however that a single physical
name server cannot be a primary and a secondary server for the same
zone, since… well, what would be the point?
 Key Concept: The master DNS server for a zone is its primary server, which maintains the master copy of DNS information. Most DNS zones also have at least one slave or secondary DNS server. These are important because they serve as backups for the primary server, and they can also help share the load of responding to requests in busy zones. Secondary name servers get their information from primary servers on a routine basis. Both master and slave servers are considered authoritative for the zones whose data they maintain.
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Caching-Only Name Servers
We'll see later in this section that
for efficiency reasons, all DNS servers perform caching
of DNS information so it can be used again
if requested in the near future. This includes both master and slave
name servers. The importance of caching is so significant that there
are some servers that are set up only to cache information from other
DNS servers. Unsurprisingly, these are called caching-only name
servers.
These name servers are not authoritative
for any zone or domain, and don't maintain any resource records of their
own. They can only answer name resolution requests by contacting other
name servers that are authoritative and then relaying the information.
They then store the information for future requests. Why bother? The
reason is performance. Through strategic placement, a caching-only server
can increase DNS resolution performance substantially in some networks
by cutting down on requests to authoritative servers.
Key Concept: There are DNS servers that do not maintain DNS resource records of their own but solely hold recently-used information from other zones. These are called caching-only name servers and are not authoritative for any zone. |
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