Back To Basics: DNS Server Roles — Caching-only Servers

Thomas Shinder

Last week we began our discussion of DNS Server
roles by examining some of the important characteristics of Primary and
Secondary DNS Server. If you missed out on that discussion, you can check it out
HERE.

Last week we began our discussion of DNS Server roles by examining some of the important characteristics of Primary and Secondary DNS Server. If you missed out on that discussion, you can check it out HERE.

This week well take a look at some of the other
important roles that DNS Servers take on:

• Caching Only Servers
• Forwarding Servers
• Slave Servers
• Dynamic DNS Servers

Caching Only Servers

All DNS Servers cache the results of their
queries. However, some DNS Servers are put into place to provide only this
caching function. The Caching-only DNS server does not contain zone
information or a zone database file. The Caching-only server only contains
information based on the results of queries that it has already performed. In
this case, the cache takes the place of the zone database file. These
Caching-only DNS Servers can be set up quickly, and are an important ally in
your network and Internet security design.

All DNS servers have a cache.dns file that
contains the IP addresses of all Internet root servers. The Windows 2000
cache.dns file is also referred to as the root hints file. The caching only server uses this list to begin building its
cache. It adds to the cache as it issues iterative queries when responding to
client requests to resolve Fully Qualified Domain Names to IP addresses. After
the FQDNs are resolved to IP addresses, this information is stored in the DNS
Server cache.

Caching only servers are valuable because:

• They do not participate in zone transfer, and
  therefore there is no zone transfer traffic
• They can be placed on the far side of a slow
  WAN link and provide host name resolution for remote offices that do not
  require a high level of host name resolution support
• They can be implemented to provide secure host
  name resolution when configured as Forwarders

Remote offices are often connected to the
main office via slow WAN links. These locations benefit from Caching-only
servers because:

1. There is no zone transfer traffic. For large
   corporate intranets with small remote offices, eliminating zone transfer
   traffic can be very beneficial since zone transfer traffic could have a
   negative effect on their slow link.
2. There is a reduction in the amount of DNS
   query traffic that traverses the WAN to the corporate DNS Servers.

These Caching-only servers do not require expert
administration. A satellite office is unlikely to have trained DNS
administrative staff on-site. This saves the cost of having an experienced DNS
administrator visit the site. However, in order to gain the most benefit from a
Caching-only DNS Server, you must not reboot the computer. Since the DNS Cache
only remains in RAM (or sometimes on disk in the page file), the contents of the
cache will be lost if the server is rebooted. Be sure to include fault-tolerance
mechanisms such as an UPS, Disk Mirroring, and redundant power supplies on such
a machine.