Posted by arlene

For each domain on the Internet, there must be a primary server and a secondary server. The primary DNS server for the domain contains a collection of resource records that contain the address mappings for hostnames in the domain. The primary DNS server is the final authority for these mappings. The secondary DNS server contains a copy of the database maintained by the primary server and can continue to resolve names when the primary server is offline. It’s important to note that the primary DNS server is where changes are made to the database. Through the use of the zone transfer mechanism, the data is copied to secondary servers.

In many cases, a DNS server answers name queries for domains for which it isn’t the authority. In that case, the DNS server contacts a DNS server further up the hierarchy until one is found that can resolve the name or that can point to another DNS server that is the authority for the name. The DNS server maintains a cache of names that have been resolved by this method so that it doesn’t have to continually poll other servers for names that are frequently queried.

A third type of DNS server is a caching-only server. This type of server does not maintain a database for a particular zone. To put it in other terms, a caching-only server isn’t authoritative for any zone or domain and does not use the zone transfer mechanism to keep a current copy of the entire database. Instead, a caching-only name server has to contact another DNS server to initially resolve a name, but like the other servers it maintains a cache of names it has resolved so that it doesn’t have to keep forwarding the query to another server. This type of server is normally used on a network segment that’s connected to the rest of the network by a slower link (or a more expensive one) and is used to reduce network traffic.

Zones

In many cases, it isn’t efficient to have a single server maintain the database for an entire domain. Instead, a primary DNS server can be authoritative for only a zone in the domain. A zone is a partition of the domain into subdomains. For example, one DNS server might be the authority for the zone zira.twoinc.com, whereas another might serve as the authority for the zone researchtwoinc.corn. Both subdomains exist within the same domain: twoinc.com. However, by dividing the domain into subdomains, it becomes easier to manage not only the DNS servers but also the individual business or organizational units that the domain services.

A zone transfer occurs when a secondary DNS server contacts a server that is primary for the zone and finds that it needs to obtain changes to the database. This is accomplished by using serial numbers contained in the database. If the secondary server has a lower serial number, a new copy of the database is copied to it.

Standard DNS Database Files

There are three basic types of files used by DNS servers. In most DNS implementations, you need to use a text editor to make changes to these files. Most newer DNS servers, such as Microsoft’s DNS server, provide a graphical interface that can be used for adding or changing information in the DNS files. The basic files are

• Database File—This is the file that stores the resource records for the zones for which the DNS server is responsible. The first record in this file is the Start of Authority (SOA) record.

• Cache File—This file contains information for other name servers that can be used to resolve queries that are outside the zone or domain for which the server is responsible.

• Reverse Lookup File—This file is used to provide a hostname when the client only knows the IP address. This can be useful for security purposes. For example, a Web server that receives a request from a client can query the DNS with the name of the client to find out whether the hostname associated with the IP address is correct.

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