Types of DNS Servers

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Types of DNS Servers

A DNS Server has information about the domain name space that it has already obtained either from a local copy of a zone file or by making a query of another DNS Server. A name server can have more than one zone file installed on it. A name server can have the original copy of a zone, or it can receive a copy of a zone file from another name server. If a name server has any copy of a zone file, it has authority for that zone.

There are three types of name servers: Primary, Secondary, and Master.

A primary server has the original copy of a zone file. Any changes made to the zone file are made to the file on the primary server. When a primary server receives a query about a host name in its own zone, it retrieves the host resolution locally from its own zone files.

A secondary server gets a copy of zone files from another server. This secondary zone file is a read-only copy of the file; any changes made to the zone are made at the originating zone file. Then the changes are copied down to the secondary server through replication. When zone files are copied from another server it is called a zone transfer.

There are several reasons you should have a secondary server for each zone. A secondary server provides redundancy, enabling host names in the zone to be resolved even if the primary server goes down. A secondary server can also reduce the load on a primary server or reduce network traffic. For example, placing a secondary server on a remote site can reduce network traffic generated when clients cross the WAN link to resolve host names. With a secondary server at this remote site, client queries can be handled locally. The only traffic from DNS is generated when the zone file on the primary server changes and the secondary server downloads a new copy. Also, the primary server sees less activity because it communicates with only one host at the remote site (the secondary server) rather than resolve queries from all the clients at the site.

A server can have any number of zone files stored on it. The primary and secondary designation applies to each zone file rather than to the server itself. A server can be the primary for one zone (it has the master copy of that zone) and a secondary for another zone (it gets a read-only copy of the zone file through a zone transfer).

The server from which a secondary server receives a zone transfer is called the Master Name Server. The TCP/IP address of the Master Name Server is configured at the secondary server. The master server can be a primary or a secondary server. If the master is a primary, then the zone transfer comes directly from the source. If the master name server is a secondary server, the file received from the master server via a zone transfer is a copy of the read-only zone file. In this scenario, there can be a delay in receiving changes made to the zone file because the file must first be transferred to the master server and then transferred again to the next server in line.

As Figure 12.5 illustrates, however, using secondary servers as master servers can reduce the load on a primary server by limiting the number of secondary servers to which the primary server must send zone transfers. In this figure, the primary server sends a copy of the zone to three servers in total while only communicating directly with one server. The master server for Secondary2 and Secondary3 is on the same side of a slow WAN link. The zone file is transferred once over the slow link to Secondary1, and then is transferred to the other servers on the same LAN.

Both primary and secondary servers are considered authoritative for their zones because they have the zone information. In other words, either the primary or the secondary can respond to a request for information about the part of the domain that is stored in that zone file.

A DNS Server doesn’t have to have any zone files, either as a primary or a secondary server. If it has no zone files, the DNS Server is known as a caching-only server. The only responsibility of a caching server is to make DNS queries, return the results, and cache any results it obtains. Caching servers are not authoritative for any domains because they don’t store copies of any zone files locally. When a caching-only server first starts, it does not have any DNS information stored. A caching server builds information only when it caches results of queries made after the server starts. However, installing DNS as a caching-only server may be a good choice across a slow WAN link, because entire zone files don’t need to be transferred. The caching server can make a query across the link, but only one record is transmitted, not the full zone file. After the server has resolved a query, a future query for the same information can be resolved locally from the cache. Resolving locally eliminates the need to communicate across the WAN link (at least until the cached entry expires). The time to live of cached entries is determined by the server that answered the query. It returns a time to live for the query along with the name resolution.

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