Servers do not really have ``types''. A server can be a primary for some zones and a secondary for others, or it can be only a primary, or only a secondary, or it can serve no zones and just answer queries via its ``cache''. Previous versions of this document referred to servers as ``master'' and ``slave'' but we now feel that those distinctions -- and the assignment of a ``type'' to a name server -- are not useful.
Caching Only Server
All servers are caching servers. This means that the server caches the information that it receives for use until the data expires. A Caching Only Server is a server that is not authoritative for any zone. This server services queries and asks other servers, who have the authority, for the information needed. All servers keep data in their cache until the data expires, based on a TTL (``Time To Live'') field which is maintained for all resource records.
Remote Server
A Remote Server is an option given to people who would like to use a name server from their workstation or on a machine that has a limited amount of memory and CPU cycles. With this option you can run all of the networking programs that use the name server without the name server running on the local machine. All of the queries are serviced by a name server that is running on another machine on the network. A host which has an /etc/resolv.conf file listing only remote hosts, and which does not run a name server of its own, is sometimes called a Remote Server (because the actual server is remote?) but more often it is called simply a DNS Client. This kind of host is technically not a ``server'', since it has no cache and does not answer queries.
Slave Server
A Slave Server is a server that always forwards queries it cannot satisfy from its cache, to a fixed list of forwarding servers instead of interacting with the name servers for the root and other domains. The queries to the forwarding servers are recursive queries. There may be one or more forwarding servers, and they are tried in turn until the list is exhausted. A Slave and forwarder configuration is typically used when you do not wish all the servers at a given site to interact with the rest of the Internet servers. A typical scenario would involve a number of workstations and a departmental timesharing machine with Internet access. The workstations might be administratively prohibited from having Internet access. To give the workstations the appearance of access to the Internet domain system, the workstations could be Slave servers to the timesharing machine which would forward the queries and interact with other name servers to resolve the query before returning the answer.
Caching Only Server
All servers are caching servers. This means that the server caches the information that it receives for use until the data expires. A Caching Only Server is a server that is not authoritative for any zone. This server services queries and asks other servers, who have the authority, for the information needed. All servers keep data in their cache until the data expires, based on a TTL (``Time To Live'') field which is maintained for all resource records.
Remote Server
A Remote Server is an option given to people who would like to use a name server from their workstation or on a machine that has a limited amount of memory and CPU cycles. With this option you can run all of the networking programs that use the name server without the name server running on the local machine. All of the queries are serviced by a name server that is running on another machine on the network. A host which has an /etc/resolv.conf file listing only remote hosts, and which does not run a name server of its own, is sometimes called a Remote Server (because the actual server is remote?) but more often it is called simply a DNS Client. This kind of host is technically not a ``server'', since it has no cache and does not answer queries.
Slave Server
A Slave Server is a server that always forwards queries it cannot satisfy from its cache, to a fixed list of forwarding servers instead of interacting with the name servers for the root and other domains. The queries to the forwarding servers are recursive queries. There may be one or more forwarding servers, and they are tried in turn until the list is exhausted. A Slave and forwarder configuration is typically used when you do not wish all the servers at a given site to interact with the rest of the Internet servers. A typical scenario would involve a number of workstations and a departmental timesharing machine with Internet access. The workstations might be administratively prohibited from having Internet access. To give the workstations the appearance of access to the Internet domain system, the workstations could be Slave servers to the timesharing machine which would forward the queries and interact with other name servers to resolve the query before returning the answer.
Source: http://www.dns.net/dnsrd/docs/bog/bog-sh-4.html
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