Name Server Types

There are three types of name servers:

1. The primary master builds its database from files that were preconfigured on its hosts, called zone or database files. The name server reads these files and builds a database for the zone it is authoritative for.
2. Secondary masters can provide information to resolvers just like the primary masters, but they get their information from the primary. Any updates to the database are provided by the primary.
3. Caching name server - It gets all its answers to queries from other name servers and saves (caches) the answers. It is a non-authoritative server.

The caching only name server generates no zone transfer traffic. A DNS Server that can communicate outside of the private network to resolve a DNS name query is referred to as forwarder.

DNS Query Types

There are two types of queries issued:

1. Recursive queries received by a server forces that server to find the information requested or post a message back to the querier that the information cannot be found.
2. Iterative queries allow the server to search for the information and pass back the best information it knows about. This is the type that is used between servers. Clients used the recursive query.
3. Reverse - The client provides the IP address and asks for the name. In other queries the name is provided, and the IP address is returned to the client. Reverse lookup entries for a network 192.168.100.0 is "100.168.192.in-addr arpa".

Generally (but not always), a server-to-server query is iterative and a client-resolver-to-server query is recursive. You should also note that a server can be queried or it can be the person placing a query. Therefore, a server contains both the server and client functions. A server can transmit either type of query. If it is handed a recursive query from a remote source, it must transmit other queries to find the specified name, or send a message back to the originator of the query that the name could not be found.

DNS Transport protocol

DNS resolvers first attempt to use UDP for transport, then use TCP if UDP fails.

The DNS Database

A database is made up of records and the DNS is a database. Therefore, common resource record types in the DNS database are:

• A - Host's IP address. Address record allowing a computer name to be translated into an IP address. Each computer must have this record for its IP address to be located. These names are not assigned for clients that have dynamically assigned IP addresses, but are a must for locating servers with static IP addresses.
• PTR - Host’s domain name, host identified by its IP address
• CNAME - Host’s canonical name allows additional names or aliases to be used to locate a computer.
• MX - Host’s or domain’s mail exchanger.
• NS - Host’s or domain’s name server(s).
• SOA - Indicates authority for the domain
• TXT - Generic text record
• SRV - Service location record
• RP - Responsible person
• HINFO - Host information record with CPU type and operating system.

When a resolver requests information from the server, the DNS query message indicates one of the preceding types.

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