DNS Name Server Functions, Name Server Architecture and General Operation
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The three major functions of a name system are creating a name space, performing name registration and providing name resolution services. We've seen earlier in this larger section on the Domain Name System that DNS uses a hierarchical tree structure for its name space, and also a hierarchical tree for name authorities and registration. I'm sure that, given this, you will have to struggle to contain your surprise when I tell you that name resolution is also oriented around the notion of a hierarchical structure.
The devices that are primarily charged with performing the functions required to enable name resolution are name servers. They are arranged in a hierarchy that is closely related to the authority structure of the name system. Just as the authority structure complements the name structure but is not exactly the same as it, the name server architecture complements both the authority structure and the name structure, but may be different in its actual composition from both.
In a large DNS implementation, information about domains is not centralized in a single database run by one authority. Instead, it is distributed across many different authorities that manage particular top-level domains (TLDs), second-level domains or lower-level subdomains. In the case of the global Internet, literally millions of different authorities, many of them responsible only for their own local domain space, participate cooperatively in running the DNS system.
With authority for registration distributed in this manner, this means that the information about domains is similarly spread amongst many entities, resulting in a distributed database. A key concept in DNS name resolution is that each entity that maintains responsibility for a part of the name space must also arrange to have that information stored on a DNS server. This is required so that the server can provide the information about that part of the name space when resolution is performed. As you can see, then, the existence of a structured hierarchy of authorities directly implies the need for a hierarchy of servers that store that hierarchical name information.
Each DNS zone of authority is required to have one or more DNS servers that are in charge of managing information about that zone. These servers are said to be authoritative for the zone. Storing information about the domains, subdomains and objects in the zone is done by recording the data in special resource records that are read from DNS master lists maintained by administrators. Servers then respond to requests for this information.
Since information in DNS is stored in a distributed form, there is no single server that has information about every domain in the system. As we'll see in the section on name resolvers, the process of resolution instead relies on the hierarchy of name servers described just above. At the top of the DNS hierarchy is the root domain, and so we also see there the root name servers. These are the most important servers, because they maintain information about the top-level domains within the root. They also have knowledge of the servers that can be used to resolve domains one level below them. Those servers in turn are responsible for the TLDs and can reference servers that are responsible for second-level domains. Thus, a DNS resolution may require that requests be sent to more than one server.
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