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How DNS works in theory

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Domain_name_space

The domain name space consists of a tree of domain names. Each node or leaf in the tree has one or more resource records, which hold information associated with the domain name. The tree sub-divides into zones. A zone consists of a collection of connected nodes authoritatively served by an authoritative DNS nameserver. (Note that a single nameserver can host several zones.)

When a system administrator wants to let another administrator control a part of the domain name space within his or her zone of authority, he or she can delegate control to the other administrator. This splits a part of the old zone off into a new zone, which comes under the authority of the second administrator’s nameservers. The old zone becomes no longer authoritative for what goes under the authority of the new zone.

A resolver looks up the information associated with nodes. A resolver knows how to communicate with name servers by sending DNS requests, and heeding DNS responses. Resolving usually entails iterating through several name servers to find the needed information.

Some resolvers function simplistically and can only communicate with a single name server. These simple resolvers rely on a recursing name server to perform the work of finding information for them.

Parts of a domain name

A domain name usually consists of two or more parts (technically labels), separated by dots. For example wikipedia.org.

  • The rightmost label conveys the top-level domain (for example, the address en.wikipedia.org has the top-level domain org).
  • Each label to the left specifies a subdivision or subdomain of the domain above it. Note that “subdomain” expresses relative dependence, not absolute dependence: for example, wikipedia.org comprises a subdomain of the org domain, and en.wikipedia.org comprises a subdomain of the domain wikipedia.org. In theory, this subdivision can go down to 127 levels deep, and each label can contain up to 63 characters, as long as the whole domain name does not exceed a total length of 255 characters. But in practice some domain registries have shorter limits than that.
  • A hostname refers to a domain name that has one or more associated IP addresses. For example, the en.wikipedia.org and wikipedia.org domains are both hostnames, but the org domain is not.

The Domain Name System consists of a hierarchical set of DNS servers. Each domain or subdomain has one or more authoritative DNS servers that publish information about that domain and the name servers of any domains “beneath” it. The hierarchy of authoritative DNS servers matches the hierarchy of domains. At the top of the hierarchy stand the root nameservers: the servers to query when looking up (resolving) a top-level domain name (TLD).

Iterative and recursive queries:

  • An Iterative query is one where the DNS server may provide a partial answer to the query (or give an error). DNS servers must support non-recursive queries.
  • A recursive query is one where the DNS server will fully answer the query (or give an error). DNS servers are not required to support recursive queries and both the resolver (or another DNS acting recursively on behalf of another resolver) negotiate use of recursive service using bits in the query headers.

Translated and adapted from Wikipedia.

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