Книга: Fedora™ Unleashed, 2008 edition

How DNS Provides Name Service Information to Users

How DNS Provides Name Service Information to Users

Users of DNS need not be aware of these details. To them, the namespace is just a single tree — any part of which they can request information about. The task of finding the requested RRs from the resource set for a domain is left to programs called resolvers. Resolvers are aware of the distributed structure of the database. They know how to contact the root nameservers (which are authoritative for the root domain) and how to follow the chain of delegations until they find an authoritative nameserver that can give them the information for which they are looking.

As an analogy, you can think of domains as directories in a file system and RRs as files in these directories. The delegation of authority over subdomains is similar to having an NFS file system mounted under a subdirectory: Requests for files under that directory would go to the NFS server, rather than this file system. The resolver's job is to start from the root directory and walk down the directory tree (following mount points) until it reaches the directory that contains the files in which the user is interested. For efficiency, the nameservers can then cache the information they find for some time. This is why things appear to be listed in reverse order. This process is examined in detail next.

In practice, there are several authoritative nameservers for a domain. One of them is the master (or primary) nameserver, where the domain's data is held. The others are known as slave (or secondary) nameservers, and they hold automatically updated copies of the master data. Both the master and the slaves serve the same information, so it doesn't matter which one a resolver asks. The distinction between master and slave is made purely for reasons of reliability—to ensure that the failure of a single nameserver does not result in the loss of authoritative data for the domain. As a bonus, this redundancy also distributes the network load between several hosts so that no one nameserver is over whelmed with requests for authoritative information.

Despite these precautions, the load on DNS servers would be crushing without the extensive use of local caches. As mentioned before, nameservers are allowed to cache the results of queries and intermediate referrals for some time so that they can serve repeated requests for data without referring to the source each time. If they did not do this, root nameservers (and the nameservers for other popular zones) would be contacted by clients all over the world for every name lookup, wasting enormous resources.

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