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Dns rotation is also used for e mail so that multiple

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DNS rotation is also used for e-mail so that multiple mail servers can have thesame alias name. Also, content distribution companies such as Akamai haveused DNS in more sophisticated ways [Dilley 2002] to provide Web contentdistribution (see Chapter 7).The DNS is specified in RFC 1034 and RFC 1035, and updated in severaladditional RFCs. It is a complex system, and we only touch upon key aspects ofits operation here. The interested reader is referred to these RFCs and the bookby Albitz and Liu [Albitz 1993]; see also the retrospective paper [Mockapetris1988], which provides a nice description of the what and why of DNS, and[Mockapetris 2005].2.5.2 Overview of How DNS WorksWe now present a high-level overview of how DNS works. Our discussion willfocus on the hostname-to-IP-address translation service.Suppose that some application (such as a Web browser or a mail reader)running in a user’s host needs to translate a hostname to an IP address. Theapplication will invoke the client side of DNS, specifying the hostname thatneeds to be translated. (On many UNIX-based machines, gethostbyname() isthe function call that an application calls in order to perform the translation.)DNS in the user’s host then takes over, sending a query message into thenetwork. All DNS query and reply messages are sent within UDP datagrams toport 53. After a delay, ranging from milliseconds to seconds, DNS in the user’shost receives a DNS reply message that provides the desired mapping. This
mapping is then passed to the invoking application. Thus, from the perspectiveof the invoking application in the user’s host, DNS is a black box providing asimple, straightforward translation service. But in fact, the black box thatimplements the service is complex, consisting of a large number of DNS serversdistributed around the globe, as well as an application-layer protocol thatspecifies how the DNS servers and querying hosts communicate.A simple design for DNS would have one DNS server that contains all themappings. In this centralized design, clients simply direct all queries to thesingle DNS server, and the DNS server responds directly to the queryingclients. Although the simplicity of this design is attractive, it is inappropriate fortoday’s Internet, with its vast (and growing) number of hosts. The problemswith a centralized design include:• A single point of failure.If the DNS server crashes, so does the entireInternet!• Traffic volume.A single DNS server would have to handle all DNS queries(for all the HTTP requests and e-mail messages generated from hundreds ofmillions of hosts).Distant centralized database.A single DNS server cannot be “close to” allthe querying clients. If we put the single DNS server in New York City, then allqueries from Australia must travel to the other side of the globe, perhaps overslow and congested links. This can lead to significant delays.

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