How DNS Works

How DNS Works


You are about to learn how DNS works. Our discussion will focus on the hostname-to-IP address translation service.

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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. The application will invoke the client side of DNS, specifying the hostname that needs to be translated. ( On many UNIX-based machines, gethostbyname() is the 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 the network. All DNS query & reply messages are sent within UDP datagrams to port 53. After a delay, ranging from milliseconds to seconds, DNS in the user’s host receives a DNS reply message that provides the desired mapping. This mapping is then passed to the invoking application. Thus, from the perspective of the invoking application in the user’s host, DNS is a black box providing a simple, straightforward translation service. But in fact, the black box that implements the service is complex, consisting of a large number of DNS servers distributed around the globe , we well as an application-layer protocol that specifies how the DNS servers and querying hosts communicate.

A simple design for DNS would have one DNS server that contains all the mappings. In this centralized design, clients simply direct all queries to the single DNS server, and the DNS server responds directly to the query clients. Although the simplicity of this design is attractive, it is inappropriate for today’s internet, with its vast (and growing ) number of hosts. The problem with a centralized design include:

  1. A single point of failure : If the DNS server crashes, so does the entire Internet!
  2. 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 of millions of hosts).
  3. Distant centralized database : A single DNS server cannot be “close to” all the querying clients. If we put the single DNS server in New York City, then all queries from Australia must travel to the other side of the globe, perhaps over slow and congested links. This can lead to significant delays.
  4. Maintenance : The single DNS server would have to keep records for all internet hosts. Not only would this centralized database be huge, but it would have to be updated frequently to account for every new host.

In summary, a centralized database in a single DNS server simply doesn’t scale. Consequently, the DNS is distributed by design. In fact, the DNS is a wonderful example of how a distributed database can be implemented in the internet.