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Overview of How DNS (Domain Name System) Works

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 and 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, as 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 querying 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 problems with a centralized design include:

• A single point of failure. If the DNS server crashes, so does the entire Internet!
• 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).
• 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.
• 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.

A Distributed, Hierarchical Database In order to deal with the issue of scale, the DNS uses a large number of servers, organized in a hierarchical fashion and distributed around the world. No single DNS server has all of the mappings for all of the hosts in the Internet. Instead, the mappings are distributed across the DNS servers. To a first approximation, there are three classes of DNS servers-root DNS servers, top-level domain (TLD) DNS servers, and authoritative DNS servers-organized in a hierarchy as shown in Figure 2.17. To understand how these three classes of servers interact, suppose a DNS client wants to determine the IP address for the hostname www.amazon.com. To a first approximation, the following events will take place. The client first contacts one of the root servers, which returns IP addresses for TLD servers for the top-level domain com. The client then contacts one of these TLD servers, which returns the IP address of an authoritative server for amazon.com. Finally, the client contacts one of the authoritative servers for amazon.com, which returns the IP address for the host name www.amazon.com. We'll soon examine this DNS lookup process in more detail. But let's first take a closer look at these three classes of DNS servers:

• Root DNS servers. There are over 400 root name servers scattered all over the world. Figure 2.18 shows the countries that have root names servers, with countries having more than ten darkly shaded. These root name servers are managed by 13 different organizations. The full list of root name servers, along with the organizations that manage them and their IP addresses can be found at [Root Servers 2016]. Root name servers provide the IP addresses of the TLD servers.

• Top-level domain (TLD) servers. For each of the top-level domains - top-level domains such as com, org, net, edu, and gov, and all of the country top-level domains such as uk, fr, ca, and jp - there is TLD server (or server cluster). The company Verisign Global Registry Services maintains the TLD servers for the com top-level domain, and the company Educause maintains the TLD servers for the edu top-level domain. The network infrastructure supporting a TLD can be large and complex; see [Osterweil 2012] for a nice overview of the Verisign network. See [TLD list 2016] for a list of all top-level domains. TLD servers provide the IP addresses for authoritative DNS servers.

• Authoritative DNS servers. Every organization with publicly accessible hosts (such as Web servers and mail servers) on the Internet must provide publicly accessible DNS records that map the names of those hosts to IP addresses. An organization's authoritative DNS server houses these DNS records. An organization can choose to implement its own authoritative DNS server to hold these records; alternatively, the organization can pay to have these records stored in an authoritative DNS server of some service provider. Most universities and large companies implement and maintain their own primary and secondary (backup) authoritative DNS server.

The root, TLD, and authoritative DNS servers all belong to the hierarchy of DNS servers, as shown in Figure 2.17. There is another important type of DNS server called the local DNS server. A local DNS server does not strictly belong to the hierarchy of servers but is nevertheless central to the DNS architecture. Each ISP-such as a residential ISP or an institutional ISP-has a local DNS server (also called a default name server). When a host connects to an ISP, the ISP provides the host with the IP addresses of one or more of its local DNS servers (typically through DHCP, which is discussed in Chapter 4). You can easily determine the IP address of your local DNS server by accessing network status windows in Windows or UNIX. A host's local DNS server is typically "close to" the host. For an institutional ISP, the local DNS server may be on the same LAN as the host; for a residential ISP, it is typically separated from the host by no more than a few routers. When a host makes a DNS query, the query is sent to the local DNS server, which acts a proxy, forwarding the query into the DNS server hierarchy, as we'll discuss in more detail below.

Let's take a look at a simple example. Suppose the host cse.nyu.edu desires the IP address of gaia.cs.umass.edu. Also suppose that NYU's ocal DNS server for cse.nyu.edu is called dns.nyu.edu and that an authoritative DNS server for gaia.cs.umass.edu is called dns.umass.edu. As shown in Figure 2.19, the host cse.nyu.edu first sends a DNS query message to its local DNS server, dns.nyu.edu. The query message contains the hostname to be translated, namely, gaia.cs.umass.edu. The local DNS server forwards the query message to a root DNS server. The root DNS server takes note of the edu suffix and returns to the local DNS server a list of IP addresses for TLD servers responsible for edu. The local DNS server then resends the query message to one of these TLD servers. The TLD server takes note of the umass.edu suffix and responds with the IP address of the authoritative DNS server for the University of Massachusetts, namely, dns.umass.edu. Finally, the local DNS server resends the query message directly to dns.umass.edu, which responds with the IP address of gaia .cs.umass.edu. Note that in this example, in order to obtain the mapping for one hostname, eight DNS messages were sent: four query messages and four reply messages! We'll soon see how DNS caching reduces this query traffic.

Our previous example assumed that the TLD server knows the authoritative DNS server for the hostname. In general this not always true. Instead, the TLD server may know only of an intermediate DNS server, which in turn knows the authoritative DNS server for the hostname. For example, suppose again that the University of Massachusetts has a DNS server for the university, called dns.umass.edu. Also suppose that each of the departments at the University of Massachusetts has its own DNS server, and that each departmental DNS server is authoritative for all hosts in the department. In this case, when the intermediate DNS server, dns.umass.edu, receives a query for a host with a hostname ending with cs.umass.edu, it returns to dns.nyu.edu the IP address of dns.cs.umass.edu, which is authoritative for all hostnames ending with cs.umass.edu. The local DNS server dns.nyu .edu then sends the query to the authoritative DNS server, which returns the desired mapping to the local DNS server, which in turn returns the mapping to the requesting host. In this case, a total of 10 DNS messages are sent!

The example shown in Figure 2.19 makes use of both recursive queries and iterative queries. The query sent from cse.nyu.edu to dns.nyu.edu is a recursive query, since the query asks dns.nyu.edu to obtain the mapping on its behalf. But the subsequent three queries are iterative since all of the replies are directly returned to dns.nyu.edu. In theory, any DNS query can be iterative or recursive. For example, Figure 2.20 shows a DNS query chain for which all of the queries are recursive. In practice, the queries typically follow the pattern in Figure 2.19: The query from the requesting host to the local DNS server is recursive, and the remaining queries are iterative.

About the Authors

Jim Kurose is a Distinguished University Professor of Computer Science at the University of Massachusetts, Amherst. He is currently on leave from the University of Massachusetts, serving as an Assistant Director at the US National Science Foundation, where he leads the Directorate of Computer and Information Science and Engineering.

Dr. Kurose has received a number of recognitions for his educational activities including Outstanding Teacher Awards from the National Technological University (eight times), the University of Massachusetts, and the Northeast Association of Graduate Schools. He received the IEEE Taylor Booth Education Medal and was recognized for his leadership of Massachusetts' Commonwealth Information Technology Initiative. He has won several conference best paper awards and received the IEEE Infocom Achievement Award and the ACM Sigcomm Test of Time Award.

Dr. Kurose is a former Editor-in-Chief of IEEE Transactions on Communications and of IEEE/ACM Transactions on Networking. He has served as Technical Program co-Chair for IEEE Infocom, ACM SIGCOMM, ACM Internet Measurement Conference, and ACM SIGMETRICS. He is a Fellow of the IEEE and the ACM. His research interests include network protocols and architecture, network measurement, multimedia communication, and modeling and performance evaluation. He holds a PhD in Computer Science from Columbia University.

Keith Ross is the Dean of Engineering and Computer Science at NYU Shanghai and the Leonard J. Shustek Chair Professor in the Computer Science and Engineering Department at NYU. Previously he was at University of Pennsylvania (13 years), Eurecom Institute (5 years) and Polytechnic University (10 years). He received a B.S.E.E from Tufts University, a M.S.E.E. from Columbia University, and a Ph.D. in Computer and Control Engineering from The University of Michigan. Keith Ross is also the co-founder and original CEO of Wimba, which develops online multimedia applications for e-learning and was acquired by Blackboard in 2010.

Professor Ross's research interests are in privacy, social networks, peer-to-peer networking, Internet measurement, content distribution networks, and stochastic modeling. He is an ACM Fellow, an IEEE Fellow, recipient of the Infocom 2009 Best Paper Award, and recipient of 2011 and 2008 Best Paper Awards for Multimedia Communications (awarded by IEEE Communications Society). He has served on numerous journal editorial boards and conference program committees, including IEEE/ACM Transactions on Networking, ACM SIGCOMM, ACM CoNext, and ACM Internet Measurement Conference. He also has served as an advisor to the Federal Trade Commission on P2P file sharing.

Unique among computernetworking texts, the 8th Edition, Global Edition, of the popular Computer Networking: A Top Down Approach build son the authors' long tradition of teaching this complex subject through alayered approach in a "top-down manner." The text works its way from the application layer down toward the physical layer, motivating students by exposing them to important concepts early in their study of networking. Focusing on the Internet and the fundamentally important issues of networking, this text provides an excellent foundation for students in computer science and electrical engineering, without requiring extensive knowledge of programming ormathematics. The 8th Edition, Global Edition, has been updated to reflect the most important and exciting recent advances in networking, including the importance of software-defined networking (SDN) and the rapidadoption of 4G/5G networks and the mobile applications they enable.

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