The protocols we have studied so far operate using IP addresses to identify the hosts that participate in a distributed application. These addresses (especially IPv6 addresses) are cumbersome for humans to use and remember, so the Internet sup-ports the use of host names to identify hosts, both clients and servers. In order to be used by protocols such as TCP and IP, host names are converted into IP addresses using a process known as name resolution. There are different forms of name reso-lution in the Internet, but the most prevalent and important one uses a distributed database system known as the Domain Name System (DNS) [MD88]. DNS runs as an application on the Internet, using IPv4 or IPv6 (or both). For scalability, DNS names are hierarchical, as are the servers that support name resolution.

DNS is a distributed client/server networked database that is used by TCP/IP applications to map between host names and IP addresses (and vice versa), to pro- vide electronic mail routing information, service naming, and other capabilities. We use the term distributed because no single site on the Internet knows all of the information. Each site (university department, campus, company, or department within a company, for example) maintains its own database of information and runs a server program that other systems across the Internet (clients) can query.

The DNS provides the protocol that allows clients and servers to communicate with each other and also a protocol for allowing servers to exchange information. From an application’s point of view, access to the DNS is through an applica- tion library called a resolver. In general, an application must convert a host name to an IPv4 and/or IPv6 address before it can ask TCP to open a connection or send a unicast datagram using UDP. The TCP and IP protocol implementations know nothing about the DNS; they operate only with the addresses. 511 512 Name Resolution and the Domain Name System (DNS) In this chapter we will take a look at how the names in DNS are set up, how resolvers and servers communicate using the Internet protocols (mainly UDP), and some of the other resolution mechanisms that are used in Internet environments. We do not cover all of the administrative details of running a name server or all of the options available with resolvers and servers. Such information is available from various other sources, including Albitz and Liu’s DNS and BIND text [AL06] and in [RFC6168]. We discuss the details of DNS security (DNSSEC) in Chapter 18.