across a wide range of machines, each of which can exert control over a portion of the database. DNS is a standard set of protocols that defines the following:
■ A mechanism for querying and updating address information in the database
■ A mechanism for replicating the information in the database among servers
■ A schema of the database
DNS was originally developed in the early days of the Internet (called ARPAnet at the time) when it was a small network created by the Department of Defense for research purposes. Before DNS, computer names, or hostnames, were manually entered into a HOSTS file located on a centrally administered server. Each site that needed to resolve hostnames outside of its organization had to download this file. As the number of computers on the Internet grew, so did the size of this HOSTS file – and along with it the problems of its management. The need for a new system that would offer features such as scalability, decentralized administration, and support for various data types became more and more obvious. DNS, introduced in 1984, became this new system.
With DNS, the hostnames reside in a database that can be distributed among multiple servers, decreasing the load on any one server and providing the ability to administer this naming system on a per-partition basis. DNS supports hierarchical names and allows for the registration of various data types in addition to the hostname-to-IP-address mapping used in HOSTS files. Database performance is ensured through its distributed nature as well as through caching.
The DNS distributed database establishes an inverted logical tree structure called the domain namespace. Each node, or domain, in that space has a unique name. At the top of the tree is the root. This may not sound quite right, which is why the DNS hierarchical model is described as being an inverted tree, with the root at the top. The root is represented by the null set "". When written, the root node is represented by a single dot (.).
Each node in the DNS can branch out to any number of nodes below it. For example, below the root node are a number of other nodes, commonly referred to as top-level domains (TLDs). These are the familiar .com, net, org, gov, edu, and other such names. Table 2.1 lists some of these TLDs.
TABLE 2.1 Common top-level DNS domains
Each of these nodes then branches out into another set of domains, and they combine to form what we refer to as domain names, such as microsoft.com. A domain name identifies the domain’s position in the logical DNS hierarchy in relation to its parent domain by separating each branch of the tree with a dot. Figure 2.2 shows a few of the top-level domains, where the Microsoft domain fits, and a host called Tigger within the microsoft.com domain. If someone wanted to contact that host, they would use the fully qualified domain name (FQDN), tigger.microsoft.com.
FIGURE 2.2 The DNS hierarchy
An FQDN includes the trailing dot (.) to indicate the root node, but it’s commonly left off in practice.
As previously stated, one of the strengths of DNS is the ability to delegate control over portions of the DNS namespace to multiple organizations. For example, the Internet Corporation for Assigned Names and Numbers (ICANN) assigns the control over TLDs to one or more organizations. In turn, those organizations delegate portions of the DNS namespace to other organizations. For example, when you register a domain name, let’s call it example.com, you control the DNS for the portion of the DNS namespace within example.com. The registrar controlling the .com TLD has delegated control over the example.com node in the DNS tree. No other node can be named example directly below the .com within the DNS database.
Within the portion of the domain namespace that you control (example.com), you could create host and other records (more on these later). You could also further subdivide example.com and delegate control over those divisions to other organizations or departments. These divisions are called subdomains. For example, you might create subdomains named for the cities in which the company has branch offices and then delegate control over those subdomains to the branch offices. The subdomains might be named losangeles.example.com, chicago.example.com, portsmouth.example.com, and so on.
Each domain (or delegated subdomain) is associated with DNS name servers. In other words, for every node in the DNS, one or more servers can give an authoritative answer to queries about that domain. At the root of the domain namespace are the root servers. More on these later.
DNS servers work together to resolve hierarchical names. If a server already has information about a name, it simply fulfills the query for the client. Otherwise, it queries other DNS servers for the appropriate information. The system works well because it distributes the authority over separate parts of the DNS structure to specific servers. A DNS zone is a portion of the DNS namespace over which a specific DNS server has authority (DNS zone types are discussed in detail later in this chapter).
Within a given DNS zone, resource records (RRs) contain the hosts and other database information that make up the data for the zone. For example, an RR might contain the host entry for www.example.com, pointing it to the IP address 192.168.1.10.
Understanding Servers, Clients, and Resolvers
You will need to know a few terms and concepts in order to manage a DNS server. Understanding these terms will make it easier to understand how the Windows Server 2012 R2 DNS server works.
DNS Server Any computer providing domain name services is a DNS name server. No matter where the server resides in the DNS namespace, it’s still a DNS name server. For example, 13 root name servers at the top of the DNS tree are responsible for delegating the TLDs. The root servers provide referrals to name servers for the TLDs, which in turn provide referrals to an authoritative name server for a given domain.
Any DNS server implementation supporting Service Location Resource Records (see RFC 2782) and Dynamic Updates (RFC 2136) is sufficient to provide the name service for any operating system running Windows 2003 software and newer.
DNS Client A DNS client is any machine that issues queries to a DNS server. The client hostname may or may not be registered in a DNS database. Clients issue DNS requests through processes called resolvers. You’ll sometimes see the terms client and resolver used synonymously.
Resolver Resolvers are software processes, sometimes implemented in software libraries that handle the actual process of finding the answers to queries for