DNS is not an objective that is covered in depth on the System Administration Certification exam Part II. You need to know the definition of DNS, but a working knowledge of DNS is not required until you take the Sun Certified Network Administrator examination.
The DNS is the name service provided by the Internet for Transmission Control Protocol/Internet Protocol (TCP/IP) networks. It was developed so that workstations on the network could be identified with common names instead of Internet addresses. DNS is a program that converts domain names to their IP addresses. Without it, users would have to remember numbers instead of words to get around the Internet. The process of finding the IP address of a computer by using its host name as an index is referred to as name-to-address resolution or "mapping."
The collection of networked systems that use DNS are referred to as the DNS namespace. The DNS namespace can be divided into a hierarchy of domains. A DNS domain is simply a group of systems. Each domain is supported by two or more name servers: the primary, secondary, or cache-only server. Each domain must have one primary server and should have at least one secondary server to provide backup.
Each server implements DNS by running a daemon called in.named. On the client side, DNS is implemented through the "resolver." The resolver's function is to resolve users' queries. The resolver is neither a daemon nor a single program; rather, it is a set of dynamic library routines used by applications that need to know system names. After the resolver is configured, a system can request DNS service from a name server. If a system's /etc/nsswitch.conf file specifies hosts: dns, the resolver libraries are automatically used. If the nsswitch.conf file specifies some other name service before DNS, such as NIS, then that name service is consulted first for host information, and then only if that name service does not find the host in question are the resolver libraries used.
For example, if the hosts line in the nsswitch.conf file specifies hosts: nis dns, the NIS name service is first searched for host information. If the information is not found in NIS, the DNS resolver is used. Because name services, such as NIS and NIS+, contain only information about hosts in their own network, the effect of a hosts:nis dns line in a switch file is to specify the use of NIS for local host information and DNS for information on remote hosts out on the Internet. If the resolver queries a name server, the server returns either the requested information or a referral to another server.
Name-to-address mapping occurs if a program running on your local system needs to contact a remote computer. The program most likely knows the host name of the remote computer but might not know how to locate it, particularly if the remote system is in another company, miles from your site. To obtain the remote system's address, the program requests assistance from the DNS software running on your local system, which is considered a DNS client.
Your system sends a request to a DNS name server, which maintains the distributed DNS database. The files in the DNS database bear little resemblance to the NIS+ Host Table or even to the local /etc/hosts file, although they maintain similar information: the host names, IP addresses, and other information about a particular group of computers. The name server uses the host name your system sent as part of its request to find or "resolve" the IP address of the remote system. It then returns this IP address to your local system if the host name is in its DNS database.
If the host name is not in that name server's DNS database, this indicates that the system is outside of its authority, or, to use DNS terminology, outside the local administrative domain.
Because maintaining a central list of domain name/IP address correspondences would be impractical, the lists of domain names and IP addresses are distributed throughout the Internet in a hierarchy of authority. A DNS server that maps the domain names in your Internet requests or forwards them to other servers in the Internet is probably within close geographic proximity to your Internet access provider.
In this chapter, I covered all the name service topics that appear on the Certified Solaris Administrator Examination for Solaris 2.6, Part II. Of course, better understanding of the topics will come as you use the products described and become experienced over time. Many large networks that use a name service are heterogeneous, meaning they have more than just Solaris systems connected to it. You need to refer to the vendor's documentation for each particular system type to understand how each different operating system implements name services. You see that most are similar in their implementation, with only subtle differences. As you gain experience, and complete the Solaris Administrator Certification exams, your next goal should be to become certified as a Sun Network Administrator for Solaris. Certification in both fields is valuable for any UNIX system administrator.