Lesson 11

Computers are useless unless they perform some service. Red Hat Linux machines come ready to perform many services, making them powerful choices for Internet application servers. IP addresses and ports form the foundation for these Internet services. To help users, the Domain Name System assigns names to IP address, and Linux itself assigns names to ports.
Finally, servers may operate as either standalone or transient; deciding which method a particular server should use is a frequent administrator concern.

Having completed this module, you should now be able to:

1. Identify the properties of Internet Protocol
2. Describe an IP address
3. Relate TCP and UDP protocols to IP
4. Define port numbers
5. Explain the relationship between port names and numbers
6. Configure port names
7. Describe the function of the Domain Name System
8. State the differences between a standalone and a transient network server
9. List the advantages and disadvantages of standalone and transient network servers

The following terms were introduced in this module:

1. Checksum: A value computed from a quantity of data and transmitted alongside the data. The receiver can verify the data's integrity by computing the checksum again and comparing the newly computed value to the transmitted one.
2. Domain Name System (DNS): The Internet system for translating IP addresses into names and vice-versa.
3. Fully-Qualified Domain Names (FQDN): A domain name that has all required DNS information: the host, network, and top-level domain.
4. Internet Assigned Numbers Authority (IANA): The IANA is responsible for assigned standard port numbers to well-known Internet services, such as HTTP, FTP, and DNS.
5. Internet Protocol (IP): The fundamental unit for sending data across the Internet.
6. Protocol: A grammar for communicating an idea between two end points.
7. Standalone server: A server that opens, maintains, and closes its own network connections.
8. Superserver: A server process responsible for listening on a group of ports and spawning other servers when connections arrive for them.
9. Transient servers: A server that performs its service for a single client, starting fresh every time a client request arrives. Transient servers do not listen for client requests, but rather are spawned by another program.
10. Transmission Control Protocol (TCP: A connected, reliable method for sending data across a network.
11. User Datagram Protocol (UDP: A connectionless, unreliable method for delivering packets across the Internet.
12. Well-known services (WKS): WKSs are services that are prevalent across the Internet. Examples include HTTP for World Wide Web traffic, FTP for File Transfer Protocol, and DNS for the Domain Name System.

The next module describes how Linux provides network services.

The Domain Name System (DNS) is essentially a distributed database that translates hostnames into IP addresses (and IP addresses back to hostnames). That database also contains information related to each domain, such as how the domain is organized into zones, where to route mail for that domain, and whom to contact with questions associated with the domain. By setting up a DNS server, you become part of a hierarchy of DNS servers that make up the Internet.
At the top of this hierarchy is the root server, represented by a dot (.). Below the root server are the Top Level Domains, or TLDs (such as .com, .org, and so on). Domains that individual organizations own and maintain lie below the TLDs, branching in a way that looks like an upside-down tree structure. That is where you come in.
As someone who is setting up a DNS server, you are responsible for managing the hostnames and IP addresses for the computers in the domain (or domains) for which you are responsible. Keeping your DNS information correct means that people can access the services that you want to share, and the Internet as a whole works that much better as a result.
Besides using your DNS server to help people from the Internet find the public servers in your domain, you can also use DNS to provide name and IP address mapping for computers on your private network.

Warning: Setting up a DNS server can be a complex and potentially dangerous undertaking. A compromised DNS server can cause requests for host addresses to be directed to a hacker's server. The sample DNS server in this section is one created as an example of a DNS server for a home or small office environment.
For information on the many different ways to set up a DNS server, go to the BIND 9 Administrator Reference manual at /usr/share/doc/bind-9*/arm/ Bv9ARM.html.

Before moving to the next module, click the Quiz link below to check your understanding of the concepts presented in this module.
Domain Name System - Quiz