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Domain Name System

A Network Programming Lecture by Steven Choy

Overview: Introduction - Name Space - Domain Name Space - DNS in the Internet - Resolution - Implementation of DNS in the Internet - Common DNS Record Types - A DNS Lookup Example

Introduction to DNS (Domain Name System)

• IP addresses are used to enable computers and routers to identify each other efficiently. However, the human mind is not designed to remember IP addresses. People prefer to work with names rather than numbers. The Domain Name System (DNS) is such a system that supports the best of both worlds of humans and machines.
• Roughly and simply speaking, as you may also be told by some people, the DNS is nothing more than a system that stores pairs of IP addresses and names, so that when you request a website by name it can send you directly to the correct address. However, that statement cannot give you the whole picture of the DNS. You need to learn more in order to appreciate the concept and the implementation of the DNS.
• DNS is a hierarchical system, a globally distributed system, that is highly scalable and reliable.

• The DNS contains three important components. They are:

• The DNS is a hierarchical system.
• DNS organizes all registered names in a tree structure.
• The hierarchical naming scheme used by the DNS is known as the domain name space.

Name Space

• Flat Name Space

• Hierarchical Name Space

Domain Name Space

• Label - Each node in the tree has a label, which is a string with a maximum of 63 characters. The root label is a null string.
• Domain - a domain is a subtree of the domain name space.
• Domain Name - Each node in the tree has a domain name.
• Fully Qualified Domain name (FQDN) - a sequence of labels separated by dots and ends in a null label.
• Partially Qualified Domain Name (PQDN) - If a label is not termined by a null string, it is called a PQDN.

Distribution of Name Space

• Zone - When a name server is responsible for, or has authority over, is called a zone.
• Root Server - A root server is a server whose zone consists of the whole tree.
• Primary Server - is a server that stores a file about the zone for which it is an authority
• Secondary Server - it loads all information from the primary server

Class Discussions

• What is the main disadvantage of a flat name space?
• What is the main advantage of a hierarchical name space over a flat name space?
• Why it is not a good idea to have a single powerful computer store all the information contained in the domain name space?
• What are FQDN and PQDN? Give an example for each one.
• Describe the current way that the Internet uses to store all the information contained in the domain name space.
• What are a root name server, a primary name server and a secondary name server? Can you differentiate them clearly?

Summary : DNS Concepts and Design

• In the design of DNS, the name space needs to be made hierarchical rather than a flat structure in order to make naming of machines scalable. The idea of hierarchical name space is to divide the naming into several parts. The naming can be based on the location of the machine, and then on the nature of the organizations, and then on the unit name within the selected area. That idea is quite similar to what we humans have done in addressing a real world location.
• The domain names can be mapped to a tree structure like the figure shown below.

• There is no restriction to the number of child nodes at each parent node. However, DNS requires that child nodes of a parent node have different labels. In this way, the uniqueness of the domain names is guaranteed. For example, we cannot have two child nodes named the same, say ouhk, under the parent node .edu.hk. We can, however, have one child node named ouhk under the parent node .edu.hk and another child node also named ouhk under the parent node .com.hk.
• In the design of domain name space, a domain is a namespace. If you think the concept of namespaces is too abstract to you, think about the following examples. Everything below .hk is in the hk domain. Everything below .com.hk is in the com.hk domain and in the hk domain.
• The information contained in the domain name space is stored in a hierarchy of servers based on the structure of the name space. Those servers are called DNS name servers. Each name server maintains just one piece of the overall hierarchy.
• While domains are namespaces, zones, which are subdivisions of a domain, are administrative spaces in which zone administrators are responsible for just a portion of a domain. Therefore, it is possible that a name server does not store all the information about a domain. Instead, it stores the information about a zone and delegates part of its authority to other servers.

DNS in the Internet

• Generic Domains - define registered hosts according to their generic behavior.

• Country Domains - uses two-character country abbreviations at the first levels. second-level labels can be organizational, or they can be more specific, national designations.
• Inverse Domains - is used to map an address to a name.

Resolution

• Recursive Resolution

• Iterative Resolution

• Caching

Summary: Implementation of DNS in the Internet

• The DNS is implemented as a distributed system. The DNS requires a coordinated effort among many computers called DNS servers to store and maintain a huge list of registered domain names and their corresponding IP addresses. Each DNS server maintains just one piece of the overall DNS hierarchy (one level of the hierarchy and then only a zone with that level).
• DNS is designed as a client-server application. DNS servers respond to requests from DNS clients. DNS clients are called resolvers as they access the closest DNS server with a request for name-address resolution. DNS servers eventually return the requested resolution (either address-to-name or name-to-address) to the resolver.
• Please note that the process by which a DNS client maps a hostname to an IP address is called name-address resolution or resolution for short.
• We need some kind of protocol for communication between DNS clients and servers. To be brief, there are only two types of DNS messages: query and response. The query message, which consists of a header and a question section, is sent from a client to a server. The response message is then sent from the server to the client, and consists of a header, a question section, an answer section, and two other sections containing additional information about the resolution.

Class Discussions

• The domain name space in the Internet is divided into three different sections. What are they?
• Give any five first-level labels in the generic domains.
• What does a resolver do?
• What is the difference between recursive resolution and iterative resolution?

Common DNS Record Types

• Address Record (A) - It returns a 32-bit IPv4 address, most commonly used to map hostnames to an IP address of the host.
• Canonical Name Record (CNAME) - Alias of one name to another: the DNS lookup will continue by retrying the lookup with the new name.
• Mail Exchange Record (MX) - It maps a domain name to a list of message transfer agents for that domain.
• Name Server Record (NS) - It delegates a DNS zone to use the given authoritative name servers.
• Start of Authority Record (SOA) - It specifies authoritative information about a DNS zone, including the primary name server, the email of the domain administrator, the domain serial number, and several timers relating to refreshing the zone.
• More information about DNS record types: http://en.wikipedia.org/wiki/List_of_DNS_record_types

A DNS Lookup Example

• The following details some important information that you can get by making use the DNS Lookup service.
• The figure below shows you one set of results. Please note that the results returned may be different for each DNS query.

References and Resources

• How DNS Works
• Resolution (DNS and BIND, 4th Edition)
• IANA (Internet Assigned Numbers Authority), URL: http://www.iana.org/

• InternetFrog.com, URL: http://www.internetfrog.com/

Thanks for Reading

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If you find any problem in this lecture note, please feel free to tell Steven via steven@findaway.hk.