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SENG2220 Web Development II. Mohammed A. Saleh http://ifm.ac.tz/staff/msaleh/teach/ 29 th October 2009. Domain Name Service (DNS). TCP/IP facility that lets you use names rather than numbers to refer to host computers.

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SENG2220 Web Development II

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Seng2220 web development ii

SENG2220Web Development II

Mohammed A. Saleh

http://ifm.ac.tz/staff/msaleh/teach/

29th October 2009


Domain name service dns

Domain Name Service (DNS)

  • TCP/IP facility that lets you use names rather than numbers to refer to host computers.

  • Without DNS, you’d buy books from 207.171.182.16 instead of from www.amazon.comand search the Web at 216.239.51.100 instead of at www.google.com.

  • If you want to review the complete official specifications for DNS, look up RFC 1034 and 1035 at www.ietf.org/rfc/rfc1034.txt and www.ietf/rfc/rfc1035.txt.


Understanding domain names

Understanding Domain Names

  • Provides a standardized system for providing names to identify TCP/IP hosts and a way to look up the IP address of a host given the host’s DNS name

  • For example, if you use DNS to look up the name www.ebay.com, you get the IP address of Ebay’s Web host: 66.135.192.87.

  • Thus, DNS allows you to access Ebay’s Web site using the DNS name www.ebay.com rather than the site’s IP address


Concepts of dns

Concepts of DNS

  • To provide a unique DNS name for every host computer on the Internet, DNS uses a time-tested technique: divide and conquer

  • Uses a hierarchical naming system

  • It organizes names into domains

  • Each domain includes all the names that appear directly beneath it in the DNS hierarchy

  • Figure 1.0: DNS Domain Tree


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  • At the very top of the tree is the root domain

  • Beneath the root domain are four top-level domains, named edu, com, org, and gov (exist more top level domains)

  • LoweWriter is a personal domain, you have to combine it with the name of its parent domain in order to identify it, hence LoweWriter.com

  • The parts of the domain name are separated from each other with periods

  • Beneath the LoweWriter node are four host nodes

  • The host name with the domain name are combined to get the complete DNS name, server1.LoweWriter.com


Nitty gritty about dns names

Nitty-gritty about DNS Names

  • DNS names are not case-sensitive

  • The name of each DNS node can be up to 63 characters long (not including the dot) and can include letters, numbers, and hyphens

  • A subdomain is a domain that’s beneath an existing domain

  • DNS is a hierarchical naming system that’s similar to the hierarchical folder system used by Windows.

  • one crucial difference exists between DNS and the Windows naming convention

  • DNS names start at the bottom of the tree and work your way up to the root ; doug.LoweWriter.com


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  • Windows paths start at the root and work their way down Windows\System32\dns, dns is the lowest node.

  • The DNS tree can be up to 127 levels deep. Most DNS names have just three levels (not counting the root)


Fully qualified domain names

Fully Qualified Domain Names

  • If a domain name ends with a trailing dot, that trailing dot represents the root domain, and the domain name is said to be a fully qualified domain name (FQDN)

  • FQDN is also called an absolute name

  • DNS names that don’t end with a trailing dot are called relative names

  • relative and FQDN are interchangeable, because the software that interprets them always interprets relative names in the context of the root domain

  • you can type www.wiley.com — without the trailing dot — rather than www.wiley.com. to go to Wiley’s home page in a Web browser


Top level domain

Top Level Domain

  • Appears immediately beneath the root domain

  • come in two categories: generic domains and geographic domains

    Generic Domains

  • Popular top-level domains that you see most often on the Internet

  • Table 2.0: Summarizes the original seven generic top-level domains


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(source: www.isc.org)

  • Due to an almost unmanageable size in the late 1990s, the Internet authorities approved seven new top-level domains in an effort to take some of the heat off of the com domain


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  • They are not very popular.


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Geographic Domains

  • They correspond to international country designations

  • About 150 geographic top-level domains exist

  • Some them are tz (Tanzania), us (United states), za (South Africa), jp (Japan) and many more

    The Hosts Files

  • Long ago, network administrators could keep track of it all in a simple text file, called hosts files

  • Simply listed the name and IP address of every host on the network

  • The key was to keep the hosts files up to date whenever a new host is added


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  • With the growth of the Internet, it became daunting handling the host files and a better solution was needed.

  • DNS was invented to solve this problem

  • Why is it important to understand the host files

  • Hosts files are still used for small networks, can coexist with DNS

  • It is the precursor to DNS

  • The exact location of the hosts file depends on the client operating system


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Dns servers and zones

DNS Servers and Zones

  • A DNS server is a computer that runs DNS server software, helps to maintain the DNS database, and responds to DNS name resolution requests.

  • Most popular DNS servers are Bind (UNIX-based) and the Windows DNS service (Windows)

  • DNS database — that is, the list of all the domains, sub-domains, and host mappings — is a massively distributed database

  • No single DNS server contains the entire DNS database

  • The database is delegated to different servers throughout the Internet

  • What happens if someone requests the IP address of coyote.acme.com from LoweWrite.com DNS server?


Zones

Zones

  • Simplify the management of the DNS database

  • The entire DNS namespace is divided into zones

  • Responsibility for each zone is delegated to a particular DNS server

  • Zones correspond directly to domains

  • Example, the LoweWriter.com domain will have LoweWriter.com zone responsible for the whole domain

  • The sub-domains that make up a domain can be parceled out to separate zones


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  • Figure 3.0 show how zones can be separated

  • A domain named LoweWriter.com has been divided into two zones

  • One zone, us.LoweWriter.com

  • The other zone, LoweWriter.com, is responsible for the entire LoweWriter.com domain except the us.LoweWriter.com sub-domain

  • Why would you do that? The main reason is to delegate authority for the zone to separate servers


Types of zones

Types of zones

  • Primary zone

  • Master copy of a zone

  • Data is stored in the local database of the DNS server

  • Only one DNS server can host a particular primary zone

  • Secondary zone

  • Read-only copy of a zone

  • It obtains its copy of the zone from the zone’s primary server by using a process called zone transfer

  • must periodically check primary servers for updates


Servers

Servers

  • Primary servers

  • The DNS server hosts a primary zone.

  • Every zone must have one primary server

  • Secondary servers

  • The DNS server obtains the data for a secondary zone from a primary server

    TIP: A secondary server should be on a different subnet

    than the zone’s primary server.


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  • Root servers

  • The core of DNS

  • Authoritative for the entire Internet

  • The main function of the root servers is to provide the address of the DNS servers that are responsible for each of the top-level domains

  • A total of13 root servers are located throughout the world

  • DNS servers learn how to reach the root servers


Caching

Caching

  • An intelligent way of handling the same user requests

  • A user visits www.wiley.com today, he’ll probably do it again tomorrow. As a result, name servers keep a cache of query results

  • Cached data can quickly become obsolete

  • DNS data is given a relatively short expiration time

  • The expiration value for DNS data is called the TTL, which stands for time to live


Dns queries

DNS Queries

  • When a DNS client needs to resolve a DNS name to an IP address, it uses a library routine called a resolver to handle the query.

  • DNS client can make two basic types of queries: recursive and iterative

    ✦Recursive queries: the server must reply with either the IP address of the requested host name or an error message indicating that the host name doesn’t exist.

    ✦Iterative queries: it returns the IP address of the requested host name if it knows the address. If it doesn’t know the address, it returns a referral; the address of a DNS server that should know


Questions

Questions


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