SENG2220Web Development II

Mohammed A. Salehhttp://ifm.ac.tz/staff/msaleh/teach/

29th October 2009

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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.com and 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.

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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

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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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Cont …

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Cont … 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

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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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Cont …- 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)

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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

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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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Cont …

(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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Cont …

They are not very popular.

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Cont …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 added13

Cont … 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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Cont …

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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? 16

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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Cont …

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Cont … 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

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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

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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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Cont … 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

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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

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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

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Questions