Communication and Networks Assignment
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0_Lesson6DomainNameSystem.pptxCommunications and Networks
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Diploma in Information Technology
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Lesson 6: Domain Name System
1
Lesson 6 Learning Outcomes
Explain what is DNS
Describe the purpose of DNS
Explain how DNS works
Describe how DNS cache
Describe the different types of DNS entries
Identify how abbreviations is handled in DNS
2
Lesson 6 Outline
DNS Basics
Implementing DNS Servers
DNS Mapping Process
3
Human-Readable Names
Domain Name System (DNS): provides a service that maps human-readable names to computer addresses
Browsers, mail software, most other Internet applications use the DNS
DNS is an example of client-server interaction!
4
Mapping of Domain Names
Traditionally, mapping of names was done via centrally managed text file
helloworld.com => 123.151.121.131
Inefficient: Text file is distributed to all hosts
DNS allows applications to use domain names
Domain names is then translated into network layer address (IP address)
Hierarchical and fully distributed
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Resolving Hostname
Mapping is not performed by a single server
Fully distributed: Naming information is distributed to multiple servers on the Internet
Whenever an application needs to translate a name, it becomes a client of the naming system
client sends request message to name server
server finds corresponding address and sends a reply message
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Temporary Client
Mapping may not be always available
If a server cannot answer a request,
Server temporarily becomes the client of another name server
Until a server is found that can answer the request
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DNS Syntax
Each name consists of a series of alpha-numeric segments separated by periods
Domain names are hierarchical, most significant part of the name is on the right
Top layer of the hierarchy is the last field
.com, .org, .edu, .net
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Domain Name Hierarchy Example
Example: mordred.cs.purdue.edu, anakin.cisco.com
Left-most segment “mordred” and “anakin” is the name of individual computer
Other segments identifies the group that owns it
“purdue” gives the name of a university
“cisco” gives the name of a company
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Top-level Domain
DNS specifies values for the most significant segment called a top-level domain (TLD)
Controlled by Internet Corporation for Assigned Names and Numbers (ICANN)
ICANN designates one or more domain registrars to administer a given top-level domain and approve specific names
But ICANN is responsible for managing 13 top DNS root server
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TLDs Servers
For each top-level domains are at least 2 DNS servers
Below this level are other delegated authorities to managed them
Some TLDs are generic: they are generally available
Other TLDs are restricted to specific groups or government agencies
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Domain Registration
An organisation applies for a name under one of the existing top-level domains
most US corporations choose to register under the “.com” domain
DNS allows organisations to use a geographic registration
Corporation For National Research Initiatives
cnri.reston.va.us
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List of TLDs
Source: Douglas, C (2016) Computer Networks and Internets
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Mnemonic Names
Domain names are assigned to reflect the service provided
FTP server: ftp.foobar.com
Web server: www.foobar.com
Such names are mnemonic, but not compulsory
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Using WWW
Use of www for a web server is merely a convention
Any computer can run a web server, even if it does not contain www
A computer with domain name beginning with www is not required to run a web server
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DNS Example
doc.gold.ac.uk
.uk managed by UK registration authority (Nominet)
.ac.uk and gov.uk managed by UK Education and Research Networking Assocation (UKERNA)
.gold.ac.uk managed by Goldsmiths (a school in a university)
doc.gold.ac.uk managed by Department of Computing in Goldsmiths
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DNS Explained
Source: https://www.youtube.com/watch?v=72snZctFFtA
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DNS Process Summary
If local server can resolve name, returns the address
If unable, make request to one of the root servers
If root server can resolve, returns the address
Otherwise, return address of another server that can help
Repeats until name is resolved OR firmly established that name cannot be resolved
Recursive or iterative
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Practice 6.1
What happens to a request when a name server receives it but does not have an answer to it?
In the URL, “cnri.reston.va.us”, which segment is the TLD? What does the TLD represent?
What does it mean when domain names are mnemonic?
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Lesson 6 Outline
DNS Basics
Implementing DNS Servers
DNS Mapping Process
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Implementing the DNS
Each organization is free to choose the details of its servers
A small organization that only has a few computers can contract with an ISP to run a DNS server
Computer
Computer
Computer
Hub
DNS Computer
Computer
Computer
Computer
Hub
ISP DNS Computer
ISP Hub
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DNS Choices
An organization that runs its own DNS
Can choose to place all names in a single physical server
Can choose to divide its names among multiple servers
candy.foobar.com
soap.foobar.com
foobar.com
candy
soap
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DNS Server Hierarchy Example
A hypothetical Foobar Corporation could choose to structure servers if the corporation had a candy division and a soap division
Source: Douglas, C (2016) Computer Networks and Internets
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DNS Autonomy
DNS is designed to allow each organisation to assign or modify domain names without informing a central authority
Autonomy: each organization is permitted to operate DNS servers for its part of the hierarchy
Purdue University operates a server for names ending with purdue.edu
IBM Corporation operates a server for names ending with ibm.com
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Replicating DNS Servers
Each DNS server can link to other domain name servers up and down the hierarchy
a server can be replicated; multiple physical copies of the server exist
Replication is useful for heavily used servers (like root servers) that provide information about TLDs
Administrators must guarantee all copies are coordinated to provide identical information
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Name Resolution
Translation of a domain name into an address is called name resolution
The name is said to be resolved to an address
Software that perform this is known as a name resolver or resolver
In socket API, the resolver is invoked by calling function gethostbyname
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DNS Request and Reply
Resolver becomes a client by contacting a DNS server
DNS server returns an answer to the caller
Each resolver is configured with the address of one or more local domain name servers
Resolver forms a DNS request message
Sends the message to the local server
Waits for server to send DNS reply message
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Name Resolution Paradigm
Resolve can choose to use either stream or message paradigm when communicating with a DNS server
Most resolvers use message paradigm as it imposes less overhead for a small request
Example: chocolate.candy.foobar.com
Resolver will send request to local DNS server for foobar.com
Even if it cannot answer the request, the server knows to contact the server for candy.foobar.com, which can generate an answer
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Locality of Reference
Locality of reference principle that forms the basis for caching applies to the Domain Name System in two ways
Spatial: A user tends to look up the names of local computers more often than the names of remote computers
Temporal: A user tends to look up the same set of domain names repeatedly
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Exploiting Locality
DNS exploits spatial locality
Resolver contacts a local server first
To exploit temporal locality
a DNS server caches all lookups
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DNS Lookup Algorithm
Source: Douglas, C (2016) Computer Networks and Internets
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Caching in DNS Server
When a request arrives for a name outside the set for which the server is an authority
Server temporarily becomes a client of another name server
When the other server returns an answer, original server caches the answer
Sends a copy of the answer back to the resolver from which the request arrived
Server 1
(authority)
Resolver
i) Help resolve name
ii) Can’t resolve name
Server 2
iii) Ask others
iv) Can resolve name
v) Returns answer
vi) Cache answer
vii) Returns answer
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Caching Consideration
In addition to knowing the address of all servers down the hierarchy
Each DNS server must know address of a root server
How long items should be cached?
If cached too long, the item will become stale (may become outdated)
DNS will specify a cache timeout for each item
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Practice 6.2
How does DNS allow autonomy?
Why is there a need to replicate a DNS server?
What kind of communication paradigm does name resolution uses? Which is more commonly used and why?
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Lesson 6 Outline
DNS Basics
Implementing DNS Servers
DNS Mapping Process
35
Entry Fields
Each entry in a DNS database consists of three fields:
Domain name: human readable names
Record type: specifies how the value is to be interpreted
Value: such as IP address
A query specifies both a domain name and a type
Server only returns a binding that matches the type of the query
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Binding Classifications
Principal type maps a domain name to an IP address
Known as type A bindings
used by applications such as FTP, or a browser
ftp.web.com -> 192.168.1.0
Another is type MX that specifies a Mail eXchanger
SMTP uses type MX to look up the domain name in an email address
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Entry Type
Each entry in a DNS server has a type
Resolver must specify the type desired when looking up a name
DNS server returns only entries that match the specified type
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Mapping Entry Type Efficiently
DNS type system can produce unexpected results
Address returned depends on the type
the name corporation.com can be used for both web and email services
Possible to divide the workload between separate computers
Mapping of type A lookups to one computer and type MX lookups to another
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Alias and CNAME
The DNS offers a CNAME
Analogous to shortcut in file systems
This provides an alias for another DNS entry
aliases can be useful, suppose Foobar Corporation has two computers, named as:
hobbes.foobar.com and calvin.foobar.com
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Alias and CNAME Example
Suppose that foobar runs a web server on computer hobbes, and wants to follow the convention of using the name www
EITHER: rename computer hobbes
OR: create a CNAME entry for www.foobar.com that points to hobbes
When resolver sends a request for www.foobar.com, server returns address of computer hobbes
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Alias Benefits (1/2)
Alias permits an organization to change the computer used for a service without changing the names or addresses:
Foobar Corporation can move its web service from hobbes calvin
changing the CNAME record in DNS server allows two computers retain their original names and IP addresses
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Alias Benefits (2/2)
Aliases allows an organization to associate multiple aliases with a single computer
Can run an FTP server and a web server on the same computer
Create CNAME records for:
www.foobar.com
ftp.foobar.com
43
Abbreviations
DNS does not incorporate abbreviations
Server only responds to a full name
Most resolvers can be configured with a set of suffixes that allow a user to abbreviate names
Each resolver at Foobar Coporation might be programmed to look up a name twice:
once with no change
once with the suffix foobar.com appended
44
Abbreviations and DNS
If user enters full domain name
Local server will return address and proceed
If user enters an abbreviated name
Will first try to resolve the name
Receive error because no such name exists
Then it will try appending suffix and looking up the resulting name
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DNS Character Set Limitations
DNS uses the ASCII character set
Can represent mostly English characters
Languages like Russian, Greek, Chinese, and Japanese each contain special characters
No ASCII representation exists
Many European languages use diacritical (accent) marks that cannot be represented in ASCII
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IETF Approach to DNS Characters
IETF debated modifications and extensions of the DNS to accommodate international domain names
Chose an approach known as Internationalizing Domain Names in Applications (IDNA)
IDNA uses ASCII to store all names
If a domain name contains a non-ASCII character, IDNA translates the name into a sequence of ASCII characters
Stores the result in the DNS
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IDNA Translation
IDNA relies on applications to translate between international characters and internal ASCII form
Rules for translating international domain names are complex and uses Unicode
Latest versions of Firefox and Internet Explorer can accept and display non-ASCII domain names as they each implement IDNA
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Practice 6.3
Describe each of the three fields in a DNS entry.
Describe the type of domain name binding that can be used for a mail exchanger. Give an application example that uses such binding.
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Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 4
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End of Lesson
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