An introduction to Domain Name System: DNS

1
An introduction to Domain Name System DNS
2
Why use DNS Domain Name System?

• Internet Hosts
• IP address (32 bit for IPV4) - used for
  addressing datagrams (used by computer)
• name, e.g., www.yahoo.com - used by humans
• DNS provides translation between host name and
  IP address
• distributed database implemented in hierarchy of
  many name servers
• distributed for scalability reliability

3
DNS Services

• Hostname to IP address translation
• Host aliasing
• Canonical and alias names
• Mail server aliasing
• Load distribution
• Replicated Web servers set of IP addresses for
  one canonical name

4
DNS Infrastructure
root DNS server
2

• Host at cis.hku.edu wants IP address for
  gaia.cs.umass.edu
• Infrastructure
• Client resolver
• Local DNS server
• Authoritative DNS Server
• Root DNS Server
• Top-Level Domain DNS Server

3
TLD DNS server
4
5
6
7
1
8
authoritative DNS server dns.cs.umass.edu
requesting host cis.hku.edu
gaia.cs.umass.edu
5
Distributed, Hierarchical Database

• Root servers and TLD servers typically do not
  contain hostname to IP mappings they contain
  mappings for locating authoritative servers.

6
DNS Root name servers

• contacted by local name server that cannot
  resolve name directly
• root name server
• contacts authoritative name server if name
  mapping is not known
• gets mapping
• returns mapping to local name server

7
TLD and Authoritative Servers

• Top-level domain (TLD) servers responsible for
  .com, .org, .net, .edu, .gov, .mil, and all
  top-level country domains (e.g., .hk, .cn, .uk,
  .fr, .ca, .jp)
• Different companies in different countries
  maintain servers for different top-level domain
  servers.
• Authoritative DNS servers organizations DNS
  servers, providing authoritative hostname to IP
  mappings for organizations servers (e.g., Web
  and mail).
• Can be maintained by organization or service
  provider

8
Local Name Server

• Each ISP (residential ISP, company, university)
  has one.
• Also called default name server
• When a host makes a DNS query, query is sent to
  its local DNS server
• Acts as a proxy (for request handling and data
  caching), forwards query into hierarchy.
• Reduces lookup latency for commonly searched
  hostnames

9
Recursive queries

• recursive query
• puts burden of name resolution on contacted name
  server
• heavy load?
• iterated query
• contacted server replies with name of server to
  contact
• I dont know this name, but ask some other
  server

10
DNS caching and updating records

• once (any) name server learns mapping, it caches
  mapping
• cache entries timeout (disappear) after some time
  called the Time To Live (TTL)
• TLD servers typically cached in local name
  servers
• Thus root name servers not often visited
• update/notify mechanisms under design by IETF
  (optional)
• RFC 2136
• http//www.ietf.org/html.charters/dnsind-charter.h
  tml

11
DNS records
Optional

• DNS distributed database storing resource
  records (RR)

• TypeCNAME
• name is alias name for some canonical (the
  real) name
• www.ibm.com is really servereast.backup2.ibm.com
  
• value is canonical name

• TypeA
• name is hostname
• value is IP address

• TypeNS
• name is domain (e.g. foo.com)
• value is IP address of authoritative name server
  for this domain

• TypeMX
• value is name of mail server associated with name

12
DNS protocol, messages
Optional

• DNS protocol query and reply messages, both
  with same message format

• message header
• identification 16 bit for query, reply to
  query uses same
• flags
• query or reply
• recursion desired
• recursion available
• reply is authoritative

13
DNS protocol, messages
Optional
Name, type fields for a query
RRs in response to query
records for authoritative servers
additional helpful info that may be used
DNS messages are carried using UDP on port 53
14
Inserting records into DNS
Optional

• Example just created startup Network Utopia
• Register name networkutopia.com at a registrar
  (e.g., AsiaRegistry for HK69.0 per year, really
  cheap!)
• Need to provide registrar with names and IP
  addresses of your authoritative name server
  (primary and secondary)
• Registrar inserts two RRs into the com TLD
  server
• (networkutopia.com, dns1.networkutopia.com, NS)
• (dns1.networkutopia.com, 212.212.212.1, A)
• Put in authoritative server Type A record for
  www.networkutopia.com and Type MX record for
  networkutopia.com
• How do people get the IP address of your Web
  site?

15
Introduction to HTTP

• Refs
• RFC 1945 (HTTP 1.0)
• RFC 2616 (HTTP 1.1)

16
What is HTTP?
17
Web Components

• In order to start communicating with one anothers
  we need the following web components

• User, Author
• User Agent Browser (MS IE, Firefox)
• Network
• Web Server Apache, IIS
• Proxy Squid, ISA, Fiddler (beyond the scope of
  this course)

H
Proxy
www.hku.hk
HTTP Message
Cache
Content Delivery Network
18
What is HTTP?

• HTTP is the protocol that supports communication
  between web browsers and web servers.
• A Web Server is a HTTP server
• Most clients/servers today speak version 1.1, but
  1.0 is also in use.

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Netscape
A Brief Timeline of HTTP
Marc Andreessen
NN Mozilla
Firefox Mozilla/5.0
TEXT
IMAGE
WorldWideWeb Nexus
NCSA Mosaic
MS IE
w3C
CERN
Berners-Lee
NCSA HTTPD
Apache
R. Fielding
httpd
HTTP/1.1
HTTP/0.9
HTTP/1.0
1989
1999
1991
1996
GET
20
From the RFC

• HTTP is an application-level protocol with the
• lightness and speed necessary for distributed,
• hypermedia information systems.

21
Domain Name Mapping
nslookup www.hku.hk nslookup www.hku.edu.hk
Domain Name 1
IP
Domain Name 2
www.hku.hk
147.8.145.43
www.hku.edu.hk
TCP/IP
Client
147.8.145.43
Host www.hku.edu.hk
Content of www.hku.edu.hk
HTTP
22
Transport Independence

• The RFC states that the HTTP protocol generally
  takes place over a TCP connection, but the
  protocol itself is not dependent on a specific
  transport layer.

23
From Plain Text to Hypertext
space

• Text to Hypertext
• eBook eDocument (internal link)
• Gopher external links
• Multimedia Images Audio Video
• Interactive Page
• JavaScript
• Java Applet
• ActiveX Object
• Flash
• More App than just static Doc

Transcending space and time
time
24
HTTP Request - Response
25
Request - Response

• HTTP has a simple structure
• client sends a request
• server returns a reply.
• HTTP can support multiple request-reply exchanges
  over a single TCP connection.

26
Well Known Address

• The well known TCP port for HTTP servers is
  port 80.
• Other ports can be used as well... Depending how
  the web server has been set up.

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HTTP Connection Process

• DNS Lookup for IP
• Use IP to Build TCP Connection on port 80
• Send HTTP Request
• Parse Servers Response
• Get Inline Resources
• Rendering UI

Domain name
DNS

• hkuspace.hku.hk ? ?10.10.100.103

IP
GET / HTTP/1.1
httpd
HTTP
80
httpd
TCP
IP
10.10.100.103
28
HTTP Versions

• The original version now goes by the name HTTP
  Version 0.9
• HTTP 0.9 was used for many years.
• Starting with HTTP 1.0 the version number is part
  of every request.
• tells the server what version the client can talk
  (what options are supported, etc).

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HTTP 1.0 plus Request
Request-Line
Headers . . .
A Simplified Model good enough for us to
understand how it works.

• Lines of text (ASCII).
• Lines end with CRLF (control linefeed) \r\n
• First line is called Request-Line

blank line
Content...
30
Request Line

• Method URI HTTP-Version\r\n
• The request line contains 3 tokens (words).
  Depending on the front side bus (FSB) of the
  machine, normally a word is 32-bit long.
• space characters separate the tokens.
• Newline (\n) seems to work by itself (but the
  protocol requires CRLF)

31
Request Method

• The Request Method can be
• GET HEAD PUT
• POST DELETE TRACE
• OPTIONS
• future expansion is supported

32
HTTP Methods
Browser
open
page1

• Message Request Response
• Start Line
• Header Fields
• Body
• Request GET Method
• Get a document from the server
• Start Line GET / HTTP/1.1
• Response
• Start Line HTTP/1.1 200 OK

page2
Session
close
33
Methods

• GET retrieve information identified by the URI.
• HEAD retrieve meta-information about the URI.
• POST send information to a URI and retrieve
  result.

34
Methods (cont.)

• PUT Store information in location named by URI.
• DELETE remove entity identified by URI.

35
More Methods

• TRACE used to trace HTTP forwarding through
  proxies, tunnels, etc.
• OPTIONS used to determine the capabilities of
  the server, or characteristics of a named
  resource.

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Requests
Method OPTIONS GET HEAD POST PUT DELETE TRACE CONN
ECT

• Start Line
• ltmethodgt lturigt HTTP/ltversiongt
• GET / HTTP/1.1
• GET /index.html HTTP/1.1
• Header Field
• ltnamegt ltvaluegt
• User-Agent Mozilla/4.0
• Host www.hku.hk

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Responses
Status Code 1xx Informational 2xx
Successful 3xx Redirection 4xx Client
Error 5xx Server Error

• Start Line
• ltstatus codegt ltreason phrasegt
• 200 OK
• Header Fields
• Cache
• Date Sun, 04 Jun 2006 125615 GMT
• Misc
• Server Apache/1.3.26 (Unix) PHP/4.2.1
• Entity
• Content-Type text/html
• Last-Modified Sat, 17 Mar 2001 155516 GMT
• ETag "2d693e-5d55-3ab388e4"

38
HTTP Status Code
39
Status Codes

• 1xx Informational
• 2xx Success
• 3xx Redirection
• 4xx Client Error
• 5xx Server Error

40
Status Code List More details

• 404 - Not Found
• 405 - Method Not Allowed
• 406 - Not Acceptable
• 407 - Proxy Authentication Required
• 408 - Request Timeout
• 409 - Conflict
• 410 - Gone
• 411 - Length Required
• 412 - Precondition Failed
• 413 - Request Entity Too Large
• 414 - Request URI Too Long
• 415 - Unsupported Media Type
• 416 - Requested Range Not Certifiable
• 417 - Expectation Failed
• 500 - Internal Server Error
• 501 - Not Implemented
• 502 - Bad Gateway
• 503 - Service Unavailable
• 504 - Gateway Timeout

• 100 - Continue
• 101 - Switching Protocols
• 200 - OK
• 201 - Created
• 202 - Accepted
• 203 - Non-Authoritative Information
• 204 - No Content
• 205 - Reset Content
• 206 - Partial Content
• 207 - Multi-Status
• 300 - Multiple Choices
• 301 - Moved Permanently
• 302 - Found
• 303 - See Other
• 304 - Not Modified
• 305 - Use Proxy
• 306 - No Longer Used
• 307 - Temporary Redirect
• 400 - Bad Request

41
Example Status Lines

• HTTP/1.0 200 OK
• HTTP/1.0 301 Moved Permanently
• HTTP/1.0 400 Bad Request
• HTTP/1.0 500 Internal Server Error

42
Web Log Analysis
Some status seldom appears but it may be
important
43
Response Headers

• Provide the client with information about the
  returned entity (document).
• what kind of document
• how big the document is
• how the document is encoded
• when the document was last modified
• Response headers end with blank line

44
Web Log Analysis
4,078
Some status seldom appears but it may be
important
45
Response Header Examples

• Date Wed, 30 Jan 2002 124817 EST
• Server Apache/1.17
• Content-Type text/html
• Content-Length 1756
• Content-Encoding gzip

46
Content

• Content can be anything (sequence of raw bytes).
• Content-Length header is required for any
  response that includes content.
• Content-Type header also required.

47
Single Request/Reply

• The client sends a complete request.
• The server sends back the entire reply.
• The server closes its socket.
• If the client needs another document it must open
  a new connection.

This was the default for HTTP 1.0
48
Persistent Connections

• HTTP 1.1 supports persistent connections (this is
  the default).
• Multiple requests can be handled over a single
  TCP connection.
• The Connection header is used to exchange
  information about persistence (HTTP/1.1)
• 1.0 Clients used a Keep-alive header
• It may however, waste up expensive network
  bandwidth.

49
GET vs POST

• A more detailed discussion

50
GET or Post? Which to use?

• Whats the difference between GET and POST to the
  beginner?
• How is GET treated differently from POST
• Why would you use one over the other?
• Not many web programmers can really answer this
  question?

51
A Closer Look

• Lets backtrack a little and look at the form
  elements method attribute
• Form method"get" action""
• There are other methods but for the most part GET
  and POST are your two basic alternatives.
• This is the method that the form will use to send
  the information to the server. Note the form data
  will be sent in plain text regardless of whether
  GET or POST is used.

52
Explaining the GET Method

• The simple thing to note about GET is that when
  you use GET the content of your form will become
  URL encoded into a query string.
• That means in the address bar of your browser
  where you type in URLs the content of your form
  will become a query string appended after a ? as
  namevalue pairs.
• For example if your form took your name and your
  age the corresponding query string attached to
  the URL would be
• ?namestevenage43
• In general, GET is meant for when you get from
  the server rather than POST being meant to
  post to the server.

53
Get Uses the Header

• One useful thing to note about query
  strings appended to your URL and the GET method
  is that search terms used with GET can be
  bookmarked as a result set.
• Its worth keeping in mind. Its perhaps worth
  noting that GET data is sent in the header part
  of a request to the server.
• Also, if no method attribute is put
  forward then the default method is GET.
• A limitation of GET, to be aware of, is that its
  size is limited to the buffer input size of the
  server so you might send too much data in your
  request.

54
URL Encoding of GET

• So what is URL encoding? Well, URL encoding is an
  important function that makes the query string
  valid. If I entered my full name in the previous
  example the space between steven and clark would
  be URL encoded to a sign.
• ?namestevenclarkage43
• To see this URL encoded output make a small form
  with two or three fields and send it to your
  email account using an action of mailto.
• You can easily see the GET data turned into a
  query string in the browser address bar whereas
  the POST data is not turned into a query string
  in the browser address bar.

55
POST uses the Body

• POST, on the other hand, is sent in the body of
  the request after the headers and is not visible
  to the user as a query string. The content is
  sent as namevalue pairs but not made visible in
  the address bar of the browser.
• In that way, although its still only text, it can
  be a little bit more secure (from the person
  reading your age over your shoulder) than GET.
• The main thing to put in your head about using
  methodpost is that you use it to POST data to
  the server for processing - for example entry
  into a database or contact form details.

56
How does GET and POST exist on server-side?

• A further note, if youre looking at CGI
  programming, GET and POST are treated
  differently. GET is available as an environment
  variable whereas POST is available through the
  standard input stream (stdin). (Please revise
  your C programming notes if you have forgotten
  what is an input stream).

57
A Rule of Thumb

• A good basic rule of thumb is
• If youre sending off form data you should be
  using POST.
• If you are just running search queries then GET
  might be appropriate.
• Please consult W3C for more accurate answers if
  you wish.

58
A Simple HTTP Lab

• (Optional)

59
telnet
Optional

• Step 1 run telnet
• begin\run\executetelnet
• Step 2 Setting up Buffer Size (Optional)
• Pull Down Menu\Content\
• format Screen buffer Size\Resolution 500
• Option/Command Record
• Buffer Size 999 and number of connections 40
• Step 3 Establish Connection
• Microsoft Telnetgt open www.hku.hk 80

60
Test it with telnet
Optional

• gt telnet www.hku.hk 80
• GET / HTTP/1.0
• HTTP/1.0 200 OK
• Server Apache
• ...

Request-line
Blank Line (end of headers)
Response
61
Test it with telnet 1.1
Optional

• gt telnet www.hku.hk 80
• GET / HTTP/1.1
• Host www.hku.hk
• HTTP/1.0 200 OK
• Server Apache
• ...

Required!
62
Reference

• W3C School
• ???Linux ???-- DNS Server
• DNS Resources Directory