How does the Internet work ? by Kannojuonline


									What we will cover
 What is the Internet
 Client/server examples
 Deciphering alphabet soup: ISP, DHCP, DNS, http,
  https, HTML, smtp, pop3, telnet, ftp, sftp, ssh
 Network security:
   How to send anonymous emails
   How to hack into a system
   How a Firewall works
   Encrypted communications
Internet History
The Internet is a global network of interconnected computers, enabling
users to share information along multiple channels.
 Started in 1969 as ARPANET funded by
  DARPA (Defense Advanced Research
  Projects Agency)
 Build as packet switching network to
  recover from a nuclear attack by
  automatically rerouting data through
  surviving links

“When I took office, only high energy physicists
had ever heard of what is called the World Wide
Web... Now even my cat has it's own page.”
Bill Clinton
The User Perspective
 have little laptop at home in NJ
 want to exchange information
 with big server in California
 (or Japan or …)
Basic Setup
 Each computer must have a unique identifier
    IP number and IP name
 Computers must be able to exchange data (electrons,
 photons, drum beats)
   Wireless cards, fiber optics, or Ethernet connections
   Unit of data is “bit” (“zero” or “one”, on/off, 2 states)
 Everyone involved must speak the same language
    TCPIP (Transmission Control Protocol/Internet Protocol)
Network Member Identifier
 Every computer on the Internet has at least one unique
  identifier, usually two:
   IP Number: #.#.#.#, where # is an 8 bit number
     What is the range for each sub-number?
     How many machines can be on the Internet?
     What is your computer’s IP number?
  • IP Name for easy reference
     What is your computer’s IP name?

  • Homework:
    • What is the IP number of “google”
    • What is the IP number of “”
    • What is the IP name of “”
             Router A
                                             Router B

                        fiber optic


                                      ISP:  Internet Service
                                      DHCP: Dynamic Host
                          DNS               Config. Protocol
                                      DNS: Domain Name
   The Data: IP Packet

   The Data: IP Packet

The Protocol
A mutually agreed-upon convention or standard that
controls or enables the connection, communication, and
data transfer between computing endpoints.

 Regulates the data exchange and interpretation
 Defines who says what at which time
 Defines how to interpret data that is exchanged
 Regulates what constitutes an error and what to do if
 one occurs
Client – Server Model
 Communication on the Internet usually takes place
 between a client and a server program/computer:
   Server program: program without a user interface
    running on a “large” computer with access to many
    resources (also called the server computer)
   Client program: a program with extensive interface
    capabilities but few resources running on a “small”
    computer (also called the client computer)
 One server computer usually run several server
 programs, each of which can service multiple client
 programs simultaneously
Client – Server Example (1)
I want to view the main web page from

 Server computer:
 Client computer:

 Server program: web server program (httpd) running on with access to lots of stored web pages
 Client program: Internet Explorer or Firefox with
  extensive formatting and display capabilities but no data to
Client – Server Example (2)
I want to view the main web page from

 Client: start IE and enter:
    Client sends packet to DNS: who is
    DNS sends packet back: =
    Client sends packet to give me main page
 Server: receives request for page from
    Retrieves the web page from disk (or database)
    Sends data to here is the data
 Client: formats data and display it nicely
Client – Server Example (3)
I want to view the main web page from

 Client: sends “give me main page”
 Server: sends data to
   http (Hypertext Transport Protocol): regulates how a
    web server and client communicate
 Client: formats data and display it nicely
    HTML (Hypertext Markup Language): defines how text
     is supposed to look and where to place it
Client – Server Example (4)
I want to view the main web page from
      Action                        http 1.0 specs

                http uses port 80 by general agreement

      Client: “give me main page”   GET / HTTP/1.0
      Server: returns data          fixed header:
                                       how many bytes
                                       are coming,
                                       type of data
                                    page text ...
Client – Server Example (5)
 universal text client used to connect to another
  computer and work on that computer in text-based
 usually connects to a “telnet server” but can also
  connect to any server computer and any server
 shows text data in ‘raw’ unformatted form
Client – Server: Telnet
A Telnet client is build into Windows:
   Click on “Start”
   Pick “Run …” and type “cmd”
   Type “telnet”
    if you get error message in Vista, open Control Panel, select
    “Programs”, click “Turn Windows features on or off”, and check
    “Telnet client” (not “Telnet server”). Then try again.
   To open a connection to, type:
      open (will this work – why not?)
   Optional: to save a log of your session, type:
      set logfile log.txt
Telnet’ing to a Web Server
 Start “telnet” and type:
    open 80

 Next type carefully and without errors (you might not
  see what you type on the screen – type anyway,
  including the empty line and capitals):
    GET / HTTP/1.0

 You have issued a request according to the Hypertext
  Transfer Protocol, version 1.0, for the root web page /
How email works
Email systems have two parts, and consequently work
with two server programs and two protocols:

 Retrieving email
   uses either pop3 (Post Office Protocol version 3 on port
   110) or IMAP (Internet message access protocol on port
 Sending email
   uses smtp (simple mail transport protocol on port 25)
An smtp Conversation
Speaker     Text
  Server:   220 Simple Mail Transfer Service ready
  Client:   HELO mycomputer.mydomain
  Server:   250
  Client:   MAIL FROM:<>
  Server:   250 OK
  Client:   RCPT TO:<>
  Server:   250 OK
  Client:   DATA
  Server:   354 Start mail input; end with <CRLF>.<CRLF>
  Client:   Blah blah blah..

  Server:   250 OK
  Client:   QUIT
1.   Capture a web page from a web server
2.   Send me an (anonymous) email using Telnet (note that
     the SHU smtp server will only allow a connection if you are located
     on campus – how does it know?)
Hacking 101
 Identify a target system

 Find an open port

 Check which server program services that port

 Learn about vulnerabilities of that server program

 Exploit vulnerability for evil purposes
 Firewall Protection                           ISP

 Every packet transported over the
  Internet contains the protocol (port),
  sender address, and destination address
 A firewall is a device that inspects every
  incoming (and outgoing) packet and
  includes rules to block data depending
  on the port, sender, or destination
 A firewall is typically integrated into a
  home wireless router
 Most firewalls do not check the content
  of a packet
 SPAM Protection                               ISP

 Every Internet packet contains the port,
  sender, destination, and content (data)
  – usually unencrypted
 A SPAM filter is a device that inspects
  all email packets and includes rules to
  block messages depending on the
  content of the email.
 A SPAM filter is typically integrated into
  you ISP or email provider
 SPAM filters typically read your email!
 Privacy/Content Protection                                  ISP

 All Internet traffic is public!
 To protect content, the data portion of IP packets
  must be encrypted
    To protect data between your wireless router and
     your laptop, use e.g. WPA wireless encryption
    To protect data after it leaves your router, you must
     use encrypted services (https instead of http or
     sftp instead of ftp)
    Note: no standard encryption for email!

Sending email is like passing a postcard along a chain of
hundreds of people with a note: “to Jane – do not read if
you’re not Jane”
Encryption is usually based on a key that used to encrypt and
decrypt a message.
 Pre-Shared Key (PSK) Encryption:
    both parties know a single key (e.g. WPA encryption: both your
     router and your computers know the key)
 Public Key Encryption:
    each party has two keys, a public and a private one. They swap
     public keys: Bob uses Annie’s public key to send her an
     encrypted message, she can decrypt it using her private key
    based on difficulty of factoring huge numbers into large primes
     and ease of multiplying them
Public Key Encryption requires:
   large “semi”-prime number x = p q
   public key x = 15 => private key is:
  Homework: Find factorization of RSA-100, which is
  More Info:
   Enigma by Robert Harris, and Enigma the Movie

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