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How does the Internet work ?

How does the Internet work ?






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    How does the Internet work ? How does the Internet work ? Presentation Transcript

    • 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.
      • http://en.wikipedia.org/wiki/Internet
      • 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 “www.shu.edu”
          • What is the IP name of “”
    • ISP DHCP Router A Router B DNS www.google.com fiber optic lines ISP: Internet Service Provider DHCP: Dynamic Host Config. Protocol DNS: Domain Name Server
    • The Data: IP Packet www.google.com ( (
    • The Data: IP Packet www.google.com ( (
    • The Protocol
      • A mutually agreed-upon convention or standard that controls or enables the connection, communication, and data transfer between computing endpoints.
      • http://en.wikipedia.org/wiki/Protocol_(computing)
      • 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 www.shu.edu
      • Server computer : www.shu.edu
      • Client computer :
      • Server program : web server program (httpd) running on www.shu.edu with access to lots of stored web pages
      • Client program : Internet Explorer or Firefox with extensive formatting and display capabilities but no data to display
    • Client – Server Example (2)
      • I want to view the main web page from www.shu.edu
      • Client : start IE and enter: http://www.shu.edu/
        • Client sends packet to DNS: who is www.shu.edu
        • DNS sends packet back: www.shu.edu =
        • 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 www.shu.edu
      • Client: sends “ give me main page ” Server: sends data to
        • http ( H yper t ext T ransport P rotocol): 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 www.shu.edu
      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)
      • Telnet:
      • universal text client used to connect to another computer and work on that computer in text-based mode
      • usually connects to a “telnet server” but can also connect to any server computer and any server program
      • 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 www.shu.edu, type:
            • open www.shu.edu (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 www.shu.edu 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 143)
      • 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 kitten.shu.edu Client: MAIL FROM:<Smith@shu.edu> Server: 250 OK Client: RCPT TO:<Jones@shu.edu> Server: 250 OK Client: DATA Server: 354 Start mail input; end with <CRLF>.<CRLF> Client: Blah blah blah.. . Server: 250 OK Client: QUIT
    • Homework
      • Capture a web page from a web server
      • 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
      • 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
      • 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
      • 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
      • 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
        • See http://www.explainthatstuff.com/encryption.html
    • Public Key Encryption requires:
        • large “semi”-prime number x = p q
        • Example :
        • public key x = 15 => private key is:
        • Homework : Find factorization of RSA-100, which is 1522605027922533360535618378132637429718068114961380688657908494580122963258952897654000350692006139
        • More Info:
        • http://en.wikipedia.org/wiki/Public-key_cryptography
        • http://en.wikipedia.org/wiki/RSA_numbers
        • http://primes.utm.edu/mersenne/
        • Enigma by Robert Harris, and Enigma the Movie