1. How does the Internet work ?

2. 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 s
How to hack into a system
How a Firewall works
Encrypted communications

3. 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

4. The User Perspective have little laptop at home in NJ
want to exchange information with big server in California (or Japan or …)

5. 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)

6. 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 “ ”

7. Router A Router B DHCP ISP fiber optic lines www.google.com DNS
ISP: Internet Service Provider
DHCP: Dynamic Host Config. Protocol
DNS: Domain Name Server
DNS

9. The Data: IP Packet
( )
( )

10. The Data: IP Packet
( )
( )

11. 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

12. 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

13. 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 display

14. 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

15. 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

16. 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 ...

17. 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

18. 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

19. 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 /

20. How works
systems have two parts, and consequently work with two server programs and two protocols:
Retrieving
uses either pop3 (Post Office Protocol version 3 on port 110) or IMAP (Internet message access protocol on port 143)
Sending
uses smtp (simple mail transport protocol on port 25)

21. An smtp Conversation Speaker Text Server:
220 Simple Mail Transfer Service ready
Client:
HELO mycomputer.mydomain
250 kitten.shu.edu
MAIL
250 OK
RCPT
DATA
354 Start mail input; end with <CRLF>.<CRLF>
Blah blah blah.. .
QUIT

22. Homework Capture a web page from a web server
Send me an (anonymous) using Telnet (note that the SHU smtp server will only allow a connection if you are located on campus – how does it know?)

23. 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

24. 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

25. 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 packets and includes rules to block messages depending on the content of the .
A SPAM filter is typically integrated into you ISP or provider
SPAM filters typically read your !

26. 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 !
Sending 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”

27. 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

28. 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
More Info:
Enigma by Robert Harris, and Enigma the Movie