Transcript How does the Internet work

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 “149.150.254.102”
Router A
DHCP
Router B
ISP
fiber optic
lines
www.google.com
ISP:
DNS
Internet Service
Provider
DHCP: Dynamic Host
Config. Protocol
DNS: Domain Name
Server
The Data: IP Packet
www.google.com
(66.102.1.147)
(149.150.254.102)
The Data: IP Packet
www.google.com
(66.102.1.147)
(149.150.254.102)
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: 192.168.1.2
 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 = 149.150.51.69
 Client sends packet to 149.150.51.69: give me main page
 Server: receives request for page from 192.168.1.2
 Retrieves the web page from disk (or database)
 Sends data to 192.168.1.2: 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 192.168.1.2
 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 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
Server:
Text
220 Simple Mail Transfer Service ready
Client:
HELO mycomputer.mydomain
Server:
250 kitten.shu.edu
Client:
MAIL FROM:<[email protected]>
Server:
250 OK
Client:
RCPT TO:<[email protected]>
Server:
250 OK
Client:
DATA
Server:
354 Start mail input; end with <CRLF>.<CRLF>
Client:
Blah blah blah..
.
Server:
250 OK
Client:
QUIT
Homework
1.
2.
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
ISP
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!
ISP
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”
ISP
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
1522605027922533360535618378132637429718
0681149613806886579084945801229632589528
97654000350692006139
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