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About your assignment 5 -layers Model Application Layer (HTTP, DNS, . . .) TCP Layer (add sequence number to packets) IP Layer (for addressing) Data Link Layer (Ethernet, PPP) Physical Layer (wire, wireless, optical fibre, . . . ) Hacking Common security problems virus

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about your assignment
About your assignment
  • 5 -layers Model

Application Layer (HTTP, DNS, . . .)

TCP Layer (add sequence number to packets)

IP Layer (for addressing)

Data Link Layer (Ethernet, PPP)

Physical Layer (wire, wireless, optical fibre, . . . )

  • Common security problems
    • virus
      • malicious computer program that makes copies of itself and attaches those copies to other programs
    • Trojan horse
      • a program that appears to have one ubiquitous function, but actually has a hidden malicious function
  • Dictionary attack
    • try all possible words in a dictionary and compare with the encrypted password in the server
    • need to get the password file first



Password file






denial of service attack
Denial of Service Attack
  • Sends fake IP packets to the web server
    • server sends an reply to the client, waiting for the HTTP message to arrive
    • but the source address is faked, so the server has to wait forever
  • a server can only support a limited number of concurrent requests, if all these requests are faked, then the server cannot do any useful work
denial of service attack5
Denial of Service Attack
  • How to prevent it?
    • charge the packets !
    • check the IP address at the source
denial of service attack6
Denial of Service Attack
  • Details of the attack
    • client > initial packet with a sequence no to server
    • server> return its own sequence no to client
    • client> OK, I got your sequence number
    • client> the request
    • server> return the requested file
    • client> disconnect
    • server> disconnect
christmas 1994
Christmas 1994
  • 25th December, 1994, 14:09:32 . . .
    • Security expert Tsutomu Shimomura was on a skiing trip
    • meanwhile his computer in was break-in by a hacker
  • 15th February, 1995
    • By tracing the traffic log, Kevin Mitnick, the most wanted computer outlaw, was caught
  • Paperbacks and movie about the story
    • Takedown by Shimomura and John Markoff
    • The Fugitive Game by Jonathan Littman
    • Takedown (movie) by Miramax
ip spoofing
IP spoofing
  • Security hole in remote login (rlogin)
    • authentication based on IP address
    • password is not required
  • IP spoofing attack
    • host A: attacker

host B: the machine to be attacked

host C: the machine impersonated by the attacker, C is trusted by B

    • A sends the 1st packet to host B using the false IP source address (host C)
    • B checks the IP, verifies it, find nothing wrong, so let the hacker login without checking the password
3 way handshakes

B (machine under attacked)




(friend of B)

A (hacker)

3-way handshakes
  • Problem: 3-way handshakes
    • 1st segment: A --------------> B
    • 2nd segment: A <-- ------------ B
    • 3rd segment: A --- ACK ---> B
    • BUT B sends the 2nd segment to C, so that A cannot return the ACK to B without knowing the sequence number used by B
sequence number prediction
Sequence number prediction
  • BUT the ISN of B can be guessed!!
  • First, the hacker makes a real connection to B
    • because the connection is real, the 2nd segment sent by B will be received by the hacker
    • the 2nd TCP segment contains the number of B
    • after getting the number of B, the hacker launches the attack
    • return the 3rd packet using the guessed ISN
    • how to guess the number?
    • the number is incremented by 128000 for every new connection
sequence number prediction11
Sequence number prediction
  • so that for two consecutive connection requests, the ISN will be differed by 128,000
  • if there is no other connection request to the server during that time, the ISN can be guessed accurately and easily
  • the best time to attack is when there is little connection requests to the server
    • Christmas
  • so after sending the 1st packet to B,

A sends the the 3rd packet to B with the guessed number

syn flooding
SYN flooding
  • but the story does not end here, note that when B sends the 2nd packet to C, C will return a packet (reset) and spoils the attack
  • To prevent C from returning the packet, A sends a large number of SYN packets (the first packet) to C using dead site as source address
  • C receives a lot of connection requests and opens up many half-open connections (waiting the the 3rd packet to return)
  • but since the source is a non-existing host, the 3rd handshake never come back
  • prevent C from receiving anything from B