Telecommunications Networking II

Lecture 41d

Denial-of-Service Attacks

Network Denial-of-Service Attacks

andOther Network-Application-

Based Attacks

Network Denial-of-Service Attacks

• Attacker’s objectiveTo interrupt or reduce the quality of services…as experienced by legitimate users

• Many attacks have innocent counterparts (e.g., someone sends me a very large E-mail attachment, and blocks my access to other messages)

Network Denial-of-Service Attacks

• The “SYN” Flooding attack:-In TCP, one establishes a connection by sending a synchronization (SYN) message to the host one wishes to communicate with-The attack: send a large number of SYN messages (with phony source addresses) to a host. This overloads the buffer in the host that keeps track of TCP connections (and half-connections) in progress

TCP SYN Flooding Attack

SYN(500)

SYN(1024), ACK(501)

No acknowledgement of prior SYN segment….

...More new SYN segments

More SYN acknowledgements ...

Network Denial-of-Service Attacks

• The “SYN” Flooding attack:-Some protection can be gained by configuring networks so that they will not accept IP packets from external (to the network) sources whose source addresses are internal to the network; and which will not allow internal sources to send IP packets to external destinations if the source addresses used are not internal addresses

Sequence Number Attacks

• Disable a host that is trusted by the target (intended victim) machine

• Initiate a TCP connection by impersonating the disabled host (I.e., use it’s IP address) and sending a SYN message.

• Guess the initial sequence number that the target system will use; and respond with an acknowledgement.

TCP Sequence Number Attack

SYN(500)

SYN(800), ACK(501)

ACK(801)

ACK(801), data

ACK(801), FIN(1012)

ACK(1013)

ACK(1013), FIN(800)

ACK(801)

ACK( )

Ref: “Firewalls and Internet Security”

Other Network-based Attacks

• See Cheswick and Bellovin Chapter 2

• Many network-based attacks are caused by the lack of strong authentication of sources (e.g., it is easy to impersonate another machine by using its IP address) and lack of encryption on IP network links