A denial of service (DoS) attack is an attack that
clogs up so much memory on the target system that it can not serve it's
users, or it causes the target system to crash, reboot, or otherwise deny
services to legitimate users.There are several different kinds of dos attacks
as discussed below:-
1) Ping Of Death :-
The ping of death attack sends oversized ICMP datagrams
(encapsulated in IP packets) to the victim.The Ping command makes use of
the ICMP echo request and echo reply messages and it's commonly used to
determine whether the remote host is alive. In a ping of death attack, however,
ping causes the remote system to hang, reboot or crash. To do so the attacker
uses, the ping command in conjuction with -l argument (used to specify the
size of the packet sent) to ping the target system that exceeds the maximum
bytes allowed by TCP/IP (65,536).
example:- c:/>ping -l 65540 hostname
Fortunately, nearly all operating systems these days are not vulnerable
to the ping of death attack.
2) Teardrop Attack
:- Whenever data is sent over the internet,
it is broken into fragments at the source system and reassembled at the
destination system. For example you need to send 3,000 bytes of data from
one system to another. Rather than sending the entire chunk in asingle packet,
the data is broken down into smaller packets as given below:
* packet 1 will carry bytes 1-1000.
* packet 2 will carry bytes 1001-2000.
* packet 3 will carry bytes 2001-3000.
In teardrop attack, however, the data packets sent to the target computer
contais bytes that overlaps with each other.
(bytes 1-1500) (bytes 1001-2000) (bytes 1500-2500)
When the target system receives such a series of packets, it can not reassemble
the data and therefore will crash, hang, or reboot.
Old Linux systems, Windows NT/95 are vulnerable.
3) SYN - Flood Attack
:- In SYN flooding attack, several SYN packets
are sent to the target host, all with an invalid source IP address. When
the target system receives these SYN packets, it tries to respond to each
one with a SYN/ACK packet but as all the source IP addresses are invalid
the target system goes into wait state for ACK message to receive from source.
Eventually, due to large number of connection requests, the target systems'
memory is consumed. In order to actually affect the target system, a large
number of SYN packets with invalid IP addresses must be sent.
