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User Guide 951
Configuring DoS Defend DoS Defend Configuration
Step 2 ip dos-prevent
Globally enable the DoS defend feature.
Step 3 ip dos-prevent type { land | scan-synfin | xma-scan | null-scan | port-less-1024 | blat | ping-
flood | syn-flood | win-nuke | ping-of-death | smurf }
Configure one or more defend types according to your needs. The types of DoS attack are
introduced as follows.
land:
The attacker sends a specic fake SYN (synchronous) packet to the destination host.
Because both the source IP address and the destination IP address of the SYN packet are
set to be the IP address of the host, the host will be trapped in an endless circle of building
the initial connection.
scan-synfin:
The attacker sends the packet with its SYN field and the FIN field set to 1.
The SYN eld is used to request initial connection whereas the FIN eld is used to request
disconnection. Therefore, a packet of this type is illegal.
xma-scan:
The attacker sends the illegal packet with its TCP index, FIN, URG and PSH eld
set to 1.
null-scan:
The attacker sends the illegal packet with its TCP index and all the control elds
set to 0. During the TCP connection and data transmission, the packets with all the control
elds set to 0 are considered as the illegal packets.
port-less-1024:
The attacker sends the illegal packet with its TCP SYN field set to 1 and
source port smaller than 1024.
blat:
The attacker sends the illegal packet with the same source port and destination port on
Layer 4 and with its URG eld set to 1. Similar to the Land Attack, the system performance
of the attacked host is reduced because the Host circularly attempts to build a connection
with the attacker.
ping-flood:
The attacker floods the destination system with Ping packets, creating a
broadcast storm that makes it impossible for system to respond to legal communication.
syn-flood:
The attacker uses a fake IP address to send TCP request packets to the server.
Upon receiving the request packets, the server responds with SYN-ACK packets. Since the
IP address is fake, no response will be returned. The server will keep on sending SYN-ACK
packets. If the attacker sends overîž°owing fake request packets, the network resource will
be occupied maliciously and the requests of the legal clients will be denied.
win-nuke:
An Operation System with bugs cannot process the URG (Urgent Pointer) of TCP
packets. If the attacker sends TCP packets to port139 (NetBIOS) of the host with Operation
System bugs, it will cause blue screen.
ping-of-death:
Ping of Death attack means that the attacker sends abnormal ping packets
larger than 65535 bytes to cause system crash on the target computer.
Note: Ping of Death is only available on certain devices.
smurf:
Smurf attack is a distributed denial-of-service attack in which large numbers of
Internet Control Message Protocol (ICMP) packets with the intended victim’s spoofed
source IP are broadcast to a computer network using an IP broadcast address. Most
devices on a network will, by default, respond to this by sending a reply to the source IP
address. If the number of machines on the network that receive and respond to these
packets is very large, the victim’s computer will be flooded with traffic.
Note: Smurf is only available on certain devices.
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