Brocade Communications Systems 6650 manual Avoiding being an intermediary in a Smurf attack

Smurf attacks

For each ICMP echo request packet sent by the attacker, a number of ICMP replies equal to the number of hosts on the intermediary network are sent to the victim. If the attacker generates a large volume of ICMP echo request packets, and the intermediary network contains a large number of hosts, the victim can be overwhelmed with ICMP replies.

Avoiding being an intermediary in a Smurf attack

A Smurf attack relies on the intermediary to broadcast ICMP echo request packets to hosts on a target subnet. When the ICMP echo request packet arrives at the target subnet, it is converted to a Layer 2 broadcast and sent to the connected hosts. This conversion takes place only when directed broadcast forwarding is enabled on the device.

To avoid being an intermediary in a Smurf attack, make sure forwarding of directed broadcasts is disabled on the Brocade device. Directed broadcast forwarding is disabled by default. To disable directed broadcast forwarding, do one of the following.

Brocade(config)# no ip directed-broadcast

Syntax: [no] ip directed-broadcast

Avoiding being a victim in a Smurf attack

You can configure the Brocade device to drop ICMP packets when excessive numbers are encountered, as is the case when the device is the victim of a Smurf attack. You can set threshold values for ICMP packets that are targeted at the router itself or passing through an interface, and drop them when the thresholds are exceeded.

For example, to set threshold values for ICMP packets targeted at the router, enter the following command in global CONFIG mode.

Brocade(config)# ip icmp burst-normal 5000 burst-max 10000 lockup 300

To set threshold values for ICMP packets received on interface 1/1/3, enter the following commands.

Brocade(config)# interface ethernet 1/1/3 Brocade(config-if-e10000-1/1/3)# ip icmp burst-normal 5000 burst-max 10000 lockup 300

For Layer 3 router code, if the interface is part of a VLAN that has a router VE, you must configure ICMP attack protection at the VE level. Otherwise, you can configure this feature at the interface level as shown in the previous example. When ICMP attack protection is configured at the VE level, it will apply to routed traffic only. It will not affect switched traffic.

NOTE

You must configure VLAN information for the port before configuring ICMP attack protection. You cannot change the VLAN configuration for a port on which ICMP attack protection is enabled.

To set threshold values for ICMP packets received on VE 31, enter commands such as the following.

Brocade(config)# interface ve 31

Brocade(config-vif-31)# ip icmp burst-normal 5000 burst-max 10000 lockup 300

268	Brocade ICX 6650 Security Configuration Guide
	53-1002601-01