Cisco WS-SUP32-GE-3B Software Configuration Guide - Page 465

Chapter 33 Configuring Denial of Service Protection Understanding How DoS Protection Works Router(config)# access-list 101 permit ip any any Security ACLs also protect against the spoofing of addresses. For example, assume that a source address A is on the inside of a network and a switch interface that is pointing to the Internet. You can apply an inbound ACL on the switch Internet interface that denies all addresses with a source of A (the inside address). This action stops attacks where the attackers spoof inside source addresses. When the packet arrives at the switch interface, it matches on that ACL and drops the packet before it causes damage. When the Catalyst 6500 series switch is used with a Cisco Intrusion Detection Module (CIDM), you can dynamically install the security ACL as a response to the detection of the attack by the sensing engine. VACLs are a security enforcement tool based on Layer 2, Layer 3, and Layer 4 information. The result of a VACL lookup against a packet can be a permit, a deny, a permit and capture, or a redirect. When you associate a VACL with a particular VLAN, all traffic must be permitted by the VACL before the traffic is allowed into the VLAN. VACLs are enforced in hardware, so there is no performance penalty for applying VACLs to a VLAN on the Catalyst 6500 series switches. QoS Rate Limiting QoS ACLs limit the amount of a particular type of traffic that is processed by the PISA. If a DoS attack is initiated against the PISA, QoS ACLs can prevent the DoS traffic from reaching the PISA data path and congesting it. The PFC3B performs QoS in hardware, which offers an efficient means of limiting DoS traffic (once that traffic has been identified) to protect the switch from impacting the PISA. For example, if the network is experiencing ping-of-death or smurf attacks, the administrator should rate limit the ICMP traffic to counteract the DoS attack and still allow legitimate traffic through the processor, or allow it to be forwarded to the PISA or host. This rate limiting configuration must be done for each flow that should be rate limited and the rate-limiting policy action should be applied to the interface. In the following example, the access-list 101 permits and identifies ping (echo) ICMP messages from any source to any destination as traffic. Within the policy map, a policing rule defines a specified committed information rate (CIR) and burst value (96000 bps and 16000 bps) to rate limit the ping (ICMP) traffic through the chassis. The policy map then is applied to an interface or VLAN. If the ping traffic exceeds the specified rate on the VLAN or interface where the policy map is applied, it is dropped as specified in the markdown map (the markdown map for the normal burst configurations is not shown in the example). Router(config)# access-list 101 permit icmp any any echo Router(config)# class-map match-any icmp_class Router(config-cmap)# match access-group 101 Router(config-cmap)# exit Router(config)# policy-map icmp_policer Router(config-pmap)# class icmp_class Router(config-pmap-c)# police 96000 16000 conform-action transmit exceed-action policed-dscp-transmit drop Router(config-pmap-c)# exit Router(config-pmap)# exit uRPF Check When you enable the unicast reverse path forwarding (uRPF) check, packets that lack a verifiable source IP address, such as spoofed IP source addresses, are discarded. Cisco Express Forwarding (CEF) tables are used to verify that the source addresses and the interfaces on which they were received are consistent with the FIB tables on the supervisor engine. OL-11439-03 Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY 33-3