Allied Telesis AT-S63 - Manual (Page 384)

Chapter 18: Denial of Service Defense
384 Section II: Advanced Operations
Note
This defense mechanism requires some involvement by the switch’s
CPU, though not as much as the Teardrop defense. This does not
impact the forwarding of traffic between the switch ports, but it can
affect the handling of CPU events, such as the processing of IGMP
packets and spanning tree BPDUs. For this reason, Allied Telesyn
recommends limiting the use of this defense, activating it only on
those ports where an attack is most likely to originate.
Also note that an attacker can circumvent the defense by sending a
stream of ICMP Echo (Ping) requests with a size of 63,488 to 65,534 bits.
A large number of requests could overwhelm the switch’s CPU.
IP Options
Attack
In the basic scenario of an IP attack, an attacker sends packets containing
bad IP options. There are several types of IP option attacks and the
AT-S63 management software does not distinguish between them.
Rather, the defense mechanism counts the number of ingress IP packets
containing IP options received on a port. If the number exceeds 20
packets per second, the switch considers this a possible IP options attack
and does the following occurs:
ˆ It sends an SNMP trap to the management stations.
ˆ The switch port discards all ingress packets containing IP options for
one minute.
This defense mechanism does not involve the switch’s CPU. You can
activate it on as many ports as you want without it impacting switch
performance.
Note
This defense does not actually check IP packets for bad IP options;
it can only alert you to a possible attack.
Denial of Service
Defense
Guidelines
Below are guidelines to observe when using this feature:
ˆ A switch port can support more than one DoS defense at a time.
ˆ The Teardrop and the Ping of Death defenses are CPU intensive. Use
these defenses with caution.
ˆ Some defenses allow you to specify a mirror port where offending
traffic is copied.