Cisco Systems IPS4510K9 manual IP Fragmentation Normalization, TCP Normalization, IPv6 Fragments

Appendix B Signature Engines

Normalizer Engine

The Normalizer engine deals with IP fragment reassembly and TCP stream reassembly. With the Normalizer engine you can set limits on system resource usage, for example, the maximum number of fragments the sensor tries to track at the same time. Sensors in promiscuous mode report alerts on violations. Sensors in inline mode perform the action specified in the event action parameter, such as produce-alert, deny-packet-inline, and modify-packet-inline.

Caution For signature 3050 Half Open SYN Attack, if you choose modify-packet-inline as the action, you can see as much as 20 to 30% performance degradation while the protection is active. The protection is only active during an actual SYN flood.

IP Fragmentation Normalization

Intentional or unintentional fragmentation of IP datagrams can hide exploits making them difficult or impossible to detect. Fragmentation can also be used to circumvent access control policies like those found on firewalls and routers. And different operating systems use different methods to queue and dispatch fragmented datagrams. If the sensor has to check for all possible ways that the end host can reassemble the datagrams, the sensor becomes vulnerable to DoS attacks. Reassembling all fragmented datagrams inline and only forwarding completed datagrams and refragmenting the datagram if necessary, prevents this. The IP Fragmentation Normalization unit performs this function.

TCP Normalization

Through intentional or natural TCP session segmentation, some classes of attacks can be hidden. To make sure policy enforcement can occur with no false positives and false negatives, the state of the two TCP endpoints must be tracked and only the data that is actually processed by the real host endpoints should be passed on. Overlaps in a TCP stream can occur, but are extremely rare except for TCP segment retransmits. Overwrites in the TCP session should not occur. If overwrites do occur, someone is intentionally trying to elude the security policy or the TCP stack implementation is broken. Maintaining full information about the state of both endpoints is not possible unless the sensor acts as a TCP proxy. Instead of the sensor acting as a TCP proxy, the segments are ordered properly and the Normalizer engine looks for any abnormal packets associated with evasion and attacks.

IPv6 Fragments

The Normalizer engine can reassemble IPv6 fragments and forward the reassembled buffer for inspection and actions by other engines and processors. The following differences exist between IPv4 and IPv6:

•modify-packet-inline for Normalizer engine signatures has no effect on IPv6 datagrams.

•Signature 1206 (IP Fragment Too Small) does not fire for IPv6 datagrams. Signature 1741 in the Atomic IP Advanced engine fires for IPv6 fragments that are too small.

•Signature 1202 allows 48 additional bytes beyond the max-datagram-size for IPv6 because of the longer IPv6 header fields.

TCP Normalizer Signature Warning

You receive the following warning if you disable a default-enabled TCP Normalizer signature or remove a default-enabled modify packet inline, deny packet inline, or deny connection inline action:

Use caution when disabling, retiring, or changing the event action settings of a <Sig ID> TCP Normalizer signature for a sensor operating in IPS mode. The TCP Normalizer signature default values are essential for proper operation of the sensor.

If the sensor is seeing duplicate packets, consider assigning the traffic to multiple virtual sensors. If you are having problems with asymmetric or out-of-order TCP packets, consider changing the normalizer mode from strict evasion protection to asymmetric mode protection. Contact Cisco TAC if you require further assistance.

	Cisco Intrusion Prevention System Sensor CLI Configuration Guide for IPS 7.2
OL-29168-01		B-37