Triggers on receipt of an IP datagram in which the IP option list for the datagram includes option 9 (Strict Source Routing), the value of which is 137. The Strict Source Route option may be used to specify the exact path a packet must traverse in route to its destination.The IP options list contains one or more options that perform various network management or debugging tasks. The first field of each option in the list consists of an eight bit code field that is broken into three subfields:COPY (Bit 0):Specifies to routers if the option information should be included in fragment headers. CLASS (Bits 1-2): Specifies 1 of 2 valid option classes: Network Control, DebuggingNUMBER (Bits 3-7): Specifies one of eight valid IP options. Option zero indicates end of list.This option may be misused to defeat authentication mechanisms that rely on IP addresses as their basis for trust relationships. The limited number of routes that may be stored in the options field minimize the usefulness of this option as a mode of attack across large internets.
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While network troubleshooting may require the legitimate use of this feature, this type of traffic is rarely if ever noted and should comprise much less than 1% of network traffic. Small amounts of source routed traffic most probably indicates that a network problem is being investigated. Large amounts of source routed traffic is more suspicious and a thorough investigation of the source and reason is indicated.
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