Rule:  

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Sid:
502

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Summary:
This event is generated when an IPv4 packet set the strict source record
route IP option.

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Impact:
Information could be gathered about network topology, and machines
routing packets onto trusted links could be abused.

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Detailed Information:
Strict source record routing specifies a series of machines which must 
be exclusively used in the routing of a datagram.  This can be useful to
map out routes ala the traceroute program by adding discovered 
intermediary routers one at a time.  Furthermore, while a machine may 
normally be unreachable due to default gateways, a compliant router can 
be forced to hand off source routed packets to an intermediary capable 
of speaking both to the outside world and target machines; the packet 
may then be forwarded on to its destination.

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Affected Systems:
Any machine fully implementing RFC 791 set up as a router.

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Attack Scenarios:
By incrementing the TTL of successive packets, the topology of routes to
a host can be determined.  Each compliant node along the way will reply
with an ICMP Time Exceeded bearing their address and the recorded route.

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Ease of Attack:
Tools are readily available to employ source routing for the purpose of
network discovery; the bounce attack described is unlikely to surface in
a properly configured network.

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False Positives:
None

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False Negatives:
Network discovery can be done using other means than source routing.

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Corrective Action:
Redesign network topologies so that routers are kept to a minimum;
disable routing by other machines.  To prevent network mapping, don't
allow source-routed packets at all. 

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Contributors:
Snort documentation contributed by by Nick Black, Reflex Security <dank@reflexsecurity.com>
Original Rule Writer Unknown
Sourcefire Research Team
Nigel Houghton <nigel.houghton@sourcefire.com>

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Additional References:

IP RFC:
www.faqs.org/rfcs/rfc791.html

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