Rule:  

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Sid:
500
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Summary:
This event is generated when an IPv4 packet has the loose source record 
route IP option set.
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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:
Loose source record routing specifies a series of machines which must be
used in the routing of a datagram.  This can be useful to map out routes
using 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 known.
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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:
Original Rule Writer Unknown
Sourcefire Research Team
Nigel Houghton <nigel.houghton@sourcefire.com>
Snort documentation contributed by Nick Black, Reflex Security <dank@reflexsecurity.com>
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Additional References:

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

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