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27-20
Cisco ASDM User Guide
OL-16647-01
Chapter 27 Configuring Advanced Firewall Protection
Configuring Anti-Spoofing
Configuring Anti-Spoofing
The Anti-Spoofing window lets you enable Unicast Reverse Path Forwarding on an interface. Unicast
RPF guards against IP spoofing (a packet uses an incorrect source IP address to obscure its true source)
by ensuring that all packets have a source IP address that matches the correct source interface according
to the routing table.
Normally, the security appliance only looks at the destination address when determining where to
forward the packet. Unicast RPF instructs the security appliance to also look at the source address; this
is why it is called Reverse Path Forwarding. For any traffic that you want to allow through the security
appliance, the security appliance routing table must include a route back to the source address. See
RFC 2267 for more information.
For outside traffic, for example, the security appliance can use the default route to satisfy the
Unicast RPF protection. If traffic enters from an outside interface, and the source address is not known
to the routing table, the security appliance uses the default route to correctly identify the outside
interface as the source interface.
If traffic enters the outside interface from an address that is known to the routing table, but is associated
with the inside interface, then the security appliance drops the packet. Similarly, if traffic enters the
inside interface from an unknown source address, the security appliance drops the packet because the
matching route (the default route) indicates the outside interface.
Unicast RPF is implemented as follows:
• ICMP packets have no session, so each packet is checked.
• UDP and TCP have sessions, so the initial packet requires a reverse route lookup. Subsequent
packets arriving during the session are checked using an existing state maintained as part of the
session. Non-initial packets are checked to ensure they arrived on the same interface used by the
initial packet.
Fields
• Interface—Lists the interface names.
• Anti-Spoofing Enabled—Shows whether an interface has Unicast RPF enabled, Yes or No.
• Enable—Enables Unicast RPF for the selected interface.
• Disable—Disables Unicast RPF for the selected interface.
Modes
The following table shows the modes in which this feature is available:
Configuring TCP Options
The TCP Options pane lets you set parameters for TCP connections.
Firewall Mode Security Context
Routed Transparent Single
Multiple
Context System
• — • •—