Cisco Systems CL-28826-01 Add/Edit General Security Configuration Dialog Box

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User Guide for Cisco Security Manager 4.4

OL-28826-01

Chapter55 Configuring Security Policies on Firewall Devices

General Page

Anti-spoofing

Unicast Reverse Path Forwarding (RPF) guards against IP spoofing—a packet using 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 looks only 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.

With 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, 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 look-up. 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.

Fragment Settings

Fragment settings provide management of packet fragmentation and improve compatibility with the

Network File System (NFS). By default, the security appliance allows up to 24 fragments per IP packet,

and up to 200 fragments awaiting reassembly. You might need to allow fragments on your network if you

have an application that routinely fragments packets, such as NFS over UDP. However, if you do not have

an application that fragments traffic, we recommend that you do not allow fragments through the security

appliance, as fragmented packets are often used as DoS attacks.