Set Security Configuration Parameters

This section describes how to prevent IP spoofing, allow full fragment reassembly, and override the default fragment setting set for at the device level in Platform Settings .

Anti-Spoofing

This section 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 threat defense device only looks at the destination address when determining where to forward the packet. Unicast RPF instructs the device 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 threat defense device, the device routing table must include a route back to the source address. See RFC 2267 for more information.

For outside traffic, for example, the threat defense device 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 device 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 threat defense device drops the packet. Similarly, if traffic enters the inside interface from an unknown source address, the device 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.

Fragment per Packet

By default, the threat defense device allows up to 24 fragments per IP packet, and up to 200 fragments awaiting reassembly. You might need to let 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 threat defense device. Fragmented packets are often used as DoS attacks.

Fragment Reassembly

The threat defense device performs the following fragment reassembly processes:

• IP fragments are collected until a fragment set is formed or until a timeout interval has elapsed.

• If a fragment set is formed, integrity checks are performed on the set. These checks include no overlapping, no tail overflow, and no chain overflow.

• IP fragments that terminate at the threat defense device are always fully reassembled.

• If Full Fragment Reassembly is disabled (the default), the fragment set is forwarded to the transport layer for further processing.

• If Full Fragment Reassembly is enabled, the fragment set is first coalesced into a single IP packet. The single IP packet is then forwarded to the transport layer for further processing.