Contents

Configuring uRPF· 1

About uRPF· 1

uRPF application scenario· 1

uRPF check modes· 1

uRPF operation· 1

Network application· 3

Restrictions and guidelines: uRPF configuration· 3

Enabling uRPF on an interface· 3

Display and maintenance commands for uRPF· 4

Configuring uRPF

About uRPF

Unicast Reverse Path Forwarding (uRPF) protects a network against source address spoofing attacks, such as DoS and DDoS attacks.

uRPF application scenario

Attackers send packets with a forged source address to access a system that uses IPv4-based authentication, in the name of authorized users or even the administrator. Even if the attackers or other hosts cannot receive any response packets, the attacks are still disruptive to the attacked target.

Figure 1 Source address spoofing attack

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As shown in Figure 1, an attacker on Router A sends the server (Router B) requests with a forged source IP address 2.2.2.1 at a high rate. Router B sends response packets to IP address 2.2.2.1 (Router C). Consequently, both Router B and Router C are attacked. If the administrator disconnects Router C by mistake, the network service is interrupted.

Attackers can also send packets with different forged source addresses or attack multiple servers simultaneously to block connections or even break down the network.

uRPF can prevent these source address spoofing attacks. It checks whether an interface that receives a packet is the output interface of the FIB entry that matches the source address of the packet. If not, uRPF considers it a spoofing attack and discards the packet.

uRPF check modes

uRPF supports strict and loose modes.

Strict uRPF check

To pass strict uRPF check, the source address of a packet and the receiving interface must match the destination address and output interface of a FIB entry. In some scenarios (for example, asymmetrical routing), strict uRPF might discard valid packets.

Strict uRPF is often deployed between a PE and a CE.

Loose uRPF check

To pass loose uRPF check, the source address of a packet must match the destination address of a FIB entry. Loose uRPF can avoid discarding valid packets, but might let go attack packets.

Loose uRPF is often deployed between ISPs, especially in asymmetrical routing.

uRPF operation

Figure 2 shows how uRPF works.

Figure 2 uRPF work flow

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1.     uRPF checks address validity:

¡     uRPF permits a packet with a multicast destination address.

¡     For a packet with an all-zero source address, uRPF permits the packet if it has a broadcast destination address. (A packet with source address 0.0.0.0 and destination address 255.255.255.255 might be a DHCP or BOOTP packet and cannot be discarded.) The packet is dropped if it does not have a broadcast destination address.

¡     uRPF proceeds to step 2 for other packets.

2.     uRPF checks whether the source address matches a unicast route:

¡     If yes, uRPF proceeds to step 3.

¡     If no, the packet is dropped. A non-unicast source address matches a non-unicast route.

3.     uRPF checks whether the matching route is to the host itself:

¡     If yes, the output interface of the matching route is an InLoop interface. uRPF checks whether the receiving interface of the packet is an InLoop interface. If yes, it does not check the packet. If no, the packet is dropped.

¡     If no, uRPF proceeds to step 4.

4.     uRPF checks whether the receiving interface matches the output interface of the matching FIB entry:

¡     If yes, the packet passes the check.

¡     If no, uRPF checks whether the check mode is loose. If yes, the packet passes the check. If no, the packet is dropped.

Network application

As shown in Figure 3, strict uRPF check is configured between an ISP network and a customer network. Loose uRPF check is configured between ISPs.

Figure 3 Network diagram

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Restrictions and guidelines: uRPF configuration

If you enable uRPF on an interface, you can use the display ip interface command to display statistics about packets discarded by uRPF (displayed as "Drops" and "Suppressed drops").

Do not use strict uRPF if ECMP routing is available in the network. Service packets that travel along ECMP routes cannot pass the strict uRPF check and will be dropped.

Enabling uRPF on an interface

1.     Enter system view.

system-view

2.     Enter interface view.

interface interface-type interface-number

3.     Enable uRPF.

ip urpf { loose | strict }

By default, uRPF is disabled.

Display and maintenance commands for uRPF

Execute display commands in any view.

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