Contents

Configuring uRPF· 1

About uRPF· 1

uRPF application scenario· 1

uRPF check modes· 1

Network application· 1

Restrictions and guidelines: uRPF configuration· 2

Enabling uRPF globally· 2

Enabling uRPF on an interface· 2

Display and maintenance commands for uRPF· 3

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.

Network application

As shown in Figure 2, strict uRPF check is configured between an ISP network and a customer network. Loose uRPF check is configured between ISPs.

Figure 2 Network diagram

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Restrictions and guidelines: uRPF configuration

If you configure uRPF globally and on an interface, the interface preferentially uses the interface-specific settings.

Do not configure both strict uRPF check and loose IPv6 uRPF check, or loose uRPF check and strict IPv6 uRPF check on an interface.

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 globally

Restrictions and guidelines

Global uRPF takes effect on all interfaces of the device.

Procedure

1.     Enter system view.

system-view

2.     Enable uRPF globally.

ip urpf { loose | strict }

By default, uRPF is disabled.

Enabling uRPF on an interface

1.     Enter system view.

system-view

2.     Enter Layer 3 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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