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The Unicast Reverse
Path Forwarding feature limits the malicious traffic on a network. This feature
enables devices to verify the reachability of the source address in packets
that are being forwarded and limit the appearance of spoofed or malformed
addresses on a network. If the source IP address is not valid, Unicast Reverse
Path Forwarding (RPF) discards the packet.
This module describes
the Unicast Reverse Path Forwarding feature.
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information,
see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module,
and to see a list of the releases in which each feature is supported, see the feature information table.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature
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Prerequisites for Unicast
Reverse Path Forwarding
Unicast Reverse
Path Forwarding (RPF) requires Cisco Express Forwarding to function properly on
a device.
Prior to
configuring Unicast RPF, you must configure the following access control lists
(ACLs):
Configure standard or
extended ACL to mitigate the transmission of invalid IP addresses (by
performing egress filtering). Configuring standard or extended ACLs permit only
valid source addresses to leave your network and enter the Internet.
Configure standard or
extended ACL entries to drop (deny) packets that have invalid source IP
addresses (by performing ingress filtering). Invalid source IP addresses
include the following types:
Private addresses (RFC 1918,
Address
Allocation for Private Internets)
Reserved addresses
Source addresses that fall
outside the range of valid addresses that are associated with the protected
network
Restrictions for Unicast
Reverse Path Forwarding
Unicast RPF does
not support access control list (ACL) templates.
The following basic
restrictions apply to multihomed clients:
Clients should
not be multihomed on the same device because multihoming defeats the purpose of
creating a redundant service for a client.
Ensure that
packets that flow up the link (out to the Internet) match the route advertised
out of the link. Otherwise, Unicast RPF filters these packets as malformed
packets.
Information About Unicast Reverse Path Forwarding
Overview of Unicast Reverse Path Forwarding
The Unicast Reverse Path Forwarding feature helps to mitigate problems that are caused by the introduction of malformed or
forged (spoofed) IP source addresses into a network by discarding IP packets that lack verifiable IP source addresses. For
example, a number of common types of denial-of-service (DoS) attacks, including Smurf and Tribal Flood Network (TFN), can
take advantage of forged or rapidly changing source IP addresses to allow attackers to thwart efforts to locate or filter
these attacks. For ISPs that provide public access, Unicast RPF deflects such attacks by forwarding only packets that have
source addresses that are valid and consistent with the IP routing table, thereby protecting the network of the ISP, ISP customers,
and the Internet.
Unicast RPF
Operation
When Unicast RPF is
enabled on an interface of a device, the device examines all packets received
as input on that interface to ensure that the source address and source
interface information appears in the routing table and matches the interface on
which packets are received. This ability to “look backwards” is available only
when Cisco Express Forwarding is enabled on a device because the lookup relies
on the presence of a Forwarding Information Base (FIB). Cisco Express
Forwarding generates a FIB as part of its operation.
Note
Unicast RPF is an
input function and is applied only on the input interface of a device at the
upstream end of a connection.
Unicast RPF does a
reverse lookup in the Cisco Express Forwarding table to check if any packet
received at the interface of a device arrives on the best return path (or
return route) to the source of the packet. If the packet was received from one
of the best reverse path routes, the packet is forwarded as normal. No reverse
path route on the interface from which the packet was received can mean that
the source address was modified.
Note
With Unicast RPF,
all equal-cost “best” return paths are considered valid. Unicast RPF supports
multiple return paths, provided that each path is equal to the others in terms
of the routing cost (such as number of hops, weights, and so on) and the route
is available in the FIB. Unicast RPF also functions where Enhanced Interior
Gateway Routing Protocol (EIGRP) variants are used.
Before forwarding a
packet that is received at the interface on which Unicast RPF and ACLs have
been configured, Unicast RPF does the following checks:
If input ACLs
are configured on the inbound interface.
If the packet
has arrived on the best return path to the source by doing a reverse lookup in
the FIB table.
Does a lookup of
the Cisco Express Forwarding table for packet forwarding.
Checks output
ACLs on the outbound interface.
Forwards the
packet.
Access Control Lists and
Logging
When you configure an
access control list (ACL) and a packet fails the Unicast RPF check, the Unicast
RPF checks the ACL to see if the packet should be dropped (by using a deny
statement in the ACL) or forwarded (by using a permit statement in the ACL).
Regardless of whether the packet is dropped or forwarded, the packet is counted
in the global IP traffic statistics for Unicast RPF drops and in the interface
statistics for Unicast RPF.
If no ACL is
configured, the device drops the forged or malformed packet immediately, and no
ACL logging occurs. The device and the interface Unicast RPF logging counters
are updated.
To log Unicast RPF
events, specify the logging option for ACL entries. Using the log information,
administrators can view source addresses that are used in an attack, the time
at which packets arrived at an interface, and so on.
Caution
Logging requires CPU and memory resources. Logging Unicast RPF
events for attacks that have a high rate of forged packets can degrade the
performance of a device.
Per-Interface
Statistics
Each time a packet is
dropped or forwarded at an interface, that information is counted two ways:
globally on the router and at each interface where you have applied Unicast
RPF. Global statistics on dropped packets provide information about potential
attacks on the network; however, these global statistics do not help to specify
which interface is the source of the attack.
Per-interface
statistics allow network administrators to track two types of information about
malformed packets: Unicast RPF drops and Unicast RPF suppressed drops.
Statistics on the number of packets that Unicast RPF drops help to identify the
interface that is the entry point of the attack. The Unicast RPF drop count
tracks the number of drops at the interface. The Unicast RPF suppressed drop
count tracks the number of packets that failed the Unicast RPF check but were
forwarded because of the permit permission set up in the ACL. Using the drop
count and suppressed drop count statistics, a network administrator can takes
steps to isolate the attack at a specific interface.
Note
Judicious use of ACL logging can further identify the address or
addresses that are being dropped by Unicast RPF.
The figure below
illustrates how Unicast RPF and CEF work together to validate IP source
addresses by verifying packet return paths. In this example, a customer has
sent a packet having a source address of 192.168.1.1 from interface FDDI 2/0/0.
Unicast RPF checks the FIB to see if 192.168.1.1 has a path to FDDI 2/0/0. If
there is a matching path, the packet is forwarded. If there is no matching
path, the packet is dropped.
Figure 1. Unicast RPF Validating IP Source Addresses
The figure below
illustrates how Unicast RPF drops packets that fail validation. In this
example, a customer has sent a packet having a source address of
209.165.200.225, which is received at interface FDDI 2/0/0. Unicast RPF checks
the FIB to see if 209.165.200.225 has a return path to FDDI 2/0/0. If there is
a matching path, the packet is forwarded. In this case, there is no reverse
entry in the routing table that routes the customer packet back to source
address 209.165.200.225 on interface FDDI 2/0/0, and so the packet is dropped.
Figure 2. Unicast RPF Dropping Packets That Fail Verification
Rules for Implementing
Unicast RPF
The following rules
apply when implementing Unicast Reverse Path Forwarding (RPF):
Packets must be
received at an interface that has the best return path (route) to the packets’
source. This process is called symmetric routing. A route in the Forwarding
Information Base (FIB) must match the route to the receiving interface. Add a
route in the FIB through dynamic or static routing or by using a network
statement.
IP source
addresses at the receiving interface must match the routing entry for the
interface.
Unicast RPF is an
input function and can be applied at the input interface of a device at the
upstream end of a connection.
Network
administrators can use Unicast RPF for their customers and also for their
downstream network or ISP, even if the downstream network or ISP has other
connections to the Internet.
Caution
Using optional
Border Gateway Protocol (BGP) attributes, such as weight and local preference,
the best path back to source addresses can be modified. The best path
modification will affect the operation of Unicast RPF.
The following
sections provides information about the implementation of Unicast RPF:
Security Policy and Unicast RPF
When determining how to deploy Unicast Reverse Path Forwarding (RPF), consider the following points:
Apply Unicast RPF at the downstream interface, away from the larger portion of the network, preferably at the edges of your
network. The further you apply Unicast RPF, the finer the granularity you have in mitigating address spoofing and in identifying
sources of spoofed addresses. For example, applying Unicast RPF on an aggregation device helps to mitigate attacks from many
downstream networks or clients and is simple to administer, but Unicast RPF does not help in identifying the source of the
attack. Applying Unicast RPF at the network access server helps to limit the scope of the attack and trace the source of the
attack. However, deploying Unicast RPF across many sites adds to the administration cost of operating a network.
When you deploy Unicast RPF on many entities on a network (for example, across the Internet, intranet, and extranet resources),
you have better chances of mitigating large-scale network disruptions throughout the Internet community, and of tracing the
source of an attack.
Unicast RPF does not inspect IP packets that are encapsulated in tunnels, such as the generic routing encapsulation (GRE),
Layer 2 Tunneling Protocol (L2TP), or Point-to-Point Tunneling Protocol (PPTP). Configure Unicast RPF on a home gateway so
that Unicast RPF processes network traffic only after tunneling and encryption layers are stripped off from the packets.
Ingress and Egress Filtering Policy for Unicast RPF
Unicast Reverse Path Forwarding (RPF) can be more effective at mitigating spoofing attacks when combined with a policy of
ingress and egress filtering by using access control lists (ACLs).
Ingress filtering applies filters to traffic that is received at a network interface from either internal or external networks.
With ingress filtering, packets that arrive from other networks or the Internet and that have a source address that matches
a local network or private or broadcast addresses are dropped. For example, in ISP environments, ingress filtering can be
applied to traffic that is received at a device from either a client (customer) or the Internet.
Egress filtering applies filters to the traffic that exits a network interface (the sending interface). By filtering packets
on devices that connect your network to the Internet or to other networks, you can permit only packets with valid source IP
addresses to leave your network.
For more information on network filtering, refer to RFC 2267,
Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing.
Where to Use Unicast RPF
Unicast Reverse Path Forwarding (RPF) can be used in any “single-homed” environment where there is essentially only one access
point out of the network, which means that there is only one upstream connection to the network. Networks having one access
point offer the best example of symmetric routing, which means that the interface where a packet enters the network is also
the best return path to the source of the IP packet. Unicast RPF is best used at the network perimeter for Internet, intranet,
or extranet environments, or in ISP environments for customer network terminations.
The following sections describe two sample network environments in which Unicast RPF is implemented:
Enterprise Networks with a
Single Connection to an ISP
In enterprise
networks, you can use Unicast Reverse Path Forwarding (RPF) to filter traffic
at the input interface (a process called ingress filtering) to protect from
malformed packets that arrive from the Internet. Traditionally, local networks
that have one connection to the Internet use access control lists (ACLs) at the
receiving interface to prevent spoofed packets from entering their local
network.
ACLs work well for
single-homed customers. However, when ACLs are used as ingress filters, the
following two commonly referenced limitations apply:
Packet-per-second
(PPS) performance at very high packet rates
ACL maintenance
(whenever there are new addresses added to the network)
Unicast RPF addresses
both the limitations described above. With Unicast RPF, ingress filtering is
done at Cisco Express Forwarding PPS rates. Because Unicast RPF uses the
Forwarding Information Base (FIB), ACL maintenance is not required, and thus,
the administration overhead of traditional ACLs is reduced. The following
figure and example demonstrate how Unicast RPF is configured for ingress
filtering.
The figure below
illustrates an enterprise network that has a single link to an upstream ISP. In
this example, Unicast RPF is applied at GigabitEthernet interface 1/0/2 on the
enterprise device for protection from malformed packets arriving from the
Internet. Unicast RPF is also applied at GigabitEthernet interface 1/0/2 on the
ISP device for protection from malformed packets arriving from the enterprise
network.
Figure 3. Enterprise Network Using
Unicast RPF for Ingress Filtering
A typical
configuration on an ISP device that uses the topography in the figure above
would be as follows:
ip cef
interface loopback 0
description Loopback interface on Gateway Device 2
ip address 192.168.3.1 255.255.255.255
no ip redirects
no ip directed-broadcast
no ip proxy-arp
!
interface GigabitEthernet 1/0/2
description 128K HDLC link to ExampleCorp WT50314E R5-0
bandwidth 128
ip unnumbered loopback 0
no ip redirects
no ip directed-broadcast
no ip proxy-arp
!
ip route 192.168.10.0 255.255.252.0 GigabitEthernet 1/0/2
The gateway device
configuration of the enterprise network will be similar to the following:
ip cef
interface FastEthernet 0/0/0
description ExampleCorp LAN
ip address 192.168.10.1 255.255.252.0
no ip redirects
no ip directed-broadcast
no ip proxy-arp
!
interface GigabitEthernet 1/0/2
description 128K HDLC link to ExampleCorp Internet Inc WT50314E C0
bandwidth 128
ip unnumbered FastEthernet 0/0/0
no ip redirects
no ip directed-broadcast
no ip proxy-arp
!
ip route 0.0.0.0 0.0.0.0 GigabitEthernet 1/0/2
Notice that Unicast
RPF works with a single default route. There are no additional routes or
routing protocols. Network 192.168.10.0/22 is a connected network. Hence,
packets coming from the Internet with a source address in the network
192.168.10.0/22 will be dropped by Unicast RPF.
Applying Unicast RPF to Network Access Servers
If a network access server supports Cisco Express Forwarding, Unicast RPF will work on that network. A network access server
(NAS) allows users to access a network by checking the credentials of the users accessing the network. Aggregation devices
support Unicast RPF with single-homed clients. Unicast RPF works well on leased lines or on a digital subscriber line (DSL),
ISDN, or public switched telephone network (PSTN) customer connections that are connected to the Internet. Dialup connections
are a big source of denial of service (Dos) attacks that use forged IP addresses.
Aggregation devices need routing prefixes information (IP address block) for routing traffic. In the topology described below,
aggregation devices do not have a full Internet routing table, and as a result, Unicast RPF uses the information configured
or redistributed by the Interior Gateway Protocol (IGP) or Internal Border Gateway Protocol (IBGP) (based on how customer
routes are added to the network) to route traffic. Unicast RPF is applied upstream on the customer dialup connection device
that is on the receiving (input) interfaces of ISP aggregation devices.
The figure below illustrates how Unicast RPF is applied to aggregation and access devices for an ISP or point of presence
(PoP) with ISP devices providing dialup connections.
Figure 4. Unicast RPF Applied to PSTN/ISDN Customer Connections
Routing Table Requirements
Unicast Reverse Path Forwarding (RPF) uses the routing information in Cisco Express Forwarding tables for routing traffic.
The amount of routing information that must be available in Cisco Express Forwarding tables depends on the device where Unicast
RPF is configured and the functions the device performs in the network. For example, in an ISP environment where a device
is a leased-line aggregation device for customers, the information about static routes that are redistributed into the Interior
Gateway Protocol (IGP) or Internal Border Gateway Protocol (IBGP) (depending on which technique is used in the network) is
required in the routing table. Because Unicast RPF is configured on customer interfaces, only minimal routing information
is required. If a single-homed ISP configures Unicast RPF on the gateway to the Internet, the full Internet routing table
information is required by Unicast RPF to help protect the ISP from external denial of service (DoS) attacks that use addresses
that are not in the Internet routing table.
Where Not to Use Unicast RPF
Do not use Unicast Reverse Path Forwarding (RPF) on interfaces that are internal to a network. Internal interfaces are likely
to have routing asymmetry (see the figure below), which means that there can be multiple routes to the source of a packet.
Unicast RPF is applied only where there is a natural or configured symmetry.
For example, devices at the edge of an ISP network are more likely to have symmetrical reverse paths than devices that are
in the core of an ISP network. The best forwarding path to forward packets from devices that are at the core of an ISP network
may not be the best forwarding path that is selected for packets that are returned to the device.
We recommend that you do not apply Unicast RPF where there is a chance of asymmetric routing, unless you configure access
control lists (ACLs) to allow the device to accept incoming packets. ACLs permit the use of Unicast RPF when packets arrive
through specific, less-optimal asymmetric input paths.
The figure below illustrates how Unicast RPF can block legitimate traffic in an asymmetric routing environment.
Figure 5. Unicast RPF Blocking Legitimate Traffic in an Asymmetric Routing Environment
Unicast RPF with BOOTP and DHCP
Unicast RPF allows packets with 0.0.0.0 as the source IP address and 255.255.255.255 as the destination IP address to pass
through a network to enable Bootstrap Protocol (BOOTP) and DHCP functions to work properly when Unicast RPF is configured.
How to Configure Unicast Reverse Path Forwarding
Configuring Unicast
RPF
Before you begin
To use Unicast
Reverse Path Forwarding, you must configure a device for Cisco Express
Forwarding switching or distributed Cisco Express Forwarding switching. If
Cisco Express Forwarding is not enabled globally on a device, Unicast RPF will
not work on that device. If Cisco Express Forwarding is running on a device,
individual interfaces on the device can be configured with other switching
modes. Unicast RPF is an input-side function that is enabled on an interface or
subinterface that supports any type of encapsulation, and Unicast RPF operates
on IP packets that are received by the device.
SUMMARY STEPS
enable
configure
terminal
ip cef
distributed
interface slot/ subslot/ port
exit
end
show cef interface
[type
number]
DETAILED STEPS
Command or Action
Purpose
Step 1
enable
Example:
Device> enable
Enables
privileged EXEC mode.
Enter your
password if prompted.
Step 2
configure
terminal
Example:
Device# configure terminal
Enters global
configuration mode.
Step 3
ip cef
distributed
Example:
Device(config)# ip cef distributed
Enables Cisco
Express Forwarding or distributed Cisco Express Forwarding on a device.
Step 4
interface slot/ subslot/ port
Example:
Device(config)# interface GigabitEthernet 0/0
Selects the
input interface on which you want to apply Unicast Reverse Path Forwarding and
enters interface configuration mode.
The
interface that is configured is the receiving interface, which allows Unicast
RPF to verify the best return path before forwarding a packet to the next
destination.
Step 5
exit
Example:
Device(config-if)# exit
Exits interface
configuration mode.
Step 6
end
Example:
Device(config-if)# end
Exits interface
configuration mode and enters privileged EXEC mode.
Step 7
show cef interface
[type
number]
Example:
Device# show cef interface
Displays
detailed Cisco Express Forwarding information for a specified interface or for
all interfaces.
Example:
Troubleshooting Tips
HSRP Failure
The failure to
disable Unicast RPF before disabling Cisco Express Forwarding can cause a Hot
Standby Router Protocol (HSRP) failure. If you want to disable Cisco Express
Forwarding on a device, you must first disable Unicast RPF.
Configuration Examples for Unicast Reverse Path Forwarding
Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing
Technical Assistance
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Link
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Feature Information for
Unicast Reverse Path Forwarding
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for
Unicast Reverse Path Forwarding
Feature
Name
Releases
Feature
Information
Unicast
Reverse Path Forwarding
Cisco IOS
XE Release 2.1
The Unicast
Reverse Path Forwarding feature limits the malicious traffic on a network. This
feature enables devices to verify the reachability of the source address in
packets that are being forwarded and limit the appearance of spoofed or
malformed addresses on a network. If the source IP address is not valid,
Unicast Reverse Path Forwarding (RPF) discards the packet.
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