Configuring Unicast Reverse Path Forwarding

For a complete description of the Unicast RPF commands in this chapter, refer to the "Unicast Reverse Path Forwarding Commands" chapter of the Cisco IOS Security Command Reference. To locate documentation of other commands that appear in this chapter, use the command reference master index or search online.

In This Chapter

This chapter has the following sections:

Feature Overview

The Unicast RPF 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 a verifiable IP source address. For example, a number of common types of denial-of-service (DoS) attacks, including Smurf and Tribe Flood Network (TFN), can take advantage of forged or rapidly changing source IP addresses to allow attackers to thwart efforts to locate or filter the attacks. For Internet service providers (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. This action protects the network of the ISP, its customer, and the rest of the Internet.

This rest of this section covers the following information:

How Unicast RPF Works

When Unicast RPF is enabled on an interface, the router examines all packets received as input on that interface to make sure that the source address and source interface appear in the routing table and match the interface on which the packet was received. This "look backwards" ability is available only when Cisco express forwarding (CEF) is enabled on the router, because the lookup relies on the presence of the Forwarding Information Base (FIB). CEF generates the FIB as part of its operation.

Note Unicast RPF is an input function and is applied only on the input interface of a router at the upstream end of a connection.

Unicast RPF checks to see if any packet received at a router interface arrives on the best return path (return route) to the source of the packet. Unicast RPF does this by doing a reverse lookup in the CEF table. If the packet was received from one of the best reverse path routes, the packet is forwarded as normal. If there is no reverse path route on the same interface from which the packet was received, it might mean that the source address was modified. If Unicast RPF does not find a reverse path for the packet, the packet is dropped.

Note With Unicast RPF, all equal-cost "best" return paths are considered valid. This means that Unicast RPF works in cases where multiple return paths exist, provided that each path is equal to the others in terms of the routing cost (number of hops, weights, and so on) and as long as the route is in the FIB. Unicast RPF also functions where EIGRP variants are being used and unequal candidate paths back to the source IP address exist.

When a packet is received at the interface where Unicast RPF and access control lists (ACLs) have been configured, the following actions occur:

Step 1 Input ACLs configured on the inbound interface are checked.

Step 2 Unicast RPF checks to see if the packet has arrived on the best return path to the source, which it does by doing a reverse lookup in the FIB table.

Step 3 CEF table (FIB) lookup is carried out for packet forwarding.

Step 4 Output ACLs are checked on the outbound interface.

Step 5 The packet is forwarded.

Figure 40 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 40: Unicast RPF Validating IP Source Addresses

Figure 41 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 41: Unicast RPF Dropping Packets That Fail Verification

Implementing Unicast RPF

Unicast RPF has several key implementation principles:

The packet must be received at an interface that has the best return path (route) to the packet source (a process called symmetric routing). There must be a route in the FIB matching the route to the receiving interface. Adding a route in the FIB can be done via static route, network statement, or dynamic routing.
IP source addresses at the receiving interface must match the routing entry for the interface.
Unicast RPF is an input function and is applied only on the input interface of a router at the upstream end of a connection.

Given these implementation principles, Unicast RPF becomes a tool that network administrators can use not only for their customers but also for their downstream network or ISP, even if the downstream network or ISP has other connections to the Internet.

Caution Using optional BGP attributes such as weight, local preference, and so on, the best path back to the source address can be modified, which would affect the operation of Unicast RPF.

This section provides information about the implementation of Unicast RPF:

Security Policy and Unicast RPF

Consider the following points in determining your policy for deploying Unicast RPF:

Unicast RPF must be applied at the interface downstream from the larger portion of the network, preferably at the edges of your network.
The further downstream you apply Unicast RPF, the finer the granularity you have in mitigating address spoofing and in identifying the sources of spoofed addresses. For example, applying Unicast RPF on an aggregation router helps mitigate attacks from many downstream networks or clients and is simple to administer, but it does not help identify the source of the attack. Applying Unicast RPF at the network access server helps limit the scope of the attack and trace the source of the attack; however, deploying Unicast RPF across many sites does add to the administration cost of operating the network.
The more entities that deploy Unicast RPF across Internet, intranet, and extranet resources, the better the chances of mitigating large-scale network disruptions throughout the Internet community, and the better the chances of tracing the source of an attack.
Unicast RPF will not inspect IP packets encapsulated in tunnels, such as GRE, LT2P, or PPTP. Unicast RPF must be configured at a home gateway so that Unicast RPF processes network traffic only after the tunneling and encryption layers have been stripped off the packets.

Where to Use Unicast RPF

Unicast RPF can be used in any "single-homed" environment where there is essentially only one access point out of the network; that is, one upstream connection. 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 provide a look at implementing Unicast RPF in two network environments:

Enterprise Networks with a Single Connection to an ISP

In enterprise networks, one objective of using Unicast RPF for filtering traffic at the input interface (a process called ingress filtering) is for protection from malformed packets arriving from the Internet. Traditionally, local networks that have one connection to the Internet would use ACLs at the receiving interface to prevent spoofed packets from the Internet from entering their local network.

ACLs work well for many single-homed customers; however, there are trade-offs when ACLs are used as ingress filters, including two commonly referenced limitations:

Packet per second (PPS) performance at very high packet rates
Maintenance of the ACL (whenever there are new addresses added to the network)

Unicast RPF is one tool that addresses both of these limitations. With Unicast RPF, ingress filtering is done at CEF PPS rates. This processing speed makes a difference when the link is more than 1 Mbps. Additionally, since Unicast RPF uses the FIB, no ACL maintenance is necessary, and thus the administration overhead of traditional ACLs is reduced. The following figure and example demonstrate how this is configured.

Figure 42 illustrates an enterprise network that has a single link to an upstream ISP. In this example, Unicast RPF is applied at interface S0 on the enterprise router for protection from malformed packets arriving from the Internet. Unicast RPF is also applied at interface S5/0 on the ISP router for protection from malformed packets arriving from the enterprise network.

Figure 42: Enterprise Network Using Unicast RPF for Ingress Filtering

Using Figure 42, a typical configuration (assuming that CEF is turned on) on the ISP router would be as follows:

ip cef
interface loopback 0
  description Loopback interface on Gateway Router 2
  ip address 192.168.3.1 255.255.255.255
  no ip redirects
  no ip directed-broadcast
  no ip proxy-arp
interface Serial 5/0
  description 128K HDLC link to ExampleCorp WT50314E  R5-0
  bandwidth 128
  ip unnumbered loopback 0
  ip verify unicast reverse-path
  no ip redirects
  no ip directed-broadcast
  no ip proxy-arp
ip route 192.168.10.0 255.255.252.0 Serial 5/0

The gateway router configuration of the enterprise network (assuming that CEF is turned on) would look similar to the following:

ip cef
interface Ethernet 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 Serial 0
 description 128K HDLC link to ExampleCorp Internet Inc WT50314E  C0
 bandwidth 128
 ip unnumbered ethernet 0
 ip verify unicast reverse-path
 no ip redirects
 no ip directed-broadcast
 no ip proxy-arp
ip route 0.0.0.0 0.0.0.0 Serial 0

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 range 192.168.10.0/22 will be dropped by Unicast RPF.

Network Access Server Application (Applying Unicast RPF in PSTN/ISDN PoP Aggregation Routers)

Aggregation routers are ideal places to use Unicast RPF with single-homed clients. Unicast RPF works equally well on leased-line or PSTN/ISDN/xDSL customer connections into the Internet. In fact, dialup connections are reputed to be the greatest source of DoS attacks using forged IP addresses. As long as the network access server supports CEF, Unicast RPF will work. In this topology, the customer aggregation routers need not have the full Internet routing table. Aggregation routers need the routing prefixes information (IP address block); hence, information configured or redistributed in the Interior Gateway Protocol (IGP) or Internal Border Gateway Protocol (IBGP) (depending on the way that you add customer routes into your network) would be enough for Unicast RPF to do its job.

Figure 43 illustrates the application of Unicast RPF to the aggregation and access routers for an Internet service provider (ISP) point-of-presence (POP), with the ISP routers providing dialup customer connections. In this example, Unicast RPF is applied upstream from the customer dialup connection router on the receiving (input) interfaces of the ISP aggregation routers.

Figure 43: Unicast RPF Applied to PSTN/ISDN Customer Connections

Routing Table Requirements

To work correctly, Unicast RPF needs proper information in the CEF tables. This requirement does not mean that the router must have the entire Internet routing table. The amount of routing information needed in the CEF tables depends on where Unicast RPF is configured and what functions the router performs in the network. For example, in an ISP environment, a router that is a leased-line aggregation router for customers needs only the information based on the static routes redistributed into the IGP or IBGP (depending on which technique is used in the network). Unicast RPF would be configured on the customer interfaces---hence the requirement for minimal routing information. In another scenario, a single-homed ISP could place Unicast RPF on the gateway link to the Internet. The full Internet routing table would be required. Requiring the full routing table would help protect the ISP from external DoS attacks that use addresses that are not in the Internet routing table.

Where Not to Use Unicast RPF

Unicast RPF should not be used on interfaces that are internal to the network. Internal interfaces are likely to have routing asymmetry (see Figure 44), meaning multiple routes to the source of a packet. Unicast RPF should be applied only where there is natural or configured symmetry. As long as administrators carefully plan which interfaces they activate Unicast RPF on, routing asymmetry is not a serious problem.

For example, routers at the edge of the network of an ISP are more likely to have symmetrical reverse paths than routers that are in the core of the ISP network. Routers that are in the core of the ISP network have no guarantee that the best forwarding path out of the router will be the path selected for packets returning to the router. Hence, it is not recommended that you apply Unicast RPF where there is a chance of asymmetric routing. It is simplest to place Unicast RPF only at the edge of a network or, for an ISP, at the customer edge of the network.

Figure 44 illustrates how Unicast RPF can block legitimate traffic in an asymmetrical routing environment.

Figure 44: Unicast RPF Blocking Traffic in an Asymmetrical Routing Environment

Unicast RPF with BOOTP and DHCP

Unicast RPF will allow packets with 0.0.0.0 source and 255.255.255.255 destination to pass so that Bootstrap Protocol (BOOTP) and Dynamic Host Configuration Protocol (DHCP) functions work properly. This enhancement was added in Cisco IOS Release 12.0 and later, but it is not in Cisco IOS Release 11.1CC.

Restrictions

There are some basic restrictions to applying Unicast RPF to multihomed clients:

Clients should not be multihomed to the same router because multihoming defeats the purpose of building a redundant service for the client.
Customers must ensure that the packets flowing up the link (out to the Internet) match the route advertised out the link. Otherwise, Unicast RPF filters those packets as malformed packets.
Unicast RPF is available only for platform images that support CEF. Unicast RPF is supported in Cisco IOS Releases 11.1(17)CC and 12.0 and later. It is not available in Cisco IOS Releases 11.2 or 11.3.

Related Features and Technologies

For more information about Unicast RPF related features and technologies, review the following:

Unicast RPF requires Cisco express forwarding (CEF) to function properly on the router. For more information about CEF, refer to the Cisco IOS Switching Services Configuration Guide.
Unicast RPF can be more effective at mitigating spoofing attacks when combined with a policy of ingress and egress filtering using Cisco IOS access control lists (ACLs).
- Ingress filtering applies filters to traffic 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, private, or broadcast address are dropped. In ISP environments, for example, ingress filtering can apply to traffic received at the router from either the client (customer) or the Internet.
- Egress filtering applies filters to traffic exiting a network interface (the sending interface). By filtering packets on routers 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 and to the Cisco IOS IP and IP Routing Configuration Guide.

Cisco IOS software provides additional features that can help mitigate DoS attacks:
- Committed Access Rate (CAR). CAR allows you to enforce a bandwidth policy against network traffic that matches an access list. For example, CAR allows you to rate-limit what should be low-volume traffic, such as ICMP traffic. To find out more about CAR, refer to the Cisco IOS Quality of Service Solutions Configuration Guide.
- Context-based Access Control (CBAC). CBAC selectively blocks any network traffic not originated by a protected network. CBAC uses timeout and threshold values to manage session state information, helping to determine when to drop sessions that do not become fully established. Setting timeout values for network sessions helps mitigate DoS attacks by freeing up system resources, dropping sessions after a specified amount of time. For more information on CBAC, refer to the Cisco IOS Security Configuration Guide.
- TCP Intercept. The TCP Intercept feature implements software to protect TCP servers from TCP SYN-flooding attacks, which are a type of DoS attack. A SYN-flooding attack occurs when a hacker floods a server with a barrage of requests for connection. Like CBAC, the TCP Intercept feature also uses timeout and threshold values to manage session state information, helping to determine when to drop sessions that do not become fully established. For more information on TCP Intercept, refer to the Cisco IOS Security Configuration Guide.

Prerequisites to Configuring Unicast RPF

Prior to configuring Unicast RPF, configure ACLs:

Configure ACLs to mitigate transmission of invalid IP addresses (perform egress filtering). Permit only valid source addresses to leave your network and get onto the Internet. Prevent all other source addresses from leaving your network for the Internet.
Configure ACLs to prevent (deny) reception of invalid IP addresses (perform ingress filtering). Invalid addresses include the following types:
- Reserved addresses
- Loopback addresses
- Private addresses
- Broadcast addresses
- Source addresses that match any addresses on the protected network (prevents spoofing)

Unicast RPF Configuration Task List

The following sections describe the configuration tasks for Unicast RPF. Each task in the list is identified as either optional or required.

Configuring Unicast RPF (Required)
Verifying Unicast RPF (Optional)

See the end of this chapter for the section "Unicast RPF Configuration Examples."

Configuring Unicast RPF

To use Unicast RPF, you must configure the router for CEF switching or CEF distributed switching. There is no need to configure the input interface for CEF switching because Unicast RPF has been implemented as a search through the FIB using the source IP address. As long as CEF is running on the router, individual interfaces 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 operates on IP packets received by the router. It is very important that CEF be turned on globally in the router---Unicast RPF will not work without CEF.

To configure Unicast RPF, use the following commands beginning in global configuration mode:

Command Purpose

Step 1

Router(config)#ip cef
or

Router(config)#ip cef distributed
Enables CEF or distributed CEF on the router. Distributed CEF is required for routers that use a Route/Switch Processor (RSP) and Versatile Interface Processor (VIP), which includes Unicast RPF.

You might want to disable CEF or distributed CEF (dCEF) on a particular interface if that interface is configured with a feature that CEF or dCEF does not support. In this case, you would enable CEF globally, but disable CEF on a specific interface using the noip route-cache cef interface command, which enables all but that specific interface to use express forwarding. If you have disabled CEF or dCEF operation on an interface and want to reenable it, you can do so by using the iproute-cache cef command in interface configuration mode.

Step2

Router(config-if)#interface type
Selects the input interface on which you want to apply Unicast RPF. This is the receiving interface, which allows Unicast RPF to verify the best return path before forwarding the packet on to the next destination.

The interface type is specific to your router and the types of interface cards installed on the router. To display a list of available interface types, enter the interface ? command.

Step3

Router(config-if)#ip verify unicast reverse-path
Enables Unicast RPF on the interface.

Step4

Router(config-if)#exit
Exits interface configuration mode. Repeat Steps2 and 3 for each interface on which you want to apply Unicast RPF.

	Command	Purpose
Step 1	Router(config)#ip cef or Router(config)#ip cef distributed	Enables CEF or distributed CEF on the router. Distributed CEF is required for routers that use a Route/Switch Processor (RSP) and Versatile Interface Processor (VIP), which includes Unicast RPF. You might want to disable CEF or distributed CEF (dCEF) on a particular interface if that interface is configured with a feature that CEF or dCEF does not support. In this case, you would enable CEF globally, but disable CEF on a specific interface using the noip route-cache cef interface command, which enables all but that specific interface to use express forwarding. If you have disabled CEF or dCEF operation on an interface and want to reenable it, you can do so by using the iproute-cache cef command in interface configuration mode.
Step2	Router(config-if)#interface type	Selects the input interface on which you want to apply Unicast RPF. This is the receiving interface, which allows Unicast RPF to verify the best return path before forwarding the packet on to the next destination. The interface type is specific to your router and the types of interface cards installed on the router. To display a list of available interface types, enter the interface ? command.
Step3	Router(config-if)#ip verify unicast reverse-path	Enables Unicast RPF on the interface.
Step4	Router(config-if)#exit	Exits interface configuration mode. Repeat Steps2 and 3 for each interface on which you want to apply Unicast RPF.

Verifying Unicast RPF

To verify that Unicast RPF is operational, use the show cef interface command. The following example shows that Unicast RPF is enabled at interface serial2/0/0.

Router-3# show cef interface serial 2/0/0
Serial2/0/0 is up (if_number 8)
Internet address is 192.168.10.2/30
ICMP redirects are never sent
Per packet loadbalancing is disabled
IP unicast RPF check is enabled
Inbound access list is not set
Outbound access list is not set
Interface is marked as point to point interface
Packets switched to this interface on linecard are dropped to next slow path
Hardware idb is Serial2/0/0
Fast switching type 4, interface type 6
IP Distributed CEF switching enabled
IP LES Feature Fast switching turbo vector
IP Feature CEF switching turbo vector
Input fast flags 0x40, Output fast flags 0x0, ifindex 7(7)
Slot 2 Slot unit 0 VC -1
Transmit limit accumulator 0x48001A02 (0x48001A02)
IP MTU 1500

Troubleshooting Tips

If you experience problems while using Unicast RPF, check the following items.

HSRP Failure

Failure to disable Unicast RPF before disabling CEF can cause Hot Standby Router Protocol (HSRP) failure. If you want to disable CEF on the router, you must first disable Unicast RPF. To disable Unicast RPF, see the section "Monitoring and Maintaining Unicast RPF."

Dropped Boot Requests

In CiscoIOS Release11.1(17)CC, Unicast RPF can drop BOOTP request packets that have a source address of 0.0.0.0 due to source address verification at the interface. To enable boot requests to work on the interface, you must use ACLs instead of Unicast RPF.

Monitoring and Maintaining Unicast RPF

This section describes commands used to monitor and maintain Unicast RPF.

Command Purpose

Router#show ip traffic
Displays statistics about IP traffic.

Router(config-if)#no ip verify unicast reverse-path
Disables Unicast RPF at the interface.

Command	Purpose
Router#show ip traffic	Displays statistics about IP traffic.
Router(config-if)#no ip verify unicast reverse-path	Disables Unicast RPF at the interface.

CautionTo disable CEF, you must first disable Unicast RPF. Failure to disable Unicast RPF before disabling CEF can cause HSRP failure. If you want to disable CEF on the router, you must first disable Unicast RPF.

A counter is maintained to count the number of packets discarded because of Unicast RPF. The value of the counter is displayed as part of the output from the show ip traffic command. The value of the counter is the total of dropped packets for all router interfaces. The Unicast RPF drop count is included in the IP statistics section.

Router# show ip traffic
IP statistics:
  Rcvd:  1471590 total, 887368 local destination
         0 format errors, 0 checksum errors, 301274 bad hop count
         0 unknown protocol, 0 not a gateway
         0 security failures, 0 bad options, 0 with options
  Opts:  0 end, 0 nop, 0 basic security, 0 loose source route
         0 timestamp, 0 extended security, 0 record route
         0 stream ID, 0 strict source route, 0 alert, 0 other
  Frags: 0 reassembled, 0 timeouts, 0 couldn't reassemble
         0 fragmented, 0 couldn't fragment
  Bcast: 205233 received, 0 sent
  Mcast: 463292 received, 462118 sent
  Sent:  990158 generated, 282938 forwarded
  Drop:  3 encapsulation failed, 0 unresolved, 0 no adjacency
         0 no route, 0 unicast RPF, 0 forced drop

If the drop counter (router drop count) is not zero, a value indicates that packets were dropped by Unicast RPF. Dropped packets can mean two things:

Unicast RPF is dropping packets that have a bad source address (normal operation).
Unicast RPF is dropping legitimate packets because the route is misconfigured to use Unicast RPF in environments where asymmetric routing exists; that is, where multiple paths can exist as the best return path for a source address.

Unicast RPF Configuration Examples

This section provides the following configuration examples:

Unicast RPF on a Leased-Line Aggregation Router Example

The following commands enable Unicast RPF on a serial interface:

ip cef
! or "ip cef distributed" for RSP+VIP based routers
!
interface serial 5/0/0
ip verify unicast reverse-path

Unicast RPF on the Cisco AS5800 Using Dialup Ports Example

The following example enables Unicast RPF on a CiscoAS5800. The interface Group-Async command makes it easy to apply Unicast RPF on all the dialup ports.

ip cef
!
interface Group-Async1
ip verify unicast reverse-path

Unicast RPF with Inbound and Outbound Filters Example

The following example uses a very simple single-homed ISP to demonstrate the concepts of ingress and egress filters used in conjunction with Unicast RPF. The example illustrates an ISP-allocated classless interdomain routing (CIDR) block 209.165.202.128/28 that has both inbound and outbound filters on the upstream interface. Be aware that ISPs are usually not single-homed. Hence, provisions for asymmetrical flows (when outbound traffic goes out one link and returns via a different link) need to be designed into the filters on the border routers of the ISP.

ip cef distributed 
!
interface Serial 5/0/0
description Connection to Upstream ISP
ip address 209.165.200.225 255.255.255.252
no ip redirects
no ip directed-broadcast
no ip proxy-arp
ip verify unicast reverse-path
ip access-group 111 in
ip access-group 110 out
!
access-list 110 permit ip 209.165.202.128 0.0.0.31 any
access-list 110 deny ip any any log 
access-list 111 deny ip host 0.0.0.0 any log
access-list 111 deny ip 127.0.0.0 0.255.255.255 any log
access-list 111 deny ip 10.0.0.0 0.255.255.255 any log
access-list 111 deny ip 172.16.0.0 0.15.255.255 any log
access-list 111 deny ip 192.168.0.0 0.0.255.255 any log
access-list 111 deny ip 209.165.202.128 0.0.0.31 any log
access-list 111 permit ip any any

Table of Contents

Configuring Unicast Reverse Path Forwarding