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IETF RFC 5635
Remote Triggered Black Hole Filtering with Unicast Reverse Path Forwarding (uRPF)
Last modified on Wednesday, August 12th, 2009
 Permanent link to RFC 5635
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Network Working Group W. Kumari
Request for Comments: 5635 Google
Category: Informational D. McPherson
Arbor Networks
August 2009
Remote Triggered Black Hole Filtering
with Unicast Reverse Path Forwarding (uRPF)
 Abstract
Remote Triggered Black Hole (RTBH) filtering is a popular and
effective technique for the mitigation of denial-of-service attacks.
This document expands upon destination-based RTBH filtering by
outlining a method to enable filtering by source address as well.
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Kumari & McPherson Informational PAGE 1 
RFC 5635 RTBH Filtering with uRPF August 2009
Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................3
3. Destination Address RTBH Filtering ..............................3
3.1. Overview ...................................................3
3.2. Detail .....................................................4
4. Source Address RTBH Filtering ...................................7
4.1. Steps to Deploy RTBH Filtering with uRPF for Source
Filtering ..................................................8
5. Security Considerations .........................................9
6. Acknowledgments .................................................9
7. References ......................................................9
7.1. Normative References .......................................9
7.2. Informative References ....................................10
Appendix A. Cisco Router Configuration Sample .....................11
Appendix B. Juniper Configuration Sample ..........................12
Appendix C. A Brief History of RTBH ...............................14
1. Introduction
This document expands upon the technique outlined in "Configuring BGP
to Block Denial-of-Service Attacks" [RFC 3882] to demonstrate a method
that allows for filtering by source address(es).
Network operators have developed a variety of techniques for
mitigating denial-of-service (DoS) attacks. While different
techniques have varying strengths and weaknesses, from an
implementation perspective, the selection of which method to use for
each type of attack involves evaluating the tradeoffs associated with
each method.
A common DoS attack directed against a customer of a service provider
involves generating a greater volume of attack traffic destined for
the target than will fit down the links from the service provider(s)
to the victim (customer). This traffic "starves out" legitimate
traffic and often results in collateral damage or negative effects to
other customers or the network infrastructure as well. Rather than
having all destinations on their network be affected by the attack,
the customer may ask their service provider to filter traffic
destined to the target destination IP address(es), or the service
provider may determine that this is necessary themselves, in order to
preserve network availability.
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RFC 5635 RTBH Filtering with uRPF August 2009
One method that the service provider can use to implement this
filtering is to deploy access control lists on the edge of their
network. While this technique provides a large amount of flexibility
in the filtering, it runs into scalability issues, both in terms of
the number of entries in the filter and the packet rate.
Most routers are able to forward traffic at a much higher rate than
they are able to filter, and they are able to hold many more
forwarding table entries and routes than filter entries. RTBH
filtering leverages the forwarding performance of modern routers to
filter more entries and at a higher rate than access control lists
would otherwise allow.
However, with destination-based RTBH filtering, the impact of the
attack on the target is complete. That is, destination-based RTBH
filtering injects a discard route into the forwarding table for the
target prefix. All packets towards that destination, attack traffic
AND legitimate traffic, are then dropped by the participating
routers,thereby taking the target completely offline. The benefit is
that collateral damage to other systems or network availability at
the customer location or in the ISP network is limited, but the
negative impact to the target itself is arguably increased.
By coupling unicast Reverse Path Forwarding (uRPF) [RFC 3704]
techniques with RTBH filtering, BGP can be used to distribute discard
routes that are based not on destination or target addresses, but on
source addresses of unwanted traffic. Note that this will drop all
traffic to/from the address, and not just the traffic to the victim.
This document is broken up into three logical parts: the first
outlines how to configure destination-based RTBH, the second covers
source-based RTBH, and the third part has examples and a history of
the technique.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC 2119].
3. Destination Address RTBH Filtering
3.1. Overview
A discard route is installed on each edge router in the network with
the destination set to the discard (or null) interface. In order to
use RTBH filtering for a single IP address (or prefix), a BGP route
for the address to be filtered is announced, with the next-hop set as
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RFC 5635 RTBH Filtering with uRPF August 2009
the discard route. This causes traffic to the announced network
prefix to be forwarded to the discard interface so that it does not
transit the network wasting resources or triggering collateral damage
to other resources along the path towards the target.
While this does "complete" the attack in that the target address(es)
are made unreachable, collateral damage is minimized. It may also be
possible to move the host or service on the target IP address(es) to
another address and keep the service up, for example, by updating
associated DNS resource records.
3.2. Detail
Before deploying RTBH filtering, there is some preparation and
planning that needs to occur and decisions that need to be made.
These include:
- What are the discard addresses?
- What are the discard BGP communities?
- What is the largest prefix that can be black-holed?
- What is the smallest advertisement that your provider will accept?
Steps to configure destination-based RTBH filtering:
Step 1. Select Your Discard Address Schema
An address is chosen to become the "discard address". This is often
chosen from 192.0.2.0/24 (TEST-NET [RFC 3330]), or from RFC 1918
[RFC 1918] space. Multiple addresses allow an operator to configure
multiple static routes, one for each incident:
192.0.2.1 = Incident #1
192.0.2.2 = Incident #2
192.0.2.3 = Incident #3
192.0.2.4 = Incident #4
192.0.2.5 = Incident #5
Customer #1, who has a DDoS (Distributed DoS) attack can be pointed
to discard route 192.0.2.1. Customer #2 can be pointed to discard
route 192.0.2.2. If capable, the router can then count the drops for
each, providing some level of telemetry on the volume of drops as
well as status of an ongoing attack. A consistent address schema
facilitates operations.
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Step 2. Configure the Discard Route(s) on Each Router
A route for the "discard address" is installed on the routers that
form the edge/perimeter of the network in all routers in the network
or some subset (e.g., peering, but not customer, etc.). The
destination of the route is set to the "discard" or "null" interface.
This route is called the "discard route". Implementation experience
demonstrates the value of configuring each ingress router with a
capability for dropping traffic via RTBH filtering.
Step 3. Configure the RTBH BGP Policy on Each Router
A BGP policy is configured on all routers that have the discard route
so that routes announced with a chosen community will have their
next-hop set to the discard address. The BGP policy should be made
restrictive so that only BGP routes covering a defined number of
hosts addresses will be accepted. That is, typically, only specific
/32s are necessary. Shorter prefix blocks may also be required or
desirable, for example, if larger numbers of attack targets are
located within a single prefix, or the employment of this mechanism
is to drop traffic bound for specific networks. When filtering based
on shorter prefixes, extreme caution should be used as to avoid
collateral damage to other hosts that reside within those address
blocks. Full implementations will have multiple communities, with
each community used for different parts of a provider's network and
for different security incidents.
Step 4. Configure the Safety Egress Prefix Filters
There is always a chance that the triggering BGP update could leak
from the network and so egress prefix filters are strongly
recommended. These egress prefix filter details may vary, but
experience has demonstrated that the following works:
- Deny all prefixes longer than the longest prefix that you expect
to announce. For example, if the longest prefix that you expect
to announce is /24, deny all prefixes of length /25 though /32.
If your triggering BGP update is only /32s, then this egress
prefix filter will add a safe measure in case the NO_EXPORT
community does not work.
- Deny all communities used for triggering RTBH filtering. This is
also a "safety" measure in case the NO_EXPORT community does not
work.
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Step 5: Configure the Trigger Router
Configure the trigger router, workstation, or other device so that
adding and removing the triggers can be done easily and quickly. The
BGP update should have the NO_EXPORT community as a mandatory
attribute. An egress prefix filter or policy that prevents RTBH
filtering prefixes in the /8 to /24 range is also recommended as a
safety tool. The trigger router can be set up as an iBGP (Internal
BGP) route reflector client that does not receive any prefixes from
its BGP peer. This allows a low-cost router/workstation to be used
as the trigger router.
Using the RTBH filtering:
1: When RTBH filtering is desired for a specific address, that
address is announced from a trigger router (or route server),
tagged with the chosen "RTBH" community and with the NO_EXPORT
community, and passed via iBGP. The receiving routers check the
BGP policy, set the next-hop to be the discard route, resulting in
a Forwarding Information Base (FIB) entry pointing to a discard
address.
2: Traffic entering the network will now be forwarded to the discard
interface on all edge routers and will therefore be dropped at the
edge of the network, saving resources.
2.1: Multiple Discard Addresses for Incident Granularity. This
technique can be expanded by having multiple discard
addresses, routes, and communities to allow for monitoring of
the discarded traffic volume on devices that support multiple
discard interfaces. As mentioned earlier, each router can
have a discard address schema to allow the operator to
distinguish multiple incidents from each other -- making it
easier to monitor the life-cycle of the incidents.
2.2: Multiple "Trigger Communities" for Network-Wide Granularity.
The network can be sectioned into multiple communities,
providing the operator with an ability to drop in discrete
parts of their network. For example, the network can be
divided into the following communities (where XXX represents
the operator's AS number):
XXX:600 RTBH filtering on all routers
XXX:601 RTBH filtering on only peering routers
XXX:602 RTBH filtering on only customers who peer BGP
XXX:603 RTBH filtering on data centers (to see if the
data center is the source of attack)
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RFC 5635 RTBH Filtering with uRPF August 2009
XXX:604 RTBH filtering on all customers (to see how many
customers are being used by the attacker)
Some diligent thinking is required to develop a community
schema that provides flexibility while reflecting topological
considerations.
2.3: "Customer-Triggered" RTBH filtering. The technique can also
be expanded by relaxing the Autonomous System (AS) path rule
to allow customers of a service provider to enable RTBH
filtering without interacting with the service provider's
trigger routers. If this is configured, an operator MUST
only accept announcements from the customer for prefixes that
the customer is authorized to advertise, in order to prevent
the customer from accidentally (or intentionally) black-
holing space that they are not allowed to advertise.
A common policy for this type of setup would first permit any
longer prefix within an authorized prefix only if the black
hole communities are attached, append NO_EXPORT,
NO_ADVERTISE, or similar communities, and then also accept
from the customer the original aggregate prefix that will be
advertised as standard policy permits.
Extreme caution should be used in order to avoid leaking any
more specifics beyond the local routing domain, unless policy
explicitly aims at doing just that.
4. Source Address RTBH Filtering
In many instances, denial-of-service attacks sourced from botnets are
being configured to "follow DNS". (The attacking machines are
instructed to attack www.example.com, and re-resolve this
periodically. Historically, the attacks were aimed simply at an IP
address and so renumbering www.example.com to a new address was an
effective mitigation.) This makes it desirable to employ a technique
that allows black-holing to be based on source address.
By combining traditional RTBH filtering with unicast Reverse Path
Forwarding (uRPF), a network operator can filter based upon the
source address. uRPF performs a route lookup of the source address
of the packet and checks to see if the ingress interface of the
packet is a valid egress interface for the packet source address
(strict mode) or if any route to the source address of the packet
exists (loose mode). If the check fails, the packet is typically
dropped. In loose mode, some vendors also verify that the
destination route does not point to an invalid next-hop -- this
allows source-based RTBH filtering to be deployed in networks that
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RFC 5635 RTBH Filtering with uRPF August 2009
cannot implement strict (or feasible path) mode uRPF. Before
enabling uRPF (in any mode), it is vital that you fully understand
the implications of doing so:
- Strict mode will cause the router to drop all ingress traffic if
the best path back to the source address of the traffic is not the
interface from which the traffic was received. Asymmetric routing
will cause strict mode uRPF to drop legitimate traffic.
- Loose mode causes the router to check if a route for the source
address of the traffic exists. This may also cause legitimate
traffic to be discarded.
It is hoped that in the future, vendors will implement a "DoS-
mitigation" mode in addition to the loose and strict modes -- in this
mode, the uRPF check will only fail if the next-hop for the source of
the packet is a discard interface.
By enabling the uRPF feature on interfaces at predetermined points in
their network and announcing the source address(es) of attack
traffic, a network operator can effectively drop the identified
attack traffic at specified devices (ideally ingress edge) of their
network based on source address.
While administrators may choose to drop traffic from any prefix they
wish, it is recommended when employing source-based RTBH filtering
inter-domain that explicit policy be defined that enables peers to
only announce source-based RTBH routes for prefixes that they
originate.
4.1. Steps to Deploy RTBH Filtering with uRPF for Source Filtering
The same steps that are required to implement destination address
RTBH filtering are taken with the additional step of enabling unicast
Reverse Path Forwarding on predetermined interfaces. When a source
address (or network) needs to be blocked, that address (or network)
is announced using BGP tagged with a community. This will cause the
route to be installed with a next-hop of the discard interface,
causing the uRPF check to fail and the packets to be discarded. The
destination-based RTBH filtering community should not be used for
source-based RTBH filtering, and the routes tagged with the selected
community should be carefully filtered.
The BGP policy will need to be relaxed to accept announcements tagged
with this community to be accepted even though they contain addresses
not controlled by the network announcing them. These announcements
must NOT be propagated outside the local AS and should carry the
NO_EXPORT community.
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RFC 5635 RTBH Filtering with uRPF August 2009
As a matter of policy, operators SHOULD NOT accept source-based RTBH
announcements from their peers or customers, they should only be
installed by local or attack management systems within their
administrative domain.
5. Security Considerations
The techniques presented here provide enough power to cause
significant traffic forwarding loss if incorrectly deployed. It is
imperative that the announcements that trigger the black-holing are
carefully checked and that the BGP policy that accepts these
announcements is implemented in such a manner that the announcements:
- Are not propagated outside the AS (NO_EXPORT).
- Are not accepted from outside the AS (except from customers).
- Except where source-based filtering is deployed, that the network
contained in the announcement falls within the address ranges
controlled by the announcing AS (i.e., for customers that the
address falls within their space).
6. Acknowledgments
I would like to thank Joe Abley, Ron Bonica, Rodney Dunn, Alfred
Hoenes, Donald Smith, Joel Jaeggli, and Steve Williams for their
assistance, feedback and not laughing *too* much at the quality of
the initial versions.
I would also like to thank all of the regular contributors to the
OPSEC Working Group and Google for 20% time :-)
The authors would also like to thank Steven L Johnson and Barry
Greene for getting this implemented and Chris Morrow for publicizing
the technique in multiple talks.
7. References
7.1. Normative References
[RFC 1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot,
G., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, February 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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RFC 5635 RTBH Filtering with uRPF August 2009
[RFC 3330] IANA, "Special-Use IPv4 Addresses", RFC 3330, September
2002.
[RFC 3704] Baker, F. and P. Savola, "Ingress Filtering for
Multihomed Networks", BCP 84, RFC 3704, March 2004.
[RFC 3882] Turk, D., "Configuring BGP to Block Denial-of-Service
Attacks", RFC 3882, September 2004.
7.2. Informative References
[Greene2001] Greene Barry Raveendran and Jarvis Neil, "Unicast
Reverse Path Forwarding (uRPF) Enhancements for the
ISP-ISP Edge", ftp://ftp-eng.cisco.com/
cons/isp/documents/uRPF_Enhancement.pdf, 2001.
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RFC 5635 RTBH Filtering with uRPF August 2009
Appendix A. Cisco Router Configuration Sample
This section provides a partial configuration for configuring RTBH
filtering on a Cisco router. This is not a complete configuration
and should be customized before being used.
Announcing router:
! The discard route
ip route 192.0.2.1 255.255.255.255 Null0
!
! Matches and empty AS-PATH only.
ip as-path access-list 10 permit ^$
!
! This route-map matches routes with tag 666 and sets the next-hop
! to be the discard route.
route-map remote-trigger-black-hole permit 10
match tag 666
set ip next-hop 192.0.2.1
set local-preference 200
set community no-export
! The community used internally to tag RTBH announcements.
set community 65505:666
set origin igp
!
route-map remote-trigger-black-hole permit 20
!
router bgp 65505
no synchronization
bgp log-neighbor-changes
redistribute static route-map remote-trigger-black-hole
no auto-summary
!
! An example IP that we are applying RTBH filtering to.
! All traffic destined to 10.0.0.1 will now be dropped!
ip route 10.0.0.1 255.255.255.255 null0 tag 666
!
Filtering router:
!
! The discard route
ip route 192.0.2.1 255.255.255.255 Null0
!
! Matches and empty AS-PATH only.
ip as-path access-list 10 permit ^$
!
route-map black-hole-filter permit 10
match ip address prefix-list only-host-routes
match as-path 10
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RFC 5635 RTBH Filtering with uRPF August 2009
match community 65505:666 no-export
!
! Don't accept any other announcements with the RTBH community.
route-map black-hole-filter deny 20
match community 65505:666
!
route-map black-hole-filter permit 30
!
! An interface for source-based RTBH filtering with uRPF loose mode.
interface FastEthernet 0/0
ip verify unicast source reachable-via any
Appendix B. Juniper Configuration Sample
This section provides a partial configuration for configuring RTBH
filtering on a Juniper router. This is not a complete configuration
and should be customized before being used.
Announcing router:
routing-options {
static {
/* EXAMPLE ATTACK SOURCE */
route 10.11.12.66/32 {
next-hop 192.0.2.1;
resolve;
tag 666;
}
/* EXAMPLE ATTACK DESTINATION */
route 10.128.0.2/32 {
next-hop 192.0.2.1;
resolve;
tag 666;
}
}
autonomous-system 100;
}
protocols {
bgp {
group ibgp {
type internal;
export rtbh;
neighbor 172.16.0.2;
}
}
}
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RFC 5635 RTBH Filtering with uRPF August 2009
policy-options {
policy-statement rtbh {
term black-hole-filter {
from {
tag 666;
route-filter 0.0.0.0/0 prefix-length-range /32-/32;
}
then {
local-preference 200;
origin igp;
community set black-hole;
community add no-export;
next-hop 192.0.2.1;
accept;
}
}
}
community black-hole members 100:666;
community no-export members no-export;
}
Filtering router:
policy-statement black-hole-filter {
from {
protocol bgp;
as-path LocalOnly;
community black-hole;
route-filter 0.0.0.0/0 prefix-length-range /32-/32;
}
then {
community set no-export;
next-hop 192.0.2.1;
}
}
community black-hole members 100:666;
community no-export members no-export;
routing-options {
forwarding-table {
unicast-reverse-path feasible-paths;
}
static {
route 192.0.2.1/32 discard;
}
}
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RFC 5635 RTBH Filtering with uRPF August 2009
interfaces {
xe-1/0/0 {
vlan-tagging;
mtu 9192;
unit 201 {
vlan-id 201;
family inet {
rpf-check;
address 10.11.12.1/24;
}
}
}
}
Appendix C. A Brief History of RTBH Filtering
Understanding the history and motivation behind the development of a
technique often helps with understanding how to best utilize the
technique. In this spirit, we present a history of unicast RPF and
RTBH filtering.
This section has been provided by Barry Raveendran Greene:
Unicast RPF Loose Check (uRPF Loose Check) was created by Neil Jarvis
and Barry Greene to be used with destination-based RTBH as a rapid
reaction tool to DDoS attacks. The requirements for this rapid
reaction tool was based on post mortem conversation after the
February 2000 attacks on several big content hosting companies. The
summary of the requirement became the "Exodus Requirement" which
stated:
We need a tool to drop packets based on source IP address that can
be pushed out to over 60 routers within 60 seconds, be longer than
a thousand lines, be modified on the fly, and work in all your
platforms filtering at line rate.
A variety of options were looked at to meet this requirement, from
reviving Common Open Policy Service (COPS), to pushing out Access
Control Lists (ACLs) with BGP, creating a new protocol. In 2000, the
quickest way to meet the "Exodus requirement" was to marry two
functions. First, modify unicast RPF so that the interface check was
no longer required and to make sure that a "null" or discard route
would drop the packet (i.e., loose check). Second, the technique
where BGP is used to trigger a distributed drop is dusted off and
documented. Combining these two techniques was deemed a fast way to
put a distributed capability to drop packets out into the industry.
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To clarify and restate, uRPF loose check was created as one part of a
rapid reaction tool to DDoS attacks that "drop packets based on
source IP address that can be pushed out to over 60 routers with in
60 seconds, be longer than a thousand lines, be modified on the fly,
and work in all your platforms filtering at line rate". The
secondary benefits of using uRPF Loose Check for other functions is a
secondary benefit -- not the primary goal for its creation.
To facilitate the adoption to the industry, uRPF Loose Check was not
patented. It was publicly published and disclosed in "Unicast
Reverse PathForwarding (uRPF) Enhancements for the ISP-ISP Edge"
[Greene2001].
Authors' Addresses
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
EMail: warren@kumari.net
Danny McPherson
Arbor Networks, Inc.
EMail: danny@arbor.net
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Remote Triggered Black Hole Filtering with Unicast Reverse Path Forwarding (uRPF)
RFC TOTAL SIZE: 30232 bytes
PUBLICATION DATE: Wednesday, August 12th, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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