Internet Draft W. Kumari <draft-kumari-blackhole-urpf-02.txt> Google Category: Informational D. McPherson Expires: April 27, 2009 Arbor Networks October 27, 2008 Remote Triggered Black Hole filtering with uRPF <draft-kumari-blackhole-urpf-02.txt> Status of this Memo Distribution of this memo is unlimited. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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. It also defines a standard BGP community for black hole prefixes to simplify associated semantics. Kumari Expires April 27, 2009 [Page 1]
Internet Draft blackhole-urpf-02.txt October 27 2008 Table of Contents 1. Introduction ....................................................2 2. Background ......................................................2 3. Destination address RTBH filtering ..............................3 3.1. Overview ...................................................3 3.2. Detail .....................................................3 4. Source address RTBH filtering ...................................4 5. Security Considerations .........................................6 6. IANA Considerations .............................................6 7. Acknowledgments .................................................7 8. References ......................................................7 A. Cisco Router Configuration Sample................................8 B. Juniper Configuration Sample....................................10 1. Introduction This document expands upon the technique outlined in "Configuring BGP to Block Denial-of-Service Attacks" [RFC3882] and presents a method to allow filtering by source address. It also defines a standard BGP community to signal that Remote Triggered Black Hole (RTBH) filtering should occur for a network. 2. Background Denial of Service (DoS) attacks that starve out legitimate traffic by saturating circuits are becoming an increasingly common occurrence. Network operators have developed a selection of techniques for mitigating these attacks. Different techniques have different strengths and weaknesses -- the selection of which technique to use for each type of attack involves tradeoffs. A common attack directed against a customer of a service provider involves generating more traffic at the customer than will fit down the links from the service provider to the customer. This traffic "starves out" legitimate traffic. Rather than having all of their network affected the customer may ask their service provider to filter traffic destined to the attacked IP address(es). 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 are able to hold many more forwarding table entries and routes than filter entries. RTBH leverages the Kumari Expires April 27, 2009 [Page 2]
Internet Draft blackhole-urpf-02.txt October 27 2008 forwarding performance of modern routers to filter both more entries and at a higher rate than access control lists would allow. However, with destination-based RTBH filtering, the impact is that you effectively complete the attack. That is, with destination-based RTBH filtering you inject a discard route into the forwarding table for the prefix in question. 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 (RPF) [RFC3704] techniques with RTBH, BGP can be used to distribute discard routes that are based not on destination or target addresses, but based on source addresses. 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 be the discard (or null) interface. In order to use RTBH filtering for an IP address (or network) a BGP route for the address to be filtered is announced, with the next-hop set as the "discard" route. This causes traffic to the announced network to be forwarded to the discard interface and so it does not transit the network and waste resources or trigger collateral damage or negative impact to other resources along the path towards the target. While this does "complete" the attack in that the attacked address(es) are made unreachable, it cuts down on collateral damage. It may also be possible to move the host / service on the attacked IP address(es) to another address and keep the service up. 3.2. Detail Steps to configure destination based RTBH filtering: 1: An address is chosen to become the "discard address". This is often chosen from 192.0.2.0/24 (TEST-NET [RFC3330]), or from RFC 1918 [RFC1918] space. 2: A route for the "discard address" is installed on the routers that form the edge/perimeter of the network, in all routers Kumari Expires April 27, 2009 [Page 3]
Internet Draft blackhole-urpf-02.txt October 27 2008 in the network, or some subset (e.g., peering, but not customer, etc.), with the destination of the route being the "discard" or "null" interface. This route is called the "discard route". 3: A BGP policy is configured on all routers that have the discard route so that routes announced with the community [ TBD1 ] 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, unless shorter prefix blocks are required. This might occur where larger numbers of attacking sources are located within a single prefix, or the employment of this mechanism is to drop traffic to 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. 4: When RTBH filtering is desired for a specific address, that address is announced from a central router (or route server), tagged with the community [ TBD1 ]. The receiving routers check the BGP policy, setting the next-hop to be the discard route, which resolves to the discard interface. 5: Traffic entering the network will now be forwarded to the discard interface on all edge routers and so will be dropped at the edge of the network, saving resources. 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. The technique can also be expanded by relaxing the AS path rule to allow customers of a service provider to enable RTBH filtering without interacting with the service provider. If this is configured, an operator MUST only accept announcements for prefixes from the customer that the customer is authorized to advertise, to prevent the customer accidentally (or intentionally) black-holing space that is not theirs. A common policy for this type of setup would first permit any more specific of an authorized prefix only if the blackhole communities is attached, append NO_EXPORT, NO_ADVERTISE, or similar communities, and then also accept from the customer the original aggregate prefix that will be advertised to 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 Kumari Expires April 27, 2009 [Page 4]
Internet Draft blackhole-urpf-02.txt October 27 2008 aims at doing just that. 4. Source address RTBH filtering. In many instances the denial of service attacks are being sourced from botnets and 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 a technique that allows black- holing based upon source address desirable. 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 or the source address of the packet exists (loose mode). If the check fails, the packet is (generally) dropped. In loose mode some vendors also verify that the destination route does not point to a discard interface - this allows source based RTBH filtering to be deployed in networks that cannot implement strict (or feasible path) mode uRPF. By enabling the uRPF feature on interfaces on pre-determined points of their network and announcing the source address(es) of attack traffic, a network operator can effectively drop the attack traffic at the edge of their network based on source addresses. While administrators may choose to drop any prefixes they wish, it is recommended when employing source-based RTBH inter-domain that explicit policy be defined that enables peers to only announce source-based RTBH routes for prefixes which they originate. 4.1 Steps to deploy RTBH w/ 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. The destination based RTBH filtering community ([ TBD1 ]) should not be used for source based RTBH filtering, and the routes tagged with the selected community should be carefully filtered. Kumari Expires April 27, 2009 [Page 5]
Internet Draft blackhole-urpf-02.txt October 27 2008 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 current AS and should carry the no-export community. 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 severe unhappiness. 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 within the announcement falls with the address ranges controlled by the announcing AS (i.e.: for customer that the address falls within their space). 6. IANA Considerations This document requests registration of a regular Type, non-transitive BGP Extended Communities Attribute [RFC4360] from the First Come, First Served range to be named "Remote Triggered Black Hole Filter". This community will provide a standard method to signal a provider that RTBH filtering should occur for a destination and will eliminate the need for customers to track different communities for each provider. Kumari Expires April 27, 2009 [Page 6]
Internet Draft blackhole-urpf-02.txt October 27 2008 7. Acknowledgments I would like to thank Alfred Hoenes, Danny McPherson, Donald Smith, Joe Abley and Joel Jaeggli for their assistance, feedback and not laughing *too* much at the quality of the initial drafts. I would also like to thank all of the regular contributors to the OpSec Working Group and Google for 20% time :-) The authors are not aware of who initially developed this technique, but would like to thank Chris Morrow for publicizing it. 8. References 8.1. Normative References [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., and G. de Groot, "Address Allocation for Private Internets", RFC 1597, March 1994. [RFC3330] IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002. [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed Networks", BCP 84, RFC 3704, March 2004. [RFC3882] Turk, D., "Configuring BGP to Block Denial-of-Service Attacks", RFC 3882, September 2004. Requirement Levels", BCP 14, RFC 2119, March 1997. 8.2. Informative References [2223BIS] Reynolds, J. and R. Braden, "Instructions to Request for Comments (RFC) Authors", draft-rfc-editor- rfc2223bis-08.txt, August 2004. Kumari Expires April 27, 2009 [Page 7]
Internet Draft blackhole-urpf-02.txt October 27 2008 Appendix A: Cisco Router Configuration Sample This section provides a partial configuration for configuring RTBH 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 the 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 Kumari Expires April 27, 2009 [Page 8]
Internet Draft blackhole-urpf-02.txt October 27 2008 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 with uRPF loose mode. interface FastEthernet 0/0 ip verify unicast source reachable-via any Kumari Expires April 27, 2009 [Page 9]
Internet Draft blackhole-urpf-02.txt October 27 2008 Appendix B: Juniper Configuration Sample This section provides a partial configuration for configuring RTBH on a Juniper router. This is not a complete configuration and should be customized for before being used. Announcing router: routing-options { static { # The discard route. route 192.0.2.1/32 discard; # An example route to be RTBH filtered. route 10.0.0.1/32 { next-hop 192.0.2.1; resolve; tag 666; } } autonomous-system 65505; } protocols { bgp { import remote-trigger-black-hole; local-as 65505; } } policy-options { policy-statement remote-trigger-black-hole { 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; } } } community black-hole members 65505:666; community no-export members no-export; } Kumari Expires April 27, 2009 [Page 10]
Internet Draft blackhole-urpf-02.txt October 27 2008 Filtering router: routing-options { static { # The discard route. route 192.0.2.1/32 discard; } autonomous-system 65505; # Enable feasible-paths mode uRPF. forwarding-table { unicast-reverse-path feasible-paths; } } protocols { bgp { group iBGP { import black-hole-filter; } } } policy-options { 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 65505:666; community no-export members no-export; as-path LocalOnly "^$"; } # Enable uRPF on an interface for source based uRPF. interfaces { so-0/0/0 { unit 0 { family inet { rpf-check; } } } } Kumari Expires April 27, 2009 [Page 11]
Internet Draft blackhole-urpf-02.txt October 27 2008 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 Kumari Expires April 27, 2009 [Page 12]
Internet Draft blackhole-urpf-02.txt October 27 2008 Full Copyright Statement Copyright (C) The IETF Trust (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. Kumari Expires April 27, 2009 [Page 13]