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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Opsec Working Group W. Kumari 3 Internet Draft Google 4 D. McPherson 5 Category: Informational Arbor Networks 6 Expires: September 5, 2009 7 March 5, 2009 9 Remote Triggered Black Hole filtering with uRPF 10 12 Status of this Memo 14 This Internet-Draft is submitted to IETF in full conformance with the 15 provisions of BCP 78 and BCP 79. 17 Internet-Drafts are working documents of the Internet Engineering 18 Task Force (IETF), its areas, and its working groups. Note that 19 other groups may also distribute working documents as Internet- 20 Drafts. 22 Internet-Drafts are draft documents valid for a maximum of six months 23 and may be updated, replaced, or obsoleted by other documents at any 24 time. It is inappropriate to use Internet-Drafts as reference 25 material or to cite them other than as "work in progress." 27 The list of current Internet-Drafts can be accessed at 28 http://www.ietf.org/ietf/1id-abstracts.txt. 30 The list of Internet-Draft Shadow Directories can be accessed at 31 http://www.ietf.org/shadow.html. 33 This Internet-Draft will expire on September 5, 2009. 35 Copyright Notice 37 Copyright (c) 2009 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (http://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. 47 Abstract 49 Remote Triggered Black Hole (RTBH) filtering is a popular and 50 effective technique for the mitigation of denial-of-service attacks. 51 This document expands upon destination-based RTBH filtering by 52 outlining a method to enable filtering by source address as well. 54 Table of Contents 56 1. Introduction ....................................................2 57 2. Background ......................................................2 58 3. Destination address RTBH filtering ..............................3 59 3.1. Overview ...................................................3 60 3.2. Detail .....................................................3 61 4. Source address RTBH filtering ...................................4 62 5. Security Considerations .........................................6 63 6. IANA Considerations .............................................6 64 7. Acknowledgments .................................................7 65 8. References ......................................................7 66 A. Cisco Router Configuration Sample................................8 67 B. Juniper Configuration Sample....................................10 68 C. A brief history of RTBH.........................................12 70 1. Introduction 72 This document expands upon the technique outlined in "Configuring BGP 73 to Block Denial-of-Service Attacks" [RFC3882] to demonstrate a method 74 that allows for filtering by source address(es). 76 1.2 Terminology 78 The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" 79 in this document are to be interpreted as described in BCP 14, RFC 80 2119 [RFC2119]. 82 2. Background 84 Network operators have developed a variety of techniques for 85 mitigating denial of service attacks. While different techniques have 86 varying strengths and weaknesses, from an implementation perspective 87 the selection of which method to use for each type of attack involves 88 evaluating the tradeoffs associated with each method. 90 A common DoS attack directed against a customer of a service provider 91 involves generating a greater volume of attack traffic destined for 92 the target than will fit down the links from the service provider(s) 93 to the victim (customer). This traffic "starves out" legitimate 94 traffic and often results in collateral damage or negative effects to 95 other customers or the network infrastructure as well. Rather than 96 having all destinations on their network be affected the attack, the 97 customer may ask their service provider to filter traffic destined to 98 the target destination IP address(es), or the service provider may 99 determine that this is necessary themselves, in order to preserve 100 network availability. 102 One method that the service provider can use to implement this 103 filtering is to deploy access control lists on the edge of their 104 network. While this technique provides a large amount of flexibility 105 in the filtering, it runs into scalability issues, both in terms of 106 the number of entries in the filter and the packet rate. 108 Most routers are able to forward traffic at a much higher rate than 109 they are able to filter, and are able to hold many more forwarding 110 table entries and routes than filter entries. RTBH leverages the 111 forwarding performance of modern routers to filter both more entries 112 and at a higher rate than access control lists would otherwise allow. 114 However, with destination-based RTBH filtering, the impact of the 115 attack on the target is complete. That is, destination-based RTBH 116 filtering injects a discard route into the forwarding table for the 117 target prefix. All packets towards that destination, attack traffic 118 AND legitimate traffic, are then dropped by the participating 119 routers,thereby taking the target completely offline. The benefit is 120 that collateral damage to other systems or network availability at 121 the customer location or in the ISP network is limited, but negative 122 impact to the target itself is arguably increased. 124 By coupling unicast reverse path forwarding (RPF) [RFC3704] 125 techniques with RTBH, BGP can be used to distribute discard routes 126 that are based not on destination or target addresses, but based on 127 source addresses of unwanted traffic. 129 3. Destination address RTBH filtering 131 3.1. Overview 133 A "discard" route is installed on each edge router in the network 134 with the destination set to be the discard (or null) interface. In 135 order to use RTBH filtering for a single IP address (or prefix) a BGP 136 route for the address to be filtered is announced, with the next-hop 137 set as the "discard" route. This causes traffic to the announced 138 network prefix to be forwarded to the discard interface so that it 139 does not transit the network wasting resources or triggering 140 collateral damage to other resources along the path towards the 141 target. 143 While this does "complete" the attack in that the target address(es) 144 are made unreachable, collateral damage in minimized. It may also be 145 possible to move the host or service on the target IP address(es) to 146 another address and keep the service up, for example by updating 147 associated DNS resource records. 149 3.2. Detail 150 Steps to configure destination based RTBH filtering: 152 1. Select your Discard Address schema. An address is chosen to become 153 the "discard address". This is often chosen from 192.0.2.0/24 (TEST- 154 NET [RFC3330]), or from RFC 1918 [RFC1918] space. Test-Net is popular 155 as a network which an operator will know never show up operationally 156 on the network. It also allows an operator to configure multiple 157 static routes, one for each incident: 159 192.0.2.1 = Incident #1 160 192.0.2.2 = Incident #2 161 192.0.2.3 = Incident #3 162 192.0.2.4 = Incident #4 163 192.0.2.5 = Incident #5 165 Customer #1 who has a DDOS attack can be pointed to discard route 166 192.0.2.1. Customer #2 can be pointed to discard route 192.0.2.2. If 167 capable, the router can then count the drops for each, providing some 168 level of telemetry on the volume of drops as well status of an 169 ongoing attack. A consistent address schema facilitate operations. 171 2. Set the Discard Route(s) on each router. A route for the "discard 172 address" is installed on the routers that form the edge/perimeter of 173 the network, in all routers in the network, or some subset (e.g., 174 peering, but not customer, etc.). The destination of the route is set 175 to the "discard" or "null" interface. This route is called the 176 "discard route". Implementation experience demonstrates the value of 177 configuring each ingress router with a capability for dropping 178 traffic via RTBH. 180 3. Configure the RTBH BGP Policy on each router. A BGP policy is 181 configured on all routers that have the discard route so that routes 182 announced with a chosen community will have their next hop set to the 183 discard address. The BGP policy should be made restrictive so that 184 only BGP routes covering a defined number of hosts addresses will be 185 accepted. That is, typically, only specific /32s are necessary. 186 Shorter prefix blocks may also be required or desirable, for example 187 if larger numbers of attack targets are located within a single 188 prefix, or the employment of this mechanism is to drop traffic bound 189 for specific networks. When filtering based on shorter prefixes, 190 extreme caution should be used as to avoid collateral damage to other 191 hosts that reside within those address blocks. Full implementations 192 will have multiple communities, with each community used for 193 different parts of a provider's network and for different security 194 incidents. 196 4. Configure the Safety Egress Prefix Filters (Murphy Prefix 197 Filters). There is always a chance that the triggering BGP Update 198 could leak from the network. This has happened [ Youtube/Pakistan 199 incident ]. Egress prefix filters are recommended. These egress 200 prefix filter be many things, but experience has demonstrated that 201 the following works: 203 4.1 Deny all prefixes /30 through /32 from leaving your network. If 204 your triggering BGP update is only /32s, then this egress prefix 205 filter will add a safe measure in case the "no-export" community does 206 not work. 208 4.2 Deny all communities used for triggering RTBH. This is also a 209 "safety" measure in case the "no-export" community does not work. 211 5: Configure the Trigger Router. Set the trigger router, workstation, 212 or other device so that adding and removing the triggers can be easy 213 and quickly added and removed. The BGP Update should have the no- 214 export community as a mandatory attribute. A egress prefix filter or 215 policy which prevent RTBHing prefixes in the /8 to /24 ranges is also 216 recommended as a safety tool. The trigger router can be set up as a 217 iBGP route reflector client which does not receive any prefixes from 218 its BGP peer. This allows a low cost router/workstation to be used as 219 the trigger router. 221 6. When RTBH filtering is desired for a specific address, that 222 address is announced from a trigger router (or route server), tagged 223 with the chosen "RTBH" community and with the community "no-export," 224 and passed via iBGP. The receiving routers check the BGP policy, set 225 the next-hop to be the discard route, restling in a fib entry pointed 226 to the discard interface. 228 7. Traffic entering the network will now be forwarded to the discard 229 interface on all edge routers and will therefore be dropped at the 230 edge of the network, saving resources. 232 7.1 Multiple Discard Address for Incident Granularity. This technique 233 can be expanded by having multiple discard addresses, routes and 234 communities to allow for monitoring of the discarded traffic volume 235 on devices that support multiple discard interfaces. As mentioned 236 earlier, each router can have a discard address schema that allows 237 for multiple incident - making it easier to monitor the life-cycle of 238 the incident. 240 7.2 Multiple "Trigger Communities" for Network Wide Granularity. The 241 network can be sectioned into multiple communities, providing the 242 operator with an ability to drop in discreete parts of their network. 243 For example, the network can be divided into the following 244 communities: 246 XXX:600 RTBH on all router 247 XXX:601 RTBH on only peering router 248 XXX:602 RTBH on only customers who peer BGP 249 XXX:603 RTBH on Datacenters (to see if the data center is the 250 source of attack) 251 XXX:604 RTBH on all customers (to see how many customers are 252 BOTed) 254 Some diligent thinking is required to develop a community schema 255 which provides flexibility while reflecting topological 256 considerations. 258 7.3 "Customer Triggered" RTBH. The technique can also be expanded by 259 relaxing the AS path rule to allow customers of a service provider to 260 enable RTBH filtering without interacting with the service provider's 261 trigger routers. If this is configured, an operator MUST only accept 262 announcements for prefixes from the customer that the customer is 263 authorized to advertise, in order to prevent the customer from 264 accidentally (or intentionally) black-holing space that is not theirs 265 to advertise.. 267 A common policy for this type of setup would first permit any more 268 specific of an authorized prefix only if the blackhole communities is 269 attached, append NO_EXPORT, NO_ADVERTISE, or similar communities, and 270 then also accept from the customer the original aggregate prefix that 271 will be advertised as standard policy permits. 273 Extreme caution should be used in order to avoid leaking any more 274 specifics beyond the local routing domain, unless policy explicitly 275 aims at doing just that. 277 4. Source address RTBH filtering. 279 In many instances denial-of-service attacks sourced from are being 280 configured to "follow DNS" (the attacking machines are instructed to 281 attack www.example.com, and re-resolve this periodically. 282 Historically the attacks were aimed simply at an IP address and so 283 renumbering www.example.com to a new address was an effective 284 mitigation). This makes employing technique that allows black-holing 285 to be based on source address desirable. 287 By combining traditional RTBH filtering with unicast Reverse Path 288 Forwarding (uRPF) a network operator can filter based upon the source 289 address. uRPF performs a route lookup of the source address of the 290 packet and checks to see if the ingress interface of the packet is a 291 valid egress interface for the packet source address (strict mode) or 292 if any route to the source address of the packet exists (loose mode). 293 If the check fails, the packet is typically dropped. In loose mode 294 some vendors also verify that the destination route does not point to 295 a discard interface - this allows source based RTBH filtering to be 296 deployed in networks that cannot implement strict (or feasible path) 297 mode uRPF. 299 By enabling the uRPF feature on interfaces at pre-determined points 300 in their network and announcing the source address(es) of attack 301 traffic, a network operator can effectively drop the identified 302 attack traffic at specified devices (ideally ingress edge) of their 303 network based on source address. 305 While administrators may choose to drop traffic from any prefix they 306 wish, it is recommended when employing source-based RTBH inter-domain 307 that explicit policy be defined that enables peers to only announce 308 source-based RTBH routes for prefixes which they originate. 310 4.1 Steps to deploy RTBH with uRPF for source filtering. 312 The same steps that are required to implement destination address 313 RTBH filtering are taken with the additional step of enabling unicast 314 reverse path forwarding on predetermined interfaces. When a source 315 address (or network) needs to be blocked, that address (or network) 316 is announced using BGP tagged with a community. This will cause the 317 route to be installed with a next hop of the discard interface, 318 causing the uRPF check to fail. The destination based RTBH filtering 319 community should not be used for source based RTBH filtering, and the 320 routes tagged with the selected community should be carefully 321 filtered. 323 The BGP policy will need to be relaxed to accept announcements tagged 324 with this community to be accepted even though they contain addresses 325 not controlled by the network announcing them. These announcements 326 must NOT be propagated outside the local AS and should carry the 327 NO_EXPORT community. 329 As a matter of policy, operators SHOULD NOT accept source-based RTBH 330 announcements from their peers or customers, they should only be 331 installed by local or attack management systems within their 332 administrative domain. 334 5. Security Considerations 336 The techniques presented here provide enough power to cause 337 significant traffic forwarding loss if incorrectly deployed. It is 338 imperative that the announcements that trigger the black-holing are 339 carefully checked and that the BGP policy that accepts these 340 announcements is implemented in such a manner that the announcements: 342 - Are not propagated outside the AS (NO_EXPORT). 343 - Are not accepted from outside the AS (except from customers). 344 - Except where source based filtering is deployed, that the network 345 contained in the announcement falls within the address ranges 346 controlled by the announcing AS (i.e.: for customers that the 347 address falls within their space). 349 6. IANA Considerations 351 No action required. 353 7. Acknowledgments 355 I would like to thank Joe Abley, Rodney Dunn, Alfred Hoenes, Donald 356 Smith, Joel Jaeggli and Steve Williams for their assistance, feedback 357 and not laughing *too* much at the quality of the initial drafts. 359 I would also like to thank all of the regular contributors to the 360 OpSec Working Group and Google for 20% time :-) 362 The authors would also like to thank Steven L Johnson and Barry Greene 363 for getting this implemented and Chris Morrow for publicizing the 364 technique in multiple talks. 366 8. References 368 8.1. Normative References 370 [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G., 371 and E. Lear, "Address Allocation for Private Internets", 372 BCP 5, RFC 1918, February 1996. 374 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 375 Requirement Levels", BCP 14, RFC 2119, March 1997. 377 [RFC3330] IANA, "Special-Use IPv4 Addresses", RFC 3330, September 378 2002. 380 [RFC4360] Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended 381 Communities Attribute", RFC 4360, February 2006. 383 [RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed 384 Networks", BCP 84, RFC 3704, March 2004. 386 [RFC3882] Turk, D., "Configuring BGP to Block Denial-of-Service 387 Attacks", RFC 3882, September 2004. 389 8.2. Informative References 391 [2223BIS] Reynolds, J. and R. Braden, "Instructions to Request for 392 Comments (RFC) Authors", draft-rfc-editor- 393 rfc2223bis-08.txt, August 2004. 395 [Greene2001] Greene Barry Raveendran and Jarvis Neil "Unicast Reverse 396 Path Forwarding (uRPF) Enhancements for the ISP-ISP Edge", 397 [ftp://ftp- 398 eng.cisco.com/cons/isp/documents/uRPF_Enhancement.pdf], 399 2001. 401 Appendix A: Cisco Router Configuration Sample 403 This section provides a partial configuration for configuring RTBH on 404 a Cisco router. This is not a complete configuration and should be 405 customized before being used. 407 Announcing router: 408 ! The discard route 409 ip route 192.0.2.1 255.255.255.255 Null0 410 ! 411 ! Matches and empty AS-PATH only. 412 ip as-path access-list 10 permit ^$ 413 ! 414 ! This route-map matches routes with tag 666 and sets the next-hop 415 ! to be the discard route. 416 route-map remote-trigger-black-hole permit 10 417 match tag 666 418 set ip next-hop 192.0.2.1 419 set local-preference 200 420 set community no-export 421 ! The community used internally to tag RTBH announcements. 422 set community 65505:666 423 set origin igp 424 ! 425 route-map remote-trigger-black-hole permit 20 426 ! 427 router bgp 65505 428 no synchronization 429 bgp log-neighbor-changes 430 redistribute static route-map remote-trigger-black-hole 431 no auto-summary 432 ! 433 ! An example IP that we are applying RTBH filtering to. 434 ! All traffic destined to 10.0.0.1 will now be dropped! 435 ip route 10.0.0.1 255.255.255.255 null0 tag 666 436 ! 438 Filtering router: 439 ! 440 ! The discard route 441 ip route 192.0.2.1 255.255.255.255 Null0 442 ! 443 ! Matches and empty AS-PATH only. 444 ip as-path access-list 10 permit ^$ 445 ! 446 route-map black-hole-filter permit 10 447 match ip address prefix-list only-host-routes 448 match as-path 10 449 match community 65505:666 no-export 450 ! 451 ! Don't accept any other announcements with the RTBH community. 452 route-map black-hole-filter deny 20 453 match community 65505:666 454 ! 455 route-map black-hole-filter permit 30 456 ! 457 ! An interface for source-based RTBH with uRPF loose mode. 458 interface FastEthernet 0/0 459 ip verify unicast source reachable-via any 461 Appendix B: Juniper Configuration Sample 463 This section provides a partial configuration for configuring RTBH on 464 a Juniper router. This is not a complete configuration and should be 465 customized for before being used. 467 Announcing router: 469 routing-options { 470 static { 471 /* EXAMPLE ATTACK SOURCE */ 472 route 10.11.12.66/32 { 473 next-hop 192.0.2.1; 474 resolve; 475 tag 666; 476 } 477 /* EXAMPLE ATTACK DESTINATION */ 478 route 10.128.0.2/32 { 479 next-hop 192.0.2.1; 480 resolve; 481 tag 666; 482 } 483 } 484 autonomous-system 100; 485 } 487 protocols { 488 bgp { 489 group ibgp { 490 type internal; 491 export rtbh; 492 neighbor 172.16.0.2; 493 } 494 } 495 } 497 policy-options { 498 policy-statement rtbh { 499 term black-hole-filter { 500 from { 501 tag 666; 502 route-filter 0.0.0.0/0 prefix-length-range /32-/32; 503 } 504 then { 505 local-preference 200; 506 origin igp; 507 community set black-hole; 508 community add no-export; 509 next-hop 192.0.2.1; 510 accept; 511 } 512 } 513 } 514 community black-hole members 100:666; 515 community no-export members no-export; 516 } 518 Filtering router: 520 policy-statement black-hole-filter { 521 from { 522 protocol bgp; 523 as-path LocalOnly; 524 community black-hole; 525 route-filter 0.0.0.0/0 prefix-length-range /32-/32; 526 } 527 then { 528 community set no-export; 529 next-hop 192.0.2.1; 530 } 531 } 532 community black-hole members 100:666; 533 community no-export members no-export; 535 routing-options { 536 forwarding-table { 537 unicast-reverse-path feasible-paths; 538 } 539 static { 540 route 192.0.2.1/32 discard; 541 } 542 } 544 interfaces { 545 xe-1/0/0 { 546 vlan-tagging; 547 mtu 9192; 548 unit 201 { 549 vlan-id 201; 550 family inet { 551 rpf-check; 552 address 10.11.12.1/24; 553 } 554 } 556 } 557 } 559 Appendix C: A Brief History of RTBH 561 Understanding the history and motivation behind the development of a 562 technique often helps with understanding how to best utilize the 563 technique. In this spirit we present a history of Unicast RPF and 564 RTBH. 566 This section provided by Barry Raveendran Greene: 568 Unicast RPF Loose Check (uRPF Loose Check) was created by Neil Jarvis 569 and Barry Greene to be used with dRTBH as a rapid reaction tool to 570 DDOS Attacks. The requirements for this rapid reaction tool was based 571 on post mortem conversation after the Feb 2000 attacks on several big 572 content hosting companies. The summary of the requirement became the 573 "Exodus Requirement" which stated: 575 "We need a tool to drop packets based on source IP address that can 576 be pushed out to over 60 routers within 60 seconds, be longer than a 577 thousand lines, be modified on the fly, and work in all your 578 platforms filtering at line rate." 580 A variety of options were looked at to meet this requirement, from 581 reviving COPS, to pushing out ACLs with BGP, creating a new protocol. 582 In 2000, the quickest way to meet the "Exodus requirement" was to 583 marry two functions. First, modify Unicast RPF so that the interface 584 check was no longer required and to make sure that a "null" or 585 discard route would drop the packet (i.e. loose check). Second, the 586 technique where BGP is used to trigger a distributed drop is dusted 587 off and documented. Combining these two techniques was deemed to 588 fast way to put a distributed capability to drop packets out into the 589 industry. 591 To clarify and restate, uRPF Loose Check was created as one part of a 592 rapid reaction tool to DDOS attacks that "drop packets based on 593 source IP address that can be pushed out to over 60 routers with in 594 60 seconds, be longer than a thousand lines, be modified on the fly, 595 and work in all your platforms filtering at line rate." The secondary 596 benefits of using uRPF Loose Check for other functions is a secondary 597 benefit - not the primary goal for its creation. 599 To facilitate the adoption to the industry, uRPF Loose Check was not 600 patent. It was publicly published and disclosed in "Unicast Reverse 601 Path Forwarding (uRPF) Enhancements for the ISP-ISP Edge "Remote 602 Triggering Black Hole Filtering," by Barry Raveendran Greene and Neil 603 Jarvis [ftp://ftp- 604 eng.cisco.com/cons/isp/documents/uRPF_Enhancement.pdf]. 606 Authors' Addresses 608 Warren Kumari 609 Google 610 1600 Amphitheatre Parkway 611 Mountain View, CA 94043 612 Email: warren@kumari.net 614 Danny McPherson 615 Arbor Networks, Inc. 616 Email: danny@arbor.net