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Network Working Group                                            H. Chen
Internet-Draft                                             China Telecom
Intended status: Standards Track                                   D. Ma
Expires: September 10, 2020                                         ZDNS
Y. Gu
S. Zhuang
H. Wang
Huawei
March 09, 2020

Enhanced AS-Loop Detection for BGP
draft-chen-grow-enhanced-as-loop-detection-04

Abstract

Misconfiguration and malicious manipulation of BGP AS_Path may lead
to route hijack.  This document proposes to enhance the BGP [RFC4271]
Inbound/ Outbound route processing in the case of detecting an AS
loop.  It is an enhancement to the current BGP's Inbound/Outbound
processing and can be implemented directly on the device, and this
document also proposes a centralized usecase.  This could empower
networks to quickly and accurately figure out they're being
victimized.

Two options are proposed for the enhancement, a) a local check at the
device; b) data collection/analysis at the remote network controller/
server.  Both approaches are beneficial for route hijack detection.

Requirements Language

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 [RFC2119].

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF).  Note that other groups may also distribute
working documents as Internet-Drafts.  The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any

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time.  It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

This Internet-Draft will expire on September 10, 2020.

Copyright (c) 2020 IETF Trust and the persons identified as the

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document.  Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document.  Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
3.  Forged AS_PATH Examples . . . . . . . . . . . . . . . . . . .   4
3.1.  AS Loop Detected at Inbound Processing  . . . . . . . . .   4
3.2.  AS Loop Detected at Outbound Processing . . . . . . . . .   5
4.  Enhancement to BGP Inbound/Outbound Processing  . . . . . . .   6
4.1.  Enhancement for AS Loop Detected at Inbound Process . . .   6
4.2.  Enhancement for AS Loop Detected at Outbound Process  . .   7
5.  Centralized AS-Loop Detection for BGP . . . . . . . . . . . .   7
5.1.  BMP Support for Monitoring AS Path Looped Update Message    7
5.2.  Application Example . . . . . . . . . . . . . . . . . . .   8
6.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
10. Normative References  . . . . . . . . . . . . . . . . . . . .  11
Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

The Border Gateway Protocol (BGP) [RFC4271], as an inter-autonomous
(AS) routing protocol, is used to exchange network reachability
information between BGP systems.  BGP is widely used by Internet
Service Providers (ISPs) and large organizations.

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As a distance-vector based protocol, BGP is used to exchange
reachable inter-AS routes, establish inter-AS paths, avoid routing
loops, and apply routing policies between ASs.  BGP loop detection
mechanism is defined in section 9.1.2. of RFC4271:

...

If the AS_PATH attribute of a BGP route contains an AS loop, the
BGP route should be excluded from the Phase 2 decision function.
AS loop detection is done by scanning the full AS path (as
specified in the AS_PATH attribute), and checking that the
autonomous system number of the local system does not appear in
the AS path.  Operations of a BGP speaker that is configured to
accept routes with its own autonomous system number in the AS path
are outside the scope of this document.

...

In ordinary BGP, every AS announces its route information with
different prefixes.  However, its neighboring ASes cannot validate
this route information, but rather directly propagate it across the
Internet or simply discard AS-Loop routes directly.  Obviously, this
weak trust model allows forged route announcement propagations and
rarely been found, which is a fundamental security weakness of BGP.
Forged routes, which can be generated by configuration errors or
malicious attacks, can lead to large-scale network connectivity
issues.

Some cases can be worse, hackers exploit this property of BGP to
achieve their ulterior motives.  They can add some providers' AS
number into the forged AS-Path and attempt to make it look like the
route had passed through these ASNs, or perhaps they are there to
prevent those providers from carrying the route.  These cases are
also being known As-Path Poisoning Attacks.

ASPA [I-D.ietf-sidrops-aspa-verification] can be used to verify the
AS_PATH attribute of routes advertised in the Border Gateway
Protocol, and it is a systematic deployment based on RPKI system.
This mechanism requires a series of infrastructure implementations.

This document proposes to enhance AS-Loop Detection for BGP Inbound/
Outbound Route Processing when detecting AS loop in order to identify
possible BGP hijacks.  It is an enhancement to the current BGP's
Inbound/Outbound processing and can be implemented directly on the
device, and this document also proposes a centralized usecase.  This
could empower networks to quickly and accurately figure out they're
being victimized.

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2.  Terminology

The following terminology is used in this document.

AS: Autonomous System

ASPA: Autonomous System Provider Authorization

BGP: Border Gateway Protocol

BGP hijacking : is the illegitimate takeover of groups of IP
addresses by corrupting Internet routing tables maintained using the
Border Gateway Protocol (BGP).  (Sometimes referred to as prefix
hijacking, route hijacking or IP hijacking)

EBGP: External BGP

ISP: Internet Service Provider

BMP: BGP Monitoring Protocol

ROA: Route Origin Authorization

3.  Forged AS_PATH Examples

3.1.  AS Loop Detected at Inbound Processing

o  Forged Case 1: AS shown in Figure 1, an upstream AS of AS64596
forged a route with the ASN 64596 as the origin ASN in the AS-
Path.

o  Forged Case 2: AS shown in Figure 1, an upstream AS of AS64596
forged a route with the ASN 64596 as the transit ASN in the AS-
Path.

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AS-Loop-Detecting at this point
Discard AS-Loop Routes directly that contains AS64596
|
|                                 x.y.z.0/24
v                                 Origin AS 64600
AS64595---AS64596---AS64597---AS64598---AS64599----AS64600
Normal Case:
<-- x.y.z.0/24, AS-Path: 64597 64598 64599 64600

Forged Case 1:
<-- x.y.z.0/24, AS-Path: 64597 64596
(Or: 64597 64598 64596 etc.)

Forged Case 2:
<-- x.y.z.0/24, AS-Path: 64597 64596 64600
(Or: 64597 64596 64599 64600 etc.)

Figure 1: BGP Inbound Route Processing

After receiving the above routes, AS64596 treats them as normal loop
routes during the loop detecting phase and discards them directly.
In most NOSes (Network Operation Systems), such rejected routes are
not logged and only visible by putting the router into debugging
mode.  If the AS64596 is slightly enhanced, it can find that someone
has faked himself, which may cause unnecessary trouble for himself.

3.2.  AS Loop Detected at Outbound Processing

Split-Horizon for EBGP is an optional function that a BGP sender will
AS.  In some current implementation, the BGP outbound route
processing step will simply discard the route if AS-Loop being
detected.

o  Forged Case 3: AS shown in Figure 2, an upstream AS of AS64597
forged a route with the ASN 64596 as the origin ASN in the AS-
Path.

o  Forged Case 4: AS shown in Figure 2, an upstream AS of AS64597
forged a route with the ASN 64596 as the transit ASN in the AS-
Path.

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Split-Horizon Enable & AS-Loop-Detecting at this point
Discard AS-Loop Routes directly if sending AS-Path contains AS64596
|
|                               x.y.z.0/24
v                               Origin AS 64600
AS64595---AS64596---AS64597---AS64598---AS64599----AS64600
Normal Case:
<-- x.y.z.0/24, AS-Path: 64597 64598 64599 64600

Forged Case 3:
<-- x.y.z.0/24, AS-Path: 64597 64596
(Or: 64597 64598 64596 etc.)

Forged Case 4:
<-- x.y.z.0/24, AS-Path: 64597 64596 64600
(Or: 64597 64596 64599 64600 etc.)

Figure 2: BGP Outbound Route Processing

When sending the above routes, AS64597 treats them as normal loop
routes and discards them directly.  If AS64597 is slightly enhanced,
it can find that someone has faked AS64596, which may cause large-
scale network connectivity problems.

4.  Enhancement to BGP Inbound/Outbound Processing

4.1.  Enhancement for AS Loop Detected at Inbound Process

Currently, ROV [RFC6811] and ASPA verification
[I-D.ietf-sidrops-aspa-verification] can be adopted for BGP leak/
hijack detection.  However, for the forged case 1&2, the conventional
BGP inbound process would simply discard the routes with AS loop
before any further leak/hajack detection.

This document suggests further analysis of such routes.  The analysis
may include mechanisms that apply to normal routes for hijack
detection, such as ROV, ASPA and so on.  The detailed analyzing
mechanisms as well as the corresponding actions w.r.t. the analysis
are outside the scope of this document.  Two options of where the
analysis of the inbound processing enhancement takes place is
proposed.

o  Option 1: Analyze the routes with AS loop based on local database.

o  Option 2: Collect the routes with AS loop with BMP and analyze
them at the remote controller/server.

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4.2.  Enhancement for AS Loop Detected at Outbound Process

Currently, the egress ROV can be adopted for BGP hijack detection.
However, for forged case 3&4, when eBGP Split-Horizon is enabled, the
routes with AS loop could possibly be discarded before any hijack
detection.

This document suggests further analysis of such routes.  The analysis
may include mechanisms that apply to normal routes for hijack
detection, such as egress ROV, ASPA and so on.  The detailed
analyzing mechanisms as well as the corresponding actions w.r.t. the
analysis are outside the scope of this document.

Two options of where the analysis of the outbound processing
enhancement takes place is proposed.

o  Option 1: Analyze the routes with AS loop based on local database.

o  Option 2: Collect the routes with AS loop with BMP and analyze
them at the remote controller/server.

5.  Centralized AS-Loop Detection for BGP

Considering the challenges facing the existing approaches, this
section proposes a centralized method.  It utilizes the BGP
Monitoring Protocol (BMP) to convey the AS Path Looped Update message
from the monitored device to the BMP server to realize centralized
attack detection.

BMP is currently deployed by OTT and Carriers to monitor the BGP
routes, such as monitoring BGP Adj-RIB-In using the process defined
in RFC7854 [RFC7854], and monitoring BGP Adj-RIB-Out using the
process defined in draft-ietf-grow-bmp-adj-rib-out [RFC8671].  This
document extends Route Mirroring message to mirror AS Path Looped
update message to the BMP Server.

5.1.  BMP Support for Monitoring AS Path Looped Update Message

Per RFC7854, Route Mirroring messages can be used to mirror the
messages that have been treated-as-withdraw [RFC7606], for debugging
purposes.  This document extends Route Mirroring message to mirror AS
Path Looped update message to the BMP Server.

This document adds a new code for Type 1 Information TLV:

o  Code = TBD: AS Path Looped.  The BGP Message TLV occurs in the
Route Mirroring message and whose loop includes the local AS.

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TLV (Type = 1) with Code = TBD: AS Path Looped, and then a BGP
Message TLV (Type = 0) contain an AS Path Looped Update Message.

+---------------------------------------------------------------+
|         Common BMP Header (Message Type = 6)                  |
+---------------------------------------------------------------+
+---------------------------------------------------------------+
| Information TLV (Type = 1) with Code = TBD: AS Path Looped    |
+---------------------------------------------------------------+
|         BGP Message TLV (Type = 0)                            |
+---------------------------------------------------------------+

Figure 3: AS Path Looped Update Message Carrying in the Route
Mirroring Message

5.2.  Application Example

This section describe a centralized application example.  As shown in
Figure 4, when receiving the routes from AS64597, AS64596 should
check whether its own AS number is already in the AS-Path, If yes, it
further encapsulate the AS Path Looped Update Message in the Route
Mirroring message and sends the Route Mirroring message to the BMP
Server.

The Analyzer gets the AS Path Looped Update Messages from the BMP
Server and further processes them.

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+------------+
| BMP server |
|     +      |
|  Analyzer  |
+------+-----+
BMP Mirroring Message:      \
AS Path Looped Update Message\to BMP Server
\
\
********************|****
*                   |   *
*          AS64596  |   *        "AS64597 Send Routes
*                   |   *  <----- to AS64596"
+-------+    *  +---+         +---+  *  +-------+
+AS64595+-------+ R1+---------+ R2|-----+AS64597+---64598 64599 64600
+-------+    *  +-+-+\        +---+  *  +-------+
*    |   \\    //  |\   *
*    |     \\//    | \  *
*    |     //\     |  \ *      +-------+
*    |   //   \\   |   \+------+AS64593+
*    |  /       \  |    *      +-------+
+-------+    *  +-+-+         +-+-+  *      +-------+
+AS64594+----+--+ R3+---------+ R4+---------+AS64592+
+-------+    *  +---+         +---+  *      +-------+
*                       *
*************************

Figure 4: Centralized AS-Loop Detection

From the perspective of the local AS, it can manage/hold the AS-
relationship database between the local AS and each of its
neighboring ASs (such as C2P, P2P, P2C, etc.).

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+--------------------------------------------------+
|   Neighboring AS  |  AS-relationship to AS64596  |
+--------------------------------------------------+
|   64592           |  P2P                         |
+--------------------------------------------------+
|   64593           |  S2S                         |
+--------------------------------------------------+
|   64594           |  C2P                         |
+--------------------------------------------------+
|   64595           |  P2C                         |
+--------------------------------------------------+
|   64597           |  P2P                         |
+--------------------------------------------------+

Figure 5: AS64596's AS-Relationship Database

When AS 64596 is listed as transit AS in the AS-Path, for example,
AS-Path looks like the following form AS64596's perspective:

(possible other ASes), left AS, local AS(64596), right AS, (possible
other ASes)

At this point, AS64596's Analyzer can lookup the local resource
database and check whether there is a real AS relationship between
the local AS and the left AS and the right AS.

6.  Benefits

After the enhancements of the AS Loop Detection for BGP Inbound/
Outbound Route Processing are added, the stability and security of
the network can be improved.

7.  Acknowledgements

The authors would like to acknowledge the review and inputs from Gang
Yan, Zhenbin Li, Aijun Wang, Jeff Haas, Robert Raszuk, Chris Morrow,
Alexander Asimov, Ruediger Volk, Jescia Chen and the working group.

8.  IANA Considerations

This document defines one type for information carried in the Route
Mirroring Information (Section 4.7 of RFC7854) code:

o  Code = TBD: AS Path Looped.

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9.  Security Considerations

This document does not change the underlying security issues in the
BGP protocol.  It however, does provide an additional mechanism to
protect against attacks based on the forged AS-Path in the BGP
routes.

10.  Normative References

[I-D.ietf-sidrops-aspa-verification]
Azimov, A., Bogomazov, E., Bush, R., Patel, K., and J.
Snijders, "Verification of AS_PATH Using the Resource
Certificate Public Key Infrastructure and Autonomous
System Provider Authorization", draft-ietf-sidrops-aspa-
verification-03 (work in progress), November 2019.

[RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.

[RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
"Multiprotocol Extensions for BGP-4", RFC 4760,
DOI 10.17487/RFC4760, January 2007,
<https://www.rfc-editor.org/info/rfc4760>.

[RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
Austein, "BGP Prefix Origin Validation", RFC 6811,
DOI 10.17487/RFC6811, January 2013,
<https://www.rfc-editor.org/info/rfc6811>.

[RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
Patel, "Revised Error Handling for BGP UPDATE Messages",
RFC 7606, DOI 10.17487/RFC7606, August 2015,
<https://www.rfc-editor.org/info/rfc7606>.

[RFC7854]  Scudder, J., Ed., Fernando, R., and S. Stuart, "BGP
Monitoring Protocol (BMP)", RFC 7854,
DOI 10.17487/RFC7854, June 2016,
<https://www.rfc-editor.org/info/rfc7854>.

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[RFC8671]  Evens, T., Bayraktar, S., Lucente, P., Mi, P., and S.
Zhuang, "Support for Adj-RIB-Out in the BGP Monitoring
Protocol (BMP)", RFC 8671, DOI 10.17487/RFC8671, November
2019, <https://www.rfc-editor.org/info/rfc8671>.

Huanan Chen
China Telecom
109, West Zhongshan Road, Tianhe District
Guangzhou  510000
China

Email: chenhuan6@chinatelecom.cn

Di Ma
ZDNS
4 South 4th St. Zhongguancun
Beijing, Haidian
China

Yunan Gu
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing  100095
China

Email: guyunan@huawei.com

Shunwan Zhuang
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing  100095
China

Email: zhuangshunwan@huawei.com

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Haibo Wang
Huawei
Huawei Bld., No.156 Beiqing Rd.
Beijing  100095
China

Email: rainsword.wang@huawei.com

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