Network Working Group P. Mohapatra, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track J. Scudder, Ed.
Expires: September 6, 2009 Juniper Networks
March 5, 2009
BGP Prefix Origin Validation
draft-pmohapat-sidr-pfx-validate-01
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Abstract
A BGP route associates an address prefix with a set of autonomous
systems (AS) that identify the interdomain path the prefix has
traversed in the form of BGP announcements. This set is represented
as the AS_PATH attribute in BGP and starts with the AS that
originated the prefix. To help reduce well-known threats against BGP
including prefix hijacking and monkey-in-the-middle attacks, one of
the security requirements is the ability to validate the origination
AS of BGP routes. More specifically, one needs to validate that the
AS number claiming to originate an address prefix (as derived from
the AS_PATH attribute of the BGP route) is in fact authorized. This
document describes a simple validation mechanism to partially satisfy
this requirement.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
2. Prefix-to-AS Mapping Database . . . . . . . . . . . . . . . . 5
3. Changes to the BGP Decision Process . . . . . . . . . . . . . 6
3.1. Policy Control . . . . . . . . . . . . . . . . . . . . . . 7
4. Route Aggregation . . . . . . . . . . . . . . . . . . . . . . 7
5. Interaction with Local Cache . . . . . . . . . . . . . . . . . 7
6. Deployment Considerations . . . . . . . . . . . . . . . . . . 7
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
10. Security Considerations . . . . . . . . . . . . . . . . . . . 9
11. Normative References . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
A BGP route associates an address prefix with a set of autonomous
systems (AS) that identify the interdomain path the prefix has
traversed in the form of BGP announcements. This set is represented
as the AS_PATH attribute in BGP and starts with the AS that
originated the prefix. To help reduce well-known threats against BGP
including prefix hijacking and monkey-in-the-middle attacks, one of
the security requirements is the ability to validate the origination
AS of BGP routes. More specifically, one needs to validate that the
AS number claiming to originate an address prefix (as derived from
the AS_PATH attribute of the BGP route) is in fact authorized. This
document describes a simple validation mechanism to partially satisfy
this requirement.
The Resource Public Key Infrastructure (RPKI) describes an approach
to build a formally verifyable database of IP addresses and AS
numbers as resources. The overall architecture of RPKI as defined in
[I-D.ietf-sidr-arch] consists of three main components:
o A public key infrastructure (PKI) with the necessary certificate
objects,
o Digitally signed routing objects,
o A distributed repository system to hold the objects that would
also support periodic retrieval.
The RPKI system is based on resource certificates that define
extensions to X.509 to represent IP addresses and AS identifiers
[RFC3779], thus the name RPKI. Route Origin Authorizations (ROA)
[I-D.ietf-sidr-roa-format] are separate digitally signed objects that
define associations between ASes and IP address blocks. Finally the
repository system is operated in a distributed fashion through the
IANA, RIR hierarchy, and ISPs.
In order to benefit from the RPKI system, it is envisioned that
relying parties either at AS or organization level obtain a local
copy of the signed object collection, verify the signatures, and
process them. The cache must also be refreshed periodically. The
exact access mechanism used to retrieve the local cache is beyond the
scope of this document.
Once the cache is made local, individual BGP speakers can utilize the
processed data to validate BGP announcements. The protocol details
to retrieve the processed data from the local cache to the BGP
speakers is beyond the scope of this document (refer to
[I-D.ymbk-rpki-rtr-protocol] for such a mechanism). This document
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proposes a simple modification to the BGP decision process that makes
use of the processed data from signed objects and validates prefix
origination of received BGP UPDATE messages.
Note that the complete path attestation against the AS_PATH attribute
of a route is outside the scope of this document.
Although RPKI provides the context for this draft, it is equally
possible to use any other database which is able to map prefixes to
their authorized origin ASes. Each distinct database will have its
own particular operational and security characteristics; such
characteristics are beyond the scope of this document.
1.1. 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].
2. Prefix-to-AS Mapping Database
The resource certificates and other signed objects (e.g. ROAs) as
received from the RPKI repository and stored in the local cache are
not in a suitable format to be matched against the prefixes received.
Moreover, further processing of the objects is necessary -- e.g. ROA
validation is required, which involves checking against the
corresponding EE certificate and so on up to configured trust
anchors, presumably for the IANA and/or other registries. But a
validated and normalized database can be created on the router for
efficient lookup purposes. The primary key for this database is a
prefix set represented as (IP prefix)/[min. length, max. length].
The value stored against each prefix set is the set of AS numbers
that is assigned or sub-allocated the corresponding IP address block.
This database can be implemented as a prefix trie structure.
Whenever UPDATEs are received from peers, a BGP speaker is expected
to perform a lookup in this database for each of the prefixes in the
UPDATE message. To aid with better description, we define terms
"UPDATE prefix" and "UPDATE origin AS number" to denote the values
derived from the received UPDATE message, and "database prefix set"
and "database origin AS number set" to mean the values derived from
the database lookup. Note that in the presence of overlapping
prefixes, the database lookup against the "UPDATE prefix" may yield
multiple matches.
The following are the different types of results expected from such a
lookup operation:
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o If the "UPDATE prefix" finds no matching or covering prefixes in
the database, the lookup result is returned as "not found". Due
to incremental deployment model of the RPKI repository, it is
expected that a complete registry of all IP address blocks and
their AS associations is not available at a given point of time.
o If there are "database prefix sets" that cover the "UPDATE
prefix", and one of them has the "UPDATE origin AS number" in the
"database origin AS number sets", then the lookup result is
returned as "valid".
o If there are "database prefix sets" which cover the "UPDATE
prefix", but none of them has the "UPDATE origin AS number" in the
"database origin AS number set", then the lookup result is
returned as "invalid".
Depending on the lookup result, we define a property for each "UPDATE
prefix", called as the "validity state" of the prefix. It can assume
the following values:
+-------+-----------------------------+
| Value | Meaning |
+-------+-----------------------------+
| 0 | Lookup result = "valid" |
| 1 | Lookup result = "not found" |
| 2 | Lookup result = "invalid" |
+-------+-----------------------------+
Note that all the routes, regardless of their "validity state" will
be stored in the local BGP speaker's Adj-RIB-In.
3. Changes to the BGP Decision Process
If a BGP router supports prefix validation and is configured to do
so, the validation check MUST be performed prior to any of the steps
defined in the decision process of [RFC4271]. The validation step is
stated as follows:
When comparing routes for a BGP destination, if both routes have
had their "validity state" computed, the route with the lowest
"validity state" value is preferred.
In all other respects, the decision process remains unchanged.
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3.1. Policy Control
It MUST be possible to enable or disable the validation step as
defined in Section 3 through configuration. The default SHOULD be
for the validation step to be enabled.
It MUST be possible to exclude routes from the BGP decision process
based on their validation state. In particular it is anticipated
that it will be desirable to exclude routes from consideration when
their validation state is "invalid"; however it may also be desirable
to exclude routes whose validation state is "not found" as well.
4. Route Aggregation
When an UPDATE message carries AGGREGATOR attribute, the "UPDATE
origin AS number" is set to the value encoded in the AGGREGATOR
instead of being derived from the AS_PATH attribute.
5. Interaction with Local Cache
Each BGP speaker supporting prefix validation as described in this
document is expected to communicate with one or multiple local caches
that store a database of RPKI signed objects. The protocol
mechanisms used to fetch the data and store them locally at the BGP
speaker is beyond the scope of this document. Irrespective of the
protocol, the prefix validation algorithm as outlined in this
document is expected to function correctly in the event of failures
and other timing conditions that may result in an empty and/or
partial prefix-to-AS mapping database. Indeed, if the (in-PoP) cache
is not available and the mapping database is empty on the BGP
speaker, all the lookups will result in "not found" state and the
prefixes will be advertised to rest of the network (unless restricted
by policy configuration). Similarly, if BGP UPDATEs arrive at the
speaker while the fetch operation from the cache is in progress, some
prefix lookups will also result in "not found" state. The
implementation is expected to handle these timing conditions and re-
validate the prefixes once the fetch operation is complete (in an
event-driven manner).
6. Deployment Considerations
It is critical that IBGP speakers within an AS have a consistent
routing view of the BGP destinations and do not make conflicting
decisions regarding the BGP best path selection that might cause
forwarding loops. Thus, the best practice in BGP deployment does not
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run any policy on IBGP sessions which could potentially create an
inconsistent view. Going by the same rules, the prefix validation
procedures SHOULD NOT be performed on IBGP learnt routes in an AS.
As a general principle, prefix validation SHOULD be executed on EBGP
boundaries. In some cases, it may be desirable to run the validation
on centralized route servers within an AS to offload the computation.
Care should be taken to ensure routing consistency in such cases.
7. Contributors
David Ward dward@cisco.com
Cisco Systems
Rex Fernando rex@juniper.net
Miya Kohno mkohno@juniper.net
Juniper Networks
Shin Miyakawa miyakawa@nttv6.jp
Taka Mizuguchi
Tomoya Yoshida
NTT Communications
Randy Bush randy@psg.com
Internet Initiative Japan
Rob Austein sra@isc.org
ISC
Russ Housley housley@vigilsec.com
Vigil Security
Junaid Israr jisra052@uottawa.ca
Mouhcine Guennoun mguennou@uottawa.ca
Hussein Mouftah mouftah@site.uottawa.ca
University of Ottawa School of Information Technology and
Engineering(SITE) 800 King Edward Avenue, Ottawa, Ontario, Canada,
K1N 6N5
8. Acknowledgements
Junaid Israr's contribution to this specification is part of his PhD
research work and thesis at University of Ottawa, Canada.
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9. IANA Considerations
10. Security Considerations
Although this specification discusses one portion of a system to
validate BGP routes, it should be noted that it relies on a database
(RPKI or other) to provide validation information. As such, the
security properties of that database must be considered in order to
determine the security provided by the overall solution. If
"invalid" routes are blocked as this specification suggests, the
overall system provides a possible denial-of-service vector, for
example if an attacker is able to inject one or more spoofed records
into the validation database which lead a good route to be declared
invalid. In addition, this system is only able to provide limited
protection against a determined attacker -- the attacker need only
prepend the "valid" source AS to a forged BGP route announcement in
order to defeat the protection provided by this system. This
mechanism does not protect against "AS in the middle attacks" or
provide any path validation. It only attempts to verify the origin.
In general, this system should be thought of more as a protection
against misconfiguration than as true "security" in the strong sense.
11. Normative References
[I-D.ietf-sidr-arch]
Lepinski, M., Kent, S., and R. Barnes, "An Infrastructure
to Support Secure Internet Routing",
draft-ietf-sidr-arch-03 (work in progress), February 2008.
[I-D.ietf-sidr-roa-format]
Kent, S., "A Profile for Route Origin Authorizations
(ROAs)", draft-ietf-sidr-roa-format-03 (work in progress),
July 2008.
[I-D.ymbk-rpki-rtr-protocol]
Bush, R., "The RPKI/Router Protocol",
draft-ymbk-rpki-rtr-protocol-00 (work in progress),
March 2009.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3779] Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779, June 2004.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
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Protocol 4 (BGP-4)", RFC 4271, January 2006.
Authors' Addresses
Pradosh Mohapatra (editor)
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
Email: pmohapat@cisco.com
John Scudder (editor)
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
USA
Email: jgs@juniper.net
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