Internet Engineering Task Force (IETF) P. Mohapatra
Request for Comments: 6811 Cisco Systems
Category: Standards Track J. Scudder
ISSN: 2070-1721 Juniper Networks
D. Ward
Cisco Systems
R. Bush
Internet Initiative Japan
R. Austein
Dragon Research Labs
January 2013
BGP Prefix Origin Validation
Abstract
To help reduce well-known threats against BGP including prefix mis-
announcing and monkey-in-the-middle attacks, one of the security
requirements is the ability to validate the origination Autonomous
System (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 by the prefix holder to do so. This document describes a
simple validation mechanism to partially satisfy this requirement.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6811.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . . 4
2. Prefix-to-AS Mapping Database . . . . . . . . . . . . . . . . . 4
2.1. Pseudo-Code . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Policy Control . . . . . . . . . . . . . . . . . . . . . . . . 7 6
4. Interaction with Local Cache . . . . . . . . . . . . . . . . . 7
5. Deployment Considerations . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 8 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informational References . . . . . . . . . . . . . . . . . 9
1. Introduction
A BGP route associates an address prefix with a set of Autonomous
Systems (ASes) 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 [RFC4271] and starts with the AS that
originated the prefix. To help reduce well-known threats against BGP
including prefix mis-announcing 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 by the prefix holder to do so. 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 verifiable database of IP addresses and AS
numbers as resources. The overall architecture of RPKI as defined in
[RFC6480] consists of three main components:
o a public key infrastructure (PKI) with the necessary certificate
objects,
o digitally signed routing objects, and
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 (ROAs)
[RFC6482] 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, Regional Internet Registry (RIR) hierarchy, and ISPs.
In order to benefit from the RPKI system, it is envisioned that
relying parties at either the 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.
Individual BGP speakers can utilize the processed data contained in
the local cache 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 [RFC6810] for
such a mechanism). This document proposes a means by which a BGP
speaker can make use of the processed data in order to assign a
"validation state" to each prefix in a received BGP UPDATE message.
Note that the complete path attestation against the AS_PATH attribute
of a route is outside the scope of this document.
Like the DNS, the global RPKI presents only a loosely consistent
view, depending on timing, updating, fetching, etc. Thus, one cache
or router may have different data about a particular prefix than
another cache or router. There is no 'fix' for this; it is the
nature of distributed data with distributed caches.
Although RPKI provides the context for this document, it is equally
possible to use any other database that 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" are to
be interpreted as described in RFC 2119 [RFC2119] only when they
appear in all upper case. They may also appear in lower or mixed
case as English words, without special meaning.
2. Prefix-to-AS Mapping Database
The BGP speaker loads validated objects from the cache into local
storage. The objects loaded have the content (IP address, prefix
length, maximum length, origin AS number). We refer to such a
locally stored object as a "Validated ROA Payload" or "VRP".
We define several terms in addition to "VRP". Where these terms are
used, they are capitalized:
o Prefix: (IP address, prefix length), interpreted as is customary
(see [RFC4632]).
o Route: Data derived from a received BGP UPDATE, as defined in
[RFC4271], Section 1.1. The Route includes one Prefix and an
AS_PATH; it may include other attributes to characterize the
prefix.
o VRP Prefix: The Prefix from a VRP.
o VRP ASN: The origin AS number from a VRP.
o Route Prefix: The Prefix derived from a route.
o Route Origin ASN: The origin AS number derived from a Route as
follows:
* the rightmost AS in the final segment of the AS_PATH attribute
in the Route if that segment is of type AS_SEQUENCE, or
* the BGP speaker's own AS number if that segment is of type
AS_CONFED_SEQUENCE or AS_CONFED_SET or if the AS_PATH is empty,
or
* the distinguished value "NONE" if the final segment of the
AS_PATH attribute is of any other type.
o Covered: A Route Prefix is said to be Covered by a VRP when the
VRP prefix length is less than or equal to the Route prefix
length, and the VRP prefix address and the Route prefix address
are identical for all bits specified by the VRP prefix length.
(That is, the Route prefix is either identical to the VRP prefix
or more specific than the VRP prefix.)
o Matched: A Route Prefix is said to be Matched by a VRP when the
Route Prefix is Covered by that VRP, the Route prefix length is
less than or equal to the VRP maximum length, and the Route Origin
ASN is equal to the VRP ASN.
Given these definitions, any given BGP Route will be found to have
one of the following validation states:
o NotFound: No VRP Covers the Route Prefix.
o Valid: At least one VRP Matches the Route Prefix.
o Invalid: At least one VRP Covers the Route Prefix, but no VRP
Matches it.
We observe that no VRP can have the value "NONE" as its VRP ASN.
Thus, a Route whose Origin ASN is "NONE" cannot be Matched by any
VRP. Similarly, no valid Route can have an Origin ASN of zero [AS0].
Thus, no Route can be Matched by a VRP whose ASN is zero.
When a BGP speaker receives an UPDATE from a neighbor, it SHOULD
perform a lookup as described above for each of the Routes in the
UPDATE message. The lookup SHOULD also be applied to routes that are
redistributed into BGP from another source, such as another protocol
or a locally defined static route. An implementation MAY provide
configuration options to control which routes the lookup is applied
to. The validation state of the Route MUST be set to reflect the
result of the lookup. The implementation should consider the
validation state as described in the document as a local property or
attribute of the Route. If validation is not performed on a Route,
the implementation SHOULD initialize the validation state of such a
route to "NotFound".
Use of the validation state is discussed in Sections 3 and 5. An
implementation MUST NOT exclude a route from the Adj-RIB-In or from
consideration in the decision process as a side effect of its
validation state, unless explicitly configured to do so.
We observe that a Route can be Matched or Covered by more than one
VRP. This procedure does not mandate an order in which VRPs must be
visited; however, the validation state output is fully determined.
2.1. Pseudo-Code
The following pseudo-code illustrates the procedure above. In case
of ambiguity, the procedure above, rather than the pseudo-code,
should be taken as authoritative.
result = BGP_PFXV_STATE_NOT_FOUND;
//Iterate through all the Covering entries in the local VRP
//database, pfx_validate_table.
entry = next_lookup_result(pfx_validate_table, route_prefix);
while (entry != NULL) {
prefix_exists = TRUE;
if (route_prefix_length <= entry->max_length) {
if (route_origin_as != NONE
&& entry->origin_as != 0
&& route_origin_as == entry->origin_as) {
result = BGP_PFXV_STATE_VALID;
return (result);
}
}
entry = next_lookup_result(pfx_validate_table, input.prefix);
}
//If one or more VRP entries Covered the route prefix, but
//none Matched, return "Invalid" validation state.
if (prefix_exists == TRUE) {
result = BGP_PFXV_STATE_INVALID;
}
return (result);
3. Policy Control
An implementation MUST provide the ability to match and set the
validation state of routes as part of its route policy filtering
function. Use of validation state in route policy is elaborated in
Section 5. For more details on operational policy considerations,
see [ORIGIN-OPS].
An implementation MUST also support four-octet AS numbers [RFC4893].
4. Interaction with Local Cache
Each BGP speaker supporting prefix validation as described in this
document is expected to communicate with one or more RPKI caches,
each of which stores a local copy of the global RPKI database. The
protocol mechanisms used to gather and validate these data and
present them to BGP speakers are described in [RFC6810].
The prefix-to-AS mappings used by the BGP speaker are expected to be
updated over time. When a mapping is added or deleted, the
implementation MUST re-validate any affected prefixes and run the BGP
decision process if needed. An "affected prefix" is any prefix that
was matched by a deleted or updated mapping, or could be matched by
an added or updated mapping.
5. Deployment Considerations
Once a Route is selected for validation, it is categorized according
the procedure given in Section 2. Subsequently, routing policy as
discussed in Section 3 can be used to take action based on the
validation state.
Policies that could be implemented include filtering routes based on
validation state (for example, rejecting all "invalid" routes) or
adjusting a route's degree of preference in the selection algorithm
based on its validation state. The latter could be accomplished by
adjusting the value of such attributes as LOCAL_PREF. Considering
invalid routes for BGP decision process is a purely local policy
matter and should be done with utmost care.
In some cases (particularly when the selection algorithm is
influenced by the adjustment of a route property that is not
propagated into Internal BGP (IBGP)) it could be necessary for
routing correctness to propagate the validation state to the IBGP
peer. This can be accomplished on the sending side by setting a
community or extended community based on the validation state, and on
the receiving side by matching the (extended) community and setting
the validation state.
6. 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 (or remove) one or more
records into (or from) the validation database, it could lead an
otherwise valid route to be marked as 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.
7. Acknowledgments
The authors wish to thank Rex Fernando, Hannes Gredler, Mouhcine
Guennoun, Russ Housley, Junaid Israr, Miya Kohno, Shin Miyakawa, Taka
Mizuguchi, Hussein Mouftah, Keyur Patel, Tomoya Yoshida, Kannan
Varadhan, Wes George, Jay Borkenhagen, and Sandra Murphy. The
authors are grateful for the feedback from the members of the SIDR
working group.
Junaid Israr's contribution to this specification was part of his PhD
research work and thesis at University of Ottawa.
8. References
8.1. Normative References
[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
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC4632] Fuller, V. and T. Li, "Classless Inter-domain Routing
(CIDR): The Internet Address Assignment and Aggregation
Plan", BCP 122, RFC 4632, August 2006.
[RFC4893] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
Number Space", RFC 4893, May 2007.
[RFC6482] Lepinski, M., Kent, S., and D. Kong, "A Profile for
Route Origin Authorizations (ROAs)", RFC 6482,
February 2012.
8.2. Informational References
[AS0] Kumari, W., Bush, R., Schiller, H., and K. Patel,
"Codification of AS 0 processing.", Work in Progress,
August 2012.
[ORIGIN-OPS] Bush, R., "RPKI-Based Origin Validation Operation",
Work in Progress, August 2012.
[RFC6480] Lepinski, M. and S. Kent, "An Infrastructure to Support
Secure Internet Routing", RFC 6480, February 2012.
[RFC6810] Bush, R. and R. Austein, "The RPKI/Router Protocol",
RFC 6810, January 2013.
Authors' Addresses
Pradosh Mohapatra
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
EMail: pmohapat@cisco.com
John Scudder
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
USA
EMail: jgs@juniper.net
David Ward
Cisco Systems
170 W. Tasman Drive
San Jose, CA 95134
USA
EMail: dward@cisco.com
Randy Bush
Internet Initiative Japan
5147 Crystal Springs
Bainbridge Island, WA 98110
USA
EMail: randy@psg.com
Rob Austein
Dragon Research Labs
EMail: sra@hactrn.net