Routing Protocol Security B. Christian, Ed.
Requirements KMC Telecom Solutions
Internet-Draft Feb 2004
Expires: August 1, 2005
BGP Security Requirements
draft-ietf-rpsec-bgpsecrec-01
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Abstract
The security of BGP is critical to the proper operation of
large-scale internetworks, both public and private. While securing
the information transmitted between two BGP speakers is a relatively
easy technical matter, securing BGP, as a routed system, is more
complex. This document describes a set of requirements for securing
BGP, including securing peering relationships between BGP speakers,
and authenticating the routing information carried within BGP.
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1. Introduction
1.1 System Description
BGP is described in RFC1771 [3] as a routing protocol. Essentially,
BGP speakers exchange information about reachable destinations in an
internetwork through a peering session. Once this information has
been exchanged, each BGP speaker locally determines a loop free path
to each reachable destination, based on local policy, policy
indicators (or policies) carried in the update, and the AS Path
carried in the BGP update.
1.2 Threats
Threats to networking protocols generally fall under one of the three
categories as defined in RFC 2196 [1]:
o Unauthorized access to resources and/or information
o Unintended and/or unauthorized disclosure of information
o Denial of service
A number of attacks can be realized which, if exploited, can lead to
one of the above mentioned threats. These are typically classified
as passive attacks and active attacks. Passive attacks are ones
where an attacker simply reads information off the network and
obtains confidential and/or private information. Active attacks are
ones where the attacker writes data to the network and can include
replay attacks, message insertion, message deletion, message
modification and man-in-the-middle attacks. These attacks are often
combined.
Attacks that do not involve direct manipulation of BGP, and the
information contained within it, are outside the scope of this
document. When possible, the requirements will attempt to minimize
the extent of the damage that occurs when end systems come under
attack.
The intent of this requirements document is to prevent attacks that
originate false data or create invalid routing paths and therefore
addresses issues relating to data integrity and peer entity
authentication. As described in RFC 3552 [2], data integrity
protection ensures that data is not modified in transit and peer
entity authentication ensures that there is a reasonable guarantee
that the sender and recipient of the data are the intended parties.
Guaranteed packet delivery is not part of the BGP protocol security
model. Just because a packet is addressed to a specific destination
does not mean it will be received, even with a "secure" route. For
example: an attacker could have compromised an intermediate router
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and installed a static route for target address A.B.C.D pointing to
an inappropriate direction or an attacker might splice into a circuit
between two secure routers and install a device that diverts A.B.C.D
traffic without requiring the compromise of control plane devices.
1.3 Areas to secure
There are two primary points where BGP may be secured. If we examine
the system description presented above those points are as follows.
o The session between two BGP speakers can be secured such that the
BGP data received by the BGP speakers can by cryptographically
verified to have been transmitted by the other speaker.
o The originator and the propagators of prefix information may have
their policy information verified such that the intent of the
policy with respect to the specific prefix is preserved
There are also several questions we can ask about the information
contained within a received update.
o Is the originating Autonomous system authorized to propagate the
prefix we have received?
o Is the AS path, received via an UPDATE, valid?
The verification of AS-Path validity falls into three distinct
categories.
o Does the AS-Path specified actually exist and, based on the
AS-PATH, is it possible to traverse that path to reach a given
prefix?
o Has the update actually travelled the path?
o Was the update authorized to traverse the given path by the
originator of the prefix?
In this draft we do not attempt to set requirements dealing with the
third question presented above. However, the needs of the operators
of the systems that use BGP are such that the first two questions are
of key importance to any secured interdomain routing system.
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2. Application Concepts
In order to properly identify security requirements it is important
to cover the fundamental aspects of BGP as related to security
requirements. The following list presents the basic parameters and
application concepts of BGP that will be covered by this document.
o Peer Communication: BGP traffic travels over TCP between peers, so
BGP speakers assume the TCP data delivery guarantees of TCP in a
benign environment. This includes ordered, error-free delivery of
application traffic from a peer identified by an IP address, plus
integrity of the control aspects of TCP. From a security
perspective, these guarantees need to be enforced in the context
of possible active wiretapping.
o Routing and Reachability: BGP is a protocol used to convey routing
and reachability information both internal and external to an
Autonomous System. Typically, internal BGP is used to distribute
prefix reachability information in conjunction with an IGP and is
used by a distinct network administrative entity to convey
internal routing policy regarding external and internal
information. External BGP is typically used to distribute route/
prefix reachability information between two distinct routing
entities and is used to signal forwarding preferences and policy
decisions.
o Inter-AS UPDATE Message assumptions: When an AS distributes
reachability information to a peer it is done with the intent of
affecting routing decisions by the peer. For example, an AS-A
sends peer AS-B a less specific advertisement and peer AS-C a
"more" specific advertisement. This prefix distribution decision
may have been made to provide a means for failure resolution
between AS-A and AS-C. Update messages are sent between AS peers
with the implicit assumption that those messages will be forwarded
to others. A notable exception to this assumption is the use of
various policy based mechanisms between peers such as the
NO-EXPORT community. In this document an important aspect of the
UPDATE message to note is that the specific UPDATE message itself
is typically not re-transmitted. Instead, the specific UPDATE
message is regenerated continually as it passes from BGP speaker
to BGP speaker. Furthermore, UPDATE messages have no mechanism
for freshness (i.e timestamps or sequence numbers). This
indicates that messages may appear valid at any point in the life
of a BGP peering session.
o Inter-AS withdraw message assumptions: The processing model of BGP
RFC1771 [3] indicates that only the peer advertising NLRI
information may withdraw it. There are several instances where a
withdraw may occur. Typical reasons for withdraw include the
determination of a better path, peer session failure, or local
policy change. There is no specified mechanism for indicating, to
an external peer, the reason for a route withdraw. Each withdraw
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received from a valid peering session must be taken at face value.
There is no, existing, method to ensure that an AS will properly
propagate withdraw messages received from it's external peers.
o AS-Path assumptions: Aside from the use of AS-Set, the AS-Path is
typically considered to be an ordered list of the Autonomous
Systems that an update has traversed. In most cases the first AS
in the list is the origin AS, or at least the AS responsible for
the management of the NLRI information associated with the first
AS. Specifications state that the AS topology must be loop free.
This indicates that the appearance of the local AS in an update
received from an external peer is generally not permitted. The
prepending of AS information for received updates and transmitted
updates is generally permitted and is common practice. Prepended
AS information on inbound advertisements (where the external peers
AS is prepended) and outbound advertisements (where the local AS
number is prepended) is a commonly used method to effect
forwarding changes.
o Route Origination: Originating a route without the ability to
forward the traffic associated with that route is, in most cases,
in conflict with the intent of the BGP specification. BGP
speakers may originate routes based on various internal and
external data. An Autonomous System should only originate a
prefix to it's external peers if that prefix has been somehow
allocated to the administrators of that system, or authorized by
the prefix holders.
o Aggregation: Aggregation, and deaggregation, of prefixes can cause
significant issues with proposed security mechanisms. According
to RFC 1771, if a BGP speaker chooses to aggregate a set of more
specific prefixes into a less specific prefix then the
ATOMIC_AGGREGATE attribute SHOULD be set. This creates a
significant potential loophole in an attempt to secure BGP based
on the RFC specifications.
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3. Deployment Requirements
We have determined, through discussion with several large
internetwork operators and equipment vendors, that the following
attributes are important to the ongoing performance of interdomain
routing systems such as BGP
3.1 Convergence speed
Convergence speed is a major concern to many operators of large scale
internetworking systems. Networks, and internetworks, are carrying
ever increasing amounts of information that is time and delay
sensitive; increasing convergence times can adversely affect the
usability of the network, and the ability of an internetwork to grow.
BGP's convergence speed, with a security system in operation, SHOULD
be equivalent to BGP running without the security system in
operation. This includes the preservation of optimizations currently
used to produce acceptable convergence speeds on current hardware,
including update packing, peer groups, and others. Current timers,
including hold timers, keepalive timers, and the peering process,
SHOULD NOT be impacted by the security system. Two types of
verification MAY be offered for the NLRI and the AS_PATH in order to
allow for a selection of optimizations:
o Contents of the UPDATE message SHOULD be authenticated in
real-time as the UPDATE message is processed.
o The route information base MAY be authenticated periodically or in
an event driven manner by scanning the data and verifying the
originating AS and the verifiability of the AS-PATH list.
All BGP implementations that implement security MUST utilize at least
one of the above methods for validating routing information. Real
time verification is preferred in order to prevent transitive
failures based on periodic or event driven scan intervals.
3.2 Incremental deployment
We will not be able to deploy a newly secured BGP protocol
instantaneously and will be unable to dictate a partitioning of large
internetworks by the operators. Because of this, there are several
requirements that any proposed mechanism to secure BGP must consider.
o BGP MUST support transmitting, receiving, and acting on both
secured and unsecured routing information with the security system
in place.
o The security system MUST allow the forming of peering
relationships between peers regardless of whether the security
system is in place
o MUST be backward compatible in the message formatting,
transmission, and processing of routing information carried
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through a mixed security environment. Message formatting in a
fully secured environment MAY be handled in a non-backward
compatible fashion.
o In an environment where both secured and non-secured systems are
interoperating a mechanism MUST exist for secured systems to
identify whether an originator intended the information to be
secured.
3.3 Conditions for initialization
A key factor in the robust nature of the existing internal and
external relationships maintained in todays Internet provider space
is the ability to maintain and return to a significantly converged
state without the need to rely on systems external to the routing
system (the physical equipment that is performing the forwarding).
In order to ensure the rapid initialization and/or return to service
of failed nodes it is important to reduce reliance on external
systems to the greatest extent possible. Therefore, proposed systems
SHOULD NOT require connections to external systems, beyond those
directly involved in peering relationships, in order to return to
full service. Proposed systems MAY require post initialization
synchronization with external systems in order to synchronize
security information.
3.4 Trust level variability
Each secured environment may have different levels of requirements in
terms of what is acceptable or unacceptable. In environments that
require strict security it may not be acceptable to temporarily route
to a destination while waiting for security verification to be
performed. However, in many commercial environments the rapidity of
route installation may be of paramount importance; in order to
facilitate the more common occurence of route withdrawl due to
network failure. Based on the two divergent requirements, the
following criteria apply.
o The security system MUST support a range of possible outputs for
local determination of the trust level for a specific route. Any
given route should be trustable to a locally configured degree,
based on the completeness of security information for the update
and other factors.
o The security system SHOULD allow the operator to determine whether
the speed of convergence is more important than security
operations, or security operations are more important than the
speed of convergence. This facilitates the incremental deployment
of security on systems not designed to support increased
processing requirements imposed by the security system.
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4. The Trust Model
In examining the various environments in which BGP is deployed, and
through discussions with various operators working with the context
of the public Internet, and other internetworks, it is apparent that
trust models are largely environment specific. For instance, in the
public Internet, a distributed trust model, following the current
transitive trust pattern of contractual and peering arrangements,
would fit the the business models of the participants. In other
environments a hierarchical trust model would work better. Thus, any
trust system specified in a security mechanism designed for BGP must
be flexible, and support both a true distributed trust model and a
fully hierarchical trust model.
Since hierarchical trust models are a subset (or a special case of) a
distributed trust model, any security system designed for BGP MUST
support a distributed trust model, and MUST also support a
hierarchical trust model, if desired.
If two internetworks using differing trust models are interconnected
they MUST be able to interoperate using locally determined levels of
trust to compensate for differences in their trust models.
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5. The AS-Path Attribute and NLRI Authentication
BGP distributes routing information across the Internet (between BGP
speakers) using BGP UPDATE messages. The UPDATE message contains
withdrawn routes, path attributes and one or more NLRIs (Network
Layer Reachability Information is synonymous with advertised prefix).
For the remainder of this section, we will focus on the AS-Path
Attribute and the NLRI. Attributes such as local pref are locally
specific and, as such, are protected by BGP session security.
The AS_PATH for specific prefixes must be protected in any proposed
security system in three ways:
o Authorization of Originating AS: For all prefixes announced in
BGP, the originating AS MUST be verifiable through the trust model
as the authorized announcer of the prefix. The verification
mechanism must account for existing BGP mechanisms such as
summarization. For the purpose of this document the term
verifiable is defined as the resultant of a secured routing
systems as described in this document. The term specifically
indicates the ability to validate the originator of a specific
prefix (or block of IP addresses) and the ability to validate the
session through which the prefix was received
o The AS_PATH list MUST correspond to a verifiable list of
autonomous systems based on the peering topology of the network.
o Announcing AS Check: For all BGP peers, a BGP Implementation MUST
ensure that the first element of the AS_PATH list corresponds to
the locally configured AS of that peer.
o Routing information carried through BGP SHOULD include information
that can be used to verify the readvertisement or modification by
each autonomous system through which the UPDATE has passed.
There are many ways in which a differential between the speed of
prefix/AS path attribute propagation and the information validating
the the prefix/AS path attribute information can be exploited to
attack the routing system on a temporary basis. These types of
attacks are dominantly exploitative of the moment in time it takes to
follow the withdraw of a NLRI with an update. As a result of this
potential for temporary disruption, BGP security solutions MUST
propagate security information at the same rate as the BGP updates
and withdrawls. The following items are required to propagate at the
same rate:
o The distribution of key information used by individual actors
within the system, including the keys used by individual
autonomous systems to sign certificates and other objects
o The distribution of information about the AS authorized to
advertise a given block of IP addresses (or an address space)
o The distribution of information about connectivity between
autonomous systems and autonomous system polices, if such
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information is to be distributed within the security system.
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6. Address Allocation and advertisement
As part of the regular operation of the Internet, addresses that are
allocated to an organization may be, and are quite commonly,
advertised by a different organizations. Common reasons for this
practice include multi-homing and route reduction for the purposes of
resource conservation. There are two modes of delegation:
o A BGP speaker and listener have chosen to restrict the amount of
received prefixes for the listener. The listener has chosen to
honor route announcements sent in a summary fashion by the
speaker.
o Address space that is being delegated is part of a larger
allocation that is owned by an autonomous system. The owner then
delegates the smaller block to another AS for purposes of
advertisement. This mode is commonly observed in multi-homing.
These two modes lead to a single common requirement: Any BGP Security
solution MUST support delegation of an address block of any size
regardless of its relationship to other address blocks to another
entity via verifiable means.
An associated delegation criteria is the requirement to allow for
non-BGP IP end user implementations. As a result, all secured BGP
implementations MUST allow for the propagation of a prefix by more
than one originator AS within normal network convergence times.
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7. NLRI and Path Attribute Tracking
The ability for a receiver to know exactly who originated and
forwarded a routing update, is a desirable trait. In order to
rapidly identify agressors and parties at fault for route table
disruption it is important to track and log prefix origination
information along with associated security information.
Any security system SHOULD provide a method to allow the receiver of
an update to verify that the originator actually originated the
update, and that the AS's listed in the AS_PATH actually forwarded
the update.
The data generated by logging may be very large depending on the
number of peers, the number of prefixes received, the authentication
model used, and routing policies. As such, efficient data structures
and storage mechanisms MUST be developed to allow for an effective
means of reproducing incidents and outages
Path and NLRI attributes MUST be logged using a standard format. The
format must be scalable with the amount of data logged and the
frequency of log generation. The frequency of log generation should
be controllable by the operator. The logging mechanisms for the
tracked information MUST be standardized across all platforms.
Logging ability both on and off line is considered highly desirable.
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8. Transport Protection
Transport protection is an important aspect of BGP routing protocol
security. The potential to create a linked transport/NLRI/AS-PATH
authentication mechanism should not be overlooked and may provide for
the accelerated deployment of a BGP security system. Current
security mechanisms for BGP transport are inadequate and require
significant operator interaction to maintain a respectable level of
security.
Any proposed security mechanism MUST include provisions for securing
both internal BGP and external BGP peering sessions. Key maintenance
can be especially onerous to the operators. The number of keys
required and the maintenance of keys (update/withdraw/renew) may have
an additive affect to a barrier to deployment. A highly securable
BGP routing system SHOULD require no more than three keys and each
key should be updateable within similar timeframes as prefix
propagation. The preferred number of keys is ONE per AS.
Transport protection systems SHOULD function as a component of the
BGP routing protocol security mechanism. This includes the use of
the same key generation/management systems as the rest of the
security system.
9 References
[1] Fraser, "RFC 2196 - Site Security Handbook", September 1997.
[2] Rescorla, Korver and Internet Architecture Board, "RFC 3552 -
Guidelines for Writing RFC Text on Security Considerations",
July 2003.
[3] Rekhter and Li, "RFC 1771 - A Border Gateway Protocol 4
(BGP-4)", March 1995.
Author's Address
Blaine Christian (editor)
KMC Telecom Solutions
1545 U.S. Highway 206
Bedminster, NJ 07921
US
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Appendix A. Acknowledgements
The following individuals contributed to the development and review
of this draft. Steve Kent, Russ White, Sandy Murphy, Jeff Haas, Bora
Akyol, Susan Hares, Mike Tibodeau, Thomas Renzy, Kaarthik Sivakumar,
Tao Wan, Radia Perlman, and Merike Kaeo.
This draft was developed based on conversations with various network
operators including Chris Morrow, Jared Mauch, Tim Battles, and Ryan
McDowell.
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