Internet Engineering Task Force (IETF) M. Bhatia

Internet Engineering Task Force (IETF) M. Bhatia
Request for Comments: 7492 Ionos Networks
Category: Informational D. Zhang
ISSN: 2070-1721 Huawei
M. Jethanandani
Ciena Corporation
March 2015

Analysis of Bidirectional Forwarding Detection (BFD) Security
According to the Keying and Authentication for Routing Protocols (KARP)
Design Guidelines

Abstract

This document analyzes the Bidirectional Forwarding Detection (BFD)
protocol according to the guidelines set forth in Section 4.2 of RFC
6518, "Keying and Authentication for Routing Protocols (KARP) Design
Guidelines".

Status of This Memo

This document is not an Internet Standards Track specification; it is
published for informational purposes.

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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc7492.

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Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements to Meet . . . . . . . . . . . . . . . . . . . . 3
3. Current State of Security Methods . . . . . . . . . . . . . . 3
4. Impacts of BFD Replays . . . . . . . . . . . . . . . . . . . 5
5. Impact of New Authentication Requirements . . . . . . . . . . 6
6. Considerations for Improvement . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9

1. Introduction

This document performs a gap analysis of the current state of
Bidirectional Forwarding Detection [RFC5880] according to the
requirements of KARP Design Guidelines [RFC6518]. Previously, the
OPSEC working group has provided an analysis of cryptographic issues
with BFD in "Issues with Existing Cryptographic Protection Methods
for Routing Protocols" [RFC6039].

The existing BFD specifications provide a basic security solution.
Key ID is provided so that the key used in securing a packet can be
changed on demand. Two cryptographic algorithms (MD5 and SHA-1) are
supported for integrity protection of the control packets; the
algorithms are both demonstrated to be subject to collision attacks.
Routing protocols like "RIPv2 Cryptographic Authentication"
[RFC4822], "IS-IS Generic Cryptographic Authentication" [RFC5310],
and "OSPFv2 HMAC-SHA Cryptographic Authentication" [RFC5709] have
started to use BFD for liveliness checks. Moving the routing

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protocols to a stronger algorithm while using a weaker algorithm for
BFD would allow the attacker to bring down BFD in order to bring down
the routing protocol. BFD therefore needs to match the routing
protocols in its strength of algorithm.

While BFD uses a non-decreasing, per-packet sequence number to
protect itself from intra-connection replay attacks, it still leaves
the protocol vulnerable to the inter-session replay attacks.

2. Requirements to Meet

There are several requirements described in Section 4 of [RFC6862]
that BFD, as defined in BFD [RFC5880], does not currently meet:

Replay Protection: BFD provides an incomplete intra-session and no
inter-session replay attack protection; this creates significant
denial-of-service opportunities.

Strong Algorithms: The cryptographic algorithms adopted for
message authentication in BFD are MD5 or SHA-1 based. However,
both algorithms are known to be vulnerable to collision attacks.
"BFD Generic Cryptographic Authentication" [BFD-CRYPTO] and
"Authenticating BFD using HMAC-SHA-2 procedures" [BFD-HMAC]
together propose a solution to support Hashed Message
Authentication Code (HMAC) with the SHA-2 family of hash functions
for BFD.

Preventing DoS Attacks: BFD packets can be sent at millisecond
intervals (the protocol uses timers at microsecond intervals).
When malicious packets are sent at short intervals, with the
authentication bit set, it can cause a DoS attack. There is
currently no lightweight mechanism within BFD to address this
issue and is one of the reasons BFD authentication is still not
widely deployed in the field.

The remainder of this document explains the details of how these
requirements fail to be met and proposes mechanisms for addressing
them.

3. Current State of Security Methods

BFD [RFC5880] describes five authentication mechanisms for the
integrity protection of BFD control packets: Simple Password, Keyed
MD5 [RFC1321], Meticulous Keyed MD5, Keyed SHA-1, and Meticulous
Keyed SHA-1. In the simple password mechanism, every control packet
is associated with a password transported in plain text; attacks
eavesdropping the network traffic can easily learn the password and
compromise the security of the corresponding BFD session. In the

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Keyed MD5 and the Meticulous Keyed MD5 mechanisms, BFD nodes use
shared secret keys to generate Keyed MD5 digests for control packets.
Similarly, in the Keyed SHA-1 and the Meticulous Keyed SHA-1
mechanisms, BFD nodes use shared secret keys to generate Keyed SHA-1
digests for control packets. Note that in the keyed authentication
mechanisms, every BFD control packet is associated with a non-
decreasing, 32-bit sequence number to resist replay attacks. In the
Keyed MD5 and the Keyed SHA-1 mechanisms, the sequence member is only
required to increase occasionally. However, in the Meticulous Keyed
MD5 and the Meticulous Keyed SHA-1 mechanisms, the sequence member is
required to increase with each successive packet.

Additionally, limited key updating functionality is provided. There
is a Key ID in every authenticated BFD control packet indicating the
key used to hash the packet. However, there is no mechanism
described to provide a smooth key rollover that the BFD routers can
use when moving from one key to the other.

The BFD session timers are defined with the granularity of
microseconds, and it is common in practice to send BFD packets at
millisecond intervals. Since the cryptographic sequence number space
is only 32 bits, a sequence number used in a BFD session may reach
its maximum value and roll over within a limited period. For
instance, if a sequence number is increased by one every 3.3
milliseconds, then it will reach its maximum value in less than 24
weeks. This can result in potential inter-session replay attacks,
especially when BFD uses the non-meticulous authentication modes.

Note that when using authentication mechanisms, BFD drops all packets
that fall outside the limited range (3 times the Detection Time
multiplier). Therefore, when meticulous authentication modes are
used, a replayed BFD packet will be rejected if it cannot fit into a
relatively short window (3 times the detection interval of the
session). This introduces some difficulties for replaying packets.
However, in a non-meticulous authentication mode, such windows can be
large (as sequence numbers are only increased occasionally), thus
making it easier to perform replay attacks .

In a BFD session, each node needs to select a 32-bit discriminator to
identify itself. Therefore, a BFD session is identified by two
discriminators. If a node randomly selects a new discriminator for a
new session and uses authentication mechanisms to secure the control
packets, inter-session replay attacks can be mitigated to some
extent. However, in existing BFD demultiplexing mechanisms, the
discriminators used in a new BFD session may be predictable. In some
deployment scenarios, the discriminators of BFD routers may be
decided by the destination and source addresses. So, if the sequence
number of a BFD router rolls over for some reason (e.g., reboot), the

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discriminators used to identify the new session will be identical to
the ones used in the previous session. This makes performing a
replay attack relatively simple.

BFD allows a mode called the echo mode. Echo packets are not defined
in the BFD specification, though they can keep the BFD session up.
The format of the echo packet is local to the sending side, and there
are no guidelines on the properties of these packets beyond the
choice of the source and destination addresses. While the BFD
specification recommends applying security mechanisms to prevent
spoofing of these packets, there are no guidelines on what type of
mechanisms are appropriate.

4. Impacts of BFD Replays

As discussed, BFD cannot meet the requirements of inter-session or
intra-session replay protection. This section discusses the impacts
of BFD replays.

When cryptographic authentication mechanisms are adopted for BFD, a
non-decreasing, 32-bit-long sequence number is used. In the Keyed
MD5 and the Keyed SHA-1 mechanisms, the sequence member is not
required to increase for every packet. Therefore, an attacker can
keep replaying the packets with the latest sequence number until the
sequence number is updated. This issue is eliminated in the
Meticulous Keyed MD5 and the Meticulous Keyed SHA-1 mechanisms.
However, note that a sequence number may reach its maximum and be
rolled over in a session. In this case, without the support from a
automatic key management mechanism, the BFD session will be
vulnerable to replay attacks performed by sending the packets before
the roll over of the sequence number. For instance, an attacker can
replay a packet with a sequence number that is larger than the
current one. If the replayed packet is accepted, the victim will
reject the legal packets whose sequence members are less than the one
in the replayed packet. Therefore, the attacker can get a good
chance to bring down the BFD session. This kind of attack assumes
that the attacker has access to the link when the BFD session is on a
point-to-point link or can inject packets for a BFD session with
multiple hops.

Additionally, the BFD specification allows for the change of
authentication state based on the state of a received packet. For
instance, according to BFD [RFC5880], if the state of an accepted
packet is down, the receiver of the packet needs to transfer its
state to down as well. Therefore, a carefully selected replayed
packet can cause a serious denial-of-service attack.

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BFD does not provide any solution to deal with inter-session replay
attacks. If two subsequent BFD sessions adopt an identical
discriminator pair and use the same cryptographic key to secure the
control packets, it is intuitive to use a malicious authenticated
packet (stored from the past session) to perform interconnection
replay attacks.

Any security issues in the BFD echo mode will directly affect the BFD
protocol and session states, and hence the network stability. For
instance, any replay attacks would be indistinguishable from normal
forwarding of the tested router. An attack would still cause a
faulty link to be believed to be up, but there is little that can be
done about it. However, if the echo packets are guessable, it may be
possible to spoof from an external source and cause BFD to believe
that a one-way link is really bidirectional. As a result, it is
important that the echo packets contain random material that is also
checked upon reception.

5. Impact of New Authentication Requirements

BFD can be run in software or hardware. Hardware implementations run
BFD at a much smaller timeout, typically in the order of few
milliseconds. For instance, with a timeout of 3.3 milliseconds, a
BFD session is required to send or receive 3 packets every 10
milliseconds. Software implementations typically run with a timeout
in hundreds of milliseconds.

Additionally, it is not common to find hardware support for computing
the authentication data for the BFD session in hardware or software.
In the Keyed MD5 and Keyed SHA-1 implementation where the sequence
number does not increase with every packet, software can be used to
compute the authentication data. This is true if the time between
the increasing sequence number is long enough to compute the data in
software. The ability to compute the hash in software is difficult
with Meticulous Keyed MD5 and Meticulous Keyed SHA-1 if the time
interval between transmits or between receives is small. The
computation problem becomes worse if hundred or thousands of sessions
require the hash to be recomputed every few milliseconds.

Smaller and cheaper boxes that have to support a few hundred BFD
sessions are boxes that also use a slower CPU. The CPU is used for
running the entire control plane software in addition to supporting
the BFD sessions. As a general rule, no more than 40-45% of the CPU
can be dedicated towards supporting BFD. Adding computation of the
hash for every BFD session can easily cause the CPU to exceed the
40-45% limit even with a few tens of sessions. On higher-end boxes
with faster and more CPU cores, the expectation is that the number of

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sessions that need to be supported are in the thousands, but the
number of BFD sessions with authentication that CPU can support is
still in the hundreds.

Implementors should assess the impact of authenticating BFD sessions
on their platform.

6. Considerations for Improvement

This section suggests changes that can be adopted to improve the
protection of BFD.

The security risks brought by SHA-1 and MD5 have been well
understood. However, when using a stronger digest algorithm, e.g.,
SHA-2, the imposed computing overhead will seriously affect the
performance of BFD implementation. In order to make the trade-off
between the strong algorithm requirement and the imposed overhead,
Galois Message Authentication Code (GMAC) can be a candidate option.
This algorithm is relatively effective and has been supported by
IPsec for data origin authentication. More detailed information can
be found in "The Use of Galois Message Authentication Code (GMAC) in
IPsec ESP and AH" [RFC4543].

There has been some hallway conversation around the idea of using BFD
cryptographic authentication only when some data in the BFD payload
changes. The other BFD packets can be transmitted and received
without authentication enabled. The bulk of the BFD packets that are
transmitted and received have no state change associated with them.
Limiting authentication to BFD packets that affect a BFD session
state allows for more sessions to be supported for authentication.
This change can significantly help the routers since they don't have
to compute and verify the authentication digest for the BFD packets
coming at the millisecond intervals. This proposal needs some more
discussion in the BFD working group and is certainly a direction that
BFD could look at.

7. Security Considerations

This document discusses vulnerabilities in the existing BFD protocol
and suggests possible mitigations.

In analyzing the improvements for BFD, the ability to repel a replay
attack is discussed. For example, increasing the sequence number to
a 64-bit value makes the wrap-around time much longer, and a replay
attack can be easily prevented.

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Mindful of the impact that stronger algorithms can have on the
performance of BFD, the document suggests GMAC as a possible
candidate for MAC function.

8. References

8.1. Normative References

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992, <http://www.rfc-editor.org/info/rfc1321>.

[RFC5880] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD)", RFC 5880, June 2010,
<http://www.rfc-editor.org/info/rfc5880>.

[RFC6039] Manral, V., Bhatia, M., Jaeggli, J., and R. White, "Issues
with Existing Cryptographic Protection Methods for Routing
Protocols", RFC 6039, October 2010,
<http://www.rfc-editor.org/info/rfc6039>.

8.2. Informative References

[BFD-CRYPTO]
Bhatia, M., Manral, V., Zhang, D., and M. Jethanandani,
"BFD Generic Cryptographic Authentication", Work in
Progress, draft-ietf-bfd-generic-crypto-auth-06, April
2014.

[BFD-HMAC] Zhang, D., Bhatia, M., Manral, V., and M. Jethanandani,
"Authenticating BFD using HMAC-SHA-2 procedures", Work in
Progress, draft-ietf-bfd-hmac-sha-05, July 2014.

[RFC4543] McGrew, D. and J. Viega, "The Use of Galois Message
Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
May 2006, <http://www.rfc-editor.org/info/rfc4543>.

[RFC4822] Atkinson, R. and M. Fanto, "RIPv2 Cryptographic
Authentication", RFC 4822, February 2007,
<http://www.rfc-editor.org/info/rfc4822>.

[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
and M. Fanto, "IS-IS Generic Cryptographic
Authentication", RFC 5310, February 2009,
<http://www.rfc-editor.org/info/rfc5310>.

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[RFC5709] Bhatia, M., Manral, V., Fanto, M., White, R., Barnes, M.,
Li, T., and R. Atkinson, "OSPFv2 HMAC-SHA Cryptographic
Authentication", RFC 5709, October 2009,
<http://www.rfc-editor.org/info/rfc5709>.

[RFC6518] Lebovitz, G. and M. Bhatia, "Keying and Authentication for
Routing Protocols (KARP) Design Guidelines", RFC 6518,
February 2012, <http://www.rfc-editor.org/info/rfc6518>.

[RFC6862] Lebovitz, G., Bhatia, M., and B. Weis, "Keying and
Authentication for Routing Protocols (KARP) Overview,
Threats, and Requirements", RFC 6862, March 2013,
<http://www.rfc-editor.org/info/rfc6862>.

Acknowledgements

We would like to thank Alexander Vainshtein for his comments on this
document.

Authors' Addresses

Manav Bhatia
Ionos Networks
Bangalore
India

EMail: [email protected]

Dacheng Zhang
Huawei

EMail: [email protected]

Mahesh Jethanandani
Ciena Corporation
3939 North 1st Street
San Jose, CA 95134
United States

Phone: 408.904.2160
Fax: 408.436.5582
EMail: [email protected]

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