IPv6 Operations Working Group (v6ops) F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Intended status: Informational N. Hilliard
Expires: January 2, 2016 INEX
G. Doering
SpaceNet AG
W. Liu
Huawei Technologies
W. Kumari
Google
July 1, 2015
Operational Implications of IPv6 Packets with Extension Headers
draft-gont-v6ops-ipv6-ehs-packet-drops-00
Abstract
This document summarizes the security and operational implications of
IPv6 extension headers, and attempts to analyze reasons why packets
with IPv6 extension headers may be dropped in the public Internet.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Previous Work on IPv6 Extension Headers . . . . . . . . . . . 3
3. Security Implications . . . . . . . . . . . . . . . . . . . . 3
4. Operational Implications . . . . . . . . . . . . . . . . . . 5
4.1. Enforcing infrastructure ACLs . . . . . . . . . . . . . . 5
4.2. Route-Processor Protection . . . . . . . . . . . . . . . 5
4.3. DDoS Management and Customer Requests for Filtering . . . 5
4.4. ECMP and Hash-based Load-Sharing . . . . . . . . . . . . 6
4.5. Packet Forwarding Engine Constraints . . . . . . . . . . 6
5. A Possible Attack Vector . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
IPv6 Extension Headers (EHs) allow for the extension of the IPv6
protocol, and provide support for core functionality such as IPv6
fragmentation. However, widespread implementation limitations
suggest that EHs present a challenge for IPv6 packet routing
equipment, and evidence exists to suggest that IPv6 with EHs may be
intentionally dropped on the public Internet in some network
deployments.
Discussions about the security and operational implications of IPv6
extension headers are a regular feature in IETF working groups and
other places. Often in these discussions, important security and
operational issues are overlooked.
This document tries to raise awareness about the security and
operational implications of IPv6 Extension Headers, and presents
reasons why some networks drop packets containing IPv6 Extension
Headers.
Section 2 of this document summarizes the work that has been done in
the area of IPv6 extension headers. Section 3 discusses the security
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implications of IPv6 Extension Headers, while Section 4 discusses
their operational implications.
2. Previous Work on IPv6 Extension Headers
Some of the implications of IPv6 Extension Headers have been
discussed in IETF circles. For example, [I-D.taylor-v6ops-fragdrop]
discusses a rationale for which operators drop IPv6 fragments.
[I-D.wkumari-long-headers] discusses possible issues arising from
"long" IPv6 header chains. [RFC7045] clarifies how intermediate
nodes should deal with IPv6 extension headers. [RFC7112] discusses
the issues arising in a specific case where the IPv6 header chain is
fragmented into two or more fragments (and formally forbids such
case). [I-D.kampanakis-6man-ipv6-eh-parsing] describes how
inconsistencies in the way IPv6 packets with extension headers are
parsed by different implementations may result in evasion of security
controls, and presents guidelines for parsing IPv6 extension headers
with the goal of providing a common and consistent parsing
methodology for IPv6 implementations. [RFC6980] analyzes the
security implications of employing IPv6 fragmentation with Neighbor
Discovery for IPv6, and formally recommends against such usage.
Finally, [RFC7123] discusses how some popular RA-Guard
implementations are subject to evasion by means of IPv6 extension
headers.
Some preliminary measurements regarding the extent to which packet
containing IPv6 EHs are dropped in the public Internet have been
presented in [PMTUD-Blackholes], [Gont-IEPG88], [Gont-Chown-IEPG89],
and [Linkova-Gont-IEPG90]. [I-D.ietf-v6ops-ipv6-ehs-in-real-world]
presents more comprehensive results and documents the methodology for
obtaining the presented results.
3. Security Implications
The security implications of IPv6 Extension Headers generally fall
into one or more of these categories:
o Evasion of security controls
o DoS due to processing requirements
o DoS due to implementation errors
o Extension Header-specific issues
Unlike IPv4 packets where the upper-layer protocols can be trivially
found by means of the "IHL" ("Internet Header Length") IPv4 header
field, the structure of IPv6 packets is more flexible and complex.
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Locating upper-layer protocol information requires that all IPv6
extension headers be examined. This has presented implementation
difficulties, and packet filtering mechanisms on several security
devices can be trivially evaded by inserting IPv6 Extension Headers
between the main IPv6 header and the upper layer protocol. [RFC7113]
describes this issue for the RA-Guard case, but the same techniques
can be employed to circumvent other IPv6 firewall and packet
filtering mechanisms. Additionally, implementation inconsistencies
in packet forwarding engines may result in evasion of security
controls [I-D.kampanakis-6man-ipv6-eh-parsing] [Atlasis2014]
[BH-EU-2014].
As noted in Section 4, packets that use IPv6 Extension Headers may
have a negative performance impact on the handling devices. Unless
appropriate mitigations are put in place (e.g., packet filtering and/
or rate-limiting), an attacker could simply send a large amount of
IPv6 traffic employing IPv6 Extension Headers with the purpose of
performing a Denial of Service (DoS) attack.
NOTE: In the most trivial case, a packet that includes a Hop-by-
Hop Options header will typically go through the slow forwarding
path, and be processed by the router's CPU. An implementation-
dependent case might be that in which a router that has been
configured to enforce an ACL based on upper-layer information
(e.g., upper layer protocol or TCP Destination Port), needs to
process the entire IPv6 header chain (in order to find the
required information) and this causes the packet to be processed
in the slow path [Cisco-EH-Cons]. We note that, for obvious
reasons, the aforementioned performance issues may also affect
other devices such as firewalls, Network Intrusion Detection
Systems (NIDS), etc. [Zack-FW-Benchmark]. The extent to which
these devices are affected will typically be implementation-
dependent.
IPv6 implementations, like all other software, tend to mature with
time and wide-scale deployment. While the IPv6 protocol itself has
existed for almost 20 years, serious bugs related to IPv6 Extension
Header processing continue to be discovered. Because there is
currently little operational reliance on IPv6 Extension headers, the
corresponding code paths are rarely exercised, and there is the
potential that bugs still remain to be discovered in some
implementations.
IPv6 Fragment Headers are employed to allow fragmentation of IPv6
packets. While many of the security implications of the
fragmentation / reassembly mechanism are known from the IPv4 world,
several related issues have crept into IPv6 implementations. These
range from denial of service attacks to information leakage, for
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example [I-D.ietf-6man-predictable-fragment-id], [Bonica-NANOG58] and
[Atlasis2012]).
4. Operational Implications
Intermediate systems and middleboxes often need to process the entire
IPv6 extension header chain to find the layer-4 header. The
following subsections discuss some of reasons for which such layer-4
information may be needed by an intermediate systems or middlebox,
and why packets containing IPv6 extension headers may represent a
challenge in such scenarios.
4.1. Enforcing infrastructure ACLs
Generally speaking, infrastructure ACLs drop unwanted packets
destined to parts of a provider's infrastructure, because they are
not operationally needed and can be used for attacks of different
sorts against the router's control plane. Some traffic needs to be
differentiated depending on layer-3 or layer-4 criteria to achieve a
useful balance of protection and functionality, for example:
o Permit some amount of ICMP echo (ping) traffic towards the
router's addresses for troubleshooting.
o Permit BGP sessions on the shared network of an exchange point
(potentially differentiating between the amount of packets/seconds
permitted for established sessions and connection establishment),
but do not permit other traffic from the same peer IP addresses.
4.2. Route-Processor Protection
Most modern routers have a fast hardware-assisted forwarding plane
and a loosely coupled control plane, connected together with a link
that has much less capacity than the forwarding plane could handle.
Traffic differentiation cannot be done by the control plane side,
because this would overload the internal link connecting the
forwarding plane to the control plane.
4.3. DDoS Management and Customer Requests for Filtering
The case of customer DDoS protection and edge-to-core customer
protection filters is similar in nature to the infrastructure ACL
protection. Similar to iACL protection, layer-4 ACLs generally need
to be applied as close to the edge of the network as possible, even
though the intent is to protect the customer edge rather than the
provider core. Application of layer-4 DDoS protection to a network
edge is often automated using Flowspec [RFC5575].
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For example, a web site which normally only handled traffic on TCP
ports 80 and 443 could be subject to a volumetric DDoS attack using
NTP and DNS packets with randomised source IP address, thereby
rendering useless traditional [RFC5635] source-based real-time black
hole mechanisms. In this situation, DDoS protection ACLs could be
configured to block all UDP traffic at the network edge without
impairing the web server functionality in any way. Thus, being able
to filter out arbitrary protocols at the network edge can avoid DDoS-
related problems both in the provider network and on the customer
edge link.
4.4. ECMP and Hash-based Load-Sharing
In the case of ECMP (equal cost multi path) load sharing, the router
on the sending side of the link needs to make a decision regarding
which of the links to use for a given packet. Since round-robin
usage of the links is usually avoided in order to prevent packet
reordering, forwarding engines need to use a mechanism which will
consistently forward the same data streams down the same forwarding
paths. Most forwarding engines achieve this by calculating a simple
hash using an n-tuple gleaned from a combination of layer-2 through
to layer-4 packet header information. This n-tuple will typically
use the src/dst MAC address, src/dst IP address, and if possible
further layer-4 src/dst port information. As layer-4 port
information increases the entropy of the hash, it is highly desirable
to use it where possible.
4.5. Packet Forwarding Engine Constraints
Most modern routers use dedicated hardware (e.g. ASICs or NPUs) to
determine how to forward packets across their internal fabrics. One
of the common methods of handling next-hop lookup is to send a small
portion of the ingress packet to a lookup engine with specialised
hardware (e.g. Tertiary CAM or RLDRAM) to determine the packet's
next-hop. Technical constraints mean that there is a trade-off
between the amount of data sent to the lookup engine and the overall
performance of the lookup engine. If more data is sent, the lookup
engine can inspect further into the packet, but the overall
performance of the system will be reduced. If less data is sent, the
overall performance of the router will be increased but the packet
lookup engine may not be able to inspect far enough into a packet to
determine how it should be handled.
Note: For example, current high-end routers at the time of
authorship of this document can use up to 192 bytes of header
(Cisco ASR9000 Typhoon) or 384 bytes of header (Juniper MX Trio)
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If a hardware forwarding engine on a modern router cannot make a
forwarding decision about a packet because critical information is
not sent to the look-up engine, then the router will normally drop
the packet. Historically, some packet forwarding engines punted
packets of this form to the control plane for more in-depth analysis,
but this is unfeasible on most current router architectures as a
result of the vast difference between the hardware forwarding
capacity of the router and the size of the management link which
connects the control plane to the forwarding plane.
If an IPv6 header chain is sufficiently long that its header exceeds
the packet look-up capacity of the router, then it may be dropped due
to hardware inability to determine how it should be handled.
5. A Possible Attack Vector
The widespread drop of IPv6 packets employing IPv6 Extension Headers
can, in some scenarios, be exploited for malicious purposes: if
packets employing IPv6 EHs are known to be dropped on the path from
system A to system B, an attacker could cause packets sent from A to
B to be dropped by sending a forged ICMPv6 Packet Too Big (PTB)
[RFC4443] error message to A (advertising an MTU smaller than 1280),
such that subsequent packets from A to B include a fragment header
(i.e., they result in atomic fragments [RFC6946]).
Possible scenarios where this attack vector could be exploited
include (but are not limited to):
o Communication between any two systems through to public network
(e.g., client from/to server or server from/to server), where
packets with IPv6 extension headers are dropped by some
intermediate router
o Communication between two BGP peers employing IPv6 transport,
where these BGP peers implement ACLs to drop IPv6 fragments (to
avoid control-plane attacks)
The aforementioned attack vector is exacerbated by the following
factors:
o The attacker does not need to forge the IPv6 Source Address of his
attack packets. Hence, deployment of simple BCP38 filters will
not help as a counter-measure.
o Only the IPv6 addresses of the IPv6 packet embedded in the ICMPv6
payload need to be forged. While one could envision filtering
devices enforcing BCP38-style filters on the ICMPv6 payload, the
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use of extension headers (by the attacker) could make this
difficult, if not impossible.
o Many implementations fail to perform validation checks on the
received ICMPv6 error messages, as recommended in Section 5.2 of
[RFC4443] and documented in [RFC5927]. It should be noted that in
some cases, such as when an ICMPv6 error message has (supposedly)
been elicited by a connection-less transport protocol (or some
other connection-less protocol being encapsulated in IPv6), it may
be virtually impossible to perform validation checks on the
received ICMPv6 error messages. And, because of IPv6 extension
headers, the ICMPv6 payload might not even contain any useful
information on which to perform validation checks.
o Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"
error messages, the Destination Cache [RFC4861] is usually updated
to reflect that any subsequent packets to such destination should
include a Fragment Header. This means that a single ICMPv6
"Packet Too Big" error message might affect multiple communication
instances (e.g. TCP connections) with such destination.
o A router or other middlebox cannot simply drop all incoming ICMPv6
Packet Too Big error messages, as this would create a PMTUD
blackhole.
Possible mitigations for this issue include:
o Filtering incoming ICMPv6 Packet Too Big error messages that
advertise a Next-Hop MTU smaller than 1280 bytes.
o Artificially reducing the MTU to 1280 bytes and filter incoming
ICMPv6 PTB error messages.
Both of these mitigations come at the expense of possibly preventing
communication through SIIT [RFC6145] that rely on IPv6 atomic
fragments (see [I-D.ietf-6man-deprecate-atomfrag-generation]), and
also implies that the filtering device has the ability to filter ICMP
PTB messages based on the contents of the MTU field.
[I-D.ietf-6man-deprecate-atomfrag-generation] has recently proposed
to deprecate the generation of IPv6 atomic fragments, and update SIIT
[RFC6145] such that it does not rely on ICMPv6 atomic fragments.
Thus, any of the above mitigations would eliminate the attack vector
without any interoperability implications.
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6. IANA Considerations
There are no IANA registries within this document. The RFC-Editor
can remove this section before publication of this document as an
RFC.
7. Security Considerations
The security implications of IPv6 extension headers are discussed in
Section 3. A specific attack vector that could leverage the
widespread filtering of packets with IPv6 EHs (along with possible
countermeasures) is discussed in Section 5. This document does not
introduce any new security issues.
8. Acknowledgements
The authors would like to thank (in alphabetical order) [TBD] for
providing valuable comments on earlier versions of this document.
Additionally, the authors would like to thank participants of the
v6ops working group for their valuable input on the topics that led
to the publication of this document.
Fernando Gont would like to thank Fernando Gont would like to thank
Jan Zorz / Go6 Lab <http://go6lab.si/>, and Jared Mauch / NTT
America, for providing access to systems and networks that were
employed to perform experiments and measurements involving packets
with IPv6 Extension Headers. Additionally, he would like to thank
SixXS <https://www.sixxs.net> for providing IPv6 connectivity.
9. References
9.1. Normative References
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", RFC 6145, April 2011.
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[RFC6946] Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC
6946, May 2013.
9.2. Informative References
[Atlasis2012]
Atlasis, A., "Attacking IPv6 Implementation Using
Fragmentation", BlackHat Europe 2012. Amsterdam,
Netherlands. March 14-16, 2012,
<https://media.blackhat.com/bh-eu-12/Atlasis/bh-eu-12-
Atlasis-Attacking_IPv6-Slides.pdf>.
[Atlasis2014]
Atlasis, A., "A Novel Way of Abusing IPv6 Extension
Headers to Evade IPv6 Security Devices", May 2014,
<http://www.insinuator.net/2014/05/a-novel-way-of-abusing-
ipv6-extension-headers-to-evade-ipv6-security-devices/>.
[BH-EU-2014]
Atlasis, A., Rey, E., and R. Schaefer, "Evasion of High-
End IDPS Devices at the IPv6 Era", BlackHat Europe 2014,
2014, <https://www.ernw.de/download/eu-14-Atlasis-Rey-
Schaefer-briefings-Evasion-of-HighEnd-IPS-Devices-wp.pdf>.
[Bonica-NANOG58]
Bonica, R., "IPv6 Extension Headers in the Real World
v2.0", NANOG 58. New Orleans, Louisiana, USA. June 3-5,
2013, <https://www.nanog.org/sites/default/files/
mon.general.fragmentation.bonica.pdf>.
[Cisco-EH-Cons]
Cisco, , "IPv6 Extension Headers Review and
Considerations", October 2006,
<http://www.cisco.com/en/US/technologies/tk648/tk872/
technologies_white_paper0900aecd8054d37d.pdf>.
[Gont-Chown-IEPG89]
Gont, F. and T. Chown, "A Small Update on the Use of IPv6
Extension Headers", IEPG 89. London, UK. March 2, 2014,
<http://www.iepg.org/2014-03-02-ietf89/
fgont-iepg-ietf89-eh-update.pdf>.
[Gont-IEPG88]
Gont, F., "Fragmentation and Extension header Support in
the IPv6 Internet", IEPG 88. Vancouver, BC, Canada.
November 13, 2013, <http://www.iepg.org/2013-11-ietf88/
fgont-iepg-ietf88-ipv6-frag-and-eh.pdf>.
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[I-D.ietf-6man-deprecate-atomfrag-generation]
Gont, F., LIU, S., and T. Anderson, "Deprecating the
Generation of IPv6 Atomic Fragments", draft-ietf-6man-
deprecate-atomfrag-generation-01 (work in progress), April
2015.
[I-D.ietf-6man-predictable-fragment-id]
Gont, F., "Security Implications of Predictable Fragment
Identification Values", draft-ietf-6man-predictable-
fragment-id-08 (work in progress), June 2015.
[I-D.ietf-v6ops-ipv6-ehs-in-real-world]
Gont, F., Linkova, J., Chown, T., and S. LIU,
"Observations on IPv6 EH Filtering in the Real World",
draft-ietf-v6ops-ipv6-ehs-in-real-world-00 (work in
progress), April 2015.
[I-D.kampanakis-6man-ipv6-eh-parsing]
Kampanakis, P., "Implementation Guidelines for parsing
IPv6 Extension Headers", draft-kampanakis-6man-ipv6-eh-
parsing-01 (work in progress), August 2014.
[I-D.taylor-v6ops-fragdrop]
Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
M., and T. Taylor, "Why Operators Filter Fragments and
What It Implies", draft-taylor-v6ops-fragdrop-02 (work in
progress), December 2013.
[]
Kumari, W., Jaeggli, J., Bonica, R., and J. Linkova,
"Operational Issues Associated With Long IPv6 Header
Chains", draft-wkumari-long-headers-03 (work in progress),
June 2015.
[Linkova-Gont-IEPG90]
Linkova, J. and F. Gont, "IPv6 Extension Headers in the
Real World v2.0", IEPG 90. Toronto, ON, Canada. July 20,
2014, <http://www.iepg.org/2014-07-20-ietf90/
iepg-ietf90-ipv6-ehs-in-the-real-world-v2.0.pdf>.
[PMTUD-Blackholes]
De Boer, M. and J. Bosma, "Discovering Path MTU black
holes on the Internet using RIPE Atlas", July 2012,
<http://www.nlnetlabs.nl/downloads/publications/
pmtu-black-holes-msc-thesis.pdf>.
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[RFC5575] Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
and D. McPherson, "Dissemination of Flow Specification
Rules", RFC 5575, August 2009.
[RFC5635] Kumari, W. and D. McPherson, "Remote Triggered Black Hole
Filtering with Unicast Reverse Path Forwarding (uRPF)",
RFC 5635, August 2009.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
[RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation
with IPv6 Neighbor Discovery", RFC 6980, August 2013.
[RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", RFC 7045, December 2013.
[RFC7112] Gont, F., Manral, V., and R. Bonica, "Implications of
Oversized IPv6 Header Chains", RFC 7112, January 2014.
[RFC7113] Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)", RFC 7113, February 2014.
[RFC7123] Gont, F. and W. Liu, "Security Implications of IPv6 on
IPv4 Networks", RFC 7123, February 2014.
[RIPE-Atlas]
RIPE, , "RIPE Atlas", <https://atlas.ripe.net/>.
[Zack-FW-Benchmark]
Zack, E., "Firewall Security Assessment and Benchmarking
IPv6 Firewall Load Tests", IPv6 Hackers Meeting #1,
Berlin, Germany. June 30, 2013,
<http://www.ipv6hackers.org/meetings/ipv6-hackers-1/zack-
ipv6hackers1-firewall-security-assessment-and-
benchmarking.pdf>.
Authors' Addresses
Fernando Gont
SI6 Networks / UTN-FRH
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
URI: http://www.si6networks.com
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Nick Hilliard
INEX
4027 Kingswood Road
Dublin 24
IE
Email: nick@inex.ie
Gert Doering
SpaceNet AG
Joseph-Dollinger-Bogen 14
Muenchen D-80807
Germany
Email: gert@space.net
Will (Shucheng) Liu
Huawei Technologies
Bantian, Longgang District
Shenzhen 518129
P.R. China
Email: liushucheng@huawei.com
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
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