Internet DRAFT - draft-ietf-opsec-dhcpv6-shield
draft-ietf-opsec-dhcpv6-shield
opsec F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Intended status: Best Current Practice W. Liu
Expires: January 7, 2016 Huawei Technologies
G. Van de Velde
Alcatel-Lucent
July 6, 2015
DHCPv6-Shield: Protecting Against Rogue DHCPv6 Servers
draft-ietf-opsec-dhcpv6-shield-08
Abstract
This document specifies a mechanism for protecting hosts connected to
a switched network against rogue DHCPv6 servers. It is based on
DHCPv6 packet-filtering at the layer-2 device at which the packets
are received. A similar mechanism has been widely deployed in IPv4
networks ('DHCP snooping'), and hence it is desirable that similar
functionality be provided for IPv6 networks. This document specifies
a Best Current Practice for the implementation of DHCPv6 Shield.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 7, 2016.
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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. DHCPv6-Shield Configuration . . . . . . . . . . . . . . . . . 4
5. DHCPv6-Shield Implementation Requirements . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. Normative References . . . . . . . . . . . . . . . . . . 9
9.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
This document specifies DHCPv6-Shield: a mechanism for protecting
hosts connected to a switched network against rogue DHCPv6 servers
[RFC3315]. The basic concept behind DHCPv6-Shield is that a layer-2
device filters DHCPv6 messages intended for DHCPv6 clients
(henceforth "DHCPv6-server messages"), according to a number of
different criteria. The most basic filtering criterion is that
DHCPv6-server messages are discarded by the layer-2 device unless
they are received on specific ports of the layer-2 device.
Before the DHCPv6-Shield device is deployed, the administrator
specifies the layer-2 port(s) on which DHCPv6-server messages are to
be allowed. Only those ports to which a DHCPv6 server or relay is to
be connected should be specified as such. Once deployed, the
DHCPv6-Shield device inspects received packets, and allows (i.e.
passes) DHCPv6-server messages only if they are received on layer-2
ports that have been explicitly configured for such purpose.
DHCPv6-Shield is analogous to the RA-Guard mechanism [RFC6104]
[RFC6105] [RFC7113], intended for protection against rogue Router
Advertisement [RFC4861] messages.
We note that DHCPv6-Shield mitigates only DHCPv6-based attacks
against hosts. Attack vectors based on other messages meant for
network configuration (such as ICMPv6 Router Advertisements) are not
addressed by DHCPv6-Shield itself. In a similar vein,
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DHCPv6-Shielddoes not mitigate attacks against DHCPv6 servers (e.g.,
Denial of Service).
2. 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].
3. Terminology
DHCPv6-Shield:
the set of filtering rules specified in this document, meant to
mitigate attacks that employ DHCPv6-server packets.
DHCPv6-Shield device:
A layer-2 device (typically a layer-2 switch) that enforces the
filtering policy specified in this document.
For the purposes of this document, the terms Extension Header, Header
Chain, First Fragment, and Upper-layer Header are used as specified
in [RFC7112]:
IPv6 Extension Header:
Extension Headers are defined in Section 4 of [RFC2460]. As a
result of [RFC7045], [IANA-PROTO] provides a list of assigned
Internet Protocol Numbers and designates which of those protocol
numbers also represent extension headers.
First Fragment:
An IPv6 fragment with fragment offset equal to 0.
IPv6 Header Chain:
The header chain contains an initial IPv6 header, zero or more
IPv6 extension headers, and optionally, a single upper-layer
header. If an upper-layer header is present, it terminates the
header chain; otherwise the "No Next Header" value (Next Header =
59) terminates it.
The first member of the header chain is always an IPv6 header.
For a subsequent header to qualify as a member of the header
chain, it must be referenced by the "Next Header" field of the
previous member of the header chain. However, if a second IPv6
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header appears in the header chain, as is the case when IPv6 is
tunneled over IPv6, the second IPv6 header is considered to be an
upper-layer header and terminates the header chain. Likewise, if
an Encapsulating Security Payload (ESP) header appears in the
header chain it is considered to be an upper-layer header and it
terminates the header chain.
Upper-layer Header:
In the general case, the upper-layer header is the first member of
the header chain that is neither an IPv6 header nor an IPv6
extension header. However, if either an ESP header, or a second
IPv6 header occur in the header chain, they are considered to be
upper layer headers and they terminate the header chain.
Neither the upper-layer payload, nor any protocol data following
the upper-layer payload, is considered to be part of the header
chain. In a simple example, if the upper-layer header is a TCP
header, the TCP payload is not part of the header chain. In a
more complex example, if the upper-layer header is an ESP header,
neither the payload data, nor any of the fields that follow the
payload data in the ESP header are part of the header chain.
4. DHCPv6-Shield Configuration
Before being deployed for production, the DHCPv6-Shield device is
explicitly configured with respect to which layer-2 ports are allowed
to receive DHCPv6 packets destined to DHCPv6 clients (i.e.
DHCPv6-server messages). Only those layer-2 ports explicitly
configured for such purpose will be allowed to receive DHCPv6 packets
to DHCPv6 clients.
5. DHCPv6-Shield Implementation Requirements
The following are the filtering rules that are enforced as part of a
DHCPv6-Shield implementation on those ports that are not allowed to
receive DHCPv6 packets to DHCPv6 clients:
1. DHCPv6-Shield implementations MUST parse the entire IPv6 header
chain present in the packet, to identify whether it is a DHCPv6
packet meant for a DHCPv6 client (i.e., a DHCPv6-server message).
RATIONALE: DHCPv6-Shield implementations MUST NOT enforce a
limit on the number of bytes they can inspect (starting from
the beginning of the IPv6 packet), since this could introduce
false-negatives: DHCP6-server packets received on ports not
allowed to receive such packets could be allowed simply
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because the DHCPv6-Shield device does not parse the entire
IPv6 header chain present in the packet.
2. When parsing the IPv6 header chain, if the packet is a first-
fragment (i.e., a packet containing a Fragment Header with the
Fragment Offset set to 0) and it fails to contain the entire IPv6
header chain (i.e., all the headers starting from the IPv6 header
up to, and including, the upper-layer header), DHCPv6-Shield MUST
drop the packet, and ought to log the packet drop event in an
implementation-specific manner as a security fault.
RATIONALE: Packets that fail to contain the entire IPv6 header
chain could otherwise be leveraged for circumventing
DHCPv6-Shield. [RFC7112] requires that the first-fragment
(i.e., the fragment with the Fragment Offset set to 0)
contains the entire IPv6 header chain, and allows intermediate
systems such as routers to drop those packets that fail to
comply with this requirement.
NOTE: This rule should only be applied to IPv6 fragments with
a Fragment Offset of 0 (non-first fragments can be safely
passed, since they will never reassemble into a complete
datagram if they are part of a DHCPv6 packet meant for a
DHCPv6 client received on a port where such packets are not
allowed).
3. DHCPv6-Shield MUST provide a configuration knob that controls
whether packets with unrecognized Next Header values are dropped;
this configuration knob MUST default to "drop". When parsing the
IPv6 header chain, if the packet contains an unrecognized Next
Header value and the configuration knob is configured to "drop",
DHCPv6-Shield MUST drop the packet, and ought to log the packet
drop event in an implementation-specific manner as a security
fault.
RATIONALE: An unrecognized Next Header value could possibly
identify an IPv6 Extension Header, and thus be leveraged to
conceal a DHCPv6-server packet (since there is no way for
DHCPv6-Shield to parse past unrecognized Next Header values
[I-D.gont-6man-rfc6564bis]). [RFC7045] requires that nodes be
configurable with respect to whether packets with unrecognized
headers are forwarded, and allows the default behavior to be
that such packets be dropped.
4. When parsing the IPv6 header chain, if the packet is identified
to be a DHCPv6 packet meant for a DHCPv6 client, DHCPv6-Shield
MUST drop the packet, and SHOULD log the packet drop event in an
implementation-specific manner as a security alert.
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RATIONALE: Ultimately, the goal of DHCPv6-Shield is drop
DHCPv6 packets destined to DHCPv6 clients (i.e. DHCPv6-server
messages) that are received on ports that have not been
explicitly configured to allow the receipt of such packets.
5. In all other cases, DHCPv6-Shield MUST pass the packet as usual.
NOTE: For the purpose of enforcing the DHCPv6-Shield filtering
policy, an ESP header [RFC4303] should be considered to be an
"upper-layer protocol" (that is, it should be considered the last
header in the IPv6 header chain). This means that packets
employing ESP would be passed by the DHCPv6-Shield device to the
intended destination. If the destination host does not have a
security association with the sender of the aforementioned IPv6
packet, the packet would be dropped. Otherwise, if the packet is
considered valid by the IPsec implementation at the receiving host
and encapsulates a DHCPv6 message, it is up to the receiving host
what to do with such packet.
The above indicates that if a packet is dropped due to this filtering
policy, the packet drop event be logged in an implementation-specific
manner as a security fault. It is useful for the logging mechanism
to include a per-port drop counter dedicated to DHCPv6-Shield packet
drops.
In order to protect current end-node IPv6 implementations, Rule #2
has been defined as a default rule to drop packets that cannot be
positively identified as not being DHCPv6-server packets (because the
packet is a fragment that fails to include the entire IPv6 header
chain). This means that, at least in theory, DHCPv6-Shield could
result in false-positive blocking of some legitimate (non
DHCPv6-server) packets. However, as noted in [RFC7112], IPv6 packets
that fail to include the entire IPv6 header chain are virtually
impossible to police with state-less filters and firewalls, and hence
are unlikely to survive in real networks. [RFC7112] requires that
hosts employing fragmentation include the entire IPv6 header chain in
the first fragment (the fragment with the Fragment Offset set to 0),
thus eliminating the aforementioned false positives.
The aforementioned filtering rules implicitly handle the case of
fragmented packets: if the DHCPv6-Shield device fails to identify the
upper-layer protocol as a result of the use of fragmentation, the
corresponding packets would be dropped.
Finally, we note that IPv6 implementations that allow overlapping
fragments (i.e. that do not comply with [RFC5722]) might still be
subject of DHCPv6-based attacks. However, a recent assessment of
IPv6 implementations [SI6-FRAG] with respect to their fragment
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reassembly policy seems to indicate that most current implementations
comply with [RFC5722].
6. IANA Considerations
This document has no actions for IANA.
7. Security Considerations
The recommendations in this document represent the ideal behavior of
a DHCPv6 shield device. However, in order to implement DHCPv6 shield
on the fast path, it may be necessary to limit the depth into the
packet that can be scanned before giving up. In circumstances where
there is such a limitation, it is recommended that implementations
drop packets after attempting to find a protocol header up to that
limit, whatever it is. Ideally, such devices should be configurable
with a list of protocol header identifiers so that if new transport
protocols are standardized after the device is released, they can be
added to the list of protocol header types that the device
recognizes. Since any protocol header that is not a UDP header would
be passed by the DHCPv6 shield algorithm, this would allow such
devices to avoid blocking the use of new transport protocols. When
an implementation must stop searching for recognizable header types
in a packet due to such limitations, whether the device passes or
drop that packet SHOULD be configurable.
The mechanism specified in this document can be used to mitigate
DHCPv6-based attacks against hosts. Attack vectors based on other
messages meant for network configuration (such as ICMPv6 Router
Advertisements) are out of the scope of this document. Additionally,
the mechanism specified in this document does not mitigate attacks
against DHCPv6 servers (e.g., Denial of Service).
If deployed in layer-2 domain with several cascading switches, there
will be an ingress port on the host's local switch which will need to
be enabled for receiving DHCPv6-server messages. However, this local
switch will be reliant on the upstream devices to have filtered out
rogue DHCPv6-server messages, as the local switch has no way of
determining which upstream DHCP-server messages are valid.
Therefore, in order to be effective DHCPv6 Shield should be deployed
and enabled on all layer-2 switches of a given layer-2 domain.
As noted in Section 5, IPv6 implementations that allow overlapping
fragments (i.e. that do not comply with [RFC5722]) might still be
subject of DHCPv6-based attacks. However, most current
implementations seem to comply with [RFC5722], and hence forbid IPv6
overlapping fragments.
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We note that if an attacker sends a fragmented DHCPv6 packet on a
port not allowed to receive such packets, the first-fragment would be
dropped, and the rest of the fragments would be passed. This means
that the victim node would tie memory buffers for the aforementioned
fragments, which would never reassemble into a complete datagram. If
a large number of such packets were sent by an attacker, and the
victim node failed to implement proper resource management for the
fragment reassembly buffer, this could lead to a Denial of Service
(DoS). However, this does not really introduce a new attack vector,
since an attacker could always perform the same attack by sending
forged fragmented datagram in which at least one of the fragments is
missing. [CPNI-IPv6] discusses some resource management strategies
that could be implemented for the fragment reassembly buffer.
Additionally, we note that the security of a site employing DHCPv6
Shield could be further improved by deploying [I-D.ietf-savi-dhcp],
to mitigate IPv6 address spoofing attacks.
Finally, we note that other mechanisms for mitigating attacks based
on DHCPv6-server messages are available that have different
deployment considerations. For example, [I-D.ietf-dhc-secure-dhcpv6]
allows for authentication of DHCPv6-server packets if the IPv6
addresses of the DHCPv6 servers can be pre-configured at the client
nodes.
8. Acknowledgements
The authors would like to thank Mike Heard, who provided detailed
feedback on earlier versions of this document and helped a lot in
producing a technically-sound document throughout the whole
publication process.
The authors would like to thank (in alphabetical order) Ben Campbell,
Jean-Michel Combes, Sheng Jiang, Ted Lemon, Pete Resnick, Juergen
Schoenwaelder, Carsten Schmoll, Robert Sleigh, Donald Smith, Mark
Smith, Hannes Tschofenig, Eric Vyncke, and Qin Wu, for providing
valuable comments on earlier versions of this document.
Part of Section 3 of this document was borrowed from [RFC7112],
authored by Fernando Gont, Vishwas Manral, and Ron Bonica.
This document is heavily based on the document [RFC7113] authored by
Fernando Gont. Thus, the authors would like to thank Ran Atkinson,
Karl Auer, Robert Downie, Washam Fan, David Farmer, Mike Heard, Marc
Heuse, Nick Hilliard, Ray Hunter, Joel Jaeggli, Simon Perreault,
Arturo Servin, Gunter van de Velde, James Woodyatt, and Bjoern A.
Zeeb, for providing valuable comments on [RFC7113], on which this
document is based.
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The authors would like to thank Joel Jaeggli for his advice and
guidance throughout the IETF process.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
and M. Carney, "Dynamic Host Configuration Protocol for
IPv6 (DHCPv6)", RFC 3315, July 2003.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, December 2009.
[RFC7112] Gont, F., Manral, V., and R. Bonica, "Implications of
Oversized IPv6 Header Chains", RFC 7112, January 2014.
[RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", RFC 7045, December 2013.
9.2. Informative References
[I-D.ietf-dhc-secure-dhcpv6]
Jiang, S. and S. Shen, "Secure DHCPv6 Using CGAs", draft-
ietf-dhc-secure-dhcpv6-07 (work in progress), September
2012.
[I-D.gont-6man-rfc6564bis]
Gont, F., Will, W., Krishnan, S., and H. Pfeifer, "IPv6
Universal Extension Header", draft-gont-6man-rfc6564bis-00
(work in progress), April 2014.
[RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
Problem Statement", RFC 6104, February 2011.
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[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
February 2011.
[RFC7113] Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)", RFC 7113, February 2014.
[IANA-PROTO]
Internet Assigned Numbers Authority, "Protocol Numbers",
February 2013, <http://www.iana.org/assignments/protocol-
numbers/protocol-numbers.txt>.
[SI6-FRAG]
SI6 Networks, "IPv6 NIDS evasion and improvements in IPv6
fragmentation/reassembly", 2012,
<http://blog.si6networks.com/2012/02/
ipv6-nids-evasion-and-improvements-in.html>.
[I-D.ietf-savi-dhcp]
Bi, J., Wu, J., Yao, G., and F. Baker, "SAVI Solution for
DHCP", draft-ietf-savi-dhcp-34 (work in progress),
February 2015.
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
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
Will Liu
Huawei Technologies
Bantian, Longgang District
Shenzhen 518129
P.R. China
Email: liushucheng@huawei.com
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Gunter Van de Velde
Alcatel-Lucent
Copernicuslaan 50
Antwerp, Antwerp 2018
Belgium
Phone: +32 476 476 022
Email: gunter.van_de_velde@alcatel-lucent.com
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