opsec F. Gont
Internet-Draft SI6 Networks / UTN-FRH
Intended status: Best Current Practice W. Liu
Expires: April 25, 2014 Huawei Technologies
G. Van de Velde
Cisco Systems
October 22, 2013
DHCPv6-Shield: Protecting Against Rogue DHCPv6 Servers
draft-ietf-opsec-dhcpv6-shield-01
Abstract
This document specifies a mechanism for protecting hosts connected to
a broadcast network against rogue DHCPv6 servers. The aforementioned
mechanism is based on DHCPv6 packet-filtering at the layer-2 device
at which the packets are received. The aforementioned mechanism has
been widely deployed in IPv4 networks ('DHCP snooping'), and hence it
is desirable that similar functionality be provided for IPv6
networks.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 25, 2014.
Copyright Notice
Copyright (c) 2013 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
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publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 2
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. DHCPv6-Shield Configuration . . . . . . . . . . . . . . . . . 4
5. DHCPv6-Shield Implementation Advice . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
This document specifies a mechanism for protecting hosts connected to
a broadcast network against rogue DHCPv6 servers [RFC3315]. This
mechanism is analogous to the RA-Guard mechanism [RFC6104] [RFC6105]
[I-D.ietf-v6ops-ra-guard-implementation] intended for protection
against rogue Router Advertisement [RFC4861] messages.
The basic concept behind DHCPv6-Shield is that a layer-2 device
filters DHCPv6 messages meant to 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 a specified port of the layer-2 device.
Before the DCHPv6-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 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.
2. Requirements Language
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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
For the purposes of this document, the terms Extension Header, Header
Chain, First Fragment, and Upper-layer Header are used as follows
[I-D.ietf-6man-oversized-header-chain]:
Extension Header:
Extension Headers are defined in Section 4 of [RFC2460]. As a
result of [I-D.ietf-6man-ext-transmit], [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
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.
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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 MUST
be explicitly configured with respect to which layer-2 ports are
allowed to send DHCPv6 packets to DHCPv6 clients (i.e. DHCPv6-server
messages). Only those layer-2 ports explicitly configured for such
purpose will be allowed to send DHCPv6 packets to DHCPv6 clients.
5. DHCPv6-Shield Implementation Advice
The following filtering rules MUST be enforced as part of a
DHCPv6-Shield implementation on those ports that are not allowed to
send DHCPv6 packets to DHCPv6 clients:
1. DHCPv6-Shield MUST parse the IPv6 entire 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: [RFC6564] specifies a uniform format for IPv6
Extension Headers, thus meaning that an IPv6 node can parse
an IPv6 header chain even if it contains Extension Headers
that are not currently supported by that node.
Additionally, [I-D.ietf-6man-oversized-header-chain]
requires that if a packet is fragmented, the first fragment
contains the entire IPv6 header chain.
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-positives: legitimate packets could be dropped simply
because the DHCPv6-Shield device does not parse the entire
IPv6 header chain present in the packet. An implementation
that has such an implementation-specific limit MUST NOT
claim compliance with this specification, and MUST pass the
packet when such implementation-specific limit is reached.
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
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up to, and including, the upper-layer header), DHCPv6-Shield MUST
drop the packet, and SHOULD log the packet drop event in an
implementation-specific manner as a security fault.
RATIONALE: [I-D.ietf-6man-oversized-header-chain] specifies
that the first-fragment (i.e., the fragment with the
Fragment Offset set to 0) MUST contain 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. 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 fault.
4. 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.
If a packet is dropped due to this filtering policy, then the packet
drop event SHOULD be logged in an implementation-specific manner as a
security fault. The logging mechanism SHOULD include a 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
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DHCPv6-server) packets. However, as noted in
[I-D.ietf-6man-oversized-header-chain], 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. [I-D.ietf-6man-oversized-header-chain]
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
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 mechanism specified in this document can be used to mitigate
DHCPv6-based attacks. Attack vectors based on other messages (such
as ICMPv6 Router Advertisements) are out of the scope of this
document.
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.
We note that if an attacker sends a fragmented DHCPv6 packet on a
port not allowed to send 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,
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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.
8. Acknowledgements
The authors would like to thank (in alphabetical order) Jean-Michel
Combes, Juergen Schoenwaelder, and Mark Smith, for providing valuable
comments on earlier versions of this document.
Section 3 of this document was borrowed from
[I-D.ietf-6man-oversized-header-chain], authored by Fernando Gont,
Vishwas Manral, and Ron Bonica.
This document is heavily based on the document
[I-D.ietf-v6ops-ra-guard-implementation] authored by Fernando Gont.
Thus, the authors would like to thank Ran Atkinson, Karl Auer, Robert
Downie, Washam Fan, David Farmer, 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 [I-D.ietf-v6ops-ra-guard-implementation], on which this
document is based.
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.
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[RFC6564] Krishnan, S., Woodyatt, J., Kline, E., Hoagland, J., and
M. Bhatia, "A Uniform Format for IPv6 Extension Headers",
RFC 6564, April 2012.
[I-D.ietf-6man-ext-transmit]
Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", draft-ietf-6man-ext-
transmit-05 (work in progress), October 2013.
9.2. Informative References
[RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
Problem Statement", RFC 6104, February 2011.
[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
February 2011.
[]
Gont, F., Manral, V., and R. Bonica, "Implications of
Oversized IPv6 Header Chains", draft-ietf-6man-oversized-
header-chain-08 (work in progress), October 2013.
[I-D.ietf-v6ops-ra-guard-implementation]
Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)", draft-ietf-v6ops-ra-
guard-implementation-07 (work in progress), November 2012.
[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>.
[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
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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
Gunter Van de Velde
Cisco Systems
De Kleetlaan 6a
Diegem 1831
Belgium
Phone: +32 2704 5473
Email: gunter@cisco.com
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