Internet Engineering Task Force (IETF) F. Gont Request for Comments: 7610 SI6 Networks / UTN-FRH BCP: 199 W. Liu Category: Best Current Practice Huawei Technologies ISSN: 2070-1721 G. Van de Velde Alcatel-Lucent August 2015 DHCPv6-Shield: Protecting against Rogue DHCPv6 Servers
AbstractThis 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'); 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 memo documents an Internet Best Current Practice. 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). Further information on BCPs is available in 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/rfc7610.
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 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 1. Introduction ....................................................3 2. Requirements Language ...........................................3 3. Terminology .....................................................3 4. DHCPv6-Shield Configuration .....................................5 5. DHCPv6-Shield Implementation Requirements .......................5 6. Security Considerations .........................................7 7. References ......................................................9 7.1. Normative References .......................................9 7.2. Informative References ....................................10 Acknowledgements ..................................................11 Authors' Addresses ................................................12
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 Router Advertisement Guard (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, DHCPv6-Shield does not mitigate attacks against DHCPv6 servers (e.g., Denial of Service). RFC2119].
For the purposes of this document, the terms "IPv6 Extension Header", "First Fragment", "IPv6 Header Chain", and "Upper-Layer Header" are used as specified in [RFC7112]: IPv6 Extension Header: IPv6 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 IPv6 Extension Headers. First Fragment: An IPv6 fragment with a Fragment Offset equal to 0. IPv6 Header Chain: The IPv6 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 IPv6 Header Chain; otherwise, the "No Next Header" value (Next Header = 59) terminates it. The first member of the IPv6 Header Chain is always an IPv6 header. For a subsequent header to qualify as a member of the IPv6 Header Chain, it must be referenced by the "Next Header" field of the previous member of the IPv6 Header Chain. However, if a second IPv6 header appears in the IPv6 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 IPv6 Header Chain. Likewise, if an Encapsulating Security Payload (ESP) header appears in the IPv6 Header Chain, it is considered to be an Upper-Layer Header, and it terminates the IPv6 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 occurs in the IPv6 Header Chain, it is considered to be an Upper-Layer Header, and it terminates the IPv6 Header Chain. Neither the upper-layer payload nor any protocol data following the upper-layer payload is considered to be part of the IPv6 Header Chain. In a simple example, if the Upper-Layer Header is a TCP header, the TCP payload is not part of the IPv6 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 IPv6 Header Chain. RFC7112] requires that the First Fragment (i.e., the fragment with the Fragment Offset set to 0) contain the entire IPv6 Header Chain. [RFC7112] also allows intermediate systems such as routers to drop 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 or not 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 [IPV6-UEH]). [RFC7045] requires that nodes be configurable with respect to whether or not 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. RATIONALE: Ultimately, the goal of DHCPv6-Shield is to 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, what to do with such a packet is up to the receiving host.
The rules above indicate that if a packet is dropped due to this filtering policy, the packet drop event should 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 such that the default is for 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) to be dropped. 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 stateless filters and firewalls; hence, they 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 reassembly policy seems to indicate that most current implementations comply with [RFC5722].
an implementation must stop searching for recognizable header types in a packet due to such limitations, the device SHOULD be configurable to either pass or drop that packet. 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 a layer 2 domain with several cascading switches, there will be an ingress port on the host's local switch that will need to be enabled for receiving DHCPv6-server messages. However, this local switch will be reliant on the upstream devices filtering 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 to 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 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 a 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 [RFC7513] 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, [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. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, <http://www.rfc-editor.org/info/rfc2460>. [RFC3315] Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July 2003, <http://www.rfc-editor.org/info/rfc3315>. [RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303, DOI 10.17487/RFC4303, December 2005, <http://www.rfc-editor.org/info/rfc4303>. [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007, <http://www.rfc-editor.org/info/rfc4861>. [RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments", RFC 5722, DOI 10.17487/RFC5722, December 2009, <http://www.rfc-editor.org/info/rfc5722>. [RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing of IPv6 Extension Headers", RFC 7045, DOI 10.17487/RFC7045, December 2013, <http://www.rfc-editor.org/info/rfc7045>. [RFC7112] Gont, F., Manral, V., and R. Bonica, "Implications of Oversized IPv6 Header Chains", RFC 7112, DOI 10.17487/RFC7112, January 2014, <http://www.rfc-editor.org/info/rfc7112>.
[CPNI-IPv6] Gont, F., "Security Assessment of the Internet Protocol version 6 (IPv6)", UK Centre for the Protection of National Infrastructure, (available on request). [IANA-PROTO] IANA, "Protocol Numbers", <http://www.iana.org/assignments/protocol-numbers>. [IPV6-UEH] Gont, F., Liu, W., Krishnan, S., and H. Pfeifer, "IPv6 Universal Extension Header", Work in Progress, draft-gont-6man-rfc6564bis-00, April 2014. [RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement Problem Statement", RFC 6104, DOI 10.17487/RFC6104, February 2011, <http://www.rfc-editor.org/info/rfc6104>. [RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J. Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105, DOI 10.17487/RFC6105, February 2011, <http://www.rfc-editor.org/info/rfc6105>. [RFC7113] Gont, F., "Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)", RFC 7113, DOI 10.17487/RFC7113, February 2014, <http://www.rfc-editor.org/info/rfc7113>. [RFC7513] Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address Validation Improvement (SAVI) Solution for DHCP", RFC 7513, DOI 10.17487/RFC7513, May 2015, <http://www.rfc-editor.org/info/rfc7513>. [SECURE-DHCPV6] Jiang, S. and S. Shen, "Secure DHCPv6 Using CGAs", Work in Progress, draft-ietf-dhc-secure-dhcpv6-07, September 2012. [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>.
Section 3 of this document was borrowed from [RFC7112], authored by Fernando Gont, Vishwas Manral, and Ron Bonica. This document is heavily based on [RFC7113], authored by Fernando Gont. Thus, the authors would like to thank the following individuals for providing valuable comments on [RFC7113]: 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. The authors would like to thank Joel Jaeggli for his advice and guidance throughout the IETF process. Fernando Gont would like to thank Diego Armando Maradona for his magic and inspiration.
http://www.si6networks.com Will (Shucheng) Liu Huawei Technologies Bantian, Longgang District Shenzhen 518129 China Email: firstname.lastname@example.org Gunter Van de Velde Alcatel-Lucent Copernicuslaan 50 Antwerp, Antwerp 2018 Belgium Phone: +32 476 476 022 Email: email@example.com