draft-ietf-6man-overlap-fragment-03.txt		rfc5722.txt
	6man Working Group S. Krishnan		Network Working Group S. Krishnan
	Internet-Draft Ericsson		Request for Comments: 5722 Ericsson
	Updates: 2460 (if approved) July 2, 2009		Updates: 2460 December 2009
	Intended status: Standards Track		Category: Standards Track
	Expires: January 3, 2010
	Handling of overlapping IPv6 fragments
	draft-ietf-6man-overlap-fragment-03
	Status of this Memo
	This Internet-Draft is submitted to IETF in full conformance with the
	provisions of BCP 78 and BCP 79.
	Internet-Drafts are working documents of the Internet Engineering		Handling of Overlapping IPv6 Fragments
	Task Force (IETF), its areas, and its working groups. Note that
	other groups may also distribute working documents as Internet-
	Drafts.
	Internet-Drafts are draft documents valid for a maximum of six months		Abstract
	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."
	The list of current Internet-Drafts can be accessed at		The fragmentation and reassembly algorithm specified in the base IPv6
	http://www.ietf.org/ietf/1id-abstracts.txt.		specification allows fragments to overlap. This document
			demonstrates the security issues associated with allowing overlapping
			fragments and updates the IPv6 specification to explicitly forbid
			overlapping fragments.
	The list of Internet-Draft Shadow Directories can be accessed at		Status of This Memo
	http://www.ietf.org/shadow.html.
	This Internet-Draft will expire on January 3, 2010.		This document specifies an Internet standards track protocol for the
			Internet community, and requests discussion and suggestions for
			improvements. Please refer to the current edition of the "Internet
			Official Protocol Standards" (STD 1) for the standardization state
			and status of this protocol. Distribution of this memo is unlimited.
	Copyright Notice		Copyright Notice
	Copyright (c) 2009 IETF Trust and the persons identified as the		Copyright (c) 2009 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents in effect on the date of		Provisions Relating to IETF Documents
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	Please review these documents carefully, as they describe your rights		publication of this document. Please review these documents
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			to this document. Code Components extracted from this document must
	Abstract		include Simplified BSD License text as described in Section 4.e of
			the Trust Legal Provisions and are provided without warranty as
	The fragmentation and reassembly algorithm specified in the base IPv6		described in the BSD License.
	specification allows fragments to overlap. This document
	demonstrates the security issues with allowing overlapping fragments
	and updates the IPv6 specification to explicitly forbid overlapping
	fragments.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3		1. Introduction ....................................................2
	1.1. Conventions used in this document . . . . . . . . . . . . . 3		1.1. Conventions Used in This Document ..........................2
	2. Overlapping Fragments . . . . . . . . . . . . . . . . . . . . . 3		2. Overlapping Fragments ...........................................2
	3. The attack . . . . . . . . . . . . . . . . . . . . . . . . . . 4		3. The Attack ......................................................3
	4. Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 6		4. Node Behavior ...................................................5
	5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6		5. Security Considerations .........................................5
	6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6		6. Acknowledgements ................................................5
	7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7		7. References ......................................................6
	8. Normative References . . . . . . . . . . . . . . . . . . . . . 7		7.1. Normative References .......................................6
	Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7		7.2. Informative References .....................................6
	1. Introduction		1. Introduction
	Fragmentation is used in IPv6 when the IPv6 packet will not fit		Fragmentation is used in IPv6 when the IPv6 packet will not fit
	inside the path MTU to its destination. When fragmentation is		inside the path MTU to its destination. When fragmentation is
	performed an IPv6 node uses a fragment header as specified in section		performed, an IPv6 node uses a fragment header, as specified in
	4.5 of the IPv6 base specification [RFC2460] to break down the		Section 4.5 of the IPv6 base specification [RFC2460], to break down
	datagram into smaller fragments that will fit in the path MTU. The		the datagram into smaller fragments that will fit in the path MTU.
	destination node receives these fragments and reassembles them. The		The destination node receives these fragments and reassembles them.
	algorithm specified for fragmentation in [RFC2460] does not prevent		The algorithm specified for fragmentation in [RFC2460] does not
	the fragments from overlapping, and this can lead to some security		prevent the fragments from overlapping, and this can lead to some
	issues with firewalls [RFC4942]. This document explores the issues		security issues with firewalls [RFC4942]. This document explores the
	that can be caused by overlapping fragments.		issues that can be caused by overlapping fragments and updates the
			IPv6 specification to explicitly forbid overlapping fragments.
	1.1. Conventions used in this document		1.1. Conventions Used in This Document
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this		"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
	document are to be interpreted as described in [RFC2119].		document are to be interpreted as described in [RFC2119].
	2. Overlapping Fragments		2. Overlapping Fragments
	Commonly used firewalls use the algorithm specified in [RFC1858] to		Commonly used firewalls use the algorithm specified in [RFC1858] to
	weed out malicious packets that try to overwrite parts of the		weed out malicious packets that try to overwrite parts of the
	transport layer header to bypass inbound connection checks.		transport-layer header in order to bypass inbound connection checks.
	[RFC1858] prevents an overlapping fragment attack on an upper layer		[RFC1858] prevents an overlapping fragment attack on an upper-layer
	protocol (in this case TCP) by recommending that packets with		protocol (in this case, TCP) by recommending that packets with a
	fragment offset 1 be dropped. While this works well for IPv4		fragment offset of 1 be dropped. While this works well for IPv4
	fragments, it will not work for IPv6 fragments. This is because the		fragments, it will not work for IPv6 fragments. This is because the
	fragmentable part of the IPv6 packet can contain extension headers		fragmentable part of the IPv6 packet can contain extension headers
	before the TCP header, making this check less effective.		before the TCP header, making this check less effective.
	3. The attack		3. The Attack
	This attack describes how a malicious node can bypass a firewall		This attack describes how a malicious node can bypass a firewall
	using overlapping fragments. Consider a sufficiently large IPv6		using overlapping fragments. Consider a sufficiently large IPv6
	packet that needs to be fragmented.		packet that needs to be fragmented.
	+------------------+--------------------//-----------------------+		+------------------+--------------------//-----------------------+
	| Unfragmentable | Fragmentable |		| Unfragmentable | Fragmentable |
	| Part | Part |		| Part | Part |
	+------------------+--------------------//-----------------------+		+------------------+--------------------//-----------------------+
	Figure 1: Large IPv6 packet		Figure 1: Large IPv6 Packet
	This packet is split into several fragments by the sender so that the		This packet is split into several fragments by the sender so that the
	packet can fit inside the path MTU. Let's say the packet is split		packet can fit inside the path MTU. Let's say the packet is split
	into two fragments.		into two fragments.
	+------------------+--------+--------------------+		+------------------+--------+--------------------+
	| Unfragmentable |Fragment| first |		| Unfragmentable |Fragment| first |
	| Part | Header | fragment |		| Part | Header | fragment |
	+------------------+--------+--------------------+		+------------------+--------+--------------------+
	+------------------+--------+--------------------+		+------------------+--------+--------------------+
	| Unfragmentable |Fragment| second |		| Unfragmentable |Fragment| second |
	| Part | Header | fragment |		| Part | Header | fragment |
	+------------------+--------+--------------------+		+------------------+--------+--------------------+
	Figure 2: Fragmented IPv6 packet		Figure 2: Fragmented IPv6 Packet
	Consider the first fragment. Let's say it contains a destination		Consider the first fragment. Let's say it contains a destination
	options header (DOH) 80 octets long and is followed by a TCP header.		options header (DOH) 80 octets long and is followed by a TCP header.
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH
	|NextHdr=DOH(60)| Reserved | FragmentOffset = 0 |Res|1|		|NextHdr=DOH(60)| Reserved | FragmentOffset = 0 |Res|1|
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Identification=aaaabbbb |		| Identification=aaaabbbb |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==DOH		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==DOH
	|NextHdr=TCP(6) | HdrExtLen = 9 | |		|NextHdr=TCP(6) | HdrExtLen = 9 | |
	skipping to change at page 5, line 27		skipping to change at page 4, line 27
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP
	| Source Port | Destination Port |		| Source Port | Destination Port |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Sequence Number |		| Sequence Number |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Acknowledgment Number |		| Acknowledgment Number |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Offset| Reserved |U|A|P|R|S|F| Window |		| Offset| Reserved |U|A|P|R|S|F| Window |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Figure 3: First Fragment		Figure 3: First Fragment
	The TCP header has the following values of the flags S(YN)=1 and		The TCP header has the following values of the flags: S(YN)=1 and
	A(CK)=1. This may make an inspecting stateful firewall think that it		A(CK)=1. This may make an inspecting stateful firewall think that it
	is a response packet for a connection request initiated from the		is a response packet for a connection request initiated from the
	trusted side of the firewall. Hence it will allow the fragment to		trusted side of the firewall. Hence, it will allow the fragment to
	pass. It will also allow the following fragments with the same		pass. It will also allow the following fragments with the same
	Fragment Identification value in the fragment header to pass through.		Fragment Identification value in the fragment header to pass through.
	A malicious node can form a second fragment with a TCP header that		A malicious node can form a second fragment with a TCP header that
	changes the flags and sets S(YN)=1 and A(CK)=0. This can change the		changes the flags and sets S(YN)=1 and A(CK)=0. This can change the
	packet on the receiving end to consider the packet as a connection		packet on the receiving end to consider the packet as a connection
	request instead of a response. By doing this the malicious node has		request instead of a response. By doing this, the malicious node has
	bypassed the firewall's access control to initiate a connection		bypassed the firewall's access control to initiate a connection
	request to a node protected by a firewall.		request to a node protected by a firewall.
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH
	|NextHdr=DOH(60)| Reserved | FragmentOffset = 10 |Res|0|		|NextHdr=DOH(60)| Reserved | FragmentOffset = 10 |Res|0|
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Identification=aaaabbbb |		| Identification=aaaabbbb |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP
	| Source Port | Destination Port |		| Source Port | Destination Port |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Sequence Number |		| Sequence Number |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Acknowledgment Number |		| Acknowledgment Number |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	| Offset| Reserved |U|A|P|R|S|F| Window |		| Offset| Reserved |U|A|P|R|S|F| Window |
	+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+		+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
	Figure 4: Second Fragment		-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
			Figure 4: Second Fragment
	Note that this attack is much more serious in IPv6 than in IPv4. In		Note that this attack is much more serious in IPv6 than in IPv4. In
	IPv4 the overlapping part of the TCP header did not include the		IPv4, the overlapping part of the TCP header does not include the
	source and destination ports. In IPv6 the attack can easily work to		source and destination ports. In IPv6, the attack can easily work to
	replace the source or destination port with an overlapping fragment.		replace the source or destination port with an overlapping fragment.
	4. Recommendation		4. Node Behavior
	IPv6 nodes transmitting datagrams that need to be fragmented MUST NOT		IPv6 nodes transmitting datagrams that need to be fragmented MUST NOT
	create overlapping fragments. When reassembling an IPv6 datagram, if		create overlapping fragments. When reassembling an IPv6 datagram, if
	one or more its constituent fragments is determined to be an		one or more its constituent fragments is determined to be an
	overlapping fragment, the entire datagram (and any constituent		overlapping fragment, the entire datagram (and any constituent
	fragments, including those not yet received), MUST be silently		fragments, including those not yet received) MUST be silently
	discarded.		discarded.
			Nodes MAY also provide mechanisms to track the reception of such
			packets, for instance, by implementing counters or alarms relating to
			these events.
	5. Security Considerations		5. Security Considerations
	This document discusses an attack that can be used to bypass IPv6		This document discusses an attack that can be used to bypass IPv6
	firewalls using overlapping fragments. It recommends disallowing		firewalls using overlapping fragments. It recommends disallowing
	overlapping fragments in order to prevent this attack.		overlapping fragments in order to prevent this attack.
	6. Acknowledgements		6. Acknowledgements
	The author would like to thank Thomas Narten, Doug Montgomery,		The author would like to thank Thomas Narten, Doug Montgomery,
	Gabriel Montenegro, Remi Denis-Courmont, Marla Azinger, Arnaud		Gabriel Montenegro, Remi Denis-Courmont, Marla Azinger, Arnaud
	Ebalard, Seiichi Kawamura, Behcet Sarikaya, Vishwas Manral, Christian		Ebalard, Seiichi Kawamura, Behcet Sarikaya, Vishwas Manral, Christian
	Vogt, and Alfred Hoenes for their reviews and suggestions that made		Vogt, Bob Hinden, Carl Wallace, Jari Arkko, Pasi Eronen, Francis
	this document better.		Dupont, Neville Brownlee, Dan Romascanu, Lars Eggert, Cullen
			Jennings, and Alfred Hoenes for their reviews and suggestions that
	7. IANA Considerations		made this document better.
	This document does not require any action from the IANA.
	8. Normative References		7. References
	[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security		7.1. Normative References
	Considerations for IP Fragment Filtering", RFC 1858,
	October 1995.
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119, March 1997.		Requirement Levels", BCP 14, RFC 2119, March 1997.
	[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6		[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
	(IPv6) Specification", RFC 2460, December 1998.		(IPv6) Specification", RFC 2460, December 1998.
	[RFC4942] Davies, E., Krishnan, S., and P. Savola, "IPv6 Transition/		7.2. Informative References
	Co-existence Security Considerations", RFC 4942,
	September 2007.		[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
			Considerations for IP Fragment Filtering", RFC 1858,
			October 1995.
			[RFC4942] Davies, E., Krishnan, S., and P. Savola, "IPv6
			Transition/Co-existence Security Considerations", RFC
			4942, September 2007.
	Author's Address		Author's Address
	Suresh Krishnan		Suresh Krishnan
	Ericsson		Ericsson
	8400 Blvd Decarie		8400 Blvd Decarie
	Town of Mount Royal, Quebec		Town of Mount Royal, Quebec
	Canada		Canada
	Email: suresh.krishnan@ericsson.com		EMail: suresh.krishnan@ericsson.com
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