Implications of Oversized IPv6 Header Chains
IPv6 packets must keep their whole header chain in the first fragment, so firewalls can do their job.
Gont, F., Manral, V., and R. Bonica, "Implications of Oversized IPv6 Header Chains", RFC 7112, DOI 10.17487/RFC7112, January 2014, <https://www.rfc-editor.org/info/rfc7112>.
@misc{rfc7112,
series = {Request for Comments},
number = 7112,
howpublished = {RFC 7112},
publisher = {RFC Editor},
doi = {10.17487/RFC7112},
url = {https://www.rfc-editor.org/info/rfc7112},
author = {F. Gont and V. Manral and R. Bonica},
title = {{Implications of Oversized IPv6 Header Chains}},
pagetotal = 8,
year = 2014,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) F. Gont
Request for Comments: 7112 Huawei Technologies
Updates: 2460 V. Manral
Category: Standards Track Ionos Networks
ISSN: 2070-1721 R. Bonica
Juniper Networks
January 2014
Implications of Oversized IPv6 Header Chains
Abstract
The IPv6 specification allows IPv6 Header Chains of an arbitrary
size. The specification also allows options that can, in turn,
extend each of the headers. In those scenarios in which the IPv6
Header Chain or options are unusually long and packets are
fragmented, or scenarios in which the fragment size is very small,
the First Fragment of a packet may fail to include the entire IPv6
Header Chain. This document discusses the interoperability and
security problems of such traffic, and updates RFC 2460 such that the
First Fragment of a packet is required to contain the entire IPv6
Header Chain.
Status of This Memo
This is an Internet Standards Track document.
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
Internet Standards 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/rfc7112.
Copyright Notice
Copyright (c) 2014 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
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Requirements Language ...........................................3
3. Terminology .....................................................3
4. Motivation ......................................................4
5. Updates to RFC 2460 .............................................5
6. IANA Considerations .............................................5
7. Security Considerations .........................................6
8. Acknowledgements ................................................6
9. References ......................................................7
9.1. Normative References .......................................7
9.2. Informative References .....................................7
1. Introduction
With IPv6, optional internet-layer information is carried in one or
more IPv6 Extension Headers [RFC2460]. Extension Headers are placed
between the IPv6 header and the Upper-Layer Header in a packet. The
term "Header Chain" refers collectively to the IPv6 header, Extension
Headers, and Upper-Layer Header occurring in a packet. In those
scenarios in which the IPv6 Header Chain is unusually long and
packets are fragmented, or scenarios in which the fragment size is
very small, the Header Chain may span multiple fragments.
While IPv4 had a fixed maximum length for the set of all IPv4 options
present in a single IPv4 packet, IPv6 does not have any equivalent
maximum limit at present. This document updates the set of IPv6
specifications to create an overall limit on the size of the
combination of IPv6 options and IPv6 Extension Headers that is
allowed in a single IPv6 packet. Namely, it updates RFC 2460 such
that the First Fragment of a fragmented datagram is required to
contain the entire IPv6 Header Chain.
It should be noted that this requirement does not preclude the use of
large payloads but, instead, merely requires that all headers,
starting from the IPv6 base header and continuing up to the Upper-
Layer Header (e.g., TCP or the like) be present in the First
Fragment.
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 [RFC2119].
3. Terminology
For the purposes of this document, the terms Extension Header, IPv6
Header Chain, First Fragment, and Upper-Layer Header are used as
follows:
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 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
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
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.
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.
4. Motivation
Many forwarding devices implement stateless firewalls. A stateless
firewall enforces a forwarding policy on a packet-by-packet basis.
In order to enforce its forwarding policy, the stateless firewall may
need to glean information from both the IPv6 and upper-layer headers.
For example, assume that a stateless firewall discards all traffic
received from an interface unless it is destined for a particular TCP
port on a particular IPv6 address. When this firewall is presented
with a fragmented packet that is destined for a different TCP port,
and the entire header chain is contained within the First Fragment,
the firewall discards the First Fragment and allows subsequent
fragments to pass. Because the First Fragment was discarded, the
packet cannot be reassembled at the destination. Insomuch as the
packet cannot be reassembled, the forwarding policy is enforced.
However, when the firewall is presented with a fragmented packet and
the header chain spans multiple fragments, the First Fragment does
not contain enough information for the firewall to enforce its
forwarding policy. Lacking sufficient information, the stateless
firewall either forwards or discards that fragment. Regardless of
the action that it takes, it may fail to enforce its forwarding
policy.
5. Updates to RFC 2460
When a host fragments an IPv6 datagram, it MUST include the entire
IPv6 Header Chain in the First Fragment.
A host that receives a First Fragment that does not satisfy the
above-stated requirement SHOULD discard the packet and SHOULD send an
ICMPv6 error message to the source address of the offending packet
(subject to the rules for ICMPv6 errors specified in [RFC4443]).
However, for backwards compatibility, implementations MAY include a
configuration option that allows such fragments to be accepted.
Likewise, an intermediate system (e.g., router or firewall) that
receives an IPv6 First Fragment that does not satisfy the above-
stated requirement MAY discard that packet, and it MAY send an ICMPv6
error message to the source address of the offending packet (subject
to the rules for ICMPv6 error messages specified in [RFC4443]).
Intermediate systems having this capability SHOULD support
configuration (e.g., enable/disable) of whether or not such packets
are dropped by the intermediate system.
If a host or intermediate system discards a First Fragment because it
does not satisfy the above-stated requirement and sends an ICMPv6
error message due to the discard, then the ICMPv6 error message MUST
be Type 4 ("Parameter Problem") and MUST use Code 3 ("First Fragment
has incomplete IPv6 Header Chain"). The Pointer field contained by
the ICMPv6 Parameter Problem message MUST be set to zero. The format
for the ICMPv6 error message is the same regardless of whether a host
or intermediate system originates it.
As a result of the above-mentioned requirement, a packet's header
chain length cannot exceed the Path MTU associated with its
destination. Hosts discover the Path MTU using procedures such as
those defined in [RFC1981] and [RFC4821]. Hosts that do not discover
the Path MTU MUST limit the IPv6 Header Chain length to 1280 bytes.
Limiting the IPv6 Header Chain length to 1280 bytes ensures that the
header chain length does not exceed the IPv6 minimum MTU [RFC2460].
6. IANA Considerations
IANA has added the following "Type 4 - Parameter Problem" message to
the "Internet Control Message Protocol version 6 (ICMPv6) Parameters"
registry:
CODE NAME/DESCRIPTION
3 IPv6 First Fragment has incomplete IPv6 Header Chain
7. Security Considerations
No new security exposures or issues are raised by this document.
This document describes how undesirably fragmented packets can be
leveraged to evade stateless packet filtering. Having made that
observation, this document updates [RFC2460] so that undesirably
fragmented packets are forbidden. Therefore, a security
vulnerability is removed.
This specification allows nodes that drop the aforementioned packets
to signal such packet drops with ICMPv6 "Parameter Problem, IPv6
First Fragment has incomplete IPv6 header chain" (Type 4, Code 3)
error messages.
As with all ICMPv6 error/diagnostic messages, deploying Source
Address Forgery Prevention filters helps reduce the chances of an
attacker successfully performing a reflection attack by sending
forged illegal packets with the victim's/target's IPv6 address as the
IPv6 source address of the illegal packet [RFC2827] [RFC3704].
A firewall that performs stateless deep packet inspection (i.e.,
examines application payload content) might still be unable to
correctly process fragmented packets, even if the IPv6 Header Chain
is not fragmented.
8. Acknowledgements
The authors would like to thank Ran Atkinson for contributing text
and ideas that were incorporated into this document.
The authors would like to thank (in alphabetical order) Ran Atkinson,
Fred Baker, Stewart Bryant, Brian Carpenter, Benoit Claise, Dominik
Elsbroek, Wes George, Mike Heard, Bill Jouris, Suresh Krishnan, Dave
Thaler, Ole Troan, Eric Vyncke, and Peter Yee, for providing valuable
comments on earlier versions of this document.
9. References
9.1. Normative References
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[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.
[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.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC7045] Carpenter, B. and S. Jiang, "Transmission and Processing
of IPv6 Extension Headers", RFC 7045, December 2013.
9.2. Informative References
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC3704] Baker, F. and P. Savola, "Ingress Filtering for Multihomed
Networks", BCP 84, RFC 3704, March 2004.
[IANA-PROTO]
Internet Assigned Numbers Authority, "Protocol Numbers",
<http://www.iana.org/assignments/protocol-numbers>.
Authors' Addresses
Fernando Gont
Huawei Technologies
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
EMail: fgont@si6networks.com
Vishwas Manral
Ionos Networks
Sunnyvale, CA 94089
US
Phone: 408-447-1497
EMail: vishwas@ionosnetworks.com
Ronald P. Bonica
Juniper Networks
2251 Corporate Park Drive
Herndon, VA 20171
US
Phone: 571 250 5819
EMail: rbonica@juniper.net
AI ELI5
On this page
§1The Header Chain Problem
IPv6 carries optional internet-layer information in Extension Headers, placed between the IPv6 header and the Upper-Layer Header. Together these form the IPv6 Header Chain. IPv4 had a fixed maximum length for all its options, but IPv6 has no equivalent limit. If the chain is unusually long and the packet is fragmented, or the fragments are very small, the chain can spill across several fragments.
This document sets an overall limit by updating RFC 2460 so that the First Fragment of a fragmented datagram must contain the entire chain. The rule does not forbid large payloads. It only requires that every header from the IPv6 base header up to the Upper-Layer Header, such as TCP, be in the First Fragment.
A packet has a stack of labels on the front. The rule says that when a packet is cut into pieces, all of the labels must be on the first piece.
§2Key Terms
A First Fragment is an IPv6 fragment with Fragment Offset equal to 0. The IPv6 Header Chain holds an initial IPv6 header, zero or more Extension Headers, and optionally a single Upper-Layer Header. An Upper-Layer Header ends the chain; if there is none, the "No Next Header" value (Next Header = 59) ends it.
A header joins the chain only if the previous header's Next Header field points to it. Two cases end the chain early: a second IPv6 header (as when IPv6 is tunneled over IPv6) and an Encapsulating Security Payload (ESP) header are both treated as the Upper-Layer Header. The upper-layer payload, such as TCP data, is not part of the chain.
The header chain is the whole row of labels from the first one down to the one that says what is inside, like TCP. The stuff inside the box does not count as a label.
§3Why Firewalls Need It
Many forwarding devices run stateless firewalls, which judge each packet on its own and may need to read both IPv6 and upper-layer headers. Suppose a firewall drops everything except traffic to one TCP port on one IPv6 address. If a fragmented packet aimed at another port has its whole chain in the First Fragment, the firewall drops that fragment and lets the rest pass. The packet can never be reassembled, so the policy holds.
If the chain spans several fragments, the First Fragment lacks the information the firewall needs. It must either forward or discard blindly, and either way it may fail to enforce its policy.
A guard checks the first page for the destination. If the destination is not on the first page, the guard has to guess, and might let in a letter that should have been stopped.
§4The Updated Rule
When a host fragments an IPv6 datagram, it MUST include the entire Header Chain in the First Fragment. A host that receives a non-compliant First Fragment SHOULD discard the packet and SHOULD send an ICMPv6 error to the source, subject to the rules of RFC 4443. For backwards compatibility, implementations MAY offer a setting to accept such fragments.
An intermediate system such as a router or firewall MAY discard such a packet and MAY send the ICMPv6 error. Systems with this capability SHOULD let the operator enable or disable dropping. Any ICMPv6 error sent MUST be Type 4 (Parameter Problem) with Code 3, "First Fragment has incomplete IPv6 Header Chain", and the Pointer field MUST be zero.
One consequence is that a chain cannot be longer than the Path MTU to its destination. Hosts discover this using RFC 1981 or RFC 4821. Hosts that do not discover the Path MTU MUST limit the chain to 1280 bytes, which is the IPv6 minimum MTU.
Senders must put all the labels on page one. Receivers can send back a note saying the labels were missing. If the sender does not know how big the envelope can be, it keeps the labels small enough for the smallest envelope.
- Fragmenting hostMUST put the entire Header Chain in the First Fragment.
- Receiving hostSHOULD discard a bad First Fragment and SHOULD send an ICMPv6 error.
- Router or firewallMAY discard it and MAY send the error; SHOULD allow this to be configured.
- ICMPv6 errorType 4, Code 3, with the Pointer field set to zero.
§5Security and IANA Notes
The RFC says it raises no new security exposures. Hiding the upper-layer header behind fragmentation can evade stateless packet filtering, and by forbidding such packets the update removes that vulnerability. It also warns that deploying source address forgery prevention filters ( RFC 2827, RFC 3704) reduces the chance that an attacker uses the new ICMPv6 errors for a reflection attack with a forged victim address.
A firewall that does stateless deep packet inspection of application payload might still be unable to process fragmented packets correctly, even when the header chain is not fragmented. IANA added Code 3, "IPv6 First Fragment has incomplete IPv6 Header Chain", to the Type 4 Parameter Problem entries of the ICMPv6 Parameters registry.
Because the labels are always on the first page, a sneaky sender cannot hide them on later pages to fool the guard.