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Versions: 00 01 02 03 04 05                                             
IPv6 Working Group                                           S. Krishnan
Internet-Draft                                                  Ericsson
Intended status: Standards Track                            July 6, 2007
Expires: January 7, 2008

                  The case against Hop-by-Hop options

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Copyright Notice

   Copyright (C) The IETF Trust (2007).

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   The Hop-by-Hop option header is a type of IPv6 extension header that
   has been defined in the IPv6 protocol specification.  The contents of
   this header need to be processed by every node along the path of an
   IPv6 datagram.This draft highlights the characteristics of this
   extension header which make it prone to Denial of Service attacks and
   proposes solutions to minimize such attacks.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Conventions used in this document  . . . . . . . . . . . .  3
   2.  Details of the attack  . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Effects of the attack  . . . . . . . . . . . . . . . . . .  4
   3.  Proposed Solutions . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Deprecation  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Rate limiting  . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Impact on deployed IPv6 nodes  . . . . . . . . . . . . . . . .  6
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Normative References . . . . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Intellectual Property and Copyright Statements . . . . . . . . . . 11

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1.  Introduction

   The IPv6 base specification [RFC2460] defines the hop-by-hop
   extension header.  This extension header carries the options which
   need to be processed by every node along the path of the datagram.
   Certain characteristics of the specification make it especially
   vulnerable to Denial of Service attacks.  The characteristics are:

   o  All the ipv6 nodes on the path need to process the options in this

   o  The option TLVs in the hop-by-hop options header need to be
      processed in order

   o  A sub range of option types in this header will not cause any
      errors even if the node does not recognize them.

   o  There is no restriction as to how many occurences of an option
      type can be present in the hop-by-hop header.

   This document details a low bandwidth Denial of Service attack on
   ipv6 routers/hosts using the hop-by-hop options extension header and
   possible ways of mitigating these attacks.

1.1.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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2.  Details of the attack

   The denial of service attack can be carried out by forming an IP
   datagram with a large number of TLV encoded options with random
   option type identifiers in the hop-by-hop options header.  The option
   type is a 8 bit field with special meaning attached to the three most
   significant bits.  The attack is most effective when all the nodes in
   the path are affected, meaning we do not want any node to drop the
   packet and send ICMP errors regarding unrecognized options.  If the
   two most significant bits are cleared(0), the receiving node will
   silently ignore the option if it does not recognize the option type.
   The third most significant bit is used to denote whether the option
   data can change en-route.  If the bit is set to 1 the option data can
   change en route.  The attack is equally effective whether or not an
   IPSec Authentication Header(AH) treats the option data as zero valued
   octets.  Hence we can include this bit in generating option types.
   The acceptable option types would be laid out like below

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
   |  Option Type  |  Opt Data Len |  Option Data
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
   |0 0 x x x x x x|...............|.................
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

                       Figure 1: Option type layout

   Since the option types 0(0x00) and 1(0x01) are reserved for the Pad1
   and PadN options in [RFC2460] we exclude these from the acceptable
   range as well.  So we choose the option type identifiers for each of
   these options to be in the range 0x02-0x63.  More option types
   defined by other RFCs can be excluded from the attack as and when
   they are allocated by the IANA.  Examples are Tunnel Encapsulation
   limit (0x04) and Router Alert (0x05).

2.1.  Effects of the attack

   The attack can be used to cripple the routers by attacking the
   control processor rather than the forwarding plane.  Since the
   control traffic, like the routing protocols, shares the same
   resources with this traffic, this kind of attack may be hard to
   control.  On routers having separate Control and Forwarding elements
   only the Control traffic would be affected.  For routers whose the
   Control and Forwarding elements are fused together this would lead to
   problems with forwarding packets as well.

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3.  Proposed Solutions

   There are at least possible solutions to handle the DoS attack
   mentioned in this draft.  The first one is to get rid of the feature
   altogether and prevent the attacks.  The second is to let the attacks
   occur, but limit the damage.

3.1.  Deprecation

   The first solution is to deprecate hop-by-hop options from the IPv6
   specification and to stop allocation of any new ones.  The existing
   hop-by-hop options MAY be grandfathered but new ones MUST NOT be
   allocated.  This allows existing protocols depending on hop-by-hop
   options to continue working, but discourages the development of new
   solutions based on hop-by-hop options.

3.2.  Rate limiting

   A less severe (and less effective) solution is to simply rate limit
   packets with hop-by-hop option headers and start dropping them
   randomly when the CPU load becomes very high.  While this solution is
   very simple and has no impact on deployed IPv6 nodes, it is sub-
   optimal.  A legitimate packet with a hop-by-hop option header has the
   same probability of being dropped as an attack packet.  Implementing
   the solution proposed in this draft does not preclude the use of rate
   limiting.  In fact it gives a legitimate packet a lower probability
   of being dropped, since most of the obvious attack traffic would have
   been dropped by the receiving algorithm.

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4.  Impact on deployed IPv6 nodes

   The proposed changes can affect all currently IPv6 nodes which need
   to send and receive packets with hop-by-hop options.  If the
   deprecation option is chosen, the IPv6 stack on both sending and
   receiving nodes needs to be modified to not send or receive hop-by-
   hop options.  In addition, transit nodes need to be modified as well
   in order to not inspect these options.

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5.  Security Considerations

   This document highlights the possible security issues with the IPv6
   hop-by-hop option header specified in [RFC2460] which can lead to
   denial of service attacks and suggests some changes to reduce the
   effect of the DoS attacks.

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6.  IANA Considerations

   This requests IANA to stop allocation of new entries for IPv6 hop-by-
   hop option types.

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7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

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Author's Address

   Suresh Krishnan
   8400 Decarie Blvd.
   Town of Mount Royal, QC

   Email: suresh.krishnan@ericsson.com

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