IPv6 Working Group S. Krishnan
Internet-Draft Ericsson
Intended status: Standards Track July 14, 2009
Expires: January 15, 2010
The case against Hop-by-Hop options
draft-krishnan-ipv6-hopbyhop-03
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Abstract
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. Skipping . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. 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
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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
header
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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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 three 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 one is to limit the
attacks to nodes that need to process hop-by-hop options. The third
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. Skipping
This option allows nodes to skip over the hop-by-hop extension header
without processing any of the options contained in the header. If a
node receives an IPv6 datagram with a hop-by-hop header, and it does
not support any hop-by-hop options at all, it can just skip over the
header.
3.3. 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
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
Email: suresh.krishnan@ericsson.com
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