IPv6 Working Group Suresh Krishnan
Internet-Draft Ericsson
Expires: December 14, 2004 June 15, 2004
Arrangement of Hop-by-Hop options
draft-krishnan-ipv6-hopbyhop-00
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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 an arrangement of options to minimize such attacks.
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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. Optimal arrangement of options . . . . . . . . . . . . . . . . 5
4. Proposed arrangement of options . . . . . . . . . . . . . . . 6
5. Deployment Considerations . . . . . . . . . . . . . . . . . . 7
5.1 Impact on deployed IPv6 nodes . . . . . . . . . . . . . . 7
5.2 Alternate solutions . . . . . . . . . . . . . . . . . . . 7
5.3 Quantitative analysis . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . 9
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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. Optimal arrangement of options
This attack can be mitigated by restricting each option type to occur
only once in a given extension header. Since it would be
computationally expensive for each ipv6 node to remember all the
option types which have already occurred in the header, it makes
sense to specify some kind of ordering of the option type identifiers
within the hop by hop options header. The most efficient arrangement
is to arrange the options in descending order of the numerical value
of the option type identifier. With this arrangement it is trivial
to check if an option has occurred before. The IPv6 node has to
remember and compare to only the last encountered option in addition
to the current one rather than remembering and comparing all the
previously encountered options. The reserved option types 0x00(Pad1)
and 0x01(PadN) options MAY occur anywhere in the header and they are
not considered for this check.
This exception leaves the door open for another class of DoS attacks.
The attacker can use just the Pad1 and PadN options multiple times in
the header to achieve the same effect as the previously detailed
attack. Hence it becomes necessary to add one more restriction. Two
padding options MUST NOT appear together in the header. Since there
is no legitimate case where two pad options have to appear together
this will not cause too many problems.
The option type identifier space is shared between the hop-by-hop
options and the destination options extension headers. Therefore the
attack is equally applicable to the destination options header but is
not as effective because only the destination node processes the
header. Similar language MAY be used to specify the destination
options header as well.
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4. Proposed arrangement of options
Within an IPv6 hop-by-hop option header each option type MUST NOT
occur more than once with the exception of the Pad1(0x00) and
PadN(0x01) options. The options MUST be arranged in descending order
of the numerical value of the option type identifier with the
exception of the Pad1 and the PadN options. The pad options MAY
occur anywhere in the header but two pad options MUST be seperated by
an option which is not a pad option.
So the final receiving algorithm looks like this
/* Receiving algorithm for hop-by-hop options */
last_option_was_pad=0;
first_option=1;
while (more_options) {
if (current_option_id & 0xfe) {
if (last_option_was_pad) {
/* Error: Cannot have two pad options together */
/* Send ICMPv6 message */
} else {
last_option_was_pad=1;
continue;
}
}
if (current_option_id<previous_option_id) {
previous_option_id=current_option_id;
last_option_was_pad=0;
/* Process the option */
} else {
if (first_option) {
first_option=0;
previous_option_id=current_option_id;
last_option_was_pad=0;
/* Process the option */
}
/* Error option is duplicate or out of order*/
/* Send ICMPv6 message */
}
}
Figure 2: Receiving algorithm
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5. Deployment Considerations
5.1 Impact on deployed IPv6 nodes
The proposed changes affect all currently IPv6 nodes which need to
send packets with hop-by-hop options. The IPv6 stack on these nodes
needs to be modified to send out the options in the correct order.
Since there are not too many option types which are currently
supported by deployed nodes, it is very likely that the nodes need to
be updated anyway for supporting more option types as they are
assigned.
5.2 Alternate solutions
There are other possible solutions to handle the DoS attack mentioned
in this draft. The first one that comes to mind 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.
5.3 Quantitative analysis
The proposed solution gives cuts processing times in worst case
scenarios by between 2x-8x depending on how many options in the
0x64-0xff range are allocated by the IANA. The reduction in
processing times is inversely proportional to the number of options
allocated in this range. The 2x number is valid when all the 192
options have been allocated and the 8x number applies when none of
them are allocated
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6. Security Considerations
This document higlights 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.
7 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.
Author's Address
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC
Canada
EMail: suresh.krishnan@ericsson.com
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