Network Working Group E. Davies
Request for Comments: 4942 Consultant
Category: Informational S. Krishnan
Ericsson
P. Savola
CSC/Funet
September 2007
IPv6 Transition/Coexistence Security Considerations
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Abstract
The transition from a pure IPv4 network to a network where IPv4 and
IPv6 coexist brings a number of extra security considerations that
need to be taken into account when deploying IPv6 and operating the
dual-protocol network and the associated transition mechanisms. This
document attempts to give an overview of the various issues grouped
into three categories:
o issues due to the IPv6 protocol itself,
o issues due to transition mechanisms, and
o issues due to IPv6 deployment.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Issues Due to IPv6 Protocol . . . . . . . . . . . . . . . . . 4
2.1. IPv6 Protocol-Specific Issues . . . . . . . . . . . . . . 5
2.1.1. Routing Headers and Hosts . . . . . . . . . . . . . . 5
2.1.2. Routing Headers for Mobile IPv6 and Other Purposes . . 6
2.1.3. Site-Scope Multicast Addresses . . . . . . . . . . . . 7
2.1.4. ICMPv6 and Multicast . . . . . . . . . . . . . . . . . 7
2.1.5. Bogus Errored Packets in ICMPv6 Error Messages . . . . 8
2.1.6. Anycast Traffic Identification and Security . . . . . 9
2.1.7. Address Privacy Extensions Interact with DDoS
Defenses . . . . . . . . . . . . . . . . . . . . . . . 10
2.1.8. Dynamic DNS: Stateless Address Autoconfiguration,
Privacy Extensions, and SEND . . . . . . . . . . . . . 10
2.1.9. Extension Headers . . . . . . . . . . . . . . . . . . 11
2.1.10. Fragmentation: Reassembly and Deep Packet
Inspection . . . . . . . . . . . . . . . . . . . . . . 14
2.1.11. Fragmentation Related DoS Attacks . . . . . . . . . . 15
2.1.12. Link-Local Addresses and Securing Neighbor
Discovery . . . . . . . . . . . . . . . . . . . . . . 16
2.1.13. Securing Router Advertisements . . . . . . . . . . . . 17
2.1.14. Host-to-Router Load Sharing . . . . . . . . . . . . . 18
2.1.15. Mobile IPv6 . . . . . . . . . . . . . . . . . . . . . 18
2.2. IPv4-Mapped IPv6 Addresses . . . . . . . . . . . . . . . . 19
2.3. Increased End-to-End Transparency . . . . . . . . . . . . 20
2.3.1. IPv6 Networks without NATs . . . . . . . . . . . . . . 20
2.3.2. Enterprise Network Security Model for IPv6 . . . . . . 21
2.4. IPv6 in IPv6 Tunnels . . . . . . . . . . . . . . . . . . . 22
3. Issues Due to Transition Mechanisms . . . . . . . . . . . . . 23
3.1. IPv6 Transition/Coexistence Mechanism-Specific Issues . . 23
3.2. Automatic Tunneling and Relays . . . . . . . . . . . . . . 23
3.3. Tunneling IPv6 through IPv4 Networks May Break IPv4
Network Security Assumptions . . . . . . . . . . . . . . . 24
4. Issues Due to IPv6 Deployment . . . . . . . . . . . . . . . . 26
4.1. Avoiding the Trap of Insecure IPv6 Service Piloting . . . 26
4.2. DNS Server Problems . . . . . . . . . . . . . . . . . . . 28
4.3. Addressing Schemes and Securing Routers . . . . . . . . . 28
4.4. Consequences of Multiple Addresses in IPv6 . . . . . . . . 28
4.5. Deploying ICMPv6 . . . . . . . . . . . . . . . . . . . . . 29
4.5.1. Problems Resulting from ICMPv6 Transparency . . . . . 30
4.6. IPsec Transport Mode . . . . . . . . . . . . . . . . . . . 30
4.7. Reduced Functionality Devices . . . . . . . . . . . . . . 31
4.8. Operational Factors when Enabling IPv6 in the Network . . 31
4.9. Security Issues Due to Neighbor Discovery Proxies . . . . 32
5. Security Considerations . . . . . . . . . . . . . . . . . . . 32
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 32
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
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7.1. Normative References . . . . . . . . . . . . . . . . . . . 33
7.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. IPv6 Probing/Mapping Considerations . . . . . . . . . 37
Appendix B. IPv6 Privacy Considerations . . . . . . . . . . . . . 38
B.1. Exposing MAC Addresses . . . . . . . . . . . . . . . . . . 38
B.2. Exposing Multiple Devices . . . . . . . . . . . . . . . . 39
B.3. Exposing the Site by a Stable Prefix . . . . . . . . . . . 39
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1. Introduction
The transition from a pure IPv4 network to a network where IPv4 and
IPv6 coexist brings a number of extra security considerations that
need to be taken into account when deploying IPv6 and operating the
dual-protocol network with its associated transition mechanisms.
This document attempts to give an overview of the various issues
grouped into three categories:
o issues due to the IPv6 protocol itself,
o issues due to transition mechanisms, and
o issues due to IPv6 deployment.
It is important to understand that deployments are unlikely to be
replacing IPv4 with IPv6 (in the short term), but rather will be
adding IPv6 to be operated in parallel with IPv4 over a considerable
period, so that security issues with transition mechanisms and dual
stack networks will be of ongoing concern. This extended transition
and coexistence period stems primarily from the scale of the current
IPv4 network. It is unreasonable to expect that the many millions of
IPv4 nodes will be converted overnight. It is more likely that it
will take two or three capital equipment replacement cycles (between
nine and 15 years) for IPv6 capabilities to spread through the
network, and many services will remain available over IPv4 only for a
significant period whilst others will be offered either just on IPv6
or on both protocols. To maintain current levels of service,
enterprises and service providers will need to support IPv4 and IPv6
in parallel for some time.
This document also describes two matters that have been wrongly
identified as potential security concerns for IPv6 in the past and
explains why they are unlikely to cause problems: considerations
about probing/mapping IPv6 addresses (Appendix A) and considerations
with respect to privacy in IPv6 (Appendix B).
2. Issues Due to IPv6 Protocol
Administrators should be aware that some of the rules suggested in
this section could potentially lead to a small amount of legitimate
traffic being dropped because the source has made unusual and
arguably unreasonable choices when generating the packet. The IPv6
specification [RFC2460] contains a number of areas where choices are
available to packet originators that will result in packets that
conform to the specification but are unlikely to be the result of a
rational packet generation policy for legitimate traffic (e.g.,
sending a fragmented packet in a much larger than necessary number of
small segments). This document highlights choices that could be made
by malicious sources with the intention of damaging the target host
or network, and suggests rules that try to differentiate
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specification-conforming packets that are legitimate traffic from
conforming packets that may be trying to subvert the specification to
cause damage. The differentiation tries to offer a reasonable
compromise between securing the network and passing every possible
conforming packet. To avoid loss of important traffic,
administrators are advised to log packets dropped according to these
rules and examine these logs periodically to ensure that they are
having the desired effect, and are not excluding traffic
inappropriately.
The built-in flexibility of the IPv6 protocol may also lead to
changes in the boundaries between legitimate and malicious traffic as
identified by these rules. New options may be introduced in the
future, and rules may need to be altered to allow the new
capabilities to be (legitimately) exploited by applications. The
document therefore recommends that filtering needs to be configurable
to allow administrators the flexibility to update rules as new pieces
of IPv6 specification are standardized.
2.1. IPv6 Protocol-Specific Issues
There are significant differences between the features of IPv6 and
IPv4: some of these specification changes may result in potential
security issues. Several of these issues have been discussed in
separate documents but are summarized here to avoid normative
references that may not become RFCs. The following specification-
related problems have been identified, but this is not necessarily a
complete list.
2.1.1. Routing Headers and Hosts
All IPv6 nodes must be able to process routing headers [RFC2460].
This RFC can be interpreted, although it is not explicitly stated, to
mean that all nodes (including hosts) must have this processing
enabled. The "Requirements for Internet Hosts" [RFC1122] permits
implementations to perform "local source routing", that is,
forwarding a packet with a routing header through the same interface
on which it was received: no restrictions are placed on this
operation even on hosts. In combination, these rules can result in
hosts forwarding received traffic to another node if there are
segments left in the Routing Header when it arrives at the host.
A number of potential security issues associated with this behavior
have been identified. Some of these issues have been resolved (a
separate routing header (Type 2) has been standardized for Mobile
IPv6 [RFC3775], and ICMP Traceback has not been standardized), but
two issues remain:
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o Both existing types of routing header can be used to evade access
controls based on destination addresses. This could be achieved
by sending a packet ostensibly to a publicly accessible host
address but with a routing header containing a 'forbidden'
address. If the publicly accessible host is processing routing
headers, it will forward the packet to the destination address in
the routing header that would have been forbidden by the packet
filters if the address had been in the destination field when the
packet was checked.
o If the packet source address can be spoofed when using a Type 0
routing header, the mechanism described in the previous bullet
could be used with any host to mediate an anonymous reflection
denial-of-service attack by having any publicly accessible host
redirect the attack packets. (This attack cannot use Type 2
routing headers because the packet cannot be forwarded outside the
host that processes the routing header, i.e., the original
destination of the packet.)
To counteract these threats, if a device is enforcing access controls
based on destination addresses, it needs to examine both the
destination address in the base IPv6 header and any waypoint
destinations in a routing header that have not yet been reached by
the packet at the point where it is being checked.
Various forms of amplification attack on routers and firewalls using
the routing header could be envisaged. A simple form involves
repeating the address of a waypoint several times in the routing
header. More complex forms could involve alternating waypoint
addresses that would result in the packet re-transiting the router or
firewall. These attacks can be counteracted by ensuring that routing
headers do not contain the same waypoint address more than once, and
performing ingress/egress filtering to check that the source address
is appropriate to the destination: packets made to reverse their path
will fail this test.
2.1.2. Routing Headers for Mobile IPv6 and Other Purposes
In addition to the basic Routing Header (Type 0), which is intended
to influence the trajectory of a packet through a network by
specifying a sequence of router waypoints, Routing Header (Type 2)
has been defined as part of the Mobile IPv6 specifications in
[RFC3775]. The Type 2 Routing Header is intended for use by hosts to
handle 'interface local' forwarding needed when packets are sent to
the care-of address of a mobile node that is away from its home
address.
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It is important that nodes treat the different types of routing
header appropriately. It should be possible to apply separate
filtering rules to the different types of Routing Header. By design,
hosts must process Type 2 Routing Headers to support Mobile IPv6 but
routers should not: to avoid the issues in Section 2.1.1, it may be
desirable to forbid or limit the processing of Type 0 Routing Headers
in hosts and some routers.
Routing Headers are an extremely powerful and general capability.
Alternative future uses of Routing Headers need to be carefully
assessed to ensure that they do not open new avenues of attack that
can be exploited.
2.1.3. Site-Scope Multicast Addresses
IPv6 supports multicast addresses with site scope that can
potentially allow an attacker to identify certain important resources
on the site if misused.
Particular examples are the 'all routers' (FF05::2) and 'all Dynamic
Host Configuration Protocol (DHCP) servers' (FF05::1:3) addresses
defined in [RFC2375]. An attacker that is able to infiltrate a
message destined for these addresses on to the site will potentially
receive in return information identifying key resources on the site.
This information can then be the target of directed attacks ranging
from simple flooding to more specific mechanisms designed to subvert
the device.
Some of these addresses have current legitimate uses within a site.
The risk can be minimized by ensuring that all firewalls and site
boundary routers are configured to drop packets with site-scope
destination addresses. Also, nodes should not join multicast groups
for which there is no legitimate use on the site, and site routers
should be configured to drop packets directed to these unused
addresses.
2.1.4. ICMPv6 and Multicast
It is possible to launch a Denial-of-Service (DoS) attack using IPv6
that could be amplified by the multicast infrastructure.
Unlike ICMP for IPv4, ICMPv6 [RFC4443] allows error notification
responses to be sent when certain unprocessable packets are sent to
multicast addresses.
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The cases in which responses are sent are:
o The received packet is longer than the next link Maximum
Transmission Unit (MTU): 'Packet Too Big' responses are needed to
support Path MTU Discovery for multicast traffic.
o The received packet contains an unrecognized option in a hop-by-
hop or destination options extension header with the first two
bits of the option type set to binary '10': 'Parameter Problem'
responses are intended to inform the source that some or all of
the recipients cannot handle the option in question.
If an attacker can craft a suitable packet sent to a multicast
destination, it may be possible to elicit multiple responses directed
at the victim (the spoofed source of the multicast packet). On the
other hand, the use of 'reverse path forwarding' checks (to eliminate
loops in multicast forwarding) automatically limits the range of
addresses that can be spoofed.
In practice, an attack using oversize packets is unlikely to cause
much amplification unless the attacker is able to carefully tune the
packet size to exploit a network with smaller MTU in the edge than
the core. Similarly, a packet with an unrecognized hop-by-hop option
would be dropped by the first router without resulting in multiple
responses. However, a packet with an unrecognized destination option
could generate multiple responses.
In addition to amplification, this kind of attack would potentially
consume large amounts of forwarding state resources in routers on
multicast-enabled networks.
2.1.5. Bogus Errored Packets in ICMPv6 Error Messages
Apart from the spurious load on the network, routers, and hosts,
bogus ICMPv6 error messages (types 0 to 127) containing a spoofed
errored packet can impact higher-layer protocols when the alleged
errored packet is referred to the higher layer at the destination of
the ICMPv6 packet [RFC4443]. The potentially damaging effects on TCP
connections, and some ways to mitigate the threats, are documented in
[ICMP-ATT].
Specific countermeasures for particular higher-layer protocols are
beyond the scope of this document, but firewalls may be able to help
counter the threat by inspecting the alleged errored packet embedded
in the ICMPv6 error message. Measures to mitigate the threat
include:
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o The receiving host should test that the embedded packet is all or
part of a packet that was transmitted by the host.
o The firewall may be able to test that the embedded packet contains
addresses that would have been legitimate (i.e., would have passed
ingress/egress filtering) for a packet sent from the receiving
host, but the possibility of asymmetric routing of the outgoing
and returning packets may prevent this sort of test depending on
the topology of the network, the location of the firewall, and
whether state synchronization between firewalls is in use.
o If the firewall is stateful and the test is not prevented by
asymmetric routing, the firewall may also be able to check that
the embedded packet is all or part of a packet that recently
transited the firewall in the opposite direction.
o Firewalls and destination hosts should be suspicious of ICMPv6
error messages with unnecessarily truncated errored packets (e.g.,
those that only carry the address fields of the IPv6 base header).
The specification of ICMPv6 requires that error messages carry as
much of the errored packet as possible (unlike ICMP for IPv4 which
requires only a minimum amount of the errored packet) and IPv6
networks must have a guaranteed minimum MTU of 1280 octets.
Accordingly, the ICMPv6 message should normally carry all the
header fields of the errored packet, together with a significant
amount of the payload, in order to allow robust comparison against
the outgoing packet.
2.1.6. Anycast Traffic Identification and Security
IPv6 introduces the notion of anycast addresses and services.
Originally the IPv6 standards disallowed using an anycast address as
the source address of a packet. Responses from an anycast server
would therefore supply a unicast address for the responding server.
To avoid exposing knowledge about the internal structure of the
network, it is recommended that anycast servers now take advantage of
the ability to return responses with the anycast address as the
source address if possible.
If the server needs to use a unicast address for any reason, it may
be desirable to consider using specialized addresses for anycast
servers, which are not used for any other part of the network, to
restrict the information exposed. Alternatively, operators may wish
to restrict the use of anycast services from outside the domain, thus
requiring firewalls to filter anycast requests. For this purpose,
firewalls need to know which addresses are being used for anycast
services: these addresses are arbitrary and not distinguishable from
any other IPv6 unicast address by structure or pattern.
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One particular class of anycast addresses that should be given
special attention is the set of Subnet-Router anycast addresses
defined in "IP Version 6 Addressing Architecture" [RFC4291]. All
routers are required to support these addresses for all subnets for
which they have interfaces. For most subnets using global unicast
addresses, filtering anycast requests to these addresses can be
achieved by dropping packets with the lower 64 bits (the Interface
Identifier) set to all zeros.
2.1.7. Address Privacy Extensions Interact with DDoS Defenses
The purpose of the privacy extensions for stateless address
autoconfiguration [RFC4941] is to change the interface identifier
(and hence the global scope addresses generated from it) from time to
time. By varying the addresses used, eavesdroppers and other
information collectors find it more difficult to identify which
transactions actually relate to a specific node.
A security issue may result from this if the frequency of node
address change is sufficiently great to achieve the intended aim of
the privacy extensions: with a relatively high rate of change, the
observed behavior has some characteristics of a node or nodes
involved in a Distributed Denial-of-Service (DDoS) attack. It should
be noted, however, that addresses created in this way are
topologically correct and that the other characteristics of the
traffic may reveal that there is no malicious intent.
This issue can be addressed in most cases by tuning the rate of
change in an appropriate manner.
Note that even if a node is well behaved, a change in the address
could make it harder for a security administrator to define an
address-based policy rule (e.g., access control list). However,
nodes that employ privacy addresses do not have to use them for all
communications.
2.1.8. Dynamic DNS: Stateless Address Autoconfiguration, Privacy
Extensions, and SEND
The introduction of Stateless Address Autoconfiguration (SLAAC)
[RFC2462] with IPv6 provides an additional challenge to the security
of Dynamic Domain Name System (DDNS). With manual addressing or the
use of DHCP, the number of security associations that need to be
maintained to secure access to the Domain Name Service (DNS) server
is limited, assuming any necessary updates are carried out by the
DHCP server. This is true equally for IPv4 and IPv6.
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Since SLAAC does not make use of a single and potentially trusted
DHCP server, but depends on the node obtaining the address, securing
the insertion of updates into DDNS may need a security association
between each node and the DDNS server. This is discussed further in
[RFC4472].
Using the Privacy Extensions to SLAAC [RFC4941] may significantly
increase the rate of updates of DDNS. Even if a node using the
Privacy Extensions does not publish its address for 'forward' lookup
(as that would effectively compromise the privacy that it is
seeking), it may still need to update the reverse DNS records. If
the reverse DNS records are not updated, servers that perform reverse
DNS checks will not accept connections from the node and it will not
be possible to gain access to IP Security (IPsec) keying material
stored in DNS [RFC4025]. If the rate of change needed to achieve
real privacy has to be increased (see Section 2.1.7), the update rate
for DDNS may be excessive.
Similarly, the cryptographically generated addresses used by SEND
[RFC3971] are expected to be periodically regenerated in line with
recommendations for maximum key lifetimes. This regeneration could
also impose a significant extra load on DDNS.
2.1.9. Extension Headers
A number of security issues relating to IPv6 Extension headers have
been identified. Several of these are a result of ambiguous or
incomplete specification in the base IPv6 specification [RFC2460].
2.1.9.1. Processing Extension Headers in Middleboxes
In IPv4, deep packet inspection techniques are used to implement
policing and filtering both as part of routers and in middleboxes
such as firewalls. Fully extending these techniques to IPv6 would
require inspection of all the extension headers in a packet. This is
essential to ensure that policy constraints on the use of certain
headers and options are enforced and to remove, at the earliest
opportunity, packets containing potentially damaging unknown options.
This requirement appears to conflict with Section 4 of the IPv6
specification in [RFC2460] which requires that only hop-by-hop
options are processed at any node through which the packet passes
until the packet reaches the appropriate destination (either the
final destination or a routing header waypoint).
Also, [RFC2460] forbids processing the headers other than in the
order in which they appear in the packet.
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A further ambiguity relates to whether an intermediate node should
discard a packet that contains a header or destination option which
it does not recognize. If the rules above are followed slavishly, it
is not (or may not be) legitimate for the intermediate node to
discard the packet because it should not be processing those headers
or options.
Therefore, [RFC2460] does not appear to take account of the behavior
of middleboxes and other non-final destinations that may be
inspecting the packet, and thereby potentially limits the security
protection of these boxes. Firewall vendors and administrators may
choose to ignore these rules in order to provide enhanced security as
this does not appear to have any serious consequences with the
currently defined set of extensions. However, administrators should
be aware that future extensions might require different treatment.
2.1.9.2. Processing Extension Header Chains
There is a further problem for middleboxes that want to examine the
transport headers that are located at the end of the IPv6 header
chain. In order to locate the transport header or other protocol
data unit, the node has to parse the header chain.
The IPv6 specification [RFC2460] does not mandate the use of the
Type-Length-Value (TLV) format with a fixed layout for the start of
each header although it is used for the majority of headers currently
defined. (Only the Type field is guaranteed in size and offset.)
Therefore, a middlebox cannot guarantee to be able to process header
chains that may contain headers defined after the box was
manufactured. As discussed further in Section 2.1.9.3, middleboxes
ought not to have to know the detailed layout of all header types in
use but still need to be able to skip over such headers to find the
transport payload start. If this is not possible, it either limits
the security policy that can be applied in firewalls or makes it
difficult to deploy new extension header types.
At the time of writing, only the Fragment Header does not fully
conform to the TLV format used for other extension headers. In
practice, many firewalls reconstruct fragmented packets before
performing deep packet inspection, so this divergence is less
problematic than it might have been, and is at least partially
justified because the full header chain is not present in all
fragments.
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Hop-by-hop and destination options may also contain unknown options.
However, the options are required to be encoded in TLV format so that
intermediate nodes can skip over them during processing, unlike the
enclosing extension headers.
2.1.9.3. Unknown Headers/Destination Options and Security Policy
A strict security policy might dictate that packets containing either
unknown headers or destination options are discarded by firewalls or
other filters. This requires the firewall to process the whole
extension header chain, which may be currently in conflict with the
IPv6 specification as discussed in Section 2.1.9.1.
Even if the firewall does inspect the whole header chain, it may not
be sensible to discard packets with items unrecognized by the
firewall: the intermediate node has no knowledge of which options and
headers are implemented in the destination node and IPv6 has been
deliberately designed to be extensible through adding new header
options. This poses a dilemma for firewall administrators. On the
one hand, admitting packets with 'unknown' options is a security
risk, but dropping them may disable a useful new extension. The best
compromise appears to be to select firewalls that provide a
configurable discard policy based on the types of the extensions.
Then, if a new extension is standardized, administrators can
reconfigure firewalls to pass packets with legitimate items that they
would otherwise not recognize because their hardware or software is
not aware of a new definition. Provided that the new extensions
conform to the TLV layout followed by current extensions, the
firewall would not need detailed knowledge of the function or layout
of the extension header.
2.1.9.4. Excessive Hop-by-Hop Options
IPv6 does not limit the number of hop-by-hop options that can be
present in a hop-by-hop option header, and any option can appear
multiple times. The lack of a limit and the provision of
extensibility bits that force nodes to ignore classes of options that
they do not understand can be used to mount denial-of-service attacks
affecting all nodes on a path. A packet with large numbers of
unknown hop-by-hop options will be processed at every node through
which it is forwarded, consuming significant resources to determine
what action should be taken for each option. Current options with
the exception of Pad1 and PadN should not appear more than once so
that packets with inappropriately repeated options can be dropped.
However, keeping track of which options have been seen adds
complexity to firewalls and may not apply to future extensions. See
Section 2.1.9.3 for a discussion of the advisability of dropping
packets with unknown options in firewalls.
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2.1.9.5. Misuse of Pad1 and PadN Options
IPv6 allows multiple padding options of arbitrary sizes to be placed
in both Hop-by-Hop and Destination option headers.
PadN options are required to contain zero octets as 'payload'; there
is, however, no incentive for receivers to check this. It may
therefore be possible to use the 'payload' of padding options as a
covert channel. Firewalls and receiving hosts should actively check
that PadN only has zero octets in its 'payload'.
There is no legitimate reason for padding beyond the next eight octet
boundary since the whole option header is aligned on an eight-octet
boundary but cannot be guaranteed to be on a 16 (or higher power of
two)-octet boundary. The IPv6 specification allows multiple Pad1 and
PadN options to be combined in any way that the source chooses to
make up the required padding. Reasonable design choices would appear
to be using however many Pad1 options (i.e., zero octets) are needed
or using a single PadN option of the required size (from two up to
seven octets). Administrators should consider at least logging
unusual padding patterns, and may consider dropping packets that
contain unusual patterns if they are certain of expected source
behavior.
2.1.9.6. Overuse of Router Alert Option
The IPv6 router alert option specifies a hop-by-hop option that, if
present, signals the router to take a closer look at the packet.
This can be used for denial-of-service attacks. By sending a large
number of packets containing a router alert option, an attacker can
deplete the processor cycles on the routers available to legitimate
traffic.
2.1.10. Fragmentation: Reassembly and Deep Packet Inspection
The current specifications of IPv6 in [RFC2460] do not mandate any
minimum packet size for the fragments of a packet before the last
one, except for the need to carry the unfragmentable part in all
fragments.
The unfragmentable part does not include the transport port numbers,
so it is possible that the first fragment does not contain sufficient
information to carry out deep packet inspection involving the port
numbers.
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Packets with overlapping fragments are considered to be a major
security risk, but the reassembly rules for fragmented packets in
[RFC2460] do not mandate behavior that would minimize the effects of
overlapping fragments.
In order to ensure that deep packet inspection can be carried out
correctly on fragmented packets, many firewalls and other nodes that
use deep packet inspection will collect the fragments and reassemble
the packet before examining it. Depending on the implementation of
packet reassembly and the treatment of packet fragments in these
nodes, the specification issues mentioned potentially leave IPv6 open
to the sort of attacks described in [RFC1858] and [RFC3128] for IPv4.
The following steps can be taken to mitigate these threats:
o Although permitted in [RFC2460], there is no reason for a source
to generate overlapping packet fragments, and overlaps could be
prohibited in a future revision of the protocol specification.
Firewalls should drop all packets with overlapped fragments:
certain implementations both in firewalls and other nodes already
drop such packets.
o Specifying a minimum size for packet fragments does not help in
the same way as it does for IPv4 because IPv6 extension headers
can be made to appear very long: an attacker could insert one or
more undefined destination options with long lengths and the
'ignore if unknown' bit set. Given the guaranteed minimum MTU of
IPv6, it seems reasonable that hosts should be able to ensure that
the transport port numbers are in the first fragment in almost all
cases and that deep packet inspection should be very suspicious of
first fragments that do not contain them (see also the discussion
of fragment sizes in Section 2.1.11).
2.1.11. Fragmentation Related DoS Attacks
Packet reassembly in IPv6 hosts also opens up the possibility of
various fragment-related security attacks. Some of these are
analogous to attacks identified for IPv4. Of particular concern is a
DoS attack based on sending large numbers of small fragments without
a terminating last fragment that would potentially overload the
reconstruction buffers and consume large amounts of CPU resources.
Mandating the size of packet fragments could reduce the impact of
this kind of attack by limiting the rate at which fragments could
arrive and limiting the number of fragments that need to be
processed, but this is not currently specified by the IPv6 standard.
In practice, reasonable design choices in protocol stacks are likely
to either maximize the size of all fragments except the final one
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using the path MTU (most likely choice), or distribute the data
evenly in the required minimum number of fragments. In either case,
the smallest non-final fragment would be at least half the guaranteed
minimum MTU (640 octets) -- the worst case arises when a payload is
just too large for a single packet and is divided approximately
equally between two packets. Administrators should consider
configuring firewalls and hosts to drop non-final fragments smaller
than 640 octets.
2.1.12. Link-Local Addresses and Securing Neighbor Discovery
All IPv6 nodes are required to configure a link-local address on each
interface. This address is used to communicate with other nodes
directly connected to the link accessed via the interface, especially
during the neighbor discovery and autoconfiguration processes. Link-
local addresses are fundamental to the operation of the Neighbor
Discovery Protocol (NDP) [RFC2461] and Stateless Address
Autoconfiguration (SLAAC) [RFC2462]. NDP also provides the
functionality of associating link-layer and IP addresses provided by
the Address Resolution Protocol (ARP) in IPv4 networks.
The standard version of NDP is subject to a number of security
threats related to ARP spoofing attacks on IPv4. These threats are
documented in [RFC3756], and mechanisms to combat them are specified
in SEcure Neighbor Discovery (SEND) [RFC3971]. SEND is an optional
mechanism that is particularly applicable to wireless and other
environments where it is difficult to physically secure the link.
Because the link-local address can, by default, be acquired without
external intervention or control, it allows an attacker to commence
communication on the link without needing to acquire information
about the address prefixes in use or communicate with any authorities
on the link. This feature gives a malicious node the opportunity to
mount an attack on any other node that is attached to this link; this
vulnerability exists in addition to possible direct attacks on NDP.
Link-local addresses may also facilitate the unauthorized use of the
link bandwidth ('bandwidth theft') to communicate with another
unauthorized node on the same link.
The vulnerabilities of IPv6 link-local addresses in NDP can be
mitigated in several ways. A general solution will require
o authenticating the link-layer connectivity, for example, by using
IEEE 802.1X functionality [IEEE.802-1X] or physical security, and
o using SEcure Neighbor Discovery (SEND) to create a
cryptographically generated link-local address (as described in
[RFC3971]) that is tied to the authenticated link-layer address.
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This solution would be particularly appropriate in wireless LAN
deployments where it is difficult to physically secure the
infrastructure, but it may not be considered necessary in wired
environments where the physical infrastructure can be kept secure by
other means.
Limiting the potentiality for abuse of link-local addresses in
general packet exchanges is more problematic because there may be
circumstances, such as isolated networks, where usage is appropriate
and discrimination between use and abuse requires complex filtering
rules which have to be implemented on hosts. The risk of misuse may
be deemed too small compared with the effort needed to control it,
but special attention should be paid to tunnel end-points (see 2.4,
3.2, and 3.3).
Any filtering has to be provided by a host-based or bridging
firewall. In general, link-local addresses are expected to be used
by applications that are written to deal with specific interfaces and
links. Typically these applications are used for control and
management. A node which is attached to multiple links has to deal
with the potentially overlapping link-local address spaces associated
with these links. IPv6 provides for this through zone identifiers
that are used to discriminate between the different address scopes
[RFC4007] and the scope identifier that can be associated with a
socket address structure [RFC3493]. Most users are unfamiliar with
these issues and general purpose applications are not intended to
handle this kind of discrimination. link-local addresses are not
normally used with the Domain Name System (DNS), and DNS cannot
supply zone identifiers. If it is considered necessary to prevent
the use of link-local addresses by means other than control and
management protocols, administrators may wish to consider limiting
the protocols that can be used with link-local addresses. At a
minimum, ICMPv6 and any intra-domain routing protocol in use (such as
Open Shortest Path First (OSPF) or Routing Information Protocol
(RIP)) need to be allowed, but other protocols may also be needed.
RIP illustrates the complexity of the filtering problem: its messages
are encapsulated as User Datagram Protocol (UDP) payloads, and
filtering needs to distinguish RIP messages addressed to UDP port 521
from other UDP messages.
2.1.13. Securing Router Advertisements
As part of the Neighbor Discovery process, routers on a link
advertise their capabilities in Router Advertisement messages. The
version of NDP defined in [RFC2461] does not protect the integrity of
these messages or validate the assertions made in the messages with
the result that any node that connects to the link can maliciously
claim to offer routing services that it will not fulfill, and
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advertise inappropriate prefixes and parameters. These threats have
been documented in [RFC3756].
A malicious node may also be able to carry out a DoS attack by
deprecating an established valid prefix (by advertising it with a
zero lifetime). Similar DoS attacks are possible if the optional
Router Selection mechanism is implemented as described in the
security considerations of [RFC4191].
SEND [RFC3971] can be used to provide verification that routers are
authorized to provide the services they advertise through a
certificate-based mechanism. This capability of SEND is also
particularly appropriate for wireless environments where clients are
reliant on the assertions of the routers rather than a physically
secured connection.
2.1.14. Host-to-Router Load Sharing
If a host deploys the optional host-to-router load-sharing mechanism
[RFC4311], a malicious application could carry out a DoS attack on
one or more of the load-sharing routers if the application is able to
use knowledge of the load-sharing algorithm to synthesize traffic
that subverts the load-sharing algorithm and directs a large volume
of bogus traffic towards a subset of the routers. The likelihood of
such an attack can be reduced if the implementation uses a
sufficiently sophisticated load sharing algorithm as described in the
security considerations of [RFC4311].
2.1.15. Mobile IPv6
Mobile IPv6 offers significantly enhanced security compared with
Mobile IPv4 especially when using optimized routing and care-of
addresses. Return routability checks are used to provide relatively
robust assurance that the different addresses that a mobile node uses
as it moves through the network do indeed all refer to the same node.
The threats and solutions are described in [RFC3775], and a more
extensive discussion of the security aspects of the design can be
found in [RFC4225].
2.1.15.1. Obsolete Home Address Option in Mobile IPv6
The Home Address option specified in early versions of Mobile IPv6
would have allowed a trivial source spoofing attack: hosts were
required to substitute the source address of incoming packets with
the address in the option, thereby potentially evading checks on the
packet source address. The version of Mobile IPv6 as standardized in
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[RFC3775] has removed this issue by ensuring that the Home Address
destination option is only processed if there is a corresponding
binding cache entry and securing Binding Update messages.
A number of pre-standard implementations of Mobile IPv6 were
available that implemented this obsolete and insecure option: care
should be taken to avoid running such obsolete systems.
2.2. IPv4-Mapped IPv6 Addresses
Overloaded functionality is always a double-edged sword: it may yield
some deployment benefits, but often also incurs the price that comes
with ambiguity.
One example of such is IPv4-mapped IPv6 addresses (::ffff/96): a
representation of an IPv4 address as an IPv6 address inside an
operating system as defined in [RFC3493]. Since the original
specification, the use of IPv4-mapped addresses has been extended to
a transition mechanism, Stateless IP/ICMP Translation algorithm
(SIIT) [RFC2765], where they are potentially used in the addresses of
packets on the wire.
Therefore, it becomes difficult to unambiguously discern whether an
IPv4 mapped address is really an IPv4 address represented in the IPv6
address format (basic API behavior) *or* an IPv6 address received
from the wire (which may be subject to address forgery, etc.). (SIIT
behavior). The security issues that arise from the ambiguous
behavior when IPv4-mapped addresses are used on the wire include:
o If an attacker transmits an IPv6 packet with ::ffff:127.0.0.1 in
the IPv6 source address field, he might be able to bypass a node's
access controls by deceiving applications into believing that the
packet is from the node itself (specifically, the IPv4 loopback
address, 127.0.0.1). The same attack might be performed using the
node's IPv4 interface address instead.
o If an attacker transmits an IPv6 packet with IPv4-mapped addresses
in the IPv6 destination address field corresponding to IPv4
addresses inside a site's security perimeter (e.g., ::ffff:
10.1.1.1), he might be able to bypass IPv4 packet filtering rules
and traverse a site's firewall.
o If an attacker transmits an IPv6 packet with IPv4-mapped addresses
in the IPv6 source and destination fields to a protocol that swaps
IPv6 source and destination addresses, he might be able to use a
node as a proxy for certain types of attacks. For example, this
might be used to construct broadcast multiplication and proxy TCP
port scan attacks.
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In addition, special cases like these, while giving deployment
benefits in some areas, require a considerable amount of code
complexity (e.g., in the implementations of bind() system calls and
reverse DNS lookups) that is probably undesirable but can be managed
in this case.
In practice, although the packet translation mechanisms of SIIT are
specified for use in "Network Address Translator - Protocol
Translator (NAT-PT)" [RFC2766], NAT-PT uses a mechanism different
from IPv4-mapped IPv6 addresses for communicating embedded IPv4
addresses in IPv6 addresses. Also, SIIT is not recommended for use
as a standalone transition mechanism. Given the issues that have
been identified, it seems appropriate that mapped addresses should
not be used on the wire. However, changing application behavior by
deprecating the use of mapped addresses in the operating system
interface would have significant impact on application porting
methods as described in [RFC4038], and it is expected that IPv4-
mapped IPv6 addresses will continue to be used within the API to aid
application portability.
Using the basic API behavior has some security implications in that
it adds additional complexity to address-based access controls. The
main issue that arises is that an IPv6 (AF_INET6) socket will accept
IPv4 packets even if the node has no IPv4 (AF_INET) sockets open.
This has to be taken into account by application developers and may
allow a malicious IPv4 peer to access a service even if there are no
open IPv4 sockets. This violates the security principle of "least
surprise".
2.3. Increased End-to-End Transparency
One of the major design aims of IPv6 has been to maintain the
original IP architectural concept of end-to-end transparency.
Transparency can help foster technological innovation in areas such
as peer-to-peer communication, but maintaining the security of the
network at the same time requires some modifications in the network
architecture. Ultimately, it is also likely to need changes in the
security model as compared with the norms for IPv4 networks.
2.3.1. IPv6 Networks without NATs
The necessity of introducing Network Address Translators (NATs) into
IPv4 networks, resulting from a shortage of IPv4 addresses, has
removed the end-to-end transparency of most IPv4 connections: the use
of IPv6 would restore this transparency. However, the use of NATs,
and the associated private addressing schemes, has become
inappropriately linked to the provision of security in enterprise
networks. The restored end-to-end transparency of IPv6 networks can
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therefore be seen as a threat by poorly informed enterprise network
managers. Some seem to want to limit the end-to-end capabilities of
IPv6, for example by deploying private, local addressing and
translators, even when it is not necessary because of the abundance
of IPv6 addresses.
Recommendations for designing an IPv6 network to meet the perceived
security and connectivity requirements implicit in the current usage
of IPv4 NATs whilst maintaining the advantages of IPv6 end-to-end
transparency are described in "IP Version 6 Network Architecture
Protection" [RFC4864].
2.3.2. Enterprise Network Security Model for IPv6
The favored model for enterprise network security in IPv4 stresses
the use of a security perimeter policed by autonomous firewalls and
incorporating the NATs. Both perimeter firewalls and NATs introduce
asymmetry and reduce the transparency of communications through these
perimeters. The symmetric bidirectionality and transparency that are
extolled as virtues of IPv6 may seem to be at odds with this model.
Consequently, network managers may even see them as undesirable
attributes, in conflict with their need to control threats to and
attacks on the networks they administer.
It is worth noting that IPv6 does not *require* end-to-end
connectivity. It merely provides end-to-end addressability; the
connectivity can still be controlled using firewalls (or other
mechanisms), and it is indeed wise to do so.
A number of matters indicate that IPv6 networks should migrate
towards an improved security model, which will increase the overall
security of the network while at the same time facilitating end-to-
end communication:
o Increased usage of end-to-end security especially at the network
layer. IPv6 mandates the provision of IPsec capability in all
nodes, and increasing usage of end-to-end security is a challenge
to current autonomous firewalls that are unable to perform deep
packet inspection on encrypted packets. It is also incompatible
with NATs because they modify the packets, even when packets are
only authenticated rather than encrypted.
o Acknowledgement that over-reliance on the perimeter model is
potentially dangerous. An attacker who can penetrate today's
perimeters will have free rein within the perimeter, in many
cases. Also a successful attack will generally allow the attacker
to capture information or resources and make use of them.
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o Development of mechanisms such as 'Trusted Computing' [TCGARCH]
that will increase the level of trust that network managers are
able to place on hosts.
o Development of centralized security policy repositories and secure
distribution mechanisms that, in conjunction with trusted hosts,
will allow network managers to place more reliance on security
mechanisms at the end-points. The mechanisms are likely to
include end-node firewalling and intrusion detection systems as
well as secure protocols that allow end-points to influence the
behavior of perimeter security devices.
o Review of the role of perimeter devices with increased emphasis on
intrusion detection, and network resource protection and
coordination to thwart distributed denial-of-service attacks.
Several of the technologies required to support an enhanced security
model are still under development, including secure protocols to
allow end-points to control firewalls: the complete security model
utilizing these technologies is now emerging but still requires some
development.
In the meantime, initial deployments will need to make use of similar
firewalling and intrusion detection techniques to IPv4 that may limit
end-to-end transparency temporarily, but should be prepared to use
the new security model as it develops and avoid the use of NATs by
the use of the architectural techniques described in [RFC4864]. In
particular, using NAT-PT [RFC2766] as a general purpose transition
mechanism should be avoided as it is likely to limit the exploitation
of end-to-end security and other IPv6 capabilities in the future as
explained in [RFC4966].
2.4. IPv6 in IPv6 Tunnels
IPv6 in IPv6 tunnels can be used to circumvent security checks, so it
is essential to filter packets both at tunnel ingress and egress
points (the encapsulator and decapsulator) to ensure that both the
inner and outer addresses are acceptable, and the tunnel is not being
used to carry inappropriate traffic. [RFC3964], which is primarily
about the 6to4 transition tunneling mechanism (see Section 3.1),
contains useful discussions of possible attacks and ways to
counteract these threats.
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3. Issues Due to Transition Mechanisms
3.1. IPv6 Transition/Coexistence Mechanism-Specific Issues
The more complicated the IPv6 transition/coexistence becomes, the
greater the danger that security issues will be introduced either
o in the mechanisms themselves,
o in the interaction between mechanisms, or
o by introducing unsecured paths through multiple mechanisms.
These issues may or may not be readily apparent. Hence, it would be
desirable to keep the mechanisms simple (as few in number as possible
and built from pieces as small as possible) to simplify analysis.
One case where such security issues have been analyzed in detail is
the 6to4 tunneling mechanism [RFC3964].
As tunneling has been proposed as a model for several more cases than
are currently being used, its security properties should be analyzed
in more detail. There are some generic dangers to tunneling:
o It may be easier to avoid ingress filtering checks.
o It is possible to attack the tunnel interface: several IPv6
security mechanisms depend on checking that Hop Limit equals 255
on receipt and that link-local addresses are used. Sending such
packets to the tunnel interface is much easier than gaining access
to a physical segment and sending them there.
o Automatic tunneling mechanisms are typically particularly
dangerous as there is no pre-configured association between end
points. Accordingly, at the receiving end of the tunnel, packets
have to be accepted and decapsulated from any source.
Consequently, special care should be taken when specifying
automatic tunneling techniques.
3.2. Automatic Tunneling and Relays
Two mechanisms have been specified that use automatic tunneling and
are intended for use outside a single domain. These mechanisms
encapsulate the IPv6 packet directly in an IPv4 packet in the case of
6to4 [RFC3056] or in an IPv4 UDP packet in the case of Teredo
[RFC4380]. In each case, packets can be sent and received by any
similarly equipped nodes in the IPv4 Internet.
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As mentioned in Section 3.1, a major vulnerability in such approaches
is that receiving nodes must allow decapsulation of traffic sourced
from anywhere in the Internet. This kind of decapsulation function
must be extremely well secured because of the wide range of potential
sources.
An even more difficult problem is how these mechanisms are able to
establish communication with native IPv6 nodes or between the
automatic tunneling mechanisms: such connectivity requires the use of
some kind of "relay". These relays could be deployed in various
locations such as:
o all native IPv6 nodes,
o native IPv6 sites,
o in IPv6-enabled ISPs, or
o just somewhere in the Internet.
Given that a relay needs to trust all the sources (e.g., in the 6to4
case, all 6to4 routers) that are sending it traffic, there are issues
in achieving this trust and at the same time scaling the relay system
to avoid overloading a small number of relays.
As authentication of such a relay service is very difficult to
achieve, and particularly so in some of the possible deployment
models, relays provide a potential vehicle for address spoofing,
(reflected) denial-of-service attacks, and other threats.
Threats related to 6to4 and measures to combat them are discussed in
[RFC3964]. [RFC4380] incorporates extensive discussion of the
threats to Teredo and measures to combat them.
3.3. Tunneling IPv6 through IPv4 Networks May Break IPv4 Network
Security Assumptions
NATs and firewalls have been deployed extensively in the IPv4
Internet, as discussed in Section 2.3. Operators who deploy them
typically have some security/operational requirements in mind (e.g.,
a desire to block inbound connection attempts), which may or may not
be misguided.
The addition of tunneling can change the security model that such
deployments are seeking to enforce. IPv6-over-IPv4 tunneling using
protocol 41 is typically either explicitly allowed, or disallowed
implicitly. Tunneling IPv6 over IPv4 encapsulated in UDP constitutes
a more difficult problem as UDP must usually be allowed to pass
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through NATs and firewalls. Consequently, using UDP implies the
ability to punch holes in NATs and firewalls although, depending on
the implementation, this ability may be limited or only achieved in a
stateful manner. In practice, the mechanisms have been explicitly
designed to traverse both NATs and firewalls in a similar fashion.
One possible view is that the use of tunneling is especially
questionable in home and SOHO (small office/home office) environments
where the level of expertise in network administration is typically
not very high; in these environments, the hosts may not be as tightly
managed as in others (e.g., network services might be enabled
unnecessarily), leading to possible security break-ins or other
vulnerabilities.
Holes allowing tunneled traffic through NATs and firewalls can be
punched both intentionally and unintentionally. In cases where the
administrator or user makes an explicit decision to create the hole,
this is less of a problem, although (for example) some enterprises
might want to block IPv6 tunneling explicitly if employees were able
to create such holes without reference to administrators. On the
other hand, if a hole is punched transparently, it is likely that a
proportion of users will not understand the consequences: this will
very probably result in a serious threat sooner or later.
When deploying tunneling solutions, especially tunneling solutions
that are automatic and/or can be enabled easily by users who do not
understand the consequences, care should be taken not to compromise
the security assumptions held by the users.
For example, NAT traversal should not be performed by default unless
there is a firewall producing a similar by-default security policy to
that provided by IPv4 NAT. IPv6-in-IPv4 (protocol 41) tunneling is
less of a problem, as it is easier to block if necessary; however, if
the host is protected in IPv4, the IPv6 side should be protected as
well.
As is shown in Appendix A, it is relatively easy to determine the
IPv6 address corresponding to an IPv4 address in tunneling
deployments. It is therefore vital NOT to rely on "security by
obscurity", i.e., assuming that nobody is able to guess or determine
the IPv6 address of the host especially when using automatic
tunneling transition mechanisms.
The network architecture must provide separate IPv4 and IPv6
firewalls with tunneled IPv6 traffic arriving encapsulated in IPv4
packets routed through the IPv4 firewall before being decapsulated,
and then through the IPv6 firewall as shown in Figure 1.
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+--------+ +--------+ +--------+
Site | Native | IPv6 |v6 in v4| IPv4 | Native | Public
Network <--->| IPv6 |<---->| Tunnel |<---->| IPv4 |<---> Internet
|Firewall| |Endpoint| |Firewall|
+--------+ +--------+ +--------+
Figure 1: Tunneled Traffic and Firewalls
4. Issues Due to IPv6 Deployment
4.1. Avoiding the Trap of Insecure IPv6 Service Piloting
Because IPv6 is a new service for many networks, network managers
will often opt to make a pilot deployment in a part of the network to
gain experience and understand the problems as well as the benefits
that may result from a full production quality IPv6 service.
Unless IPv6 service piloting is done in a manner that is as secure as
possible, there is a risk that if security in the pilot does not
match up to what is achievable with current IPv4 production service,
the comparison can adversely impact the overall assessment of the
IPv6 pilot deployment. This may result in a decision to delay or
even avoid deploying an IPv6 production service. For example, hosts
and routers might not be protected by IPv6 firewalls, even if the
corresponding IPv4 service is fully protected by firewalls. The use
of tunneling transition mechanisms (see Section 3.3) and the
interaction with virtual private networks also need careful attention
to ensure that site security is maintained. This is particularly
critical where IPv6 capabilities are turned on by default in new
equipment or new releases of operating systems: network managers may
not be fully aware of the security exposure that this creates.
In some cases, a perceived lack of availability of IPv6 firewalls and
other security capabilities, such as intrusion detection systems may
have led network managers to resist any kind of IPv6 service
deployment. These problems may be partly due to the relatively slow
development and deployment of IPv6-capable security equipment, but
the major problems appear to have been a lack of information, and
more importantly a lack of documented operational experience on which
managers can draw. In actual fact, at the time of writing, there are
a significant number of alternative IPv6 packet filters and firewalls
already in existence that could be used to provide sufficient access
controls.
However, there are a small number of areas where the available
equipment and capabilities may still be a barrier to secure
deployment as of the time of writing:
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o 'Personal firewalls' with support for IPv6 and intended for use on
hosts are not yet widely available.
o Enterprise firewalls are at an early stage of development and may
not provide the full range of capabilities needed to implement the
necessary IPv6 filtering rules. Network managers often expect the
same devices that support and are used for IPv4 today to also
become IPv6-capable -- even though this is not really required and
the equipment may not have the requisite hardware capabilities to
support fast packet filtering for IPv6. Suggestions for the
appropriate deployment of firewalls are given in Section 3.3 -- as
will be seen from this section, it is usually desirable that the
firewalls are in separate boxes, and there is no necessity for
them to be same the model of equipment.
o A lesser factor may be that some design decisions in the IPv6
protocol make it more difficult for firewalls to be implemented
and work in all cases, and to be fully future-proof (e.g., when
new extension headers are used) as discussed in Section 2.1.9. It
is significantly more difficult for intermediate nodes to process
the IPv6 header chains than IPv4 packets.
o Adequate Intrusion Detection Systems (IDS) are more difficult to
construct for IPv6. IDSs are now beginning to become available
but the pattern-based mechanisms used for IPv4 may not be the most
appropriate for long-term development of these systems as end-to-
end encryption becomes more prevalent. Future systems may be more
reliant on traffic flow pattern recognition.
o Implementations of high availability capabilities supporting IPv6
are also in short supply. In particular, development of the IPv6
version of the Virtual Router Redundancy Protocol (VRRP) [VRRP]
has lagged the development of the main IPv6 protocol although
alternatives may be available for some environments.
In all these areas, developments are ongoing and they should not be
considered a long-term bar to the deployment of IPv6 either as a
pilot or production service. The necessary tools are now available
to make a secure IPv6 deployment, and with careful selection of
components and design of the network architecture, a successful pilot
or production IPv6 service can be deployed. Recommendations for
secure deployment and appropriate management of IPv6 networks can be
found in the documentation archives of the European Union 6net
project [SIXNET] and in the Deployment Guide published by the IPv6
Promotion Council of Japan [JpIPv6DC].
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4.2. DNS Server Problems
Some DNS server implementations have flaws that severely affect DNS
queries for IPv6 addresses as discussed in [RFC4074]. These flaws
can be used for DoS attacks affecting both IPv4 and IPv6 by inducing
caching DNS servers to believe that a domain is broken and causing
the server to block access to all requests for the domain for a
precautionary period.
4.3. Addressing Schemes and Securing Routers
Whilst in general terms brute force scanning of IPv6 subnets is
essentially impossible due to the enormously larger address space of
IPv6 and the 64-bit interface identifiers (see Appendix A), this will
be obviated if administrators do not take advantage of the large
space to use unguessable interface identifiers.
Because of the unmemorability of complete IPv6 addresses, there is a
temptation for administrators to use small integers as interface
identifiers when manually configuring them, as might happen on point-
to-point links or when provisioning complete addresses from a DHCPv6
server. Such allocations make it easy for an attacker to find active
nodes that they can then port scan.
To make use of the larger address space properly, administrators
should be very careful when entering IPv6 addresses in their
configurations (e.g., access control lists), since numerical IPv6
addresses are more prone to human error than IPv4 due to their length
and unmemorability.
It is also essential to ensure that the management interfaces of
routers are well secured (e.g., allowing remote access using Secure
Shell (SSH) only and ensuring that local craft interfaces have non-
default passwords) as the router will usually contain a significant
cache of neighbor addresses in its neighbor cache.
4.4. Consequences of Multiple Addresses in IPv6
One positive consequence of IPv6 is that nodes that do not require
global access can communicate locally just by the use of a link-local
address (if very local access is sufficient) or across the site by
using a Unique Local Address (ULA). In either case it is easy to
ensure that access outside the assigned domain of activity can be
controlled by simple filters (which should be the default for link-
locals). However, the security hazards of using link-local addresses
for general purposes, as documented in Section 2.1.12, should be
borne in mind.
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On the other hand, the possibility that a node or interface can have
multiple global scope addresses makes access control filtering (both
on ingress and egress) more complex and requires higher maintenance
levels. Vendors and network administrators need to be aware that
multiple addresses are the norm rather than the exception in IPv6:
when building and selecting tools for security and management, a
highly desirable feature is the ability to be able to 'tokenize'
access control lists and configurations in general to cater for
multiple addresses and/or address prefixes.
The addresses could be from the same network prefix (for example,
privacy mechanisms [RFC4941] will periodically create new addresses
taken from the same prefix, and two or more of these may be active at
the same time), or from different prefixes (for example, when a
network is multihomed, when for management purposes a node belongs to
several subnets on the same link or is implementing anycast
services). In all these cases, it is possible that a single host
could be using several different addresses with different prefixes
and/or different interface identifiers. It is desirable that the
security administrator be able to identify that the same host is
behind all these addresses.
Some network administrators may find the mutability of addresses when
privacy mechanisms are used in their network to be undesirable
because of the current difficulties in maintaining access control
lists and knowing the origin of traffic. In general, disabling the
use of privacy addresses is only possible if the full stateful DHCPv6
mechanism [RFC3315] is used to allocate IPv6 addresses and DHCPv6
requests for privacy addresses are not honored.
4.5. Deploying ICMPv6
In IPv4 it is commonly accepted that some filtering of ICMP packets
by firewalls is essential to maintain security. Because of the
extended use that is made of ICMPv6 [RFC2461] with a multitude of
functions, the simple set of dropping rules that are usually applied
in IPv4 need to be significantly developed for IPv6. The blanket
dropping of all ICMP messages that is used in some very strict
environments is simply not possible for IPv6.
In an IPv6 firewall, policy needs to allow some messages through the
firewall but also has to permit certain messages to and from the
firewall, especially those with link-local sources on links to which
the firewall is attached. These messages must be permitted to ensure
that Neighbor Discovery [RFC2462], Multicast Listener Discovery
([RFC2710], [RFC3810]), and Stateless Address Configuration [RFC4443]
work as expected.
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Recommendations for filtering ICMPv6 messages can be found in
[RFC4890].
4.5.1. Problems Resulting from ICMPv6 Transparency
As described in Section 4.5, certain ICMPv6 error packets need to be
passed through a firewall in both directions. This means that some
ICMPv6 error packets can be exchanged between inside and outside
without any filtering.
Using this feature, malicious users can communicate between the
inside and outside of a firewall, thus bypassing the administrator's
inspection (proxy, firewall, etc.). For example, it might be
possible to carry out a covert conversation through the payload of
ICMPv6 error messages or to tunnel inappropriate encapsulated IP
packets in ICMPv6 error messages. This problem can be alleviated by
filtering ICMPv6 errors using a stateful packet inspection mechanism
to ensure that the packet carried as a payload is associated with
legitimate traffic to or from the protected network.
4.6. IPsec Transport Mode
IPsec provides security to end-to-end communications at the network
layer (layer 3). The security features available include access
control, connectionless integrity, data origin authentication,
protection against replay attacks, confidentiality, and limited
traffic flow confidentiality (see [RFC4301] Section 2.1). IPv6
mandates the implementation of IPsec in all conforming nodes, making
the usage of IPsec to secure end-to-end communication possible in a
way that is generally not available to IPv4.
To secure IPv6 end-to-end communications, IPsec transport mode would
generally be the solution of choice. However, use of these IPsec
security features can result in novel problems for network
administrators and decrease the effectiveness of perimeter firewalls
because of the increased prevalence of encrypted packets on which the
firewalls cannot perform deep packet inspection and filtering.
One example of such problems is the lack of security solutions in the
middlebox, including effective content-filtering, ability to provide
DoS prevention based on the expected TCP protocol behavior, and
intrusion detection. Future solutions to this problem are discussed
in Section 2.3.2. Another example is an IPsec-based DoS (e.g.,
sending malformed ESP/AH packets) that can be especially detrimental
to software-based IPsec implementations.
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4.7. Reduced Functionality Devices
With the deployment of IPv6 we can expect the attachment of a very
large number of new IPv6-enabled devices with scarce resources and
low computing capacity. The resource limitations are generally
because of a market requirement for cost reduction. Although the
[RFC4294] specifies some mandatory security capabilities for every
conformant node, these do not include functions required for a node
to be able to protect itself. Accordingly, some such devices may not
be able even to perform the minimum set of functions required to
protect themselves (e.g., 'personal' firewall, automatic firmware
update, enough CPU power to endure DoS attacks). This means a
different security scheme may be necessary for such reduced
functionality devices.
4.8. Operational Factors when Enabling IPv6 in the Network
There are a number of reasons that make it essential to take
particular care when enabling IPv6 in the network equipment:
Initially, IPv6-enabled router software may be less mature than
current IPv4-only implementations, and there is less experience with
configuring IPv6 routing, which can result in disruptions to the IPv6
routing environment and (IPv6) network outages.
IPv6 processing may not happen at (near) line speed (or at a
comparable performance level to IPv4 in the same equipment). A high
level of IPv6 traffic (even legitimate, e.g., Network News Transport
Protocol, NNTP) could easily overload IPv6 processing especially when
it is software-based without the hardware support typical in high-end
routers. This may potentially have deleterious knock-on effects on
IPv4 processing, affecting availability of both services.
Accordingly, if people don't feel confident enough in the IPv6
capabilities of their equipment, they will be reluctant to enable it
in their "production" networks.
Sometimes essential features may be missing from early releases of
vendors' software; an example is provision of software enabling IPv6
telnet/SSH access (e.g., to the configuration application of a
router), but without the ability to turn it off or limit access to
it!
Sometimes the default IPv6 configuration is insecure. For example,
in one vendor's implementation, if you have restricted IPv4 telnet to
only a few hosts in the configuration, you need to be aware that IPv6
telnet will be automatically enabled, that the configuration commands
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used previously do not block IPv6 telnet, that IPv6 telnet is open to
the world by default, and that you have to use a separate command to
also lock down the IPv6 telnet access.
Many operator networks have to run interior routing protocols for
both IPv4 and IPv6. It is possible to run them both in one routing
protocol, or have two separate routing protocols; either approach has
its tradeoffs [RFC4029]. If multiple routing protocols are used, one
should note that this causes double the amount of processing when
links flap or recalculation is otherwise needed -- which might more
easily overload the router's CPU, causing slightly slower convergence
time.
4.9. Security Issues Due to Neighbor Discovery Proxies
In order to span a single subnet over multiple physical links, a new
experimental capability is being trialed in IPv6 to proxy Neighbor
Discovery messages. A node with this capability will be called an
NDProxy (see [RFC4389]). NDProxies are susceptible to the same
security issues as those faced by hosts using unsecured Neighbor
Discovery or ARP. These proxies may process unsecured messages, and
update the neighbor cache as a result of such processing, thus
allowing a malicious node to divert or hijack traffic. This may
undermine the advantages of using SEND [RFC3971].
If a form of NDProxy is standardized, SEND will need to be extended
to support this capability.
5. Security Considerations
This memo attempts to give an overview of security considerations of
the different aspects of IPv6, particularly as they relate to the
transition to a network in which IPv4- and IPv6-based communications
need to coexist.
6. Acknowledgements
This document draws together the work of many people who have
contributed security-related documents to the IPV6 and V6OPS working
groups. Alain Durand, Alain Baudot, Luc Beloeil, Sharon Chisholm,
Tim Chown, Lars Eggert, Andras Kis-Szabo, Vishwas Manral, Janos
Mohacsi, Mark Smith, Alvaro Vives, and Margaret Wassermann provided
feedback to improve this document. Satoshi Kondo, Shinsuke Suzuki,
and Alvaro Vives provided additional inputs in cooperation with the
Deployment Working Group of the Japanese IPv6 Promotion Council and
the Euro6IX IST co-funded project, together with inputs from Jordi
Palet, Brian Carpenter, and Peter Bieringer. Michael Wittsend and
Michael Cole discussed issues relating to probing/mapping and
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RFC 4942 IPv6 Security Overview September 2007
privacy. Craig Metz and Jun-ichiro itojun Hagino did the original
work identifying the problems of using IPv4-mapped IPv6 addresses on
the wire. Vishwas Manral made further investigations of the impact
of tiny fragments on IPv6 security. Francis Dupont raised the issues
relating to IPv6 Privacy Addresses. Finally, Pekka Savola wrote a
number of documents on aspects IPv6 security which have been subsumed
into this work. His document on "Firewalling Considerations for
IPv6" (October 2003) originally identified many of the issues with
the base IPv6 specification which are documented here.
7. References
7.1. Normative References
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC2375] Hinden, R. and S. Deering, "IPv6 Multicast Address
Assignments", RFC 2375, July 1998.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version
6 (IPv6) Specification", RFC 2460, December 1998.
[RFC2461] Narten, T., Nordmark, E., and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461,
December 1998.
[RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710,
October 1999.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6
Domains via IPv4 Clouds", RFC 3056, February 2001.
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility
Support in IPv6", RFC 3775, June 2004.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3964] Savola, P. and C. Patel, "Security Considerations for
6to4", RFC 3964, December 2004.
Davies, et al. Informational [Page 33]
RFC 4942 IPv6 Security Overview September 2007
[RFC4007] Deering, S., Haberman, B., Jinmei, T., Nordmark, E.,
and B. Zill, "IPv6 Scoped Address Architecture",
RFC 4007, March 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[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.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
7.2. Informative References
[FNAT] Bellovin, S., "Technique for Counting NATted Hosts",
Proc. Second Internet Measurement Workshop ,
November 2002,
.
[ICMP-ATT] Gont, F., "ICMP attacks against TCP", Work
in Progress, May 2007.
[IEEE.802-1X] Institute of Electrical and Electronics Engineers,
"Port-Based Network Access Control", IEEE Standard for
Local and Metropolitan Area Networks 802.1X-2004,
December 2004.
[JpIPv6DC] Deployment WG, "IPv6 Deployment Guideline (2005
Edition)", IPv6 Promotion Council (Japan) Deployment
Working Group, 2005, .
[VRRP] Hinden, R. and J. Cruz, "Virtual Router Redundancy
Protocol for IPv6", Work in Progress, March 2007.
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Appendix A. IPv6 Probing/Mapping Considerations
One school of thought wanted the IPv6 numbering topology (either at
network or node level) to match IPv4 as exactly as possible, whereas
others see IPv6 as giving more flexibility to the address plans, not
wanting to constrain the design of IPv6 addressing. Mirroring the
address plans is now generally seen as a security threat because an
IPv6 deployment may have different security properties from IPv4.
Given the relatively immature state of IPv6 network security, if an
attacker knows the IPv4 address of the node and believes it to be
dual-stacked with IPv4 and IPv6, he might want to try to probe the
corresponding IPv6 address, based on the assumption that the security
defenses might be lower. This might be the case particularly for
nodes which are behind a NAT in IPv4, but globally addressable in
IPv6. Naturally, this is not a concern if similar and adequate
security policies are in place.
On the other hand, brute-force scanning or probing of addresses is
computationally infeasible due to the large search space of interface
identifiers on most IPv6 subnets (somewhat less than 64 bits wide,
depending on how identifiers are chosen), always provided that
identifiers are chosen at random out of the available space, as
discussed in [SCAN-IMP].
For example, automatic tunneling mechanisms typically use
deterministic methods for generating IPv6 addresses, so probing/
port-scanning an IPv6 node is simplified. The IPv4 address is
embedded at least in 6to4, Teredo, and ISATAP addresses.
Additionally, it is possible (in the case of 6to4 in particular) to
learn the address behind the prefix; for example, Microsoft 6to4
implementation uses the address 2002:V4ADDR::V4ADDR while older Linux
and FreeBSD implementations default to 2002:V4ADDR::1. This could
also be used as one way to identify an implementation and hence
target any specific weaknesses.
One proposal has been to randomize the addresses or subnet identifier
in the address of the 6to4 router. This does not really help, as the
6to4 router (whether a host or a router) will return an ICMPv6 Hop
Limit Exceeded message, revealing the IP address. Hosts behind the
6to4 router can use methods such as privacy addresses [RFC4941] to
conceal themselves, provided that they are not meant to be reachable
by sessions started from elsewhere; they would still require a
globally accessible static address if they wish to receive
communications initiated elsewhere.
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To conclude, it seems that when an automatic tunneling mechanism is
being used, given an IPv4 address, the corresponding IPv6 address
could possibly be guessed with relative ease. This has significant
implications if the IPv6 security policy is less adequate than that
for IPv4.
Appendix B. IPv6 Privacy Considerations
The generation of IPv6 addresses from MAC addresses potentially
allows the behavior of users to be tracked in a way which may
infringe their privacy. [RFC4941] specifies mechanisms which can be
used to reduce the risk of infringement. It has also been claimed
that IPv6 harms the privacy of the user, either by exposing the MAC
address, or by exposing the number of nodes connected to a site.
Additional discussion of privacy issues can be found in [RFC4864].
B.1. Exposing MAC Addresses
Using stateless address autoconfiguration results in the MAC address
being incorporated in an EUI64 that exposes the model of network
card. The concern has been that a user might not want to expose the
details of the system to outsiders, e.g., fearing a resulting
burglary if a thief identifies expensive equipment from the vendor
identifier embedded in MAC addresses, or allowing the type of
equipment in use to be identified, thus facilitating an attack on
specific security weaknesses.
In most cases, this seems completely unfounded. First, such an
address must be learned somehow -- this is a non-trivial process; the
addresses are visible, e.g., in Web site access logs, but the chances
that a random Web site owner is collecting this kind of information
(or whether it would be of any use) are quite slim. Being able to
eavesdrop the traffic to learn such addresses (e.g., by the
compromise of DSL (Digital Subscriber Line) or Cable modem physical
media) seems also quite far-fetched. Further, using statically
configured interface identifiers or privacy addresses [RFC4941] for
such purposes is straightforward if worried about the risk. Second,
the burglar would have to be able to map the IP address to the
physical location; typically this would only be possible with
information from the private customer database of the Internet
Service Provider (ISP) and, for large sites, the administrative
records of the site, although some physical address information may
be available from the WHOIS database of Internet registries.
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B.2. Exposing Multiple Devices
Another concern that has been aired involves the user wanting to
conceal the presence of a large number of computers or other devices
connected to a network; NAT can "hide" all this equipment behind a
single address, but it is not perfect either [FNAT].
One practical reason why some administrators may find this desirable
is being able to thwart certain ISPs' business models. These models
require payment based on the number of connected computers, rather
than the connectivity as a whole.
Similar feasibility issues as described above apply. To a degree,
the number of machines present could be obscured by the sufficiently
frequent reuse of privacy addresses [RFC4941] -- that is, if during a
short period, dozens of generated addresses seem to be in use, it's
difficult to estimate whether they are generated by just one host or
multiple hosts.
B.3. Exposing the Site by a Stable Prefix
When an ISP provides IPv6 connectivity to its customers, including
home or consumer users, it delegates a fixed global routing prefix
(usually a /48) to them. This is in contrast to the typical IPv4
situation where home users typically receive a dynamically allocated
address that may be stable only for a period of hours.
Due to this fixed allocation, it is easier to correlate the global
routing prefix to a network site. With consumer users, this
correlation leads to a privacy issue, since a site is often
equivalent to an individual or a family in such a case. Consequently
some users might be concerned about being able to be tracked based on
their /48 allocation if it is static [RFC4941]. On the other hand,
many users may find having a static allocation desirable as it allows
them to offer services hosted in their network more easily.
This situation is not affected even if a user changes his/her
interface ID or subnet ID, because malicious users can still discover
this binding. On larger sites, the situation can be mitigated by
using "untraceable" IPv6 addresses as described in [RFC4864], and it
is possible that in the future ISPs might be prepared to offer
untraceable addresses to their consumer customers to minimize the
privacy issues.
This privacy issue is common to both IPv4 and IPv6 and is inherent in
the use of IP addresses as both identifiers for node interfaces and
locators for the nodes.
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Authors' Addresses
Elwyn B. Davies
Consultant
Soham, Cambs
UK
Phone: +44 7889 488 335
EMail: elwynd@dial.pipex.com
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal, QC H4P 2N2
Canada
Phone: +1 514-345-7900
EMail: suresh.krishnan@ericsson.com
Pekka Savola
CSC/Funet
EMail: psavola@funet.fi
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Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
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This document and the information contained herein are provided on an
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Davies, et al. Informational [Page 41]
RFC, FYI, BCP