IPv6 Operations E. Davies
Internet-Draft Nortel Networks
Expires: April 25, 2005 S. Krishnan
Ericsson
P. Savola
CSC/Funet
October 25, 2004
IPv6 Transition/Co-existence Security Considerations
draft-savola-v6ops-security-overview-03.txt
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
The transition from a pure IPv4 network to a network where IPv4 and
IPv6 co-exist 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
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document attempts to give an overview of the various issues grouped
into three categories: Issues due to the IPv6 protocol itself, due to
transition mechanisms, and due to the way in which IPv6 is being
deployed.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Issues Due to IPv6 Protocol . . . . . . . . . . . . . . . . 4
2.1 IPv6 Protocol-specific Issues . . . . . . . . . . . . . . 4
2.1.1 Routing Headers and Hosts . . . . . . . . . . . . . . 4
2.1.2 Routing Headers for Mobile IPv6 and Other Purposes . . 5
2.1.3 Obsolete Home Address Option in Mobile IPv6 . . . . . 5
2.1.4 Site(and Larger)-scope Multicast Addresses . . . . . . 5
2.1.5 ICMPv6 and Multicast . . . . . . . . . . . . . . . . . 6
2.1.6 Anycast traffic Identification and Security . . . . . 7
2.1.7 Address Privacy Extensions Interact with DDoS
Defenses . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.8 Dynamic DNS, Stateless Address Auto-Configuration
and Privacy Extensions . . . . . . . . . . . . . . . . 8
2.1.9 Extension Headers . . . . . . . . . . . . . . . . . . 8
2.1.10 Fragmentation: Reassembly and Deep Packet
Inspection . . . . . . . . . . . . . . . . . . . . . 10
2.1.11 Fragmentation Related DoS Attacks . . . . . . . . . 11
2.1.12 Areas of Improved Security in IPv6 . . . . . . . . . 11
2.2 IPv4-mapped IPv6 Addresses . . . . . . . . . . . . . . . . 12
2.3 Increased End-to-End Transparency . . . . . . . . . . . . 12
3. Issues Due to Transition Mechanisms . . . . . . . . . . . . 13
3.1 IPv6 Transition/Co-existence Mechanism-specific Issues . . 13
3.2 Automatic Tunneling and Relays . . . . . . . . . . . . . . 13
3.3 Tunneling May Break Security Assumptions . . . . . . . . . 14
4. Issues Due to IPv6 Deployment . . . . . . . . . . . . . . . 15
4.1 IPv6 Service Piloting Done Insecurely . . . . . . . . . . 15
4.2 Enabling IPv6 by Default Brings the Usability Down . . . . 16
4.3 Addressing Schemes and Securing Routers . . . . . . . . . 16
4.4 Consequences of Multiple Addresses in IPv6 . . . . . . . . 17
4.5 Deploying ICMPv6 . . . . . . . . . . . . . . . . . . . . . 17
4.6 Operational Factors when Enabling IPv6 in the Network . . 18
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
6. Security Considerations . . . . . . . . . . . . . . . . . . 19
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.1 Normative References . . . . . . . . . . . . . . . . . . . . 19
7.2 Informative References . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 22
A. IPv6 Probing/Mapping Considerations . . . . . . . . . . . . 22
B. IPv6 Privacy Considerations . . . . . . . . . . . . . . . . 23
B.1 Exposing MAC Addresses . . . . . . . . . . . . . . . . . . 23
B.2 Exposing Multiple Devices . . . . . . . . . . . . . . . . 24
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Intellectual Property and Copyright Statements . . . . . . . 25
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1. Introduction
The transition from a pure IPv4 network to a network where IPv4 and
IPv6 co-exist 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.
An architectural view of the transition has been presented in a
separate document [I-D.savola-v6ops-transarch]; it is important to
read it at least cursorily to understand that we have to be concerned
not about replacing IPv4 with IPv6 (in the short term), but with
adding IPv6 to be operated in parallel with IPv4.
This document also (at the moment, may be removed in future versions)
describes two "non-issues", in Appendix A and Appendix B:
considerations about probing/mapping IPv6 addresses, and
considerations with respect to privacy in IPv6.
2. Issues Due to IPv6 Protocol
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 drafts but are summarised here to avoid normative references
which 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 clearly stated, to
mean that all nodes (including hosts) must have this processing
enabled. This can result in hosts forwarding received traffic 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
were documented in [I-D.savola-ipv6-rh-hosts]. Some of these issues
have been resolved (a separate routing header type is now used for
Mobile IPv6 [RFC3775] and ICMP Traceback has not been standardized),
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but two issues remain:
o Routing headers can be used to evade access controls based on
destination addresses. This could be achieved by sending a packet
ostensibly to a publically accessible host address but with a
routing header which will cause the publically accessible host to
forward the packet to a destination which 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 in the previous case can be spoofed,
any host could be used to mediate an anonymous reflection
denial-of-service attack by having any publically accessible host
redirect the attack packets.
2.1.2 Routing Headers for Mobile IPv6 and Other Purposes
A new type of Routing Header (type 2) has been defined in [RFC3775]
to handle 'interface local' forwarding needed when packets are sent
to the care-of address of a mobile node which is away from its home
address.
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 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 Obsolete Home Address Option in Mobile IPv6
The Home Address option specified in early drafts of Mobile IPv6
would have allowed a trivial source spoofing attack as discussed in
[I-D.savola-ipv6-rh-ha-security]. The version of Mobile IPv6 as
standardised in [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.
2.1.4 Site(and Larger)-scope Multicast Addresses
IPv6 supports multicast addresses with site scope which can
potentially allow an attacker to identify certain important resources
on the site if misused. In principle these addresses also have
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equivalents for organization-scope and global-scope which could also
be misused.
Particular examples are the 'all routers' (FF05::2) and 'all 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 from external sources can be minimised by ensuring that all
firewalls and site boundary routers are configured to drop packets
with site-scope and organization-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.
An attacker internal to the site could potentially use these
addresses as part of a scanning attack.
TBD: There needs to be more discussion of possible defences against
these attacks and ways that they could be carried out from outside
the site (use of source-specific join). Also consideration of the
difficulties of applying appropriate filtering for multicast
addresses at site boundaries.
2.1.5 ICMPv6 and Multicast
It is possible to launch a denial-of-service (DoS) attack using IPv6
which could be amplified by the multicast infrastructure.
Unlike ICMP for IPv4, ICMPv6 [RFC2463] allows error notification
responses to be sent when certain unprocessable packets are sent to
multicast addresses.
The cases in which responses are sent are:
o The received packet is longer than the next link MTU: 'Packet Too
Big' responses are needed to support Path MTU Discovery for
multicast traffic.
o The received packet contains an unrecognised 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
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destination, it may be possible to elicit multiple responses directed
at the victim (the spoofed source of the multicast packet).
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 unrecognised hop-by-hop option
would be dropped by the first router. However a packet with an
unrecognised destination option could generate multiple responses.
On the other hand, the use of 'reverse path forwarding' checks to
eliminate loops in multicast forwarding limits the range of addresses
which can be spoofed, except where unicast-encapsulated register
messages are used.
In addition to amplification, this kind of attack would potentially
consume large amounts of forwarding state resources in routers on
multicast-enabled networks. See [I-D.savola-v6ops-firewalling].
2.1.6 Anycast traffic Identification and Security
IPv6 introduces the notion of anycast addresses and services. A
request to an anycast service will return the global unicast address
of the server that actually implements the service thereby exposing
some knowledge about the internal structure of the network. It may
be desirable to consider using specialised 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 look just like any other
IPv6 unicast address.
It is also difficult to secure anycast communications using IPsec and
IKE.
2.1.7 Address Privacy Extensions Interact with DDoS Defenses
The purpose of the privacy extensions for stateless address
auto-configuration [RFC3041] is to change the interface identifier
(and hence the global scope addresses generated from it) from time to
time in order to make it more difficult for eavesdroppers and other
information collectors to identify when different addresses used in
different transactions actually correspond to the same node.
The security issue resulting from this is that if the frequency of
change of the addresses used by a node is sufficiently great to
achieve the intended aim of the privacy extensions, the observed
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behavior of the node could look very like that of a compromised node
which was being used as the source of a distributed denial-of-service
(DDoS). It would thus be difficult to for any future defenses
against DDoS attacks to distinguish between a high rate change of
addresses resulting from genuine use of the privacy extensions and a
compromised node being used as the source of a DDoS with 'in-prefix'
spoofed source addresses as described in
[I-D.dupont-ipv6-rfc3041harmful].
2.1.8 Dynamic DNS, Stateless Address Auto-Configuration and Privacy
Extensions
The introduction of Stateless Address Auto-Configuration (SLAAC) with
IPv6 provides an additional challenge to the security of Dynamic DNS.
With manual addressing or the use of DHCP, the number of hosts
trusted to make updates to the DNS server is limited, assuming any
necessary updates are carried out by the DHCP server. This is true
equally for IPv4 and IPv6.
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
[I-D.ietf-dnsop-ipv6-dns-issues].
Using the Privacy Extensions to SLAAC [RFC3041] may significantly
increase the rate of updates of Dynamic DNS, assuming a node which
wishes to use the privacy extensions wishes to publish its address in
some DNS server. If the rate of change needed to achieve real
privacy has to be increased as is mentioned in Section 2.1.7 the
update rate for DDNS may be excessive.
2.1.9 Extension Headers
A number of issues relating to the specification of IPv6 Extension
headers have been identified. Several of these are discussed in
[I-D.savola-v6ops-firewalling].
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 to ensure
that policy constraints on the use of certain headers and options
were enforced and to remove packets containing potentially damaging
unknown options at the earliest opportunity.
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This requirement appears to conflict with Section 4 of the IPv6
specification in [RFC2460] which requires that destination options
are not processed at all 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.
A further ambiguity relates to whether an intermediate node should
discard a packet which contains a header or destination option which
it does not recognise. If the rules above are followed slaveishly,
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.
[RFC2460] therefore does not appear to take account of the behavior
of middleboxes and other non-final destinations which may be
inspecting the packet, and thereby potentially limits the security
protection of these boxes.
2.1.9.2 Processing Extension Header Chains
There is a further problem for middleboxes that want to examine the
transport headers which 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 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).
For example the fragment header does not conform to the TLV format
used for all the other headers.
A middlebox cannot therefore guarantee to be able to process header
chains which may contain headers defined after the box was
manufactured. As noted in Section 2.1.9.1, middleboxes ought not to
have to know about all header types in use but still need to be able
to skip over such headers to find the transport PDU start. This
either limits the security which can be applied in firewalls or makes
it difficult to deploy new extension header types.
As noted in Section 2.1.9.1, Destination Options may contain unknown
options. However, the options are encoded in TLV format so that
intermediate nodes can skip over them during processing, unlike the
enclosing extension headers.
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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 unrecognised by the
firewall because the intermediate node has no knowledge of which
options and headers are implemented in the destination node. Hence
it is highly desirable to make the discard policy configurable to
avoid firewalls dropping packets with legitimate items that they do
not recognise because their hardware or software is not aware of a
new definition.
2.1.9.4 Excessive Hop-by-Hop Options
IPv6 does not limit the number of hop by hop options which can be
present in a hop-by-hop option header. This can be used for mounting
denial of service attacks affecting all nodes on a path as described
in [I-D.krishnan-ipv6-hopbyhop].
2.1.9.5 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 with the router alert option present 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 that it is possible that the first fragment does not contain
sufficient information to carry out deep packet inspection involving
the port numbers.
Also the reassembly rules for fragmented packets in [RFC2460] do not
mandate behavior which would minimise the effects of overlapping
fragments.
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Depending on the implementation of packet reassembly and the
treatment of packet fragments in firewalls and other nodes which use
deep packet inspection for traffic filtering, this potentially leaves
IPv6 open to the sort of attacks described in [RFC1858] and [RFC3128]
for IPv4.
There is no reason to allow overlapping packet fragments and overlaps
could be prohibited in a future revision of the protocol
specification. Some implementations already drop all packets with
overlapped fragments.
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.
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
analagous 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 which 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.
2.1.12 Areas of Improved Security in IPv6
There are several areas where IPv4 security is weak which have been
made stronger either in the base IPv6 specifications or by additional
specifications. These areas include:
o Combatting threats related to local links, comparable to various
ARP spoofing techniques associated with IPv4; the Neighbor
Discovery (ND) threats have been documented in [RFC3756] and
mechanisms to combat them specified in Secure Neighbor Discovery
(SEND) [I-D.ietf-send-ndopt]. SEND is an optional mechanism which
is particularly applicable to wireless and other environments
where it is difficult to physically secure the link.
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o Improving Mobile IP security: 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 which 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 be found
in [I-D.ietf-mip6-ro-sec].
Appendix A lists (typically bogus) considerations related to IPv6
network mapping or probing. Appendix B lists mainly unfounded claims
about the lack of privacy in IPv6.
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 which comes
with ambiguity.
One example of such is IPv4-mapped IPv6 addresses: a representation
of an IPv4 address as an IPv6 address inside an operating system.
Since the original specification, IPv4-mapped addresses have been
extended to be used with a transition mechanism [RFC2765], 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 *or* an IPv6 address received from the wire (which may
be subject to address forgery, etc.).
In addition, special cases like these, while giving deployment
benefits in some arenas, require a considerable amount of code
complexity (e.g. in the implementations of bind() system calls)
which is probably undesirable. These issues are discussed in
[I-D.cmetz-v6ops-v4mapped-api-harmful] and
[I-D.itojun-v6ops-v4mapped-harmful].
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 and needs further study.
2.3 Increased End-to-End Transparency
With IPv6, increased end-to-end transparency in general can sometimes
be seen as a threat. Some seem to want limited end-to-end
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capabilities, e.g. in the form of private, local addressing, even
when it is not necessary.
People have gotten used to the perceived, dubious security benefits
of NATs and perimeter firewalls, and the bidirectionality and
transparency that IPv6 can provide may seem undesirable at times.
This is a really important issue especially for most enterprise
network managers.
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.
3. Issues Due to Transition Mechanisms
3.1 IPv6 Transition/Co-existence Mechanism-specific Issues
The more complicated the IPv6 transition/co-existence becomes, the
greater the danger that security issues will be introduced in the
mechanisms (which may or may not be readily apparent). Therefore it
would be desirable to keep the mechanisms simple, and in as small
pieces as possible.
One case where such security issues have been analyzed is
[I-D.ietf-v6ops-6to4-security] .
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 the other end-point is unspecified, and packets have
to be accepted and decapsulated from everywhere. Therefore,
special care should be observed when specifying automatic
tunneling techniques.
3.2 Automatic Tunneling and Relays
Two mechanisms have been (or are being) specified which use automatic
tunneling over IPv4 or UDP/IPv4 between the nodes enabling the same
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mechanism for connectivity: 6to4 and Teredo (respectively).
The first obvious issue (as mentioned above) in such approaches is
that such nodes must allow decapsulation of traffic from anywhere in
the Internet. That kind of decapsulation function must be extremely
well secured as it's so wide open.
Even more difficult problem is how these mechanisms are able to
communicate with native IPv6 nodes or between the automatic tunneling
mechanisms: such connectivity requires the use of some kind of
"relays". These relays could be deployed e.g., in all native IPv6
nodes, native IPv6 sites, IPv6 ISPs, or just somewhere in the
Internet. This has some obvious trust and scaling issues. As
authentication of such a relay service is very difficult, and more so
in some of those deployment models, relays provide a means to for
address spoofing, (reflected) Denial-of-Service attacks, and other
threats.
Threats related to 6to4 are discussed in
[I-D.ietf-v6ops-6to4-security].
3.3 Tunneling May Break Security Assumptions
NATs and firewalls have been deployed extensively in the IPv4
Internet, for the good or the bad. People who deploy them typically
have some security/operational requirements in mind (e.g. a desire
to block inbound connection attempts), whether misguided or not.
Tunneling can change that model. IPv6-over-IPv4 tunneling is
typically explicitly allowed or disallowed implicitly. Tunneling
IPv6 over IPv4 with UDP, however, is often an entirely different
thing: as UDP must usually be allowed through, at least in part and
in a possibly stateful manner, one can "punch holes" in NAT's and
firewalls using UDP. Actually, the mechanisms have been explicitly
designed to traverse both NATs and firewalls in a similar fashion.
One could say that tunneling is especially questionable in home/SOHO
environments where the level of network administration is not that
high; in these environments the hosts may not be as managed as in
others (e.g., network services might be enabled unnecessarily),
leading to possible security break-ins or other vulnerabilities.
Holes can be punched both intentionally and unintentionally. In case
it is a willing choice from the administrator/user, this is less of a
problem (but e.g., enterprises might want to block IPv6 tunneling
explicitly if some employees would do something like this willingly
on their own). On the other hand, if a hole is punched
transparently, without people understanding the consequences, it will
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very probably result in a serious threat sooner or later.
When deploying tunneling solutions, especially tunneling solutions
which are automatic and/or can be enabled easily by users not
understanding 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 as
IPv4 NAT provides. 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 has been shown in Appendix A, it is relatively easy to find out
IPv6 address of corresponding to an IPv4 address, so one should never
rely on "security by obscurity" i.e., relying that nobody is able to
guess or know the IPv6 address of the host.
4. Issues Due to IPv6 Deployment
4.1 IPv6 Service Piloting Done Insecurely
In many cases, IPv6 service piloting is done in a manner which is
considered to be less secure than as one would do with IPv4. For
example, hosts and routers might not be protected by IPv6 firewalls,
even if in IPv4 firewalls are being used.
The other possible alternative, in some places, is that no service
piloting is done at all because IPv6 firewalls may not be widely used
-- and IPv6 deployment suffers (of course, this is also one of the
nice excuses for not doing IPv6).
This problem may be partially due to a slow speed of IPv6-capable
firewall development and deployment. However, it is also a problem
with a lack of information: actually, there are quite a few IPv6
packet filters and firewalls already, which could be used for
sufficient access controls, but network administrators may not be
aware of them yet.
However, there appears to be a real lack in two areas: 'personal
firewalls' and enterprise firewalls; the same devices that support
and are used for IPv4 today are often expected to also become
IPv6-capable -- even though this is not really required. That is,
IPv4 access could be filtered by one firewall, and when IPv6 access
is added, it could be protected by another firewall; they don't have
to be the same, and even their models don't have to be the same.
Another, smaller factor may be that due to a few decisions on how
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IPv6 was built, it's more difficult for firewalls to be implemented
and work under all the cases (e.g. when new extension headers etc.
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.
A similar argument, which is often quoted as hindering IPv6
deployment, has been the lack of Intrusion Detection Systems (IDS).
It is not clear whether this is more of an excuse than a real reason.
4.2 Enabling IPv6 by Default Brings the Usability Down
A practical disadvantage of enabling IPv6 as of this writing is that
it typically brings the observed service level down a bit; that is,
the usability suffers.
This is due to at least three reasons:
o global IPv6 routing is still rather unstable, leading to packet
loss, lower throughput, and higher delay
[I-D.savola-v6ops-6bone-mess]
o some applications cannot properly handle both IPv4 and IPv6 or may
have problems handling all the fallbacks and failure modes (and in
some cases, e.g. if the TCP timeout kicks in, this may be very
difficult)
o some DNS server implementations have flaws that severely affect
DNS queries for IPv6 addresses
[I-D.ietf-dnsop-misbehavior-against-aaaa]
Actually, some would be 100% ready to release IPv6 services (e.g.
web) today, but that would mean trouble for many of their
dual-stacked customers or users all over the world so they don't:
these are often published under a separate domain or subdomain, and
are practically not used that often.
These issues are also described at some length in
[I-D.ietf-v6ops-v6onbydefault] .
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 the unmemorability of complete IPv6 addresses there is a
temptation for administrators to use small integers as interface
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identifiers when manually configuring them, as might happen on
point-to-point links. Such allocations make it easy for an attacker
to find active nodes that they can then port scan.
It is also essential to ensure that the management interfaces of
routers are well secured 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 which 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 may be the default for link
locals).
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.
4.5 Deploying ICMPv6
In IPv4 it is generally accepted that stringent filtering of ICMP
packets by firewalls is essential to maintain security. Because of
the extended use that is made of ICMPv6 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.
To support effective functioning of IPv6, firewalls should typically
allow the following messages to pass through the firewall (the first
four are equivalent to the typical IPv4 filtering allowance):
o ICMPv6 Type 1, Code 0 - No route to destination error
o ICMPv6 Type 3 - Time exceeded error
o ICMPv6 Type 128 - Echo request
o ICMPv6 Type 129 - Echo response
o ICMPv6 Type 2 - Packet too big (required for Path MTU Discovery)
o ICMPv6 Type 4 - Parameter problem (this type needs to be
investigated further as it is possible that it can be abused.
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Additionally the following ICMPv6 messages may be required to be
supported to and from a firewall:
o ICMPv6 Type 2 - packet too big - The firewall itself has to
participate in Path MTU Discovery.
o ICMPv6 Type 130-132 - Multicast Listener Discovery messages have
to be accepted by routing devices to replace IGMP which is used in
IPv4[Check for MLDv2]
o ICMPv6 Type 133/134 - Router Solicitations and Advertisements -
assuming the firewall is also a router, it needs to support router
discovery and host auto-configuration.
o ICMPv6 Type 135/136 - Neigbor Solicitation and Advertisement -
Needed for duplicate address detection and Layer 2 address
resolution.
o ICMPv6 Type 4 - parameter Problem - Needs further investigation
because of possible abuse.
4.6 Operational Factors when Enabling IPv6 in the Network
You have to be careful when enabling IPv6 in the network gear for
multiple reasons:
IPv6-enabled router software may be unstable(r) yet; either IPv6 is
unstable, or the software you have to run to be able to run IPv6 is
different (from non-IPv6 parts) from the one you would run otherwise,
making the software in practice more unstable -- and raising the bar
for IPv6 adoption.
IPv6 processing may not happen at (near) line speed (or in the same
level as IPv4). A high amount of IPv6 traffic (even legitimate, e.g.
NNTP) could easily overload the software-based IPv6 processing and
cause harm also to IPv4 processing, affecting availability. That is,
if people don't feel confident enough in the IPv6 support, they will
be hesitant to enable it in their "production" networks.
Sometimes required features may be missing from the vendors' software
releases; an example is a software enabling IPv6 telnet/SSH access,
but having no ability to turn it off or limit access to it!
Sometimes the default IPv6 configuration is insecure. For example,
in one vendor, 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 used
previously do not block IPv6 telnet, 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's possible to run the both in one routing
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protocol, or have two separate routing protocols; either approach has
its tradeoffs. If multiple routing protocols are used, one should
note that this causes double the number of processing when links flap
or recalculation is otherwise needed -- which might more easily
overload the routers' CPU, causing slightly slower convergence time.
5. Acknowledgements
Alain Durand, Alain Baudot, Luc Beloeil, and Andras Kis-Szabo
provided feedback to improve this memo. Michael Wittsend and Michael
Cole discussed issues relating to probing/mapping and privacy.
6. Security Considerations
This memo tries to give an overview of security considerations of the
different aspects of IPv6.
7. References
7.1 Normative References
[I-D.savola-v6ops-transarch]
Savola, P., "A View on IPv6 Transition Architecture",
draft-savola-v6ops-transarch-03 (work in progress),
January 2004.
[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.
[RFC2463] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
[RFC3041] Narten, T. and R. Draves, "Privacy Extensions for
Stateless Address Autoconfiguration in IPv6", RFC 3041,
January 2001.
[RFC3775] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
7.2 Informative References
[I-D.dupont-ipv6-rfc3041harmful]
Dupont, F. and P. Savola, "RFC 3041 Considered Harmful",
draft-dupont-ipv6-rfc3041harmful-05 (work in progress),
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June 2004.
[I-D.savola-v6ops-6bone-mess]
Savola, P., "Moving from 6bone to IPv6 Internet",
draft-savola-v6ops-6bone-mess-01 (work in progress),
November 2002.
[I-D.ietf-v6ops-6to4-security]
Savola, P., "Security Considerations for 6to4",
draft-ietf-v6ops-6to4-security-04 (work in progress), July
2004.
[I-D.ietf-dnsop-misbehavior-against-aaaa]
Morishita, Y. and T. Jinmei, "Common Misbehavior against
DNS Queries for IPv6 Addresses",
draft-ietf-dnsop-misbehavior-against-aaaa-01 (work in
progress), April 2004.
[FNAT] Bellovin, S., "Technique for Counting NATted Hosts", Proc.
Second Internet Measurement Workshop , November 2002,
<http://www.research.att.com/~smb/papers/fnat.pdf>.
[I-D.savola-v6ops-firewalling]
Savola, P., "Firewalling Considerations for IPv6",
draft-savola-v6ops-firewalling-02 (work in progress),
October 2003.
[I-D.schild-v6ops-guide-v4mapping]
Schild, C., "Guide to Mapping IPv4 to IPv6 Subnets",
draft-schild-v6ops-guide-v4mapping-00 (work in progress),
January 2004.
[I-D.ietf-v6ops-v6onbydefault]
Roy, S., Durand, A. and J. Paugh, "Issues with Dual Stack
IPv6 on by Default", draft-ietf-v6ops-v6onbydefault-03
(work in progress), July 2004.
[I-D.chown-v6ops-port-scanning-implications]
Chown, T., "IPv6 Implications for TCP/UDP Port Scanning",
draft-chown-v6ops-port-scanning-implications-01 (work in
progress), July 2004.
[I-D.savola-ipv6-rh-ha-security]
Savola, P., "Security of IPv6 Routing Header and Home
Address Options", draft-savola-ipv6-rh-ha-security-02
(work in progress), March 2002.
[I-D.savola-ipv6-rh-hosts]
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Savola, P., "Note about Routing Header Processing on IPv6
Hosts", draft-savola-ipv6-rh-hosts-00 (work in progress),
February 2002.
[I-D.ietf-mip6-ro-sec]
Nikander, P., "Mobile IP version 6 Route Optimization
Security Design Background", draft-ietf-mip6-ro-sec-02
(work in progress), October 2004.
[I-D.ietf-send-ndopt]
Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P.
Nikander, "SEcure Neighbor Discovery (SEND)",
draft-ietf-send-ndopt-06 (work in progress), July 2004.
[RFC3756] Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May
2004.
[RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm
(SIIT)", RFC 2765, February 2000.
[RFC1858] Ziemba, G., Reed, D. and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128, June 2001.
[I-D.cmetz-v6ops-v4mapped-api-harmful]
Metz, C. and J. Hagino, "IPv4-Mapped Address API
Considered Harmful",
draft-cmetz-v6ops-v4mapped-api-harmful-01 (work in
progress), October 2003.
[I-D.itojun-v6ops-v4mapped-harmful]
Metz, C. and J. Hagino, "IPv4-Mapped Addresses on the Wire
Considered Harmful",
draft-itojun-v6ops-v4mapped-harmful-02 (work in progress),
October 2003.
[I-D.ietf-dnsop-ipv6-dns-issues]
Durand, A., Ihren, J. and P. Savola, "Operational
Considerations and Issues with IPv6 DNS",
draft-ietf-dnsop-ipv6-dns-issues-09 (work in progress),
August 2004.
[I-D.krishnan-ipv6-hopbyhop]
Krishnan, S., "Arrangement of Hop-by-Hop options",
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draft-krishnan-ipv6-hopbyhop-00 (work in progress), June
2004.
Authors' Addresses
Elwyn B. Davies
Nortel Networks
Harlow Laboratories
London Road
Harlow, Essex CM17 9NA
UK
Phone: +44 1279 405 498
EMail: elwynd@nortelnetworks.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
Appendix A. IPv6 Probing/Mapping Considerations
Some want the IPv6 numbering topology (either at network or node
level) [I-D.schild-v6ops-guide-v4mapping] match IPv4 as exactly as
possible, the others see this as a security threat because IPv6 could
have different security properties than IPv4.
That is, if an attacker knows the IPv4 address of the node, 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
the similar security policies are in place.
On the other hand, brute-force scanning or probing is unfeasible
[I-D.chown-v6ops-port-scanning-implications].
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For example, automatic tunneling mechanisms use rather 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 address. Further than that, it's 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 Linux and BSD implementations default to
2002:V4ADDR::1. This could also be used as one way to identify an
implementation.
One proposal has been to randomize the addresses or Subnet identifier
in the address of the 6to4 router. This doesn't 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 RFC 3041 addresses to conceal
themselves, though.
To conclude, it seems that with an IPv4 address, the respective IPv6
address, when automatic tunneling mechanism is being used, could
possibly be guessed with relative ease. This has significant
implications if the IPv6 security policy isn't the same as IPv4.
Appendix B. IPv6 Privacy Considerations
It has 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.
B.1 Exposing MAC Addresses
The MAC address, which with stateless address autoconfiguration,
results in an EUI64, 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., in the fear of a resulting burglary (e.g.,
if a crook identifies expensive equipment from the MAC addresses).
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 or Cable modem physical media) seems also quite
far-fetched. Further, using RFC 3041 addresses 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; this
is typically only available in the private customer database of the
ISP.
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B.2 Exposing Multiple Devices
Another presented concern is whether the user wants to show off as
having a lot of computers or other devices at a network; NAT "hides"
everything behind an address, but is not perfect either [FNAT].
One practical reason why some may find this desirable is being able
to thwart certain ISPs' business models, where one should pay extra
for additional computers (and not the connectivity as a whole).
Similar feasibility issues as described above apply. To a degree,
the counting avoidance could be performed by the sufficiently
frequent re-use of RFC 3041 addresses -- 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.
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