Network Working Group P. Savola
Request for Comments: 3964 CSC/FUNET
Category: Informational C. Patel
All Play, No Work
December 2004
Security Considerations for 6to4
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.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
The IPv6 interim mechanism 6to4 (RFC3056) uses automatic
IPv6-over-IPv4 tunneling to interconnect IPv6 networks. The
architecture includes 6to4 routers and 6to4 relay routers, which
accept and decapsulate IPv4 protocol-41 ("IPv6-in-IPv4") traffic from
any node in the IPv4 internet. This characteristic enables a number
of security threats, mainly Denial of Service. It also makes it
easier for nodes to spoof IPv6 addresses. This document discusses
these issues in more detail and suggests enhancements to alleviate
the problems.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Different 6to4 Forwarding Scenarios . . . . . . . . . . . . . 4
2.1. From 6to4 to 6to4 . . . . . . . . . . . . . . . . . . . 4
2.2. From Native to 6to4 . . . . . . . . . . . . . . . . . . 5
2.3. From 6to4 to Native . . . . . . . . . . . . . . . . . . 5
2.4. Other Models . . . . . . . . . . . . . . . . . . . . . . 6
2.4.1. BGP between 6to4 Routers and Relays . . . . . . 6
2.4.2. 6to4 as an Optimization Method . . . . . . . . . 7
2.4.3. 6to4 as Tunnel End-Point Addressing Mechanism . . 8
3. Functionalities of 6to4 Network Components . . . . . . . . . . 9
3.1. 6to4 Routers . . . . . . . . . . . . . . . . . . . . . . 9
3.2. 6to4 Relay Routers . . . . . . . . . . . . . . . . . . . 10
4. Threat Analysis . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. Attacks on 6to4 Networks . . . . . . . . . . . . . . . . 12
4.1.1. Attacks with ND Messages . . . . . . . . . . . . 13
4.1.2. Spoofing Traffic to 6to4 Nodes . . . . . . . . . 14
4.1.3. Reflecting Traffic to 6to4 Nodes . . . . . . . . 17
4.1.4. Local IPv4 Broadcast Attack . . . . . . . . . . 19
4.2. Attacks on Native IPv6 Internet . . . . . . . . . . . . 20
4.2.1. Attacks with ND Messages . . . . . . . . . . . . 21
4.2.2. Spoofing Traffic to Native IPv6 Nodes. . . . . . 21
4.2.3. Reflecting Traffic to Native IPv6 Nodes . . . . 23
4.2.4. Local IPv4 Broadcast Attack . . . . . . . . . . 24
4.2.5. Theft of Service . . . . . . . . . . . . . . . . 25
4.2.6. Relay Operators Seen as Source of Abuse . . . . 26
4.3. Attacks on IPv4 Internet . . . . . . . . . . . . . . . . 28
4.4. Summary of the Attacks . . . . . . . . . . . . . . . . . 28
5. Implementing Proper Security Checks in 6to4 . . . . . . . . . 30
5.1. Encapsulating IPv6 into IPv4 . . . . . . . . . . . . . . 31
5.2. Decapsulating IPv4 into IPv6 . . . . . . . . . . . . . . 31
5.3. IPv4 and IPv6 Sanity Checks . . . . . . . . . . . . . . 32
5.3.1. IPv4 . . . . . . . . . . . . . . . . . . . . . . 32
5.3.2. IPv6 . . . . . . . . . . . . . . . . . . . . . . 33
5.3.3. Optional Ingress Filtering . . . . . . . . . . . 33
5.3.4. Notes about the Checks . . . . . . . . . . . . . 33
6. Issues in 6to4 Implementation and Use . . . . . . . . . . . . 34
6.1. Implementation Considerations with Automatic Tunnels . . 34
6.2. A Different Model for 6to4 Deployment . . . . . . . . . 35
7. Security Considerations . . . . . . . . . . . . . . . . . . . 36
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 36
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
A. Some Trivial Attack Scenarios Outlined . . . . . . . . . . . . 39
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
The IPv6 interim mechanism "6to4" [1] specifies automatic
IPv6-over-IPv4 tunneling to interconnect isolated IPv6 clouds by
embedding the tunnel IPv4 address in the IPv6 6to4 prefix.
Two characteristics of the 6to4 mechanism introduce most of the
security considerations:
1. All 6to4 routers must accept and decapsulate IPv4 packets from
every other 6to4 router, and from 6to4 relays.
2. 6to4 relay routers must accept traffic from any native IPv6 node.
As the 6to4 router must accept traffic from any other 6to4 router or
relay, a certain requirement for trust is implied, and there are no
strict constraints on what the IPv6 packet may contain. Thus,
addresses within the IPv4 and IPv6 headers may be spoofed, and this
leads to various types of threats, including different flavors of
Denial of Service attacks.
The 6to4 specification outlined a few security considerations and
rules but was ambiguous as to their exact requirement level.
Moreover, the description of the considerations was rather short, and
some of them have proven difficult to understand or impossible to
implement.
This document analyzes the 6to4 security issues in more detail and
outlines some enhancements and caveats.
Sections 2 and 3 are more or less introductory, rehashing how 6to4 is
used today based on the 6to4 specification, so that it is easier to
understand how security could be affected. Section 4 provides a
threat analysis for implementations that already implement most of
the security checks. Section 5 describes the optimal
decapsulation/encapsulation rules for 6to4 implementations, and
Section 6 provides further discussion on a few issues that have
proven difficult to implement. Appendix A outlines a few possible
trivial attack scenarios in which very little or no security has been
implemented.
For the sake of simplicity, in this document, the native Internet is
assumed to encompass IPv6 networks formed by using other transition
mechanisms (e.g., RFC 2893 [4]), as these mechanisms cannot talk
directly to the 6to4 network.
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC 2119 [2].
Throughout this memo, IPv4 addresses from blocks 7.0.0.0/24,
8.0.0.0/24, and 9.0.0.0/24 are used for demonstrative purposes, to
represent global IPv4 addresses that have no relation to each other.
This approach was chosen instead of just using addresses from
192.0.2.0/24 [5] for two reasons: to use addresses whose 6to4 mapping
is glaringly obvious, and to make it obvious that the IPv4 addresses
of different 6to4 gateways need not have any relation to each other.
2. Different 6to4 Forwarding Scenarios
Note that when one communicates between 6to4 and native domains, the
6to4 relays that will be used in the two directions are very likely
different; routing is highly asymmetric. Because of this, it is not
feasible to limit relays from which 6to4 routers may accept traffic.
The first three subsections introduce the most common forms of 6to4
operation. Other models are presented in the fourth subsection.
2.1. From 6to4 to 6to4
6to4 domains always exchange 6to4 traffic directly via IPv4
tunneling; the endpoint address V4ADDR is derived from 6to4 prefix
2002:V4ADDR::/48 of the destination.
.--------. _----_ .--------.
| 6to4 | _( IPv4 )_ | 6to4 |
| router | <====> ( Internet ) <===> | router |
'--------' (_ _) '--------'
^ '----' ^
| Direct tunneling over IPv4 |
V V
.--------. .-------.
| 6to4 | | 6to4 |
| host | | host |
'--------' '--------'
Figure 1
It is required that every 6to4 router consider every other 6to4
router it wants to talk to be "on-link" (with IPv4 as the
link-layer).
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2.2. From Native to 6to4
When native domains send traffic to 6to4 prefix 2002:V4ADDR::/48, it
will be routed to the topologically nearest advertising 6to4 relay
(advertising route to 2002::/16). The 6to4 relay will tunnel the
traffic over IPv4 to the corresponding IPv4 address V4ADDR.
Note that IPv4 address 9.0.0.1 here is just an example of a global
IPv4 address, and it is assigned to the 6to4 router's
pseudo-interface.
Closest to
"Native IPv6 node"
.--------. _----_ .------------. .--------.
| Native | _( IPv6 )_ | 6to4 relay | Tunneled | 6to4 |
| IPv6 | -> ( Internet ) --> | router | =========> | router |
| node | (_ _) '------------' 9.0.0.1 '--------'
'--------' '----' dst_v6=2002:0900:0001::1 |
V
.-------.
| 6to4 |
| host |
'--------'
Figure 2
2.3. From 6to4 to Native
6to4 domains send traffic to native domains by tunneling it over IPv4
to their configured 6to4 relay router, or the closest one found by
using 6to4 IPv4 Anycast [3]. The relay will decapsulate the packet
and forward it to native IPv6 Internet, as would any other IPv6
packet.
Note that the destination IPv6 address in the packet is a non-6to4
address and is assumed to be 2001:db8::1 in the example.
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Configured
-or-
found by IPv4 Anycast
.--------. _----_ .------------. .--------.
| Native | _( IPv6 )_ | 6to4 relay | Tunneled | 6to4 |
| Client | <- ( Internet ) <-- | router | <========= | router |
'--------' (_ _) '------------' 192.88.99.1'--------'
2001:db8::1 '----' (or configured) ^
|
.-------.
| 6to4 |
| client |
'--------'
Figure 3
2.4. Other Models
These are more or less special cases of 6to4 operations. In later
chapters, unless otherwise stated, only the most generally used
models (above) will be considered.
2.4.1. BGP between 6to4 Routers and Relays
Section 5.2.2.2 in [1] presents a model where, instead of static
configuration, BGP [6] is used between 6to4 relay routers and 6to4
routers (for outgoing relay selection only).
Going further than [1], if the 6to4 router established a BGP session
between all the possible 6to4 relays and advertised its /48 prefix to
them, the traffic from non-6to4 sites would always come from a
"known" relay. Alternatively, the 6to4 relays might advertise the
more specific 6to4 routes between 6to4 relays.
Both of these approaches are obviously infeasible due to scalability
issues.
Neither of these models are known to be used at the time of writing;
this is probably because parties that need 6to4 are not able to run
BGP, and because setting up these sessions would be much more work
for relay operators.
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2.4.2. 6to4 as an Optimization Method
Some sites seem to use 6to4 as an IPv6 connectivity "optimization
method"; that is, they also have non-6to4 addresses on their nodes
and border routers but also employ 6to4 to reach 6to4 sites.
This is typically done to be able to reach 6to4 destinations by
direct tunneling without using relays at all.
These sites also publish both 6to4 and non-6to4 addresses in DNS to
affect inbound connections. If the source host's default address
selection [7] works properly, 6to4 sources will use 6to4 addresses to
reach the site and non-6to4 nodes use non-6to4 addresses. If this
behavior of foreign nodes can be assumed, the security threats to
such sites can be significantly simplified.
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2.4.3. 6to4 as Tunnel End-Point Addressing Mechanism
6to4 addresses can also be used only as an IPv6-in-IPv4 tunnel
endpoint addressing and routing mechanism.
An example of this is interconnecting 10 branch offices where nodes
use non-6to4 addresses. Only the offices' border routers need to be
aware of 6to4, and use 6to4 addresses solely for addressing the
tunnels between different branch offices. An example is provided in
the figure below.
2001:db8:0:10::/60 2001:db8:0:20::/60
.--------. .--------.
( Branch 1 ) ( Branch 2 )
'--------' '--------'
| |
.--------. _----_ .--------.
| 6to4 | _( IPv4 )_ | 6to4 |
| router | <====> ( Internet ) <===> | router |
'--------' (_ _) '--------'
9.0.0.1 '----' 8.0.0.2
^^
||
vv
.--------.
| 6to4 | 7.0.0.3
| router |
'--------'
| 2001:db8::/48
.-----------.
( Main Office )
'-----------'
^
|
v
_----_
_( IPv6 )_
( Internet )
(_ _)
'----'
Figure 4
In the figure, the main office sets up two routes:
2001:db8:0:10::/60 -> 2002:0900:0001::1
2001:db8:0:20::/60 -> 2002:0800:0002::1
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And a branch office sets up two routes as well:
2001:db8:0:20::/60 -> 2002:0800:0002::1
default -> 2002:0700:0003::1
Thus, the IPv4 Internet is treated as an NBMA link-layer for
interconnecting 6to4-enabled sites; with explicit routes, 6to4
addressing need not be used in routers other than the 6to4 edge
routers. However, note that if a branch office sends a packet to any
6to4 destination, it will not go through the main office, as the 6to4
2002::/16 route overrides the default route.
This approach may make addressing and routing slightly easier in some
circumstances.
3. Functionalities of 6to4 Network Components
This section summarizes the main functionalities of the 6to4 network
components (6to4 routers, and 6to4 relays) and the security checks
they must do. The pseudo-code for the security checks is provided in
Section 5.
This section summarizes the main functions of the various components
of a 6to4 network: 6to4 relay routers and 6to4 routers. Refer to
Section 1.1 of RFC 3056 [1] for 6to4-related definitions.
3.1. 6to4 Routers
The 6to4 routers act as the border routers of a 6to4 domain. It does
not have a native global IPv6 address except in certain special
cases. Since the specification [1] uses the term "6to4 router", this
memo does the same; however, note that in this definition, we also
include a single host with a 6to4 pseudo-interface, which doesn't
otherwise act as a router. The main functions of the 6to4 router are
as follows:
o Provide IPv6 connectivity to local clients and routers.
o Tunnel packets sent to foreign 6to4 addresses to the destination
6to4 router using IPv4.
o Forward packets sent to locally configured 6to4 addresses to the
6to4 network.
o Tunnel packets sent to non-6to4 addresses to the configured/
closest-by-anycast 6to4 relay router.
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o Decapsulate directly received IPv4 packets from foreign 6to4
addresses.
o Decapsulate IPv4 packets received via the relay closest to the
native IPv6 sources. Note that it is not easily distinguishable
whether the packet was received from a 6to4 relay router or from a
spoofing third party.
The 6to4 router should also perform security checks on traffic that
it receives from other 6to4 relays, or 6to4 routers, or from within
the 6to4 site. These checks include the following:
o Disallow traffic that has private, broadcast or certain specific
reserved IPv4 addresses (see Section 5.3.1 for details) in
tunnels, or the matching 6to4 prefixes.
o Disallow traffic from 6to4 routers in which the IPv4 tunnel source
address does not match the 6to4 prefix. (Note that the
pseudo-interface must pick the IPv4 address corresponding to the
prefix when encapsulating, or problems may ensue, e.g., on
multi-interface routers.)
o Disallow traffic in which the destination IPv6 address is not a
global address; in particular, link-local addresses, mapped
addresses, and such should not be used.
o Disallow traffic transmission to other 6to4 domains through 6to4
relay router or via some third party 6to4 router. (To avoid
transmission to the relay router, the pseudo-interface prefix
length must be configured correctly to /16. Further, to avoid the
traffic being discarded, 6to4 source addresses must always
correspond to the IPv4 address encapsulating the traffic.)
o Discard traffic received from other 6to4 domains via a 6to4 relay
router.
o Discard traffic received for prefixes other than one's own 6to4
prefix(es).
3.2. 6to4 Relay Routers
The 6to4 relay router acts as a relay between all 6to4 domains and
native IPv6 networks; more specifically, it
o advertises the reachability of the 2002::/16 prefix to native IPv6
routing, thus receiving traffic to all 6to4 addresses from the
closest native IPv6 nodes,
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o advertises (if RFC 3068 [3] is implemented) the reachability of
IPv4 "6to4 relay anycast prefix" (192.88.99.0/24) to IPv4 routing,
thus receiving some tunneled traffic to native IPv6 nodes from
6to4 routers.
o decapsulates and forwards packets received from 6to4 addresses
through tunneling, by using normal IPv6 routing, and
o tunnels packets received through normal IPv6 routing from native
addresses that are destined for 2002::/16 to the corresponding
6to4 router.
The 6to4 relay should also perform security checks on traffic that it
receives from 6to4 routers, or from native IPv6 nodes. These checks
are as follows:
o Disallow traffic that has private, broadcast, or certain specific
reserved IPv4 addresses in tunnels, or in the matching 6to4
prefixes.
o Disallow traffic from 6to4 routers in which the IPv4 tunnel source
address does not match the 6to4 prefix. (Note that the
pseudo-interface must pick the IPv4 address corresponding to the
prefix when encapsulating, or problems may ensue, e.g., on
multi-interface routers.)
o Disallow traffic in which the destination IPv6 address is not a
global address; in particular, link-local addresses, mapped
addresses, and such should not be used.
o Discard traffic received from 6to4 routers with the destination as
a 6to4 prefix.
4. Threat Analysis
This section discusses attacks against the 6to4 network or attacks
caused by the 6to4 network. The threats are discussed in light of
the 6to4 deployment models defined in Section 2.
There are three general types of threats:
1. Denial-of-Service (DoS) attacks, in which a malicious node
prevents communication between the node under attack and other
nodes.
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2. Reflection Denial-of-Service (DoS) attacks, in which a malicious
node reflects the traffic off unsuspecting nodes to a particular
node (node under attack) in order to prevent communication
between the node under attack and other nodes.
3. Service theft, in which a malicious node/site/operator may make
unauthorized use of service.
6to4 also provides a means for a "meta-threat", traffic laundering,
in which some other attack is channeled through the third parties to
make tracing the real origin of the attack more difficult. This is
used in conjunction with other threats, whether specific to 6to4 or
not.
At this point it is important to reiterate that the attacks are
possible because
1. 6to4 routers have to consider all 6to4 relays, and other 6to4
routers, as "on-link",
2. 6to4 relays have to consider all 6to4 routers as "on-link", and
3. it has been discovered that at least a couple of major 6to4
implementations do not implement all the security checks.
The attacks' descriptions are classified based on the target of the
attack:
1. Attacks on 6to4 networks.
2. Attacks on IPv6 networks.
3. Attacks on IPv4 networks.
Note that one of the mitigation methods listed for various attacks is
based on the premise that 6to4 relays could have a feature limiting
traffic to/from specific 6to4 sites. At the time of this writing,
this feature is speculative, and more work needs to be done to
determine the logistics.
4.1. Attacks on 6to4 Networks
This section describes attacks against 6to4 networks. Attacks that
leverage 6to4 networks, but for which the ultimate victim is
elsewhere (e.g., a native IPv6 user, an IPv4 user), are described
later in the memo.
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6to4 relays and routers are IPv4 nodes, and there is no way for any
6to4 router to confirm the identity of the IPv4 node from which it
receives traffic -- whether from a legitimate 6to4 relay or some
other node. A 6to4 router has to process traffic from all IPv4
nodes. Malicious IPv4 nodes can exploit this property and attack
nodes within the 6to4 network.
It is possible to conduct a variety of attacks on the 6to4 nodes.
These attacks are as follows:
1. Attacks with Neighbor Discovery (ND) Messages
2. Spoofing traffic to 6to4 nodes
3. Reflecting traffic from 6to4 nodes
4. Local IPv4 broadcast attack
4.1.1. Attacks with ND Messages
ATTACK DESCRIPTION
Since the 6to4 router assumes that all the other 6to4 routers and
6to4 relays are "on-link", it is possible to attack the 6to4 router
by using ND messages from any node in the IPv4 network, unless a
prior trust relationship has been established.
The attacks target the 6to4 pseudo-interface. As long as the 6to4
addresses are not used in the source or destination address, the
security checks specified by 6to4 take no stance on these packets.
Typically they use link-local addresses.
For example, an attack could be a Route Advertisement or Neighbor
Advertisement message crafted specifically to cause havoc; the
addresses in such a packet could resemble to the following:
src_v6 = fe80::2 (forged address)
dst_v6 = fe80::1 (valid or invalid address)
src_v4 = 8.0.0.1 (valid or forged address)
dst_v4 = 9.0.0.2 (valid address, matches dst_v6)
These attacks are exacerbated if the implementation supports more
tunneling mechanisms than 6to4 (or configured tunneling) because it
is impossible to disambiguate such mechanisms, making it difficult to
enable strict security checks (see Section 6.1).
The Neighbor Discovery threats (Redirect DoS, or DoS) are described
in [8]. Note that all attacks may not be applicable, as the 6to4
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pseudo-interface is assumed not to have a link-layer address (Section
3.8 RFC 2893 [4]). However, note that the 6to4 router can be either
a router or host from the Neighbor Discovery perspective.
THREAT ANALYSIS AND MITIGATION METHODS
The attacks can be mitigated by using any of the following methods:
o The usage of ND messages could be prohibited. This implies that
all packets using addresses of scope link-local will be silently
discarded. Section 3.1 of RFC 3056 [1] leaves scope for future
uses of link-local address. This method has its pitfalls: It
would prohibit any sort of ND message and thus close the doors on
development and use of other ND options. Whether this is a
significant problem is another thing.
o The 6to4 pseudo-interface could be insulated from the other
interfaces, particularly the other tunnel interfaces (if any), for
example by using a separate neighbor cache.
o If ND messages are needed, either IPsec [4] or an extension of
SEND could be used [9] to secure packet exchange using the
link-local address; vanilla SEND would not work, as the link-layer
does not have an address -- and IPsec would be rather complex.
COMPARISON TO SITUATION WITHOUT 6to4
Even though rather simply fixed, this attack is not new as such; the
same is possible by using automatic tunneling [4] or configured
tunneling (if one is able to spoof source IPv4 address to that of the
tunnel end-point).
However, as 6to4 provides open decapsulation, and automatic tunneling
is being deprecated [10], 6to4 provides an easy means, which would
not exist without it.
4.1.2. Spoofing Traffic to 6to4 Nodes
ATTACK DESCRIPTION
The attacker - a malicious IPv4 or IPv6 node - can send packets that
are difficult to trace (e.g., due to spoofing or going through a
relay) to a 6to4 node. This can be used e.g., to accomplish a DoS
attack.
The IPv6 and IPv4 addresses of the packets will be similar to the
following:
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src_v6 = 2001:db8::1 (forged address)
dst_v6 = 2002:0900:0002::1 (valid address)
src_v4 = 8.0.0.1 (valid or forged address)
dst_v4 = 9.0.0.2 (valid address, matches dst_v6)
For attacks launched from a native IPv6 node, the src_v4 will be the
address of the relay through which the traffic will reach the 6to4
node. From IPv4 nodes, src_v4 can be either a spoofed source address
or the real one.
The 6to4 router receives these packets from 8.0.0.1, decapsulates
them, discards the IPv4 header containing the source address 8.0.0.1,
and processes them as normal (the attacker has guessed or obtained
"dst_v6" by using one of a number of techniques).
This is a DoS attack on 6to4 nodes.
This attack is similar to those shown in [11].
EXTENSIONS
Replies to the traffic will be directed to the src_v6 address,
resulting in 6to4 nodes participating in a reflection DoS. This
attack is described in more detail in Section 4.2.3. The replies
(e.g., TCP SYN ACK, TCP RST, ICMPv6 Echo Reply, input sent to UDP
echo service, ICMPv6 Destination Unreachable) are sent to the victim
(src_v6), above. All the traces from the original attacker (src_v4)
have been discarded. These return packets will go through a relay.
Certain 6to4 networks may have a trivial ACL (Access Control List)
based firewall that allows traffic to pass through if it comes from
particular source(s). Such a firewalling mechanism can be bypassed
by address spoofing. This attack can therefore be used for trivial
ACL avoidance as well. These attacks might be hampered because the
replies from the 6to4 node to the spoofed address will be lost.
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
The Denial-of-Service attack based on traffic spoofing is not new;
the only twists come from the fact that traces of an attack are more
easily lost, and that spoofing the IPv6 address is possible even to
those who are unable to do so in their current networks. The 6to4
router typically does not log IPv4 addresses (as they would be
treated as L2 addresses), and thus the source of the attack (if
launched from an IPv4 node) is lost. Because traces to the src_v4
address are easily lost, these attacks can also be launched from IPv4
nodes whose connections are ingress-filtered.
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However, often this is not a real factor, as usually the attackers
are just zombies and real attackers may not even care whether the
unspoofed source address is discovered.
Malicious native IPv6 nodes could be caught easily if ingress
filtering was enabled everywhere in the IPv6 Internet.
These attacks are easy to perform, but the extent of harm is limited:
o For every packet sent, at most one reply packet is generated:
there is no amplification factor.
o Attack packets, if initiated from an IPv6 node, will pass through
choke point(s), namely a 6to4 relay; in addition to physical
limitations, these could implement some form of 6to4-site-specific
traffic limiting.
On the other hand, a variety of factors can make the attacks serious:
o The attacker may have the ability to choose the relay, and he
might employ the ones best suited for the attacks. Also, many
relays use 192.88.99.1 [3] as the source address, making tracing
even more difficult (also see Section 4.2.6).
o The relay's IPv4 address may be used as a source address for these
attacks, potentially causing a lot of complaints or other actions,
as the relay might seem to be the source of the attack (see
Section 4.2.6 for more).
Some of the mitigation methods for such attacks are as follows:
1. Ingress filtering in the native IPv6 networks to prevent packets
with spoofed IPv6 sources from being transmitted. This would,
thus, make it easy to identify the source of the attack.
Unfortunately, it would depend on significant (or even complete)
ingress filtering everywhere in other networks; while this is
highly desirable, it may not be feasible.
2. Security checks in the 6to4 relay. The 6to4 relay must drop
traffic (from the IPv6 Internet) that has 6to4 addresses as
source address; see Section 5 for more detail. This has very
little cost.
However, these mitigation methods do not address the case of an IPv4
node sending encapsulated IPv6 packets.
No simple way to prevent such attacks exists, and longer-term
solutions, such as ingress filtering [12] or itrace [13], would have
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to be deployed in both IPv6 and IPv4 networks to help identify the
source of the attacks. A total penetration is likely impossible.
(Note that itrace work has been discontinued, as of this writing in
July 2004.)
COMPARISON TO SITUATION WITHOUT 6to4
Traffic spoofing is not a new phenomenon in IPv4 or IPv6. 6to4 just
makes it easier: Anyone can, regardless of ingress filtering, spoof a
native IPv6 address to a 6to4 node, even if "maximal security" would
be implemented and deployed. Losing trails is also easier.
Therefore, depending on how much one assumes ingress filtering is
deployed for IPv4 and IPv6, this could be considered either a very
serious issue or close to irrelevant compared to the IP spoofing
capabilities without 6to4.
4.1.3. Reflecting Traffic to 6to4 Nodes
ATTACK DESCRIPTION
Spoofed traffic (as described in Section 4.2.2) may be sent to native
IPv6 nodes to perform a reflection attack against 6to4 nodes.
The spoofed traffic is sent to a native IPv6 node, either from an
IPv4 node (through a 6to4 relay) or from a native IPv6 node (unless
ingress filtering has been deployed). With the former, the sent
packets would resemble the following:
src_v6 = 2002:1234:1234::1 (forged address of the target 6to4 node)
dst_v6 = 2002:0900:0002::1 (valid address)
src_v4 = 8.0.0.1 (valid or invalid address)
dst_v4 = 9.0.0.2 (valid address, matches dst_v6)
Note that an attack through the relay is prevented if the relay
implements proper decapsulation security checks (see Section 5 for
details) unless the IPv4 node can spoof the source address to match
src_v6. Similarly, the attack from native IPv6 nodes could be
prevented by global ingress filtering deployment.
These attacks can be initiated by native IPv6, IPv4, or 6to4 nodes.
EXTENSIONS
A distributed Reflection DoS can be performed if a large number of
nodes are involved in sending spoofed traffic with the same src_v6.
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Malicious 6to4 nodes can also (try to) initiate this attack by
bouncing traffic off 6to4 nodes in other 6to4 sites. However, this
attack may not be possible, as the 6to4 router (in the site from
which the attack is launched) will filter packets with forged source
addresses (with security checks mentioned in Section 5).
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
In this case, the reverse traffic comprises replies to the messages
received by the 6to4 nodes. The attacker has less control on the
packet type, and this would inhibit certain types of attacks. For
example, flooding a 6to4 node with TCP SYN packets will not be
possible (but e.g., a SYN-ACK or RST would be).
These attacks may be mitigated in various ways:
o Implementation of ingress filtering by the IPv4 service providers.
This would prevent forging of the src_v4 address and help in
closing down on the culprit IPv4 nodes. Note that it will be
difficult to shut down the attack if a large number of IPv4 nodes
are involved.
These attacks may be also be stopped at the 6to4 sites if the
culprit src_v4 address is identified, and if it is constant, by
filtering traffic from this address. Note that it would be
difficult to implement this method if appropriate logging were not
done by the 6to4 router or if a large number of 6to4 nodes, and/or
a large number of IPv4 nodes were participating in the attack.
Unfortunately, because many IPv4 service providers don't implement
ingress filtering, for whatever reasons, this may not be a
satisfactory solution.
o Implementation of ingress filtering by all IPv6 service providers
would eliminate this attack, because src_v6 could not be spoofed
as a 6to4 address. However, expecting this to happen may not be
practical.
o Proper implementation of security checks (see Section 5) both at
the 6to4 relays and routers would eliminate an attack launched
from an IPv4 node, except when the IPv4 source address was also
spoofed -- but then the attacker would have been able to attack
the ultimate destination directly.
o Rate limiting traffic at the 6to4 relays. In a scenario where
most of the traffic is passing through few 6to4 relays, these
relays can implement traffic rate-limiting features and rate-limit
the traffic from 6to4 sites.
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COMPARISON TO SITUATION WITHOUT 6to4
This particular attack can be mitigated by proper implementation of
security checks (which is quite straightforward) and ingress
filtering; when ingress filtering is not implemented, it is typically
easier to attack directly than through reflection -- unless "traffic
laundering" is an explicit goal of the attack. Therefore, this
attack does not seem very serious.
4.1.4. Local IPv4 Broadcast Attack
ATTACK DESCRIPTION
This threat is applicable if the 6to4 router does not check whether
the IPv4 address to which it tries to send encapsulated IPv6 packets
is a local broadcast address or a multicast address.
This threat is described in the specification [1], and implementing
the checks eliminates this threat. However, as checks have not been
widely implemented, the threat is included here for completeness.
There practically two kinds of attacks: when a local 6to4 user tries
to send packets to the address corresponding to the broadcast
address, and when someone is able to do so remotely.
In the first option, assume that 9.0.0.255 is the 6to4 router's
broadcast address. After receiving the packet with a destination
address like "2002:0900:00ff::bbbb" from a local 6to4 node, if the
router doesn't check the destination address for subnet broadcast, it
would send the encapsulated protocol-41 packet to 9.0.0.255. This
would be received by all nodes in the subnet, and the responses would
be directed to the 6to4 router.
Malicious sites may also embed forged 6to4 addresses in the DNS, use
of which by a 6to4 node would result in a local broadcast by the 6to4
router. One way to perform this attack would be to send an HTML mail
containing a link to an invalid URL (for example,
http://[2002:0900:00ff::bbbb]/index.html) to all users in a 6to4
technology based network. Opening of the mail simultaneously would
result in a broadcast storm.
The second kind of attack is more complex: The attack can be
initiated by IPv4 nodes not belonging to the local network as long as
they can send traffic with invalid (for example 2002:0900:00ff::bbbb)
source address. The 6to4 router has to respond to the traffic by
sending ICMPv6 packets back to the source, (e.g., Hop Limit Exceeded
or Destination Unreachable). The packet would be as follows:
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src_v6 = 2002:0800:00ff::bbbb (broadcast address of the router)
dst_v6 = 2002:0800:0001::0001 (valid non-existent address)
This is a DoS attack.
EXTENSIONS
The attacks could also be directed at non-local broadcast addresses,
but these would be so-called "IPv4 directed broadcasts", which have
(luckily enough) already been extensively blocked in the Internet.
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
The attack is based on the premise that the 6to4 router has to send a
packet that embeds an invalid IPv4 address to an IPv6 address. Such
an attack is easily thwarted by ensuring that the 6to4 router does
not transmit packets to invalid IPv4 addresses. Specifically,
traffic should not be sent to broadcast or multicast IPv4 addresses.
COMPARISON TO SITUATION WITHOUT 6to4
The first threat is similar to what is already possible with IPv4,
but IPv6 does not have broadcast addresses.
The second, a more complex threat, is, similarly, also available in
IPv4.
In consequence, the security does not seem to be significantly worse
than with IPv4, and even that is restricted to the site(s) with 6to4
implementations that haven't been secured as described in Section 5.
4.2. Attacks on Native IPv6 Internet
This section describes attacks against native IPv6 Internet that
somehow leverage 6to4 architecture. Attacks against 6to4 nodes were
described in the previous section.
6to4 and IPv4 nodes can access native IPv6 nodes through the 6to4
relay routers. Thus, the 6to4 relays play a crucial role in any
attack on native IPv6 nodes by IPv4 nodes or 6to4 nodes.
6to4 relays have only one significant security check they must
perform for general safety: When decapsulating IPv4 packets, they
check that 2002:V4ADDR::/48 and V4ADDR match in the source address.
If this is not done, several threats become more serious; in the
following analysis, it is assumed that such checks are implemented.
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6to4 relay should not relay packets between 6to4 addresses. In
particular, packets decapsulated from 6to4 routers should not be
encapsulated toward 6to4 routers, as described in Section 5.
Similarly, packets with 6to4 source and destination addresses sent
from IPv6 nodes should not be relayed. It is not clear whether this
kind of check is typically implemented. The attacks described below
assume that such checks are not implemented.
4.2.1. Attacks with ND Messages
These attacks are the same as those employed against 6to4 routers, as
described in Section 4.1.1.
4.2.2. Spoofing Traffic to Native IPv6 Node
ATTACK DESCRIPTION
The attacker - a malicious IPv4 or 6to4 node - can send packets with
a spoofed (or not spoofed) 6to4 source address to a native IPv6 node
to accomplish a DoS attack.
The threat is similar to that involving 6to4 routers, as described in
Section 4.1.2.
The difference here is that the attack is initiated by IPv4 or 6to4
nodes. The source IPv6 address may or may not be spoofed. Note
that, as mentioned above, the relay is assumed to correlate the
source IPv4 address with the address embedded in the source IPv6
address during decapsulation. A side effect is that all spoofed
traffic will have a 6to4 source address.
EXTENSIONS
Spoofed traffic may also be sent to native IPv6 nodes either by other
native IPv6 nodes, by 6to4 nodes, or by malicious IPv4 nodes to
conduct Reflection DoS on either native IPv6 nodes or 6to4 nodes.
Certain native IPv6 networks may have a trivial ACL (Access Control
List) based firewall that allows traffic to pass through if it comes
from particular source(s). Such a firewalling mechanism can be
bypassed by address spoofing. This attack can therefore be used for
trivial ACL avoidance as well. These attacks might be hampered by
lost replies from the 6to4 node to the spoofed address.
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THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
The Denial-of-Service attack based on traffic spoofing is not new;
the only twist is that traces of an attack are more easily lost. The
6to4 relay typically does not log IPv4 addresses (as they would be
treated as L2 addresses), and thus the source of the attack (if
launched from an IPv4 node) is lost. Because traces to the src_v4
address are easily lost, these attacks can also be launched from IPv4
nodes whose connections are ingress-filtered.
These attacks might not be easy to perform and might be hampered
because of the following:
o It might be difficult to launch such attacks from 6to4 nodes
because even if the 6to4 routers allow spoofing of the source IPv6
address, the 6to4 relay would check whether the source V4ADDR is
the same as the one embedded in the source IPv6 address. Thus,
6to4 nodes will be forced to use the correct IPv6 prefix while
launching an attack, making it easy to close such attacks.
o Packets may pass through choke point(s), namely a 6to4 relay. In
addition to physical limitations, there could be some sort of
traffic rate limiting mechanisms that may be implemented, and
these could tone down the attack.
o For every packet sent, at most one reply packet is generated:
There is no amplification factor.
Some of the mitigation methods for such attacks are as follows:
1. Ingress filtering in the IPv4 Internet to prevent packets with a
spoofed IPv4 source from being transmitted. As the relay checks
that the 6to4 address embeds the IPv4 address, no spoofing can be
achieved unless IPv4 addresses can be spoofed. However, this
would probably be an unfeasible requirement.
2. Security checks in the 6to4 relay. The 6to4 relay must drop
traffic (from 6to4 nodes, or IPv4 nodes) with non-6to4 addresses
as the source address, or for which the source IPv4 address does
not match the address embedded in the source IPv6 address.
COMPARISON TO SITUATION WITHOUT 6to4
Compared to Section 4.1.2, which describes more serious threats, this
threat appears to be slightly more manageable. If the relays perform
proper decapsulation checks, the spoofing can only be achieved, to a
6to4 source address, when the IPv4 address is spoofable as well.
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4.2.3. Reflecting Traffic to Native IPv6 Nodes
ATTACK DESCRIPTION
These reflection attacks are similar to that involving 6to4 routers,
as described in Section 4.1.3. Traffic may be reflected off native
IPv6 nodes, or off 6to4 nodes. The attack can be initiated by one of
the following:
o Native IPv6 nodes. These nodes can send invalid traffic with
spoofed native IPv6 addresses to valid 6to4 nodes. Replies from
the 6to4 nodes are part of a reflection attack.
o IPv4 nodes. These nodes can send traffic with native IPv6 source
addresses (encapsulated by the IPv4 node itself into a protocol-41
packet) to 6to4 nodes. Replies from the 6to4 nodes are part of a
reflection attack.
o 6to4 nodes. These nodes can perform attacks similar to those by
IPv4 nodes, but this would require spoofing of the source address
at the 6to4 site before encapsulation, which is likely to be
difficult.
When launched from a native IPv6 node, the traffic goes through 6to4
relays twice, both before and after the reflection; when launched
from a 6to4/IPv4 node, the traffic goes through a relay only after
the reflection.
EXTENSIONS
A distributed reflection DoS can be performed if a large number of
native IPv6 nodes or IPv4/6to4 nodes are involved in sending spoofed
traffic with the same source IPv6 address.
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
Some of the mitigation methods for such attacks are as follows:
1. Attacks from the native IPv6 nodes could be stopped by
implementing ingress filtering in the IPv6 Internet; hopefully
this will become commonplace, but past experience of IPv4 ingress
filtering deployment (or lack thereof) does not promise much.
2. Two measures are needed to stop or mitigate the attacks from IPv4
nodes: 1) Implementing ingress filtering in the IPv4 internet,
and 2) logging IPv4 source addresses in the 6to4 router.
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3. Attacks from 6to4 nodes in other sites can be stopped if the 6to4
routers in those sites implement egress filtering. This could be
done by those sites, but the sites that are most likely to be
abused are typically also those most likely to neglect installing
appropriate filtering at their edges.
4. The traffic passes through one or two relays, and traffic rate
limiting in the 6to4 relays might help tone down the reflection
attack.
COMPARISON TO SITUATION WITHOUT 6to4
Even though there are means to mitigate it, the attack is still
rather efficient, especially when used by native IPv6 nodes with
spoofed addresses. Using 6to4 relays and routers could easily take
down the 6to4 relay system and/or provide an easy means for traffic
laundering. However, if the attack is intended to DoS the victim,
this can be achieved more smoothly by doing it directly (as the
source address spoofing was available as well).
Therefore, the threat to the availability and stability of the 6to4
relay system itself seems to be higher than to the native IPv6
Internet.
4.2.4. Local IPv4 Broadcast Attack
This attack is similar to the ones employed against 6to4 routers, as
described in Section 4.1.4. There are slight differences with regard
to the source of the attacks. This attack can be initiated by:
o native IPv6 nodes that may send traffic to the relay's subnet
broadcast address, and
o IPv4 nodes that may send traffic with a spoofed source IP address
(to be the relay's broadcast address) to elicit replies (e.g.,
ICMPv6 Hop Limit Exceeded) from the 6to4 relay to its local nodes.
The first approach is more dangerous than those in Section 4.1.4
because it can be initiated by any IPv6 node (allowed to use the
relay); the approach is not limited to local users.
The second approach is trickier and not really useful. For it to
succeed, the relay would have to accept native source addresses over
the 6to4 pseudo-interface (we did not assume this check was
implemented), as if coming from another relay, triggering an ICMPv6
message to the relay's local IPv4 subnet. The former method is more
lucrative.
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EXTENSIONS
None.
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
The threat is restricted to the relay's local subnet and is fixed by
tightening the 6to4 security checks.
COMPARISON TO SITUATION WITHOUT 6to4
This scenario is caused by 6to4, but fortunately the issue is not
serious.
4.2.5. Theft of Service
ATTACK DESCRIPTION
The 6to4 relay administrators would often want to use some policy to
limit the use of the relay to specific 6to4 sites and/or specific
IPv6 sites.
The policy control is usually enacted by applying restrictions to
where the routing information for 2002::/16 and/or 192.188.99.0/24
(if the anycast address used [3]) will spread.
Some users may be able to use the service regardless of these
controls, by
o configuring the address of the relay using its IPv4 address
instead of 192.88.99.1, or
o using the routing header to route IPv6 packets to reach specific
6to4 relays. (Other routing tricks, such as using static routes,
may also be used.)
EXTENSIONS
None.
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
Attempts to use the relay's IPv4 address instead of 192.88.99.1 can
be mitigated in the following ways:
1. IPv4 domains should prevent use of the actual IPv4 address of the
relay instead of 192.88.99.1.
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2. Usage of access lists in the 6to4 relay to limit access. This is
only feasible if the number of IP networks the relay is supposed
to serve is relatively low.
3. The 6to4 relay should filter out arriving tunneled packets with
protocol 41 (IPv6) that do not have 192.88.99.1 as the
destination address.
The other threat, of using routing tricks in the IPv6 networks to
reach the 6to4 relay, has similar solutions:
1. Usage of access lists in the relay to limit access.
2. Filtering out the packets with a routing header (although this
may have other implications).
3. Monitoring the source addresses going through the relay to
detect, e.g., peers setting up static routes.
Routing Header is not specific to 6to4. The main thing one could do
with it here would be to select the relay. Some generic threats
about routing header use are described in [11].
As this threat does not have implications for anything other than the
organization providing 6to4 relay, it is not analyzed any further.
COMPARISON TO SITUATION WITHOUT 6to4
These threats are specific to 6to4 relays (or in general anycast
services) and do not exist in networks without 6to4.
4.2.6. Relay Operators Seen as Source of Abuse
ATTACK DESCRIPTION
Several attacks use 6to4 relays to anonymize the traffic; this often
results in packets being tunneled from the relay to a supposedly-6to4
site.
However, as was pointed out in Section 4.2, the IPv4 source address
used by the relay could, on a cursory look, be seen as the source of
these "protocol-41" attacks.
This could cause a number of concerns for the operators deploying
6to4 relay service, including the following:
o being contacted a lot (via email, phone, fax, or lawyers) on
suspected "abuse",
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RFC 3964 Security Considerations for 6to4 December 2004
o having the whole IPv4 address range rejected as a source of abuse
or spam, causing outage to other operations as well, or
o causing the whole IPv4 address range to be blacklisted in some
"spammer databases", if the relay were used for those purposes.
This threat seems slightly similar to the outburst of SMTP abuse
caused by open relays but is more generic.
EXTENSIONS
None.
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
This problem can be avoided (or, really, "made someone else's
problem") by using the 6to4 anycast address in 192.88.99.0/24 as the
source address. Blacklisting or rejecting this should not cause
problems to the other operations.
Further, when someone files complaints to the owner of
192.88.99.0/24, depending on which registry they are querying, they
might get, for example:
o knowledge that this is a special IANA address block, with no real
contact person,
o knowledge that this is a special address block for RFC 3068, or
o knowledge that this is a special address block for RFC 3068, and
that there are multiple entries by relay operators in the
database.
Any of these, at least when processed by a human, should show that
the 6to4 relay is in fact innocent. Of course, this could result in
reports going to the closest anycast 6to4 relay as well, which had
nothing to do with the incident.
However, the widespread usage of 192.88.99.1 as the source address
may make it more difficult to disambiguate the relays, which might be
a useful feature for debugging purposes.
COMPARISON TO SITUATION WITHOUT 6to4
This threat is caused by 6to4 deployment but can be avoided, at least
in the short-term, by using 192.88.99.1 as the source address.
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4.3. Attacks on IPv4 Internet
There are two types of attacks on the IPv4 internet - spoofed
traffic, and reflection. These can be initiated by native IPv6
nodes, 6to4 nodes, and IPv4 nodes.
Attacks initiated by IPv4 nodes that send spoofed traffic, which
would not use the 6to4 infrastructure, are considered out of the
scope of this document. 6to4 infrastructure may be used in
reflection attacks initiated by IPv4 nodes.
It is difficult for these attacks to be effective, as the traffic
sent out will be IPv6-in-IPv4. Such traffic will be rejected by most
IPv4 nodes unless they have implemented some sort of IPv6-in-IPv4
tunneling.
4.4. Summary of the Attacks
Columns:
o Section number. The section that describes the attack.
o Attack name.
o Initiator. The node that initiates the attack.
* I_4 - IPv4 node
* I_6 - native IPv6 node
* 6to4 - 6to4 node
* * - All of the above
o Victim. The victim node
* I_4 - IPv4 node
* I_6 - native IPv6 node
* 6to4 - 6to4 node
* Relay - 6to4 relay
* Router - 6to4 router
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RFC 3964 Security Considerations for 6to4 December 2004
o ToA. Type of Attack
* D - DoS
* R - Reflection DoS
* T - Theft of Service
o Fix. Specified who is responsible for fixing the attack.
* 6 - The 6to4 developer and/or operator can completely mitigate
this attack.
* 6* - The 6to4 developer and/or operator can partially mitigate
this attack.
* E - This threat cannot be fixed by the 6to4 developer or the
6to4 operator.
Summary of attacks on a 6to4 network:
+-------+----------------------+---------+----------+-----+-----+
| Sec | Attack name |Initiator| Victim | ToA | Fix |
+-------+----------------------+---------+----------+-----+-----+
| 4.1.1 | Attacks with ND | I_4 | Router | D | 6 |
+-------+----------------------+---------+----------+-----+-----+
| 4.1.2 | Spoofing Traffic | I_4,I_6 | 6to4 | D | E |
+-------+----------------------+---------+----------+-----+-----+
| 4.1.3 | Reflection Attacks | * | 6to4 | R | 6* |
+-------+----------------------+---------+----------+-----+-----+
| 4.1.4 | Local IPv4 Broadcast | * | Router | D | 6 |
+-------+----------------------+---------+----------+-----+-----+
Figure 9
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RFC 3964 Security Considerations for 6to4 December 2004
Summary of attacks on the native IPv6 internet:
+-------+----------------------+---------+----------+-----+-----+
| Sec | Attack name |Initiator| Victim | ToA | Fix |
+-------+----------------------+---------+----------+-----+-----+
| 4.2.1 | Attacks with ND | I_4 | Relay | D | 6 |
+-------+----------------------+---------+----------+-----+-----+
| 4.2.2 | Spoofing Traffic | I_4,6to4| I_6 | D | 6* |
+-------+----------------------+---------+----------+-----+-----+
| 4.2.3 | Reflection Attacks | * | I_6 | R | 6* |
+-------+----------------------+---------+----------+-----+-----+
| 4.2.4 | Local IPv4 Broadcast | * | Relay | D | 6 |
+-------+----------------------+---------+----------+-----+-----+
| 4.2.5 | Theft of Service | 6to4 | Relay | T | 6 |
+-------+----------------------+---------+----------+-----+-----+
| 4.2.6 | Relay Operators ... | - | - | D | 1) |
+-------+----------------------+---------+----------+-----+-----+
Figure 10
Notes:
1) This attack is a side-effect of the other attacks and thus does
not have any Initiator, Victim, and Fix. It is a Denial of Service
attack not on the network but on the organization in-charge of the
relay.
Summary of attacks on IPv4 internet:
+-------+----------------------+---------+----------+-----+-----+
| Sec | Attack name |Initiator| Victim | ToA | Fix |
+-------+----------------------+---------+----------+-----+-----+
| 4.3 | Spoofing Traffic | * | I_4 | D | 6* |
+-------+----------------------+---------+----------+-----+-----+
| 4.3 | Reflection Attacks | * | I_4 | R | 6* |
+-------+----------------------+---------+----------+-----+-----+
Figure 11
5. Implementing Proper Security Checks in 6to4
This section describes several ways to implement the security checks
required or implied by the specification [1] or augmented by this
memo. These do not, in general, protect against most of the threats
listed above in the "Threat Analysis" section. They are only
prerequisites for a relatively safe and simple 6to4 implementation.
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Note that, in general, the 6to4 router or relay does not know whether
it is acting as a router or relay. It would be possible to include a
toggle to specify the behaviour, to be used when, e.g., the interface
is brought up, but as of February 2004, no implementations were known
to do that. Therefore, the checks are described as that which works
independently of whether the node is a router or relay.
5.1. Encapsulating IPv6 into IPv4
The checks described in this section are to be performed when
encapsulating IPv6 into IPv4.
The encapsulation rules are mainly designed to keep implementors from
"shooting themselves in the foot." For example, the source address
check would verify that the packet will be acceptable to the
decapsulator, or the sanity checks would ensure that addresses
derived from private addresses are not used (which would be equally
unacceptable).
src_v6 and dst_v6 MUST pass ipv6-sanity checks (see below) else drop
if prefix (src_v6) == 2002::/16
ipv4 address embedded in src_v6 MUST match src_v4
else if prefix (dst_v6) == 2002::/16
dst_v4 SHOULD NOT be assigned to the router
else
drop
/* we somehow got a native-native ipv6 packet */
fi
accept
5.2. Decapsulating IPv4 into IPv6
The checks described in this section are to be performed when
decapsulating IPv4 into IPv6. They will be performed in both the
6to4 router and relay.
src_v4 and dst_v4 MUST pass ipv4-sanity checks, else drop
src_v6 and dst_v6 MUST pass ipv6-sanity checks, else drop
if prefix (dst_v6) == 2002::/16
ipv4 address embedded in dst_v6 MUST match dst_v4
if prefix (src_v6) == 2002::/16
ipv4 address embedded in src_v6 MUST match src_v4
dst_v4 SHOULD be assigned to the router
fi
elif prefix (src_v6) == 2002::/16
ipv4 address embedded in src_v6 MUST match src_v4
dst_v4 SHOULD be assigned to the router (see notes below)
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else
drop
/* the we somehow got a native-native ipv6 packet */
fi
accept
5.3. IPv4 and IPv6 Sanity Checks
The encapsulation and decapsulation checks include certain sanity
checks for both IPv4 and IPv6. These are described here in detail.
5.3.1. IPv4
IPv4 address MUST be a global unicast address, as required by the
6to4 specification. The disallowed addresses include those defined
in [14], and others widely used and known not to be global. These
are
o 0.0.0.0/8 (the system has no address assigned yet)
o 10.0.0.0/8 (private)
o 127.0.0.0/8 (loopback)
o 172.16.0.0/12 (private)
o 192.168.0.0/16 (private)
o 169.254.0.0/16 (IANA Assigned DHCP link-local)
o 224.0.0.0/4 (multicast)
o 240.0.0.0/4 (reserved and broadcast)
In addition, the address MUST NOT be any of the system's broadcast
addresses. This is especially important if the implementation is
made so that it can
o receive and process encapsulated IPv4 packets arriving at its
broadcast addresses, or
o send encapsulated IPv4 packets to one of its broadcast addresses.
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5.3.2. IPv6
IPv6 address MUST NOT be
o 0::/16 (compatible, mapped addresses, loopback, unspecified, ...)
o fe80::/10 (link-local)
o fec0::/10 (site-local)
o ff00::/8 (any multicast)
Note: Only link-local multicast would be strictly required, but it is
believed that multicast with 6to4 will not be feasible, so it has
been disallowed as well.
In addition, it MUST be checked that equivalent 2002:V4ADDR::/48
checks, where V4ADDR is any of the above IPv4 addresses, will not be
passed.
5.3.3. Optional Ingress Filtering
In addition, the implementation in the 6to4 router may perform some
form of ingress filtering (e.g., Unicast Reverse Path Forwarding
checks). For example, if the 6to4 router has multiple interfaces, of
which some are "internal", receiving either IPv4 or IPv6 packets with
source address belonging to any of these internal networks from the
Internet might be disallowed.
If these checks are implemented and enabled by default, it's
recommended that there be a toggle to disable them if needed.
5.3.4. Notes about the Checks
The rule "dst_v4 SHOULD be assigned to the router" is not needed if
the 6to4 router implementation only accepts and processes
encapsulated IPv4 packets arriving to its unicast IPv4 addresses, and
when the destination address is known to be a local broadcast
address, it does not try to encapsulate and send packets to it. (See
Sections 4.1.4 and 4.2.4 about this threat.)
Some checks, especially the IPv4/IPv6 Sanity Checks, could be at
least partially implementable with system-level access lists, if one
would like to avoid placing too many restrictions in the 6to4
implementation itself. This depends on how many hooks are in place
for the access lists. In practice, it seems that this could not be
done effectively enough unless the access list mechanism is able to
parse the encapsulated packets.
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6. Issues in 6to4 Implementation and Use
This section tries to give an overview of some of the problems 6to4
implementations face, and the kind of generic problems the 6to4 users
could come up with.
6.1. Implementation Considerations with Automatic Tunnels
There is a problem with multiple transition mechanisms if strict
security checks are implemented. This may vary a bit from
implementation to implementation.
Consider three mechanisms using automatic tunneling: 6to4, ISATAP
[15], and Automatic Tunneling using Compatible Addresses [4]
(currently removed [10] but typically still supported). All of these
use IP-IP (protocol 41) [16] IPv4 encapsulation with, more or less, a
pseudo-interface.
When a router, which has any two of these enabled, receives an IPv4
encapsulated IPv6 packet
src_v6 = 2001:db8::1
dst_v6 = 2002:1010:1010::2
src_v4 = 10.0.0.1
dst_v4 = 20.20.20.20
What can it do? How should it decide which transition mechanism this
belongs to; there is no "transition mechanism number" in the IPv6 or
IPv4 header to signify this. (This can also be viewed as a
flexibility benefit.)
Without any kind of security checks (in any of the implemented
methods), these often just "work", as the mechanisms aren't
differentiated but handled in "one big lump".
Configured tunneling [4] does not suffer from this, as it is
point-to-point and based on src_v6/dst_v6 pairs of both IPv4 and IPv6
addresses, so the tunnel interface can be logically deduced.
Solutions for this include 1) not using more than one automatic
tunneling mechanism in a node and 2) binding different mechanisms to
different IPv4 addresses.
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6.2. A Different Model for 6to4 Deployment
Even though this was already discussed in Section 4.1.2, it bears
some additional elaboration, as it was the only problem that cannot
be even partially solved using the current deployment model. There
are some mitigation methods.
6to4 routers receive traffic from non-6to4 ("native") sources via
6to4 relays. 6to4 routers have no way of matching the IPv4 source
address of the relay with the non-6to4 IPv6 address of the source.
Consequently, anyone can spoof any non-6to4 IPv6 address by sending
traffic, encapsulated, directly to 6to4 routers.
It could be possible to turn the deployment assumptions of 6to4
around a bit to eliminate some threats caused by untrusted 6to4
relays:
o Every dual-stack site (or even ISP) would be required to have its
own 6to4 relay. (This assumes that IPv6-only is so far away that
6to4 would be retired by that point.) That is, there would not be
third-party relays, and 2002::/16 and 192.88.99.0/24 routes would
not need to be advertised globally.
o The security implications of 6to4 use could be pushed back to the
level of trust inside the site or ISP (or their acceptable use
policies). This is something that the sites and ISPs should
already be familiar with already.
However, this presents a number of problems:
This model would shift most of the burden of supporting 6to4 to IPv6
sites that don't employ or use 6to4 at all, i.e., "those who deploy
proper native dual-stack." It could be argued that the deployment
pain should be borne by 6to4 users, not by the others.
The main advantage of 6to4 is easy deployment and free relays. This
would require that everyone the 6to4 sites wish to communicate with
implement these measures.
The model would not fix the "relay spoofing problem", unless
everybody also deployed 6to4 addresses on the nodes (alongside with
native addresses, if necessary), which would in turn change 6to4 to
operate without relays completely.
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7. Security Considerations
This document discusses security considerations of 6to4.
Even if proper checks are implemented, there are a large number of
different security threats; these threats are analyzed in Section 4.
There are mainly four classes of potential problem sources:
1. 6to4 routers not being able to identify whether relays are
legitimate
2. Wrong or impartially implemented 6to4 router or relay security
checks
3. 6to4 architecture used to participate in DoS or reflected DoS
attacks or made to participate in "packet laundering", i.e.,
making another attack harder to trace
4. 6to4 relays being subject to "administrative abuse" e.g., theft
of service or being seen as a source of abuse.
The first is the toughest problem, still under research. The second
can be fixed by ensuring the correctness of implementations; this is
important. The third is also a very difficult problem, impossible to
solve completely; therefore it is important to be able to analyze
whether this results in a significant increase of threats. The
fourth problem seems to have feasible solutions.
These are analyzed in detail in "Threat Analysis", in Section 4.
8. Acknowledgments
Some issues were first brought up by Itojun Hagino in [17], and Alain
Durand introduced one specific problem at IETF51 in August 2001
(though there was some discussion on the list prior to that); these
two gave the authors the push to start looking into the details of
securing 6to4.
Alexey Kuznetsov brought up the implementation problem with IPv6
martian checks. Christian Huitema formulated the rules that rely on
6to4 relays using only anycast. Keith Moore brought up the point
about reduced flexibility. Brian Carpenter, Tony Hain, and Vladislav
Yasevich are acknowledged for lengthy discussions. Alain Durand
reminded the authors about relay spoofing problems. Brian Carpenter
reminded the authors about the BGP-based 6to4 router model.
Christian Huitema gave a push for a more complete threat analysis.
Itojun Hagino spelled out the operators' fears about 6to4 relay
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RFC 3964 Security Considerations for 6to4 December 2004
abuse. Rob Austein brought up the idea of a different 6to4
deployment model.
In the latter phase, discussions with Christian Huitema, Brian
Carpenter, and Alain Durand were helpful when improving the document.
David Malone, Iljitsch van Beijnum, and Tim Chown gave feedback on
the document.
9. References
9.1. Normative References
[1] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4
Clouds", RFC 3056, February 2001.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Huitema, C., "An Anycast Prefix for 6to4 Relay Routers", RFC
3068, June 2001.
9.2. Informative References
[4] Gilligan, R. and E. Nordmark, "Transition Mechanisms for IPv6
Hosts and Routers", RFC 2893, August 2000.
[5] IANA, "Special-Use IPv4 Addresses", RFC 3330, September 2002.
[6] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[7] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[8] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[9] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", Work in Progress, July 2004.
[10] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", Work in Progress, September 2004.
[11] Savola, P., "Security of IPv6 Routing Header and Home Address
Options", Work in Progress, March 2002.
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RFC 3964 Security Considerations for 6to4 December 2004
[12] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
Denial of Service Attacks which employ IP Source Address
Spoofing", BCP 38, RFC 2827, May 2000.
[13] Bellovin, S., Leech, M. and T. Taylor, "ICMP Traceback
Messages", Work in Progress, February 2003.
[14] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.
[15] Templin, F., Gleeson, T., Talwar, M. and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)", Work in
Progress, May 2004.
[16] Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995.
[17] Hagino, J., "Possible abuse against IPv6 transition
technologies", Work in Progress, July 2000.
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Appendix A. Some Trivial Attack Scenarios Outlined
Here, a few trivial attack scenarios are outlined -- ones that are
prevented by implementing checks listed in [1] or in section 6.
When two 6to4 routers send traffic to each others' domains, the
packet sent by RA to RB resembles the following:
src_v6 = 2002:0800:0001::aaaa
dst_v6 = 2002:0800:0002::bbbb
src_v4 = 8.0.0.1 (added when encapsulated to IPv4)
dst_v4 = 8.0.0.2 (added when encapsulated to IPv4)
When the packet is received by IPv4 stack on RB, it will be
decapsulated so that only src_v6 and dst_v6 remain, as originally
sent by RA:
src_v6 = 2002:0800:0001::aaaa
dst_v6 = 2002:0800:0002::bbbb
As every other node is just one hop away (IPv6-wise) and the
link-layer (IPv4) addresses are lost, this may open many
possibilities for misuse.
As an example, unidirectional IPv6 spoofing is made trivial because
nobody can check (without delving into IP-IP packets) whether the
encapsulated IPv6 addresses were authentic. (With native IPv6, this
can be done by, e.g., RPF-like mechanisms or access lists in upstream
routers.)
src_v6 = 2002:1234:5678::aaaa (forged)
dst_v6 = 2002:0800:0002::bbbb
src_v4 = 8.0.0.1 (added when encapsulated to IPv4)
dst_v4 = 8.0.0.2 (added when encapsulated to IPv4)
A similar attack with "src" being the native address is made
possible, even with the security checks, by having the sender node
pretend to be a 6to4 relay router.
More worries come into the picture if, e.g.,
src_v6 = ::ffff:[some trusted IPv4 in a private network]
src_v6/dst_v6 = ::ffff:127.0.0.1
src_v6/dst_v6 = ::1
src_v6/dst_v6 = ...
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Some implementations might have been careful enough to design the
stack so as to avoid the incoming (or reply) packets going to IPv4
packet processing through special addresses (e.g., IPv4-mapped
addresses), but who can say for all ...
Authors' Addresses
Pekka Savola
CSC/FUNET
Espoo
Finland
EMail: psavola@funet.fi
Chirayu Patel
All Play, No Work
185, Defence Colony
Bangalore, Karnataka 560038
India
Phone: +91-98452-88078
EMail: chirayu@chirayu.org
URI: http://www.chirayu.org
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RFC 3964 Security Considerations for 6to4 December 2004
Full Copyright Statement
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RFC, FYI, BCP