v6ops Working Group P. Savola
Internet-Draft CSC/FUNET
Expires: August 10, 2004 C. Patel
All Play, No Work
Feb 10, 2004
Security Considerations for 6to4
draft-ietf-v6ops-6to4-security-01.txt
Status of this Memo
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This Internet-Draft will expire on August 10, 2004.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
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 enable ones to go
around access controls and perform Denial of Service attacks using
6to4 relays or 6to4 routers. 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 . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Different 6to4 Forwarding Scenarios . . . . . . . . . . . . 5
2.1 From 6to4 to 6to4 . . . . . . . . . . . . . . . . . . . . . 5
2.2 From Native to 6to4 . . . . . . . . . . . . . . . . . . . . 5
2.3 From 6to4 to Native . . . . . . . . . . . . . . . . . . . . 6
2.4 Other Models . . . . . . . . . . . . . . . . . . . . . . . . 6
2.4.1 BGP Between 6to4 Routers and Relays . . . . . . . . . . . . 7
2.4.2 6to4 as an Optimization Method . . . . . . . . . . . . . . . 7
2.4.3 6to4 as Tunnel End-Point Addressing Mechanism . . . . . . . 7
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 . . . . . . . . . . . . . . 16
4.1.4 Local IPv4 Broadcast Attack . . . . . . . . . . . . . . . . 18
4.2 Attacks on Native IPv6 Internet . . . . . . . . . . . . . . 19
4.2.1 Attacks with ND Messages . . . . . . . . . . . . . . . . . . 20
4.2.2 Spoofing Traffic to Native IPv6 node . . . . . . . . . . . . 20
4.2.3 Reflecting Traffic to Native IPv6 nodes . . . . . . . . . . 22
4.2.4 Local IPv4 Broadcast Attack . . . . . . . . . . . . . . . . 23
4.2.5 Theft of Service . . . . . . . . . . . . . . . . . . . . . . 24
4.2.6 Relay Operators Seen as Source of Abuse . . . . . . . . . . 25
4.3 Attacks on IPv4 Internet . . . . . . . . . . . . . . . . . . 26
4.4 Summary of the Attacks . . . . . . . . . . . . . . . . . . . 27
5. Implementing Proper Security Checks in 6to4 . . . . . . . . 29
5.1 Encapsulating IPv6 into IPv4 . . . . . . . . . . . . . . . . 29
5.2 Decapsulating IPv4 into IPv6 . . . . . . . . . . . . . . . . 30
5.3 IPv4 and IPv6 Sanity Checks . . . . . . . . . . . . . . . . 30
5.3.1 IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.3.2 IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
5.3.3 Optional Ingress Filtering . . . . . . . . . . . . . . . . . 32
5.3.4 Notes About the Checks . . . . . . . . . . . . . . . . . . . 32
6. Issues in 6to4 Implementation and Use . . . . . . . . . . . 32
6.1 Implementation Considerations with Automatic Tunnels . . . . 32
6.2 Anyone Pretending to Be a 6to4 Relay . . . . . . . . . . . . 33
6.2.1 Limited Distribution of More Specific Routes . . . . . . . . 34
6.2.2 A Different Model for 6to4 Deployment . . . . . . . . . . . 35
7. Security Considerations . . . . . . . . . . . . . . . . . . 35
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 36
Normative References . . . . . . . . . . . . . . . . . . . . 36
Informative References . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 38
A. Some Trivial Attack Scenarios Outlined . . . . . . . . . . . 38
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B. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . . 40
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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 6to4 relays.
2. 6to4 relay routers must accept traffic from any native IPv6 node.
Since the 6to4 router must accept from traffic from any other 6to4
router or relay, it implies a certain level of trust, and there are
no strict constraints on what the IPv6 packet may contain. Thus,
addresses within the IPv4, and IPv6 header may be spoofed, and this
property leads to various types of threats including DoS, and
reflection DoS.
The 6to4 specification outlined quite a few security considerations,
but it has been shown that in practice some of them have been
difficult to get implemented due to their abstract nature.
This draft analyzes the 6to4 security issues in more detail and
outlines some enhancements and caveats.
Section 2, and Section 3 are more or less introductory in nature,
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 introduces some
filtering rules for 6to4 implementations, and Section 6 provides
further discussion on a few issues which have proven to be difficult.
Appendix A outlines a few possible trivial attack scenarios in the
case that very little or no security has been implemented.
For the sake of simplicity, in this document, native IPv6 Internet is
assumed to encompass IPv6 networks formed using other transition
mechanisms (e.g. RFC 2893 [4]), as these mechanisms cannot talk
directly 6to4 network.
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 RFC 2119 [2].
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2. Different 6to4 Forwarding Scenarios
It should be noted that when communicating 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 considers every other 6to4
router it wants to talk to to be "on-link" (with IPv4 as the
link-layer).
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 (advertising
route to 2002::/16) 6to4 relay. 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.
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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 using
6to4 IPv4 Anycast [3]. The relay will decapsulate the packet and
forward it to native IPv6 Internet, the same way as any other IPv6
packet.
Note that destination IPv6 address in the packet is a non-6to4
address, and is assumed to be 2001:db8::1 in the example.
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
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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 [5] is used between 6to4 relay routers and 6to4
routers.
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, as described in Section 6.2.1 in this
memo.
Neither of these models are known to be used at the time of writing;
this is probably caused by the fact that parties that need 6to4 are
those that are not able to run BGP, and because setting up these
sessions would be much more work for relay operators.
2.4.2 6to4 as an Optimization Method
Some sites seem to use 6to4 as an IPv6 connectivity "optimization
method"; that is, they have also 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 and not having to use 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 [6] 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.
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.
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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
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 NBMA link-layer for
interconnecting 6to4-enabled sites; with explicit routes, 6to4
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addressing need not be used in 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
that must be done by them. The pseudo-code for the security checks is
provided in Section 5.
This section summarizes the main functions of the various components
that are part 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 acts as the border router of a 6to4 domain. It does
not have a native, global IPv6 address. The main functions of the
6to4 router are:
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.
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, it is not easily distinguishable that
the packet was really received from a 6to4 relay router, not from
a spoofing third party.)
The 6to4 router will also perform security checks on traffic that it
will receive from other 6to4 relays, or 6to4 routers, or from within
the 6to4 site. These checks include:
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o Disallow traffic that has private, broadcast or reserved IPv4
addresses in tunnels, or the matching 6to4 prefixes.
o Disallow traffic from 6to4 routers where the IPv4 tunnel source
address does not match the 6to4 prefix.
o Disallow traffic where the destination IPv6 address is not a
global address; in particular, e.g. 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.
o Discard traffic received from other 6to4 domains via a 6to4 relay
router.
o Discard traffic received for prefixes other than self 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:
o It advertises the reachability of the 2002::/16 prefix to native
IPv6 routing, thus receiving traffic to all 6to4 addresses from
closest native IPv6 nodes.
o Advertise (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 Decapsulate, and forward packets received from 6to4 addresses
through tunneling, using normal IPv6 routing.
o Tunnels packets received through normal IPv6 routing from native
addresses, and are destined for 2002::/16, to the corresponding
6to4 router.
The 6to4 relay will also perform security checks on traffic that it
will receive from 6to4 routers, or from native IPv6 nodes. These
checks are:
o Disallow traffic that has private, broadcast or reserved IPv4
addresses in tunnels, or the matching 6to4 prefixes.
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o Disallow traffic from 6to4 routers where the IPv4 tunnel source
address does not match the 6to4 prefix.
o Disallow traffic where the destination IPv6 address is not a
global address; in particular, e.g. link-local addresses, mapped
addresses and such should not be used. Note, this check might be
incorrect if 6to4 were to 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
that are caused by the 6to4 network. The threats are discussed in
light of the 6to4 deployment models defined in the 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.
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) with the intent of preventing
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 it more difficult to trace the real origin of the attack. 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".
3. Partial implementation of the security checks in the 6to4
implementation. It has been discovered that at least a couple of
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major implementations do not implement all the 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, the IPv4 address blocks 8.0.0.0/24 and 9.0.0.0/24 are only used
for demonstrative purposes, and represent global IPv4 addresses.
Note, one of the mitigation methods listed for various attacks is
based on the premise that 6to4 relays will a have a feature that may
be able to limit traffic to/from specific 6to4 sites. At the time of
writing this document, such a feature is speculation, and more work
needs to be done to determine the logistics of such a feature.
4.1 Attacks on 6to4 Networks
This section describes attacks against 6to4 networks. Attacks which
legerate 6to4 networks, but where the ultimate victim is elsewhere
(e.g., a native IPv6 user, an IPv4 user) are described later in the
memo.
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 is
receiving traffic -- whether it is 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:
1. Attacks with Neighbor Discovery (ND) Messages
2. Spoofing traffic to 6to4 nodes
3. Reflecting traffic from 6to4 nodes
4. Local IPv4 broadcast attack
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4.1.1 Attacks with ND Messages
ATTACK DESCRIPTION
Since the 6to4 router assumes all the other 6to4 routers, and 6to4
relays are "on-link" it is possible to attack the 6to4 router using
ND messages from any node in the IPv4 network, unless a prior trust
relationship has been established.
The attacks are targeting 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 these use link-local addresses.
These attacks are exacerbated in case the implementation supports
more tunneling mechanisms than just 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 [7]. Note that all attacks may not be applicable, as the 6to4
pseudo-interface is assumed not to have a link-layer address (Section
3.8 RFC 2893 [4]). However, one should 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. It 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 (for example, using a separate neighbor cache).
o Either IPsec [4] or an extension of SEND could be used [8] to
secure packet exchange using 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 fixable, this attack is not new as such;
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the same is possible 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 [9], 6to4 provides an easy means which would not
exist without 6to4.
4.1.2 Spoofing Traffic to 6to4 Nodes
ATTACK DESCRIPTION
The attacker - a malicious IPv4 or IPv6 node - can send packets with
spoofed source address to a 6to4 node to accomplish a DoS attack.
The IPv6 and IPv4 addresses of the packets will be similar to:
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 or the real
source address.
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" using one of a number of techniques).
This is a DoS attack on 6to4 nodes.
This attack is similar to ones shown in [10].
EXTENSIONS
Replies to the traffic will be directed to the src_v6 address,
resulting in 6to4 nodes in participating in a reflection DoS. This
attack is described in more detail in Section 4.2.3. That is, the
replies (e.g., TCP SYN ACK, TCP RST, ICMPv6 Echo Reply, input sent to
UDP echo service, ICMPv6 Destination Unreachable, etc.) 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)
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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 the fact that
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. Since traces to the src_v4
address can easily be lost, these attacks can also be be launched
from IPv4 nodes whose connection is ingress-filtered.
However, often this is not a real factor, as usually the attackers
are just zombies and real attackers may not even care if 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 attack serious:
o The attacker may have the ability to choose the relay, and he
might employ the ones best suited for the attacks. Also, some
relays use 192.88.99.1 [3] as the source address making tracing
even more difficult.
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:
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1. Ingress filtering in the native IPv6 networks to prevent packets
with spoofed IPv6 source being transmitted. It would, thus, make
it easy to identify the source of the attack.
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.
However, these mitigation methods do not address the case of IPv4
node sending encapsulated IPv6 packets.
There exists no simple way to prevent such attacks, and longer term
solutions like ingress filtering [11] or itrace [12] have to be
deployed in both IPv6 and IPv4 networks to help identify the source
of the attacks.
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 to be 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 the Section 4.2.2) may be sent to
native IPv6 nodes with the aim of performing 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 look like:
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)
One should note that an attack through the relay is prevented if the
relay implements proper decapsulation security checks (see Section 5
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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 nodes
are involved in sending spoofed traffic with the same src_v6.
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 being launched) will filter packets with forged source
address (with security checks mentioned in Section 5), and thus the
attack will be prevented.
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
The reverse traffic in this case are replies to the messages received
by the 6to4 nodes. The attacker has less control on the packet type
in this case, 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 countered in various ways:
o Implementation of ingress filtering by the IPv4 service providers.
It would prevent forging of the src_v4 address, and would 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 is not
done by the 6to4 router, or if a large number of 6to4 nodes, and/
or a large number of IPv4 nodes are participating in the attack.
o Implementation of ingress filtering by all IPv6 service providers
would eliminate this attack, because src_v6 could not be spoofed
to be 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 the attack, when
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launched from an IPv4 node, except when the IPv4 source address
was also spoofed -- but then the attacker would have been able to
just 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
COMPARISON TO SITUATION WITHOUT 6to4
This particular attack can be mitigated by proper implementation of
security checks and ingress filtering; where ingress filtering is not
implemented, it's typically easier to attack directly than through
reflection -- unless "traffic laundering" is an explicit goal in 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 it tries to send encapsulated IPv6 packets to a
local broadcast address, or a multicast address. This threat is
mentioned in the specification [1].
There practically two kinds of attacks: where a local 6to4 user tries
to send packets to the address corresponding to the broadcast
address, or when someone is able to do that remotely.
In the first option, assume that 9.0.0.255 is the 6to4 router's
broadcast address. After receiving the packet with a destionation
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 will 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
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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, for example Hop Limit
Exceeded or Destination Unreachable. The packet would be as follows:
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
been (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 to an IPv6 address that embeds an invalid IPv4 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's 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 which haven't been secured as described in Section 5.
4.2 Attacks on Native IPv6 Internet
This section describes attacks against native IPv6 Internet which
leverage 6to4 architecture somehow. Attacks against 6to4 nodes were
described in the previous section.
Native IPv6 nodes can be accessed by 6to4 and IPv4 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
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perform for general safety: when decapsulating IPv4 packets, check
that 2002:V4ADDR::/48 and V4ADDR match. If this is not done, several
threats become more serious; in the following analysis, it is assumed
that such checks are implemented.
6to4 relay should not relay packets between 6to4 addresses. In
particular, packets decapsulated from 6to4 routers should not be
encapsulated again towards 6to4 routers, as described in rules in
Section 5. Similarly, packets with 6to4 source and destination
address 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 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
spoofed (or not spoofed) 6to4 source address to a native IPv6 node to
accomplish a DoS attack.
The threat is similar as the one involving 6to4 routers as described
in Section 4.1.2.
The difference here is that the attack is initiated by IPv4 nodes, or
6to4 nodes. The source IPv6 address may or may not be spoofed. Note,
as mentioned above, the relay is assumed to correlate source IPv4
address with the address embedded in the source IPv6 address during
decapsulation. A side effect is that all the spoofed traffic will
have a 6to4 source address.
EXTENSIONS
Spoofed traffic may also be sent to native IPv6 nodes by either other
native IPv6 nodes, or 6to4 nodes, or 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 the
fact that the replies from the 6to4 node to the spoofed address will
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be lost.
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
The Denial-of-Service attack based on traffic spoofing is not new;
the only twist comes from the fact 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. Since traces to the
src_v4 address can easily be lost, these attacks can also be be
launched from IPv4 nodes whose connection is ingress-filtered.
These attacks might be not be very easy to perform, and might be
hampered because of:
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 if source V4ADDR is same as
the one embedded in the source IPv6 address. Thus, 6to4 nodes
will be forced to use the correct IPv6 prefix while lauching
attack, and it is 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 which may be implemented, and it
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:
1. Ingress filtering in the IPv4 Internet to prevent packets with
spoofed IPv4 source being transmitted. As the relay checks that
the 6to4 address embeds the IPv4 address, no spoofing can be
achieved done unless IPv4 addresses can be spoofed.
2. Security checks in the 6to4 relay. The 6to4 relay must drop
traffic (from 6to4 nodes, or IPv4 nodes) that has non-6to4
addresses as source address, or where the source IPv4 address
does not match the address embebdded in the source IPv6 address.
COMPARISON TO SITUATION WITHOUT 6to4
Compared to Section 4.1.2, which is more serious, this threat appears
to be slightly more manageable. If the relays perform proper
decapsulation checks, the spoofing can only be achived, to a 6to4
source address, when 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 the one involving 6to4
routers as described in Section 4.1.3. Traffic may be reflected off
native IPv6 nodes, or 6to4 nodes. The attack can be initiated by
either:
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 similar attacks to the ones
by IPv4 nodes, but that would require spoofing of the source
address at the 6to4 site before encapsulation; that is likely to
be difficult.
When launched from a native IPv6 node, the traffic goes through 6to4
relays twice, both after and before 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:
1. Attacks from the native IPv6 nodes could be stopped by
implementing ingress filtering in the IPv6 Internet.
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 address in the 6to4 router.
3. Attacks from 6to4 nodes in other sites can be stopped if the 6to4
router in those sites implements egress filtering.
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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 thought there are means to mitigate the attack, it 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 intent of the attack is just to DoS the
victim, it can be achieved more smoothly by doing it directly (as the
source address spoofing was available as well).
Therefore, the threat seems to be higher to the availability and
stability of the 6to4 relay system itself than to 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.
o IPv4 nodes that may send traffic with spoofed source IP address
(to be the relay's broadcast address) to elicit replies (e.g.,
ICMPv6 Hop Limit Exceeded messages) from the 6to4 relay to its
local nodes.
The first is more dangerous than in Section 4.1.4 because it can be
initiated by any IPv6 node (which is allowed to use the relay, that
is), not limited to local users.
The second is trickier and not really useful. For it to succeed, the
relay would have to accept native source addresses over the 6to4
pseudo-interface (but we did not assume this check was implemented),
as if coming from another relay, and trigger an ICMPv6 message to the
relay's local IPv4 subnet. The former method is more lucrative.
EXTENSIONS
None.
THREAT ANALYSIS AND SOLUTIONS/MITIGATION METHODS
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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 done 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. (Some other routing tricks like 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 usage of the actual IPv4 address of
the relay instead of 192.88.99.1.
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) which do not have the the 192.88.99.1 as the
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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 (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 things one could do
here with it would be to select the relay; some generic threats about
Routing Header use are described in [10].
As this threat does not have implications on 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
There are several attacks which 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 already pointed out in Section 4.2, the IPv4 source
address used by the relay could, cursorily looking, be seen as the
source of these "protocol-41" attacks.
This could cause a number of concerns for the operators deploying
6to4 relay service. For example:
o Getting contacted a lot (via email, phone, fax, or lawyers) on
suspected "abuse",
o Getting 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 to be blacklisted in
some "spammer databases", if the relay would be used for those
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purposes.
This threat seems slightly similar (but more generic) to the outburst
of SMTP abuse caused by open relays.
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 that should not cause
problems to the other operations.
Further, when someone is filing complaints to the owner of
192.88.99.0/24, they notice multiple WHOIS records and see a pointer
to [3], and may learn that the 6to4 relay is in fact innocent. Of
course, this could result in these reports going to the closest
anycast 6to4 relay as well, which in fact had nothing to do with the
incident.
However, the wide-spread 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 the use of 192.88.99.1 as the
source address.
4.3 Attacks on IPv4 Internet
There are two types of attacks on the IPv4 internet - spoofed
traffic, and reflection. They can be initiated by native IPv6 nodes,
6to4 nodes, and IPv4 nodes.
Attacks initiated by IPv4 nodes that send spoofed traffic that will
not utilize the 6to4 infrastructure are considered out of scope of
this document. 6to4 infrastructure may be utilized in reflection
attacks that are 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.
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Such attacks can easily be thwarted by implementing protocol-41
filtering in IPv4 nodes or sites that do not implement IPv6 over IPv4
tunneling. Such filters should not be made permanent, as they would
act as a hurdle if IPv6 over IPv4 tunneling mechanisms were ever to
be implemented by the IPv4 node or site.
XXX: do these need to be spelled out, as in previous sections?
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
o ToA. Type of Attack
* D - DoS
* R - Reflection DoS
* T - Theft of Service
o Fix. Specified who is responsible for fixing the attack.
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* 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 8
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 9
Notes:
1) This attack is a side-effect of the other attacks, and thus does
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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 10
5. Implementing Proper Security Checks in 6to4
In this section, several ways to implement the security checks
required or implied by the specification [1] or augmented by this
memo are described. These do not, in general, protect against the
majority of the threats listed above in the "Threat Analysis"
section. They're just prerequisites for a relatively safe and simple
6to4 implementation.
Note that in 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 e.g., when the
interface is brought up, but at least in February 2004, no
implementations were known to do that. Due to that, the checks are
described as one -- which works independent 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.
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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
if prefix (dst_v6) == 2002::/16
dst_v4 SHOULD NOT be assigned to the router
fi
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)
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 included 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 [13], and others widely used and known not to be global. These
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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.
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.
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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 are enabled by default, it's
recommended that there is 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 its unicast IPv4 addresses, and
when destination address is known to be a local broadcast address, it
does not try to encapsulate and send packets to it. (see Section
4.1.4, and Section 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 for the access
lists are in place. In practice it seems that this could not be done
effectively enough unless the access list mechanism is able to parse
the encapsulated packets.
6. Issues in 6to4 Implementation and Use
This section tries to give an overview of some of the problems 6to4
implementations are faced with, 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
[14] and Automatic Tunneling using Compatible Addresses [4]. All of
these use IP-IP (protocol 41) [15] 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:
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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 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 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 it can be deduced which logical tunnel interface is in
the question.
Solutions for this include 1) not using more than one automatic
tunneling mechanism in a node or 2) binding different mechanisms to
different IPv4 addresses.
6.2 Anyone Pretending to Be a 6to4 Relay
Even though this was already discussed in Section 4.1.2, it bears
some additional elaboration as it was the only problem which cannot
be even partially solved.
That is, 6to4 routers receive traffic from non-6to4 ("native")
sources via 6to4 relays. 6to4 routers have no way of matching IPv4
source address of the relay with non-6to4 IPv6 address of the source.
Consequently, anyone can spoof any non-6to4 IPv6 address he wants by
sending traffic, encapsulated, directly to 6to4 routers.
Two different models of thinking have been proposed to fix this
problem if it is considered to be important:
o Every 6to4 relay must participate in an eBGP multi-hop peering
mesh (which can be hierarchical); it would be used to advertise
more specific routes.
o The 6to4 usage model would be turned upside-down, and the
deployment of 6to4 would be have to be borne by native IPv6 users.
These are described at a bit more length below.
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6.2.1 Limited Distribution of More Specific Routes
If 6to4 prefixes more specific than 2002::/16 could be advertised,
the traffic model between native to 6to4 and 6to4 to native could be
changed so that only one 6to4 relay would always be used, most often
the one closest to the 6to4 router.
This model was rejected in the base specification, as IPv6 routing
table was not to be polluted by 6to4 prefixes derived of IPv4
prefixes.
However, the problem could be avoided with some effort: creating a
separate, possibly hierarchical based on IPv6 connections, peering
mesh for 6to4 relay routers. This could be done by forming eBGP [5]
multi-hop peerings between 6to4 relays, and advertising more specific
routes (e.g., the same superblocks of IPv4 addresses one expects to
service) to all the other routers.
In that way, the global routing table would not be impacted at all.
This seems to have at least three potential issues:
1. Every 6to4 relay should be part of this (if one wants to have
some extra safety that the addresses haven't been spoofed),
2. The route from a native source takes the path to the first 6to4
relay, and there (possibly through other Relays) to the last 6to4
relay to tunnel the packet to the 6to4 router; this adds at least
some latency, and
3. The mechanism of redistributing IPv4 6to4 client addresses to
other relays as 6to4 prefixes needs work.
Of these, only the last requires more discussion. It could be argued
that this advertising should either be manually configured once
(i.e., relatively static prefixes, or generated from IPv4
route-objects in RADB [16] or generated automatically (e.g., when a
6to4 router first sends a "triggering" packet through the 6to4
relay). Of course, this data could even be derived in some form from
the IPv4 routing tables. Further analysis on this is TBD if
necessary.
Even if the traffic to 6to4 routers is limited to few relays, other
IPv4 nodes can still spoof both IPv4, and IPv6 address and perform
the spoofing attack. Hence, a necessary step is to use strong
cryptography-based mechanisms to ensure the 6to4 router - 6to4 relay
relationship. Alternatively, some sort of infrastructure (e.g.,
small-scale PKI) would have to be established which would have to
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include all the possible 6to4 relays in the Internet.
6.2.2 A Different Model for 6to4 Deployment
It could be possible to turn the deployment assumptions of 6to4
around a bit to eliminate some threats caused by untrusted 6to4
relays. That is:
o Every dual-stack site (or even ISP) would be required to have
their own 6to4 relay. That is, there would not be third-party
relays, and the 2002::/16 route would not need to be advertised
globally, and
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 ISP's should be
familiar with already.
However, this has a number of problems:
This model would shift the majority of burden of supporting 6to4 to
IPv6 sites which 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 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 deployed also 6to4 addresses on the nodes (alongside with
native addresses, if necessary), which in turn would change 6to4 to
operate without relays completely.
7. Security Considerations
This draft discusses security considerations of 6to4.
Even if proper checks are implemented, there are a large number of
different kind of security threats; these threats are analyzed in
Section 4.
There are mainly three 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
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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, or
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, and
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
gave the author 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 of relay spoofing problems. Brian Carpenter reminded of the
BGP-based 6to4 router model. Christian Huitema gave a push to a more
complete threat analysis. Itojun Hagino spelled out the operators'
fears about 6to4 relay abuse. Rob Austein brought up the idea of a
different 6to4 deployment model.
In the latter phase, especially discussions with Christian Huitema,
Brian Carpenter and Alain Durand were helpful when improving the
document.
David Malone and [your name could be here] gave feedback on the
document.
Normative References
[1] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4
Clouds", RFC 3056, February 2001.
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[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.
Informative References
[4] Gilligan, R. and E. Nordmark, "Transition Mechanisms for IPv6
Hosts and Routers", RFC 2893, August 2000.
[5] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[6] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[7] Nikander, P., "IPv6 Neighbor Discovery trust models and
threats", draft-ietf-send-psreq-04 (work in progress), October
2003.
[8] Arkko, J., "SEcure Neighbor Discovery (SEND)",
draft-ietf-send-ndopt-03 (work in progress), January 2004.
[9] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms for
IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-02 (work in
progress), February 2004.
[10] Savola, P., "Security of IPv6 Routing Header and Home Address
Options", draft-savola-ipv6-rh-ha-security-02 (work in
progress), March 2002.
[11] 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.
[12] Bellovin, S., Leech, M. and T. Taylor, "ICMP Traceback
Messages", draft-ietf-itrace-04 (work in progress), February
2003.
[13] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.
[14] Templin, F., Gleeson, T., Talwar, M. and D. Thaler, "Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP)",
draft-ietf-ngtrans-isatap-18 (work in progress), February 2004.
[15] Simpson, W., "IP in IP Tunneling", RFC 1853, October 1995.
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[16] Merit Network Inc., "Routing Assets Database (http://
www.radb.net)".
[17] Hagino, J., "Possible abuse against IPv6 transition
technologies", draft-itojun-ipv6-transition-abuse-01.txt
(expired, work-in-progress) , July 2000.
[18] Barros, C., "Proposal for ICMP Traceback Messages", http://
www.research.att.com/lists/ietf-itrace/2000/09/msg00044.html .
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
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, packet
sent by RA to RB is like (note: addresses from 8.0.0.0/24 are just
examples of global IPv4 addresses):
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:
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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 a lot of
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 native address is possible even
with the security checks, by having the sender node pretend to be a
6to4 relay router.
More worries come in to 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 = ...
Some implementations might have been careful enough to have designed
the stack to 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 ...
Appendix B. Change Log
[[ RFC-EDITOR note: to be removed before publication ]]
Changes from -00 to -01
1. Lots of editorial changes in other sections
2. Revamped the "Threat Analysis" section completely; ripple the
effects elsewhere in the document as well.
3. Added Chirayu Patel as a co-author.
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