RFC5909: Securing Neighbor Discovery Proxy: Problem Statement
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Related keywords:
(Secure Neighbor Discovery)
(Send)
Internet Engineering Task Force (IETF) J-M. Combes
Request for Comments: 5909 France Telecom Orange
Category: Informational S. Krishnan
ISSN: 2070-1721 Ericsson
G. Daley
Netstar Logicalis
July 2010
Securing Neighbor Discovery Proxy: Problem Statement
Abstract
Neighbor Discovery Proxies are used to provide an address presence on
a link for nodes that are no longer present on the link. They allow
a node to receive packets directed at its address by allowing another
device to perform Neighbor Discovery operations on its behalf.
Neighbor Discovery Proxy is used in Mobile IPv6 and related protocols
to provide reachability from nodes on the home network when a Mobile
Node is not at home, by allowing the Home Agent to act as proxy. It
is also used as a mechanism to allow a global prefix to span multiple
links, where proxies act as relays for Neighbor Discovery messages.
Neighbor Discovery Proxy currently cannot be secured using Secure
Neighbor Discovery (SEND). Today, SEND assumes that a node
advertising an address is the address owner and in possession of
appropriate public and private keys for that node. This document
describes how existing practice for proxy Neighbor Discovery relates
to SEND.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5909.
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. IPv6 Mobile Nodes and Neighbor Discovery Proxy . . . . . . 4
2.2. IPv6 Fixed Nodes and Neighbor Discovery Proxy . . . . . . 6
2.3. Bridge-Like ND Proxies . . . . . . . . . . . . . . . . . . 6
3. Proxy Neighbor Discovery and SEND . . . . . . . . . . . . . . 9
3.1. CGA Signatures and Proxy Neighbor Discovery . . . . . . . 9
3.2. Non-CGA Signatures and Proxy Neighbor Discovery . . . . . 10
3.3. Securing Proxy DAD . . . . . . . . . . . . . . . . . . . . 11
3.4. Securing Router Advertisements . . . . . . . . . . . . . . 11
4. Potential Approaches to Securing Proxy ND . . . . . . . . . . 12
4.1. Secured Proxy ND and Mobile IPv6 . . . . . . . . . . . . . 12
4.1.1. Mobile IPv6 and Router-Based Authorization . . . . . . 13
4.1.2. Mobile IPv6 and Per-Address Authorization . . . . . . 13
4.1.3. Cryptographic-Based Solutions . . . . . . . . . . . . 13
4.1.4. Solution Based on the 'Point-to-Point' Link Model . . 14
4.2. Secured Proxy ND and Bridge-Like Proxies . . . . . . . . . 14
4.2.1. Authorization Delegation . . . . . . . . . . . . . . . 14
4.2.2. Unauthorized Routers and Proxies . . . . . . . . . . . 14
4.2.3. Multiple Proxy Spans . . . . . . . . . . . . . . . . . 15
4.2.4. Routing Infrastructure Delegation . . . . . . . . . . 15
4.2.5. Local Delegation . . . . . . . . . . . . . . . . . . . 16
4.2.6. Host Delegation of Trust to Proxies . . . . . . . . . 17
4.3. Proxying Unsecured Addresses . . . . . . . . . . . . . . . 17
5. Two or More Nodes Defending the Same Address . . . . . . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6.1. Router Trust Assumption . . . . . . . . . . . . . . . . . 19
6.2. Certificate Transport . . . . . . . . . . . . . . . . . . 19
6.3. Timekeeping . . . . . . . . . . . . . . . . . . . . . . . 19
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
8.1. Normative References . . . . . . . . . . . . . . . . . . . 20
8.2. Informative References . . . . . . . . . . . . . . . . . . 21
1. Introduction
Neighbor Discovery Proxy is defined in IPv6 Neighbor Discovery
[RFC4861]. It is used in networks where a prefix has to span
multiple links [RFC4389] but also in Mobile IPv6 [RFC3775] (and so in
Mobile-IPv6-based protocols like Network Mobility (NEMO) [RFC3963],
Fast Handovers for Mobile IPv6 (FMIPv6) [RFC5568], or Hierarchical
Mobile IPv6 (HMIPv6) [RFC5380]) and in the Internet Key Exchange
Protocol (IKE) version 2 (IKEv2) [RFC4306]. It allows a device that
is not physically present on a link to have another advertise its
presence, and forward packets to the off-link device.
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Neighbor Discovery Proxy relies upon another device, the proxy, to
monitor for Neighbor Solicitations (NSs), and answer with Neighbor
Advertisements (NAs). These proxy Neighbor Advertisements direct
data traffic through the proxy. Proxied traffic is then forwarded to
the end destination.
2. Scenarios
This section describes the different scenarios where the interaction
between Secure Neighbor Discovery (SEND) and ND Proxy raises issues.
2.1. IPv6 Mobile Nodes and Neighbor Discovery Proxy
The goal of IPv6 mobility is to allow nodes to remain reachable while
moving around in the IPv6 Internet. The following text is focused on
Mobile IPv6 but the issue raised by the interaction between SEND and
ND Proxy may be the same with Mobile IPv6 based protocols (e.g.,
NEMO, HMIPv6).
For Mobile IPv6 Mobile Nodes (MNs), it is necessary to keep existing
sessions going or to allow new sessions even when one leaves the home
network.
In order to continue existing sessions, when nodes are present on the
home link, the Proxy (i.e., the Home Agent in Mobile IPv6) sends an
unsolicited NA to the all-nodes multicast address on the home link as
specified [RFC3775].
For new sessions, the Proxy, which listens to the MN's address
responds with a Neighbor Advertisement that originates at its own
IPv6 address and has the proxy's address as the Target Link-Layer
Address, but contains the absent mobile in the Target Address field
of the Neighbor Advertisement. In this case, SEND cannot be applied
because the address in the Target Address field is not the same as
the one in the Source Address field of the IP header.
As seen in Figure 1, solicitors send a multicast solicitation to the
solicited nodes multicast address (based on the unicast address) of
the absent node (a mobile node that is away from the home link).
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Absent Mobile Proxy Solicitor
NS:SL3=S,DL3=Sol(A),TA=A
+-----+ SL2=s,DL2=sol(a),SLL=s
| |<================
| |
| |================>
+-----+ NA:SL3=P,DL3=S,TA=A,
SL2=p,DL2=s,TLL=p
Legend:
SL3: Source IPv6 Address NS: Neighbor Solicitation
DL3: Destination IPv6 Address NA: Neighbor Advertisement
SL2: Source Link-Layer Address RS: Router Solicitation
DL2: Destination Link-Layer Address RA: Router Advertisement
TA: Target Address
SLL/TLL: Source/Target Link-Layer Address Option
Figure 1
While at home, if the MN has configured Cryptographically Generated
Addresses (CGAs) [RFC3972], it can secure establishment by its on-
link neighbors of Neighbor Cache Entries (NCEs) for its CGAs by using
SEND [RFC3971]. SEND security requires a node sending Neighbor
Advertisements for a given address to be in possession of the public/
private key pair that generated the address.
When an MN moves away from the home link, a proxy has to undertake
Neighbor Discovery signaling on behalf of the MN. In Mobile IPv6,
the role of the proxy is undertaken by the Home Agent. While the
Home Agent has a security association with the MN, it does not have
access to the public/private key pair used to generate the MN's CGA.
Thus, the Home Agent acting as an ND proxy cannot use SEND for the
address it is proxying [RFC3971].
When an MN moves from the home network to a visited network, the
proxy will have to override the MN's existing Neighbor Cache Entries
that are flagged as secure [RFC3971]. This is needed for the Home
Agent to intercept traffic sent on-link to the MN that would
otherwise be sent to the MN's link-layer address.
With the current SEND specification, any solicitation or
advertisement sent by the proxy will be unsecure and thus will not be
able to update the MN's NCE for the home address because it is
flagged as secured. These existing Neighbor Cache Entries will only
time-out after Neighbor Unreachability Detection [RFC4861] concludes
the Home Address is unreachable at the link layer recorded in the
NCE.
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Where secured proxy services are not able to be provided, a proxy's
advertisement may be overridden by a rogue proxy without the
receiving host realizing that an attack has occurred. This is
identical to what happens in a network where SEND is not deployed.
2.2. IPv6 Fixed Nodes and Neighbor Discovery Proxy
This scenario is a sub-case of the previous one. In this scenario,
the IPv6 node will never be on the link where the ND messages are
proxied. For example, an IPv6 node gains remote access to a network
protected by a security gateway that runs IKEv2 [RFC4306]. When a
node needs an IP address in the network protected by a security
gateway, the security gateway assigns an address dynamically using
Configuration Payload during IKEv2 exchanges. The security gateway
then needs to receive packets sent to this address; one way to do so
would be to proxy ND messages.
2.3. Bridge-Like ND Proxies
The Neighbor Discovery (ND) Proxy specification [RFC4389] defines an
alternative method to classic bridging. Just as with classic
bridging, multiple link-layer segments are bridged into a single
segment, but with the help of proxying at the IP layer rather than
link-layer bridging. In this case, the proxy forwards messages while
modifying their source and destination MAC addresses, and it rewrites
their solicited and override flags and Link-Layer Address Options.
This rewriting is incompatible with SEND signed messages for a number
of reasons:
o Rewriting elements within the message will break the digital
signature.
o The source IP address of each packet is the packet's origin, not
the proxy's address. The proxy is unable to generate another
signature for this address, as it doesn't have the CGA private key
[RFC3971].
Thus, proxy modification of SEND solicitations may require sharing of
credentials between the proxied node and the proxying node or
creation of new options with proxying capabilities.
While bridge-like ND proxies aim to provide as little interference
with ND mechanisms as possible, SEND has been designed to prevent
modification or spoofing of advertisements by devices on the link.
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Of particular note is the fact that ND Proxy performs a different
kind of proxy Neighbor Discovery to Mobile IPv6 [RFC3775] [RFC4389].
RFC 3775 (Mobile IPv6) specifies that the Home Agent as proxy sends
Neighbor Advertisements from its own address with the Target Address
set to the absent Mobile Node's address. The Home Agent's own link-
layer address is placed in the Target Link-Layer Address Option
[RFC3775]. On the other hand, ND Proxy resends messages containing
their original address, even after modification (i.e., the IP source
address remains the same) [RFC4389]. Figure 2 describes packet
formats for proxy Neighbor solicitation and advertisement as
specified by RFC 4389.
Advertiser Proxy Solicitor
NS:SL3=S,DL3=Sol(A),TA=A, NS:SL3=S,DL3=Sol(A),TA=A,
SL2=p,DL2=sol(a),SLL=p +-----+ SL2=s,DL2=sol(a),SLL=s
<==================| |<================
| |
==================>| |================>
NA:SL3=A,DL3=S,TA=A, +-----+ NA:SL3=A,DL3=S,TA=A
SL2=a,DL2=p,TLL=a SL2=p,DL2=s,TLL=p
Legend:
SL3: Source IPv6 Address NS: Neighbor Solicitation
DL3: Destination IPv6 Address NA: Neighbor Advertisement
SL2: Source Link-Layer Address
DL2: Destination Link-Layer Address
TA: Target Address
SLL/TLL: Source/Target Link-Layer Address Option
Figure 2
In order to use the same security procedures for both ND Proxy and
Mobile IPv6, changes may be needed to the proxying procedures in
[RFC4389], as well as changes to SEND.
An additional (and undocumented) requirement for bridge-like proxying
is the operation of router discovery. Router discovery packets may
similarly modify Neighbor Cache state, and require protection from
SEND.
In Figure 3, the router discovery messages propagate without
modification to the router address, but elements within the message
change. This is consistent with the description of Neighbor
Discovery above.
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Advertiser Proxy Solicitor
RS:SL3=S,DL3=AllR, RS:SL3=S,DL3=AllR,
SL2=p,DL2=allr,SLL=p +-----+ SL2=s,DL2=allr,SLL=s
<==================| |<================
| |
==================>| |================>
RA:SL3=A,DL3=S, +-----+ RA:SL3=A,DL3=S,
SL2=a,DL2=p,SLL=a SL2=p,DL2=s,SLL=p
Legend:
SL3: Source IPv6 Address RS: Router Solicitation
DL3: Destination IPv6 Address RA: Router Advertisement
SL2: Source Link-Layer Address
DL2: Destination Link-Layer Address
TA: Target Address
SLL/TLL: Source/Target Link-Layer Address Option
Figure 3
Once again, these messages may not be signed with a CGA signature by
the proxy, because it does not own the source address.
Additionally, Authorization Delegation Discovery messages need to be
exchanged for bridge-like ND proxies to prove their authority to
forward. Unless the proxy receives explicit authority to act as a
router, or the router knows of its presence, no authorization may be
made. This explicit authorization requirement may be at odds with
the zero configuration goal of ND proxying [RFC4389].
An alternative (alluded to in an appendix of ND Proxy [RFC4389])
suggests that the proxy send Router Advertisements (RAs) from its own
address. As described by ND Proxy, this is insufficient for
providing proxied Neighbor Advertisement service, but may be matched
with Neighbor solicitation and advertisement services using the
proxy's source address in the same way as Mobile IPv6 [RFC4389]
[RFC3775]. This means that all router and Neighbor advertisements
would come from the proxied address, but may contain a target address
that allows proxied Neighbor presence to be established with peers on
other segments. Router discovery in this case has the identity of
the original (non-proxy) router completely obscured in router
discovery messages.
The resultant proxy messages would have no identifying information
indicating their origin, which means that proxying between multiple
links would require state to be stored on outstanding solicitations
(effectively a ND only NAT). This level of state storage may be
undesirable.
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Mobile IPv6 does not experience this issue when supplying its own
address, since ND messages are never forwarded on to the absent node
(the Home Agent having sufficient information to respond itself).
Authorization from a router may still be required for Router
Advertisement, and will be discussed in Section 4.2.
3. Proxy Neighbor Discovery and SEND
There are currently no existing secured Neighbor Discovery procedures
for proxied addresses, and all Neighbor Advertisements from SEND
nodes are required to have equal source and target addresses, and be
signed by the transmitter (Section 7.4 of [RFC3971]).
Signatures over SEND messages are required to be applied on the CGA
source address of the message, and there is no way of indicating that
a message is proxied.
Even if the message is able to be transmitted from the original
owner, differences in link-layer addressing and options require
modification by a proxy. If a message is signed with a CGA-based
signature, the proxy is unable to regenerate a signature over the
changed message as it lacks the keying material.
Therefore, a router wishing to provide proxy Neighbor Advertisement
service cannot use existing SEND procedures on those messages.
A host may wish to establish a session with a device that is not on-
link but is proxied. As a SEND host, it prefers to create Neighbor
Cache Entries using secured procedures. Since SEND signatures cannot
be applied to an existing proxy Neighbor Advertisement, it must
accept non-SEND advertisements in order to receive proxy Neighbor
Advertisements.
Neighbor Cache spoofing of another node therefore becomes trivial, as
any address may be proxy-advertised to the SEND node, and overridden
only if the node is there to protect itself. When a node is present
to defend itself, it may also be difficult for the solicitor
determine the difference between a proxy-spoofing attack, and a
situation where a proxied device returns to a link and overrides
other proxy advertisers [RFC4861].
3.1. CGA Signatures and Proxy Neighbor Discovery
SEND defines one public-key and signature format for use with
Cryptographically Generated Addresses (CGAs) [RFC3972]. CGAs are
intended to tie address ownership to a particular public/private key
pair.
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In SEND as defined today, Neighbor Discovery messages (including the
IP Addresses from the IPv6 header) are signed with the same key used
to generate the CGA. This means that message recipients have proof
that the signer of the message owned the address.
When a proxy replaces the message's source IPv6 address with its own
CGA, the existing CGA option and RSA signature option would need to
be replaced with ones that correspond to the CGA of the proxy. To be
valid according to the SEND specification, the Target Address of the
Neighbor Advertisement message would need to be replaced also to be
equal to the Source Address [RFC3971].
Additional authorization information may be needed to prove that the
proxy is indeed allowed to advertise for the target address, as is
described in Section 4.
3.2. Non-CGA Signatures and Proxy Neighbor Discovery
Where a proxy retains the original source address in a proxied
message, existing security checks for SEND will fail, since fields
within the message will be changed. In order to achieve secured
proxy Neighbor Discovery in this case, extended authorization
mechanisms may be needed for SEND.
SEND provides mechanisms for extension of SEND to non-CGA-based
authorization. Messages are available for Authorization Delegation
Discovery, which is able to carry arbitrary PKIX/X.509 certificates
[RFC5280].
There is, however, no specification of keying information option
formats analogous to the SEND CGA Option [RFC3971]. The existing
option allows a host to verify message integrity by specifying a key
and algorithm for digital signature, without providing authorization
via mechanisms other than CGA ownership.
The digital signature in SEND is transported in the RSA Signature
Option. As currently specified, the signature operation is performed
over a CGA Message type, and allows for CGA verification. Updating
the signature function to support non-CGA operations may be
necessary.
Within SEND, more advanced functions such as routing may be
authorized by certificate path verification using Authorization
Delegation Discovery.
With non-CGA signatures and authentication, certificate contents for
authorization may need to be determined, as outlined in Section 4.
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While SEND provides for extensions to new non-CGA methods, existing
SEND hosts may silently discard messages with unverifiable RSA
signature options (Section 5.2.2 of [RFC3971]), if configured only to
accept SEND messages. In cases where unsecured Neighbor Cache
Entries are still accepted, messages from new algorithms will be
treated as unsecured.
3.3. Securing Proxy DAD
Initiation of proxy Neighbor Discovery also requires Duplicate
Address Detection (DAD) checks of the address [RFC4862]. These DAD
checks need to be performed by sending Neighbor Solicitations, from
the unspecified source address, with the target being the proxied
address.
In existing SEND procedures, the address that is used for CGA tests
on DAD NS is the target address. A Proxy that originates this
message while the proxied address owner is absent is unable to
generate a CGA-based signature for this address and must undertake
DAD with an unsecured NS. It may be possible that the proxy can
ensure that responding NAs are secured though.
Where bridge-like ND proxy operations are being performed, DAD NSs
may be copied from the original source, without modification
(considering they have an unspecified source address and contain no
link-layer address options) [RFC4389].
If non-CGA-based signatures are available, then the signature over
the DAD NS doesn't need to have a CGA relationship to the Target
Address, but authorization for address configuration needs to be
shown using certificates.
In case there is a DAD collision between two SEND nodes on different
interfaces of the proxy, it is possible that the proxy may not have
the authority to modify the NA defending the address. In this case,
the proxy still needs to modify the NA and pass it onto the other
interfaces even if it will fail SEND verification on the receiving
node.
3.4. Securing Router Advertisements
While Router Solicitations are protected in the same manner as
Neighbor Solicitations, the security for Router Advertisements is
mainly based on the use of certificates. Even though the mechanism
for securing RAs is different, the problems that arise due to the
modification of the L2 addresses are exactly the same: the proxy
needs to have the right security material (e.g., certificate) to sign
the RA messages after modification.
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4. Potential Approaches to Securing Proxy ND
SEND nodes already have the concept of delegated authority through
requiring external authorization of routers to perform their routing
and advertisement roles. The authorization of these routers takes
the form of delegation certificates.
Proxy Neighbor Discovery requires a delegation of authority (on
behalf of the absent address owner) to the proxier. Without this
authority, other devices on the link have no reason to trust an
advertiser.
For bridge-like proxies, it is assumed that there is no preexisting
trust between the host owning the address and the proxy. Therefore,
authority may necessarily be dynamic or based on topological roles
within the network [RFC4389].
Existing trust relationships lend themselves to providing authority
for proxying in two alternative ways.
First, the SEND router authorization mechanisms described above
provide delegation from the organization responsible for routing in
an address domain to the certified routers. It may be argued that
routers so certified may be trusted to provide service for nodes that
form part of a link's address range, but are themselves absent.
Devices which are proxies could either be granted the right to proxy
by the network's router, or be implicitly allowed to proxy by virtue
of being an authorized router.
Second, where the proxied address is itself a CGA, the holder of the
public and private keys is seen to be authoritative about the
address's use. If this address owner was able to sign the proxier's
address and public key information, it would be possible to identify
that the proxy is known and trusted by the CGA address owner for
proxy service. This method requires that the proxied address know or
learn the proxy's address and public key, and that the certificate
signed by the proxied node's is passed to the proxy, either while
they share the same link, or at a later stage.
In both methods, the original address owner's advertisements need to
override the proxy if it suddenly returns, and therefore timing and
replay protection from such messages need to be carefully considered.
4.1. Secured Proxy ND and Mobile IPv6
Mobile IPv6 has a security association between the Mobile Node and
Home Agent. The Mobile Node sends a Binding Update to the Home
Agent, to indicate that it is not at home. This implies that the
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Mobile Node wishes the Home Agent to begin proxy Neighbor Discovery
operations for its home address(es).
4.1.1. Mobile IPv6 and Router-Based Authorization
A secured Proxy Neighbor Advertisements proposal based on existing
router trust would require no explicit authorization signaling
between HA and MN to allow proxying. Hosts on the home link will
believe proxied advertisements solely because they come from a
trusted router.
Where the home agent operates as a router without explicit trust to
route from the advertising routing infrastructure (such as in a home,
with a router managed by an ISP), more explicit proxying
authorization may be required, as described in Section 4.2.
4.1.2. Mobile IPv6 and Per-Address Authorization
Where proxy Neighbor Discovery is delegated by the MN to the home
agent, the MN needs to learn the public key for the Home Agent, so
that it can generate a certificate authorizing the public/private key
pair to be used in proxying. It may conceivably do this using
Certificate Path Solicitations either over a home tunnel, when it is
away from home, or during router discovery while still at home
[RFC3971] [RFC3775].
When sending its Binding Update to the HA, the MN would need to
provide a certificate containing the subject's (i.e., proxy HA's)
public key and address, the issuer's (i.e., MN's) CGA and public key,
and timestamps indicating when the authority began and when it ends.
This certificate would need to be transmitted at binding time.
Messaging or such an exchange mechanism would have to be developed.
4.1.3. Cryptographic-Based Solutions
Specific cryptographic algorithms may help to allow trust between
entities of a same group.
This is the case, for example, with ring signature algorithms. These
algorithms generate a signature using the private key of any member
from the same group, but to verify the signature the public keys of
all group members are required. Applied to SEND, the addresses are
cryptographically generated using multiple public keys, and the
Neighbor Discovery messages are signed with an RSA ring signature
[RING]. (Note that the cryptographic algorithms that are the
foundation for [RING] and other similar solutions are not widely
accepted in the security community; additional research is needed
before a Standards Track protocol could be developed.)
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4.1.4. Solution Based on the 'Point-to-Point' Link Model
Another approach is to use the 'Point-to-Point' link model.
In this model, one prefix is provided per MN, and only an MN and the
HA are on a same link. The consequence is the HA no longer needs to
act as ND Proxy.
One way to design such a solution is to use virtual interfaces, on
the MN and the HA, and a virtual link between them. Addresses
generated on the virtual interfaces will only be advertised on the
virtual link. For Mobile IPv6, this results in a virtual Home
Network where the MN will never come back.
4.2. Secured Proxy ND and Bridge-Like Proxies
In link-extension environments, the role of a proxy is more
explicitly separated from that of a router. In SEND, routers may
expect to be authorized by the routing infrastructure to advertise
and may provide this authority to hosts in order to allow them to
change forwarding state.
Proxies are not part of the traditional infrastructure of the
Internet, and hosts or routers may not have an explicit reason to
trust them, except that they can forward packets to regions where
otherwise those hosts or routers could not reach.
4.2.1. Authorization Delegation
If a proxy can convince a device that it should be trusted to perform
proxying function, it may require that device to vouch for its
operation in dealing with other devices. It may do this by receiving
a certificate, signed by the originating device that the proxy is
believed capable of proxying under certain circumstances.
This allows nodes receiving proxied Neighbor Discovery packets to
quickly check if the proxy is authorized for the operation. There
are several bases for such trust, and requirements in proxied
environments, which are discussed below.
4.2.2. Unauthorized Routers and Proxies
Routers may be advertising on networks without any explicit
authorization, and SEND hosts will register these routers if there
are no other options [RFC3971]. While proxies may similarly attempt
to advertise without authority, this provides no security for the
routing infrastructure. Any device can be setup as a SEND proxy/
router so long as it signs its own ND messages from its CGA.
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RFC 5909 SEND ND Proxy: Problem Statement July 2010
This may not help in the case that a proxy attempts to update
Neighbor Cache Entries for a SEND node that moves between links,
since the SEND node's authority to advertise its own CGA address
would not be superseded by a proxy with no credentials.
4.2.3. Multiple Proxy Spans
Proxies may have multiple levels of nesting, which allow the network
to connect between non-adjacent segments.
In this case, authority delegated at one point will have to be
redelegated (possibly in a diluted form) to proxies further away from
the origin of the trust.
Trust Proxy A Proxy B Distant
Origin - T Node - D
+-----+ +-----+
| | | |
+-----+ +-----+ +-----+ +-----+
| | | | | |
------------| |------------| |----------
| | | |
+-----+ +-----+
==========> ==============> ==========>
Deleg(A,T) Deleg(B,Deleg(A,T)) Advertise(D, Deleg(B,
Deleg(A,T))
Figure 4
As shown in Figure 4, the Proxy A needs to redelegate authority to
proxy for T to Proxy B; this allows it to proxy advertisements that
target T back to D.
4.2.4. Routing Infrastructure Delegation
Where it is possible for the proxy to pre-establish trust with the
routing infrastructure, or at least to the local router, it may be
possible to authorize proxying as a function of routing within the
subnet. The router or CA may then be able to certify proxying for
only a subset of the prefixes for which it is itself certified.
If a router or CA provides certification for a particular prefix, it
may be able to indicate that only proxying is supported, so that
Neighbor Cache Entries of routers connected to Internet
infrastructure are never overridden by the proxy, if the router is
present on a segment.
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Hosts understanding such certificates may allow authorized proxies
and routers to override the host when assuming proxy roles, if the
host is absent.
Proxy certificate signing could be done either dynamically (requiring
exchanges of identity and authorization information) or statically
when the network is set up.
4.2.5. Local Delegation
Where no trust tie exists between the authority that provides the
routing infrastructure and the provider of bridging and proxying
services, it may still be possible for SEND hosts to trust the
bridging provider to authorize proxying operations.
SEND itself requires that routers be able to show authorization, but
doesn't require routers to have a single trusted root.
A local bridging/proxying authority trust delegation may be possible.
It would be possible for this authority to pass out local-use
certificates, allowing proxying on a specific subnet or subnets, with
a separate authorization chain to those subnets for the routers with
Internet access.
This would require little modification to SEND, other than the
addition of router-based proxy authority (as in Section 4.2.4), and
proxies would in effect be treated as routers by SEND hosts
[RFC3971]. Distribution of keying and trust material for the initial
bootstrap of proxies would not be provided though (and may be
static).
Within small domains, key management and distribution may be a
tractable problem, so long as these operations are simple enough to
perform.
Since these domains may be small, it may be necessary to provide
certificate chains for trust anchors that weren't requested in
Certificate Path Solicitations, if the proxy doesn't have a trust
chain to any requested trust anchor.
This is akin to 'suggesting' an appropriate trusted root. It may
allow for user action in allowing trust extension when visiting
domains without ties to a global keying infrastructure. In this
case, the trust chain would have to start with a self-signed
certificate from the original CA.
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4.2.6. Host Delegation of Trust to Proxies
Unlike Mobile IPv6, for bridge-like proxied networks, there is no
existing security association upon which to transport proxying
authorization credentials.
Thus, proxies need to convince Neighbors to delegate proxy authority
to them, in order to proxy-advertise to nodes on different segments.
It will be difficult without additional information to distinguish
between legitimate proxies and devices that have no need or right to
proxy (and may want to make two network segments appear connected).
When proxy advertising, proxies must not only identify that proxying
needs to occur, but provide proof that they are allowed to do so, so
that SEND Neighbor Cache Entries may be updated. Unless the
authorization to update such entries is tied to address ownership
proofs from the proxied host or the verifiable routing
infrastructure, spoofing may occur.
When a host received a proxied Neighbor advertisement, it would be
necessary to check authorization in the same way that authorization
delegation discovery is performed in SEND.
Otherwise, certificate transport will be required to exchange
authorization between proxied nodes and proxies.
Proxies would have to be able to delegate this authorization to
downstream proxies, as described in Section 4.2.3.
4.3. Proxying Unsecured Addresses
Where the original Neighbor Discovery message is unsecured, there is
an argument for not providing secured proxy service for that node.
In both the Mobile IPv6 and extended networks cases, the node may
arrive back at the network and require other hosts to map their
existing Neighbor Cache Entry to the node's link-layer address. The
re-arriving node's overriding of link-layer address mappings will
occur without SEND in this case.
It is notable that without SEND protection any node may spoof the
arrival, and effectively steal service across an extended network.
This is the same as in the non-proxy case, and is not made
significantly worse by the proxy's presence (although the identity of
the attacker may be masked if source addresses are being replaced).
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If signatures over the proxied messages were to be used, re-arrival
and override of the Neighbor Cache Entries would have to be allowed,
so the signatures would indicate that at least the proxy wasn't
spoofing (even if the original sender was).
For non-SEND routers, though, it may be possible for secured proxies
to send signed router advertisement messages, in order to ensure that
routers aren't spoofed, and subsequently switched to different parts
of the extended network.
This has problems in that the origin is again unsecured, and any node
on the network could spoof router advertisement for an unsecured
address. These spoofed messages may become almost indistinguishable
(except for the non-CGA origin address) from unspoofed messages from
SEND routers.
Given these complexities, the simplest method is to allow unsecured
devices to be spoofed from any port on the network, as is the case
today.
5. Two or More Nodes Defending the Same Address
All the previous sections of this document focused on the case where
two nodes defend the same address (i.e., the node and the proxy).
However, there are also cases where two or more nodes are defending
the same address. This is at least the case for:
o Nodes having the same address, as the Mobile Access Gateway's
(MAG's) ingress link-local address in Proxy Mobile IPv6 (PMIPv6)
[RFC5213].
o Nodes having a common anycast address [RFC4291].
The problem statement, described previously in this document, applies
for these cases, and the issues are the same from a signaling point
of view.
Multicast addresses are not mentioned here because Neighbor Discovery
Protocol is not used for them.
In the first case, [RFC5213] assumes that the security material used
by SEND (i.e., public-private key pair) is shared between all the
MAGs. For the second case, there is no solution today. But, in the
same way, it should be possible to assume that the nodes having a
common anycast address could also share the security material.
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It is important to notice that when many nodes defending the same
address are not in the same administrative domain (e.g., MAGs in
different administrative domains but in the same PMIPv6 domain
[RFC5213]), sharing the security material used by SEND may raise a
security issue.
6. Security Considerations
6.1. Router Trust Assumption
Router-based authorization for Secured Proxy ND may occur without the
knowledge or consent of a device. It is susceptible to the 'Good
Router Goes Bad' attack described in [RFC3756].
6.2. Certificate Transport
Certificate delegation relies upon transfer of the new credentials to
the proxying HA in order to undertake ND proxy on its behalf. Since
the binding cannot come into effect until DAD has taken place, the
delegation of the proxying authority necessarily predates the return
of the Binding Ack, as described in [RFC3775]. In the case above
described, the home tunnel that comes into creation as part of the
binding process may be required for transport of Certificate Path
Solicitations or Advertisements [RFC3971]. This constitutes a
potential chicken-and-egg problem. Either modifications to initial
home binding semantics or certificate transport are required. This
may be trivial if certificates are sent in the clear between the MN's
Care-of Address (CoA) and the HA without being tunneled.
6.3. Timekeeping
All of the presented methods rely on accurate timekeeping on the
receiver nodes of Neighbor Discovery Timestamp Options.
For router-authorized proxy ND, a Neighbor may not know that a
particular ND message is replayed from the time when the proxied host
was still on-link, since the message's timestamp falls within the
valid timing window. Where the router advertises its secured proxy
NA, a subsequent replay of the old message will override the NCE
created by the proxy.
Creating the NCE in this way, without reference to accurate
subsequent timing, may only be done once. Otherwise, the receiver
will notice that the timestamp of the advertisement is old or doesn't
match.
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One way of creating a sequence of replayable messages that have
timestamps likely to be accepted is to pretend to do an unsecured DAD
on the address each second while the MN is at home. The attacker
saves each DAD defense in a sequence. The granularity of SEND
timestamp matching is around one second, so the attacker has a set of
SEND NAs to advertise, starting at a particular timestamp, and valid
for as many seconds as the original NA gathering occurred.
This sequence may then be played against any host that doesn't have a
timestamp history for that MN, by tracking the number of seconds
elapsed since the initial transmission of the replayed NA to that
victim, and replaying the appropriate cached NA.
Where certificate-based authorization of ND proxy is in use, the
origination/starting timestamp of the delegated authority may be used
to override a replayed (non-proxy) SEND NA, while also ensuring that
the Proxy NA's timestamp (provided by the proxy) is fresh. A
returning MN would advertise a more recent timestamp than the
delegated authority and thus override it. This method is therefore
not subject to the above attack, since the proxy advertisement's
certificate will have a timestamp greater than any replayed messages,
preventing it from being overridden.
7. Acknowledgments
James Kempf and Dave Thaler particularly contributed to work on this
document. Contributions to discussion on this topic helped to
develop this document. The authors would also like to thank Jari
Arkko, Vijay Devarapalli, Mohan Parthasarathy, Marcelo Bagnulo,
Julien Laganier, Tony Cheneau, Michaela Vanderveen, Sean Shen, and
Sheng Jiang for their comments and suggestions.
Jean-Michel Combes is partly funded by MobiSEND, a research project
supported by the French 'National Research Agency' (ANR).
8. References
8.1. Normative References
[RFC3775] Johnson, D., Perkins, C., and J. Arkko, "Mobility Support
in IPv6", RFC 3775, June 2004.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
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RFC 5909 SEND ND Proxy: Problem Statement July 2010
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4389] Thaler, D., Talwar, M., and C. Patel, "Neighbor Discovery
Proxies (ND Proxy)", RFC 4389, April 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
8.2. Informative References
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
[RFC3963] Devarapalli, V., Wakikawa, R., Petrescu, A., and P.
Thubert, "Network Mobility (NEMO) Basic Support Protocol",
RFC 3963, January 2005.
[RFC5213] Gundavelli, S., Leung, K., Devarapalli, V., Chowdhury, K.,
and B. Patil, "Proxy Mobile IPv6", RFC 5213, August 2008.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008.
[RFC5380] Soliman, H., Castelluccia, C., ElMalki, K., and L.
Bellier, "Hierarchical Mobile IPv6 (HMIPv6) Mobility
Management", RFC 5380, October 2008.
[RFC5568] Koodli, R., "Mobile IPv6 Fast Handovers", RFC 5568,
July 2009.
[RING] Kempf, J. and C. Gentry, "Secure IPv6 Address Proxying
using Multi-Key Cryptographically Generated Addresses
(MCGAs)", Work in Progress, August 2005.
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Authors' Addresses
Jean-Michel Combes
France Telecom Orange
38 rue du General Leclerc
92794 Issy-les-Moulineaux Cedex 9
France
EMail: jeanmichel.combes@orange-ftgroup.com
Suresh Krishnan
Ericsson
8400 Decarie Blvd.
Town of Mount Royal
QC Canada
EMail: Suresh.Krishnan@ericsson.com
Greg Daley
Netstar Logicalis
Level 6/616 St Kilda Road
Melbourne, Victoria 3004
Australia
Phone: +61 401 772 770
EMail: hoskuld@hotmail.com
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