Network Working Group G. Daley
Internet-Draft Monash University CTIE
Expires: August 19, 2005 February 18, 2005
Securing Proxy Neighbour Discovery Problem Statement
draft-daley-send-spnd-prob-01.txt
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Abstract
Proxy Neighbour Discovery is used to provide an address presence on a
link from nodes which are no themselves present. It allows a node to
receive packets directed at its address by allowing another device to
neighbour advertise on its behalf.
Proxy Neighbour Discovery 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 neighbour
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discovery messages.
Proxy Neighbour Discovery currently cannot be secured using 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
Neighbour Discovery relates to Secured Neighbour Discovery.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Mobile IPv6 and Proxy Neighbour Discovery . . . . . . . . 3
1.2 Bridge-like ND proxies . . . . . . . . . . . . . . . . . . 4
2. Proxy ND and Mobility . . . . . . . . . . . . . . . . . . . 7
3. Proxy Neighbour Discovery and SEND . . . . . . . . . . . . . 9
3.1 CGA signatures and Proxy Neighbour Discovery . . . . . . . 10
3.2 Non-CGA signatures and Proxy Neighbour Discovery . . . . . 10
3.3 Securing proxy DAD . . . . . . . . . . . . . . . . . . . . 11
4. Potential Approaches to Securing Proxy ND . . . . . . . . . 12
5. Secured Proxy ND and Mobile IPv6 . . . . . . . . . . . . . . 13
5.1 Mobile IPv6 and Router-based authorization . . . . . . . . 13
5.2 Mobile IPv6 and per-address authorization . . . . . . . . 13
6. Secured Proxy ND and Bridge-like proxies . . . . . . . . . . 13
6.1 Authorization Delegation . . . . . . . . . . . . . . . . . 14
6.2 Unauthorized routers and proxies . . . . . . . . . . . . . 14
6.3 Multiple proxy spans . . . . . . . . . . . . . . . . . . . 14
6.4 Routing Infrastructure Delegation . . . . . . . . . . . . 15
6.5 Local Delegation . . . . . . . . . . . . . . . . . . . . . 15
6.6 Host delegation of trust to proxies . . . . . . . . . . . 16
7. Proxying unsecured addresses . . . . . . . . . . . . . . . . 17
8. Summary of San Diego Bar BoF Discussion . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . 18
10.1 Router Trust Assumption . . . . . . . . . . . . . . . . 18
10.2 Certificate Transport . . . . . . . . . . . . . . . . . 18
10.3 Timekeeping . . . . . . . . . . . . . . . . . . . . . . 19
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 19
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1 Normative References . . . . . . . . . . . . . . . . . . . 20
12.2 Informative References . . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . 22
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1. Introduction
Proxy Neighbour Discovery is defined in IPv6 Neighbour Discovery[2].
It is used in Mobile IPv6 [4], and networks where a prefix has to
span multiple links[5]. It allows a device which is not physically
present on a link to have another advertise its presence, and forward
on packets to the off-link device.
Proxy Neighbour Discovery relies upon another device, the proxy, to
monitor for Neighbour Solicitations (NSs), and answer with Neighbour
Advertisements (NAs). These proxy Neighbour Advertisements direct
data traffic through the proxy. Proxied traffic is then forwarded on
to the end destination.
1.1 Mobile IPv6 and Proxy Neighbour Discovery
When moving in the Internet, the aim of Mobile IPv6 is to allow a
device continued packet delivery, whether present on its home network
or not. For Mobile IPv6 Mobile Nodes (MNs), it is necessary to keep
existing sessions going even when one leaves the home network. If a
neighbour is actively delivering packets to a Mobile Node which is at
home, this neighbour will have a valid neighbour cache entry pointed
at the MN's link-layer address on the Home link.
As seen in Figure 1, solicitors send a multicast solicitation to the
solicited nodes address of the absent node (based on the unicast
address).
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: Neighbour Solicitation
SL3: Destination IPv6 Address NA: Neighbour 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
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The Proxy, which listens to this address responds with a Neighbour
Advertisement which 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 Neighbour
Advertisement. In this case, no solicitations are proxied, as the
advertisements originate within the proxy itself.
If Cryptographically Generated Addressing (CGA) is available, the MN
may be able to secure its neighbour cache bindings while at home
using Secured Neighbour Discovery (SEND) [6]. SEND assumes that the
address owner is the advertiser and therefore has access to the keys
required to sign advertisements about the address. Movement away
from the home link requires that a proxy undertake Neighbour
Discovery.
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 as proxy will not have access to the public-private key pair used
to generate the MN's cryptographic address. This prevents Proxy
Neighbour Discovery from using SEND as defined [6].
Where a host moves from the home network to a visited network, the
proxy needs to override existing valid neighbour cache entries which
may have SEND protection. This is needed in order to redirect
traffic to use the proxy's link-layer address, allowing packets to
flow onto the tunnel connecting the Home Agent/Proxy and the MN.
With current specifications, any solicitation or advertisement sent
by the proxy will not be able to update the MN's home address if the
existing NC entry is protected by SEND. Such existing neighbour
cache entries will time-out after Neighbour Unreachability Detection
[2].
Where secured proxy services are not able to be provided, a proxy's
advertisement may be overridden by a bogus proxy without it even
knowing the attack has occurred.
This document describes some of the issues in providing security for
proxy Neighbour Discovery, and how Mobile IPv6 interacts with these
requirements.
1.2 Bridge-like ND proxies
Where proxies exist between two segments, messages may be sent by the
proxy on the far link, in order to gain or pass on neighbour
information. The proxy in this case forwards messages while
modifying their source and destination MAC addresses, and rewrites
their Link-Layer Address Options solicited and override flags. This
is defined in the Bridge Like ND Proxy (NDproxy) draft [5].
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This rewriting is incompatible with SEND signed messages for a number
of reasons:
Rewriting elements within the message will break the digital
signature.
The source IP address of the packets 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
[6].
Proxy modification of SEND solicitations and advertisements require
removal of (at least) CGA and Signature options, and may also need
new options with proxy capabilities if non-CGA signatures are added
to SEND.
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.
Of particular note is the fact that the NDProxy draft performs a
different kind of proxy neighbour discovery to Mobile IPv6 [4][5].
The Mobile IPv6 RFC specifies that the Home Agent as proxy sends
Neighbour 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
[4].
The NDproxy draft resends messages containing their original address,
even after modification [5]. Figure 2 describes packet formats for
proxy neighbour solicitation and advertisement as specified by the
draft.
Advertisor 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
Figure 2
In order to use the same security procedures for both NDProxying and
Mobile IPv6, changes may be needed to the proxying procedures in [5],
as well as changes to SEND.
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An additional (and undocumented) requirement for bridge-like proxying
is the operation of router discovery. Router Discovery packets may
similarly modify neighbour 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 Neighbour
Discovery above.
Advertisor Proxy Solicitor
RS:SL3=S,DL3=AllR, RS:SL3=S,DL3=AllRr,
SL2=p,DL2=sol(A),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
Figure 3
Once again, these messages may not be signed with a CGA signature by
the re-advertisor, because it does not own the source address.
Additionally, multicast Authorization Delegation Discovery ICMPv6
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 zero configuration goal of ND proxying [5].
An alternative (alluded to in an appendix of NDproxy) suggests that
the proxy send Router Advertisements (RAs) from its own address. As
described by NDproxy this is insufficient for providing proxied
Neighbour Advertisement service, but may be matched with neighbour
solicitation and advertisement services using the proxy's source
address in the same way as Mobile IPv6 [5][4]. This means that all
router and neighbour advertisements would come from the proxied
address, but may contain a target address which allows proxied
neighbour 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 an ND only NAT). This level of state storage may be
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undesirable.
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 6.
2. Proxy ND and Mobility
Whenever a mobile device moves off a link and requires another device
to forward packets from that address to the MN's new location, proxy
Neighbour Discovery is required.
In the Mobile IPv6 case, where the Mobile Node moves away from home,
a Home Agent needs to be able to override existing neighbour cache
entries in order to redirect packet flow over a tunnel to the Mobile
Node's Care-of-Address (CoA) [4].
In Fast Handovers for Mobile IPv6, local neighbours or routers with
existing valid neighbour cache states need to be told the PAR's
link-layer address when the MN is departing for a new link, or after
arrival on the new link when tunnel forwarding begins[7]. This
allows the MN to maintain reachability to the hosts on that link
until it is able to send Mobile IPv6 Binding signalling subsequent to
address configuration on the new network.
As shown in Figure 4, after the mobile node departs, the Home Agent
or Proxy sends an overriding Neighbour advertisement, in order to
update existing neighbour cache entries.
Absent Mobile Proxy Solicitor
+-----+
Binding Update | |
---------------->| |
or Fast BU | |================>
+-----+ NA:SL3=P,DL3=AllN,TA=A,
SL2=p,DL2=alln,TLL=p
Figure 4
Where the proxy forwards between segments of a network, nodes may
move between segments [5]. For this scenario, the proxy is
responsible for updating neighbour cache entries as incorrect state
is left in them after the move.
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Devices which were on the same segment as the moving node,
subsequently have incorrect neighbour cache state, as they now need
to traverse the proxy to get to the other node. Devices which were
previously being proxied may now be on the same segment as the mobile
node, and may go direct.
As illustrated in Figure 5, the nodes may have incorrect neighbour
cache state, even if the proxy knows of the departure to another
segment.
Mobile Node Proxy Mobile Node - M
(Departed) P (New Location)
+ - - + +-----+ NC:
' ' NC: NC: | | N -> n
+ - - + N -> n+-----+M -> m +-----+
| | | |
------------------| |--------------------
| | |
+-----+NC: +-----+
| |M -> m
+-----+
Existing
Neighbour - N
Figure 5
While neighbour cache state times out, and causes devices to probe
for the location of a peer, long delays may occur before timeouts of
neighbour cache state [2]. In cases where these delays are too long,
the proxy may have to override the neighbour cache entries of hosts
which were previously on the same segment as the moving node.
Those devices now resident on the same segment as the mobile node
will have the proxy's link-layer address in its neighbour cache. In
the NDProxy draft, it is indicated that packets are never forwarded
back to the same segment upon which they arrived (potentially to
prevent forwarding loops)[5].
Similarly, if the mobile node is unaware of its movement, it too may
have incorrect neighbour cache entries for devices which it is now on
the same segment as. This is shown below in Figure 6.
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Mobile Node Proxy Mobile Node - M
(Departed) P (New Location)
+ - - + +-----+ NC:
' ' NC: NC: | | N -> p2
+ - - + +-----+M -> m +-----+
| | |N -> n |
------------------| |--------------------
| | |
P2+-----+P1 +-----+ NC:
| | M -> p1
+-----+
Existing
Neighbour - N
Figure 6
For the remaining duration of their incorrect neighbour cache entry
(up to around 35 seconds), all packets will be dropped. Therefore,
these devices may need to be updated with the present node's
link-layer address.
Procedures regarding updating caches rely upon Section 7.2.6 of IPv6
Neighbour Discovery [2], which allows proxies to neighbour advertise
to all-nodes with the override flag set when becoming a proxy or
addresses change.
For either environment, updates are required to neighbour cache
entries which may be for SEND nodes. These advertisements must
therefore have enough authority to override neighbour cache entries
even though they are secured.
3. Proxy Neighbour Discovery and SEND
There are currently no existing secured Neighbour Discovery
procedures for proxied addresses, and all Neighbour Advertisements
from SEND nodes are required to have equal source and target
addresses, and be signed by the transmitter. (section 7.4 of [6]).
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
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changed message as it lacks the keying material.
Therefore, a router wishing to provide proxy Neighbour Advertisement
service can not use existing SEND procedures on those messages.
A host may wish to establish a session with a device which is not
on-link but is proxied. As a SEND host, it prefers to create
neighbour cache entries using secured procedures. Since SEND
signatures cannot be applied to an existing proxy Neighbour
Advertisement, it must accept non-SEND advertisements in order to
receive proxy Neighbour Advertisements.
Neighbour 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 [2].
3.1 CGA signatures and Proxy Neighbour Discovery
SEND defines one public-key and signature format for use with
Cryptographically Generated Addresses (CGAs) [6]. CGAs are intended
to tie address ownership to a particular Public/Private key pair.
In SEND as defined today, Neighbour 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.
Where a proxy replaces the message source with its own CGA, the
existing CGA option and RSA signature option need to be replaced with
the proxy's. Such a message will validate using SEND, except that
the Target Address field will not match the IPv6 Source Address in
Neighbour Advertisements [6].
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 Neighbour Discovery
Where a proxy retains the original source address in a proxied
message, existing SEND-CGA checks will fail, since fields within the
message will be changed. In order to achieve secured proxy neighbour
discovery in this case, new signature authentication mechanisms may
be needed for SEND.
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SEND provides interfaces 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
[9].
There is no specification though of keying information option formats
analogous to the SEND CGA Option [6]. The existing option allows a
host to verify message integrity by specifying a key and algorithm
for digital signature, without providing authorization for functions
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 infers support for CGA verification.
clarification or changing of this issue for 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.
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 [6]), if configured only to
accept SEND messages. In cases where unsecured neighbour cache
entries are still accepted, messages from new algorithms will be
treated as unsecured.
3.3 Securing proxy DAD
Initiation of Proxy Neighbour Discovery also requires Duplicate
Address Detection (DAD) checks of the address[3]. These DAD checks
need to be performed by sending Neighbour Solicitations, from the
unspecified source address, with the target being the proxied
address.
In existing SEND procedures, the address which is used for CGA tests
on DAD NSs is the target address. A Proxy which 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 NA's are secured though.
Where bridge-like NDproxy operations are being performed, DAD NS's
may be copied from the original source, without modification
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(considering they have an unspecified source address and contain no
link-layer address options)[5]
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. Where SEND-only nodes do not understand
the signature format.
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 Neighbour 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 [5].
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
which 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' 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.
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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.
5. 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
Mobile Node wishes the Home Agent to begin proxy Neighbour Discovery
operations for its home address(es).
5.1 Mobile IPv6 and Router-based authorization
A secured Proxy Neighbour Advertisements proposal based on existing
router trust would require no explicit authorization signalling
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 6.
5.2 Mobile IPv6 and per-address authorization
Where proxy Neighbour 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 either do this
using Certificate Path Solicitations over a home tunnel, over the
Internet, or Router Discovery while still at home [6][4].
When sending its Binding Update to the HA, the MN would need to
provide a certificate containing the subject(proxy-HA)'s public key
and address, the issuer(MN)'s CGA and public key, and timestamps
indicating when the authority began and when it ends. This
certificate would need to be passed near to binding time, possibly in
a Certificate Path Advertisement[6]. Messaging or such an exchange
mechanism would have to be developed.
6. 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 provide this authority to hosts in order to allow them to change
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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 they could not reach.
6.1 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 neighbour 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.
6.2 Unauthorized routers and proxies
Routers advertising on networks without routers may have to operate
with no explicit authorization, and SEND hosts will configure these
if there's no other option [6]. While proxies may similarly attempt
to advertise without authority, this provides no security for the
routing infrastructure. Any device can set up to be a SEND proxy/
router so long as it signs its own ND messages from its CGA.
This may not help in the case that a proxy attempts to update
neighbour cache entries for SEND node which moves between links,
since the SEND node's authority to advertise its own CGA address
would not be superceded by a proxy with no credentials.
6.3 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.
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Trust ProxyA ProxyB Distant
Origin - T Node - D
+-----+ +-----+
| | | |
+-----+ +-----+ +-----+ +-----+
| | | | | |
------------| |------------| |----------
| | | |
+-----+ +-----+
==========> ==============> ==========>
Deleg(A,T) Deleg(B,Deleg(T,A)) Advertise(D, Deleg(B,
Deleg(A,T))
Figure 7
As shown in Figure 7, the Proxy A needs to redelegate authority to
proxy for T to B, this allows it to proxy advertisements back to D,
which target T.
6.4 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 which is itself certified for.
If a router or CA provides certification for a particular prefix, it
may be able to indicate that only proxying is supported, so that
neighbour cache entries of routers connected to internet
infrastructure are never overridden by the proxy, if the router is
present on a segment.
Hosts understanding such certificates may allow authorized proxies
and routers to override host SEND/CGA 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.
6.5 Local Delegation
Where no trust tie exists between the authority which 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.
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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 that for the routers with Internet
access.
This would require little modification to SEND, other than addition
of router based proxy authority (as in Section 6.4), and proxies
would in effect be treated as routers by SEND hosts [6].
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 are simple enough
to perform.
Since these domains may be small, it may be necessary to provide
certificate chains for trust anchors which 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.
6.6 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.
Proxies need then to convince neighbours 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 which have no need or right
to proxy (and may wish two network segments to appear to be
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 Neighbour 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
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infrastructure, spoofing may occur.
When a host received a proxied neighbour 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 6.3.
Movement between segments could be controlled with increasing
certificate sequence numbers and timestamps. The timestamp of the
root authority (in this case, the CGA address owner) would be most
significant. Where ties exist, the shortest chain would supercede,
as this would indicate a proxy closer to the proxied node.
7. Proxying unsecured addresses
Where the original neighbour 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 neighbour 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).
If signatures over the proxied messages were to be used, re-arrival
and override of the neighbour 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/CGA 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
being on 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
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(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.
8. Summary of San Diego Bar BoF Discussion
At the Bar BoF in San Diego, discussion followed the issue of mobile
nodes receiving proxy service from home agents and FMIPv6 access
routers.
At the time, it was considered appropriate to see if the router
authorization model could be adopted for proxy SEND. This discussion
didn't cover bridge-like proxies in any detail though.
9. IANA Considerations
No new options or messages are defined in this document.
10. Security Considerations
This document is in a developmental stage. The author actively seeks
feedback regarding the security issues for proxy Neighbour Discovery
and the potential solution space.
Please monitor this section for further security considerations as
the document develops.
10.1 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 [8].
10.2 Certificate Transport
The certification delegation relies upon transfer of the new
credentials to the proxying HA in order to undertake Proxy ND 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 [4]. In the
above described case, the home tunnel which comes into creation as
part of the binding process may be required for Certificate Path
Solicitation or Advertisement transport [6]. This constitutes a
potential chicken-and-egg problem. Either modifications to initial
home binding semantics or certificate transport are required. This
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may be trivial if signed, non-repudiable certificates are sent in the
clear between the MN's CoA and the HA without being tunneled.
10.3 Timekeeping
All of the presented methods rely on accurate timekeeping on the
receiver nodes of Neighbour Discovery Timestamp Options and
certificates.
For router-authorized proxy ND, a neighbour 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 NC entry
created by the proxy.
Creating the neighbour cache entry 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.
One way of creating a sequence of replayable messages which 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 defence in a sequence. The granularity of SEND
timestamp matching is around 1 second, so the attacker has a set of
SEND NA's 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 which 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 proxy ND 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.
11. Acknowledgments
Jean-Michel Combes brought this issue to the attention of the SEND
WG. James Kempf and Dave Thaler particularly contributed to work on
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this document. Contributions to discussion on this topic helped to
develop this document. Thanks go to Jari Arkko, Vijay Devarapalli,
and Mohan Parthasarathy.
12. References
12.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for
IP Version 6 (IPv6)", RFC 2461, December 1998.
[3] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[4] Johnson, D., Perkins, C. and J. Arkko, "Mobility Support in
IPv6", RFC 3775, June 2004.
[5] Thaler, D., "Bridge-like Neighbor Discovery Proxies (ND Proxy)",
draft-ietf-ipv6-ndproxy-00 (work in progress), December 2004.
[6] Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P. Nikander,
"SEcure Neighbor Discovery (SEND)", draft-ietf-send-ndopt-06
(work in progress), July 2004.
[7] Koodli, R., "Fast Handovers for Mobile IPv6",
draft-ietf-mipshop-fast-mipv6-03 (work in progress), October
2004.
12.2 Informative References
[8] Nikander, P., Kempf, J. and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[9] Housley, R., Polk, W., Ford, W. and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 3280, April 2002.
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Author's Address
Greg Daley
Centre for Telecommunications and Information Engineering
Department of Electrical and Computer Systems Engineering
Monash University
Clayton, Victoria 3800
Australia
Phone: +61 3 9905 4655
EMail: greg.daley@eng.monash.edu.au
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