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IETF RFC 3971
SEcure Neighbor Discovery (SEND)
Last modified on Friday, March 11th, 2005
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Network Working Group J. Arkko, Ed.
Request for Comments: 3971 Ericsson
Category: Standards Track J. Kempf
DoCoMo Communications Labs USA
B. Zill
Microsoft
P. Nikander
Ericsson
March 2005
SEcure Neighbor Discovery (SEND)
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2005).
Abstract
IPv6 nodes use the Neighbor Discovery Protocol (NDP) to discover
other nodes on the link, to determine their link-layer addresses to
find routers, and to maintain reachability information about the
paths to active neighbors. If not secured, NDP is vulnerable to
various attacks. This document specifies security mechanisms for
NDP. Unlike those in the original NDP specifications, these
mechanisms do not use IPsec.
Arkko, et al. Standards Track PAGE 1 
RFC 3971 SEcure Neighbor Discovery March 2005
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Specification of Requirements . . . . . . . . . . . . . 4
2. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Neighbor and Router Discovery Overview. . . . . . . . . . . . 6
4. Secure Neighbor Discovery Overview. . . . . . . . . . . . . . 8
5. Neighbor Discovery Protocol Options . . . . . . . . . . . . . 9
5.1. CGA Option. . . . . . . . . . . . . . . . . . . . . . . 10
5.1.1. Processing Rules for Senders. . . . . . . . . . 11
5.1.2. Processing Rules for Receivers. . . . . . . . . 12
5.1.3. Configuration . . . . . . . . . . . . . . . . . 13
5.2. RSA Signature Option. . . . . . . . . . . . . . . . . . 14
5.2.1. Processing Rules for Senders. . . . . . . . . . 16
5.2.2. Processing Rules for Receivers. . . . . . . . . 16
5.2.3. Configuration . . . . . . . . . . . . . . . . . 17
5.2.4. Performance Considerations. . . . . . . . . . . 18
5.3. Timestamp and Nonce Options . . . . . . . . . . . . . . 19
5.3.1. Timestamp Option. . . . . . . . . . . . . . . . 19
5.3.2. Nonce Option. . . . . . . . . . . . . . . . . . 20
5.3.3. Processing Rules for Senders. . . . . . . . . . 21
5.3.4. Processing Rules for Receivers. . . . . . . . . 21
6. Authorization Delegation Discovery. . . . . . . . . . . . . . 24
6.1. Authorization Model . . . . . . . . . . . . . . . . . . 24
6.2. Deployment Model. . . . . . . . . . . . . . . . . . . . 25
6.3. Certificate Format. . . . . . . . . . . . . . . . . . . 26
6.3.1. Router Authorization Certificate Profile. . . . 26
6.3.2. Suitability of Standard Identity Certificates . 29
6.4. Certificate Transport . . . . . . . . . . . . . . . . . 29
6.4.1. Certification Path Solicitation Message Format. 30
6.4.2. Certification Path Advertisement Message Format 32
6.4.3. Trust Anchor Option . . . . . . . . . . . . . . 34
6.4.4. Certificate Option. . . . . . . . . . . . . . . 36
6.4.5. Processing Rules for Routers. . . . . . . . . . 37
6.4.6. Processing Rules for Hosts. . . . . . . . . . . 38
6.5. Configuration . . . . . . . . . . . . . . . . . . . . . 39
7. Addressing. . . . . . . . . . . . . . . . . . . . . . . . . . 40
7.1. CGAs. . . . . . . . . . . . . . . . . . . . . . . . . . 40
7.2. Redirect Addresses. . . . . . . . . . . . . . . . . . . 40
7.3. Advertised Subnet Prefixes. . . . . . . . . . . . . . . 40
7.4. Limitations . . . . . . . . . . . . . . . . . . . . . . 41
8. Transition Issues . . . . . . . . . . . . . . . . . . . . . . 42
9. Security Considerations . . . . . . . . . . . . . . . . . . . 44
9.1. Threats to the Local Link Not Covered by SEND . . . . . 44
9.2. How SEND Counters Threats to NDP. . . . . . . . . . . . 45
9.2.1. Neighbor Solicitation/Advertisement Spoofing. . 45
9.2.2. Neighbor Unreachability Detection Failure . . . 46
9.2.3. Duplicate Address Detection DoS Attack. . . . . 46
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RFC 3971 SEcure Neighbor Discovery March 2005
9.2.4. Router Solicitation and Advertisement Attacks . 46
9.2.5. Replay Attacks. . . . . . . . . . . . . . . . . 47
9.2.6. Neighbor Discovery DoS Attack . . . . . . . . . 48
9.3. Attacks against SEND Itself . . . . . . . . . . . . . . 48
10. Protocol Values . . . . . . . . . . . . . . . . . . . . . . . 49
10.1. Constants . . . . . . . . . . . . . . . . . . . . . . . 49
10.2. Variables . . . . . . . . . . . . . . . . . . . . . . . 49
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 49
12. References. . . . . . . . . . . . . . . . . . . . . . . . . . 50
12.1. Normative References. . . . . . . . . . . . . . . . . . 50
12.2. Informative References. . . . . . . . . . . . . . . . . 51
Appendices. . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
A. Contributors and Acknowledgments. . . . . . . . . . . . 53
B. Cache Management. . . . . . . . . . . . . . . . . . . . 53
C. Message Size When Carrying Certificates . . . . . . . . 54
Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . . 55
Full Copyright Statements . . . . . . . . . . . . . . . . . . . . 56
1. Introduction
IPv6 defines the Neighbor Discovery Protocol (NDP) in RFCs 2461 [4]
and 2462 [5]. Nodes on the same link use NDP to discover each
other's presence and link-layer addresses, to find routers, and to
maintain reachability information about the paths to active
neighbors. NDP is used by both hosts and routers. Its functions
include Neighbor Discovery (ND), Router Discovery (RD), Address
Autoconfiguration, Address Resolution, Neighbor Unreachability
Detection (NUD), Duplicate Address Detection (DAD), and Redirection.
The original NDP specifications called for the use of IPsec to
protect NDP messages. However, the RFCs do not give detailed
instructions for using IPsec to do this. In this particular
application, IPsec can only be used with a manual configuration of
security associations, due to bootstrapping problems in using IKE
[19, 15]. Furthermore, the number of manually configured security
associations needed for protecting NDP can be very large [20], making
that approach impractical for most purposes.
The SEND protocol is designed to counter the threats to NDP. These
threats are described in detail in [22]. SEND is applicable in
environments where physical security on the link is not assured (such
as over wireless) and attacks on NDP are a concern.
This document is organized as follows. Sections 2 and 3 define some
terminology and present a brief review of NDP, respectively. Section
4 describes the overall approach to securing NDP. This approach
involves the use of new NDP options to carry public key - based
signatures. A zero-configuration mechanism is used for showing
Arkko, et al. Standards Track PAGE 3
RFC 3971 SEcure Neighbor Discovery March 2005
address ownership on individual nodes; routers are certified by a
trust anchor [7]. The formats, procedures, and cryptographic
mechanisms for the zero-configuration mechanism are described in a
related specification [11].
The required new NDP options are discussed in Section 5. Section 6
describes the mechanism for distributing certification paths to
establish an authorization delegation chain to a trust anchor.
Finally, Section 8 discusses the co-existence of secured and
unsecured NDP on the same link, and Section 9 discusses security
considerations for SEcure Neighbor Discovery (SEND).
The use of identity certificates provisioned on end hosts for
authorizing address use is out of the scope for this document, as is
the security of NDP when the entity defending an address is not the
same as the entity claiming that address (also known as "proxy ND").
These are extensions of SEND that may be treated in separate
documents, should the need arise.
1.1. Specification of Requirements
In this document, several words are used to signify the requirements
of the specification. These words are often capitalized. The key
words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", and
"MAY" are to be interpreted as described in [2].
2. Terms
Authorization Delegation Discovery (ADD)
A process through which SEND nodes can acquire a certification
path from a peer node to a trust anchor.
Certificate Revocation List (CRL)
In one method of certificate revocation, an authority periodically
issues a signed data structure called the Certificate Revocation
List. This is a time-stamped list identifying revoked
certificates, signed by the issuer, and made freely available in a
public repository.
Certification Path Advertisement (CPA)
The advertisement message used in the ADD process.
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Certification Path Solicitation (CPS)
The solicitation message used in the ADD process.
Cryptographically Generated Address (CGA)
A technique [11] whereby an IPv6 address of a node is
cryptographically generated by using a one-way hash function from
the node's public key and some other parameters.
Distinguished Encoding Rules (DER)
An encoding scheme for data values, defined in [12].
Duplicate Address Detection (DAD)
A mechanism assuring that two IPv6 nodes on the same link are not
using the same address.
Fully Qualified Domain Name (FQDN)
A fully qualified domain name consists of a host and domain name,
including the top-level domain.
Internationalized Domain Name (IDN)
Internationalized Domain Names can be used to represent domain
names that contain characters outside the ASCII set. See RFC 3490
[9].
Neighbor Discovery (ND)
The Neighbor Discovery function of the Neighbor Discovery Protocol
(NDP). NDP contains functions besides ND.
Neighbor Discovery Protocol (NDP)
The IPv6 Neighbor Discovery Protocol [7, 8].
The Neighbor Discovery Protocol is a part of ICMPv6 [6].
Neighbor Unreachability Detection (NUD)
A mechanism used for tracking the reachability of neighbors.
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Non-SEND node
An IPv6 node that does not implement this specification but uses
only the Neighbor Discovery protocol defined in RFCs 2461 and
2462, as updated, without security.
Nonce
An unpredictable random or pseudo-random number generated by a
node and used exactly once. In SEND, nonces are used to assure
that a particular advertisement is linked to the solicitation that
triggered it.
Router Authorization Certificate
An X.509v3 [7] public key certificate using the profile specified
in Section 6.3.1.
SEND node
An IPv6 node that implements this specification.
Router Discovery (RD)
Router Discovery allows the hosts to discover what routers exist
on the link, and what subnet prefixes are available. Router
Discovery is a part of the Neighbor Discovery Protocol.
Trust Anchor
Hosts are configured with a set of trust anchors to protect Router
Discovery. A trust anchor is an entity that the host trusts to
authorize routers to act as routers. A trust anchor configuration
consists of a public key and some associated parameters (see
Section 6.5 for a detailed explanation of these parameters).
3. Neighbor and Router Discovery Overview
The Neighbor Discovery Protocol has several functions. Many of these
are overloaded on a few central message types, such as the ICMPv6
Neighbor Advertisement message. In this section, we review some of
these tasks and their effects in order to better understand how the
messages should be treated. This section is not normative, and if
this section and the original Neighbor Discovery RFCs are in
conflict, the original RFCs, as updated, take precedence.
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The main functions of NDP are as follows:
o The Router Discovery function allows IPv6 hosts to discover the
local routers on an attached link. Router Discovery is described
in Section 6 of RFC 2461 [4]. The main purpose of Router
Discovery is to find neighboring routers willing to forward
packets on behalf of hosts. Subnet prefix discovery involves
determining which destinations are directly on a link; this
information is necessary in order to know whether a packet should
be sent to a router or directly to the destination node.
o The Redirect function is used for automatically redirecting a host
to a better first-hop router, or to inform hosts that a
destination is in fact a neighbor (i.e., on-link). Redirect is
specified in Section 8 of RFC 2461 [4].
o Address Autoconfiguration is used for automatically assigning
addresses to a host [5]. This allows hosts to operate without
explicit configuration related to IP connectivity. The default
autoconfiguration mechanism is stateless. To create IP addresses,
hosts use any prefix information delivered to them during Router
Discovery and then test the newly formed addresses for uniqueness.
A stateful mechanism, DHCPv6 [18], provides additional
autoconfiguration features.
o Duplicate Address Detection (DAD) is used for preventing address
collisions [5]: for instance, during Address Autoconfiguration. A
node that intends to assign a new address to one of its interfaces
first runs the DAD procedure to verify that no other node is using
the same address. As the rules forbid the use of an address until
it has been found unique, no higher layer traffic is possible
until this procedure has been completed. Thus, preventing attacks
against DAD can help ensure the availability of communications for
the node in question.
o The Address Resolution function allows a node on the link to
resolve another node's IPv6 address to the corresponding link-
layer address. Address Resolution is defined in Section 7.2 of
RFC 2461 [4], and it is used for hosts and routers alike. Again,
no higher level traffic can proceed until the sender knows the
link layer address of the destination node or the next hop router.
Note that the source link layer address on link layer frames is
not checked against the information learned through Address
Resolution. This allows for an easier addition of network
elements such as bridges and proxies and eases the stack
implementation requirements, as less information has to be passed
from layer to layer.
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o Neighbor Unreachability Detection (NUD) is used for tracking the
reachability of neighboring nodes, both hosts and routers. NUD is
defined in Section 7.3 of RFC 2461 [4]. NUD is security
sensitive, because an attacker could claim that reachability
exists when in fact it does not.
The NDP messages follow the ICMPv6 message format. All NDP functions
are realized by using the Router Solicitation (RS), Router
Advertisement (RA), Neighbor Solicitation (NS), Neighbor
Advertisement (NA), and Redirect messages. An actual NDP message
includes an NDP message header, consisting of an ICMPv6 header and ND
message-specific data, and zero or more NDP options. The NDP message
options are formatted in the Type-Length-Value format.
<------------NDP Message---------------->
*-------------------------------------------------------------*
| IPv6 Header | ICMPv6 | ND Message- | ND Message |
| Next Header = 58 | Header | specific | Options |
| (ICMPv6) | | data | |
*-------------------------------------------------------------*
<--NDP Message header-->
4. Secure Neighbor Discovery Overview
To secure the various functions in NDP, a set of new Neighbor
Discovery options is introduced. They are used to protect NDP
messages. This specification introduces these options, an
authorization delegation discovery process, an address ownership
proof mechanism, and requirements for the use of these components in
NDP.
The components of the solution specified in this document are as
follows:
o Certification paths, anchored on trusted parties, are expected to
certify the authority of routers. A host must be configured with
a trust anchor to which the router has a certification path before
the host can adopt the router as its default router.
Certification Path Solicitation and Advertisement messages are
used to discover a certification path to the trust anchor without
requiring the actual Router Discovery messages to carry lengthy
certification paths. The receipt of a protected Router
Advertisement message for which no certification path is available
triggers the authorization delegation discovery process.
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RFC 3971 SEcure Neighbor Discovery March 2005
o Cryptographically Generated Addresses are used to make sure that
the sender of a Neighbor Discovery message is the "owner" of the
claimed address. A public-private key pair is generated by all
nodes before they can claim an address. A new NDP option, the CGA
option, is used to carry the public key and associated parameters.
This specification also allows a node to use non-CGAs with
certificates that authorize their use. However, the details of
such use are beyond the scope of this specification and are left
for future work.
o A new NDP option, the RSA Signature option, is used to protect all
messages relating to Neighbor and Router discovery.
Public key signatures protect the integrity of the messages and
authenticate the identity of their sender. The authority of a
public key is established either with the authorization delegation
process, by using certificates, or through the address ownership
proof mechanism, by using CGAs, or with both, depending on
configuration and the type of the message protected.
Note: RSA is mandated because having multiple signature algorithms
would break compatibility between implementations or increase
implementation complexity by forcing the implementation of
multiple algorithms and the mechanism to select among them. A
second signature algorithm is only necessary as a recovery
mechanism, in case a flaw is found in RSA. If this happens, a
stronger signature algorithm can be selected, and SEND can be
revised. The relationship between the new algorithm and the RSA-
based SEND described in this document would be similar to that
between the RSA-based SEND and Neighbor Discovery without SEND.
Information signed with the stronger algorithm has precedence over
that signed with RSA, in the same way that RSA-signed information
now takes precedence over unsigned information. Implementations
of the current and revised specs would still be compatible.
o In order to prevent replay attacks, two new Neighbor Discovery
options, Timestamp and Nonce, are introduced. Given that Neighbor
and Router Discovery messages are in some cases sent to multicast
addresses, the Timestamp option offers replay protection without
any previously established state or sequence numbers. When the
messages are used in solicitation-advertisement pairs, they are
protected with the Nonce option.
5. Neighbor Discovery Protocol Options
The options described in this section MUST be supported.
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5.1. CGA Option
The CGA option allows the verification of the sender's CGA. The
format of the CGA option is described as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Pad Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. CGA Parameters .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
11
Length
The length of the option (including the Type, Length, Pad Length,
Reserved, CGA Parameters, and Padding fields) in units of 8
octets.
Pad Length
The number of padding octets beyond the end of the CGA Parameters
field but within the length specified by the Length field.
Padding octets MUST be set to zero by senders and ignored by
receivers.
Reserved
An 8-bit field reserved for future use. The value MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.
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RFC 3971 SEcure Neighbor Discovery March 2005
CGA Parameters
A variable-length field containing the CGA Parameters data
structure described in Section 4 of [11].
This specification requires that if both the CGA option and the
RSA Signature option are present, then the public key found from
the CGA Parameters field in the CGA option MUST be that referred
by the Key Hash field in the RSA Signature option. Packets
received with two different keys MUST be silently discarded. Note
that a future extension may provide a mechanism allowing the owner
of an address and the signer to be different parties.
Padding
A variable-length field making the option length a multiple of 8,
containing as many octets as specified in the Pad Length field.
5.1.1. Processing Rules for Senders
If the node has been configured to use SEND, the CGA option MUST be
present in all Neighbor Solicitation and Advertisement messages and
MUST be present in Router Solicitation messages unless they are sent
with the unspecified source address. The CGA option MAY be present
in other messages.
A node sending a message using the CGA option MUST construct the
message as follows:
The CGA Parameter field in the CGA option is filled according to
the rules presented above and in [11]. The public key in the
field is taken from the configuration used to generate the CGA,
typically from a data structure associated with the source
address. The address MUST be constructed as specified in Section
4 of [11]. Depending on the type of the message, this address
appears in different places, as follows:
Redirect
The address MUST be the source address of the message.
Neighbor Solicitation
The address MUST be the Target Address for solicitations sent for
Duplicate Address Detection; otherwise it MUST be the source
address of the message.
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Neighbor Advertisement
The address MUST be the source address of the message.
Router Solicitation
The address MUST be the source address of the message. Note that
the CGA option is not used when the source address is the
unspecified address.
Router Advertisement
The address MUST be the source address of the message.
5.1.2. Processing Rules for Receivers
Neighbor Solicitation and Advertisement messages without the CGA
option MUST be treated as unsecured (i.e., processed in the same way
as NDP messages sent by a non-SEND node). The processing of
unsecured messages is specified in Section 8. Note that SEND nodes
that do not attempt to interoperate with non-SEND nodes MAY simply
discard the unsecured messages.
Router Solicitation messages without the CGA option MUST also be
treated as unsecured, unless the source address of the message is the
unspecified address.
Redirect, Neighbor Solicitation, Neighbor Advertisement, Router
Solicitation, and Router Advertisement messages containing a CGA
option MUST be checked as follows:
If the interface has been configured to use CGA, the receiving
node MUST verify the source address of the packet by using the
algorithm described in Section 5 of [11]. The inputs to the
algorithm are the claimed address, as defined in the previous
section, and the CGA Parameters field.
If the CGA verification is successful, the recipient proceeds with
a more time-consuming cryptographic check of the signature. Note
that even if the CGA verification succeeds, no claims about the
validity of the use can be made until the signature has been
checked.
A receiver that does not support CGA or has not specified its use for
a given interface can still verify packets by using trust anchors,
even if a CGA is used on a packet. In such a case, the CGA property
of the address is simply left unverified.
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5.1.3. Configuration
All nodes that support the verification of the CGA option MUST record
the following configuration information:
minbits
The minimum acceptable key length for public keys used in the
generation of CGAs. The default SHOULD be 1024 bits.
Implementations MAY also set an upper limit for the amount of
computation needed when verifying packets that use these security
associations. The upper limit SHOULD be at least 2048 bits. Any
implementation should follow prudent cryptographic practice in
determining the appropriate key lengths.
All nodes that support the sending of the CGA option MUST record the
following configuration information:
CGA parameters
Any information required to construct CGAs, as described in [11].
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5.2. RSA Signature Option
The RSA Signature option allows public key-based signatures to be
attached to NDP messages. The format of the RSA Signature option is
described in the following diagram:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Key Hash |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Digital Signature .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Padding .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
12
Length
The length of the option (including the Type, Length, Reserved,
Key Hash, Digital Signature, and Padding fields) in units of 8
octets.
Reserved
A 16-bit field reserved for future use. The value MUST be
initialized to zero by the sender, and MUST be ignored by the
receiver.
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Key Hash
A 128-bit field containing the most significant (leftmost) 128
bits of a SHA-1 [14] hash of the public key used for constructing
the signature. The SHA-1 hash is taken over the presentation used
in the Public Key field of the CGA Parameters data structure
carried in the CGA option. Its purpose is to associate the
signature to a particular key known by the receiver. Such a key
can either be stored in the certificate cache of the receiver or
be received in the CGA option in the same message.
Digital Signature
A variable-length field containing a PKCS#1 v1.5 signature,
constructed by using the sender's private key over the following
sequence of octets:
1. The 128-bit CGA Message Type tag [11] value for SEND, 0x086F
CA5E 10B2 00C9 9C8C E001 6427 7C08. (The tag value has been
generated randomly by the editor of this specification.).
2. The 128-bit Source Address field from the IP header.
3. The 128-bit Destination Address field from the IP header.
4. The 8-bit Type, 8-bit Code, and 16-bit Checksum fields from the
ICMP header.
5. The NDP message header, starting from the octet after the ICMP
Checksum field and continuing up to but not including NDP
options.
6. All NDP options preceding the RSA Signature option.
The signature value is computed with the RSASSA-PKCS1-v1_5
algorithm and SHA-1 hash, as defined in [13].
This field starts after the Key Hash field. The length of the
Digital Signature field is determined by the length of the RSA
Signature option minus the length of the other fields (including
the variable length Pad field).
Padding
This variable-length field contains padding, as many bytes long as
remain after the end of the signature.
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5.2.1. Processing Rules for Senders
If the node has been configured to use SEND, Neighbor Solicitation,
Neighbor Advertisement, Router Advertisement, and Redirect messages
MUST contain the RSA Signature option. Router Solicitation messages
not sent with the unspecified source address MUST contain the RSA
Signature option.
A node sending a message with the RSA Signature option MUST construct
the message as follows:
o The message is constructed in its entirety, without the RSA
Signature option.
o The RSA Signature option is added as the last option in the
message.
o The data to be signed is constructed as explained in Section 5.2,
under the description of the Digital Signature field.
o The message, in the form defined above, is signed by using the
configured private key, and the resulting PKCS#1 v1.5 signature is
put in the Digital Signature field.
5.2.2. Processing Rules for Receivers
Neighbor Solicitation, Neighbor Advertisement, Router Advertisement,
and Redirect messages without the RSA Signature option MUST be
treated as unsecured (i.e., processed in the same way as NDP messages
sent by a non-SEND node). See Section 8.
Router Solicitation messages without the RSA Signature option MUST
also be treated as unsecured, unless the source address of the
message is the unspecified address.
Redirect, Neighbor Solicitation, Neighbor Advertisement, Router
Solicitation, and Router Advertisement messages containing an RSA
Signature option MUST be checked as follows:
o The receiver MUST ignore any options that come after the first RSA
Signature option. (The options are ignored for both signature
verification and NDP processing purposes.)
o The Key Hash field MUST indicate the use of a known public key,
either one learned from a preceding CGA option in the same
message, or one known by other means.
Arkko, et al. Standards Track PAGE 16
RFC 3971 SEcure Neighbor Discovery March 2005
o The Digital Signature field MUST have correct encoding and MUST
not exceed the length of the RSA Signature option minus the
Padding.
o The Digital Signature verification MUST show that the signature
has been calculated as specified in the previous section.
o If the use of a trust anchor has been configured, a valid
certification path (see Section 6.3) between the receiver's trust
anchor and the sender's public key MUST be known.
Note that the receiver may verify just the CGA property of a
packet, even if, in addition to CGA, the sender has used a trust
anchor.
Messages that do not pass all the above tests MUST be silently
discarded if the host has been configured to accept only secured ND
messages. The messages MAY be accepted if the host has been
configured to accept both secured and unsecured messages but MUST be
treated as an unsecured message. The receiver MAY also otherwise
silently discard packets (e.g., as a response to an apparent CPU
exhausting DoS attack).
5.2.3. Configuration
All nodes that support the reception of the RSA Signature options
MUST allow the following information to be configured for each
separate NDP message type:
authorization method
This parameter determines the method through which the authority
of the sender is determined. It can have four values:
trust anchor
The authority of the sender is verified as described in
Section 6.3. The sender may claim additional authorization
through the use of CGAs, but this is neither required nor
verified.
CGA
The CGA property of the sender's address is verified as
described in [11]. The sender may claim additional
authority through a trust anchor, but this is neither
required nor verified.
Arkko, et al. Standards Track PAGE 17
RFC 3971 SEcure Neighbor Discovery March 2005
trust anchor and CGA
Both the trust anchor and the CGA verification is required.
trust anchor or CGA
Either the trust anchor or the CGA verification is required.
anchor
The allowed trust anchor(s), if the authorization method is not
set to CGA.
All nodes that support sending RSA Signature options MUST record the
following configuration information:
keypair
A public-private key pair. If authorization delegation is in
use, a certification path from a trust anchor to this key pair
must exist.
CGA flag
A flag that indicates whether CGA is used or not. This flag
may be per interface or per node. (Note that in future
extensions of the SEND protocol, this flag may also be per
subnet prefix.)
5.2.4. Performance Considerations
The construction and verification of the RSA Signature option is
computationally expensive. In the NDP context, however, hosts
typically only have to perform a few signature operations as they
enter a link, a few operations as they find a new on-link peer with
which to communicate, or Neighbor Unreachability Detection with
existing neighbors.
Routers are required to perform a larger number of operations,
particularly when the frequency of router advertisements is high due
to mobility requirements. Still, the number of required signature
operations is on the order of a few dozen per second, some of which
can be precomputed as explained below. A large number of router
solicitations may cause a higher demand for performing asymmetric
operations, although the base NDP protocol limits the rate at which
multicast responses to solicitations can be sent.
Arkko, et al. Standards Track PAGE 18
RFC 3971 SEcure Neighbor Discovery March 2005
Signatures can be precomputed for unsolicited (multicast) Neighbor
and Router Advertisements if the timing of the future advertisements
is known. Typically, solicited neighbor advertisements are sent to
the unicast address from which the solicitation was sent. Given that
the IPv6 header is covered by the signature, it is not possible to
precompute solicited advertisements.
5.3. Timestamp and Nonce Options
5.3.1. Timestamp Option
The purpose of the Timestamp option is to make sure that unsolicited
advertisements and redirects have not been replayed. The format of
this option is described in the following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Timestamp +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
13
Length
The length of the option (including the Type, Length, Reserved,
and Timestamp fields) in units of 8 octets; i.e., 2.
Reserved
A 48-bit field reserved for future use. The value MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.
Arkko, et al. Standards Track PAGE 19
RFC 3971 SEcure Neighbor Discovery March 2005
Timestamp
A 64-bit unsigned integer field containing a timestamp. The value
indicates the number of seconds since January 1, 1970, 00:00 UTC,
by using a fixed point format. In this format, the integer number
of seconds is contained in the first 48 bits of the field, and the
remaining 16 bits indicate the number of 1/64K fractions of a
second.
Implementation note: This format is compatible with the usual
representation of time under UNIX, although the number of bits
available for the integer and fraction parts may vary.
5.3.2. Nonce Option
The purpose of the Nonce option is to make sure that an advertisement
is a fresh response to a solicitation sent earlier by the node. The
format of this option is described in the following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Nonce ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| |
. .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
14
Length
The length of the option (including the Type, Length, and Nonce
fields) in units of 8 octets.
Nonce
A field containing a random number selected by the sender of the
solicitation message. The length of the random number MUST be at
least 6 bytes. The length of the random number MUST be selected
so that the length of the nonce option is a multiple of 8 octets.
Arkko, et al. Standards Track PAGE 20
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5.3.3. Processing Rules for Senders
If the node has been configured to use SEND, all solicitation
messages MUST include a Nonce. When sending a solicitation, the
sender MUST store the nonce internally so that it can recognize any
replies containing that particular nonce.
If the node has been configured to use SEND, all advertisements sent
in reply to a solicitation MUST include a Nonce, copied from the
received solicitation. Note that routers may decide to send a
multicast advertisement to all nodes instead of a response to a
specific host. In such a case, the router MAY still include the
nonce value for the host that triggered the multicast advertisement.
(Omitting the nonce value may cause the host to ignore the router's
advertisement, unless the clocks in these nodes are sufficiently
synchronized so that timestamps function properly.)
If the node has been configured to use SEND, all solicitation,
advertisement, and redirect messages MUST include a Timestamp.
Senders SHOULD set the Timestamp field to the current time, according
to their real time clocks.
5.3.4. Processing Rules for Receivers
The processing of the Nonce and Timestamp options depends on whether
a packet is a solicited advertisement. A system may implement the
distinction in various ways. Section 5.3.4.1 defines the processing
rules for solicited advertisements. Section 5.3.4.2 defines the
processing rules for all other messages.
In addition, the following rules apply in all cases:
o Messages received without at least one of the Timestamp and Nonce
options MUST be treated as unsecured (i.e., processed in the same
way as NDP messages sent by a non-SEND node).
o Messages received with the RSA Signature option but without the
Timestamp option MUST be silently discarded.
o Solicitation messages received with the RSA Signature option but
without the Nonce option MUST be silently discarded.
o Advertisements sent to a unicast destination address with the RSA
Signature option but without a Nonce option SHOULD be processed as
unsolicited advertisements.
Arkko, et al. Standards Track PAGE 21
RFC 3971 SEcure Neighbor Discovery March 2005
o An implementation MAY use some mechanism such as a timestamp cache
to strengthen resistance to replay attacks. When there is a very
large number of nodes on the same link, or when a cache filling
attack is in progress, it is possible that the cache holding the
most recent timestamp per sender will become full. In this case,
the node MUST remove some entries from the cache or refuse some
new requested entries. The specific policy as to which entries
are preferred over others is left as an implementation decision.
However, typical policies may prefer existing entries to new ones,
CGAs with a large Sec value to smaller Sec values, and so on. The
issue is briefly discussed in Appendix B.
o The receiver MUST be prepared to receive the Timestamp and Nonce
options in any order, as per RFC 2461 [4], Section 9.
5.3.4.1. Processing Solicited Advertisements
The receiver MUST verify that it has recently sent a matching
solicitation, and that the received advertisement contains a copy of
the Nonce sent in the solicitation.
If the message contains a Nonce option but the Nonce value is not
recognized, the message MUST be silently discarded.
Otherwise, if the message does not contain a Nonce option, it MAY be
considered an unsolicited advertisement and processed according to
Section 5.3.4.2.
If the message is accepted, the receiver SHOULD store the receive
time of the message and the timestamp time in the message, as
specified in Section 5.3.4.2.
5.3.4.2. Processing All Other Messages
Receivers SHOULD be configured with an allowed timestamp Delta value,
a "fuzz factor" for comparisons, and an allowed clock drift
parameter. The recommended default value for the allowed Delta is
TIMESTAMP_DELTA; for fuzz factor TIMESTAMP_FUZZ; and for clock drift,
TIMESTAMP_DRIFT (see Section 10.2).
To facilitate timestamp checking, each node SHOULD store the
following information for each peer:
o The receive time of the last received and accepted SEND message.
This is called RDlast.
o The time stamp in the last received and accepted SEND message.
This is called TSlast.
Arkko, et al. Standards Track PAGE 22
RFC 3971 SEcure Neighbor Discovery March 2005
An accepted SEND message is any successfully verified Neighbor
Solicitation, Neighbor Advertisement, Router Solicitation, Router
Advertisement, or Redirect message from the given peer. The RSA
Signature option MUST be used in such a message before it can update
the above variables.
Receivers SHOULD then check the Timestamp field as follows:
o When a message is received from a new peer (i.e., one that is not
stored in the cache), the received timestamp, TSnew, is checked,
and the packet is accepted if the timestamp is recent enough to
the reception time of the packet, RDnew:
-Delta < (RDnew - TSnew) < +Delta
The RDnew and TSnew values SHOULD be stored in the cache as RDlast
and TSlast.
o If the timestamp is NOT within the boundaries but the message is a
Neighbor Solicitation message that the receiver should answer, the
receiver SHOULD respond to the message. However, even if it does
respond to the message, it MUST NOT create a Neighbor Cache entry.
This allows nodes that have large differences in their clocks to
continue communicating with each other by exchanging NS/NA pairs.
o When a message is received from a known peer (i.e., one that
already has an entry in the cache), the timestamp is checked
against the previously received SEND message:
TSnew + fuzz > TSlast + (RDnew - RDlast) x (1 - drift) - fuzz
If this inequality does not hold, the receiver SHOULD silently
discard the message. If, on the other hand, the inequality holds,
the receiver SHOULD process the message.
Moreover, if the above inequality holds and TSnew > TSlast, the
receiver SHOULD update RDlast and TSlast. Otherwise, the receiver
MUST NOT update RDlast or TSlast.
As unsolicited messages may be used in a Denial-of-Service attack to
make the receiver verify computationally expensive signatures, all
nodes SHOULD apply a mechanism to prevent excessive use of resources
for processing such messages.
Arkko, et al. Standards Track PAGE 23
RFC 3971 SEcure Neighbor Discovery March 2005
6. Authorization Delegation Discovery
NDP allows a node to configure itself automatically based on
information learned shortly after connecting to a new link. It is
particularly easy to configure "rogue" routers on an unsecured link,
and it is particularly difficult for a node to distinguish between
valid and invalid sources of router information, because the node
needs this information before communicating with nodes outside of the
link.
As the newly-connected node cannot communicate off-link, it cannot be
responsible for searching information to help validate the router(s).
However, given a certification path, the node can check someone
else's search results and conclude that a particular message comes
from an authorized source. In the typical case, a router already
connected beyond the link can communicate if necessary with off-link
nodes and construct a certification path.
The Secure Neighbor Discovery Protocol mandates a certificate format
and introduces two new ICMPv6 messages used between hosts and routers
to allow the host to learn a certification path with the assistance
of the router.
6.1. Authorization Model
To protect Router Discovery, SEND requires that routers be authorized
to act as routers. This authorization is provisioned in both routers
and hosts. Routers are given certificates from a trust anchor, and
the hosts are configured with the trust anchor(s) to authorize
routers. This provisioning is specific to SEND and does not assume
that certificates already deployed for some other purpose can be
used.
The authorization for routers in SEND is twofold:
o Routers are authorized to act as routers. The router belongs to
the set of routers trusted by the trust anchor. All routers in
this set have the same authorization.
o Optionally, routers may also be authorized to advertise a certain
set of subnet prefixes. A specific router is given a specific set
of subnet prefixes to advertise; other routers have an
authorization to advertise other subnet prefixes. Trust anchors
may also delegate a certain set of subnet prefixes to someone
(such as an ISP) who, in turn, delegates parts of this set to
individual routers.
Arkko, et al. Standards Track PAGE 24
RFC 3971 SEcure Neighbor Discovery March 2005
Note that while communicating with hosts, routers typically also
present a number of other parameters beyond the above. For instance,
routers have their own IP addresses, subnet prefixes have lifetimes,
and routers control the use of stateless and stateful address
autoconfiguration. However, the ability to be a router and the
subnet prefixes are the most fundamental parameters to authorize.
This is because the host needs to choose a router that it uses as its
default router, and because the advertised subnet prefixes have an
impact on the addresses the host uses. The subnet prefixes also
represent a claim about the topological location of the router in the
network.
Care should be taken if the certificates used in SEND are also used
to provide authorization in other circumstances; for example, with
routing protocols. It is necessary to ensure that the authorization
information is appropriate for all applications. SEND certificates
may authorize a larger set of subnet prefixes than the router is
authorized to advertise on a given interface. For instance, SEND
allows the use of the null prefix, which might cause verification or
routing problems in other applications. It is RECOMMENDED that SEND
certificates containing the null prefix are only used for SEND.
Note that end hosts need not be provisioned with their own certified
public keys, just as Web clients today do not require end host
provisioning with certified keys. Public keys for CGA generation do
not need to be certified, as these keys derive their ability to
authorize operations on the CGA by the tie to the address.
6.2. Deployment Model
The deployment model for trust anchors can be either a globally
rooted public key infrastructure or a more local, decentralized
deployment model similar to that currently used for TLS in Web
servers. The centralized model assumes a global root capable of
authorizing routers and, optionally, the address space they
advertise. The end hosts are configured with the public keys of the
global root. The global root could operate, for instance, under the
Internet Assigned Numbers Authority (IANA) or as a co-operative among
Regional Internet Registries (RIRs). However, no such global root
currently exists.
In the decentralized model, end hosts are configured with a
collection of trusted public keys. The public keys could be issued
from various places; for example, a) a public key for the end host's
own organization, b) a public key for the end host's home ISP and for
ISPs with which the home ISP has a roaming agreement, or c) public
keys for roaming brokers acting as intermediaries for ISPs that don't
want to run their own certification authority.
Arkko, et al. Standards Track PAGE 25
RFC 3971 SEcure Neighbor Discovery March 2005
This decentralized model works even when a SEND node is used both in
networks that have certified routers and in networks that do not. As
discussed in Section 8, a SEND node can fall back to the use of a
non-SEND router. This makes it possible to start with a local trust
anchor even if there is no trust anchor for all possible networks.
6.3. Certificate Format
The certification path of a router terminates in a Router
Authorization Certificate that authorizes a specific IPv6 node to act
as a router. Because authorization paths are not a common practice
in the Internet at the time of this writing, the path MUST consist of
standard Public Key Certificates (PKC, in the sense of [8]). The
certification path MUST start from the identity of a trust anchor
shared by the host and the router. This allows the host to anchor
trust for the router's public key in the trust anchor. Note that
there MAY be multiple certificates issued by a single trust anchor.
6.3.1. Router Authorization Certificate Profile
Router Authorization Certificates are X.509v3 certificates, as
defined in RFC 3280 [7], and SHOULD contain at least one instance of
the X.509 extension for IP addresses, as defined in [10]. The parent
certificates in the certification path SHOULD contain one or more
X.509 IP address extensions, back up to a trusted party (such as the
user's ISP) that configured the original IP address block for the
router in question, or that delegated the right to do so. The
certificates for the intermediate delegating authorities SHOULD
contain X.509 IP address extension(s) for subdelegations. The
router's certificate is signed by the delegating authority for the
subnet prefixes the router is authorized to advertise.
The X.509 IP address extension MUST contain at least one
addressesOrRanges element. This element MUST contain an
addressPrefix element containing an IPv6 address prefix for a prefix
that the router or the intermediate entity is authorized to route.
If the entity is allowed to route any prefix, the IPv6 address prefix
used is the null prefix, ::/0. The addressFamily element of the
IPAddrBlocks sequence element MUST contain the IPv6 Address Family
Identifier (0002), as specified in [10], for IPv6 subnet prefixes.
Instead of an addressPrefix element, the addressesOrRange element MAY
contain an addressRange element for a range of subnet prefixes, if
more than one prefix is authorized. The X.509 IP address extension
MAY contain additional IPv6 subnet prefixes, expressed as either an
addressPrefix or an addressRange.
Arkko, et al. Standards Track PAGE 26
RFC 3971 SEcure Neighbor Discovery March 2005
A node receiving a Router Authorization Certificate MUST first check
whether the certificate's signature was generated by the delegating
authority. Then the client SHOULD check whether all the
addressPrefix or addressRange entries in the router's certificate are
contained within the address ranges in the delegating authority's
certificate, and whether the addressPrefix entries match any
addressPrefix entries in the delegating authority's certificate. If
an addressPrefix or addressRange is not contained within the
delegating authority's subnet prefixes or ranges, the client MAY
attempt to take an intersection of the ranges/subnet prefixes and to
use that intersection. If the resulting intersection is empty, the
client MUST NOT accept the certificate. If the addressPrefix in the
certificate is missing or is the null prefix, ::/0, the parent prefix
or range SHOULD be used. If there is no parent prefix or range, the
subnet prefixes that the router advertises are said to be
unconstrained (see Section 7.3). That is, the router is allowed to
advertise any prefix.
The above checks SHOULD be done for all certificates in the path. If
any of the checks fail, the client MUST NOT accept the certificate.
The client also has to perform validation of advertised subnet
prefixes as discussed in Section 7.3.
Hosts MUST check the subjectPublicKeyInfo field within the last
certificate in the certificate path to ensure that only RSA public
keys are used to attempt validation of router signatures. Hosts MUST
disregard the certificate for SEND if it does not contain an RSA key.
As it is possible that some public key certificates used with SEND do
not immediately contain the X.509 IP address extension element, an
implementation MAY contain facilities that allow the prefix and range
checks to be relaxed. However, any such configuration options SHOULD
be switched off by default. The system SHOULD have a default
configuration that requires rigorous prefix and range checks.
The following is an example of a certification path. Suppose that
isp_group_example.net is the trust anchor. The host has this
certificate:
Certificate 1:
Issuer: isp_group_example.net
Validity: Jan 1, 2004 through Dec 31, 2004
Subject: isp_group_example.net
Extensions:
IP address delegation extension:
Prefixes: P1, ..., Pk
... possibly other extensions ...
... other certificate parameters ...
Arkko, et al. Standards Track PAGE 27
RFC 3971 SEcure Neighbor Discovery March 2005
When the host attaches to a link served by
router_x.isp_foo_example.net, it receives the following certification
path:
Certificate 2:
Issuer: isp_group_example.net
Validity: Jan 1, 2004 through Dec 31, 2004
Subject: isp_foo_example.net
Extensions:
IP address delegation extension:
Prefixes: Q1, ..., Qk
... possibly other extensions ...
... other certificate parameters ...
Certificate 3:
Issuer: isp_foo_example.net
Validity: Jan 1, 2004 through Dec 31, 2004
Subject: router_x.isp_foo_example.net
Extensions:
IP address delegation extension:
Prefixes R1, ..., Rk
... possibly other extensions ...
... other certificate parameters ...
When the three certificates are processed, the usual RFC 3280 [7]
certificate path validation is performed. Note, however, that when a
node checks certificates received from a router, it typically does
not have a connection to the Internet yet, and so it is not possible
to perform an on-line Certificate Revocation List (CRL) check, if
necessary. Until this check is performed, acceptance of the
certificate MUST be considered provisional, and the node MUST perform
a check as soon as it has established a connection with the Internet
through the router. If the router has been compromised, it could
interfere with the CRL check. Should performance of the CRL check be
disrupted or should the check fail, the node SHOULD immediately stop
using the router as a default and use another router on the link
instead.
In addition, the IP addresses in the delegation extension MUST be a
subset of the IP addresses in the delegation extension of the
issuer's certificate. So in this example, R1, ..., Rs must be a
subset of Q1,...,Qr, and Q1,...,Qr must be a subset of P1,...,Pk. If
the certification path is valid, then router_foo.isp_foo_example.com
is authorized to route the prefixes R1,...,Rs.
Arkko, et al. Standards Track PAGE 28
RFC 3971 SEcure Neighbor Discovery March 2005
6.3.2. Suitability of Standard Identity Certificates
As deployment of the IP address extension is, itself, not common, a
network service provider MAY choose to deploy standard identity
certificates on the router to supply the router's public key for
signed Router Advertisements.
If there is no prefix information further up in the certification
path, a host interprets a standard identity certificate as allowing
unconstrained prefix advertisements.
If the other certificates contain prefix information, a standard
identity certificate is interpreted as allowing those subnet
prefixes.
6.4. Certificate Transport
The Certification Path Solicitation (CPS) message is sent by a host
when it wishes to request a certification path between a router and
one of the host's trust anchors. The Certification Path
Advertisement (CPA) message is sent in reply to the CPS message.
These messages are kept separate from the rest of Neighbor and Router
Discovery to reduce the effect of the potentially voluminous
certification path information on other messages.
The Authorization Delegation Discovery (ADD) process does not exclude
other forms of discovering certification paths. For instance, during
fast movements, mobile nodes may learn information (including the
certification paths) about the next router from a previous router, or
nodes may be preconfigured with certification paths from roaming
partners.
Where hosts themselves are certified by a trust anchor, these
messages MAY also optionally be used between hosts to acquire the
peer's certification path. However, the details of such usage are
beyond the scope of this specification.
Arkko, et al. Standards Track PAGE 29
RFC 3971 SEcure Neighbor Discovery March 2005
6.4.1. Certification Path Solicitation Message Format
Hosts send Certification Path Solicitations in order to prompt
routers to generate Certification Path Advertisements.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | Component |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
A link-local unicast address assigned to the sending interface,
or to the unspecified address if no address is assigned to the
sending interface.
Destination Address
Typically the All-Routers multicast address, the Solicited-Node
multicast address, or the address of the host's default router.
Hop Limit
255
ICMP Fields:
Type
148
Code
0
Checksum
The ICMP checksum [6].
Arkko, et al. Standards Track PAGE 30
RFC 3971 SEcure Neighbor Discovery March 2005
Identifier
A 16-bit unsigned integer field, acting as an identifier to
help match advertisements to solicitations. The Identifier
field MUST NOT be zero, and its value SHOULD be randomly
generated. This randomness does not have to be
cryptographically hard, as its purpose is only to avoid
collisions.
Component
This 16-bit unsigned integer field is set to 65,535 if the
sender seeks to retrieve all certificates. Otherwise, it is
set to the component identifier corresponding to the
certificate that the receiver wants to retrieve (see Sections
6.4.2 and 6.4.6).
Valid Options:
Trust Anchor
One or more trust anchors that the client is willing to accept.
The first (or only) Trust Anchor option MUST contain a DER
Encoded X.501 Name; see Section 6.4.3. If there is more than
one Trust Anchor option, the options beyond the first may
contain any type of trust anchor.
Future versions of this protocol may define new option types.
Receivers MUST silently ignore any options they do not recognize
and continue processing the message. All included options MUST
have a length greater than zero.
ICMP length (derived from the IP length) MUST be 8 or more octets.
Arkko, et al. Standards Track PAGE 31
RFC 3971 SEcure Neighbor Discovery March 2005
6.4.2. Certification Path Advertisement Message Format
Routers send out Certification Path Advertisement messages in
response to a Certification Path Solicitation.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identifier | All Components |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Component | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options ...
+-+-+-+-+-+-+-+-+-+-+-+-
IP Fields:
Source Address
A link-local unicast address assigned to the interface from
which this message is sent. Note that routers may use multiple
addresses, and therefore this address is not sufficient for the
unique identification of routers.
Destination Address
Either the Solicited-Node multicast address of the receiver or
the link-scoped All-Nodes multicast address.
Hop Limit
255
ICMP Fields:
Type
149
Code
0
Checksum
The ICMP checksum [6].
Arkko, et al. Standards Track PAGE 32
RFC 3971 SEcure Neighbor Discovery March 2005
Identifier
A 16-bit unsigned integer field, acting as an identifier to
help match advertisements to solicitations. The Identifier
field MUST be zero for advertisements sent to the All-Nodes
multicast address and MUST NOT be zero for others.
All Components
A 16-bit unsigned integer field, used to inform the receiver of
the number of certificates in the entire path.
A single advertisement SHOULD be broken into separately sent
components if there is more than one certificate in the path,
in order to avoid excessive fragmentation at the IP layer.
Individual certificates in a path MAY be stored and used as
received before all the certificates have arrived; this makes
the protocol slightly more reliable and less prone to Denial-
of-Service attacks.
Examples of packet lengths of Certification Path Advertisement
messages for typical certification paths are listed in Appendix
C.
Component
A 16-bit unsigned integer field, used to inform the receiver
which certificate is being sent.
The first message in an N-component advertisement has the
Component field set to N-1, the second set to N-2, and so on.
A zero indicates that there are no more components coming in
this advertisement.
The sending of path components SHOULD be ordered so that the
certificate after the trust anchor is sent first. Each
certificate sent after the first can be verified with the
previously sent certificates. The certificate of the sender
comes last. The trust anchor certificate SHOULD NOT be sent.
Reserved
An unused field. It MUST be initialized to zero by the sender
and MUST be ignored by the receiver.
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Valid Options:
Certificate
One certificate is provided in each Certificate option to
establish part of a certification path to a trust anchor.
The certificate of the trust anchor itself SHOULD NOT be sent.
Trust Anchor
Zero or more Trust Anchor options may be included to help
receivers decide which advertisements are useful for them. If
present, these options MUST appear in the first component of a
multi-component advertisement.
Future versions of this protocol may define new option types.
Receivers MUST silently ignore any options they do not recognize
and continue processing the message. All included options MUST
have a length that is greater than zero.
The ICMP length (derived from the IP length) MUST be 8 or more
octets.
6.4.3. Trust Anchor Option
The format of the Trust Anchor option is described in the following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Name Type | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Name ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
15
Length
The length of the option (including the Type, Length, Name Type,
Pad Length, and Name fields), in units of 8 octets.
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Name Type
The type of the name included in the Name field. This
specification defines two legal values for this field:
1 DER Encoded X.501 Name
2 FQDN
Pad Length
The number of padding octets beyond the end of the Name field but
within the length specified by the Length field. Padding octets
MUST be set to zero by senders and ignored by receivers.
Name
When the Name Type field is set to 1, the Name field contains a
DER encoded X.501 Name identifying the trust anchor. The value is
encoded as defined in [12] and [7].
When the Name Type field is set to 2, the Name field contains a
Fully Qualified Domain Name of the trust anchor; for example,
"trustanchor.example.com". The name is stored as a string, in the
DNS wire format, as specified in RFC 1034 [1]. Additionally, the
restrictions discussed in RFC 3280 [7], Section 4.2.1.7 apply.
In the FQDN case, the Name field is an "IDN-unaware domain name
slot", as defined in [9]. That is, it can contain only ASCII
characters. An implementation MAY support internationalized
domain names (IDNs) using the ToASCII operation; see [9] for more
information.
All systems MUST support the DER Encoded X.501 Name.
Implementations MAY support the FQDN name type.
Padding
A variable-length field making the option length a multiple of 8,
beginning after the previous field ends and continuing to the end
of the option, as specified by the Length field.
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6.4.4. Certificate Option
The format of the certificate option is described in the following:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Cert Type | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Certificate ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
16
Length
The length of the option (including the Type, Length, Cert Type,
Pad Length, and Certificate fields), in units of 8 octets.
Cert Type
The type of the certificate included in the Certificate field.
This specification defines only one legal value for this field:
1 X.509v3 Certificate, as specified below
Reserved
An 8-bit field reserved for future use. The value MUST be
initialized to zero by the sender and MUST be ignored by the
receiver.
Certificate
When the Cert Type field is set to 1, the Certificate field
contains an X.509v3 certificate [7], as described in Section
6.3.1.
Padding
A variable length field making the option length a multiple of 8,
beginning after the ASN.1 encoding of the previous field [7, 15]
ends and continuing to the end of the option, as specified by the
Length field.
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6.4.5. Processing Rules for Routers
A router MUST silently discard any received Certification Path
Solicitation messages that do not conform to the message format
defined in Section 6.4.1. The contents of the Reserved field and of
any unrecognized options MUST be ignored. Future, backward-
compatible changes to the protocol may specify the contents of the
Reserved field or add new options; backward-incompatible changes may
use different Code values. The contents of any defined options that
are not specified to be used with Router Solicitation messages MUST
be ignored, and the packet processed in the normal manner. The only
defined option that may appear is the Trust Anchor option. A
solicitation that passes the validity checks is called a "valid
solicitation".
Routers SHOULD send advertisements in response to valid solicitations
received on an advertising interface. If the source address in the
solicitation was the unspecified address, the router MUST send the
response to the link-scoped All-Nodes multicast address. If the
source address was a unicast address, the router MUST send the
response to the Solicited-Node multicast address corresponding to the
source address, except when under load, as specified below. Routers
SHOULD NOT send Certification Path Advertisements more than
MAX_CPA_RATE times within a second. When there are more
solicitations, the router SHOULD send the response to the All-Nodes
multicast address regardless of the source address that appeared in
the solicitation.
In an advertisement, the router SHOULD include suitable Certificate
options so that a certification path can be established to the
solicited trust anchor (or a part of it, if the Component field in
the solicitation is not equal to 65,535). Note also that a single
advertisement is broken into separately sent components and ordered
in a particular way (see Section 6.4.2) when there is more than one
certificate in the path.
The anchor is identified by the Trust Anchor option. If the Trust
Anchor option is represented as a DER Encoded X.501 Name, then the
Name must be equal to the Subject field in the anchor's certificate.
If the Trust Anchor option is represented as an FQDN, the FQDN must
be equal to an FQDN in the subjectAltName field of the anchor's
certificate. The router SHOULD include the Trust Anchor option(s) in
the advertisement for which the certification path was found.
If the router is unable to find a path to the requested anchor, it
SHOULD send an advertisement without any certificates. In this case,
the router SHOULD include the Trust Anchor options that were
solicited.
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6.4.6. Processing Rules for Hosts
A host MUST silently discard any received Certification Path
Advertisement messages that do not conform to the message format
defined in Section 6.4.2. The contents of the Reserved field, and of
any unrecognized options, MUST be ignored. Future, backward-
compatible changes to the protocol MAY specify the contents of the
Reserved field or add new options; backward-incompatible changes MUST
use different Code values. The contents of any defined options not
specified to be used with Certification Path Advertisement messages
MUST be ignored, and the packet processed in the normal manner. The
only defined options that may appear are the Certificate and Trust
Anchor options. An advertisement that passes the validity checks is
called a "valid advertisement".
Hosts SHOULD store certification paths retrieved in Certification
Path Discovery messages if they start from an anchor trusted by the
host. The certification paths MUST be verified, as defined in
Section 6.3, before storing them. Routers send the certificates one
by one, starting from the trust anchor end of the path.
Note: Except to allow for message loss and reordering for temporary
purposes, hosts might not store certificates received in a
Certification Path Advertisement unless they contain a certificate
that can be immediately verified either to the trust anchor or to a
certificate that has been verified earlier. This measure is intended
to prevent Denial-of-Service attacks, whereby an attacker floods a
host with certificates that the host cannot validate and overwhelms
memory for certificate storage.
Note that caching this information, and the implied verification
results between network attachments for use over multiple attachments
to the network, can help improve performance. But periodic
certificate revocation checks are still needed, even with cached
results, to make sure that the certificates are still valid.
The host SHOULD retrieve a certification path when a Router
Advertisement has been received with a public key that is not
available from a certificate in the hosts' cache, or when there is no
certification path to one of the host's trust anchors. In these
situations, the host MAY send a Certification Path Solicitation
message to retrieve the path. If there is no response within
CPS_RETRY seconds, the message should be retried. The wait interval
for each subsequent retransmission MUST exponentially increase,
doubling each time. If there is no response after CPS_RETRY_MAX
seconds, the host abandons the certification path retrieval process.
If the host receives only a part of a certification path within
CPS_RETRY_FRAGMENTS seconds of receiving the first part, it MAY in
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addition transmit a Certification Path Solicitation message with the
Component field set to a value not equal to 65,535. This message can
be retransmitted by using the same process as for the initial
message. If there are multiple missing certificates, additional CPS
messages can be sent after getting a response to first one. However,
the complete retrieval process may last at most CPS_RETRY_MAX
seconds.
Certification Path Solicitations SHOULD NOT be sent if the host has a
currently valid certification path from a reachable router to a trust
anchor.
When soliciting certificates for a router, a host MUST send
Certification Path Solicitations either to the All-Routers multicast
address, if it has not selected a default router yet, or to the
default router's IP address, if a default router has already been
selected.
If two hosts want to establish trust with the CPS and CPA messages,
the CPS message SHOULD be sent to the Solicited-Node multicast
address of the receiver. The advertisements SHOULD be sent as
specified above for routers. However, the exact details are outside
the scope of this specification.
When processing possible advertisements sent as responses to a
solicitation, the host MAY prefer to process those advertisements
with the same Identifier field value as that of the solicitation
first. This makes Denial-of-Service attacks against the mechanism
harder (see Section 9.3).
6.5. Configuration
End hosts are configured with a set of trust anchors in order to
protect Router Discovery. A trust anchor configuration consists of
the following items:
o A public key signature algorithm and associated public key, which
may optionally include parameters.
o A name as described in Section 6.4.3.
o An optional public key identifier.
o An optional list of address ranges for which the trust anchor is
authorized.
If the host has been configured to use SEND, it SHOULD possess the
above information for at least one trust anchor.
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Routers are configured with a collection of certification paths and a
collection of certificates containing certified keys, down to the key
and certificate for the router itself. Certified keys are required
for routers so that a certification path can be established between
the router's certificate and the public key of a trust anchor.
If the router has been configured to use SEND, it should be
configured with its own key pair and certificate, and with at least
one certification path.
7. Addressing
7.1. CGAs
By default, a SEND-enabled node SHOULD use only CGAs for its own
addresses. Other types of addresses MAY be used in testing, in
diagnostics, or for other purposes. However, this document does not
describe how to choose between different types of addresses for
different communications. A dynamic selection can be provided by an
API, such as the one defined in [21].
7.2. Redirect Addresses
If the Target Address and Destination Address fields in the ICMP
Redirect message are equal, then this message is used to inform hosts
that a destination is, in fact, a neighbor. In this case, the
receiver MUST verify that the given address falls within the range
defined by the router's certificate. Redirect messages failing this
check MUST be treated as unsecured, as described in Section 7.3.
Note that base NDP rules prevent a host from accepting a Redirect
message from a router that the host is not using to reach the
destination mentioned in the redirect. This prevents an attacker
from tricking a node into redirecting traffic when the attacker is
not the default router.
7.3. Advertised Subnet Prefixes
The router's certificate defines the address range(s) that it is
allowed to advertise securely. A router MAY, however, advertise a
combination of certified and uncertified subnet prefixes.
Uncertified subnet prefixes are treated as unsecured (i.e., processed
in the same way as unsecured router advertisements sent by non-SEND
routers). The processing of unsecured messages is specified in
Section 8. Note that SEND nodes that do not attempt to interoperate
with non-SEND nodes MAY simply discard the unsecured information.
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Certified subnet prefixes fall into the following two categories:
Constrained
If the network operator wants to constrain which routers are
allowed to route particular subnet prefixes, routers should be
configured with certificates having subnet prefixes listed in the
prefix extension. These routers SHOULD advertise the subnet
prefixes that they are certified to route, or a subset thereof.
Unconstrained
Network operators that do not want to constrain routers this way
should configure routers with certificates containing either the
null prefix or no prefix extension at all.
Upon processing a Prefix Information option within a Router
Advertisement, nodes SHOULD verify that the prefix specified in this
option falls within the range defined by the certificate, if the
certificate contains a prefix extension. Options failing this check
are treated as containing uncertified subnet prefixes.
Nodes SHOULD use one of the certified subnet prefixes for stateless
autoconfiguration. If none of the advertised subnet prefixes match,
the host SHOULD use a different advertising router as its default
router, if one is available. If the node is performing stateful
autoconfiguration, it SHOULD check the address provided by the DHCP
server against the certified subnet prefixes and SHOULD NOT use the
address if the prefix is not certified.
7.4. Limitations
This specification does not address the protection of NDP packets for
nodes configured with a static address (e.g., PREFIX::1). Future
certification path-based authorization specifications are needed for
these nodes. This specification also does not apply to addresses
generated by the IPv6 stateless address autoconfiguration from a
fixed interface identifiers (such as EUI-64).
It is outside the scope of this specification to describe the use of
trust anchor authorization between nodes with dynamically changing
addresses. These addresses may be the result of stateful or
stateless address autoconfiguration, or may have resulted from the
use of RFC 3041 [17] addresses. If the CGA method is not used, nodes
are required to exchange certification paths that terminate in a
certificate authorizing a node to use an IP address having a
particular interface identifier. This specification does not specify
the format of these certificates, as there are currently only a few
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cases where they are provided by the link layer, and it is up to the
link layer to provide certification for the interface identifier.
This may be the subject of a future specification. It is also
outside the scope of this specification to describe how stateful
address autoconfiguration works with the CGA method.
The Target Address in Neighbor Advertisement is required to be equal
to the source address of the packet, except in proxy Neighbor
Discovery, which is not supported by this specification.
8. Transition Issues
During the transition to secured links, or as a policy consideration,
network operators may want to run a particular link with a mixture of
nodes accepting secured and unsecured messages. Nodes that support
SEND SHOULD support the use of secured and unsecured NDP messages at
the same time.
In a mixed environment, SEND nodes receive both secured and unsecured
messages but give priority to secured ones. Here, the "secured"
messages are those that contain a valid signature option, as
specified above, and "unsecured" messages are those that contain no
signature option.
A SEND node SHOULD have a configuration option that causes it to
ignore all unsecured Neighbor Solicitation and Advertisement, Router
Solicitation and Advertisement, and Redirect messages. This can be
used to enforce SEND-only networks. The default for this
configuration option SHOULD be that both secured and unsecured
messages are allowed.
A SEND node MAY also have a configuration option whereby it disables
the use of SEND completely, even for the messages it sends itself.
This configuration option SHOULD be switched off by default; that is,
SEND is used. Plain (non-SEND) NDP nodes will obviously send only
unsecured messages. Per RFC 2461 [4], such nodes will ignore the
unknown options and will treat secured messages in the same way that
they treat unsecured ones. Secured and unsecured nodes share the
same network resources, such as subnet prefixes and address spaces.
SEND nodes configured to use SEND at least in their own messages
behave in a mixed environment as explained below.
SEND adheres to the rules defined for the base NDP protocol, with the
following exceptions:
o All solicitations sent by a SEND node MUST be secured.
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o Unsolicited advertisements sent by a SEND node MUST be secured.
o A SEND node MUST send a secured advertisement in response to a
secured solicitation. Advertisements sent in response to an
unsecured solicitation MUST be secured as well, but MUST NOT
contain the Nonce option.
o A SEND node that uses the CGA authorization method to protect
Neighbor Solicitations SHOULD perform Duplicate Address Detection
as follows. If Duplicate Address Detection indicates that the
tentative address is already in use, the node generates a new
tentative CGA. If after three consecutive attempts no non-unique
address is generated, it logs a system error and gives up
attempting to generate an address for that interface.
When performing Duplicate Address Detection for the first
tentative address, the node accepts both secured and unsecured
Neighbor Advertisements and Solicitations received in response to
the Neighbor Solicitations. When performing Duplicate Address
Detection for the second or third tentative address, it ignores
unsecured Neighbor Advertisements and Solicitations. (The
security implications of this are discussed in Section 9.2.3 and
in [11].)
o The node MAY have a configuration option whereby it ignores
unsecured advertisements, even when performing Duplicate Address
Detection for the first tentative address. This configuration
option SHOULD be disabled by default. This is a recovery
mechanism for cases in which attacks against the first address
become common.
o The Neighbor Cache, Prefix List, and Default Router list entries
MUST have a secured/unsecured flag that indicates whether the
message that caused the creation or last update of the entry was
secured or unsecured. Received unsecured messages MUST NOT cause
changes to existing secured entries in the Neighbor Cache, Prefix
List, or Default Router List. Received secured messages MUST
cause an update of the matching entries, which MUST be flagged as
secured.
o Neighbor Solicitations for the purpose of Neighbor Unreachability
Detection (NUD) MUST be sent to that neighbor's solicited-nodes
multicast address if the entry is not secured with SEND.
Upper layer confirmations on unsecured neighbor cache entries
SHOULD NOT update neighbor cache state from STALE to REACHABLE on
a SEND node if the neighbor cache entry has never previously been
REACHABLE. This ensures that if an entry spoofing a valid SEND
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host is created by a non-SEND attacker without being solicited,
NUD will be done with the entry for data transmission within five
seconds of use.
As a result, in mixed mode, attackers can take over a Neighbor
Cache entry of a SEND node for a longer time only if (a) the SEND
node was not communicating with the victim node, so that there is
no secure entry for it, and (b) the SEND node is not currently on
the link (or is unable to respond).
o The conceptual sending algorithm is modified so that an unsecured
router is selected only if there is no reachable SEND router for
the prefix. That is, the algorithm for selecting a default router
favors reachable SEND routers over reachable non-SEND ones.
o A node MAY adopt a router sending unsecured messages, or a router
for which secured messages have been received but for which full
security checks have not yet been completed, while security
checking is underway. Security checks in this case include
certification path solicitation, certificate verification, CRL
checks, and RA signature checks. A node MAY also adopt a router
sending unsecured messages if a router known to be secured becomes
unreachable, but because the unreachability may be the result of
an attack it SHOULD attempt to find a router known to be secured
as soon as possible. Note that although this can speed up
attachment to a new network, accepting a router that is sending
unsecured messages or for which security checks are not complete
opens the node to possible attacks. Nodes that choose to accept
such routers do so at their own risk. The node SHOULD, in any
case, prefer a router known to be secure as soon as one is made
available with completed security checks.
9. Security Considerations
9.1. Threats to the Local Link Not Covered by SEND
SEND does not provide confidentiality for NDP communications.
SEND does not compensate for an unsecured link layer. For instance,
there is no assurance that payload packets actually come from the
same peer against which the NDP was run.
There may not be cryptographic binding in SEND between the link layer
frame address and the IPv6 address. An unsecured link layer could
allow nodes to spoof the link layer address of other nodes. An
attacker could disrupt IP service by sending out a Neighbor
Advertisement on an unsecured link layer, with the link layer source
address on the frame set as the source address of a victim, a valid
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CGA address and a valid signature corresponding to itself, and a
Target Link-layer Address extension corresponding to the victim. The
attacker could then make a traffic stream bombard the victim in a DoS
attack. This cannot be prevented just by securing the link layer.
Even on a secured link layer, SEND does not require that the
addresses on the link layer and Neighbor Advertisements correspond.
However, performing these checks is RECOMMENDED if the link layer
technology permits.
Prior to participating in Neighbor Discovery and Duplicate Address
Detection, nodes must subscribe to the link-scoped All-Nodes
Multicast Group and the Solicited-Node Multicast Group for the
address that they are claiming as their addresses; RFC 2461 [4].
Subscribing to a multicast group requires that the nodes use MLD
[16]. MLD contains no provision for security. An attacker could
send an MLD Done message to unsubscribe a victim from the Solicited-
Node Multicast address. However, the victim should be able to detect
this attack because the router sends a Multicast-Address-Specific
Query to determine whether any listeners are still on the address, at
which point the victim can respond to avoid being dropped from the
group. This technique will work if the router on the link has not
been compromised. Other attacks using MLD are possible, but they
primarily lead to extraneous (but not necessarily overwhelming)
traffic.
9.2. How SEND Counters Threats to NDP
The SEND protocol is designed to counter the threats to NDP, as
outlined in [22]. The following subsections contain a regression of
the SEND protocol against the threats, to illustrate which aspects of
the protocol counter each threat.
9.2.1. Neighbor Solicitation/Advertisement Spoofing
This threat is defined in Section 4.1.1 of [22]. The threat is that
a spoofed message may cause a false entry in a node's Neighbor Cache.
There are two cases:
1. Entries made as a side effect of a Neighbor Solicitation or Router
Solicitation. A router receiving a Router Solicitation with a
Target Link-Layer Address extension and the IPv6 source address
unequal to the unspecified address inserts an entry for the IPv6
address into its Neighbor Cache. Also, a node performing
Duplicate Address Detection (DAD) that receives a Neighbor
Solicitation for the same address regards the situation as a
collision and ceases to solicit for the address.
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In either case, SEND counters these threats by requiring that the
RSA Signature and CGA options be present in these solicitations.
SEND nodes can send Router Solicitation messages with a CGA source
address and a CGA option, which the router can verify, so that the
Neighbor Cache binding is correct. If a SEND node must send a
Router Solicitation with the unspecified address, the router will
not update its Neighbor Cache, as per base NDP.
2. Entries made as a result of a Neighbor Advertisement message.
SEND counters this threat by requiring that the RSA Signature and
CGA options be present in these advertisements.
Also see Section 9.2.5, below, for discussion about replay protection
and timestamps.
9.2.2. Neighbor Unreachability Detection Failure
This attack is described in Section 4.1.2 of [22]. SEND counters it
by requiring that a node responding to Neighbor Solicitations sent as
NUD probes include an RSA Signature option and proof of authorization
to use the interface identifier in the address being probed. If
these prerequisites are not met, the node performing NUD discards the
responses.
9.2.3. Duplicate Address Detection DoS Attack
This attack is described in Section 4.1.3 of [22]. SEND counters
this attack by requiring that the Neighbor Advertisements sent as
responses to DAD include an RSA Signature option and proof of
authorization to use the interface identifier in the address being
tested. If these prerequisites are not met, the node performing DAD
discards the responses.
When a SEND node performs DAD, it may listen for address collisions
from non-SEND nodes for the first address it generates, but not for
new attempts. This protects the SEND node from DAD DoS attacks by
non-SEND nodes or attackers simulating non-SEND nodes, at the cost of
a potential address collision between a SEND node and a non-SEND
node. The probability and effects of such an address collision are
discussed in [11].
9.2.4. Router Solicitation and Advertisement Attacks
These attacks are described in Sections 4.2.1, 4.2.4, 4.2.5, 4.2.6,
and 4.2.7 of [22]. SEND counters them by requiring that Router
Advertisements contain an RSA Signature option, and that the
signature is calculated by using the public key of a node that can
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prove its authorization to route the subnet prefixes contained in any
Prefix Information Options. The router proves its authorization by
showing a certificate containing the specific prefix or an indication
that the router is allowed to route any prefix. A Router
Advertisement without these protections is discarded.
SEND does not protect against brute force attacks on the router, such
as DoS attacks, or against compromise of the router, as described in
Sections 4.4.2 and 4.4.3 of [22].
9.2.5. Replay Attacks
This attack is described in Section 4.3.1 of [22]. SEND protects
against attacks in Router Solicitation/Router Advertisement and
Neighbor Solicitation/Neighbor Advertisement transactions by
including a Nonce option in the solicitation and requiring that the
advertisement include a matching option. Together with the
signatures, this forms a challenge-response protocol.
SEND protects against attacks from unsolicited messages such as
Neighbor Advertisements, Router Advertisements, and Redirects by
including a Timestamp option. The following security issues are
relevant only for unsolicited messages:
o A window of vulnerability for replay attacks exists until the
timestamp expires.
However, such vulnerabilities are only useful for attackers if the
advertised parameters change during the window. Although some
parameters (such as the remaining lifetime of a prefix) change
often, radical changes typically happen only in the context of
some special case, such as switching to a new link layer address
due to a broken interface adapter.
SEND nodes are also protected against replay attacks as long as
they cache the state created by the message containing the
timestamp. The cached state allows the node to protect itself
against replayed messages. However, once the node flushes the
state for whatever reason, an attacker can re-create the state by
replaying an old message while the timestamp is still valid.
Because most SEND nodes are likely to use fairly coarse-grained
timestamps, as explained in Section 5.3.1, this may affect some
nodes.
o Attacks against time synchronization protocols such as NTP [23]
may cause SEND nodes to have an incorrect timestamp value. This
can be used to launch replay attacks, even outside the normal
window of vulnerability. To protect against these attacks, it is
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recommended that SEND nodes keep independently maintained clocks
or apply suitable security measures for the time synchronization
protocols.
9.2.6. Neighbor Discovery DoS Attack
This attack is described in Section 4.3.2 of [22]. In it, the
attacker bombards the router with packets for fictitious addresses on
the link, causing the router to busy itself by performing Neighbor
Solicitations for addresses that do not exist. SEND does not address
this threat because it can be addressed by techniques such as rate
limiting Neighbor Solicitations, restricting the amount of state
reserved for unresolved solicitations, and clever cache management.
These are all techniques involved in implementing Neighbor Discovery
on the router.
9.3. Attacks against SEND Itself
The CGAs have a 59-bit hash value. The security of the CGA mechanism
has been discussed in [11].
Some Denial-of-Service attacks remain against NDP and SEND itself.
For instance, an attacker may try to produce a very high number of
packets that a victim host or router has to verify by using
asymmetric methods. Although safeguards are required to prevent an
excessive use of resources, this can still render SEND non-
operational.
When CGA protection is used, SEND deals with the DoS attacks by using
the verification process described in Section 5.2.2. In this
process, a simple hash verification of the CGA property of the
address is performed before the more expensive signature
verification. However, even if the CGA verification succeeds, no
claims about the validity of the message can be made until the
signature has been checked.
When trust anchors and certificates are used for address validation
in SEND, the defenses are not quite as effective. Implementations
SHOULD track the resources devoted to the processing of packets
received with the RSA Signature option and start selectively
discarding packets if too many resources are spent. Implementations
MAY also first discard packets that are not protected with CGA.
The Authorization Delegation Discovery process may also be vulnerable
to Denial-of-Service attacks. An attack may target a router by
requesting that a large number of certification paths be discovered
for different trust anchors. Routers SHOULD defend against such
attacks by caching discovered information (including negative
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responses) and by limiting the number of different discovery
processes in which they engage.
Attackers may also target hosts by sending a large number of
unnecessary certification paths, forcing hosts to spend useless
memory and verification resources on them. Hosts can defend against
such attacks by limiting the amount of resources devoted to the
certification paths and their verification. Hosts SHOULD also
prioritize advertisements sent as a response to solicitations the
hosts have sent about unsolicited advertisements.
10. Protocol Values
10.1. Constants
Host constants:
CPS_RETRY 1 second
CPS_RETRY_FRAGMENTS 2 seconds
CPS_RETRY_MAX 15 seconds
Router constants:
MAX_CPA_RATE 10 times per second
10.2. Variables
TIMESTAMP_DELTA 300 seconds (5 minutes)
TIMESTAMP_FUZZ 1 second
TIMESTAMP_DRIFT 1 % (0.01)
11. IANA Considerations
This document defines two new ICMP message types, used in
Authorization Delegation Discovery. These messages must be assigned
ICMPv6 type numbers from the informational message range:
o The Certification Path Solicitation message (148), described in
Section 6.4.1.
o The Certification Path Advertisement message (149), described in
Section 6.4.2.
This document defines six new Neighbor Discovery Protocol [4]
options, which must be assigned Option Type values within the option
numbering space for Neighbor Discovery Protocol messages:
o The CGA option (11), described in Section 5.1.
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o The RSA Signature option (12), described in Section 5.2.
o The Timestamp option (13), described in Section 5.3.1.
o The Nonce option (14), described in Section 5.3.2.
o The Trust Anchor option (15), described in Section 6.4.3.
o The Certificate option (16), described in Section 6.4.4.
This document defines a new 128-bit value under the CGA Message Type
[11] namespace, 0x086F CA5E 10B2 00C9 9C8C E001 6427 7C08.
This document defines a new name space for the Name Type field in the
Trust Anchor option. Future values of this field can be allocated by
using Standards Action [3]. The current values for this field are
1 DER Encoded X.501 Name
2 FQDN
Another new name space is allocated for the Cert Type field in the
Certificate option. Future values of this field can be allocated by
using Standards Action [3]. The current values for this field are
1 X.509v3 Certificate
12. References
12.1. Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[4] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[5] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
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[6] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
[7] 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.
[8] Farrell, S. and R. Housley, "An Internet Attribute Certificate
Profile for Authorization", RFC 3281, April 2002.
[9] Faltstrom, P., Hoffman, P. and A. Costello, "Internationalizing
Domain Names in Applications (IDNA)", RFC 3490, March 2003.
[10] Lynn, C., Kent, S. and K. Seo, "X.509 Extensions for IP
Addresses and AS Identifiers", RFC 3779, June 2004.
[11] Aura, T., "Cryptographically Generated Addresses (CGA)", RFC
3972, March 2005.
[12] International Telecommunications Union, "Information Technology
- ASN.1 encoding rules: Specification of Basic Encoding Rules
(BER), Canonical Encoding Rules (CER) and Distinguished
Encoding Rules (DER)", ITU-T Recommendation X.690, July 2002.
[13] RSA Laboratories, "RSA Encryption Standard, Version 2.1", PKCS
1, November 2002.
[14] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS PUB 180-1, April 1995,
<http://www.itl.nist.gov/fipspubs/fip180-1.htm>.
12.2. Informative References
[15] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[16] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
Discovery (MLD) for IPv6", RFC 2710, October 1999.
[17] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[18] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
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[19] Arkko, J., "Effects of ICMPv6 on IKE and IPsec Policies", Work
in Progress, March 2003.
[20] Arkko, J., "Manual SA Configuration for IPv6 Link Local
Messages", Work in Progress, June 2002.
[21] Nordmark, E., Chakrabarti, S. and J. Laganier, "IPv6 Socket API
for Address Selection", Work in Progress, October 2003.
[22] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[23] Bishop, M., "A Security Analysis of the NTP Protocol", Sixth
Annual Computer Security Conference Proceedings, December 1990.
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Appendix A. Contributors and Acknowledgments
Tuomas Aura contributed the transition mechanism specification in
Section 8. Jonathan Trostle contributed the certification path
example in Section 6.3.1. Bill Sommerfeld was involved with much of
the early design work.
The authors would also like to thank Tuomas Aura, Bill Sommerfeld,
Erik Nordmark, Gabriel Montenegro, Pasi Eronen, Greg Daley, Jon Wood,
Julien Laganier, Francis Dupont, Pekka Savola, Wenxiao He, Valtteri
Niemi, Mike Roe, Russ Housley, Thomas Narten, and Steven Bellovin for
interesting discussions in this problem space and for feedback
regarding the SEND protocol.
Appendix B. Cache Management
In this section, we outline a cache management algorithm that allows
a node to remain partially functional even under a cache-filling DoS
attack. This appendix is informational, and real implementations
SHOULD use different algorithms in order to avoid the dangers of a
mono-cultural code.
There are at least two distinct cache-related attack scenarios:
1. There are a number of nodes on a link, and someone launches a
cache filling attack. The goal here is to make sure that the
nodes can continue to communicate even if the attack is going on.
2. There is already a cache-filling attack going on, and a new node
arrives to the link. The goal here is to make it possible for the
new node to become attached to the network, in spite of the
attack.
As the intent is to limit the damage to existing, valid cache
entries, it is clearly better to be very selective in throwing out
entries. Reducing the timestamp Delta value is very discriminatory
against nodes with a large clock difference, as an attacker can
reduce its clock difference arbitrarily. Throwing out old entries
just because their clock difference is large therefore seems like a
bad approach.
It is reasonable to have separate cache spaces for new and old
entries, where when under attack, the newly cached entries would be
more readily dropped. One could track traffic and only allow
reasonable new entries that receive genuine traffic to be converted
into old cache entries. Although such a scheme can make attacks
harder, it will not fully prevent them. For example, an attacker
could send a little traffic (i.e., a ping or TCP syn) after each NS
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to trick the victim into promoting its cache entry to the old cache.
To counter this, the node can be more intelligent in keeping its
cache entries than it would be just by having a black/white old/new
boundary.
Distinction of the Sec parameter from the CGA Parameters when forcing
cache entries out -- by keeping entries with larger Sec parameters
preferentially -- also appears to be a possible approach, as CGAs
with higher Sec parameters are harder to spoof.
Appendix C. Message Size When Carrying Certificates
In one example scenario using SEND, an Authorization Delegation
Discovery test run was made with a certification path length of 4.
Three certificates are sent by using Certification Path Advertisement
messages, as the trust anchor's certificate is already known by both
parties. With a key length of 1024 bits, the certificate lengths in
the test run ranged from 864 to 888 bytes; the variation is due to
the differences in the certificate issuer names and address prefix
extensions. The different certificates had between 1 and 4 address
prefix extensions.
The three Certification Path Advertisement messages ranged from 1050
to 1,066 bytes on an Ethernet link layer. The certificate itself
accounts for the bulk of the packet. The rest is the trust anchor
option, ICMP header, IPv6 header, and link layer header.
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Authors' Addresses
Jari Arkko
Ericsson
Jorvas 02420
Finland
EMail: jari.arkko@ericsson.com
James Kempf
DoCoMo Communications Labs USA
181 Metro Drive
San Jose, CA 94043
USA
EMail: kempf@docomolabs-usa.com
Brian Zill
Microsoft Research
One Microsoft Way
Redmond, WA 98052
USA
EMail: bzill@microsoft.com
Pekka Nikander
Ericsson
Jorvas 02420
Finland
EMail: Pekka.Nikander@nomadiclab.com
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Full Copyright Statement
Copyright © The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
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RFC TOTAL SIZE: 123372 bytes
PUBLICATION DATE: Friday, March 11th, 2005
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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