NETWORK WORKING GROUP N. Williams
Internet-Draft Sun
Intended status: Standards Track M. Richardson
Expires: October 3, 2007 SSW
April 2007
Better-Than-Nothing-Security: An Unauthenticated Mode of IPsec
draft-ietf-btns-core-03.txt
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Abstract
This document specifies how to use the Internet Key Exchange (IKE)
protocols, such as IKEv1 and IKEv2, to setup "unauthenticated"
security associations (SAs) for use with the IPsec Encapsulating
Security Payload (ESP) and the IPsec Authentication Header (AH). No
IKE extensions are needed, but Peer Authorization Database (PAD) and
Security Policy Database (SPD) extensions are specified.
Unauthenticated IPsec is herein referred to by its popular acronym,
"BTNS" (Better Than Nothing Security).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . . 3
2. BTNS . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Example #1: sgA . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Example #2: Q . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Example #3: C . . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Miscaellaneous examples . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . 10
4.1. Connection-Latching and Channel Binding . . . . . . . . . . 10
4.2. Leap-of-Faith (LoF) for BTNS . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1. Normative References . . . . . . . . . . . . . . . . . . . . 13
6.2. Informative References . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . 15
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1. Introduction
Here we describe how to establish unauthenticated IPsec SAs using
IKEv1 [RFC2408] [RFC2409] or IKEv2 [RFC4306] and unauthenticated
public keys. No new on-the-wire protocol elements are added to IKE
or IKEv2.
The [RFC4301] processing model is assumed.
This document does not define an opportunistic BTNS mode of IPsec
whereby nodes may fallback on unprotected IP when their peers do not
support IKE or IKEv2, nor does it describe "leap-of-faith" modes, or
"connection latching."
See [I-D.ietf-btns-prob-and-applic] for the applicability and uses of
BTNS.
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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2. BTNS
The IPsec processing model, IKE and IKEv2 are hereby modified as
follows:
o A new ID type is added, 'PUBLICKEY'; IDs of this type have public
keys as values. This ID type is not used on the wire.
o A BTNS-specific PAD entry. This entry is intended to be the last
entry in the PAD when BTNS is enabled. A peer that matches no
other PAD entries is to be "authenticated" by verifying that the
signature in its AUTH (or SIG) payload in the IKEv2 (or v1)
exchange with the public key from the peer's CERT payload. The
peer's ID MUST then be coerced to be of 'PUBLICKEY' type with the
peer's public key as its value.
o A new flag for SPD entries: 'BTNS_OK'. Traffic to/from peers that
match the BTNS PAD entry will only match SPD entries that have the
BTNS_OK flag set. The SPD may be searched by address or by ID (of
type PUBLICKEY, of course, for BTNS peers), as per the IPsec
processing model [RFC4301]; searching by ID in this case requires
creation of SPD entries that are bound to public key values (this
could be used to build "leap-of-faith" behaviour, for example).
Nodes MUST reject IKE_SA proposals from peers that match non-BTNS PAD
entries but fail to authenticate properly.
Nodes wishing to be treated as BTNS nodes by their peers SHOULD use
bare RSA key CERT payloads and MAY use certificates known not to have
been pre-shared with their peers or outside their trust anchors
(e.g., self-signed certificates). RSA keys for use in BTNS may be
generated at any time, but "connection latching"
[I-D.ietf-btns-connection-latching] requires that they remain
constant between IKE exchanges setting up SAs for latched
connections.
To preserve standard IPsec access control semantics BTNS responders
MUST NOT allow BTNS peers to assert addresses which could be asserted
by non-BTNS peers. This can be achieved by processing the PAD in
order both, when peers authenticate and when BTNS peers negotiate
child SAs -- in the first case the PAD is searched for a matching PAD
entry as usual, and in the second it is searched to make sure that
BTNS peers' asserted child SA traffic selectors do not conflict with
non-BTNS PAD entries. Note that in general, if there are multiple
PAD entries with wildcard matching on peer ID then all but the last
one should constrain the traffic selectors for matching peers.
Note that nodes may unwittingly match peers' BTNS PAD entries and be
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authenticated as BTNS nodes. This may be used in specifying the
"latching" of traffic flows to peer IDs
[I-D.ietf-btns-connection-latching].
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3. Usage Scenarios
In order to explain the above rules a number of scenarios will be
postulated. The goal here is to demonstrate that the above rules are
both sufficient and required.
To explain the scenarios a reference diagram describing a canonical
network will be used. It is as follows:
[Q] [R]
AS1 . . AS2
[A]----+----[SG-A].......+....+.......[SG-B]-------[B]
...... \
..PI.. ----[btns-B]
......
[btns-C].....+....+.......[btns-D]
Figure 1: Reference Network Diagram
In this diagram, there are six end-nodes: A, B, C and D. Two of the
systems are security gateways: SG-A, SG-B, protecting networks on
which [A] and [B] reside. There is a node [Q] which is IPsec and
BTNS capable, and node [R] is a simple node, with no IPsec or BTNS
capability. Nodes [C] and [D] are BTNS capable. We will examine
interactions between the BTNS enabled nodes, and the IPsec enabled
nodes.
Nodes C and Q have a fixed addresses. Node D non-fixed addresses.
PI is the Public Internet ("The Wild").
3.1. Example #1: sgA
The machine that we will care about will be [SG-A], a firewall device
of some kind which we wish to configure to respond to BTNS
connections from [C]
SG-A has the following "VPN" PAD and SPD entries:
Child SA
Rule Remote ID IDs allowed SPD Search by
---- --------- ----------- -------------
1 <B's ID> <B's network> by-IP
2 <Q's ID> <Q's host> by-IP
3 PUBLICKEY:any ANY by-IP
The last entry is the BTNS entry.
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Figure 2: SG-A PAD table
Here <ANY> is any address that is not part of A's network, and is not
claimed by any other entry. Note that sgA's PAD entry has one and
only one wildcard PAD entry: the BTNS catch-all PAD entry, so, as
described in Section 2.
<Child SA IDs allowed> and <SPD Search by> are from [RFC4301] section
4.4.3
Rule Local Remote Next Layer BTNS Action
addr addr Protocol ok
---- ----- ------ ---------- ---- -----------------------
1 A R ANY N/A BYPASS
2 A Q ANY no PROTECT(ESP,tunnel,AES,
SHA256)
3 A B-net ANY no PROTECT(ESP,tunnel,AES,
SHA256)
4 A ANY ANY yes PROTECT(ESP,transport,
integr+conf)
Figure 3: SG-A SPD table
The processing by sgA of various peers then is as follows:
o Q does not match PAD entry #1, but does match PAD entry #2; PAD
processing stops, then the SPD is searched by Q's ID to find entry
#2; CHILD SAs are then allowed that have sgA's and Q's addresses
as the end-point addresses.
o sgB matches PAD entry #1; PAD processing stops, then the SPD is
searched by sgB's ID to find SPD entry #3; CHILD SAs are then
allowed that have sgA's address and any addresses from B's network
as the end-point addresses.
o R does not initiate any IKE_SAs; its traffic to A is bypassed by
SPD entry #1.
o C does not match PAD entries #1 or #2, but does match entry #3,
the BTNS wildcard PAD entry; the SPD is searched by C's address
and SPD entry #4 is matched. CHILD SAs are then allowed that have
sgA's address and C's address as the end-point addresses provided
that C's address is neither Q's nor any of B's (see Section 2).
o Rogue BTNS nodes attempting to assert Q's or B's addresses will
either match the PAD entries for Q or B and fail to authenticate
as Q or B, in which case they are rejected, or they will match PAD
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entry #3 but will not be allowed to create CHILD SAs with Q's or
B's addresses as traffic selectors.
o Rogue BTNS nodes attempting to assert C's address are allowed.
Protection for C requires additional bindings of C's specific BTNS
ID (that is, its public key) to its traffic flows through
connection-latching and channel binding, or leap-of-faith, none of
which are described here.
3.2. Example #2: Q
Q is either a BITS or native IPsec implementation; if it is a native
implementation it may have IPsec-aware applications, specifically
NFSv4 (TCP port 2049).
In any case, Q wants to communicate with A generally, and with BTNS
peers for NFSv4 only. It's PAD and SPD are configured as follows:
Child SA
Rule Remote ID IDs allowed SPD Search by
---- --------- ----------- -------------
1 <A's ID> <A's address> by-IP
2 PUBLICKEY:any ANY by-IP
The last entry is the BTNS entry.
Figure 4: Q PAD table
Rule Local Remote Next Layer BTNS Action
addr addr Protocol ok
---- ----- ------ ---------- ---- -----------------------
1 Q A ANY no PROTECT(ESP,tunnel,AES,
SHA256)
2 Q ANY ANY yes PROTECT(ESP,transport,
and integr+conf)
port
2049
Figure 5: SG-A SPD table
The same analysis shown above in Section 3.1 applies here with
respect to sgA, C and rogue peers, except that C is allowed only
access to the NFSv4 service on Q. Additionally sgB is treated as a
BTNS peer as it is not known to Q, and therefore any host behind sgB
can access the NFSv4 service on Q (and, because Q has no formal
relationship with sgB, rogues can impersonate B).
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3.3. Example #3: C
C only supports BTNS and wants to use BTNS to protect NFSv4 traffic.
It's PAD and SPD are configured as follows:
Child SA
Rule Remote ID IDs allowed SPD Search by
---- --------- ----------- -------------
1 PUBLICKEY:any ANY by-IP
The last entry is the BTNS entry.
Figure 6: Q PAD table
Rule Local Remote Next Layer BTNS Action
addr addr Protocol ok
---- ----- ------ ---------- ---- -----------------------
1 C ANY ANY yes PROTECT(ESP,transport,
and integr+conf)
port
2049
2 C ANY ANY N/A BYPASS
Figure 7: SG-A SPD table
3.4. Miscaellaneous examples
If sgA were not BTNS-capable then it would not have PAD and SPD
entries #3 and #4, respectively. Then C would be rejected as usual
under the standard IPsec model [RFC4301].
Similarly, if Q were not BTNS-capable then it would not have PAD and
SPD entries #2. Then C would be rejected as usual under the standard
IPsec model [RFC4301].
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4. Security Considerations
Unauthenticated security association negotiation is subject to MITM
attacks and should be used with care. Where security infrastructures
are lacking this may indeed be better than nothing.
Use with applications that bind authentication at higher network
layers to secure channels at lower layers may provide one secure way
to use unauthenticated IPsec, but this is not specified herein.
Use of multiple wildcard PAD entries can be problematic. Where it is
important that addresses and node identities be tightly bound it is
important that such PAD entries limit the addresses that matching
peers can assert for their CHILD SAs to non-overlapping address
spaces. In practice this may be difficult to configure; if it is not
feasible to configure systems in this way then either BTNS should not
be used or BTNS PAD entries should constrain matching peers only to
using services for which authentication is not normally necessary or
where IPsec-aware/connection-latching applications are used.
4.1. Connection-Latching and Channel Binding
BTNS is subject to MITM attacks. One way to protect against MITM
attacks subsequent to initial communications is to use "connection
latching" [I-D.ietf-btns-connection-latching], whereby ULPs cooperate
with IPsec in native IPsec implementations to bind individual packet
flows to sequences of SAs whose end-point IDs (public keys, in the
case of BTNS end-points) and other characteristics (e.g., quality of
protection) must all be the same.
MITMs can be detected by using application-layer authentication
frameworks and/or mechanisms, such as the GSS-API [RFC2743], with
channel binding [I-D.williams-on-channel-binding], where the channels
to be bound to application-layer authentication are latched
connections and where the channel bindings data strongly identify the
end-points of the latched connection (e.g., the public keys of the
end-points).
4.2. Leap-of-Faith (LoF) for BTNS
"Leap of faith" is the term generally used for the habit of accepting
that a given key identifies a peer that one wanted to talk to without
strong evidence for that proposition. Specifically this is a common
mode of operation for Secure Shell [RFC4251] clients where, when a
server is encountered for the first time the client may ask the user
whether to accept the server's public key as its identity and, if so,
records the server's name (as given by the user) and public key in a
database.
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Leap of Faith can work in a similar way for BTNS nodes, but it is
currently still being designed and specified by the IETF BTNS WG.
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5. IANA Considerations
This document has no IANA considerations, neither seeking to create
new registrations nor new registries. (The new ID type is not used
on the wire, therefore it need not be assigned a number from the IANA
IKEv2 Identification Payload ID Types registry.)
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6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
6.2. Informative References
[I-D.ietf-btns-connection-latching]
Williams, N., "IPsec Channels: Connection Latching",
draft-ietf-btns-connection-latching-00 (work in progress),
February 2006.
[I-D.ietf-btns-prob-and-applic]
Touch, J., "Problem and Applicability Statement for Better
Than Nothing Security (BTNS)",
draft-ietf-btns-prob-and-applic-05 (work in progress),
February 2007.
[I-D.williams-on-channel-binding]
Williams, N., "On the Use of Channel Bindings to Secure
Channels", draft-williams-on-channel-binding-00 (work in
progress), August 2006.
[RFC2408] Maughan, D., Schneider, M., and M. Schertler, "Internet
Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture", RFC 4251, January 2006.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
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Authors' Addresses
Nicolas Williams
Sun Microsystems
5300 Riata Trace Ct
Austin, TX 78727
US
Email: Nicolas.Williams@sun.com
Michael C. Richardson
Sandelman Software Works
470 Dawson Avenue
Ottawa, ON K1Z 5V7
CA
Email: mcr@sandelman.ottawa.on.ca
URI: http://www.sandelman.ottawa.on.ca/
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