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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 changes to IKEv2 bits-on-the-wire are required, 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).
1.
Introduction
1.1.
Conventions used in this document
2.
BTNS
3.
Usage Scenarios
3.1.
Example #1: A security gateway
3.2.
Example #2: A mixed end-system
3.3.
Example #3: A BTNS-only system
3.4.
Miscellaneous comments
4.
Security Considerations
4.1.
Connection-Latching and Channel Binding
4.2.
Leap-of-Faith (LoF) for BTNS
5.
IANA Considerations
6.
Acknowledgements
7.
References
7.1.
Normative References
7.2.
Informative References
§
Authors' Addresses
§
Intellectual Property and Copyright Statements
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Here we describe how to establish unauthenticated IPsec SAs using IKEv2 [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.) and unauthenticated public keys. No new on-the-wire protocol elements are added to IKEv2.
The [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.) processing model is assumed.
This document does not define an opportunistic BTNS mode of IPsec whereby nodes may fallback to unprotected IP when their peers do not support IKEv2, nor does it describe "leap-of-faith" modes, or "connection latching."
See [I‑D.ietf‑btns‑prob‑and‑applic] (Touch, J., Black, D., and Y. Wang, “Problem and Applicability Statement for Better Than Nothing Security (BTNS),” July 2008.) for the applicability and uses of BTNS and definitions of these terms.
This document describes BTNS in terms of IKEv2 and [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.)'s concepts. There is no reason why the same methods cannot be used with IKEv1 [RFC2408] (Maughan, D., Schneider, M., and M. Schertler, “Internet Security Association and Key Management Protocol (ISAKMP),” November 1998.) [RFC2409] (Harkins, D. and D. Carrel, “The Internet Key Exchange (IKE),” November 1998.) and [RFC2401] (Kent, S. and R. Atkinson, “Security Architecture for the Internet Protocol,” November 1998.), however, those specifications do not include the PAD concepts, and therefore it may not be possible to implement BTNS on all compliant RFC2401 implementations.
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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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
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The IPsec processing model is hereby modified as follows:
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 MUST include bare public key CERT payloads. Currently only bare RSA public key CERT payloads are defined, which means that BTNS works only with RSA public keys at this time (see "Raw RSA Key" in section 3.6 of [RFC4306] (Kaufman, C., “Internet Key Exchange (IKEv2) Protocol,” December 2005.)). Nodes MAY also include any number of certificates that bind the same public key. These certificates need not to have been pre-shared with their peers (e.g., because ephermal, self-signed). RSA keys for use in BTNS may be generated at any time, but "connection latching" [I‑D.ietf‑btns‑connection‑latching] (Williams, N., “IPsec Channels: Connection Latching,” August 2009.) requires that they remain constant between IKEv2 exchanges that are used to establish SAs for latched connections.
To preserve standard IPsec access control semantics:
As described above, the logical PAD ordering requirements can easily be implemented by searching the PAD twice at peer authentication time: once using the peer-asserted ID, and if that fails, once using the peer's public key as a PUBLICKEY ID. A single pass implementation that meets this requirement is permitted.
The BTNS entry ID constraint non-overlap requirement can easily be implemented by searching the PAD twice: once when BTNS peers authenticate, and again when BTNS peers negotiate child SAs. In the first pass the PAD is searched for a matching PAD entry as described above, 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. Single pass implementations that preserve these semantics are feasible.
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In order to explain the above rules a number of scenarios will be examined. The goal here is to persuade the reader that the above rules are both sufficient and necessary.
This section is informative only.
To explain the scenarios a reference diagram describing an example 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.
Nodes [C] and [Q] have fixed addresses. Node [D] has a non-fixed address.
We will examine how these various nodes communicate with node SG-A, and/or how SG-A rejects communications with some such nodes. In the first example, we examine SG-A's point of view. In the second example we look at Q's point of view. In the third example we look at C's point of view.
PI is the Public Internet ("The Wild").
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The machine that we will care in this example is [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 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.
| Figure 2: SG-A PAD table |
Note that [SG-A]'s PAD entry has one and only one wildcard PAD entry: the BTNS catch-all PAD entry as the last entry, as described in Section 2 (BTNS).
<Child SA IDs allowed> and <SPD Search by> are from [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.) 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 [SG-A] of SA establishment attempts by various peers is as follows:
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[Q] is an NFSv4 server.
[Q] is a native IPsec implementation, and it's NFSv4 implementation is IPsec-aware.
[Q] wants to protect all traffic with [A]. [Q] also wants
to protect NFSv4 traffice with all peers. 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,
with integr+conf)
port 2049
| Figure 5: [Q] SPD table |
The same analysis shown above in Section 3.1 (Example #1: A security gateway) applies here with respect to [SG-A], [C] and rogue peers. The second SPD entry permits any BTNS capable node to negotiate a port-specific SA to port 2049, the port on which NFSv4 runs. Additionally [SG-B] is treated as a BTNS peer as it is not known to [Q], and therefore any host behind [SG-B] can access the NFSv4 service on [Q]. As [Q] has no formal relationship with [SG-B], rogues can impersonate [B] (i.e., assert [B]'s addresses).
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[C] supports only 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 (and only) 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,
with integr+conf)
port
2049
2 [C] ANY ANY N/A BYPASS
| Figure 7: SG-A SPD table |
The analysis from Section 3.1 (Example #1: A security gateway) applies as follows:
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If [SG-A] were not BTNS-capable then it would not have PAD and SPD entries #3 and #4, respectively in example #1. Then [C] would be rejected as usual under the standard IPsec model [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.).
Similarly, if [Q] were not BTNS-capable then it would not have PAD and SPD entries #2 in example #2. Then [C] would be rejected as usual under the standard IPsec model [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.).
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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.
The BTNS PAD entry must be last and its child SA ID constraints must be non-overlapping with any other PAD entry, as described in section 2, in order to ensure that no BTNS peer can impersonate another IPsec non-BTNS peer.
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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] (Williams, N., “IPsec Channels: Connection Latching,” August 2009.). In connection latching, ULPs cooperate with IPsec to bind discrete packet flows to sequences of similar SAs. Connection latching requires native IPsec implementations.
MITMs can be detected by using application-layer authentication frameworks and/or mechanisms, such as the GSS-API [RFC2743] (Linn, J., “Generic Security Service Application Program Interface Version 2, Update 1,” January 2000.), with channel binding [RFC5056] (Williams, N., “On the Use of Channel Bindings to Secure Channels,” November 2007.). IPsec "channels" are nothing other than latched connnections.
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"Leap of faith" is the term generally used when a user accepts the assertion that a given key identifies a peer on the first communication, despite a lack of strong evidence for that assertion, and then remembers this association for future communications. Specifically this is a common mode of operation for Secure Shell [RFC4251] (Ylonen, T. and C. Lonvick, “The Secure Shell (SSH) Protocol Architecture,” January 2006.) client. When a server is encountered for the first time the Secure Shell client may ask the user whether to accept the server's public key. If so, records the server's name (as given by the user) and public key in a database.
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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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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Thanks for the following reviewers: Stephen Kent
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| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [RFC4301] | Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” RFC 4301, December 2005 (TXT). |
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| 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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