One document matched: draft-ietf-btns-core-07.xml
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<rfc category="std" ipr="full3978" docName="draft-ietf-btns-core-07.txt">
<front>
<title abbrev="BTNS IPsec">Better-Than-Nothing-Security: An Unauthenticated Mode of IPsec</title>
<author initials='N.' surname="Williams" fullname='Nicolas
Williams'>
<organization abbrev="Sun">Sun Microsystems</organization>
<address>
<postal>
<street>5300 Riata Trace Ct</street>
<city>Austin</city> <region>TX</region>
<code>78727</code> <country>US</country>
</postal>
<email>Nicolas.Williams@sun.com</email>
</address>
</author>
<author initials="M." surname="Richardson" fullname="Michael C.
Richardson">
<organization abbrev="SSW">Sandelman Software
Works</organization>
<address>
<postal>
<street>470 Dawson Avenue</street>
<city>Ottawa</city>
<region>ON</region>
<code>K1Z
5V7</code>
<country>CA</country>
</postal>
<email>mcr@sandelman.ottawa.on.ca</email>
<uri>http://www.sandelman.ottawa.on.ca/</uri>
</address>
</author>
<date month="August" year="2008"/>
<area>Security</area>
<workgroup>NETWORK WORKING GROUP</workgroup>
<keyword>Internet-Draft</keyword>
<abstract><t>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).</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>Here we describe how to establish unauthenticated IPsec
SAs using IKEv2 <xref target="RFC4306"/> and
unauthenticated public keys. No new on-the-wire
protocol elements are added to IKEv2.</t>
<t>The <xref target="RFC4301"/> processing model is
assumed.</t>
<t>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."</t>
<t>See <xref target="I-D.ietf-btns-prob-and-applic"/> for
the applicability and uses of BTNS and definitions of
these terms.</t>
<t>This document describes BTNS in terms of IKEv2 and <xref
target="RFC4301"/>'s concepts. There is no reason
why the same methods cannot be used with IKEv1 <xref
target="RFC2408"/> <xref target="RFC2409" /> and
<xref target="RFC2401" />, however, those specifications
do not include the PAD concepts, and therefore it may
not be possible to implement BTNS on all compliant
RFC2401 implementations.</t>
<section title="Conventions used in this document">
<t>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 <xref target="RFC2119"/>.</t>
</section>
</section>
<section anchor="core" title="BTNS">
<t>The IPsec processing model is hereby modified as follows:
<list style='symbols'>
<t>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.</t>
<t>PAD entries that match on PUBLICKEY IDs are
referred to as "BTNS PAD entries." All other
PAD entries are referred to as "non-BTNS PAD
entries."</t>
<t>BTNS PAD entries may match on specific peer
PUBLICKEY IDs (or public key fingerprints), or
on all peer public keys. The latter is referred
to as the "wildcard BTNS PAD entry."</t>
<t>BTNS PAD entries MUST logically (see below)
follow all other PAD entries (the PAD being an
ordered list).</t>
<t>At most one wildcard BTNS PAD entry may appear in
the PAD, and, if present, MUST be the last entry
in the PAD (see below).</t>
<t>Any peer that uses an IKEv2 AUTH method involving
a digital signature (made with a private key to
a public key cryptosystem) may match a BTNS PAD
entry, provided that it matches no non-BTNS PAD
entries. Suitable AUTH methods as of August
2007 are: RSA Digital Signature (method #1) and
DSS Digital Signature (method #3); see <xref
target="RFC4306"/>, section 3.8.</t>
<t>A BTNS capable implementation of IPsec will first
search the PAD for non-BTNS entries matching a
peer's ID. If no matching non-BTNS PAD entries
are found then the peer's ID MUST then be
coerced to be of 'PUBLICKEY' type with the
peer's public key as its value and the PAD is
then searched again for matching BTNS PAD
entries. This ensures that BTNS PAD entries
logically follow non-BTNS PAD entries. A single
PAD search that preserves these semantics is
allowed.</t>
<t>A peer that matches a BTNS PAD entry is referred
to as a "BTNS peer." Such a peer is
"authenticated" by verifying that the signature
in its IKEv2 AUTH payload with the public key
from the peer's CERT payload.</t>
<t>Of course, if no matching PAD entry is found,
then the IKE SA is rejected as usual.</t>
<t>A new flag for SPD entries: 'BTNS_OK'. Traffic
to/from peers that match the BTNS PAD entry will
match only SPD entries that have the BTNS_OK
flag set. The SPD may be searched by address or
by ID (of type PUBLICKEY, for BTNS peers), as
per the IPsec processing model <xref
target="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" <xref
target="I-D.ietf-btns-prob-and-applic"/>
<xref target="lof" /> behaviour, for
example).</t>
</list>
</t>
<t>Nodes MUST reject IKE_SA proposals from peers that match
non-BTNS PAD entries but fail to authenticate
properly.</t>
<t>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 <xref
target='RFC4306'/>). 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" <xref
target="I-D.ietf-btns-connection-latching"/>
requires that they remain constant between IKEv2
exchanges that are used to establish SAs for latched
connections.</t>
<t>To preserve standard IPsec access control semantics:
<list style='symbols'>
<t>BTNS PAD entries MUST logically follow all
non-BTNS PAD entries</t>
<t>the wildcard BTNS PAD entry MUST be the last
entry in the PAD, logically</t>
<t>the wildcard BTNS PAD entry MUST have ID
constraints that do not logically overlap those
of other PAD entries.</t>
</list>
</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Usage Scenarios">
<t>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.</t>
<t>This section is informative only.</t>
<figure anchor="networkdiagram" title="Reference Network Diagram">
<preamble>
To explain the scenarios a reference diagram
describing an example network will be used. It is
as follows:
</preamble>
<artwork>
[Q] [R]
AS1 . . AS2
[A]----+----[SG-A].......+....+.......[SG-B]-------[B]
...... \
..PI.. ----[btns-B]
......
[btns-C].....+....+.......[btns-D]
</artwork>
<postamble></postamble>
</figure>
<t>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.</t>
<t>Nodes [C] and [Q] have fixed addresses. Node [D] has a
non-fixed address.</t>
<t> 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.
</t>
<t>PI is the Public Internet ("The Wild").</t>
<section anchor="example_sgA" title="Example #1: A security gateway">
<t>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]. </t>
<t>SG-A has the following PAD and SPD entries:
<figure anchor="sgA_pad" title="SG-A PAD table">
<preamble>
</preamble>
<artwork>
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
</artwork>
<postamble>The last entry is the BTNS
entry.</postamble>
</figure>
</t>
<t>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 <xref target="core"/>.</t>
<t><Child SA IDs allowed> and <SPD Search
by> are from <xref target="RFC4301"/> section
4.4.3</t>
<t>
<figure anchor="sgA_spd" title="[SG-A] SPD table">
<preamble>
</preamble>
<artwork>
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)
</artwork>
<postamble></postamble>
</figure>
</t>
<t>The processing by [SG-A] of SA establishment attempts by various peers is as
follows:
<list style='symbols'>
<t>[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 [SG-A]'s
and [Q]'s addresses as the end-point
addresses.</t>
<t>[SG-B] matches PAD entry #1; PAD processing
stops, then the SPD is searched by [SG-B]'s ID
to find SPD entry #3; CHILD SAs are then
allowed that have [SG-A]'s address and any
addresses from B's network as the end-point
addresses.</t>
<t>[R] does not initiate any IKE SAs; its traffic
to [A] is bypassed by SPD entry #1.</t>
<t>[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 [SG-A]'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 <xref target="core"/>). See the
last bullet item below.</t>
<t>A rogue BTNS node 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 entry #3 but will not
be allowed to create CHILD SAs with [Q]'s or
[B]'s addresses as traffic selectors.</t>
<t>A rogue BTNS node attempting to establish an
SA whereby the rogue node asserts [C]'s
address will succeed at establishing such an
SA. 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.</t>
</list>
</t>
</section>
<section title="Example #2: A mixed end-system ">
<t>[Q] is an NFSv4 server.</t>
<t>[Q] is a native IPsec implementation, and it's
NFSv4 implementation is IPsec-aware.</t>
<t>[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:
<figure anchor="Q_pad" title="[Q] PAD table">
<preamble>
</preamble>
<artwork>
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
</artwork>
<postamble>The last entry is the BTNS
entry.</postamble>
</figure>
</t>
<t>
<figure anchor="Q_spd" title="[Q] SPD table">
<preamble>
</preamble>
<artwork>
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
</artwork>
<postamble></postamble>
</figure>
</t>
<t>The same analysis shown above in <xref
target="example_sgA"/> 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).</t>
</section>
<section title="Example #3: A BTNS-only system">
<t>[C] supports only BTNS and wants to use BTNS to protect
NFSv4 traffic. It's PAD and SPD are configured as
follows:
<figure anchor="C_pad" title="Q PAD table">
<preamble>
</preamble>
<artwork>
Child SA
Rule Remote ID IDs allowed SPD Search by
---- --------- ----------- -------------
1 PUBLICKEY:any ANY by-IP
</artwork>
<postamble>The last (and only) entry is the BTNS
entry.</postamble>
</figure>
</t>
<t>
<figure anchor="C_spd" title="SG-A SPD table">
<preamble>
</preamble>
<artwork>
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
</artwork>
<postamble></postamble>
</figure>
</t>
<t>The analysis from <xref target="example_sgA" /> applies as follows:
<list style='symbols'>
<t>Communication with [Q] on port 2049 matches
SPD entry number 1. This causes [C] to initiate
an IKEv2 exchange with [Q]. The PAD entry on [C]
causes it to not care what identity [Q]
asserts. Further authentication (and channel
binding) could occur within the NFSv4 protocol.
</t>
<t>Communication with [A], [B] or any other internet
machine (including [Q]), occurs in the clear,
so long as it is not
on port 2049.</t>
<t>All analysis about rogue BTNS nodes applies, but
they can only assert SAs for port 2049.</t>
</list>
</t>
</section>
<section title="Miscellaneous comments">
<t>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 <xref
target="RFC4301"/>.</t>
<t>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
<xref target="RFC4301"/>.</t>
</section>
</section>
<section title="Security Considerations">
<t>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.</t>
<t>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.</t>
<t>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. </t>
<section title="Connection-Latching and Channel Binding">
<t>BTNS is subject to MITM attacks. One way to protect
against MITM attacks subsequent to initial
communications is to use "connection latching" <xref
target="I-D.ietf-btns-connection-latching"/>.
In connection latching, ULPs cooperate with IPsec
to bind discrete packet flows to
sequences of similar SAs. Connection latching requires
native IPsec implementations.</t>
<t>MITMs can be detected by using application-layer
authentication frameworks and/or mechanisms, such as
the GSS-API <xref target="RFC2743"/>, with channel
binding <xref target="RFC5056"/>. IPsec "channels"
are nothing other than latched connnections.</t>
</section>
<section anchor="lof" title="Leap-of-Faith (LoF) for BTNS">
<t>"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 <xref
target="RFC4251"/> 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.</t>
<t>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.</t>
</section>
</section>
<section title="IANA Considerations">
<t>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.)</t>
</section>
<section title="Acknowledgements">
<t>Thanks for the following reviewers: Stephen Kent</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119; &rfc4301;
</references>
<references title="Informative References">
&rfc2401;
&rfc2408;
&rfc2409;
&rfc2743;
&btns-applic;
&connection-latching;
&rfc4251;
&rfc4306;
&rfc5056;
</references>
</back>
</rfc>
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