IPSECKEY WG M. Richardson
Internet-Draft SSW
Expires: January 15, 2005 July 17, 2004
A Method for Storing IPsec Keying Material in DNS
draft-ietf-ipseckey-rr-11.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes a new resource record for the Domain Name
System (DNS). This record may be used to store public keys for use in
IP security (IPsec) systems. The record also includes provisions for
indicating what system should be contacted when establishing an IPsec
tunnel with the entity in question.
This record replaces the functionality of the sub-type #1 of the KEY
Resource Record, which has been obsoleted by RFC3445.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Use of DNS address-to-name maps (IN-ADDR.ARPA and
IP6.ARPA) . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Usage Criteria . . . . . . . . . . . . . . . . . . . . . . . 4
2. Storage formats . . . . . . . . . . . . . . . . . . . . . . 5
2.1 IPSECKEY RDATA format . . . . . . . . . . . . . . . . . . . 5
2.2 RDATA format - precedence . . . . . . . . . . . . . . . . . 5
2.3 RDATA format - gateway type . . . . . . . . . . . . . . . . 5
2.4 RDATA format - algorithm type . . . . . . . . . . . . . . . 6
2.5 RDATA format - gateway . . . . . . . . . . . . . . . . . . . 6
2.6 RDATA format - public keys . . . . . . . . . . . . . . . . . 6
3. Presentation formats . . . . . . . . . . . . . . . . . . . . 8
3.1 Representation of IPSECKEY RRs . . . . . . . . . . . . . . . 8
3.2 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . 10
4.1 Active attacks against unsecured IPSECKEY resource
records . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.1 Active attacks against IPSECKEY keying materials . . . . . . 10
4.1.2 Active attacks against IPSECKEY gateway material . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . 13
6. Intellectual Property Claims . . . . . . . . . . . . . . . . 14
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 15
Normative references . . . . . . . . . . . . . . . . . . . . 16
Non-normative references . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . 18
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1. Introduction
Suppose we have a host which wishes to establish an IPsec tunnel with
some remote entity on the network. In many cases this end system
will only know a DNS name for the remote entity (whether that DNS
name be the name of the remote node, a DNS reverse tree name
corresponding to the IP address of the remote node, or perhaps a the
domain name portion of a "user@FQDN" name for a remote entity). In
these cases the host will need to obtain a public key in order to
authenticate the remote entity, and may also need some guidance about
whether it should contact the entity directly or use another node as
a gateway to the target entity.
The IPSECKEY RR provides a storage mechanism for such data as the
public key and the gateway information.
The type number for the IPSECKEY RR is TBD.
1.1 Overview
The IPSECKEY resource record (RR) is used to publish a public key
that is to be associated with a Domain Name System (DNS) name for use
with the IPsec protocol suite. This can be the public key of a
host, network, or application (in the case of per-port keying).
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 [7].
1.2 Use of DNS address-to-name maps (IN-ADDR.ARPA and IP6.ARPA)
Often a security gateway will only have access to the IP address of
the node with which communication is desired, and will not know any
other name for the target node. Because of this, it will frequently
be the case that the best way of looking up IPSECKEY RRs will be by
using the IP address as an index into one of the reverse mapping
trees (IN-ADDR.ARPA for IPv4 or IP6.ARPA for IPv6).
The lookup is done in the usual fashion as for PTR records. The IP
address' octets (IPv4) or nibbles (IPv6) are reversed and looked up
with the appropriate suffix. Any CNAMEs or DNAMEs found MUST be
followed.
Note: even when the IPsec function is the end-host, often only the
application will know the forward name used. While the case where the
application knows the forward name is common, the user could easily
have typed in a literal IP address. This storage mechanism does not
preclude using the forward name when it is available, but does not
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require it.
1.3 Usage Criteria
An IPSECKEY resource record SHOULD be used in combination with DNSSEC
unless some other means of authenticating the IPSECKEY resource
record is available.
It is expected that there will often be multiple IPSECKEY resource
records at the same name. This will be due to the presence of
multiple gateways and the need to rollover keys.
This resource record is class independent.
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2. Storage formats
2.1 IPSECKEY RDATA format
The RDATA for an IPSECKEY RR consists of a precedence value, a
gateway type, a public key, algorithm type, and an optional gateway
address.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| precedence | gateway type | algorithm | gateway |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-------------+ +
~ gateway ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
/ public key /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
2.2 RDATA format - precedence
This is an 8-bit precedence for this record. This is interpreted in
the same way as the PREFERENCE field described in section 3.3.9 of
RFC1035 [2].
Gateways listed in IPSECKEY records with lower precedence are to be
attempted first. Where there is a tie in precedence, the order should
be non-deterministic.
2.3 RDATA format - gateway type
The gateway type field indicates the format of the information that
is stored in the gateway field.
The following values are defined:
0 No gateway is present
1 A 4-byte IPv4 address is present
2 A 16-byte IPv6 address is present
3 A wire-encoded domain name is present. The wire-encoded format is
self-describing, so the length is implicit. The domain name MUST
NOT be compressed. (see section 3.3 of RFC1035 [2]).
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2.4 RDATA format - algorithm type
The algorithm type field identifies the public key's cryptographic
algorithm and determines the format of the public key field.
A value of 0 indicates that no key is present.
The following values are defined:
1 A DSA key is present, in the format defined in RFC2536 [10]
2 A RSA key is present, in the format defined in RFC3110 [11]
2.5 RDATA format - gateway
The gateway field indicates a gateway to which an IPsec tunnel may be
created in order to reach the entity named by this resource record.
There are three formats:
A 32-bit IPv4 address is present in the gateway field. The data
portion is an IPv4 address as described in section 3.4.1 of RFC1035
[2]. This is a 32-bit number in network byte order.
A 128-bit IPv6 address is present in the gateway field. The data
portion is an IPv6 address as described in section 2.2 of RFC3596
[13]. This is a 128-bit number in network byte order.
The gateway field is a normal wire-encoded domain name, as described
in section 3.3 of RFC1035 [2]. Compression MUST NOT be used.
2.6 RDATA format - public keys
Both of the public key types defined in this document (RSA and DSA)
inherit their public key formats from the corresponding KEY RR
formats. Specifically, the public key field contains the
algorithm-specific portion of the KEY RR RDATA, which is all of the
KEY RR DATA after the first four octets. This is the same portion of
the KEY RR that must be specified by documents that define a DNSSEC
algorithm. Those documents also specify a message digest to be used
for generation of SIG RRs; that specification is not relevant for
IPSECKEY RR.
Future algorithms, if they are to be used by both DNSSEC (in the KEY
RR) and IPSECKEY, are likely to use the same public key encodings in
both records. Unless otherwise specified, the IPSECKEY public key
field will contain the algorithm-specific portion of the KEY RR RDATA
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for the corresponding algorithm. The algorithm must still be
designated for use by IPSECKEY, and an IPSECKEY algorithm type number
(which might be different than the DNSSEC algorithm number) must be
assigned to it.
The DSA key format is defined in RFC2536 [10]
The RSA key format is defined in RFC3110 [11], with the following
changes:
The earlier definition of RSA/MD5 in RFC2065 limited the exponent and
modulus to 2552 bits in length. RFC3110 extended that limit to 4096
bits for RSA/SHA1 keys. The IPSECKEY RR imposes no length limit on
RSA public keys, other than the 65535 octet limit imposed by the
two-octet length encoding. This length extension is applicable only
to IPSECKEY and not to KEY RRs.
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3. Presentation formats
3.1 Representation of IPSECKEY RRs
IPSECKEY RRs may appear in a zone data master file. The precedence,
gateway type and algorithm and gateway fields are REQUIRED. The
base64 encoded public key block is OPTIONAL; if not present, then the
public key field of the resource record MUST be construed as being
zero octets in length.
The algorithm field is an unsigned integer. No mnemonics are defined.
If no gateway is to be indicated, then the gateway type field MUST be
zero, and the gateway field MUST be "."
The Public Key field is represented as a Base64 encoding of the
Public Key. Whitespace is allowed within the Base64 text. For a
definition of Base64 encoding, see RFC3548 [6] Section 5.2.
The general presentation for the record as as follows:
IN IPSECKEY ( precedence gateway-type algorithm
gateway base64-encoded-public-key )
3.2 Examples
An example of a node 192.0.2.38 that will accept IPsec tunnels on its
own behalf.
38.2.0.192.in-addr.arpa. 7200 IN IPSECKEY ( 10 1 2
192.0.2.38
AQNRU3mG7TVTO2BkR47usntb102uFJtugbo6BSGvgqt4AQ== )
An example of a node, 192.0.2.38 that has published its key only.
38.2.0.192.in-addr.arpa. 7200 IN IPSECKEY ( 10 0 2
.
AQNRU3mG7TVTO2BkR47usntb102uFJtugbo6BSGvgqt4AQ== )
An example of a node, 192.0.2.38 that has delegated authority to the
node 192.0.2.3.
38.2.0.192.in-addr.arpa. 7200 IN IPSECKEY ( 10 1 2
192.0.2.3
AQNRU3mG7TVTO2BkR47usntb102uFJtugbo6BSGvgqt4AQ== )
An example of a node, 192.0.1.38 that has delegated authority to the
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node with the identity "mygateway.example.com".
38.1.0.192.in-addr.arpa. 7200 IN IPSECKEY ( 10 3 2
mygateway.example.com.
AQNRU3mG7TVTO2BkR47usntb102uFJtugbo6BSGvgqt4AQ== )
An example of a node, 2001:0DB8:0200:1:210:f3ff:fe03:4d0 that has
delegated authority to the node 2001:0DB8:c000:0200:2::1
$ORIGIN 1.0.0.0.0.0.2.8.B.D.0.1.0.0.2.ip6.arpa.
0.d.4.0.3.0.e.f.f.f.3.f.0.1.2.0 7200 IN IPSECKEY ( 10 2 2
2001:0DB8:0:8002::2000:1
AQNRU3mG7TVTO2BkR47usntb102uFJtugbo6BSGvgqt4AQ== )
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4. Security Considerations
This entire memo pertains to the provision of public keying material
for use by key management protocols such as ISAKMP/IKE (RFC2407) [8].
The IPSECKEY resource record contains information that SHOULD be
communicated to the end client in an integral fashion - i.e. free
from modification. The form of this channel is up to the consumer of
the data - there must be a trust relationship between the end
consumer of this resource record and the server. This relationship
may be end-to-end DNSSEC validation, a TSIG or SIG(0) channel to
another secure source, a secure local channel on the host, or some
combination of the above.
The keying material provided by the IPSECKEY resource record is not
sensitive to passive attacks. The keying material may be freely
disclosed to any party without any impact on the security properties
of the resulting IPsec session: IPsec and IKE provide for defense
against both active and passive attacks.
Any derivative specification that makes use of this resource record
MUST carefully document their trust model, and why the trust model of
DNSSEC is appropriate, if that is the secure channel used.
An active attack on the DNS that caused the wrong IP address to be
retrieved (via forged address), and therefore the wrong QNAME to be
queried would also result in a man-in-the-middle attack. This
situation exists independantly of whether or not the IPSECKEY RR is
used.
4.1 Active attacks against unsecured IPSECKEY resource records
This section deals with active attacks against the DNS. These attacks
require that DNS requests and responses be intercepted and changed.
DNSSEC is designed to defend against attacks of this kind. This
section deals with the situation where DNSSEC is not available. This
is not the recommended deployment scenario.
4.1.1 Active attacks against IPSECKEY keying materials
The first kind of active attack is when the attacker replaces the
keying material with either a key under its control or with garbage.
The gateway field is either untouched, or is null. The IKE
negotiation will therefore occur with the original end-system. For
this attack to be successful, the attacker must be able to perform a
man-in-the-middle attack on the IKE negotiation. This attack requires
that the attacker be able to intercept and modify packets on the
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forwarding path for the IKE and data packets.
If the attacker is not able to perform this man-in-the-middle attack
on the IKE negotiation, then this will result in a denial of service,
as the IKE negotiation will fail.
If the attacker is able to both to mount active attacks against DNS
and is also in a position to perform a man-in-the-middle attack on
IKE and IPsec negotiations, then the attacker will be in a position
to compromise the resulting IPsec channel. Note that an attacker
must be able to perform active DNS attacks on both sides of the IKE
negotiation in order for this to succeed.
4.1.2 Active attacks against IPSECKEY gateway material
The second kind of active attack is one in which the attacker
replaces the the gateway address to point to a node under the
attacker's control. The attacker then either replaces the public key
or removes it. If they were to remove the public key, then they
could provide an accurate public key of their own in a second record.
This second form creates a simple man-in-the-middle since the
attacker can then create a second tunnel to the real destination.
Note that, as before, this requires that the attacker also mount an
active attack against the responder.
Note that the man-in-the-middle can not just forward cleartext
packets to the original destination. While the destination may be
willing to speak in the clear, replying to the original sender, the
sender will have already created a policy expecting ciphertext. Thus,
the attacker will need to intercept traffic in both directions. In
some cases, the attacker may be able to accomplish the full intercept
by use of Network Addresss/Port Translation (NAT/NAPT) technology.
This attack is easier than the first one because the attacker does
NOT need to be on the end-to-end forwarding path. The attacker need
only be able to modify DNS replies. This can be done by packet
modification, by various kinds of race attacks, or through methods
that pollute DNS caches.
In cases where the end-to-end integrity of the IPSECKEY RR is
suspect, the end client MUST restrict its use of the IPSECKEY RR to
cases where the RR owner name matches the content of the gateway
field. As the RR owner name is assumed when the gateway field is
null, a null gateway field is considered a match.
Thus, any records obtained under unverified conditions (e.g. no
DNSSEC, or trusted path to source) that have a non-null gateway field
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MUST be ignored.
This restriction eliminates attacks against the gateway field, which
are considered much easier, as the attack does not need to be on the
forwarding path.
In the case of an IPSECKEY RR with a value of three in its gateway
type field, the gateway field contains a domain name. The subsequent
query required to translate that name into an IP address or IPSECKEY
RR will also be subject to man-in-the-middle attacks. If the
end-to-end integrity of this second query is suspect, then the
provisions above also apply. The IPSECKEY RR MUST be ignored whenever
the resulting gateway does not match the QNAME of the original
IPSECKEY RR query.
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5. IANA Considerations
This document updates the IANA Registry for DNS Resource Record Types
by assigning type X to the IPSECKEY record.
This document creates two new IANA registries, both specific to the
IPSECKEY Resource Record:
This document creates an IANA registry for the algorithm type field.
Values 0, 1 and 2 are defined in Section 2.4. Algorithm numbers 3
through 255 can be assigned by IETF Consensus (see RFC2434 [5]).
This document creates an IANA registry for the gateway type field.
Values 0, 1, 2 and 3 are defined in Section 2.3. Gateway type numbers
4 through 255 can be assigned by Standards Action (see RFC2434 [5]).
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6. Intellectual Property Claims
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
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7. Acknowledgments
My thanks to Paul Hoffman, Sam Weiler, Jean-Jacques Puig, Rob
Austein, and Olafur Gurmundsson who reviewed this document carefully.
Additional thanks to Olafur Gurmundsson for a reference
implementation.
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Normative references
[1] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[4] Eastlake, D. and C. Kaufman, "Domain Name System Security
Extensions", RFC 2065, January 1997.
[5] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[6] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
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Non-normative references
[7] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[8] Piper, D., "The Internet IP Security Domain of Interpretation
for ISAKMP", RFC 2407, November 1998.
[9] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[10] Eastlake, D., "DSA KEYs and SIGs in the Domain Name System
(DNS)", RFC 2536, March 1999.
[11] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name
System (DNS)", RFC 3110, May 2001.
[12] Massey, D. and S. Rose, "Limiting the Scope of the KEY Resource
Record (RR)", RFC 3445, December 2002.
[13] Thomson, S., Huitema, C., Ksinant, V. and M. Souissi, "DNS
Extensions to Support IP Version 6", RFC 3596, October 2003.
Author's Address
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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Intellectual Property Statement
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Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
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http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
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this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
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Copyright Statement
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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