RFC2230 - Key Exchange Delegation Record for the DNS

Network Working Group R. Atkinson

Request for Comments: 2230 NRL

Category: Informational November 1997

Key Exchange Delegation Record for the DNS

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

ABSTRACT

This note describes a mechanism whereby authorisation for one node to

act as key exchanger for a second node is delegated and made

available via the Secure DNS. This mechanism is intended to be used

only with the Secure DNS. It can be used with several security

services. For example, a system seeking to use IP Security [RFC-

1825, RFC-1826, RFC-1827] to protect IP packets for a given

destination can use this mechanism to determine the set of authorised

remote key exchanger systems for that destination.

1. INTRODUCTION

The Domain Name System (DNS) is the standard way that Internet nodes

locate information about addresses, mail exchangers, and other data

relating to remote Internet nodes. [RFC-1035, RFC-1034] More

recently, Eastlake and Kaufman have defined standards-track security

extensions to the DNS. [RFC-2065] These security extensions can be

used to authenticate signed DNS data records and can also be used to

store signed public keys in the DNS.

The KX record is useful in providing an authenticatible method of

delegating authorisation for one node to provide key exchange

services on behalf of one or more, possibly different, nodes. This

note specifies the syntax and semantics of the KX record, which is

currently in limited deployment in certain IP-based networks. The

reader is assumed to be familiar with the basics of DNS, including

familiarity with [RFC-1035, RFC-1034]. This document is not on the

IETF standards-track and does not specify any level of standard.

This document merely provides information for the Internet community.

1.1 Identity Terminology

This document relies upon the concept of "identity domination". This

concept might be new to the reader and so is eXPlained in this

section. The subject of endpoint naming for security associations

has historically been somewhat contentious. This document takes no

position on what forms of identity should be used. In a network,

there are several forms of identity that are possible.

For example, IP Security has defined notions of identity that

include: IP Address, IP Address Range, Connection ID, Fully-Qualified

Domain Name (FQDN), and User with Fully Qualified Domain Name (USER

FQDN).

A USER FQDN identity dominates a FQDN identity. A FQDN identity in

turn dominates an IP Address identity. Similarly, a Connection ID

dominates an IP Address identity. An IP Address Range dominates each

IP Address identity for each IP address within that IP address range.

Also, for completeness, an IP Address identity is considered to

dominate itself.

2. APPROACH

This document specifies a new kind of DNS Resource Record (RR), known

as the Key Exchanger (KX) record. A Key Exchanger Record has the

mnemonic "KX" and the type code of 36. Each KX record is associated

with a fully-qualified domain name. The KX record is modeled on the

MX record described in [Part86]. Any given domain, subdomain, or host

entry in the DNS might have a KX record.

2.1 IPsec Examples

In these two examples, let S be the originating node and let D be the

destination node. S2 is another node on the same subnet as S. D2 is

another node on the same subnet as D. R1 and R2 are IPsec-capable

routers. The path from S to D goes via first R1 and later R2. The

return path from D to S goes via first R2 and later R1.

IETF-standard IP Security uses unidirectional Security Associations

[RFC-1825]. Therefore, a typical IP session will use a pair of

related Security Associations, one in each direction. The examples

below talk about how to setup an example Security Association, but in

practice a pair of matched Security Associations will normally be

used.

2.1.1 Subnet-to-Subnet Example

If neither S nor D implements IPsec, security can still be provided

between R1 and R2 by building a secure tunnel. This can use either

AH or ESP.

S ---+ +----D

+- R1 -----[zero or more routers]-------R2-+

S2---+ +----D2

Figure 1: Network Diagram for Subnet-to-Subnet Example

In this example, R1 makes the policy decision to provide the IPsec

service for traffic from R1 destined for R2. Once R1 has decided

that the packet from S to D should be protected, it performs a secure

DNS lookup for the records associated with domain D. If R1 only

knows the IP address for D, then a secure reverse DNS lookup will be

necessary to determine the domain D, before that forward secure DNS

lookup for records associated with domain D. If these DNS records of

domain D include a KX record for the IPsec service, then R1 knows

which set of nodes are authorised key exchanger nodes for the

destination D.

In this example, let there be at least one KX record for D and let

the most preferred KX record for D point at R2. R1 then selects a

key exchanger (in this example, R2) for D from the list oBTained from

the secure DNS. Then R1 initiates a key management session with that

key exchanger (in this example, R2) to setup an IPsec Security

Association between R1 and D. In this example, R1 knows (either by

seeing an outbound packet arriving from S destined to D or via other

methods) that S will be sending traffic to D. In this example R1's

policy requires that traffic from S to D should be segregated at

least on a host-to-host basis, so R1 desires an IPsec Security

Association with source identity that dominates S, proxy identity

that dominates R1, and destination identity that dominates R2.

In turn, R2 is able to authenticate the delegation of Key Exchanger

authorisation for target S to R1 by making an authenticated forward

DNS lookup for KX records associated with S and verifying that at

least one such record points to R1. The identity S is typically

given to R2 as part of the key management process between R1 and R2.

If D initially only knows the IP address of S, then it will need to

perform a secure reverse DNS lookup to obtain the fully-qualified

domain name for S prior to that secure forward DNS lookup.

If R2 does not receive an authenticated DNS response indicating that

R1 is an authorised key exchanger for S, then D will not accept the

SA negotiation from R1 on behalf of identity S.

If the proposed IPsec Security Association is acceptable to both R1

and R2, each of which might have separate policies, then they create

that IPsec Security Association via Key Management.

Note that for unicast traffic, Key Management will typically also

setup a separate (but related) IPsec Security Association for the

return traffic. That return IPsec Security Association will have

equivalent identities. In this example, that return IPsec Security

Association will have a source identity that dominates D, a proxy

identity that dominates R2, and a destination identity that dominates

R1.

Once the IPsec Security Association has been created, then R1 uses it

to protect traffic from S destined for D via a secure tunnel that

originates at R1 and terminates at R2. For the case of unicast, R2

will use the return IPsec Security Association to protect traffic

from D destined for S via a secure tunnel that originates at R2 and

terminates at R1.

2.1.2 Subnet-to-Host Example

Consider the case where D and R1 implement IPsec, but S does not

implement IPsec, which is an interesting variation on the previous

example. This example is shown in Figure 2 below.

S ---+

+- R1 -----[zero or more routers]-------D

S2---+

Figure 2: Network Diagram for Subnet-to-Host Example

In this example, R1 makes the policy decision that IP Security is

needed for the packet travelling from S to D. Then, R1 performs the

secure DNS lookup for D and determines that D is its own key

exchanger, either from the existence of a KX record for D pointing to

D or from an authenticated DNS response indicating that no KX record

exists for D. If R1 does not initially know the domain name of D,

then prior to the above forward secure DNS lookup, R1 performs a

secure reverse DNS lookup on the IP address of D to determine the

fully-qualified domain name for that IP address. R1 then initiates

key management with D to create an IPsec Security Association on

behalf of S.

In turn, D can verify that R1 is authorised to create an IPsec

Security Association on behalf of S by performing a DNS KX record

lookup for target S. R1 usually provides identity S to D via key

management. If D only has the IP address of S, then D will need to

perform a secure reverse lookup on the IP address of S to determine

domain name S prior to the secure forward DNS lookup on S to locate

the KX records for S.

If D does not receive an authenticated DNS response indicating that

R1 is an authorised key exchanger for S, then D will not accept the

SA negotiation from R1 on behalf of identity S.

If the IPsec Security Association is successfully established between

R1 and D, that IPsec Security Association has a source identity that

dominates S's IP address, a proxy identity that dominates R1's IP

address, and a destination identity that dominates D's IP address.

Finally, R1 begins providing the security service for packets from S

that transit R1 destined for D. When D receives such packets, D

examines the SA information during IPsec input processing and sees

that R1's address is listed as valid proxy address for that SA and

that S is the source address for that SA. Hence, D knows at input

processing time that R1 is authorised to provide security on behalf

of S. Therefore packets coming from R1 with valid IP security that

claim to be from S are trusted by D to have really come from S.

2.1.3 Host to Subnet Example

Now consider the above case from D's perspective (i.e. where D is

sending IP packets to S). This variant is sometimes known as the

Mobile Host or "roadwarrier" case. The same basic concepts apply, but

the details are covered here in hope of improved clarity.

S ---+

+- R1 -----[zero or more routers]-------D

S2---+

Figure 3: Network Diagram for Host-to-Subnet Example

In this example, D makes the policy decision that IP Security is

needed for the packets from D to S. Then D performs the secure DNS

lookup for S and discovers that a KX record for S exists and points

at R1. If D only has the IP address of S, then it performs a secure

reverse DNS lookup on the IP address of S prior to the forward secure

DNS lookup for S.

D then initiates key management with R1, where R1 is acting on behalf

of S, to create an appropriate Security Association. Because D is

acting as its own key exchanger, R1 does not need to perform a secure

DNS lookup for KX records associated with D.

D and R1 then create an appropriate IPsec Security Security

Association. This IPsec Security Association is setup as a secure

tunnel with a source identity that dominates D's IP Address and a

destination identity that dominates R1's IP Address. Because D

performs IPsec for itself, no proxy identity is needed in this IPsec

Security Association. If the proxy identity is non-null in this

situation, then the proxy identity must dominate D's IP Address.

Finally, D sends secured IP packets to R1. R1 receives those

packets, provides IPsec input processing (including appropriate

inner/outer IP address validation), and forwards valid packets along

to S.

2.2 Other Examples

This mechanism can be extended for use with other services as well.

To give some insight into other possible uses, this section discusses

use of KX records in environments using a Key Distribution Center

(KDC), such as Kerberos [KN93], and a possible use of KX records in

conjunction with mobile nodes Accessing the network via a dialup

service.

2.2.1 KDC Examples

This example considers the situation of a destination node

implementing IPsec that can only obtain its Security Association

information from a Key Distribution Center (KDC). Let the KDC

implement both the KDC protocol and also a non-KDC key management

protocol (e.g. ISAKMP). In such a case, each client node of the KDC

might have its own KX record pointing at the KDC so that nodes not

implementing the KDC protocol can still create Security Associations

with each of the client nodes of the KDC.

In the event the session initiator were not using the KDC but the

session target was an IPsec node that only used the KDC, the

initiator would find the KX record for the target pointing at the

KDC. Then, the external key management exchange (e.g. ISAKMP) would

be between the initiator and the KDC. Then the KDC would distribute

the IPsec SA to the KDC-only IPsec node using the KDC. The IPsec

traffic itself could travel directly between the initiator and the

destination node.

In the event the initiator node could only use the KDC and the target

were not using the KDC, the initiator would send its request for a

key to the KDC. The KDC would then initiate an external key

management exchange (e.g. ISAKMP) with a node that the target's KX

record(s) pointed to, on behalf of the initiator node.

The target node could verify that the KDC were allowed to proxy for

the initiator node by looking up the KX records for the initiator

node and finding a KX record for the initiator that listed the KDC.

Then the external key exchange would be performed between the KDC and

the target node. Then the KDC would distribute the resulting IPsec

Security Association to the initiator. Again, IPsec traffic itself

could travel directly between the initiator and the destination.

2.2.2 Dial-Up Host Example

This example outlines a possible use of KX records with mobile hosts

that dial into the network via PPP and are dynamically assigned an IP

address and domain-name at dial-in time.

Consider the situation where each mobile node is dynamically assigned

both a domain name and an IP address at the time that node dials into

the network. Let the policy require that each mobile node act as its

own Key Exchanger. In this case, it is important that dial-in nodes

use addresses from one or more well known IP subnets or address pools

dedicated to dial-in access. If that is true, then no KX record or

other action is needed to ensure that each node will act as its own

Key Exchanger because lack of a KX record indicates that the node is

its own Key Exchanger.

Consider the situation where the mobile node's domain name remains

constant but its IP address changes. Let the policy require that

each mobile node act as its own Key Exchanger. In this case, there

might be operational problems when another node attempts to perform a

secure reverse DNS lookup on the IP address to determine the

corresponding domain name. The authenticated DNS binding (in the

form of a PTR record) between the mobile node's currently assigned IP

address and its permanent domain name will need to be securely

updated each time the node is assigned a new IP address. There are

no mechanisms for accomplishing this that are both IETF-standard and

widely deployed as of the time this note was written. Use of Dynamic

DNS Update without authentication is a significant security risk and

hence is not recommended for this situation.

3. SYNTAX OF KX RECORD

A KX record has the DNS TYPE of "KX" and a numeric value of 36. A KX

record is a member of the Internet ("IN") CLASS in the DNS. Each KX

record is associated with a <domain-name> entry in the DNS. A KX

record has the following textual syntax:

<domain-name> IN KX <preference> <domain-name>

For this description, let the <domain-name> item to the left of the

"KX" string be called <domain-name 1> and the <domain-name> item to

the right of the "KX" string be called <domain-name 2>. <preference>

is a non-negative integer.

Internet nodes about to initiate a key exchange with <domain-name 1>

should instead contact <domain-name 2> to initiate the key exchange

for a security service between the initiator and <domain-name 2>. If

more than one KX record exists for <domain-name 1>, then the

<preference> field is used to indicate preference among the systems

delegated to. Lower values are preferred over higher values. The

<domain-name 2> is authorised to provide key exchange services on

behalf of <domain-name 1>. The <domain-name 2> MUST have a CNAME

record, an A record, or an AAAA record associated with it.

3.1 KX RDATA format

The KX DNS record has the following RDATA format:

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

PREFERENCE

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

/ EXCHANGER /

/ /

+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

where:

PREFERENCE A 16 bit non-negative integer which specifies the

preference given to this RR among other KX records

at the same owner. Lower values are preferred.

EXCHANGER A <domain-name> which specifies a host willing to

act as a mail exchange for the owner name.

KX records MUST cause type A additional section processing for the

host specified by EXCHANGER. In the event that the host processing

the DNS transaction supports IPv6, KX records MUST also cause type

AAAA additional section processing.

The KX RDATA field MUST NOT be compressed.

4. SECURITY CONSIDERATIONS

KX records MUST always be signed using the method(s) defined by the

DNS Security extensions specified in [RFC-2065]. All unsigned KX

records MUST be ignored because of the security vulnerability caused

by assuming that unsigned records are valid. All signed KX records

whose signatures do not correctly validate MUST be ignored because of

the potential security vulnerability in trusting an invalid KX

record.

KX records MUST be ignored by systems not implementing Secure DNS

because such systems have no mechanism to authenticate the KX record.

If a node does not have a permanent DNS entry and some form of

Dynamic DNS Update is in use, then those dynamic DNS updates MUST be

fully authenticated to prevent an adversary from injecting false DNS

records (especially the KX, A, and PTR records) into the Domain Name

System. If false records were inserted into the DNS without being

signed by the Secure DNS mechanisms, then a denial-of-service attack

results. If false records were inserted into the DNS and were

(erroneously) signed by the signing authority, then an active attack

results.

Myriad serious security vulnerabilities can arise if the restrictions

throuhout this document are not strictly adhered to. Implementers

should carefully consider the openly published issues relating to DNS

security [Bell95,Vixie95] as they build their implementations.

Readers should also consider the security considerations discussed in

the DNS Security Extensions document [RFC-2065].

5. REFERENCES

[RFC-1825] Atkinson, R., "IP Authentication Header", RFC1826,

August 1995.

[RFC-1827] Atkinson, R., "IP Encapsulating Security Payload",

RFC1827, August 1995.

[Bell95] Bellovin, S., "Using the Domain Name System for System

Break-ins", Proceedings of 5th USENIX UNIX Security

Symposium, USENIX Association, Berkeley, CA, June 1995.

FTP://ftp.research.att.com/dist/smb/dnshack.ps

[RFC-2065] Eastlake, D., and C. Kaufman, "Domain Name System

Security Extensions", RFC2065, January 1997.

[RFC-1510] Kohl J., and C. Neuman, "The Kerberos Network

Authentication Service", RFC1510, September 1993.

[RFC-1035] Mockapetris, P., "Domain names - implementation and

specification", STD 13, RFC1035, November 1987.

[RFC-1034] Mockapetris, P., "Domain names - concepts and

facilities", STD 13, RFC1034, November 1987.

[Vixie95] P. Vixie, "DNS and BIND Security Issues", Proceedings of

the 5th USENIX UNIX Security Symposium, USENIX

Association, Berkeley, CA, June 1995.

ftp://ftp.vix.com/pri/vixie/bindsec.psf

ACKNOWLEDGEMENTS

Development of this DNS record was primarily performed during 1993

through 1995. The author's work on this was sponsored jointly by the

Computing Systems Technology Office (CSTO) of the Advanced Research

Projects Agency (ARPA) and by the Information Security Program Office

(PD71E), Space & Naval Warface Systems Command (SPAWAR). In that

era, Dave Mihelcic and others provided detailed review and

constructive feedback. More recently, Bob Moscowitz and Todd Welch

provided detailed review and constructive feedback of a work in

progress version of this document.

AUTHOR'S ADDRESS

Randall Atkinson

Code 5544

Naval Research Laboratory

4555 Overlook Avenue, SW

Washington, DC 20375-5337

Phone: (DSN) 354-8590

EMail: atkinson@itd.nrl.navy.mil

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