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Versions: 00 01 02 03 04 05 draft-ietf-dnsop-cookies

INTERNET-DRAFT                                    Donald E. Eastlake 3rd
Intended Status: Proposed Standard                 Motorola Laboratories
Expires: August 2008                                   February 25, 2008


                    Domain Name System (DNS) Cookies

                 <draft-eastlake-dnsext-cookies-03.txt>



Status of This Document

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   This draft is intended to be become a Proposed Standard RFC.
   Distribution of this document is unlimited. Comments should be sent
   to the author or the DNSEXT working group mailing list
   <namedroppers@ops.ietf.org>.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.html

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html


Abstract
   DNS cookies are a light-weight DNS transaction security mechanism
   designed for incremental deployment.  They provide limited protection
   to DNS servers and resolvers against a variety of increasingly common
   denial-of-service and cache poisoning or forgery attacks by off-path
   attackers. DNS Cookies are tolerant of NAT, NAT-PT, and Anycast.









D. Eastlake 3rd                                                 [Page 1]


INTERNET-DRAFT                                               DNS Cookies


Table of Contents

      Status of This Document....................................1
      Abstract...................................................1

      Table of Contents..........................................2

      1. Introduction............................................3
      1.1 Contents of This Document..............................3
      1.2 Definitions............................................4

      2. Threats Considered......................................5
      2.1 Denial-of-Service Attacks..............................5
      2.1.1 DNS Server Denial-of-Service.........................5
      2.1.2 Selected Host Denial-of-Service......................5
      2.2 Cache Poisoning and Answer Forgery Attacks.............6
      3. Comments on Existing DNS Security.......................6
      3.1 Existing DNS Data Security.............................6
      3.2 DNS Message or Transaction Security....................7
      3.3 Conclusions on Existing DNS Security...................7

      4. The COOKIE OPT option...................................8
      4.1 Resolver Cookies.......................................8
      4.2 Server Cookies.........................................9
      5. DNS Cookie Policies and Implementation Requirements.....9
      5.1 Resolver Policies and Implementation..................10
      5.2 Server Policies and Implementation....................11
      5.3 Implementation Requirements...........................11
      6. NAT Considerations and AnyCast Server Considerations...12

      7. Incremental Deployment.................................14

      8. IANA Considerations....................................15
      9. Security Considerations................................15
      10. Normative References..................................16
      11. Informative References................................16

      Author's Address..........................................18
      Copyright and Disclaimer..................................18
      Additional IPR Provisions.................................18
      Expiration and File Name..................................19











D. Eastlake 3rd                                                 [Page 2]


INTERNET-DRAFT                                               DNS Cookies


1. Introduction

   As with many core Internet protocols, the Domain Name System (DNS)
   was designed at a time when the Internet had only a small pool of
   trusted users. As the Internet has exploded to a global information
   utility, the DNS has increasingly been subject to abuse and been used
   as a vector for abuse.

   This document describes DNS cookies, a light-weight DNS transaction
   security mechanism specified as an OPT [RFC2671] option.  This
   mechanism provides limited protection to DNS servers and resolvers
   against a variety of increasingly common denial-of-service and cache
   poisoning forgery attacks by off-path attackers.

   The DNS cookies mechanism is designed with a default mode which
   supports incremental deployment. If only one party to a DNS
   transaction supports the mechanism, it does not interfere or provide
   a benefit, but, if both support it, the additional security provided
   is automatically available for that and subsequent transactions.

   The DNS cookies mechanism is compatible with and can be used in
   conjunction with other DNS transaction forgery resistance measures
   such as those in [forgery].

   The DNS cookies mechanism is designed to work in the presence of NAT
   and NAT-PT boxes and guidance is provided herein on supporting the
   DNS cookies mechanism in anycast servers.



1.1 Contents of This Document

   In Section 2, we discuss the threats against which the DNS cookie
   mechanism provides some protection.

   Section 3 describes existing DNS security mechanisms and why they are
   not adequate substitutes for DNS cookies.

   Section 4 describes the COOKIE OPT option including recommendations
   for calculating Resolver and Server Cookies.

   Section 5 describes the processing of COOKIE OPT options by resolvers
   and server and policies for such processing.

   Section 6 discusses some NAT and anycast related DNS Cookies design
   considerations.

   Section 7 discusses incremental deployment considerations.

   Sections 8 and 9 describe IANA and Security Considerations.


D. Eastlake 3rd                                                 [Page 3]


INTERNET-DRAFT                                               DNS Cookies


1.2 Definitions

   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].

   An "off-path attacker", for a particular DNS resolver and server, is
   defined as an attacker which cannot observe the legitimate plain text
   DNS requests and responses between that resolver and server.

   "Soft state" indicates information learned or derived by a host which
   may be discarded when indicated by the policies of that host but can
   be later re-instantiated if needed.  For example, it could be
   discarded after a period of time or when storage for caching such
   data becomes full. If operations requiring that soft state continue
   after it has been discarded, it will be automatically re-generated,
   albeit at some cost.

   "Silently discarded" indicates that there are no DNS protocol message
   consequences; however, it is RECOMMENDED that appropriate debugging
   network management facilities be included in implementations, such as
   a counter of the occurrences of each type of such events.

   The term "IP address" is used herein in a length independent manner
   and refers to address formats including IPv4 and IPv6.



























D. Eastlake 3rd                                                 [Page 4]


INTERNET-DRAFT                                               DNS Cookies


2. Threats Considered

   DNS cookies are intended to provide significant but limited
   protection against certain denial-of-service and cache poisoning or
   answer forgery attacks by off-path attackers described below.



2.1 Denial-of-Service Attacks

   The canonical form of the denial-of-service attacks considered herein
   is to send DNS requests with forged source IP addresses to a server.
   The intent can be to attack that server or a selected host as
   described below.



2.1.1 DNS Server Denial-of-Service

   DNS requests that are accepted cause work on the part of DNS servers.
   This is particularly true for recursive servers which may issue one
   or more requests and process the responses thereto in order to
   determine their response to the initial query. And the situation is
   even worse for recursive servers implementing DNSSEC [RFC4033],
   [RFC4034], [RFC4035] because they may be induced to perform
   burdensome public key cryptographic computations in attempts to
   verify the authenticity of data they retrieve while trying to answer
   the request.

   The computational or communications burden caused by such requests
   may not dependent on a forged IP source address, but the use of such
   addresses makes
   +  the source of the requests causing the denial-of-service attack to
      be harder to find and
   +  administrative restriction of the IP addresses from which such
      requests should be honored harder or impossible to specify.

   Use of DNS cookies almost always enables a server to reject forged
   queries from an off path attacker with relative ease, certainly
   before any recursive queries or public key cryptographic operations
   are performed.



2.1.2 Selected Host Denial-of-Service

   A request with a forged IP address generally causes a response to be
   sent to that forged IP address. Thus the forging of many such
   requests with a particular source IP address can result in enough
   traffic being sent to the forged IP address to interfere with service


D. Eastlake 3rd                                                 [Page 5]


INTERNET-DRAFT                                               DNS Cookies


   to the host at the IP address. Furthermore, it is generally easy in
   the DNS to create short requests that produce much longer responses.
   Thus a DNS server can be used as not only a way to obscure the true
   source of an attack but as a traffic amplifier to make the attack
   more effective.

   Use of the DNS Cookies mechanism severely limits the traffic
   amplification that can be obtained by attackers off path for the
   server and the attacked host. Enforced DNS cookies would make it hard
   for an off path attacker to cause any more than a brief error
   response to be sent to a forged IP address.  Furthermore, DNS cookies
   make it more effective to implement a rate limiting scheme for bad
   DNS cookie error responses from the server. Such a scheme would
   further restrict selected host denial-of-service traffic from that
   server.



2.2 Cache Poisoning and Answer Forgery Attacks

   The form of the cache poisoning attacks considered is to send forged
   replies to a resolver. Modern network speeds for well connected hosts
   are such that, by forging replies from the IP addresses of heavily
   used DNS servers for popular names to a heavily used resolver, there
   can be an unacceptably high probability of randomly coming up with a
   reply that will be accepted and cause false DNS information to be
   cached by that resolver. This can be used to facilitate phishing
   attacks and other diversion of legitimate traffic to a compromised or
   malicious host such as a web server.



3. Comments on Existing DNS Security

   Two forms of security have been added to DNS, data security and
   message or transaction security.



3.1 Existing DNS Data Security

   DNS data security is one part of DNSSEC and is described in
   [RFC4033], [RFC4034], and [RFC4035]. It provides data origin
   authentication and authenticated denial of existence. It is being
   deployed slowly and, in any case, can make some denial-of-service
   attacks worse because of the high cryptographic computational load it
   can require and the increased size in DNS packets that it tends to
   produce.




D. Eastlake 3rd                                                 [Page 6]


INTERNET-DRAFT                                               DNS Cookies


3.2 DNS Message or Transaction Security

   The second form of security which has been added to DNS provides
   "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931].
   TSIG could provide near perfect protection against the attacks for
   which the DNS Cookies mechanism provide weak and incomplete
   protection; however, TSIG is hard to deploy in the general Internet
   because of the burden it imposes of pre-agreement and key
   distribution between pairs of resolvers and servers and because it
   requires time synchronization between resolver and server.

   TKEY [RFC2930] can solve the problem of key distribution for TSIG but
   some modes of TKEY impose a substantial cryptographic computation
   loads and can be dependent on the deployment of DNSSEC.

   SIG(0) provides less denial of service protection than TSIG or, in
   one way, even DNS cookies, because it does not authenticate requests,
   only complete transactions.  In any case, it also depends on the
   deployment of DNSSEC and requires computationally burdensome public
   key cryptographic operations.



3.3 Conclusions on Existing DNS Security

   Thus, none of the previous forms of DNS security are a suitable
   substitute for the DNS Cookies mechanism, which provide light weight
   message authentication of DNS requests and responses with no
   requirement for pre-configuration.























D. Eastlake 3rd                                                 [Page 7]


INTERNET-DRAFT                                               DNS Cookies


4. The COOKIE OPT option

   COOKIE is an OPT RR [RFC2671] option that can be included no more
   than once in the RDATA portion of an OPT RR in DNS requests and
   responses.

   The option is encoded into 22 bytes as shown below.

                         1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      OPTION-CODE = {TBD}      |     OPTION-LENGTH = 18        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +-                       Resolver Cookie                       -+
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +--                       Server Cookie                        -+
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Error Code          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The 64-bit Resolver and Server Cookies are stored in network byte
   order and are determined as described below.

   The Error Code field MUST be zero in requests and in responses unless
   the response is communicating a DNS cookie error. Three values are
   specified in this document for Error Code: NOCOOKIE and BADCOOKIE
   which occur with a Refused RCODE in the DNS response header, and
   MANYCOOKIE which occurs with a FormErr RCODE in the DNS header. More
   information on the generation of error responses appears in Section 5
   below.



4.1 Resolver Cookies

   The Resolver Cookie, when it occurs in an OPT in a DNS response, is
   intended to weakly assure the resolver that the response came from a
   server at the indicated source IP address.

   Servers remember the Resolver Cookie that appears in a query long
   enough to use it in the construction of the COOKIE OPT option in the
   corresponding response if such a COOKIE OPT option is included in
   that response.

   The Resolver Cookie SHOULD be a pseudo-random function of the server
   IP address and a secret quantity known only to the resolver. This


D. Eastlake 3rd                                                 [Page 8]


INTERNET-DRAFT                                               DNS Cookies


   resolver secret SHOULD have 64 bits of entropy [RFC4086] and MAY be
   changed periodically.  The RECOMMENDED method is the HMAC-SHA1-64
   [RFC1321] [RFC2104] of the server IP address and the resolver secret.
   That is

      Resolver Cookie = Truncate-64
      ( HMAC-SHA1 ( Server IP Address, Resolver Secret ) )

   where Truncate-64 takes the first 64 bits.  A resolver MUST NOT use
   the same Resolver Cookie value for queries to all servers.



4.2 Server Cookies

   The Server Cookie, when it occurs in a COOKIE OPT option in a query,
   is intended to weakly assure the server that the query legitimately
   came from a resolver at the indicated source IP address that is using
   that Resolver Cookie.

   Resolvers learn Server Cookies and retain them as soft state
   associated with the server IP address. They learn them from the
   Server Cookie that appears in the COOKIE OPT option of a reply if
   that reply has the correct Resolver Cookie, even if that reply is an
   error message.

   The Server Cookie SHOULD be a pseudo-random function of the request
   source IP address, the request Resolver Cookie, and a secret quantity
   known only to the server. This server secret SHOULD have 64 bits of
   entropy [RFC4086] and SHOULD be changed periodically such as daily.
   The RECOMMENDED method is the HMAC-SHA1-64 [RFC1321], [RFC2104] of
   the request IP address, the Resolver Cookie, and the server secret.
   That is

      Server Cookie = Truncate-64 ( HMAC-SHA1 (
      (Request IP Address | Resolver Cookie), Server Secret ) )

   where Truncate-64 takes the first 64 bits and "|" represents
   concatenation.

   A server MUST NOT use the same Server Cookie value for responses to
   all resolvers.



5. DNS Cookie Policies and Implementation Requirements

   DNS resolvers and servers will adopt one of various policies
   regarding cookies. These policies SHOULD be logically settable on a
   per server IP address basis for resolvers and a per resolver ( IP


D. Eastlake 3rd                                                 [Page 9]


INTERNET-DRAFT                                               DNS Cookies


   address, Resolver Cookie ) pair for servers.  Thus a resolver can
   have different policies for different servers, based on the server IP
   address. And a server can have different policies for different
   resolvers, based on the resolver IP address and Resolver Cookie. Of
   course, the actual implementation of setting these policies may be
   for blocks or classes of values or use sparse array techniques or the
   like.

   The policy for each value is either "Disabled", "Enabled", or
   "Enforced" as described below.



5.1 Resolver Policies and Implementation

   A resolver will logically have one of the following three modes of
   operation or "policies" for each DNS server as distinguished by
   server IP Address.

   Disabled:
      Never include a COOKIE OPT option in requests.
      Ignore COOKIE OPT options in responses.

   Enabled:
      Always include a COOKIE OPT option in requests. If a cached Server
         Cookie for the server is not available, the Server Cookie field
         can be set to any value.
      Normally process responses without a COOKIE OPT option.
      Silently ignore responses with more than one COOKIE OPT option.
      Silently ignore responses with one COOKIE OPT option if it has an
         incorrect Resolver Cookie value.
      On receipt of a response with one COOKIE OPT option carrying the
         correct Resolver Cookie value (even if it is a BADCOOKIE error
         response), the DNS client performs normal response processing,
         including caching the received Server Cookie, and it MUST
         change to the Enforced policy for DNS requests to that DNS
         server IP address. This policy change SHOULD be treated as soft
         state with the same timeout strategy as the Server Cookie value
         for that server. On timeout of that state information, the
         policy for that DNS server reverts to Enabled.

   Enforced:
      Always include a COOKIE OPT option in requests.
      Silently ignore all responses that do not include exactly one
         COOKIE OPT option having the correct Resolver Cookie value.







D. Eastlake 3rd                                                [Page 10]


INTERNET-DRAFT                                               DNS Cookies


5.2 Server Policies and Implementation

   A server will logically have one of the following three modes of
   operation or "policies" for each DNS resolver as distinguished by
   resolver IP Address and Resolver Cookie.

   Disabled:
      Ignore COOKIE OPT options in requests.
      Never include a COOKIE OPT option in responses.

   Enabled:
      Always include a COOKIE OPT option in responses.
      Normally process requests without a COOKIE OPT option.
      Ignore, other than sending a MANYCOOKIE error response, any
         request with more than one COOKIE OPT option.
      Ignore, other than sending a BADCOOKIE error response, any query
         with one COOKIE OPT option if it has an incorrect Server
         Cookie.
      On receipt of a request with a COOKIE OPT option carrying the
         correct Server Cookie value, the DNS server performs normal
         request processing and it SHOULD switch to the Enforced policy
         for DNS requests from that resolver IP address with that
         Resolver Cookie in the request. This policy change for that
         resolver SHOULD be treated as soft state. On timing out that
         state information, the policy for that resolver IP and Resolver
         Cookie pair reverts to Enabled.

   Enforced:
      Always include a COOKIE OPT option in responses.
      Ignore requests without a COOKIE OPT option or with more than one
         COOKIE OPT option, other than returning a NOCOOKIE or
         MANYCOOKIE error respectively.
      Ignore requests with one COOKIE OPT option if they have an
         incorrect Server Cookie, other than returning a BADCOOKIE error
         message.
      If a request has one COOKIE OPT option with a correct Server
         Cookie, perform normal processing of the request.



5.3 Implementation Requirements

   DNS resolvers and servers SHOULD implement DNS cookies.

   DNS resolvers SHOULD operate in and be shipped so as to default to
   the Enabled or Enforced mode for all servers.

   DNS servers SHOULD operate in and be shipped so as to default to the
   Enabled or Enforced mode for all resolvers they are willing to
   service.


D. Eastlake 3rd                                                [Page 11]


INTERNET-DRAFT                                               DNS Cookies


6. NAT Considerations and AnyCast Server Considerations

   In the Classic Internet, DNS Cookies could simply be a pseudo-random
   function of the resolver IP address and a sever secret or the server
   IP address and a resolver secret. You would want to compute the
   Server Cookie that way, so a resolver could cache its Server Cookie
   for a particular server for an indefinitely amount of time and the
   server could easily regenerate and check it. You could consider the
   Resolver Cookie to be a resolver signature over the server IP address
   which the resolver checks in responses and you could extend this
   signature to cover the request ID, for example.

   But we have this reality called NAT [RFC3022], Network Address
   Translation (including, for the purposes of this document, NAT-PT
   [RFC2766], Network Address and Protocol Translation, which has been
   declared Historic [RFC4966]).  There is no problem with DNS
   transactions between resolvers and servers behind a NAT box using
   local IP addresses. Nor is there a problem with NAT translation of
   internal addresses to external addresses or translations between IPv4
   and IPv6 addresses, as long as the address mapping is relatively
   stable. Should an internal resolver being mapped to a particular
   external IP address change occasionally, the disruption is no more
   than when a resolver rolls-over its DNS COOKIE secret. And normally
   external access to a DNS server behind a NAT box is handled by a
   fixed mapping which forwards externally received DNS requests to a
   specific host.

   However, NAT devices sometimes also map ports. This can cause
   multiple DNS requests and responses from multiple internal hosts to
   be simultaneously mapped to a smaller number of external IP
   addresses, frequently one.  There could be many resolvers behind a
   NAT box that appear to come from the same source IP address to a
   server outside that NAT box.  If one of these were an attacker (think
   Zombie or Botnet), that behind-NAT attacker could get the Server
   Cookie for some server for the outgoing IP address by just making
   some random request to that server. It could then include that Server
   Cookie in the COOKIE RR of requests to the server with the forged
   local IP address of some other host and/or resolver behind the NAT
   box. (Attacker possession of this Server Cookie will not help in
   forging responses to cause cache poisoning as such responses are
   protected by the required Resolver Cookie.)

   To fix this potential defect, it is necessary to distinguish
   different resolvers behind a NAT box from the point of view of the
   server. It is for this reason that the Server Cookie is specified as
   a pseudo-random function of both the request source IP address and
   the Resolver Cookie.  From this inclusion of the Resolver Cookie in
   the calculation of the Server Cookie, it follows that a stable
   Resolver Cookie, for any particular server, is needed. If, for
   example, the request ID was included in the calculation of the


D. Eastlake 3rd                                                [Page 12]


INTERNET-DRAFT                                               DNS Cookies


   Resolver Cookie, it would normally change with each query to a
   particular server.  This would mean that each query would have to be
   sent twice: first to learn the new Server Cookie based on this new
   Resolver Cookie based on the new ID and then again using this new
   Resolver Cookie to actually get an answer. Thus the input to the
   Resolver Cookie computation must be limited to the server IP address
   and one or more things that change slowly such as the resolver
   secret.

   In principle, there could be a similar problem for servers, not
   particularly due to NAT but due to mechanisms like anycast which may
   cause queries to a DNS server at an IP address to be delivered to any
   one of several machines. (External queries to a DNS server behind a
   NAT box usually occur via port forwarding such that all such queries
   go to one host.) However, it is impossible to solve this the way the
   similar problem was solved for NATed resolvers; if the Server Cookie
   was included in the calculation of the Resolver Cookie the same way
   the Resolver Cookie is included in the Server Cookie, you would just
   get an almost infinite series of BADCOOKIE errors as a query was
   repeatedly retried.

   For servers accessed via anycast to successfully support DNS COOKIES,
   the server clones must either all use the same server secret or the
   mechanism that distributes queries to them must cause the queries
   from a particular resolver to go to a particular server for a
   sufficiently long period of time that extra queries due to changes in
   Server Cookie resulting from accessing different server machines are
   not unduly burdensome. When such anycast accessed servers act as
   recursive servers or otherwise act as resolvers they normally use a
   different unique address to source their queries and avoid confusion
   in the delivery of responses.

   For simplicity, it is RECOMMENDED that the same server secret be used
   by each DNS server in a set of anycast servers.


















D. Eastlake 3rd                                                [Page 13]


INTERNET-DRAFT                                               DNS Cookies


7. Incremental Deployment

   The DNS cookies mechanism is designed for incremental deployment and
   to complement the orthogonal techniques in [forgery]. Either or both
   techniques can be deployed independently at each DNS server and
   resolver.

   In particular, a DNS server or resolver that implements the DNS
   cookies mechanism and is in the Enabled mode will interoperate
   successfully with a DNS resolver or server that does not implement
   this mechanism although, of course, in this case it will not get the
   benefit of the mechanism. When such a server or resolver
   interoperates with a resolver or server which also implements the DNS
   cookies mechanism, this is recognized and, for that transaction
   partner, it enters the Enforced mode and gets the full benefit of the
   DNS cookies mechanism until this soft state times out and it reverts
   to Enabled.



































D. Eastlake 3rd                                                [Page 14]


INTERNET-DRAFT                                               DNS Cookies


8. IANA Considerations

   IANA will allocate the following code points:

   The OPT option value for COOKIE is <TBD>.

   Three new RCODES are assigned values as listed below:
      NOCOOKIE is assigned the value ({TBD}, 23 suggested).
      BADCOOKIE is assigned the value ({TBD}, 24 suggested).
      MANYCOOKIE is assigned the value ({TBD}, 25 suggested).



9. Security Considerations

   DNS Cookies provide a weak form of authentication of DNS requests and
   responses. In particular, they provide no protection at all against
   "on-path" adversaries; that is, they provide no protection against
   any adversary which can observe the plain text DNS traffic, such as
   an on-path router, bridge, or any device on an on-path shared link
   (unless the DNS traffic in question on that path is appropriately
   encrypted).

   For example, if a host is connected via an unsecured IEEE 802.11 link
   (Wi-Fi), any device in the vicinity that could receive and decode the
   802.11 transmissions must be considered "on-path". On the other hand,
   in a similar situation but one where 802.11 Robust Security (WPAv2)
   is appropriately deployed on the Wi-Fi network nodes, only the Access
   Point via which the host is connecting is "on-path".

   Despite these limitations, use of DNS Cookies on the global Internet
   is expected to provide a reduction in the available launch points for
   the traffic amplification and denial of service forgery attacks
   described in Section 2 above.

   The recommended cryptographic algorithms for use in DNS Cookies is
   HMAC-SHA1-64, that is, the HMAC scheme [RFC2104] using the SHA1 hash
   function [RFC3174] [RFC4634] with the HMAC output truncated to
   64-bits. MD5 is now considered to be susceptible to collisions
   attacks. Although this does not effect the security of HMAC-MD5,
   HMAC-SHA1 is believed to be stronger.

   In light of the weak plain-text token security provided by DNS
   Cookies, stronger cryptography is probably not warranted and in many
   cases it would be acceptable to use the weaker MD5 hash function
   [RFC1321].  However, there is nothing wrong with using something
   stronger, for example, HMAC-SHA256-64 [RFC4634], assuming a DNS
   processor has adequate computational resources available. DNS
   processors that feel the need for somewhat stronger security without
   a significant increase in computational load should consider more


D. Eastlake 3rd                                                [Page 15]


INTERNET-DRAFT                                               DNS Cookies


   frequent changes in their resolver and/or server secret; however,
   this does require more frequent generation of a cryptographically
   strong random number [RFC4086] and a change in a server secret will
   result in a number of initial BADCOOKIE rejected requests from
   resolvers caching their old Server Cookie.



10. Normative References

   [RFC1321] - Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
   April 1992.

   [RFC2104] - Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
   Hashing for Message Authentication", RFC 2104, February 1997.

   [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2671] - Vixie, P., "Extension Mechanisms for DNS (EDNS0)", August
   1999.

   [RFC4086] - Eastlake, D., 3rd, Schiller, J., and S.  Crocker,
   "Randomness Requirements for Security", BCP 106, RFC 4086, June 2005.



11. Informative References

   [forgery] - "Measures for making DNS more resilient against forged
   answers", Hubert, A. , R. van Mook, draft-ietf-dnsext-forgery-
   resilience-01.txt, work in progress, July 2007.

   [RFC2766] - Tsirtsis, G., P. Srisuresh, "Network Address Translation
   - Protocol Translation (NAT-PT)", February 2000.

   [RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
   Wellington, "Secret Key Transaction Authentication for DNS (TSIG)",
   RFC 2845, May 2000.

   [RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
   RR)", RFC 2930, September 2000.

   [RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures
   ( SIG(0)s )", RFC 2931, September 2000.

   [RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network
   Address Translator (Traditional NAT)", RFC 3022, January 2001.

   [RFC3174] - Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm


D. Eastlake 3rd                                                [Page 16]


INTERNET-DRAFT                                               DNS Cookies


   1 (SHA1)", RFC 3174, September 2001.

   [RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
   Rose, "DNS Security Introduction and Requirements", RFC 4033, March
   2005.

   [RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
   Rose, "Resource Records for the DNS Security Extensions", RFC 4034,
   March 2005.

   [RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
   Rose, "Protocol Modifications for the DNS Security Extensions", RFC
   4035, March 2005.

   [RFC4634] - Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
   (SHA)", RFC 4634, July 2006.

   [RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network
   Address Translator - Protocol Translator (NAT-PT) to Historic
   Status", RFC 4966, July 2007.
































D. Eastlake 3rd                                                [Page 17]


INTERNET-DRAFT                                               DNS Cookies


Author's Address

   Donald E. Eastlake 3rd
   Motorola Laboratories
   111 Locke Drive
   Marlborough, MA 01752 USA

   Telephone:   +1-508-786-7554 (w)

   EMail:       Donald.Eastlake@motorola.com



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   Copyright (C) The IETF Trust (2008).

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D. Eastlake 3rd                                                [Page 18]


INTERNET-DRAFT                                               DNS Cookies


   The IETF invites any interested party to bring to its attention any
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Expiration and File Name

   This draft expires in August 2008.

   Its file name is draft-eastlake-dnsext-cookies-03.txt







































D. Eastlake 3rd                                                [Page 19]


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