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Domain Name System (DNS) Cookies

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7873.
Authors Donald E. Eastlake 3rd , Mark P. Andrews
Last updated 2016-05-27 (Latest revision 2016-04-05)
Replaces draft-eastlake-dnsext-cookies
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Tim Wicinski
Shepherd write-up Show Last changed 2015-11-03
IESG IESG state Became RFC 7873 (Proposed Standard)
Action Holders
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Joel Jaeggli
Send notices to (None)
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
INTERNET-DRAFT                                           Donald Eastlake
Intended Status: Proposed Standard                                Huawei
                                                            Mark Andrews
Expires: October 4, 2016                                   April 5, 2016

                    Domain Name System (DNS) Cookies


   DNS cookies are a lightweight DNS transaction security mechanism that
   provides limited protection to DNS servers and clients against a
   variety of increasingly common denial-of-service and amplification /
   forgery or cache poisoning attacks by off-path attackers. DNS Cookies
   are tolerant of NAT, NAT-PT, and anycast and can be incrementally
   deployed. (Since DNS Cookies are only returned to the IP address from
   which they were originally received, they cannot be used to generally
   track Internet users.)

Status of This Document

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Distribution of this document is unlimited. Comments should be sent
   to the author or the DNSEXT mailing list <>.

   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-

   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 The list of Internet-Draft
   Shadow Directories can be accessed at

Donald Eastlake & Mark Andrews                                  [Page 1]
INTERNET-DRAFT                                               DNS Cookies

Table of Contents

      1. Introduction............................................4
      1.1 Contents of This Document..............................4
      1.2 Definitions............................................5

      2. Threats Considered......................................6
      2.1 Denial-of-Service Attacks..............................6
      2.1.1 DNS Amplification Attacks............................6
      2.1.2 DNS Server Denial-of-Service.........................7
      2.2 Cache Poisoning and Answer Forgery Attacks.............7

      3. Comments on Existing DNS Security.......................8
      3.1 Existing DNS Data Security.............................8
      3.2 DNS Message/Transaction Security.......................8
      3.3 Conclusions on Existing DNS Security...................8

      4. DNS Cookie Option......................................10
      4.1 Client Cookie.........................................11
      4.2 Server Cookie.........................................11

      5. DNS Cookies Protocol Specification.....................12
      5.1 Originating Requests..................................12
      5.2 Responding to Request.................................12
      5.2.1 No Opt RR or No COOKIE OPT option...................13
      5.2.2 Malformed COOKIE OPT option.........................13
      5.2.3 Only a Client Cookie................................13
      5.2.4 A Client Cookie and an Invalid Server Cookie........14
      5.2.5 A Client Cookie and a Valid Server Cookie...........14
      5.3 Processing Responses..................................15
      5.4 QUERYing for a Server Cookie..........................15

      6. NAT Considerations and AnyCast Server Considerations...17

      7. Operational and Deployment Considerations..............19
      7.1 Client and Server Secret Rollover.....................19
      7.2 Counters..............................................20

      8. IANA Considerations....................................21

      9. Security Considerations................................22
      9.1 Cookie Algorithm Considerations.......................23

      10. Implementation Considerations.........................24

      Normative References......................................25
      Informative References....................................25


Donald Eastlake & Mark Andrews                                  [Page 2]
INTERNET-DRAFT                                               DNS Cookies

Table of Contents (continued)

      Appendix A: Example Client Cookie Algorithms..............28
      A.1 A Simple Algorithm....................................28
      A.2 A More Complex Algorithm..............................28

      Appendix B: Example Server Cookie Algorithms..............29
      B.1 A Simple Algorithm....................................29
      B.2 A More Complex Algorithm..............................29

      Author's Address..........................................31

Donald Eastlake & Mark Andrews                                  [Page 3]
INTERNET-DRAFT                                               DNS Cookies

1. Introduction

   As with many core Internet protocols, the Domain Name System (DNS)
   was originally designed at a time when the Internet had only a small
   pool of trusted users. As the Internet has grown exponentially to a
   global information utility, the DNS has increasingly been subject to

   This document describes DNS cookies, a lightweight DNS transaction
   security mechanism specified as an OPT [RFC6891] option.  The DNS
   cookies mechanism provides limited protection to DNS servers and
   clients against a variety of increasingly common abuses by off-path
   attackers. It is compatible with and can be used in conjunction with
   other DNS transaction forgery resistance measures such as those in
   [RFC5452]. (Since DNS Cookies are only returned to the IP address
   from which they were originally received, they cannot be used to
   generally track Internet users.)

   The protection provided by DNS cookies is similar to that provided by
   using TCP for DNS transactions. To bypass the weak protection
   provided by using TCP requires, among other things, that an off-path
   attacker guess the 32-bit TCP sequence number in use. To bypass the
   weak protection provided by DNS Cookies requires such an attacker to
   guess a 64-bit pseudo-random "cookie" quantity. Where DNS Cookies are
   not available but TCP is, falling back to using TCP is reasonable.

   If only one party to a DNS transaction supports DNS cookies, the
   mechanism does not provide a benefit or significantly interfere; but,
   if both support it, the additional security provided is automatically

   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.

   Section 5 provides a protocol description.

   Section 6 discusses some NAT and anycast related DNS Cookies design

Donald Eastlake & Mark Andrews                                  [Page 4]
INTERNET-DRAFT                                               DNS Cookies


   Section 7 discusses incremental deployment considerations.

   Sections 8 and 9 describe IANA and Security Considerations.

1.2 Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

   "Off-path attacker", for a particular DNS client and server, is
         defined as an attacker who cannot observe the DNS request and
         response messages between that client 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

   "IP address" is used herein as a length independent term and includes
         both IPv4 and IPv6 addresses.

Donald Eastlake & Mark Andrews                                  [Page 5]
INTERNET-DRAFT                                               DNS Cookies

2. Threats Considered

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

2.1 Denial-of-Service Attacks

   The typical 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 some other selected host
   as described below.

   There are also on-path denial of service attacks that attempt to
   saturate a server with DNS requests having correct source addresses.
   Cookies do not protect against such attacks but successful cookie
   validation improves the probability that the correct source IP
   address for the requests is known. This facilitates contacting the
   managers of or taking other actions for the networks from which the
   requests originate.

2.1.1 DNS Amplification Attacks

   A request with a forged IP source 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
   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 amplifying the attack.

   The DNS Cookies mechanism can severely limit the traffic
   amplification obtained by attacker requests that are off the path
   between the server and the request's source address. Enforced DNS
   cookies would make it hard for an off path attacker to cause any more
   than rate-limited short error responses to be sent to a forged IP
   address so the attack would be attenuated rather than amplified.  DNS
   cookies make it more effective to implement a rate limiting scheme
   for error responses from the server.  Such a scheme would further
   restrict selected host denial-of-service traffic from that server.

Donald Eastlake & Mark Andrews                                  [Page 6]
INTERNET-DRAFT                                               DNS Cookies

2.1.2 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 that may issue one or
   more requests and process the responses thereto, in order to
   determine their response to the initial request. And the situation
   can be even worse for recursive servers implementing DNSSEC
   ([RFC4033] [RFC4034] [RFC4035]) because they may be induced to
   perform burdensome cryptographic computations in attempts to verify
   the authenticity of data they retrieve in trying to answer the

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

   Use of DNS cookies should enable a server to reject forged requests
   from an off path attacker with relative ease and before any recursive
   queries or public key cryptographic operations are performed.

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 a DNS
   server to a resolver for names that resolver has been induced to
   resolve or for common names whose resource records have short time-
   to-live values, 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 (the Dan Kaminsky
   attack [Kaminsky]). 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.

   With the use of DNS cookies, a resolver can generally reject such
   forged replies.

Donald Eastlake & Mark Andrews                                  [Page 7]
INTERNET-DRAFT                                               DNS Cookies

3. Comments on Existing DNS Security

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

3.1 Existing DNS Data Security

   DNS data security is one part of DNSSEC and is described in
   [RFC4033], [RFC4034], [RFC4035], and updates thereto. It provides
   data origin authentication and authenticated denial of existence.
   DNSSEC is being deployed and can provide strong protection against
   forged data and cache poisoning; however, it has the unintended
   effect of making some denial-of-service attacks worse because of the
   cryptographic computational load it can require and the increased
   size in DNS response packets that it tends to produce.

3.2 DNS Message/Transaction Security

   The second form of security that has been added to DNS provides
   "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931].
   TSIG could provide strong protection against the attacks for which
   the DNS Cookies mechanism provides weaker protection; however, TSIG
   is non-trivial to deploy in the general Internet because of the
   burdens it imposes. Among these burdens are pre-agreement and key
   distribution between client and server, keeping track of server side
   key state, and required time synchronization between client and

   TKEY [RFC2930] can solve the problem of key distribution for TSIG but
   some modes of TKEY impose a substantial cryptographic computation
   load and can be dependent on the deployment of DNS data security (see
   Section 3.1).

   SIG(0) [RFC2931] 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 DNS data security and requires computationally
   burdensome public key cryptographic operations.

3.3 Conclusions on Existing DNS Security

   The existing DNS security mechanisms do not provide the services
   provided by the DNS Cookies mechanism: lightweight message
   authentication of DNS requests and responses with no requirement for

Donald Eastlake & Mark Andrews                                  [Page 8]
INTERNET-DRAFT                                               DNS Cookies

   pre-configuration or per client server side state.

Donald Eastlake & Mark Andrews                                  [Page 9]
INTERNET-DRAFT                                               DNS Cookies

4. DNS Cookie Option

   The DNS Cookie Option is an OPT RR [RFC6891] option that can be
   included in the RDATA portion of an OPT RR in DNS requests and
   responses.  The option length varies depending on the circumstances
   in which it is being used.  There are two cases as described below.
   Both use the same OPTION-CODE; they are distinguished by their

   In a request sent by a client to a server when the client does not
   know the server's cookie, its length is 8, consisting of an 8 byte
   Client Cookie as shown in Figure 1.

                           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 = 10      |       OPTION-LENGTH = 8        |
      |                                                               |
      +-+-    Client Cookie (fixed size, 8 bytes)              -+-+-+-+
      |                                                               |

              Figure 1. COOKIE Option, Unknown Server Cookie

   In a request sent by a client when a server cookie is known and in
   all responses, the length is variable from 16 to 40 bytes, consisting
   of an 8 bytes Client Cookie followed by the variable 8 to 32 bytes
   Server Cookie as shown in Figure 2.  The variability of the option
   length stems from the variable length Server Cookie.  The Server
   Cookie is an integer number of bytes with a minimum size of 8 bytes
   for security and a maximum size of 32 bytes for implementation

                         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 = 10      |   OPTION-LENGTH >= 16, <= 40   |
    |                                                               |
    +-+-    Client Cookie (fixed size, 8 bytes)              -+-+-+-+
    |                                                               |
    |                                                               |
    /       Server Cookie  (variable size, 8 to 32 bytes)           /
    /                                                               /

               Figure 2. COOKIE Option, Known Server Cookie

Donald Eastlake & Mark Andrews                                 [Page 10]
INTERNET-DRAFT                                               DNS Cookies

4.1 Client Cookie

   The Client Cookie SHOULD be a pseudo-random function of the client IP
   address, the server IP address, and a secret quantity known only to
   the client. This client secret SHOULD have at least 64 bits of
   entropy [RFC4086] and be changed periodically (see Section 7.1). The
   selection of the pseudo-random function is a matter private to the
   client as only the client needs to recognize its own DNS cookies.

   The client IP address is included so that the Client Cookie cannot be
   used (1) to track a client if the client IP address changes due to
   privacy mechanisms or (2) to impersonate the client by some network
   device that was formerly on path but is no longer on path when the
   client IP address changes due to mobility.  However, if the client IP
   address is being changed very often, it may be necessary to fix the
   Client Cookie for a particular server for several requests to avoid
   undue inefficiency due to retries caused by that server not
   recognizing the Client Cookie.

   For further discussion of the Client Cookie field, see Section 5.1.
   For example methods of determining a Client Cookie, see Appendix A.

   In order to provide minimal authentication, a client MUST send Client
   Cookies that will usually be different for any two servers at
   different IP addresses.

4.2 Server Cookie

   The Server Cookie SHOULD consist of or include a 64-bit or larger
   pseudo-random function of the request source (client) IP address, a
   secret quantity known only to the server, and the request Client
   Cookie. (See Section 6 for a discussion of why the Client Cookie is
   used as input to the Server Cookie but the Server Cookie is not used
   as an input to the Client Cookie.)  This server secret SHOULD have at
   least 64 bits of entropy [RFC4086] and be changed periodically (see
   Section 7.1).  The selection of the pseudo-random function is a
   matter private to the server as only the server needs to recognize
   its own DNS cookies.

   For further discussion of the Server Cookie field see Section 5.2.
   For example methods of determining a Server Cookie, see Appendix B.
   When implemented as recommended, the server need not maintain any
   cookie related per client state.

   In order to provide minimal authentication, a server MUST send Server
   Cookies that will usually be different for clients at any two
   different IP addresses or with different Client Cookies.

Donald Eastlake & Mark Andrews                                 [Page 11]
INTERNET-DRAFT                                               DNS Cookies

5. DNS Cookies Protocol Specification

   This section discusses using DNS Cookies in the DNS Protocol. The
   cycle of originating a request, responding to that request, and
   processing the response are covered in Sections 5.1, 5.2, and 5.3. A
   de facto extension to QUERY to allow pre-fetching a Server Cookie is
   specified in Section 5.4. Rollover of the client and server secrets
   and transient retention of the old cookie or secret is covered in
   Section 7.1.

   DNS clients and servers SHOULD implement DNS cookies to decrease
   their vulnerability to the threats discussed in Section 2.

5.1 Originating Requests

   A DNS client that implements DNS Cookies includes one DNS COOKIE OPT
   option containing a Client Cookie in every DNS request it sends
   unless DNS cookies are disabled.

   If the client has a cached Server Cookie for the server against its
   IP address it uses the longer cookie form and includes that Server
   Cookie in the option along with the Client Cookie (Figure 2).
   Otherwise it just sends the shorter form option with a Client Cookie
   (Figure 1).

5.2 Responding to Request

   The Server Cookie, when it occurs in a COOKIE OPT option in a
   request, is intended to weakly assure the server that the request
   came from a client that is both at the source IP address of the
   request and using the Client Cookie included in the option. This
   assurance is provided by the Server Cookie that server sent to that
   client in an earlier response appearing as the Server Cookie field in
   the request.

   At a server where DNS Cookies are not implemented and enabled,
   presence of a COOKIE OPT option is ignored and the server responds as
   if no COOKIE OPT option had been included in the request.

   When DNS Cookies are implemented and enabled, there are five
   possibilities: (1) there is no OPT RR at all in the request or there
   is a OPT RR but the COOKIE OPT option is absent from the OPT RR; (2)
   a COOKIE OPT is present but is not a legal length or otherwise
   malformed; (3) there is a valid length cookie option in the request
   with no Server Cookie; (4) there is a valid length COOKIE OPT in the
   request with a Server Cookie but that Server Cookie is invalid; or

Donald Eastlake & Mark Andrews                                 [Page 12]
INTERNET-DRAFT                                               DNS Cookies

   (5) there is a valid length COOKIE OPT in the request with a correct
   Server Cookie.

   The five possibilities are discussed in the subsections below.

   In all cases of multiple COOKIE OPT options in a request, only the
   first (the one closest to the DNS header) is considered. All others
   are ignored.

5.2.1 No Opt RR or No COOKIE OPT option

   If there is no OPT record or no COOKIE OPT option present in the
   request then the server responds to the request as if the server
   doesn't implement the COOKIE OPT.

5.2.2 Malformed COOKIE OPT option

   If the COOKIE OPT is too short to contain a Client Cookie then
   FORMERR is generated.  If the COOKIE OPT is longer than that required
   to hold a COOKIE OPT with just a Client Cookie (8 bytes) but is
   shorter that the minimum COOKIE OPT with both a Client and Server
   Cookie (16 bytes) then FORMERR is generated.  If the COOKIE OPT is
   longer than the maximum valid COOKIE OPT (40 bytes) then a FORMERR is

   In summary, valid cookie lengths are 8 and 16 to 40 inclusive.

5.2.3 Only a Client Cookie

   Based on server policy, including rate limiting, the server chooses
   one of the following:

   (1) Silently discard the request.

   (2) Send a BADCOOKIE error response.

   (3) Process the request and provide a normal response. The RCODE is
       NOERROR unless some non-cookie error occurs in processing the

   If the server responds, choosing 2 or 3 above, it SHALL generate its
   own COOKIE OPT containing both the Client Cookie copied from the
   request and a Server Cookie it has generated and adds this COOKIE OPT
   to the response's OPT record. Servers MUST, at least occasionally,

Donald Eastlake & Mark Andrews                                 [Page 13]
INTERNET-DRAFT                                               DNS Cookies

   respond to such requests to inform the client of the correct Server
   Cookie. This is necessary so that such a client can bootstrap to the
   more secure state where requests and responses have recognized Server
   Cookies and Client Cookies. A server is not expected to maintain per
   client state to achieve this. For example, it could respond to every
   Nth request across all clients.

   If the request was received over TCP, the server SHOULD take the
   authentication provided by the use of TCP into account and SHOULD
   choose 3. In this case, if the server is not willing to accept the
   security provided by TCP as a substitute for the security provided by
   DNS Cookies but instead chooses 2, there is some danger of an
   indefinite loop of retries (see Section 5.3).

5.2.4 A Client Cookie and an Invalid Server Cookie

   The server examines the Server Cookie to determine if it is a valid
   Server Cookie it has generated. This determination normally involves
   re-calculating the Server Cookie (or the hash part thereof) based on
   the server secret (or the previous server secret if it has just
   changed), the received Client Cookie, the client IP address, and
   possibly other fields -- see Appendix B.2 for an example. If the
   cookie is invalid, it can be because of a stale Server Cookie, or a
   client's IP address or Client Cookie changing without the DNS server
   being aware, or an anycast server cluster that is not consistently
   configured, or an attempt to spoof the client.

   The server SHALL process the request as if the invalid Server Cookie
   was not present as described in Section 5.2.3.

5.2.5 A Client Cookie and a Valid Server Cookie

   When a valid Server Cookie is present in the request the server can
   assume that the request is from a client that it has talked to before
   and defensive measures for spoofed UDP requests, if any, are no
   longer required.

   The server SHALL process the request and include a COOKIE OPT in the
   response by (a) copying the complete COOKIE OPT from the request or
   (b) generating a new COOKIE OPT containing both the Client Cookie
   copied from the request and a valid Server Cookie it has generated.

Donald Eastlake & Mark Andrews                                 [Page 14]
INTERNET-DRAFT                                               DNS Cookies

5.3 Processing Responses

   The Client Cookie, when it occurs in a COOKIE OPT option in a DNS
   reply, is intended to weakly assure the client that the reply came
   from a server at the source IP address used in the response packet
   because the Client Cookie value is the value that client would send
   to that server in a request. In a DNS reply with multiple COOKIE OPT
   options, all but the first (the one closest to the DNS Header) are

   A DNS client where DNS cookies are implemented and enabled examines
   the response for DNS cookies and MUST discard the response if it
   contains an illegal COOKIE OPT option length or an incorrect Client
   Cookie value.  If the client is expecting the response to contain a
   COOKIE OPT and it is missing the response MUST be discarded. If the
   COOKIE OPT option Client Cookie is correct, the client caches the
   Server Cookie provided even if the response is an error response
   (RCODE non-zero).

   If the reply extended RCODE is BADCOOKIE and the Client Cookie
   matches what was sent, it means that the server was unwilling to
   process the request because it did not have the correct Server Cookie
   in it. The client SHOULD retry the request using the new Server
   Cookie from the response. Repeated BADCOOKIE responses to requests
   that use the Server Cookie provided in the previous response may be
   an indication that the shared secrets / secret generation method in
   an anycast cluster of servers are inconsistent. If the reply to a
   retried request with a fresh Server Cookie is BADCOOKIE, the client
   SHOULD retry using TCP as the transport since the server will likely
   process the request normally based on the security provided by TCP
   (see Section 5.2.3).

   If the RCODE is some value other than BADCOOKIE, including zero, the
   further processing of the response proceeds normally.

5.4 QUERYing for a Server Cookie

   In many cases a client will learn the Server Cookie for a server as
   the side effect of another transaction; however, there may be times
   when this is not desirable. Therefore a means is provided for
   obtaining a Server Cookie through an extension to the QUERY opcode
   for which opcode most existing implementations require that QDCOUNT
   be one (see Section 4.1.2 of [RFC1035]).

   For servers with DNS Cookies enabled, the QUERY opcode behavior is
   extended to support queries with an empty question section (QDCOUNT
   zero) provided that an OPT record is present with a COOKIE option.
   Such servers will reply with an empty answer section and a COOKIE

Donald Eastlake & Mark Andrews                                 [Page 15]
INTERNET-DRAFT                                               DNS Cookies

   option giving the Client Cookie provided in the query and a valid
   Server Cookie.

   If such a query provided just a Client Cookie and no Server Cookie,
   the response SHALL have the RCODE NOERROR.

   This mechanism can also be used to confirm/re-establish an existing
   Server Cookie by sending a cached Server Cookie with the Client
   Cookie. In this case the response SHALL have the RCODE BADCOOKIE if
   the Server Cookie sent with the query was invalid and the RCODE
   NOERROR if it was valid.

   Servers which don't support the COOKIE option will normally send
   FORMERR in response to such a query, though REFUSED, NOTIMP, and
   NOERROR without a COOKIE option are also possible in such responses.

Donald Eastlake & Mark Andrews                                 [Page 16]
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 client IP address and a server secret or the server
   IP address and a client secret. You would want to compute the Server
   Cookie that way, so a client could cache its Server Cookie for a
   particular server for an indefinite amount of time and the server
   could easily regenerate and check it. You could consider the Client
   Cookie to be a weak client signature over the server IP address that
   the client checks in replies and you could extend this signature to
   cover the request ID, for example, or any other information that is
   returned unchanged in the reply.

   But we have this reality called NAT [RFC3022], Network Address
   Translation (including, for the purposes of this document, NAT-PT,
   Network Address and Protocol Translation, which has been declared
   Historic [RFC4966]).  There is no problem with DNS transactions
   between clients 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 the external IP address an internal client is being mapped to
   change occasionally, the disruption is little more than when a client
   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 mapped to a smaller number of external IP addresses, such as one
   address.  Thus there could be many clients 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 OPT of requests to the server with the forged local IP
   address of some other host and/or client 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 Client Cookie.)

   To fix this potential defect, it is necessary to distinguish
   different clients 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 Client Cookie.  From this inclusion of the Client Cookie in the
   calculation of the Server Cookie, it follows that a stable Client
   Cookie, for any particular server, is needed. If, for example, the
   request ID was included in the calculation of the Client Cookie, it

Donald Eastlake & Mark Andrews                                 [Page 17]
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   would normally change with each request to a particular server.  This
   would mean that each request would have to be sent twice: first to
   learn the new Server Cookie based on this new Client Cookie based on
   the new ID and then again using this new Client Cookie to actually
   get an answer. Thus the input to the Client Cookie computation must
   be limited to the server IP address and one or more things that
   change slowly such as the client secret.

   In principle, there could be a similar problem for servers, not due
   to NAT but due to mechanisms like anycast which may cause requests to
   a DNS server at an IP address to be delivered to any one of several
   machines. (External requests to a DNS server behind a NAT box usually
   occur via port forwarding such that all such requests go to one
   host.)  However, it is impossible to solve this the way the similar
   problem was solved for NATed clients; if the Server Cookie was
   included in the calculation of the Client Cookie the same way the
   Client Cookie is included in the Server Cookie, you would just get an
   almost infinite series of errors as a request 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 requests to them must cause the requests
   from a particular client to go to a particular server for a
   sufficiently long period of time that extra requests 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 clients they normally use a
   different unique address to source their requests to 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. If there is limited
   time skew in updating this secret in different anycast servers, this
   can be handled by a server accepting requests containing a Server
   Cookie based on either its old or new secret for the maximum likely
   time period of such time skew (see also Section 7.1).

Donald Eastlake & Mark Andrews                                 [Page 18]
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7. Operational and Deployment Considerations

   The DNS cookies mechanism is designed for incremental deployment and
   to complement the orthogonal techniques in [RFC5452]. Either or both
   techniques can be deployed independently at each DNS server and
   client. Thus installation at the client and server end need not be

   In particular, a DNS server or client that implements the DNS COOKIE
   mechanism can interoperate successfully with a DNS client or server
   that does not implement this mechanism although, of course, in this
   case it will not get the benefit of the mechanism and the server
   involved might choose to severely rate limit responses. When such a
   server or client interoperates with a client or server which also
   implements the DNS cookies mechanism, they get the security benefits
   of the DNS Cookies mechanism.

7.1 Client and Server Secret Rollover

   The longer a secret is used, the higher the probability it has been
   compromised. Thus clients and servers are configured with a lifetime
   for their secret and rollover to a new secret when that lifetime
   expires or earlier due to deliberate jitter as described below. The
   default lifetime is one day and the maximum permitted is one month.
   To be precise and to make it practical to stay within limits despite
   long holiday weekends and daylight savings time shifts and the like,
   clients and servers MUST NOT continue to use the same secret in new
   requests and responses for more than 36 days and SHOULD NOT continue
   to do so for more than 26 hours.

   Many clients rolling over their secret at the same time could briefly
   increase server traffic and exactly predictable rollover times for
   clients or servers might facilitate guessing attacks. For example, an
   attacker might increase the priority of attacking secrets they
   believe will be in effect for an extended period of time.  To avoid
   rollover synchronization and predictability, it is RECOMMENDED that
   pseudorandom jitter in the range of plus zero to minus at least 40%
   be applied to the time until a scheduled rollover of a DNS cookie

   It is RECOMMENDED that a client keep the Client Cookie it is
   expecting in a reply until there is no longer an outstanding request
   associated with that Client Cookie that the client is tracking. This
   avoids rejection of replies due to a bad Client Cookie right after a
   change in the client secret.

   It is RECOMMENDED that a server retain its previous secret after a
   rollover to a new secret for a configurable period of time not less

Donald Eastlake & Mark Andrews                                 [Page 19]
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   than 1 second or more than 5 minutes with default configuration of 2
   1/2 minutes. Requests with Server Cookies based on its previous
   secret are treated as a correct Server Cookie during that time. When
   a server responds to a request containing a old Server Cookie that
   the server is treating as correct, the server MUST include a new
   Server Cookie in its response.

7.2 Counters

   It is RECOMMENDED that implementations include counters of the
   occurrences of the various types of requests and responses described
   in Section 5.

Donald Eastlake & Mark Andrews                                 [Page 20]
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8. IANA Considerations

   IANA has assigned the following OPT option value:

       Value       Name      Status        Reference
      --------    ------    --------    ---------------
         10       COOKIE    Standard    [this document]

   IANA has assigned the following DNS error code as an early

       RCODE       Name       Description                 Reference
      --------  ---------  -------------------------   ---------------
         23     BADCOOKIE  Bad/missing server cookie   [this document]

Donald Eastlake & Mark Andrews                                 [Page 21]
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9. Security Considerations

   DNS Cookies provide a weak form of authentication of DNS requests and
   responses. In particular, they provide no protection against "on-
   path" adversaries; that is, they provide no protection against any
   adversary that 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 encrypted).

   For example, if a host is connected via an unsecured IEEE Std 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 (WPA2) is appropriately deployed on the Wi-Fi network nodes,
   only the Access Point via which the host is connecting is "on-path"
   as far as the 802.11 link is concerned.

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

   Work is underway in the IETF DPRIVE working group to provide
   confidentiality for DNS requests and responses which would be
   compatible with DNS cookies.

   Should stronger message/transaction security be desired, it is
   suggested that TSIG or SIG(0) security be used (see Section 3.2);
   however, it may be useful to use DNS Cookies in conjunction with
   these features. In particular, DNS Cookies could screen out many DNS
   messages before the cryptographic computations of TSIG or SIG(0) are
   required and, if SIG(0) is in use, DNS Cookies could usefully screen
   out many requests given that SIG(0) does not screen requests but only
   authenticates the response of complete transactions.

   An attacker that does not know the Server Cookie could do a variety
   of things, such as omitting the COOKIE OPT option or sending a random
   Server Cookie. In general, DNS servers need to take other measures,
   including rate limiting responses, to protect from abuse in such
   cases. See further information in Section 5.2.

   When a server or client starts receiving an increased level of
   requests with bad server cookies or replies with bad client cookies,
   it would be reasonable for it to believe it is likely under attack
   and it should consider a more frequent rollover of its secret. More
   rapid rollover decreases the benefit to a cookie guessing attacker if
   they succeed in guessing a cookie.

Donald Eastlake & Mark Andrews                                 [Page 22]
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9.1 Cookie Algorithm Considerations

   The cookie computation algorithm for use in DNS Cookies SHOULD be
   based on a pseudo-random function at least as strong as 64-bit FNV
   (Fowler-Noll-Vo [FNV]) because an excessively weak or trivial
   algorithm could enable adversaries to guess cookies.  However, in
   light of the lightweight plain-text token security provided by DNS
   Cookies, a strong cryptography hash algorithm may not be warranted in
   many cases, and would cause an increased computational burden.
   Nevertheless there is nothing wrong with using something stronger,
   for example, HMAC-SHA256 [RFC6234] truncated to 64 bits, 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
   frequent changes in their client and/or server secret; however, this
   does require more frequent generation of a cryptographically strong
   random number [RFC4086]. See Appendices A and B for specific examples
   of cookie computation algorithms.

Donald Eastlake & Mark Andrews                                 [Page 23]
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10. Implementation Considerations

   The DNS Cookie Option specified herein is implemented in BIND 9.10
   using an experimental option code.

Donald Eastlake & Mark Andrews                                 [Page 24]
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Normative References

   [RFC1035] - Mockapetris, P., "Domain names - implementation and
         specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
         November 1987, <>.

   [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
         Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119,
         March 1997, <>.

   [RFC4086] - Eastlake 3rd, D., Schiller, J., and S. Crocker,
         "Randomness Requirements for Security", BCP 106, RFC 4086, DOI
         10.17487/RFC4086, June 2005, <http://www.rfc->.

   [RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
         for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891,
         April 2013, <>.

Informative References

   [FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non-
         Cryptographic Hash Algorithm", draft-eastlake-fnv, work in

   [Kaminsky] - Olney, M., P. Mullen, K. Miklavicic, "Dan Kaminsky's
         2008 DNS Vulnerability", 25 July 2008,

   [RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
         Wellington, "Secret Key Transaction Authentication for DNS
         (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,

   [RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
         RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000,

   [RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures
         ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 2000,

   [RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network
         Address Translator (Traditional NAT)", RFC 3022, DOI
         10.17487/RFC3022, January 2001, <http://www.rfc->.

Donald Eastlake & Mark Andrews                                 [Page 25]
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   [RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
         Rose, "DNS Security Introduction and Requirements", RFC 4033,
         DOI 10.17487/RFC4033, March 2005, <http://www.rfc->.

   [RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
         Rose, "Resource Records for the DNS Security Extensions", RFC
         4034, DOI 10.17487/RFC4034, March 2005, <http://www.rfc->.

   [RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
         Rose, "Protocol Modifications for the DNS Security Extensions",
         RFC 4035, DOI 10.17487/RFC4035, March 2005, <http://www.rfc->.

   [RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network
         Address Translator - Protocol Translator (NAT-PT) to Historic
         Status", RFC 4966, DOI 10.17487/RFC4966, July 2007,

   [RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More
         Resilient against Forged Answers", RFC 5452, DOI
         10.17487/RFC5452, January 2009, <http://www.rfc->.

   [RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash
         Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI
         10.17487/RFC6234, May 2011, <http://www.rfc->.

Donald Eastlake & Mark Andrews                                 [Page 26]
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   The suggestions and contributions of the following are gratefully

      Alissa Cooper, Bob Harold, Paul Hoffman, David Malone, Yoav Nir,
      Gayle Noble, Dan Romascanu,
      Tim Wicinski, Peter Yee

   The document was prepared in raw nroff. All macros used were defined
   within the source file.

Donald Eastlake & Mark Andrews                                 [Page 27]
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Appendix A: Example Client Cookie Algorithms

A.1 A Simple Algorithm

   A simple example method to compute Client Cookies is the FNV-64 [FNV]
   of the client IP address, the server IP address, and the client
   secret. That is

      Client Cookie =
         FNV-64( Client IP Address | Server IP Address | Client Secret )

   where "|" indicates concatenation. Some computational resources may
   be saved by precomputing FNV-64 through the Client IP Address. (If
   the order of the items concatenated above is changed to put the
   Server IP Address last, it might be possible to further reduce the
   computational effort by pre-computing FNV-64 through the bytes of
   both the Client IP Address and the Client Secret but this would
   reduce the strength of the Client Cookie and is NOT RECOMMENDED.)

A.2 A More Complex Algorithm

   A more complex algorithm to calculate Client Cookies is given below.
   It uses more computational resources than the simpler algorithm shown
   in A.1.

      Client Cookie = HMAC-SHA256-64(
                         Client IP Address | Server IP Address,
                         Client Secret )

Donald Eastlake & Mark Andrews                                 [Page 28]
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Appendix B: Example Server Cookie Algorithms

B.1 A Simple Algorithm

   An example of a simple method producing a 64-bit Server Cookie is the
   FNV-64 [FNV] of the request IP address, the Client Cookie, and the
   server secret.

      Server Cookie =
         FNV-64( Client IP Address | Client Cookie | Server Secret )

   where "|" represents concatenation. (If the order of the items
   concatenated was changed, it might be possible to reduce the
   computational effort by pre-computing FNV-64 through the bytes of the
   Sever Secret and Client Cookie but this would reduce the strength of
   the Server Cookie and is NOT RECOMMENDED.)

B.2 A More Complex Algorithm

   Since the Server Cookie has a variable size, the server can store
   various information in that field as long as it is hard for an
   adversary to guess the entire quantity used for authentication. There
   should be 64 bits of entropy in the Server Cookie; for example it
   could have a sub-field of 64-bits computed pseudo-randomly with the
   server secret as one of the inputs to the pseudo-random function.
   Types of additional information that could be stored include a time
   stamp and/or a nonce.

   The example below is one variation for the Server Cookie that has
   been implemented in BIND 9.10.3 (and later) releases where the Server
   Cookie is 128 bits composed as follows:

         Sub-field      Size
         ---------   ---------
           Nonce      32 bits
           Time       32 bits
           Hash       64 bits

   With this algorithm, the server sends a new 128-bit cookie back with
   every request. The Nonce field assures a low probability that there
   would be a duplicate.

   The Time field gives the server time and makes it easy to reject old

   The Hash part of the Server Cookie is the hard-to-guess part. In BIND

Donald Eastlake & Mark Andrews                                 [Page 29]
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   9.10.3 (and later), its computation can be configured to use AES,
   HMAC-SHA1, or, as shown below, HMAC-SHA256:

      hash =
          HMAC-SHA256-64( Server Secret,
              (Client Cookie | nonce | time | Client IP Address) )

   where "|" represents concatenation.

Donald Eastlake & Mark Andrews                                 [Page 30]
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Author's Address

   Donald E. Eastlake 3rd
   Huawei Technologies
   155 Beaver Street
   Milford, MA 01757 USA

   Telephone:   +1-508-333-2270

   Mark Andrews
   Internet Systems Consortium
   950 Charter Street
   Redwood City, CA  94063 USA


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Donald Eastlake & Mark Andrews                                 [Page 31]