< draft-eastlake-dnsext-cookies-03.txt   draft-eastlake-dnsext-cookies-04.txt >
INTERNET-DRAFT Donald E. Eastlake 3rd INTERNET-DRAFT Donald Eastlake
Intended Status: Proposed Standard Motorola Laboratories Intended Status: Proposed Standard Huawei
Expires: August 2008 February 25, 2008 Expires: July 21, 2014 January 22, 2014
Domain Name System (DNS) Cookies Domain Name System (DNS) Cookies
<draft-eastlake-dnsext-cookies-04.txt>
<draft-eastlake-dnsext-cookies-03.txt> Abstract
DNS cookies are a lightweight 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 amplification/forgery or cache poisoning
attacks by off-path attackers. DNS Cookies are tolerant of NAT, NAT-
PT, and anycast.
Status of This Document Status of This Document
By submitting this Internet-Draft, each author represents that any This Internet-Draft is submitted to IETF in full conformance with the
applicable patent or other IPR claims of which he or she is aware provisions of BCP 78 and BCP 79.
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 Distribution of this document is unlimited. Comments should be sent
to the author or the DNSEXT working group mailing list to the author or the DNSEXT mailing list <dnsext@ietf.org>.
<namedroppers@ops.ietf.org>.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft
Shadow Directories can be accessed at
The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.
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.
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
Table of Contents Table of Contents
Status of This Document....................................1
Abstract...................................................1
Table of Contents..........................................2
1. Introduction............................................3 1. Introduction............................................3
1.1 Contents of This Document..............................3 1.1 Contents of This Document..............................3
1.2 Definitions............................................4 1.2 Definitions............................................4
2. Threats Considered......................................5 2. Threats Considered......................................5
2.1 Denial-of-Service Attacks..............................5 2.1 Denial-of-Service Attacks..............................5
2.1.1 DNS Server Denial-of-Service.........................5 2.1.1 DNS Amplification Attacks............................5
2.1.2 Selected Host Denial-of-Service......................5 2.1.2 DNS Server Denial-of-Service.........................5
2.2 Cache Poisoning and Answer Forgery Attacks.............6 2.2 Cache Poisoning and Answer Forgery Attacks.............6
3. Comments on Existing DNS Security.......................6
3.1 Existing DNS Data Security.............................6 3. Comments on Existing DNS Security.......................7
3.2 DNS Message or Transaction Security....................7 3.1 Existing DNS Data Security.............................7
3.2 DNS Message/Transaction Security.......................7
3.3 Conclusions on Existing DNS Security...................7 3.3 Conclusions on Existing DNS Security...................7
4. The COOKIE OPT option...................................8 4. The COOKIE OPT Option...................................8
4.1 Resolver Cookies.......................................8 4.1 Resolver Cookie........................................8
4.2 Server Cookies.........................................9 4.2 Server Cookie..........................................9
5. DNS Cookie Policies and Implementation Requirements.....9 4.3 Error Code.............................................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 5. DNS Cookies Protocol Description.......................11
5.1 Originating Requests..................................11
5.2 Responding to Requests................................11
5.3 Processing Responses..................................11
8. IANA Considerations....................................15 6. DNS Cookie Policies and Implementation.................13
9. Security Considerations................................15 6.1 Resolver Policies and Implementation..................13
10. Normative References..................................16 6.2 Server Policies and Implementation....................14
11. Informative References................................16 6.3 Resolver and Server Secret Rollover...................15
6.4 Implementation Requirement............................15
Author's Address..........................................18 7. NAT Considerations and AnyCast Server Considerations...17
Copyright and Disclaimer..................................18 8. Deployment.............................................19
Additional IPR Provisions.................................18 9. IANA Considerations....................................20
Expiration and File Name..................................19
10. Security Considerations...............................21
10.1 Cookie Algorithm Considerations......................21
Acknowledgements..........................................22
Normative References......................................23
Informative References....................................23
Author's Address..........................................25
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
1. Introduction 1. Introduction
As with many core Internet protocols, the Domain Name System (DNS) 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 was originally designed at a time when the Internet had only a small
trusted users. As the Internet has exploded to a global information pool of trusted users. As the Internet has grown exponentially to a
utility, the DNS has increasingly been subject to abuse and been used global information utility, the DNS has increasingly been subject to
as a vector for abuse. abuse.
This document describes DNS cookies, a light-weight DNS transaction This document describes DNS cookies, a lightweight DNS transaction
security mechanism specified as an OPT [RFC2671] option. This security mechanism specified as an OPT [RFC6891] option. This
mechanism provides limited protection to DNS servers and resolvers mechanism provides limited protection to DNS servers and resolvers
against a variety of increasingly common denial-of-service and cache against a variety of increasingly common abuses by off-path
poisoning forgery attacks by off-path attackers. attackers.
The DNS cookies mechanism is designed with a default mode which The DNS cookies mechanism has a default mode that supports
supports incremental deployment. If only one party to a DNS incremental deployment. If only one party to a DNS transaction
transaction supports the mechanism, it does not interfere or provide supports the mechanism, it does not provide a benefit or
a benefit, but, if both support it, the additional security provided significantly interfere, but, if both support it, the additional
is automatically available for that and subsequent transactions. security provided is automatically available.
The DNS cookies mechanism is compatible with and can be used in The DNS cookies mechanism is compatible with and can be used in
conjunction with other DNS transaction forgery resistance measures conjunction with other DNS transaction forgery resistance measures
such as those in [forgery]. such as those in [RFC5452].
The DNS cookies mechanism is designed to work in the presence of NAT 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 and NAT-PT boxes and guidance is provided herein on supporting the
DNS cookies mechanism in anycast servers. DNS cookies mechanism in anycast servers.
1.1 Contents of This Document 1.1 Contents of This Document
In Section 2, we discuss the threats against which the DNS cookie In Section 2, we discuss the threats against which the DNS cookie
mechanism provides some protection. mechanism provides some protection.
Section 3 describes existing DNS security mechanisms and why they are Section 3 describes existing DNS security mechanisms and why they are
not adequate substitutes for DNS cookies. not adequate substitutes for DNS cookies.
Section 4 describes the COOKIE OPT option including recommendations Section 4 describes the COOKIE OPT option and Section 5 provides a
for calculating Resolver and Server Cookies. protocol description including suggestions for calculating Resolver
and Server Cookies.
Section 5 describes the processing of COOKIE OPT options by resolvers Section 6 gives further details on the processing of COOKIE OPT
and server and policies for such processing. options by resolvers and server and policies for such processing.
Section 6 discusses some NAT and anycast related DNS Cookies design Section 7 discusses some NAT and anycast related DNS Cookies design
considerations. considerations.
Section 7 discusses incremental deployment considerations. Section 8 discusses incremental deployment considerations.
Sections 8 and 9 describe IANA and Security Considerations.
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
Sections 9 and 10 describe IANA and Security Considerations.
1.2 Definitions 1.2 Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
An "off-path attacker", for a particular DNS resolver and server, is An "off-path attacker", for a particular DNS resolver and server, is
defined as an attacker which cannot observe the legitimate plain text defined as an attacker who cannot observe the plain text of DNS
DNS requests and responses between that resolver and server. requests and responses between that resolver and server.
"Soft state" indicates information learned or derived by a host which "Soft state" indicates information learned or derived by a host which
may be discarded when indicated by the policies of that host but can may be discarded when indicated by the policies of that host but can
be later re-instantiated if needed. For example, it could be be later re-instantiated if needed. For example, it could be
discarded after a period of time or when storage for caching such discarded after a period of time or when storage for caching such
data becomes full. If operations requiring that soft state continue data becomes full. If operations requiring that soft state continue
after it has been discarded, it will be automatically re-generated, after it has been discarded, it will be automatically re-generated,
albeit at some cost. albeit at some cost.
"Silently discarded" indicates that there are no DNS protocol message "Silently discarded" indicates that there are no DNS protocol message
consequences; however, it is RECOMMENDED that appropriate debugging consequences; however, it is RECOMMENDED that appropriate network
network management facilities be included in implementations, such as management facilities be included in implementations, such as a
a counter of the occurrences of each type of such events. counter of the occurrences of each type of such events.
The term "IP address" is used herein in a length independent manner The term "IP address" is used herein in a length independent manner
and refers to address formats including IPv4 and IPv6. and refers to both IPv4 and IPv6.
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
2. Threats Considered 2. Threats Considered
DNS cookies are intended to provide significant but limited DNS cookies are intended to provide significant but limited
protection against certain denial-of-service and cache poisoning or protection against certain attacks by off-path attackers as described
answer forgery attacks by off-path attackers described below. below. These attacks include denial-of-service, cache poisoning and
answer forgery.
2.1 Denial-of-Service Attacks 2.1 Denial-of-Service Attacks
The canonical form of the denial-of-service attacks considered herein The typical form of the denial-of-service attacks considered herein
is to send DNS requests with forged source IP addresses to a server. 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 The intent can be to attack that server or some other selected host
described below. 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 2.1.1 DNS Amplification Attacks
A request with a forged IP address generally causes a response to be 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 sent to that forged IP address. Thus the forging of many such
requests with a particular source IP address can result in enough requests with a particular source IP address can result in enough
traffic being sent to the forged IP address to interfere with service 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 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 rate-limited short error
responses to be sent to a forged IP address so the attack would be
reduced rather than amplified. 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.
2.1.2 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 public key cryptographic computations in attempts
to verify the authenticity of data they retrieve in trying to answer
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
to the host at the IP address. Furthermore, it is generally easy in the request.
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 The computational or communications burden caused by such requests
amplification that can be obtained by attackers off path for the may not dependent on a forged IP source address, but the use of such
server and the attacked host. Enforced DNS cookies would make it hard addresses makes
for an off path attacker to cause any more than a brief error + the source of the requests causing the denial-of-service attack
response to be sent to a forged IP address. Furthermore, DNS cookies harder to find and
make it more effective to implement a rate limiting scheme for bad + restriction of the IP addresses from which such requests should
DNS cookie error responses from the server. Such a scheme would be honored hard or impossible to specify or verify.
further restrict selected host denial-of-service traffic from that
server. Use of DNS cookies should enables a server to reject forged queries
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 2.2 Cache Poisoning and Answer Forgery Attacks
The form of the cache poisoning attacks considered is to send forged The form of the cache poisoning attacks considered is to send forged
replies to a resolver. Modern network speeds for well connected hosts replies to a resolver. Modern network speeds for well-connected hosts
are such that, by forging replies from the IP addresses of heavily are such that, by forging replies from the IP addresses of heavily
used DNS servers for popular names to a heavily used resolver, there used DNS servers for popular names to a heavily used resolver, there
can be an unacceptably high probability of randomly coming up with a can be an unacceptably high probability of randomly coming up with a
reply that will be accepted and cause false DNS information to be reply that will be accepted and cause false DNS information to be
cached by that resolver. This can be used to facilitate phishing cached by that resolver (the Dan Kaminsky attack). This can be used
attacks and other diversion of legitimate traffic to a compromised or to facilitate phishing attacks and other diversion of legitimate
malicious host such as a web server. 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.
INTERNET-DRAFT DNS Cookies
3. Comments on Existing DNS Security 3. Comments on Existing DNS Security
Two forms of security have been added to DNS, data security and Two forms of security have been added to DNS, data security and
message or transaction security. message/transaction security.
3.1 Existing DNS Data Security 3.1 Existing DNS Data Security
DNS data security is one part of DNSSEC and is described in DNS data security is one part of DNSSEC and is described in
[RFC4033], [RFC4034], and [RFC4035]. It provides data origin [RFC4033], [RFC4034], and [RFC4035] and updates thereto. It provides
authentication and authenticated denial of existence. It is being data origin authentication and authenticated denial of existence.
deployed slowly and, in any case, can make some denial-of-service DNSSEC is being deployed and can provide strong protection against
attacks worse because of the high cryptographic computational load it forged data; however, it has the unintended effect of making some
can require and the increased size in DNS packets that it tends to denial-of-service attacks worse because of the cryptographic
produce. computational load it can require and the increased size in DNS
packets that it tends to produce.
INTERNET-DRAFT DNS Cookies
3.2 DNS Message or Transaction Security 3.2 DNS Message/Transaction Security
The second form of security which has been added to DNS provides The second form of security that has been added to DNS provides
"transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931]. "transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931].
TSIG could provide near perfect protection against the attacks for TSIG could provide strong protection against the attacks for which
which the DNS Cookies mechanism provide weak and incomplete the DNS Cookies mechanism provide weak protection; however, TSIG is
protection; however, TSIG is hard to deploy in the general Internet non-trivial to deploy in the general Internet because of the burden
because of the burden it imposes of pre-agreement and key it imposes of pre-agreement and key distribution between resolver-
distribution between pairs of resolvers and servers and because it server pairs, the burden of server side key state, and because it
requires time synchronization between resolver and server. requires time synchronization between resolver and server.
TKEY [RFC2930] can solve the problem of key distribution for TSIG but TKEY [RFC2930] can solve the problem of key distribution for TSIG but
some modes of TKEY impose a substantial cryptographic computation some modes of TKEY impose a substantial cryptographic computation
loads and can be dependent on the deployment of DNSSEC. loads and can be dependent on the deployment of DNS data security
(see Section 3.1).
SIG(0) provides less denial of service protection than TSIG or, in SIG(0) [RFC2931] provides less denial of service protection than TSIG
one way, even DNS cookies, because it does not authenticate requests, or, in one way, even DNS cookies, because it does not authenticate
only complete transactions. In any case, it also depends on the requests, only complete transactions. In any case, it also depends
deployment of DNSSEC and requires computationally burdensome public on the deployment of DNS data security and requires computationally
key cryptographic operations. burdensome public key cryptographic operations.
3.3 Conclusions on Existing DNS Security 3.3 Conclusions on Existing DNS Security
Thus, none of the previous forms of DNS security are a suitable The existing DNS security mechanisms do not provide the services
substitute for the DNS Cookies mechanism, which provide light weight provided by the DNS Cookies mechanism: lightweight message
message authentication of DNS requests and responses with no authentication of DNS requests and responses with no requirement for
requirement for pre-configuration. pre-configuration or server side state.
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
4. The COOKIE OPT option 4. The COOKIE OPT Option
COOKIE is an OPT RR [RFC2671] option that can be included no more COOKIE is an OPT RR [RFC6891] option that can be included no more
than once in the RDATA portion of an OPT RR in DNS requests and than once in the RDATA portion of an OPT RR in DNS requests and
responses. responses.
The option is encoded into 22 bytes as shown below. 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 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 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 | | OPTION-CODE = {TBD} | OPTION-LENGTH = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
+- Resolver Cookie -+ +- Resolver Cookie -+
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
+-- Server Cookie -+ +- Server Cookie -+
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | | Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 64-bit Resolver and Server Cookies are stored in network byte The 64-bit Resolver and Server Cookies are stored in little endian
order and are determined as described below. order and are determined as described below.
The Error Code field MUST be zero in requests and in responses unless 4.1 Resolver Cookie
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 The Resolver Cookie SHOULD be a pseudo-random function of the server
IP address and a secret quantity known only to the resolver. This IP address and a secret quantity known only to the resolver. This
resolver secret SHOULD have at least 64 bits of entropy [RFC4086bis]
and be changed periodically (see Section 6.3). The selection of the
pseudo-random function is a private matter to the resolver as only
the resolver needs to recognize its own DNS cookies. An example
method is the FNV-64 [FNV] of the server IP address and the resolver
secret. That is
INTERNET-DRAFT DNS Cookies Resolver Cookie = FNV-64 ( Resolver Secret | Server IP Address )
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 where "|" indicates concatenation.
the same Resolver Cookie value for queries to all servers.
4.2 Server Cookies A resolver MUST NOT use the same Resolver Cookie value for queries to
all servers.
The Server Cookie, when it occurs in a COOKIE OPT option in a query, INTERNET-DRAFT DNS Cookies
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 4.2 Server Cookie
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 The Server Cookie SHOULD be a pseudo-random function of the request
source IP address, the request Resolver Cookie, and a secret quantity source IP address, the request Resolver Cookie, and a secret quantity
known only to the server. This server secret SHOULD have 64 bits of known only to the server. (See Section 7 for a discussion of why the
entropy [RFC4086] and SHOULD be changed periodically such as daily. Resolver Cookie is used as input to the Server Cookie but the Server
The RECOMMENDED method is the HMAC-SHA1-64 [RFC1321], [RFC2104] of Cookie is not used as an input to the Resolver Cookie.) This server
the request IP address, the Resolver Cookie, and the server secret. secret SHOULD have 64 bits of entropy [RFC4086bis] and be changed
That is periodically (see Section 6.3). The selection of the pseudo-random
function is a private matter to the server as only the server needs
to recognize its own DNS cookies. An example method is the FNV-64
[FNV] of the request IP address, the Resolver Cookie, and the server
secret. That is
Server Cookie = Truncate-64 ( HMAC-SHA1 ( Server Cookie =
(Request IP Address | Resolver Cookie), Server Secret ) ) FNV-64 ( Server Secret | Request IP Address | Resolver Cookie )
where Truncate-64 takes the first 64 bits and "|" represents where "|" represents concatenation.
concatenation.
A server MUST NOT use the same Server Cookie value for responses to A server MUST NOT use the same Server Cookie value for responses to
all resolvers. all resolvers.
5. DNS Cookie Policies and Implementation Requirements 4.3 Error Code
DNS resolvers and servers will adopt one of various policies The Error Code field MUST have one of the values listed below.
regarding cookies. These policies SHOULD be logically settable on a
per server IP address basis for resolvers and a per resolver ( IP Requests have a COOKIE OPT Error Code equal to one of the following
two values:
Zero, if the resolver believes the Server Cookie field is
correct, or
CKPING (Cookie PING), if the resolver does not know the correct
value for the Server Cookie field.
(In all cases, the RCODE in a DNS request header is zero.)
Replies have a COOKIE OPT with an Error Code equal to one of the
following five values:
Zero, if the request they respond to had one COOKIE OPT with a
correct Server Cookie.
NOCOOKIE, in which case the DNS reply header RCODE field is
Refused.
BADCOOKIE, in which case the DNS reply header RCODE field is
Refused.
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
address, Resolver Cookie ) pair for servers. Thus a resolver can MANYCOOKIE, in which case the DNS reply header RCODE field is
have different policies for different servers, based on the server IP FormErr.
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 CKPINGR (Cookie PING Response), which case the DNS reply RCODE
field might be any value (see Section 5.2).
For more information on errors in replies see Section 6.
For further discussion of the Resolver Cookie field, see Section 5.1.
For further discussion of the Server Cookie field see Section 5.2.
INTERNET-DRAFT DNS Cookies
5. DNS Cookies Protocol Description
The sections provide a general discussion of using DNS Cookies in the
DNS Protocol. More details are provided in Section 6.
5.1 Originating Requests
A DNS resolver that implements DNS cookies includes a DNS Cookie
option in every DNS request it sends unless DNS cookies are disabled
in that resolver. The DNS Cookie in a request includes a Resolver
Cookie as discussed in Section 4.1, a Server Cookie cached as soft
state associated with that server IP address from a previous DNS
response, and a zero Error Code field.
If the resolver has no cached Server Cookie for the server, then it
sets the Server Cookie field to any value and sets the Error Code
field to CKPING (Cookie Ping); this is the only case in which the
Error Code field in a COOKIE OPT in a request is non-zero.
5.2 Responding to Requests
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 resolver at the source IP address used because the Server
Cookie value is the value that server would send to that resolver in
a response.
A DNS server that implements DNS cookies and for which DNS cookies
are not disabled always includes a DNS cookie in the response to a
DNS request that includes such a cookie. If the request did not
include a DNS cookie, inclusion of a DNS cookies in the response
depends on the server mode for that resolved (see Section 6.2). In
the DNS Cookie that the server includes, the Resolver Cookie field is
copied from that field in the request. If there was no cookie in the
request, it may be set to any value. The Server Cookie field is set
as discussed in Section 4.2 and the Error Code field is set as
specified in Section 6.
5.3 Processing Responses
The Resolver Cookie, when it occurs in a COOKIE OPT option in a DNS
reply, is intended to weakly assure the resolver that the reply came
from a server at the source IP address use in the response packet
because the Resolver Cookie value is the value that resolver would
INTERNET-DRAFT DNS Cookies
send to that server in a request.
A DNS resolver that implements DNS cookies and for which DNS cookies
are not disabled examines response for DNS cookies and will discard
the response if it contains an incorrect Resolver Cookie or has
multiple cookies. If the COOKIE OPT option Resolver Cookie is correct
and the Error Code field is not NOCOOKIE, MANYCOOKIES, it caches the
Server Cookie provided even if the response is an error response. The
rest of the response is then processed normally.
INTERNET-DRAFT DNS Cookies
6. DNS Cookie Policies and Implementation
Obviously, DNS resolvers that do not implement DNS cookies do not
include them in requests and ignore them in replies and DNS servers
that do not implement DNS cookies ignore them in requests and do not
include the in replies.
DNS resolvers and servers that implement DNS cookies will adopt one
of various policies regarding cookies. These policies SHOULD be
logically settable on a per server IP address basis in resolvers and
on a per resolver ( IP address, Resolver Cookie ) pair in 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 the
configuration of these policies may be for blocks or classes of
values or use sparse array techniques or the like.
The policy in each case is either "Disabled", "Enabled", or
"Enforced" as described below. "Enforced" as described below.
5.1 Resolver Policies and Implementation 6.1 Resolver Policies and Implementation
A resolver will logically have one of the following three modes of A resolver will logically have one of the following three modes of
operation or "policies" for each DNS server as distinguished by operation or "policies" for each DNS server as distinguished by
server IP Address. server IP Address.
Disabled: Disabled:
Never include a COOKIE OPT option in requests. Never include a COOKIE OPT option in requests.
Ignore COOKIE OPT options in responses. Ignore COOKIE OPT options in replies.
Enabled: Enabled:
Always include a COOKIE OPT option in requests. If a cached Server Always include a COOKIE OPT option in requests. If a cached Server
Cookie for the server is not available, the Server Cookie field Cookie for the server is not available, the Server Cookie field
can be set to any value. can be set to any value and the COOKIE OPT Error Code field is
Normally process responses without a COOKIE OPT option. set to CKPING (Cookie Ping); otherwise, the Error Code field is
Silently ignore responses with more than one COOKIE OPT option. set to zero.
Silently ignore responses with one COOKIE OPT option if it has an Normally process replies without a COOKIE OPT option.
Silently ignore replies with more than one COOKIE OPT option.
Silently ignore replies with one COOKIE OPT option if it has an
incorrect Resolver Cookie value. incorrect Resolver Cookie value.
On receipt of a response with one COOKIE OPT option carrying the On receipt of a reply with one COOKIE OPT option carrying the
correct Resolver Cookie value (even if it is a BADCOOKIE error correct Resolver Cookie value (even if it is a DNS error
response), the DNS client performs normal response processing, response), the DNS client performs normal response processing,
including caching the received Server Cookie, and it MUST including caching the received Server Cookie as soft state, and
change to the Enforced policy for DNS requests to that DNS it MUST change to the Enforced policy for DNS requests to that
server IP address. This policy change SHOULD be treated as soft DNS server IP address. This policy change to Enforced is
state with the same timeout strategy as the Server Cookie value treated as soft state with the same retention strategy as the
for that server. On timeout of that state information, the
policy for that DNS server reverts to Enabled. INTERNET-DRAFT DNS Cookies
Server Cookie value for that server. On discard of that state
information, the policy for that DNS server IP address reverts
to Enabled.
Enforced: Enforced:
Always include a COOKIE OPT option in requests. Always include a COOKIE OPT option in requests.
Silently ignore all responses that do not include exactly one Silently ignore all replies that do not include exactly one COOKIE
COOKIE OPT option having the correct Resolver Cookie value. OPT option having the correct Resolver Cookie value.
On receipt of a reply with one COOKIE OPT option carrying the
INTERNET-DRAFT DNS Cookies correct Resolver Cookie value (even if it is a DNS error
response), the DNS client performs normal response processing,
including caching the received Server Cookie. If a copy of the
same Server Cookie value is already cached for that server,
then its retention probability should be increased. For
example, if a time out is being used for the discard to cached
Server Cookies, that time out should be extended.
5.2 Server Policies and Implementation 6.2 Server Policies and Implementation
A server will logically have one of the following three modes of A server will logically have one of the following three modes of
operation or "policies" for each DNS resolver as distinguished by operation or "policies" for each DNS resolver as discussed below.
resolver IP Address and Resolver Cookie.
Disabled: Disabled:
Ignore COOKIE OPT options in requests. Ignore COOKIE OPT options in requests.
Never include a COOKIE OPT option in responses. Never include a COOKIE OPT option in replies.
Enabled: Enabled:
Always include a COOKIE OPT option in responses. Include a COOKIE OPT option in replies to requests that include a
Normally process requests without a COOKIE OPT option. COOKIE OPT.
Ignore, other than sending a MANYCOOKIE error response, any Normally process requests without a COOKIE OPT option except that
request with more than one COOKIE OPT option. it is RECOMMENDED that the processing of burdensome requests
Ignore, other than sending a BADCOOKIE error response, any query and requests producing replies substantially longer than the
with one COOKIE OPT option if it has an incorrect Server request be significantly rate limited.
Cookie. Ignore, other than sending a FormErr/MANYCOOKIE error reply, any
On receipt of a request with a COOKIE OPT option carrying the request with more than one COOKIE OPT option. Such replies MAY
correct Server Cookie value, the DNS server performs normal be rate limited and SHOULD be as short as practical.
request processing and it SHOULD switch to the Enforced policy Ignore, other than sending a BADCOOKIE error reply, any request
for DNS requests from that resolver IP address with that with one COOKIE OPT option if it has an incorrect Server Cookie
Resolver Cookie in the request. This policy change for that unless the request COOKIE has an Error Code of CKPING (Cookie
resolver SHOULD be treated as soft state. On timing out that Ping) in which case the response has an Error Code of CKPINGR
(Cookie Ping Response). Such replies MAY be rate limited and
SHOULD be as short as practical.
On receipt of a request with one COOKIE OPT option carrying the
correct Server Cookie value and an Error Code of zero, 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 to Enforced is treated as soft state. On discard of that
INTERNET-DRAFT DNS Cookies
state information, the policy for that resolver IP and Resolver state information, the policy for that resolver IP and Resolver
Cookie pair reverts to Enabled. Cookie pair reverts to Enabled.
Enforced: Enforced:
Always include a COOKIE OPT option in responses. Always include a COOKIE OPT option in replies.
Ignore requests without a COOKIE OPT option or with more than one Ignore requests without a COOKIE OPT option or with more than one
COOKIE OPT option, other than returning a NOCOOKIE or COOKIE OPT option, other than returning a NOCOOKIE or
MANYCOOKIE error respectively. MANYCOOKIE DNS error respectively. Such replies MAY be rate
limited to any particular IP address and SHOULD be as short as
practical.
Ignore requests with one COOKIE OPT option if they have an Ignore requests with one COOKIE OPT option if they have an
incorrect Server Cookie, other than returning a BADCOOKIE error incorrect Server Cookie, other than returning a BADCOOKIE error
message. message, unless the request has an Error Code of CKPING in
which case the response has an Error Code of CKPINGR. Such
replies MAY be rate limited and SHOULD be as short as
practical.
If a request has one COOKIE OPT option with a correct Server If a request has one COOKIE OPT option with a correct Server
Cookie, perform normal processing of the request. Cookie and an Error Code of zero, perform normal processing of
the request.
5.3 Implementation Requirements 6.3 Resolver and Server Secret Rollover
Resolvers and servers MUST NOT continue to use the same secret in new
queries and responses, respectively, for more than 14 days and SHOULD
NOT continue to do so for more than 1 day. It is RECOMMENDED that a
resolver keep the Resolver Cookie it is expecting in a reply
associated with the outstanding query to avoid rejection of replies
due to a bad Resolver Cookie right after a change in the Resolver
Secret. It is RECOMMENDED that a server retain its previous secret
for a period of time not less than 1 second or more than 3 minutes,
after a change in its secret, and consider queries with Server
Cookies based on its previous secret to have a correct Server Cookie
during that time.
Receiving a suddenly increased level of requests with bad Server
Cookies or replies with bad Resolver Cookies would be a good reason
to believe a server or resolver likely to be under attack and should
consider more frequent rollover of its secret.
6.4 Implementation Requirement
DNS resolvers and servers SHOULD implement DNS cookies. DNS resolvers and servers SHOULD implement DNS cookies.
DNS resolvers SHOULD operate in and be shipped so as to default to DNS resolvers SHOULD operate in and be shipped so as to default to
the Enabled or Enforced mode for all servers. the Enabled or Enforced mode for all servers.
INTERNET-DRAFT DNS Cookies
DNS servers SHOULD operate in and be shipped so as to default to the 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 Enabled or Enforced mode for all resolvers they are willing to
service. service.
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
6. NAT Considerations and AnyCast Server Considerations 7. NAT Considerations and AnyCast Server Considerations
In the Classic Internet, DNS Cookies could simply be a pseudo-random 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 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 IP address and a resolver secret. You would want to compute the
Server Cookie that way, so a resolver could cache its Server Cookie Server Cookie that way, so a resolver could cache its Server Cookie
for a particular server for an indefinitely amount of time and the for a particular server for an indefinitely amount of time and the
server could easily regenerate and check it. You could consider the server could easily regenerate and check it. You could consider the
Resolver Cookie to be a resolver signature over the server IP address Resolver Cookie to be a weak resolver signature over the server IP
which the resolver checks in responses and you could extend this address that the resolver checks in replies and you could extend this
signature to cover the request ID, for example. weak 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 But we have this reality called NAT [RFC3022], Network Address
Translation (including, for the purposes of this document, NAT-PT Translation (including, for the purposes of this document, NAT-PT,
[RFC2766], Network Address and Protocol Translation, which has been Network Address and Protocol Translation, which has been declared
declared Historic [RFC4966]). There is no problem with DNS Historic [RFC4966]). There is no problem with DNS transactions
transactions between resolvers and servers behind a NAT box using between resolvers and servers behind a NAT box using local IP
local IP addresses. Nor is there a problem with NAT translation of addresses. Nor is there a problem with NAT translation of internal
internal addresses to external addresses or translations between IPv4 addresses to external addresses or translations between IPv4 and IPv6
and IPv6 addresses, as long as the address mapping is relatively addresses, as long as the address mapping is relatively stable.
stable. Should an internal resolver being mapped to a particular Should the external IP address an internal resolver being mapped to
external IP address change occasionally, the disruption is no more change occasionally, the disruption is little more than when a
than when a resolver rolls-over its DNS COOKIE secret. And normally resolver rolls-over its DNS COOKIE secret. And normally external
external access to a DNS server behind a NAT box is handled by a access to a DNS server behind a NAT box is handled by a fixed mapping
fixed mapping which forwards externally received DNS requests to a which forwards externally received DNS requests to a specific host.
specific host.
However, NAT devices sometimes also map ports. This can cause However, NAT devices sometimes also map ports. This can cause
multiple DNS requests and responses from multiple internal hosts to multiple DNS requests and responses from multiple internal hosts to
be simultaneously mapped to a smaller number of external IP be mapped to a smaller number of external IP addresses, such as one
addresses, frequently one. There could be many resolvers behind a address. Thus there could be many resolvers behind a NAT box that
NAT box that appear to come from the same source IP address to a appear to come from the same source IP address to a server outside
server outside that NAT box. If one of these were an attacker (think that NAT box. If one of these were an attacker (think Zombie or
Zombie or Botnet), that behind-NAT attacker could get the Server Botnet), that behind-NAT attacker could get the Server Cookie for
Cookie for some server for the outgoing IP address by just making some server for the outgoing IP address by just making some random
some random request to that server. It could then include that Server request to that server. It could then include that Server Cookie in
Cookie in the COOKIE RR of requests to the server with the forged the COOKIE OPT of requests to the server with the forged local IP
local IP address of some other host and/or resolver behind the NAT address of some other host and/or resolver behind the NAT box.
box. (Attacker possession of this Server Cookie will not help in (Attacker possession of this Server Cookie will not help in forging
forging responses to cause cache poisoning as such responses are responses to cause cache poisoning as such responses are protected by
protected by the required Resolver Cookie.) the required Resolver Cookie.)
To fix this potential defect, it is necessary to distinguish To fix this potential defect, it is necessary to distinguish
different resolvers behind a NAT box from the point of view of the 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 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 a pseudo-random function of both the request source IP address and
the Resolver Cookie. From this inclusion of the Resolver Cookie in the Resolver Cookie. From this inclusion of the Resolver Cookie in
the calculation of the Server Cookie, it follows that a stable the calculation of the Server Cookie, it follows that a stable
Resolver Cookie, for any particular server, is needed. If, for Resolver Cookie, for any particular server, is needed. If, for
example, the request ID was included in the calculation of the example, the request ID was included in the calculation of the
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
Resolver Cookie, it would normally change with each query to a Resolver Cookie, it would normally change with each request to a
particular server. This would mean that each query would have to be particular server. This would mean that each request would have to
sent twice: first to learn the new Server Cookie based on this new 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 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 to actually get an answer. Thus the input to the
Resolver Cookie computation must be limited to the server IP address Resolver Cookie computation must be limited to the server IP address
and one or more things that change slowly such as the resolver and one or more things that change slowly such as the resolver
secret. secret.
In principle, there could be a similar problem for servers, not In principle, there could be a similar problem for servers, not due
particularly due to NAT but due to mechanisms like anycast which may to NAT but due to mechanisms like anycast which may cause queries to
cause queries to a DNS server at an IP address to be delivered to any a DNS server at an IP address to be delivered to any one of several
one of several machines. (External queries to a DNS server behind a machines. (External queries to a DNS server behind a NAT box usually
NAT box usually occur via port forwarding such that all such queries occur via port forwarding such that all such queries go to one host.)
go to one host.) However, it is impossible to solve this the way the However, it is impossible to solve this the way the similar problem
similar problem was solved for NATed resolvers; if the Server Cookie was solved for NATed resolvers; if the Server Cookie was included in
was included in the calculation of the Resolver Cookie the same way the calculation of the Resolver Cookie the same way the Resolver
the Resolver Cookie is included in the Server Cookie, you would just Cookie is included in the Server Cookie, you would just get an almost
get an almost infinite series of BADCOOKIE errors as a query was infinite series of errors as a request was repeatedly retried.
repeatedly retried.
For servers accessed via anycast to successfully support DNS COOKIES, For servers accessed via anycast to successfully support DNS COOKIES,
the server clones must either all use the same server secret or the the server clones must either all use the same server secret or the
mechanism that distributes queries to them must cause the queries mechanism that distributes queries to them must cause the queries
from a particular resolver to go to a particular server for a from a particular resolver to go to a particular server for a
sufficiently long period of time that extra queries due to changes in sufficiently long period of time that extra queries due to changes in
Server Cookie resulting from accessing different server machines are Server Cookie resulting from accessing different server machines are
not unduly burdensome. When such anycast accessed servers act as not unduly burdensome. (When such anycast accessed servers act as
recursive servers or otherwise act as resolvers they normally use a recursive servers or otherwise act as resolvers they normally use a
different unique address to source their queries and avoid confusion different unique address to source their queries to avoid confusion
in the delivery of responses. in the delivery of responses.)
For simplicity, it is RECOMMENDED that the same server secret be used For simplicity, it is RECOMMENDED that the same server secret be used
by each DNS server in a set of anycast servers. 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 6.3).
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
7. Incremental Deployment 8. Deployment
The DNS cookies mechanism is designed for incremental deployment and The DNS cookies mechanism is designed for incremental deployment and
to complement the orthogonal techniques in [forgery]. Either or both to complement the orthogonal techniques in [RFC5452]. Either or both
techniques can be deployed independently at each DNS server and techniques can be deployed independently at each DNS server and
resolver. resolver.
In particular, a DNS server or resolver that implements the DNS In particular, a DNS server or resolver that implements the DNS
cookies mechanism and is in the Enabled mode will interoperate COOKIE mechanism and is in the Enabled mode will interoperate
successfully with a DNS resolver or server that does not implement successfully with a DNS resolver or server that does not implement
this mechanism although, of course, in this case it will not get the this mechanism although, of course, in this case it will not get the
benefit of the mechanism. When such a server or resolver benefit of the mechanism. When such a server or resolver
interoperates with a resolver or server which also implements the DNS interoperates with a resolver or server which also implements the DNS
cookies mechanism, this is recognized and, for that transaction cookies mechanism and is in Enabled or Enforced mode, this is
partner, it enters the Enforced mode and gets the full benefit of the recognized and, for that transaction partner, it latches up into the
DNS cookies mechanism until this soft state times out and it reverts Enforced mode and gets the full benefit of the DNS cookies mechanism
to Enabled. until this soft state lapses and it reverts to Enabled mode.
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
8. IANA Considerations 9. IANA Considerations
IANA will allocate the following code points: IANA will assign the following code points:
The OPT option value for COOKIE is <TBD>. The OPT option value for COOKIE is <TBD>.
Three new RCODES are assigned values as listed below: Three new DNS error codes are assigned values in the range above
NOCOOKIE is assigned the value ({TBD}, 23 suggested). 15 and below 3841 as listed below:
BADCOOKIE is assigned the value ({TBD}, 24 suggested).
MANYCOOKIE is assigned the value ({TBD}, 25 suggested).
9. Security Considerations NOCOOKIE is assigned the value TBD (23 suggested).
BADCOOKIE is assigned the value TBD (24 suggested).
MANYCOOKIE is assigned the value TBD (25 suggested).
Two new DNS error codes are assigned in the range above 4095 and
below 65535:
CKPING (Cookie PING) is assigned the value TBD (4096
suggested).
CKPINGR (Cookie PING Response) is assigned the value RBD (4097
suggested).
INTERNET-DRAFT DNS Cookies
10. Security Considerations
DNS Cookies provide a weak form of authentication of DNS requests and DNS Cookies provide a weak form of authentication of DNS requests and
responses. In particular, they provide no protection at all against responses. In particular, they provide no protection at all against
"on-path" adversaries; that is, they provide no protection against "on-path" adversaries; that is, they provide no protection against
any adversary which can observe the plain text DNS traffic, such as any adversary that can observe the plain text DNS traffic, such as an
an on-path router, bridge, or any device on an on-path shared link on-path router, bridge, or any device on an on-path shared link
(unless the DNS traffic in question on that path is appropriately (unless the DNS traffic in question on that path is encrypted).
encrypted).
For example, if a host is connected via an unsecured IEEE 802.11 link 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 (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, 802.11 transmissions must be considered "on-path". On the other hand,
in a similar situation but one where 802.11 Robust Security (WPAv2) in a similar situation but one where 802.11 Robust Security (WPAv2)
is appropriately deployed on the Wi-Fi network nodes, only the Access is appropriately deployed on the Wi-Fi network nodes, only the Access
Point via which the host is connecting is "on-path". Point via which the host is connecting is "on-path".
Despite these limitations, use of DNS Cookies on the global Internet Despite these limitations, use of DNS Cookies on the global Internet
is expected to provide a reduction in the available launch points for is expected to provide a substantial reduction in the available
the traffic amplification and denial of service forgery attacks launch points for the traffic amplification and denial of service
described in Section 2 above. forgery attacks described in Section 2 above.
The recommended cryptographic algorithms for use in DNS Cookies is Should stronger message/transaction security be desired, it is
HMAC-SHA1-64, that is, the HMAC scheme [RFC2104] using the SHA1 hash suggested that TSIG or SIG(0) security be used (see Section 3.2);
function [RFC3174] [RFC4634] with the HMAC output truncated to however, it may be useful to use DNS Cookies in conjunction with
64-bits. MD5 is now considered to be susceptible to collisions these features. In particular, DNS Cookies could screen out many DNS
attacks. Although this does not effect the security of HMAC-MD5, messages before the cryptographic computations of TSIG or SIG(0) are
HMAC-SHA1 is believed to be stronger. 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.
In light of the weak plain-text token security provided by DNS 10.1 Cookie Algorithm Considerations
Cookies, stronger cryptography is probably not warranted and in many
cases it would be acceptable to use the weaker MD5 hash function The cookie computation algorithm for use in DNS Cookies SHOULD be
[RFC1321]. However, there is nothing wrong with using something FNV-64 [FNV] or some stronger algorithm because an excessively weak
stronger, for example, HMAC-SHA256-64 [RFC4634], assuming a DNS or trivial algorithm could enable adversaries to guess cookies.
However, in light of the weak plain-text token security provided by
DNS Cookies, a strong cryptography hash algorithm is probably not
warranted in many cases, and would cause an increased computational
burden. Nevertheless there is nothing wrong with using something
stronger, for example, HMAC-SHA256-64 [RFC6234], assuming a DNS
processor has adequate computational resources available. DNS processor has adequate computational resources available. DNS
processors that feel the need for somewhat stronger security without processors that feel the need for somewhat stronger security without
a significant increase in computational load should consider more a significant increase in computational load should consider more
frequent changes in their resolver and/or server secret; however,
this does require more frequent generation of a cryptographically
strong random number [RFC4086bis].
INTERNET-DRAFT DNS Cookies INTERNET-DRAFT DNS Cookies
frequent changes in their resolver and/or server secret; however, Acknowledgements
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 The contributions of the following are gratefully acknowledged:
[RFC1321] - Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, Tim Wicinski
April 1992.
[RFC2104] - Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- INTERNET-DRAFT DNS Cookies
Hashing for Message Authentication", RFC 2104, February 1997.
[RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate Normative References
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2671] - Vixie, P., "Extension Mechanisms for DNS (EDNS0)", August [FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non-
1999. Cryptographic Hash Algorithm", draft-eastlake-fnv, work in
progress.
[RFC4086] - Eastlake, D., 3rd, Schiller, J., and S. Crocker, [RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
"Randomness Requirements for Security", BCP 106, RFC 4086, June 2005. Requirement Levels", BCP 14, RFC 2119, March 1997.
11. Informative References [RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
[forgery] - "Measures for making DNS more resilient against forged [RFC4086bis] - Eastlake, D., 3rd, Schiller, J., and S. Crocker,
answers", Hubert, A. , R. van Mook, draft-ietf-dnsext-forgery- "Randomness Requirements for Security", draft-eastlake-
resilience-01.txt, work in progress, July 2007. randomness3, work in progress.
[RFC2766] - Tsirtsis, G., P. Srisuresh, "Network Address Translation Informative References
- Protocol Translation (NAT-PT)", February 2000.
[RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. [RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS (TSIG)", Wellington, "Secret Key Transaction Authentication for DNS
RFC 2845, May 2000. (TSIG)", RFC 2845, May 2000.
[RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY [RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, September 2000. RR)", RFC 2930, September 2000.
[RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures [RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, September 2000. ( SIG(0)s )", RFC 2931, September 2000.
[RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network [RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January 2001. Address Translator (Traditional NAT)", RFC 3022, January 2001.
[RFC3174] - Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm
INTERNET-DRAFT DNS Cookies
1 (SHA1)", RFC 3174, September 2001.
[RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC 4033, March Rose, "DNS Security Introduction and Requirements", RFC 4033,
2005. March 2005.
[RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions", RFC 4034, Rose, "Resource Records for the DNS Security Extensions", RFC
March 2005. 4034, March 2005.
[RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Extensions", RFC Rose, "Protocol Modifications for the DNS Security Extensions",
4035, March 2005. 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 [RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to Historic Address Translator - Protocol Translator (NAT-PT) to Historic
Status", RFC 4966, July 2007. Status", RFC 4966, July 2007.
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
Copyright and Disclaimer
Copyright (C) The IETF Trust (2008). [RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More
This document is subject to the rights, licenses and restrictions INTERNET-DRAFT DNS Cookies
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an Resilient against Forged Answers", RFC 5452, January 2009.
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Additional IPR Provisions [RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash
Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, May
2011.
The IETF takes no position regarding the validity or scope of any INTERNET-DRAFT DNS Cookies
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any Author's Address
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
INTERNET-DRAFT DNS Cookies Donald E. Eastlake 3rd
Huawei Technologies
155 Beaver Street
Milford, MA 01757 USA
The IETF invites any interested party to bring to its attention any Telephone: +1-508-333-2270
copyrights, patents or patent applications, or other proprietary EMail: d3e3e3@gmail.com
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Expiration and File Name Copyright, Disclaimer, and Additional IPR Provisions
This draft expires in August 2008. Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
Its file name is draft-eastlake-dnsext-cookies-03.txt This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
 End of changes. 131 change blocks. 
430 lines changed or deleted 540 lines changed or added

This html diff was produced by rfcdiff 1.48. The latest version is available from http://tools.ietf.org/tools/rfcdiff/