INTERNET-DRAFT Donald Eastlake
Intended Status: Proposed Standard Huawei
Expires: July 21, 2014 January 22, 2014
Domain Name System (DNS) Cookies
<draft-eastlake-dnsext-cookies-04.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
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 <dnsext@ietf.org>.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft
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http://www.ietf.org/shadow.html.
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Table of Contents
1. Introduction............................................3
1.1 Contents of This Document..............................3
1.2 Definitions............................................4
2. Threats Considered......................................5
2.1 Denial-of-Service Attacks..............................5
2.1.1 DNS Amplification Attacks............................5
2.1.2 DNS Server Denial-of-Service.........................5
2.2 Cache Poisoning and Answer Forgery Attacks.............6
3. Comments on Existing DNS Security.......................7
3.1 Existing DNS Data Security.............................7
3.2 DNS Message/Transaction Security.......................7
3.3 Conclusions on Existing DNS Security...................7
4. The COOKIE OPT Option...................................8
4.1 Resolver Cookie........................................8
4.2 Server Cookie..........................................9
4.3 Error Code.............................................9
5. DNS Cookies Protocol Description.......................11
5.1 Originating Requests..................................11
5.2 Responding to Requests................................11
5.3 Processing Responses..................................11
6. DNS Cookie Policies and Implementation.................13
6.1 Resolver Policies and Implementation..................13
6.2 Server Policies and Implementation....................14
6.3 Resolver and Server Secret Rollover...................15
6.4 Implementation Requirement............................15
7. NAT Considerations and AnyCast Server Considerations...17
8. Deployment.............................................19
9. IANA Considerations....................................20
10. Security Considerations...............................21
10.1 Cookie Algorithm Considerations......................21
Acknowledgements..........................................22
Normative References......................................23
Informative References....................................23
Author's Address..........................................25
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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
abuse.
This document describes DNS cookies, a lightweight DNS transaction
security mechanism specified as an OPT [RFC6891] option. This
mechanism provides limited protection to DNS servers and resolvers
against a variety of increasingly common abuses by off-path
attackers.
The DNS cookies mechanism has a default mode that supports
incremental deployment. If only one party to a DNS transaction
supports the mechanism, it does not provide a benefit or
significantly interfere, but, if both support it, the additional
security provided is automatically available.
The DNS cookies mechanism is compatible with and can be used in
conjunction with other DNS transaction forgery resistance measures
such as those in [RFC5452].
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 and Section 5 provides a
protocol description including suggestions for calculating Resolver
and Server Cookies.
Section 6 gives further details on the processing of COOKIE OPT
options by resolvers and server and policies for such processing.
Section 7 discusses some NAT and anycast related DNS Cookies design
considerations.
Section 8 discusses incremental deployment considerations.
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Sections 9 and 10 describe IANA and Security Considerations.
1.2 Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
An "off-path attacker", for a particular DNS resolver and server, is
defined as an attacker who cannot observe the plain text of DNS
requests and responses between that resolver and server.
"Soft state" indicates information learned or derived by a host which
may be discarded when indicated by the policies of that host but can
be later re-instantiated if needed. For example, it could be
discarded after a period of time or when storage for caching such
data becomes full. If operations requiring that soft state continue
after it has been discarded, it will be automatically re-generated,
albeit at some cost.
"Silently discarded" indicates that there are no DNS protocol message
consequences; however, it is RECOMMENDED that appropriate network
management facilities be included in implementations, such as a
counter of the occurrences of each type of such events.
The term "IP address" is used herein in a length independent manner
and refers to both IPv4 and IPv6.
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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.
2.1.1 DNS Amplification Attacks
A request with a forged IP address generally causes a response to be
sent to that forged IP address. Thus the forging of many such
requests with a particular source IP address can result in enough
traffic being sent to the forged IP address to interfere with service
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
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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
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 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
The form of the cache poisoning attacks considered is to send forged
replies to a resolver. Modern network speeds for well-connected hosts
are such that, by forging replies from the IP addresses of heavily
used DNS servers for popular names to a heavily used resolver, there
can be an unacceptably high probability of randomly coming up with a
reply that will be accepted and cause false DNS information to be
cached by that resolver (the Dan Kaminsky attack). 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.
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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], and [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; 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
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 provide weak protection; however, TSIG is
non-trivial to deploy in the general Internet because of the burden
it imposes of pre-agreement and key distribution between resolver-
server pairs, the burden of server side key state, and because it
requires time synchronization between resolver and server.
TKEY [RFC2930] can solve the problem of key distribution for TSIG but
some modes of TKEY impose a substantial cryptographic computation
loads and can be dependent on the deployment of 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
pre-configuration or server side state.
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4. The COOKIE OPT Option
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
responses.
The option is encoded into 22 bytes as shown below.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION-CODE = {TBD} | OPTION-LENGTH = 18 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Resolver Cookie -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+- Server Cookie -+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 64-bit Resolver and Server Cookies are stored in little endian
order and are determined as described below.
4.1 Resolver Cookie
The Resolver Cookie SHOULD be a pseudo-random function of the server
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
Resolver Cookie = FNV-64 ( Resolver Secret | Server IP Address )
where "|" indicates concatenation.
A resolver MUST NOT use the same Resolver Cookie value for queries to
all servers.
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4.2 Server Cookie
The Server Cookie SHOULD be a pseudo-random function of the request
source IP address, the request Resolver Cookie, and a secret quantity
known only to the server. (See Section 7 for a discussion of why the
Resolver Cookie is used as input to the Server Cookie but the Server
Cookie is not used as an input to the Resolver Cookie.) This server
secret SHOULD have 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 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 =
FNV-64 ( Server Secret | Request IP Address | Resolver Cookie )
where "|" represents concatenation.
A server MUST NOT use the same Server Cookie value for responses to
all resolvers.
4.3 Error Code
The Error Code field MUST have one of the values listed below.
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.
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MANYCOOKIE, in which case the DNS reply header RCODE field is
FormErr.
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.
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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
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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.
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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.
6.1 Resolver Policies and Implementation
A resolver will logically have one of the following three modes of
operation or "policies" for each DNS server as distinguished by
server IP Address.
Disabled:
Never include a COOKIE OPT option in requests.
Ignore COOKIE OPT options in replies.
Enabled:
Always include a COOKIE OPT option in requests. If a cached Server
Cookie for the server is not available, the Server Cookie field
can be set to any value and the COOKIE OPT Error Code field is
set to CKPING (Cookie Ping); otherwise, the Error Code field is
set to zero.
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.
On receipt of a reply with one COOKIE OPT option carrying the
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 as soft state, and
it MUST change to the Enforced policy for DNS requests to that
DNS server IP address. This policy change to Enforced is
treated as soft state with the same retention strategy as the
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Server Cookie value for that server. On discard of that state
information, the policy for that DNS server IP address reverts
to Enabled.
Enforced:
Always include a COOKIE OPT option in requests.
Silently ignore all replies that do not include exactly one COOKIE
OPT option having the correct Resolver Cookie value.
On receipt of a reply with one COOKIE OPT option carrying the
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.
6.2 Server Policies and Implementation
A server will logically have one of the following three modes of
operation or "policies" for each DNS resolver as discussed below.
Disabled:
Ignore COOKIE OPT options in requests.
Never include a COOKIE OPT option in replies.
Enabled:
Include a COOKIE OPT option in replies to requests that include a
COOKIE OPT.
Normally process requests without a COOKIE OPT option except that
it is RECOMMENDED that the processing of burdensome requests
and requests producing replies substantially longer than the
request be significantly rate limited.
Ignore, other than sending a FormErr/MANYCOOKIE error reply, any
request with more than one COOKIE OPT option. Such replies MAY
be rate limited and SHOULD be as short as practical.
Ignore, other than sending a BADCOOKIE error reply, any request
with one COOKIE OPT option if it has an incorrect Server Cookie
unless the request COOKIE has an Error Code of CKPING (Cookie
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
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state information, the policy for that resolver IP and Resolver
Cookie pair reverts to Enabled.
Enforced:
Always include a COOKIE OPT option in replies.
Ignore requests without a COOKIE OPT option or with more than one
COOKIE OPT option, other than returning a NOCOOKIE or
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
incorrect Server Cookie, other than returning a BADCOOKIE error
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
Cookie and an Error Code of zero, perform normal processing of
the request.
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 SHOULD operate in and be shipped so as to default to
the Enabled or Enforced mode for all servers.
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DNS servers SHOULD operate in and be shipped so as to default to the
Enabled or Enforced mode for all resolvers they are willing to
service.
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7. NAT Considerations and AnyCast Server Considerations
In the Classic Internet, DNS Cookies could simply be a pseudo-random
function of the resolver IP address and a sever secret or the server
IP address and a resolver secret. You would want to compute the
Server Cookie that way, so a resolver could cache its Server Cookie
for a particular server for an indefinitely amount of time and the
server could easily regenerate and check it. You could consider the
Resolver Cookie to be a weak resolver signature over the server IP
address that the resolver checks in replies and you could extend this
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
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 resolvers and servers behind a NAT box using local IP
addresses. Nor is there a problem with NAT translation of internal
addresses to external addresses or translations between IPv4 and IPv6
addresses, as long as the address mapping is relatively stable.
Should the external IP address an internal resolver being mapped to
change occasionally, the disruption is little more than when a
resolver rolls-over its DNS COOKIE secret. And normally external
access to a DNS server behind a NAT box is handled by a fixed mapping
which forwards externally received DNS requests to a specific host.
However, NAT devices sometimes also map ports. This can cause
multiple DNS requests and responses from multiple internal hosts to
be mapped to a smaller number of external IP addresses, such as one
address. Thus there could be many resolvers behind a NAT box that
appear to come from the same source IP address to a server outside
that NAT box. If one of these were an attacker (think Zombie or
Botnet), that behind-NAT attacker could get the Server Cookie for
some server for the outgoing IP address by just making some random
request to that server. It could then include that Server Cookie in
the COOKIE OPT of requests to the server with the forged local IP
address of some other host and/or resolver behind the NAT box.
(Attacker possession of this Server Cookie will not help in forging
responses to cause cache poisoning as such responses are protected by
the required Resolver Cookie.)
To fix this potential defect, it is necessary to distinguish
different resolvers behind a NAT box from the point of view of the
server. It is for this reason that the Server Cookie is specified as
a pseudo-random function of both the request source IP address and
the Resolver Cookie. From this inclusion of the Resolver Cookie in
the calculation of the Server Cookie, it follows that a stable
Resolver Cookie, for any particular server, is needed. If, for
example, the request ID was included in the calculation of the
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Resolver Cookie, it 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
Resolver Cookie based on the new ID and then again using this new
Resolver Cookie to actually get an answer. Thus the input to the
Resolver Cookie computation must be limited to the server IP address
and one or more things that change slowly such as the resolver
secret.
In principle, there could be a similar problem for servers, not due
to NAT but due to mechanisms like anycast which may cause queries to
a DNS server at an IP address to be delivered to any one of several
machines. (External queries to a DNS server behind a NAT box usually
occur via port forwarding such that all such queries go to one host.)
However, it is impossible to solve this the way the similar problem
was solved for NATed resolvers; if the Server Cookie was included in
the calculation of the Resolver Cookie the same way the Resolver
Cookie is included in the Server Cookie, you would just get an almost
infinite series of 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 queries to them must cause the queries
from a particular resolver to go to a particular server for a
sufficiently long period of time that extra queries due to changes in
Server Cookie resulting from accessing different server machines are
not unduly burdensome. (When such anycast accessed servers act as
recursive servers or otherwise act as resolvers they normally use a
different unique address to source their queries 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 6.3).
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8. Deployment
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
resolver.
In particular, a DNS server or resolver that implements the DNS
COOKIE mechanism and is in the Enabled mode will interoperate
successfully with a DNS resolver or server that does not implement
this mechanism although, of course, in this case it will not get the
benefit of the mechanism. When such a server or resolver
interoperates with a resolver or server which also implements the DNS
cookies mechanism and is in Enabled or Enforced mode, this is
recognized and, for that transaction partner, it latches up into the
Enforced mode and gets the full benefit of the DNS cookies mechanism
until this soft state lapses and it reverts to Enabled mode.
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9. IANA Considerations
IANA will assign the following code points:
The OPT option value for COOKIE is <TBD>.
Three new DNS error codes are assigned values in the range above
15 and below 3841 as listed below:
NOCOOKIE is assigned the value TBD (23 suggested).
BADCOOKIE is assigned the value TBD (24 suggested).
MANYCOOKIE is assigned the value TBD (25 suggested).
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).
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10. Security Considerations
DNS Cookies provide a weak form of authentication of DNS requests and
responses. In particular, they provide no protection at all against
"on-path" adversaries; that is, they provide no protection against
any adversary 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 802.11 link
(Wi-Fi), any device in the vicinity that could receive and decode the
802.11 transmissions must be considered "on-path". On the other hand,
in a similar situation but one where 802.11 Robust Security (WPAv2)
is appropriately deployed on the Wi-Fi network nodes, only the Access
Point via which the host is connecting is "on-path".
Despite these limitations, use of DNS Cookies on the global Internet
is expected to provide a substantial reduction in the available
launch points for the traffic amplification and denial of service
forgery attacks described in Section 2 above.
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.
10.1 Cookie Algorithm Considerations
The cookie computation algorithm for use in DNS Cookies SHOULD be
FNV-64 [FNV] or some stronger algorithm because an excessively weak
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
processors that feel the need for somewhat stronger security without
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].
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Acknowledgements
The contributions of the following are gratefully acknowledged:
Tim Wicinski
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Normative References
[FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non-
Cryptographic Hash Algorithm", draft-eastlake-fnv, work in
progress.
[RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
[RFC4086bis] - Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", draft-eastlake-
randomness3, work in progress.
Informative References
[RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
[RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, September 2000.
[RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, September 2000.
[RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, January 2001.
[RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions", RFC
4034, March 2005.
[RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Extensions",
RFC 4035, March 2005.
[RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to Historic
Status", RFC 4966, July 2007.
[RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More
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Resilient against Forged Answers", RFC 5452, January 2009.
[RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash
Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, May
2011.
D. Eastlake 3rd [Page 24]
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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
EMail: d3e3e3@gmail.com
Copyright, Disclaimer, and Additional IPR Provisions
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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D. Eastlake 3rd [Page 25]
