Internet DRAFT - draft-eastlake-dnsext-cookies
draft-eastlake-dnsext-cookies
INTERNET-DRAFT Donald Eastlake
Intended Status: Proposed Standard Huawei
Mark Andrews
ISC
Expires: April 10, 2015 October 11, 2014
Domain Name System (DNS) Cookies
<draft-eastlake-dnsext-cookies-05.txt>
Abstract
DNS cookies are a lightweight DNS transaction security mechanism that
provides limited protection to DNS servers and clients against a
variety of increasingly common denial-of-service and amplification /
forgery or cache poisoning attacks by off-path attackers. DNS Cookies
are tolerant of NAT, NAT-PT, and anycast and can be incrementally
deployed.
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 Client Cookie..........................................9
4.2 Server Cookie..........................................9
4.3 Error Code............................................10
5. DNS Cookies Protocol Description.......................11
5.1 Originating Requests..................................11
5.2 Responding to Requests................................11
5.2.1 No OPT RR...........................................12
5.2.2 No Valid Client Cookie..............................12
5.2.3 Bad or Absent Server Cookie.........................13
5.2.4 A Correct Server Cookie.............................13
5.3 Processing Responses..................................14
5.4 Client and Server Secret Rollover.....................14
5.5 Implementation Requirement............................15
6. NAT Considerations and AnyCast Server Considerations...16
7. Deployment.............................................18
8. IANA Considerations....................................19
9. Security Considerations................................20
9.1 Cookie Algorithm Considerations.......................20
Acknowledgements..........................................21
Normative References......................................22
Informative References....................................22
Appendix A: Example Client Cookie Algorithms..............24
A.1 A Simple Algorithm....................................24
A.2 A More Complex Algorithm..............................24
Appendix B: Example Server Cookie Algorithms..............25
B.1 A Simple Algorithm....................................25
B.2 A More Complex Algorithm..............................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 clients
against a variety of increasingly common abuses by off-path
attackers. It is compatible with and can be used in conjunction with
other DNS transaction forgery resistance measures such as those in
[RFC5452].
The protection provided by DNS cookies bears some resemblance to that
provided by using TCP for DNS transactions. To bypass the weak
protection provided by using TCP requires an off-path attacker
guessing the 32-bit TCP sequence number in use. To bypass the weak
protection provided by DNS Cookies requires such an attacker to guess
a 64-bit pseudo-random quantity. Where DNS Cookies are not available
but TCP is, a fall back to using TCP is a reasonable strategy.
If only one party to a DNS transaction supports DNS cookies, the
mechanism does not provide a benefit or significantly interfere; but,
if both support it, the additional security provided is automatically
available.
The DNS cookies mechanism is designed to work in the presence of NAT
and NAT-PT boxes and guidance is provided herein on supporting the
DNS cookies mechanism in anycast servers.
1.1 Contents of This Document
In Section 2, we discuss the threats against which the DNS cookie
mechanism provides some protection.
Section 3 describes existing DNS security mechanisms and why they are
not adequate substitutes for DNS cookies.
Section 4 describes the COOKIE OPT option.
Section 5 provides a protocol description.
Section 6 discusses some NAT and anycast related DNS Cookies design
considerations.
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Section 7 discusses incremental deployment considerations.
Sections 8 and 9 describe IANA and Security Considerations.
1.2 Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"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 client and server, is
defined as an attacker who cannot observe the DNS request and
response messages between that client and server.
"Soft state" indicates information learned or derived by a host which
may be discarded when indicated by the policies of that host but can
be later re-instantiated if needed. For example, it could be
discarded after a period of time or when storage for caching such
data becomes full. If operations requiring that soft state continue
after it has been discarded, it will be automatically re-generated,
albeit at some cost.
"Silently discarded" indicates that there are no DNS protocol message
consequences; however, it is RECOMMENDED that appropriate network
management facilities be included in implementations, such as a
counter of the occurrences of each such event type.
"IP address" is used herein as a length independent term and includes
both IPv4 and IPv6 addresses.
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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
attenuated rather than amplified. DNS cookies make it more effective
to implement a rate limiting scheme for error responses from the
server. Such a scheme would further restrict selected host denial-
of-service traffic from that server.
2.1.2 DNS Server Denial-of-Service
DNS requests that are accepted cause work on the part of DNS servers.
This is particularly true for recursive servers that may issue one or
more requests and process the responses thereto, in order to
determine their response to the initial request. And the situation
can be even worse for recursive servers implementing DNSSEC
([RFC4033] [RFC4034] [RFC4035]) because they may be induced to
perform burdensome cryptographic computations in attempts to verify
the authenticity of data they retrieve in trying to answer the
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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 a DNS
server to a resolver for common names or names that resolver has been
induced to resolve, 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
response packets that it tends to produce.
3.2 DNS Message/Transaction Security
The second form of security that has been added to DNS provides
"transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931].
TSIG could provide strong protection against the attacks for which
the DNS Cookies mechanism 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 client-
server pairs, the burden of server side key state, and because it
requires time synchronization between client 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 per client server side state.
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4. The COOKIE OPT Option
COOKIE is an OPT RR [RFC6891] option that can be included in the
RDATA portion of an OPT RR in DNS requests and responses. The option
length varies depending on the circumstance in which it is being
used. There are two cases as described below. Both use the same
OPTION-CODE; they are distinguished by their length.
In a request sent by a client to a server when the client does not
know the server cookie, its length is 10, consisting of a 2 bytes DNS
error code field followed by the 8 byte Client Cookie as shown in
Figure 1.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION-CODE = {TBD} | OPTION-LENGTH = 10 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code (2 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. COOKIE Option, Unknown Server Cookie
In a request sent by a client when a server cookie is known and in
all responses, the length is variable from 18 to 42 bytes, consisting
of a 2 byte DNS error field followed by the 8 bytes Client Cookie and
then the variable 8 to 32 bytes Server Cookie as shown in Figure 2.
The variability of the option length stems from the variable length
Server Cookie. The Server Cookie is an integer number of bytes with a
minimum size of 64 bits for security and a maximum size of 256 bits
for implementation convenience.
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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, <= 42 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code (2 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Server Cookie (variable size, 8 to 32 bytes) /
/ /
+-+-+-+-...
Figure 2. COOKIE Option, Known Server Cookie
4.1 Client Cookie
The Client Cookie SHOULD be a pseudo-random function of the server IP
address and a secret quantity known only to the client. This client
secret SHOULD have at least 64 bits of entropy [RFC4086] and be
changed periodically (see Section 5.4). The selection of the pseudo-
random function is a matter private to the client as only the client
needs to recognize its own DNS cookies.
For further discussion of the Client Cookie field, see Section 5.1.
For example methods of determining a Client Cookie, see Appendix A.
A client MUST NOT use the same Client Cookie value for queries to all
servers.
4.2 Server Cookie
The Server Cookie SHOULD consist of or include a 64-bit or larger
pseudo-random function of the request source IP address, the request
Client Cookie, and a secret quantity known only to the server. (See
Section 6 for a discussion of why the Client Cookie is used as input
to the Server Cookie but the Server Cookie is not used as an input to
the Client Cookie.) This server secret SHOULD have at least 64 bits
of entropy [RFC4086] and be changed periodically (see Section 5.4).
The selection of the pseudo-random function is a matter private to
the server as only the server needs to recognize its own DNS cookies.
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For further discussion of the Server Cookie field see Section 5.2.
For example methods of determining a Server Cookie, see Appendix B.
A server MUST NOT use the same Server Cookie value for responses to
all clients.
4.3 Error Code
In requests, the Error Code field MUST be zero and is ignored on
receipt. Replies have a COOKIE OPT with an Error Code equal to one of
the following four values: Zero (if the request they respond to had a
COOKIE OPT with a correct Server Cookie), NOCOOKIE, MFCOOKIE, or
BADCOOKIE.
NOCOOKIE and MFCOOKIE indicate that the server did not receive a
Client Cookie, either because there was no COOKIE OPT option in the
request (NOCOOKIE) or one was present but the COOKIE OPT option was
malformed as not being a valid length (MFCOOKIE). BADCOOKIE indicates
that the server did receive a Client Cookie but did not receive the
correct Server Cookie either because there was no Server Cookie
present or because it was not a valid value.
A server may choose to normally process a request, for example
returning the normal answer information for a QUERY, notwithstanding
a cookie error condition. For more information on error processing,
see Section 5.
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5. DNS Cookies Protocol Description
This section discusses using DNS Cookies in the DNS Protocol.
5.1 Originating Requests
A DNS client that implements DNS cookies includes one DNS Cookie
option in every DNS request it sends unless DNS cookies are disabled.
The COOKIE OPT option in a request always includes a zero Error Code
field and a Client Cookie as discussed in Section 4.1.
If the client has no Server Cookie obtained from a previous DNS
response and cached under the server's IP address, it uses the
shorter form of COOKIE OPT shown in Figure 1. If the client does have
such a cached Server Cookie, it uses the form of COOKIE OPT shown in
Figure 2 and also includes that cached Server Cookie in the DNS
option it sends.
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 client that is both at the source IP address of the
request and using the Client Cookie included in the option. This weak
assurance is provided by the Server Cookie that server would send to
that client in an earlier response appearing as the Server Cookie
field in the request.
At a server where DNS Cookies are not implemented and enabled, the
presence of a COOKIE OPT option is ignored and the server responds as
before.
When DNS Cookies are implemented and enabled, there are four
possibilities: (1) there is no OPT RR at all in the request; (2)
there is no valid Client Cookie in the request because the COOKIE OPT
option in absent from the request or one is present but not a legal
length; (3) there is a valid length cookie option in the request with
no Server Cookie or an incorrect Server Cookie; or (4) there is a
cookie option in the request with a correct Server Cookie. The four
possibilities are discussed in the subsections below.
In the case of multiple COOKIE OPT options in a request, only the
first (the one closest to the DNS header) is considered. All others
are ignored.
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5.2.1 No OPT RR
If there is no OPT RR in the request, the client does not support
EDNS since [RFC6891] requires that an OPT RR be included in a request
if the requester supports that feature. Under these circumstances,
the server cannot expect to ever receive a correct COOKIE OPT option
from the client as in Section 5.2.4.
The situation and server options available are the same as those in
Section 5.2.2 except that no OPT RR can be included in any response.
5.2.2 No Valid Client Cookie
A request with an OPT RR but no COOKIE OPT option or with a COOKIE
that is not a valid length (10 or 18 through 42) could be from a
client that does not implement DNS cookies or on which they are
disabled or it could be some form of abuse or broken client
implementation. A server on which DNS cookies are enabled has the
following three choices in responding to such a request:
(1) Silently discard the request.
(2) Not process the request other than returning a minimal length
error response. Because of the absence of a validly formatted
COOKIE OPT option in the request, it cannot be assumed that the
client would understand any new RCODE values. An RCODE of Refused
is returned and the Error Field of the returned COOKIE OPT option
is set to NOCOOKIE if there was no COOKIE OPT option in the
request and set to MFCOOKIE if such an option was present but not
a valid length.
(3) Process the request normally and provide a normal response except
that a COOKIE OPT option with a non-zero Error Field is included
as in point 2 above. The RCODE in the DNS Header is zero unless
some non-cookie error occurs in processing the request.
Server policy determines how often the server selects each of the
above response choices; however, if the request was received over
TCP, the server may wish to take the weak authentication provided by
the use of TCP into account, increasing the probability of choice 3
and decreasing the probability of choice 1 perhaps to the extent of
never choosing 1. For both response choices 2 and 3, the server
should consider setting TC=1 in the response so that future requests
from the client are more likely to be received with the weak
authentication that can be provided by TCP.
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5.2.3 Bad or Absent Server Cookie
If a request is received with the COOKIE OPT option having no Server
Cookie value (length 10) or a bad Server Cookie value (length 18 to
42), it could be some attempted abuse or it could just be that the
client does not know a currently valid Server Cookie for the server
to which the request was sent. For example, the client might have an
old, no longer recognized Server Cookie
Servers MUST, at least occasionally, respond to such requests to
inform the client of the correct Server Cookie. This is necessary so
that such a client can bootstrap to the weakly secure state where
requests and responses have recognized Server Cookies and Client
Cookies.
In responding to such a request, the server has the following three
choices:
(1) Silently discard the request.
(2) Not process the request other than returning a minimal length
error response. Because of the correct length COOKIE OPT option
in the request, the client can be assume to understand the new
error codes assigned in this document. Both the Error Field in
the returned COOKIE OPT option and the extended RCODE are set to
BADCOOKIE.
(3) Processes the request normally and sends its usual response
including a COOKIE OPT option with an Error field of BADCOOKIE
and a zero RCODE (unless there was also a non-cookie error in
processing the request).
Server policy determines how often the server selects each of the
above response choices; however, if the request was received over
TCP, the server may wish to take the weak authentication provided by
the use of TCP into account, increasing the probability of choice 3
and decreasing the probability of choice 1 perhaps to the extent of
never choosing 1.
5.2.4 A Correct Server Cookie
If a server with enabled DNS cookies receives a request where the
COOKIE OPT option has a valid length and correct Server Cookie, it
processes the request normally and includes a COOKIE OPT option with
a zero Error Field in the response. Such a response might have a
non-zero RCODE if a non-cookie error occurs in processing the
request.
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5.3 Processing Responses
The Client Cookie, when it occurs in a COOKIE OPT option in a DNS
reply, is intended to weakly assure the client that the reply came
from a server at the source IP address use in the response packet
because the Client Cookie value is the value that client would send
to that server in a request. If there are multiple COOKIE OPT options
in a DNS reply, all but the first (the one closest to the DNS Header)
are ignored.
A DNS client where DNS cookies are implemented and enabled examines
response for DNS cookies and MUST discard the response if it contains
an illegal COOKIE OPT option length or an incorrect Client Cookie
value. If the COOKIE OPT option Client Cookie is correct, the client
caches the Server Cookie provided even if the response is an error
response (RCODE non-zero).
If the reply extended RCDOE is BADCOOKIE, it means that the server
was unwilling to process the request because it did not have the
correct Server Cookie in it. The client should retry the request
using the new Server Cookie from the response.
If the RCODE is some value other than BADCOOKIE, including zero, the
response is then processed normally.
5.4 Client and Server Secret Rollover
Clients 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. Many clients rolling over
their secret at the same time could briefly increase server traffic
and exactly predictable rollover times for clients or servers might
facilitate guessing attacks. For example, an attacker might increase
the priority of attacking secrets they believe will be in effect for
an extended period of time. To avoid rollover synchronization and
predictability, it is RECOMMENDED that pseudorandom jitter of at
least 30% be applied to the time of a scheduled rollover of a DNS
cookie secret.
It is RECOMMENDED that a client keep the Client Cookie it is
expecting in a reply associated with the outstanding query to avoid
rejection of replies due to a bad Client Cookie right after a change
in the Client Secret. It is RECOMMENDED that a server retain its
previous secret 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.
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Receiving a sudden increased level of requests with bad Server
Cookies or replies with bad Client Cookies would be a good reason to
believe a server or client is likely to be under attack and should
consider more frequent rollover of its secret.
5.5 Implementation Requirement
DNS clients and servers SHOULD implement DNS cookies to decrease
their vulnerability to the threats discussed in Section 2.
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6. NAT Considerations and AnyCast Server Considerations
In the Classic Internet, DNS Cookies could simply be a pseudo-random
function of the client IP address and a sever secret or the server IP
address and a client secret. You would want to compute the Server
Cookie that way, so a client could cache its Server Cookie for a
particular server for an indefinitely amount of time and the server
could easily regenerate and check it. You could consider the Client
Cookie to be a weak client signature over the server IP address that
the client checks in replies and you could extend this 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 clients and servers behind a NAT box using local IP
addresses. Nor is there a problem with NAT translation of internal
addresses to external addresses or translations between IPv4 and IPv6
addresses, as long as the address mapping is relatively stable.
Should the external IP address an internal client is being mapped to
change occasionally, the disruption is little more than when a client
rolls-over its DNS COOKIE secret. And normally external access to a
DNS server behind a NAT box is handled by a fixed mapping which
forwards externally received DNS requests to a specific host.
However, NAT devices sometimes also map ports. This can cause
multiple DNS requests and responses from multiple internal hosts to
be mapped to a smaller number of external IP addresses, such as one
address. Thus there could be many clients behind a NAT box that
appear to come from the same source IP address to a server outside
that NAT box. If one of these were an attacker (think Zombie or
Botnet), that behind-NAT attacker could get the Server Cookie for
some server for the outgoing IP address by just making some random
request to that server. It could then include that Server Cookie in
the COOKIE OPT of requests to the server with the forged local IP
address of some other host and/or client behind the NAT box.
(Attacker possession of this Server Cookie will not help in forging
responses to cause cache poisoning as such responses are protected by
the required Client Cookie.)
To fix this potential defect, it is necessary to distinguish
different clients behind a NAT box from the point of view of the
server. It is for this reason that the Server Cookie is specified as
a pseudo-random function of both the request source IP address and
the Client Cookie. From this inclusion of the Client Cookie in the
calculation of the Server Cookie, it follows that a stable Client
Cookie, for any particular server, is needed. If, for example, the
request ID was included in the calculation of the Client Cookie, it
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would normally change with each request to a particular server. This
would mean that each request would have to be sent twice: first to
learn the new Server Cookie based on this new Client Cookie based on
the new ID and then again using this new Client Cookie to actually
get an answer. Thus the input to the Client Cookie computation must
be limited to the server IP address and one or more things that
change slowly such as the client secret.
In principle, there could be a similar problem for servers, not due
to NAT but due to mechanisms like anycast which may cause 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 clients; if the Server Cookie was included in
the calculation of the Client Cookie the same way the Client Cookie
is included in the Server Cookie, you would just get an almost
infinite series of errors as a request was repeatedly retried.
For servers accessed via anycast to successfully support DNS COOKIES,
the server clones must either all use the same server secret or the
mechanism that distributes queries to them must cause the queries
from a particular client 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 clients 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 5.4).
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7. 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
client.
In particular, a DNS server or client that implements the DNS COOKIE
mechanism can interoperate successfully with a DNS client or server
that does not implement this mechanism although, of course, in this
case it will not get the benefit of the mechanism and the server
involved might choose to severely rate limit responses. When such a
server or client interoperates with a client or server which also
implements the DNS cookies mechanism, they get the weak security
benefits of the DNS Cookies mechanism.
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8. IANA Considerations
IANA is requested to assign the following four code points:
The OPT option value for COOKIE is <TBD> [10 suggested].
Three new DNS error codes in the range above 16 and below 3,840 as
shown below:
RCODE Name Description Reference
-------- --------- ----------------- ---------------
TBD1[23] NOCOOKIE No client cookie. [this document]
TBD2[24] MFCOOKIE Malformed cookie. [this document]
TBD3[25] BADCOOKIE Bad/missing server cookie. [this document]
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9. Security Considerations
DNS Cookies provide a weak form of authentication of DNS requests and
responses. In particular, they provide no protection against "on-
path" adversaries; that is, they provide no protection against any
adversary that can observe the plain text DNS traffic, such as an on-
path router, bridge, or any device on an on-path shared link (unless
the DNS traffic in question on that path is encrypted).
For example, if a host is connected via an unsecured IEEE 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" as far as the
802.11 link is concerned.
Despite these limitations, deployment of DNS Cookies on the global
Internet is expected to provide a 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.
9.1 Cookie Algorithm Considerations
The cookie computation algorithm for use in DNS Cookies SHOULD be
based on a pseudo-random function at least as strong as [FNV] 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 may not be warranted in many cases, and would cause an
increased computational burden. Nevertheless there is nothing wrong
with using something stronger, for example, HMAC-SHA256-64 [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 client and/or server secret;
however, this does require more frequent generation of a
cryptographically strong random number [RFC4086]. See Appendices A
and B for specific examples of cookie computation algorithms.
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Acknowledgements
The contributions of the following are gratefully acknowledged:
Tim Wicinski
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Normative References
[RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4086] - Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086, June
2005.
[RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
Informative References
[FNV] - G. Fowler, L. C. Noll, K.-P. Vo, D. Eastlake, "The FNV Non-
Cryptographic Hash Algorithm", draft-eastlake-fnv, work in
progress.
[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
Donald Eastlake & Mark Andrews [Page 22]
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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.
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Appendix A: Example Client Cookie Algorithms
A.1 A Simple Algorithm
An simple example method to compute Client Cookies is the FNV-64
[FNV] of the server IP address and the client secret. That is
Client Cookie = FNV-64 ( Client Secret | Server IP Address )
where "|" indicates concatenation.
A.2 A More Complex Algorithm
A more complex algorithm to calculate Client Cookies is give below.
It uses more computational resources than the simpler algorithm shown
in A.1.
Client Cookie = HMAC-SHA256-64 ( Client Secret, Server IP Address
)
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Appendix B: Example Server Cookie Algorithms
B.1 A Simple Algorithm
An example simple method producing a 64-bit Server Cookie is the
FNV-64 [FNV] of the request IP address, the Client Cookie, and the
server secret. That is
Server Cookie =
FNV-64 ( Server Secret | Request IP Address | Client Cookie )
where "|" represents concatenation.
B.2 A More Complex Algorithm
Since the Server Cookie is variable size, the server can store
various information in that field as long as it is hard for an
adversary to guess the entire quantity used for weak authentication.
There should be 64 bits of entropy in the Server Cookie; for example
it could have a sub-field of 64-bits computed pseudo-randomly with
the server secret as one of the inputs to the pseudo-random function.
Types of additional information that could be stored include a time
stamp and/or a nonce.
The example below is one variation for the Server Cookie that has
been implemented in a beta release of BIND where the Server Cookie is
128 bits composed as follows:
Sub-field Size
--------- ---------
Nonce 32 bits
Time 32 bits
Hash 64 bits
With this algorithm, the server sends a new 128-bit cookie back with
every request. The Nonce field assures a low probability that there
would be a duplicate.
The Time field gives the server time and makes it easy to reject old
cookies.
The Hash part of the Server Cookie is the hard-to-guess part. In the
beta release of BIND, its computation can be configured to use AES,
HMAC-SHA1, or, as shown below, HMAC-SHA256:
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hash =
HMAC-SHA256-64 ( Server Secret,
(Client Cookie | nonce | time | client IP Address) )
where "|" represents concatenation.
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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
Mark Andrews
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063 USA
Email: marka@isc.org
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Donald Eastlake & Mark Andrews [Page 27]