Internet DRAFT - draft-ietf-dnsop-cookies
draft-ietf-dnsop-cookies
INTERNET-DRAFT Donald Eastlake
Intended Status: Proposed Standard Huawei
Mark Andrews
ISC
Expires: October 4, 2016 April 5, 2016
Domain Name System (DNS) Cookies
<draft-ietf-dnsop-cookies-10.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. (Since DNS Cookies are only returned to the IP address from
which they were originally received, they cannot be used to generally
track Internet users.)
Status of This Document
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Distribution of this document is unlimited. Comments should be sent
to the author or the DNSEXT mailing list <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-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html. The list of Internet-Draft
Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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Table of Contents
1. Introduction............................................4
1.1 Contents of This Document..............................4
1.2 Definitions............................................5
2. Threats Considered......................................6
2.1 Denial-of-Service Attacks..............................6
2.1.1 DNS Amplification Attacks............................6
2.1.2 DNS Server Denial-of-Service.........................7
2.2 Cache Poisoning and Answer Forgery Attacks.............7
3. Comments on Existing DNS Security.......................8
3.1 Existing DNS Data Security.............................8
3.2 DNS Message/Transaction Security.......................8
3.3 Conclusions on Existing DNS Security...................8
4. DNS Cookie Option......................................10
4.1 Client Cookie.........................................11
4.2 Server Cookie.........................................11
5. DNS Cookies Protocol Specification.....................12
5.1 Originating Requests..................................12
5.2 Responding to Request.................................12
5.2.1 No Opt RR or No COOKIE OPT option...................13
5.2.2 Malformed COOKIE OPT option.........................13
5.2.3 Only a Client Cookie................................13
5.2.4 A Client Cookie and an Invalid Server Cookie........14
5.2.5 A Client Cookie and a Valid Server Cookie...........14
5.3 Processing Responses..................................15
5.4 QUERYing for a Server Cookie..........................15
6. NAT Considerations and AnyCast Server Considerations...17
7. Operational and Deployment Considerations..............19
7.1 Client and Server Secret Rollover.....................19
7.2 Counters..............................................20
8. IANA Considerations....................................21
9. Security Considerations................................22
9.1 Cookie Algorithm Considerations.......................23
10. Implementation Considerations.........................24
Normative References......................................25
Informative References....................................25
Acknowledgements..........................................27
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Table of Contents (continued)
Appendix A: Example Client Cookie Algorithms..............28
A.1 A Simple Algorithm....................................28
A.2 A More Complex Algorithm..............................28
Appendix B: Example Server Cookie Algorithms..............29
B.1 A Simple Algorithm....................................29
B.2 A More Complex Algorithm..............................29
Author's Address..........................................31
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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. The DNS
cookies mechanism provides limited protection to DNS servers and
clients against a variety of increasingly common abuses by off-path
attackers. It is compatible with and can be used in conjunction with
other DNS transaction forgery resistance measures such as those in
[RFC5452]. (Since DNS Cookies are only returned to the IP address
from which they were originally received, they cannot be used to
generally track Internet users.)
The protection provided by DNS cookies is similar to that provided by
using TCP for DNS transactions. To bypass the weak protection
provided by using TCP requires, among other things, that an off-path
attacker guess the 32-bit TCP sequence number in use. To bypass the
weak protection provided by DNS Cookies requires such an attacker to
guess a 64-bit pseudo-random "cookie" quantity. Where DNS Cookies are
not available but TCP is, falling back to using TCP is reasonable.
If only one party to a DNS transaction supports DNS cookies, the
mechanism does not provide a benefit or significantly interfere; but,
if both support it, the additional security provided is automatically
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
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considerations.
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", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
"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.
"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.
There are also on-path denial of service attacks that attempt to
saturate a server with DNS requests having correct source addresses.
Cookies do not protect against such attacks but successful cookie
validation improves the probability that the correct source IP
address for the requests is known. This facilitates contacting the
managers of or taking other actions for the networks from which the
requests originate.
2.1.1 DNS Amplification Attacks
A request with a forged IP source address generally causes a response
to be sent to that forged IP address. Thus the forging of many such
requests with a particular source IP address can result in enough
traffic being sent to the forged IP address to interfere with service
to the host at the IP address. Furthermore, it is generally easy in
the DNS to create short requests that produce much longer responses,
thus amplifying the attack.
The DNS Cookies mechanism can severely limit the traffic
amplification obtained by attacker requests that are off the path
between the server and the request's source address. Enforced DNS
cookies would make it hard for an off path attacker to cause any more
than rate-limited short error responses to be sent to a forged IP
address so the attack would be attenuated rather than amplified. DNS
cookies make it more effective to implement a rate limiting scheme
for error responses from the server. Such a scheme would further
restrict selected host denial-of-service traffic from that server.
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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
request.
The computational or communications burden caused by such requests
may not depend on a forged IP source address, but the use of such
addresses makes
+ the source of the requests causing the denial-of-service attack
harder to find and
+ restriction of the IP addresses from which such requests should
be honored hard or impossible to specify or verify.
Use of DNS cookies should enable a server to reject forged requests
from an off path attacker with relative ease and before any recursive
queries or public key cryptographic operations are performed.
2.2 Cache Poisoning and Answer Forgery Attacks
The form of the cache poisoning attacks considered is to send forged
replies to a resolver. Modern network speeds for well-connected hosts
are such that, by forging replies from the IP addresses of a DNS
server to a resolver for names that resolver has been induced to
resolve or for common names whose resource records have short time-
to-live values, there can be an unacceptably high probability of
randomly coming up with a reply that will be accepted and cause false
DNS information to be cached by that resolver (the Dan Kaminsky
attack [Kaminsky]). This can be used to facilitate phishing attacks
and other diversion of legitimate traffic to a compromised or
malicious host such as a web server.
With the use of DNS cookies, a resolver can generally reject such
forged replies.
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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], [RFC4035], and updates thereto. It provides
data origin authentication and authenticated denial of existence.
DNSSEC is being deployed and can provide strong protection against
forged data and cache poisoning; however, it has the unintended
effect of making some denial-of-service attacks worse because of the
cryptographic computational load it can require and the increased
size in DNS response packets that it tends to produce.
3.2 DNS Message/Transaction Security
The second form of security that has been added to DNS provides
"transaction" security through TSIG [RFC2845] or SIG(0) [RFC2931].
TSIG could provide strong protection against the attacks for which
the DNS Cookies mechanism provides weaker protection; however, TSIG
is non-trivial to deploy in the general Internet because of the
burdens it imposes. Among these burdens are pre-agreement and key
distribution between client and server, keeping track of server side
key state, and required time synchronization between client and
server.
TKEY [RFC2930] can solve the problem of key distribution for TSIG but
some modes of TKEY impose a substantial cryptographic computation
load and can be dependent on the deployment of DNS data security (see
Section 3.1).
SIG(0) [RFC2931] provides less denial of service protection than TSIG
or, in one way, even DNS cookies, because it does not authenticate
requests, only complete transactions. In any case, it also depends
on the deployment of DNS data security and requires computationally
burdensome public key cryptographic operations.
3.3 Conclusions on Existing DNS Security
The existing DNS security mechanisms do not provide the services
provided by the DNS Cookies mechanism: lightweight message
authentication of DNS requests and responses with no requirement for
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pre-configuration or per client server side state.
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4. DNS Cookie Option
The DNS Cookie Option is an OPT RR [RFC6891] option that can be
included in the RDATA portion of an OPT RR in DNS requests and
responses. The option length varies depending on the circumstances
in which it is being used. There are two cases as described below.
Both use the same OPTION-CODE; they are distinguished by their
length.
In a request sent by a client to a server when the client does not
know the server's cookie, its length is 8, consisting of an 8 byte
Client Cookie as shown in Figure 1.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION-CODE = 10 | OPTION-LENGTH = 8 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. COOKIE Option, Unknown Server Cookie
In a request sent by a client when a server cookie is known and in
all responses, the length is variable from 16 to 40 bytes, consisting
of an 8 bytes Client Cookie followed by the variable 8 to 32 bytes
Server Cookie as shown in Figure 2. The variability of the option
length stems from the variable length Server Cookie. The Server
Cookie is an integer number of bytes with a minimum size of 8 bytes
for security and a maximum size of 32 bytes for implementation
convenience.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| OPTION-CODE = 10 | OPTION-LENGTH >= 16, <= 40 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Server Cookie (variable size, 8 to 32 bytes) /
/ /
+-+-+-+-...
Figure 2. COOKIE Option, Known Server Cookie
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4.1 Client Cookie
The Client Cookie SHOULD be a pseudo-random function of the client IP
address, the server IP address, and a secret quantity known only to
the client. This client secret SHOULD have at least 64 bits of
entropy [RFC4086] and be changed periodically (see Section 7.1). The
selection of the pseudo-random function is a matter private to the
client as only the client needs to recognize its own DNS cookies.
The client IP address is included so that the Client Cookie cannot be
used (1) to track a client if the client IP address changes due to
privacy mechanisms or (2) to impersonate the client by some network
device that was formerly on path but is no longer on path when the
client IP address changes due to mobility. However, if the client IP
address is being changed very often, it may be necessary to fix the
Client Cookie for a particular server for several requests to avoid
undue inefficiency due to retries caused by that server not
recognizing the Client Cookie.
For further discussion of the Client Cookie field, see Section 5.1.
For example methods of determining a Client Cookie, see Appendix A.
In order to provide minimal authentication, a client MUST send Client
Cookies that will usually be different for any two servers at
different IP addresses.
4.2 Server Cookie
The Server Cookie SHOULD consist of or include a 64-bit or larger
pseudo-random function of the request source (client) IP address, a
secret quantity known only to the server, and the request Client
Cookie. (See Section 6 for a discussion of why the Client Cookie is
used as input to the Server Cookie but the Server Cookie is not used
as an input to the Client Cookie.) This server secret SHOULD have at
least 64 bits of entropy [RFC4086] and be changed periodically (see
Section 7.1). The selection of the pseudo-random function is a
matter private to the server as only the server needs to recognize
its own DNS cookies.
For further discussion of the Server Cookie field see Section 5.2.
For example methods of determining a Server Cookie, see Appendix B.
When implemented as recommended, the server need not maintain any
cookie related per client state.
In order to provide minimal authentication, a server MUST send Server
Cookies that will usually be different for clients at any two
different IP addresses or with different Client Cookies.
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5. DNS Cookies Protocol Specification
This section discusses using DNS Cookies in the DNS Protocol. The
cycle of originating a request, responding to that request, and
processing the response are covered in Sections 5.1, 5.2, and 5.3. A
de facto extension to QUERY to allow pre-fetching a Server Cookie is
specified in Section 5.4. Rollover of the client and server secrets
and transient retention of the old cookie or secret is covered in
Section 7.1.
DNS clients and servers SHOULD implement DNS cookies to decrease
their vulnerability to the threats discussed in Section 2.
5.1 Originating Requests
A DNS client that implements DNS Cookies includes one DNS COOKIE OPT
option containing a Client Cookie in every DNS request it sends
unless DNS cookies are disabled.
If the client has a cached Server Cookie for the server against its
IP address it uses the longer cookie form and includes that Server
Cookie in the option along with the Client Cookie (Figure 2).
Otherwise it just sends the shorter form option with a Client Cookie
(Figure 1).
5.2 Responding to Request
The Server Cookie, when it occurs in a COOKIE OPT option in a
request, is intended to weakly assure the server that the request
came from a client that is both at the source IP address of the
request and using the Client Cookie included in the option. This
assurance is provided by the Server Cookie that server sent to that
client in an earlier response appearing as the Server Cookie field in
the request.
At a server where DNS Cookies are not implemented and enabled,
presence of a COOKIE OPT option is ignored and the server responds as
if no COOKIE OPT option had been included in the request.
When DNS Cookies are implemented and enabled, there are five
possibilities: (1) there is no OPT RR at all in the request or there
is a OPT RR but the COOKIE OPT option is absent from the OPT RR; (2)
a COOKIE OPT is present but is not a legal length or otherwise
malformed; (3) there is a valid length cookie option in the request
with no Server Cookie; (4) there is a valid length COOKIE OPT in the
request with a Server Cookie but that Server Cookie is invalid; or
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(5) there is a valid length COOKIE OPT in the request with a correct
Server Cookie.
The five possibilities are discussed in the subsections below.
In all cases of multiple COOKIE OPT options in a request, only the
first (the one closest to the DNS header) is considered. All others
are ignored.
5.2.1 No Opt RR or No COOKIE OPT option
If there is no OPT record or no COOKIE OPT option present in the
request then the server responds to the request as if the server
doesn't implement the COOKIE OPT.
5.2.2 Malformed COOKIE OPT option
If the COOKIE OPT is too short to contain a Client Cookie then
FORMERR is generated. If the COOKIE OPT is longer than that required
to hold a COOKIE OPT with just a Client Cookie (8 bytes) but is
shorter that the minimum COOKIE OPT with both a Client and Server
Cookie (16 bytes) then FORMERR is generated. If the COOKIE OPT is
longer than the maximum valid COOKIE OPT (40 bytes) then a FORMERR is
generated.
In summary, valid cookie lengths are 8 and 16 to 40 inclusive.
5.2.3 Only a Client Cookie
Based on server policy, including rate limiting, the server chooses
one of the following:
(1) Silently discard the request.
(2) Send a BADCOOKIE error response.
(3) Process the request and provide a normal response. The RCODE is
NOERROR unless some non-cookie error occurs in processing the
request.
If the server responds, choosing 2 or 3 above, it SHALL generate its
own COOKIE OPT containing both the Client Cookie copied from the
request and a Server Cookie it has generated and adds this COOKIE OPT
to the response's OPT record. Servers MUST, at least occasionally,
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respond to such requests to inform the client of the correct Server
Cookie. This is necessary so that such a client can bootstrap to the
more secure state where requests and responses have recognized Server
Cookies and Client Cookies. A server is not expected to maintain per
client state to achieve this. For example, it could respond to every
Nth request across all clients.
If the request was received over TCP, the server SHOULD take the
authentication provided by the use of TCP into account and SHOULD
choose 3. In this case, if the server is not willing to accept the
security provided by TCP as a substitute for the security provided by
DNS Cookies but instead chooses 2, there is some danger of an
indefinite loop of retries (see Section 5.3).
5.2.4 A Client Cookie and an Invalid Server Cookie
The server examines the Server Cookie to determine if it is a valid
Server Cookie it has generated. This determination normally involves
re-calculating the Server Cookie (or the hash part thereof) based on
the server secret (or the previous server secret if it has just
changed), the received Client Cookie, the client IP address, and
possibly other fields -- see Appendix B.2 for an example. If the
cookie is invalid, it can be because of a stale Server Cookie, or a
client's IP address or Client Cookie changing without the DNS server
being aware, or an anycast server cluster that is not consistently
configured, or an attempt to spoof the client.
The server SHALL process the request as if the invalid Server Cookie
was not present as described in Section 5.2.3.
5.2.5 A Client Cookie and a Valid Server Cookie
When a valid Server Cookie is present in the request the server can
assume that the request is from a client that it has talked to before
and defensive measures for spoofed UDP requests, if any, are no
longer required.
The server SHALL process the request and include a COOKIE OPT in the
response by (a) copying the complete COOKIE OPT from the request or
(b) generating a new COOKIE OPT containing both the Client Cookie
copied from the request and a valid Server Cookie it has generated.
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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 used in the response packet
because the Client Cookie value is the value that client would send
to that server in a request. In a DNS reply with multiple COOKIE OPT
options, all but the first (the one closest to the DNS Header) are
ignored.
A DNS client where DNS cookies are implemented and enabled examines
the response for DNS cookies and MUST discard the response if it
contains an illegal COOKIE OPT option length or an incorrect Client
Cookie value. If the client is expecting the response to contain a
COOKIE OPT and it is missing the response MUST be discarded. If the
COOKIE OPT option Client Cookie is correct, the client caches the
Server Cookie provided even if the response is an error response
(RCODE non-zero).
If the reply extended RCODE is BADCOOKIE and the Client Cookie
matches what was sent, it means that the server was unwilling to
process the request because it did not have the correct Server Cookie
in it. The client SHOULD retry the request using the new Server
Cookie from the response. Repeated BADCOOKIE responses to requests
that use the Server Cookie provided in the previous response may be
an indication that the shared secrets / secret generation method in
an anycast cluster of servers are inconsistent. If the reply to a
retried request with a fresh Server Cookie is BADCOOKIE, the client
SHOULD retry using TCP as the transport since the server will likely
process the request normally based on the security provided by TCP
(see Section 5.2.3).
If the RCODE is some value other than BADCOOKIE, including zero, the
further processing of the response proceeds normally.
5.4 QUERYing for a Server Cookie
In many cases a client will learn the Server Cookie for a server as
the side effect of another transaction; however, there may be times
when this is not desirable. Therefore a means is provided for
obtaining a Server Cookie through an extension to the QUERY opcode
for which opcode most existing implementations require that QDCOUNT
be one (see Section 4.1.2 of [RFC1035]).
For servers with DNS Cookies enabled, the QUERY opcode behavior is
extended to support queries with an empty question section (QDCOUNT
zero) provided that an OPT record is present with a COOKIE option.
Such servers will reply with an empty answer section and a COOKIE
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option giving the Client Cookie provided in the query and a valid
Server Cookie.
If such a query provided just a Client Cookie and no Server Cookie,
the response SHALL have the RCODE NOERROR.
This mechanism can also be used to confirm/re-establish an existing
Server Cookie by sending a cached Server Cookie with the Client
Cookie. In this case the response SHALL have the RCODE BADCOOKIE if
the Server Cookie sent with the query was invalid and the RCODE
NOERROR if it was valid.
Servers which don't support the COOKIE option will normally send
FORMERR in response to such a query, though REFUSED, NOTIMP, and
NOERROR without a COOKIE option are also possible in such responses.
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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 server secret or the server
IP address and a client secret. You would want to compute the Server
Cookie that way, so a client could cache its Server Cookie for a
particular server for an indefinite amount of time and the server
could easily regenerate and check it. You could consider the Client
Cookie to be a weak client signature over the server IP address that
the client checks in replies and you could extend this signature to
cover the request ID, for example, or any other information that is
returned unchanged in the reply.
But we have this reality called NAT [RFC3022], Network Address
Translation (including, for the purposes of this document, NAT-PT,
Network Address and Protocol Translation, which has been declared
Historic [RFC4966]). There is no problem with DNS transactions
between clients and servers behind a NAT box using local IP
addresses. Nor is there a problem with NAT translation of internal
addresses to external addresses or translations between IPv4 and IPv6
addresses, as long as the address mapping is relatively stable.
Should the external IP address an internal client is being mapped to
change occasionally, the disruption is little more than when a client
rolls-over its DNS COOKIE secret. And normally external access to a
DNS server behind a NAT box is handled by a fixed mapping which
forwards externally received DNS requests to a specific host.
However, NAT devices sometimes also map ports. This can cause
multiple DNS requests and responses from multiple internal hosts to
be mapped to a smaller number of external IP addresses, such as one
address. Thus there could be many clients behind a NAT box that
appear to come from the same source IP address to a server outside
that NAT box. If one of these were an attacker (think Zombie or
Botnet), that behind-NAT attacker could get the Server Cookie for
some server for the outgoing IP address by just making some random
request to that server. It could then include that Server Cookie in
the COOKIE OPT of requests to the server with the forged local IP
address of some other host and/or client behind the NAT box.
(Attacker possession of this Server Cookie will not help in forging
responses to cause cache poisoning as such responses are protected by
the required Client Cookie.)
To fix this potential defect, it is necessary to distinguish
different clients behind a NAT box from the point of view of the
server. It is for this reason that the Server Cookie is specified as
a pseudo-random function of both the request source IP address and
the Client Cookie. From this inclusion of the Client Cookie in the
calculation of the Server Cookie, it follows that a stable Client
Cookie, for any particular server, is needed. If, for example, the
request ID was included in the calculation of the Client Cookie, it
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would normally change with each request to a particular server. This
would mean that each request would have to be sent twice: first to
learn the new Server Cookie based on this new Client Cookie based on
the new ID and then again using this new Client Cookie to actually
get an answer. Thus the input to the Client Cookie computation must
be limited to the server IP address and one or more things that
change slowly such as the client secret.
In principle, there could be a similar problem for servers, not due
to NAT but due to mechanisms like anycast which may cause requests to
a DNS server at an IP address to be delivered to any one of several
machines. (External requests to a DNS server behind a NAT box usually
occur via port forwarding such that all such requests go to one
host.) However, it is impossible to solve this the way the similar
problem was solved for NATed clients; if the Server Cookie was
included in the calculation of the Client Cookie the same way the
Client Cookie is included in the Server Cookie, you would just get an
almost infinite series of errors as a request was repeatedly retried.
For servers accessed via anycast to successfully support DNS COOKIES,
the server clones must either all use the same server secret or the
mechanism that distributes requests to them must cause the requests
from a particular client to go to a particular server for a
sufficiently long period of time that extra requests due to changes
in Server Cookie resulting from accessing different server machines
are not unduly burdensome. (When such anycast-accessed servers act
as recursive servers or otherwise act as clients they normally use a
different unique address to source their requests to avoid confusion
in the delivery of responses.)
For simplicity, it is RECOMMENDED that the same server secret be used
by each DNS server in a set of anycast servers. If there is limited
time skew in updating this secret in different anycast servers, this
can be handled by a server accepting requests containing a Server
Cookie based on either its old or new secret for the maximum likely
time period of such time skew (see also Section 7.1).
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7. Operational and Deployment Considerations
The DNS cookies mechanism is designed for incremental deployment and
to complement the orthogonal techniques in [RFC5452]. Either or both
techniques can be deployed independently at each DNS server and
client. Thus installation at the client and server end need not be
synchronized.
In particular, a DNS server or client that implements the DNS COOKIE
mechanism can interoperate successfully with a DNS client or server
that does not implement this mechanism although, of course, in this
case it will not get the benefit of the mechanism and the server
involved might choose to severely rate limit responses. When such a
server or client interoperates with a client or server which also
implements the DNS cookies mechanism, they get the security benefits
of the DNS Cookies mechanism.
7.1 Client and Server Secret Rollover
The longer a secret is used, the higher the probability it has been
compromised. Thus clients and servers are configured with a lifetime
for their secret and rollover to a new secret when that lifetime
expires or earlier due to deliberate jitter as described below. The
default lifetime is one day and the maximum permitted is one month.
To be precise and to make it practical to stay within limits despite
long holiday weekends and daylight savings time shifts and the like,
clients and servers MUST NOT continue to use the same secret in new
requests and responses for more than 36 days and SHOULD NOT continue
to do so for more than 26 hours.
Many clients rolling over their secret at the same time could briefly
increase server traffic and exactly predictable rollover times for
clients or servers might facilitate guessing attacks. For example, an
attacker might increase the priority of attacking secrets they
believe will be in effect for an extended period of time. To avoid
rollover synchronization and predictability, it is RECOMMENDED that
pseudorandom jitter in the range of plus zero to minus at least 40%
be applied to the time until a scheduled rollover of a DNS cookie
secret.
It is RECOMMENDED that a client keep the Client Cookie it is
expecting in a reply until there is no longer an outstanding request
associated with that Client Cookie that the client is tracking. This
avoids rejection of replies due to a bad Client Cookie right after a
change in the client secret.
It is RECOMMENDED that a server retain its previous secret after a
rollover to a new secret for a configurable period of time not less
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than 1 second or more than 5 minutes with default configuration of 2
1/2 minutes. Requests with Server Cookies based on its previous
secret are treated as a correct Server Cookie during that time. When
a server responds to a request containing a old Server Cookie that
the server is treating as correct, the server MUST include a new
Server Cookie in its response.
7.2 Counters
It is RECOMMENDED that implementations include counters of the
occurrences of the various types of requests and responses described
in Section 5.
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8. IANA Considerations
IANA has assigned the following OPT option value:
Value Name Status Reference
-------- ------ -------- ---------------
10 COOKIE Standard [this document]
IANA has assigned the following DNS error code as an early
allocation:
RCODE Name Description Reference
-------- --------- ------------------------- ---------------
23 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 Std 802.11
link (Wi-Fi), any device in the vicinity that could receive and
decode the 802.11 transmissions must be considered "on-path". On the
other hand, in a similar situation but one where 802.11 Robust
Security (WPA2) is appropriately deployed on the Wi-Fi network nodes,
only the Access Point via which the host is connecting is "on-path"
as far as the 802.11 link is concerned.
Despite these limitations, deployment of DNS Cookies on the global
Internet is expected to provide a significant reduction in the
available launch points for the traffic amplification and denial of
service forgery attacks described in Section 2 above.
Work is underway in the IETF DPRIVE working group to provide
confidentiality for DNS requests and responses which would be
compatible with DNS cookies.
Should stronger message/transaction security be desired, it is
suggested that TSIG or SIG(0) security be used (see Section 3.2);
however, it may be useful to use DNS Cookies in conjunction with
these features. In particular, DNS Cookies could screen out many DNS
messages before the cryptographic computations of TSIG or SIG(0) are
required and, if SIG(0) is in use, DNS Cookies could usefully screen
out many requests given that SIG(0) does not screen requests but only
authenticates the response of complete transactions.
An attacker that does not know the Server Cookie could do a variety
of things, such as omitting the COOKIE OPT option or sending a random
Server Cookie. In general, DNS servers need to take other measures,
including rate limiting responses, to protect from abuse in such
cases. See further information in Section 5.2.
When a server or client starts receiving an increased level of
requests with bad server cookies or replies with bad client cookies,
it would be reasonable for it to believe it is likely under attack
and it should consider a more frequent rollover of its secret. More
rapid rollover decreases the benefit to a cookie guessing attacker if
they succeed in guessing a cookie.
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9.1 Cookie Algorithm Considerations
The cookie computation algorithm for use in DNS Cookies SHOULD be
based on a pseudo-random function at least as strong as 64-bit FNV
(Fowler-Noll-Vo [FNV]) because an excessively weak or trivial
algorithm could enable adversaries to guess cookies. However, in
light of the lightweight plain-text token security provided by DNS
Cookies, a strong cryptography hash algorithm may not be warranted in
many cases, and would cause an increased computational burden.
Nevertheless there is nothing wrong with using something stronger,
for example, HMAC-SHA256 [RFC6234] truncated to 64 bits, assuming a
DNS processor has adequate computational resources available. DNS
processors that feel the need for somewhat stronger security without
a significant increase in computational load should consider more
frequent changes in their client and/or server secret; however, this
does require more frequent generation of a cryptographically strong
random number [RFC4086]. See Appendices A and B for specific examples
of cookie computation algorithms.
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10. Implementation Considerations
The DNS Cookie Option specified herein is implemented in BIND 9.10
using an experimental option code.
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Normative References
[RFC1035] - Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2119] - Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119,
March 1997, <http://www.rfc-editor.org/info/rfc2119>.
[RFC4086] - Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086, DOI
10.17487/RFC4086, June 2005, <http://www.rfc-
editor.org/info/rfc4086>.
[RFC6891] - Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/RFC6891,
April 2013, <http://www.rfc-editor.org/info/rfc6891>.
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.
[Kaminsky] - Olney, M., P. Mullen, K. Miklavicic, "Dan Kaminsky's
2008 DNS Vulnerability", 25 July 2008,
<https://www.ietf.org/mail-
archive/web/dnsop/current/pdf2jgx6rzxN4.pdf>.
[RFC2845] - Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000,
<http://www.rfc-editor.org/info/rfc2845>.
[RFC2930] - Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, DOI 10.17487/RFC2930, September 2000,
<http://www.rfc-editor.org/info/rfc2930>.
[RFC2931] - Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 2000,
<http://www.rfc-editor.org/info/rfc2931>.
[RFC3022] - Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022, DOI
10.17487/RFC3022, January 2001, <http://www.rfc-
editor.org/info/rfc3022>.
Donald Eastlake & Mark Andrews [Page 25]
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[RFC4033] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC 4033,
DOI 10.17487/RFC4033, March 2005, <http://www.rfc-
editor.org/info/rfc4033>.
[RFC4034] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions", RFC
4034, DOI 10.17487/RFC4034, March 2005, <http://www.rfc-
editor.org/info/rfc4034>.
[RFC4035] - Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security Extensions",
RFC 4035, DOI 10.17487/RFC4035, March 2005, <http://www.rfc-
editor.org/info/rfc4035>.
[RFC4966] - Aoun, C. and E. Davies, "Reasons to Move the Network
Address Translator - Protocol Translator (NAT-PT) to Historic
Status", RFC 4966, DOI 10.17487/RFC4966, July 2007,
<http://www.rfc-editor.org/info/rfc4966>.
[RFC5452] - Hubert, A. and R. van Mook, "Measures for Making DNS More
Resilient against Forged Answers", RFC 5452, DOI
10.17487/RFC5452, January 2009, <http://www.rfc-
editor.org/info/rfc5452>.
[RFC6234] - Eastlake 3rd, D. and T. Hansen, "US Secure Hash
Algorithms (SHA and SHA-based HMAC and HKDF)", RFC 6234, DOI
10.17487/RFC6234, May 2011, <http://www.rfc-
editor.org/info/rfc6234>.
Donald Eastlake & Mark Andrews [Page 26]
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Acknowledgements
The suggestions and contributions of the following are gratefully
acknowledged:
Alissa Cooper, Bob Harold, Paul Hoffman, David Malone, Yoav Nir,
Gayle Noble, Dan Romascanu,
Tim Wicinski, Peter Yee
The document was prepared in raw nroff. All macros used were defined
within the source file.
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Appendix A: Example Client Cookie Algorithms
A.1 A Simple Algorithm
A simple example method to compute Client Cookies is the FNV-64 [FNV]
of the client IP address, the server IP address, and the client
secret. That is
Client Cookie =
FNV-64( Client IP Address | Server IP Address | Client Secret )
where "|" indicates concatenation. Some computational resources may
be saved by precomputing FNV-64 through the Client IP Address. (If
the order of the items concatenated above is changed to put the
Server IP Address last, it might be possible to further reduce the
computational effort by pre-computing FNV-64 through the bytes of
both the Client IP Address and the Client Secret but this would
reduce the strength of the Client Cookie and is NOT RECOMMENDED.)
A.2 A More Complex Algorithm
A more complex algorithm to calculate Client Cookies is given below.
It uses more computational resources than the simpler algorithm shown
in A.1.
Client Cookie = HMAC-SHA256-64(
Client IP Address | Server IP Address,
Client Secret )
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Appendix B: Example Server Cookie Algorithms
B.1 A Simple Algorithm
An example of a simple method producing a 64-bit Server Cookie is the
FNV-64 [FNV] of the request IP address, the Client Cookie, and the
server secret.
Server Cookie =
FNV-64( Client IP Address | Client Cookie | Server Secret )
where "|" represents concatenation. (If the order of the items
concatenated was changed, it might be possible to reduce the
computational effort by pre-computing FNV-64 through the bytes of the
Sever Secret and Client Cookie but this would reduce the strength of
the Server Cookie and is NOT RECOMMENDED.)
B.2 A More Complex Algorithm
Since the Server Cookie has a variable size, the server can store
various information in that field as long as it is hard for an
adversary to guess the entire quantity used for authentication. There
should be 64 bits of entropy in the Server Cookie; for example it
could have a sub-field of 64-bits computed pseudo-randomly with the
server secret as one of the inputs to the pseudo-random function.
Types of additional information that could be stored include a time
stamp and/or a nonce.
The example below is one variation for the Server Cookie that has
been implemented in BIND 9.10.3 (and later) releases where the Server
Cookie is 128 bits composed as follows:
Sub-field Size
--------- ---------
Nonce 32 bits
Time 32 bits
Hash 64 bits
With this algorithm, the server sends a new 128-bit cookie back with
every request. The Nonce field assures a low probability that there
would be a duplicate.
The Time field gives the server time and makes it easy to reject old
cookies.
The Hash part of the Server Cookie is the hard-to-guess part. In BIND
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9.10.3 (and later), its computation can be configured to use AES,
HMAC-SHA1, or, as shown below, HMAC-SHA256:
hash =
HMAC-SHA256-64( Server Secret,
(Client Cookie | nonce | time | Client IP Address) )
where "|" represents concatenation.
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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
Copyright, Disclaimer, and Additional IPR Provisions
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Donald Eastlake & Mark Andrews [Page 31]
