So DNS is changing.
DNS is the heart of the Internet. But today as everything else weaknesses have been found and take advantage of. We need to make it as secure as the the websites are and The article below from Johannes B. Ullrich, at SANS talks to that. This is somewhat old news but sheds some light on where DNS is going.
I consider DNS one of the great success stories as far as internet protocols go. I remember (for a short time) updating host files and the emergence of DNS. DNS was created for a very different type of Internet then what we have today. It assumed a more “friendly” and cooperative Internet. Without amplified DDoS attacks, MITM attacks, and spoofing. Overall, DNS has survived pretty much in its original form with a couple of relatively small additions like EDNS 0 and DNSSEC (which nobody uses).
But recently, in particular, pushed by the need for more privacy and the more bold and unregulated data trading ISPs participate in, the need has come up for more privacy. Also, the failure of DNSSEC to gain substantial traction has led other, simpler, ideas to provide most of the benefit of DNSSEC.
In this article, I want to highlight two changes that I see showing up in DNS traffic:
1 – DNS Cookies
DNS Cookies have been introduced in RFC 7873 . They try to address many of the problems that DNSSEC tried to solve (like cache poisoning), and they are addressing issues like spoofed DNS amplification attacks that DNSSEC didn’t prevent. In some cases, DNSSEC may have made these attacks worse. To make it a total “win”: DNS cookies are much easier to implement than DNSSEC. The security afforded by DNS cookies is supposed to be similar to the security gained by using TCP instead of UDP. To successfully spoof TCP, an attacker needs to guess a 32-bit sequence number. DNSSEC is much harder to break. So DNS cookies are not as good as DNSSEC. But they may be “good enough.”
So how does it work?
A client sending a DNS request will attach a cookie as an option. The cookie is a hash of client IP, server IP, and a secret. So the server will see consistent cookies from a particular client. The secret should be at least 64 Bits long. The details do not matter as long as the cookie is consistent with a specific server/client combination.
The server cookie uses the server IP address, the client cookie and again a secret that is only known to the server. The client IP is not used since it may change due to NAT. Using the client cookie instead will again lead to consistent cookies for a particular server-client combination.
A client that supports cookies will send them with all requests. This *should* not cause any problems. Unsupported options will be ignored. But some testing has shown in the past that there are non-compliant DNS servers that may reject the request as malformed. This test from a few years ago showed that 10% of name servers had problems. Not sure what this looks like right now, but the number is likely smaller.
The client will include a server cookie if it communicated with this server in the past.
Once a server receives a request with client cookie, one of several options may occur:
- If the server doesn’t support cookies, then it will respond as usual ignoring the cookie.
- If the server does support cookies, and the client sent a request with only a client cookie, then the server will respond, but it may not include anything but the server cookie. Now the client may re-send the query and include the server cookie. However, servers may also send a complete response and apply different rate limits for server-cookie-less requests. Servers may also be more lenient responding to TCP requests that do not include the cookie.
- If the client included a server cookie, and the cookie is genuine, then the server will send a response.
For a badly formatted cookie, an error is returned (FORMERR). Requests that include an invalid server cookie are treated like requests that do not include a server cookie at all. This feature allows a client to recover if IP addresses changed, or if the server restarted and selected a new secret.
Cookies have been implemented in BIND 9.11. If you installed Ubuntu 18.04 LTS, you may have seen BIND use them. In BIND, cookies are enabled by default, but they are not enforced by default. Also, the “dig” tool now supports cookies with the +cookie option.
Here is a quick sample packet showing the cookie option in a DNS request. I haven’t found any of the large DNS providers supporting the option yet, but I haven’t tested them all.
To filter for these queries, you can use the Wireshark display filter “dns.opt.code == 10”. There is no great BPF expresion for it, but “tcpdump -r dns -n ‘udp[18:2]>0 && udp&0x80=0′” will show all queries with DNS options (some may just be the EDNS option 0).
2 – DNS over TLS
The second DNS innovation I see more and more is DNS over TLS. Unlike DNS cookies, DNS over TLS attempts to solve the privacy issue in DNS. It has found more followers after Cloudflare’s “188.8.131.52” DNS service started to support it. I set it up in my PFSense firewall and am including some sample packets below.
The protocol is pretty “straightforward”: Setup a TCP TLS connection, then send the DNS query across this TLS tunnel. The problem is that the TLS connection takes quite a bit of overhead to establish. But it can be reused for multiple queries to limit the overhead. In real life, I find the TLS connections to last only a very short time, so the overhead is substantial as far as the number of packets exchanged goes. Also, note that the TLS endpoint will be able to inspect all your queries. Cloudflare states that they will not use the data.
The tricky part with DNS over TLS is that it renders many enterprise systems blind that take advantage of DNS traffic. Your best bet is to block DNS over TLS (it uses port 853/TCP) and to require users use an internal recursive DNS server. You can then do all the logging you need on that DNS server, and you may even use DNS over TLS from the recursive DNS server to an entity like Cloudflare if you trust them more then you do trust your ISP. Based on my observation, DNS over TLS also doesn’t use the ALPN or the SNI options in TLS, which are used by more “regular” TLS connections like HTTPS.
Johannes B. Ullrich, Ph.D., Dean of Research, SANS Technology Institute
rfc 7873 at the bottom.
|Author: Mark Andrews|
DNS COOKIE is an Extended DNS (EDNS) option which, when both the client and server support it, allows the client to detect and ignore off-path spoofed responses, and the server to determine that a client’s address is not spoofed. It is supported as an experimental compile-time option in BIND9.10.3 and higher, under the name Server Identity Tokens (SIT), but it enabled by default in BIND 9.11.0.
The COOKIE option is defined in RFC7873,”Domain Name System (DNS) Cookies”.
Why would I want to use DNS Cookies?
Using cookies allows the client to do less work when sending queries, such as having to manage multiple sockets to get source port randomization, to prevent spoofed responses being received. DNS Cookie also allows a server to pick out legitimate repeat clients from a sea of spoofed-source queries, so long as the client supports the DNS Cookie option. Some of these features that can make use of DNS Cookies are still under development. They are also dependent on both client and server supporting the feature, however, support for DNS Cookies will increase along with the benefits available to those deploying them. COOKIE, like all EDNS options, is theoretically incrementally and independently deployable, so as to encourage early adoption.
How does BIND use DNS Cookies?
In BIND 9.11.0, we use DNS Cookies to white-list known clients for Response Rate Limiting (RRL). Client queries associated with valid Cookies will not be rate-limited. This makes rate limiting more effective in mitigating abusive traffic, because rate limiting is able to focus on only those client queries not associated with Cookies.
For DNS Cookies to be effective, both server and client need to support them
From BIND 9.11.0 DNS Cookie functionality is enabled by default and will be used by servers that deploy Response Rate Limiting. Administrators that upgrade their Authoritative servers to BIND 9.11.0 will have this feature automatically enabled and used with their RRL configuration. Administrators of Recursive servers that resolve queries on behalf of end user clients, when they upgrade to BIND 9.11.0, will be able to take advantage of DNS Cookies when communicating with Authoritative servers that also support their use.
What configuration options exist for DNS Cookies?
Please see the Administrator Reference Manuals (BIND 9.11.0 and newer) for more detail, but in summary:
What if my server supports DNS Cookies but other servers don’t?
DNS servers that are properly compliant with regards to EDNS options (RFC6891), even if they don’t support DNS Cookies, should simply ignore it. The use of DNS Cookies is established by negotiation between the client and the server; if the negotiation does not take place then DNS Cookies will not be used and DNS queries and query responses will be sent and received as they would be before DNS Cookies were available.
If I want to enable DNS Cookies on my server, is there anything else I need to do in my network infrastructure?
How do I generate a cookie secret string (instead of having it automatically generated at start-up)?
The typical use-case for this would be for all the servers in an anycast set-up to share the same server cookie. If you want to specify cookie-secret yourself instead of allowing the system to generate a random cookie at start-up then you first need to know how many bits of secret that you need – and this is determined by the cookie-algorithm, either default or specified explicitly.
The default for cookie-algorithm is:
If you explicitly set SHA1, then the cookie secret would need 160 bits.
The following command (as an example) will generate 128 bits that can be used as a cookie-secret with AES:
$ dd if=/dev/random bs=16 count=1 | od -x
This can then be used in named.conf like this:
The advantage of generating cookie-secret locally is that you might, for ease of system administration, want to script a process for updating it periodically.
Another method for generating a secret string would be to use a publicly available source of randomness such as random.org:
I don’t want to enable DNS Cookies on my servers; do I need to do anything?
We strongly advise anyone running Authoritative DNS services to check that they are providing RFC-compliant services.
You can test whether your servers fully support EDNS via: https://ednscomp.isc.org/
Because EDNS Cookie is being deployed by many DNS server and client code providers, your servers will increasingly be encountering this EDNS option in received queries. You should ensure that your servers and any load-balancing, proxying and firewalling devices that you operate are fully compliant with (RFC6891).
Failure to support RFC6891 will cause DNS resolution failures and delays
This warning is primarily aimed at those running Authoritative DNS services. Failure to respond appropriately to queries that contain EDNS options, whether or not they are options that your servers support, may mean that your customers are unable to reach your servers as a result of DNS resolution failures and delays.
COOKIE, like all EDNS options, is theoretically incrementally and independently deployable. Servers that don’t know about an EDNS option are supposed to ignore it (RFC6891). In practice, this is not always the case; about 10% of servers (as of June 2016) mishandle queries with unknown EDNS options in various ways. This is not normally fatal for DNS COOKIE; it just results in slightly slower lookups from these servers.
Nevertheless, mishandling of the COOKIE option has been known to cause errors that are fatal to name resolution when the resolver is validating responses coming from a signed zone, and the authoritative server returns either FORMERR or BADVERS, or fails to respond to the query. namedtreats these answers as if the server does not support EDNS (which it doesn’t) so it stops sending any EDNS queries at all, which makes it impossible to get a DNSSEC response back. When testing against servers for the Alexa Top 1 Million names only a handful of servers behaved in this manner with signed zones, and the owners of these servers have been notified of the issue.
Mishandling of the COOKIE option can also trigger incorrect responses (such as NXDOMAIN or no NOERROR/NODATA, when there should have been a positive answer). This is usually caused by a misconfigured load-balancer.
I’ve enabled DNS Cookies on my Recursive Server and now I’m experiencing problems with some domains – what can I do?
Unfortunately, not all DNS server implementations are correct (this is not limited to support for EDNS – there are many circumstances in which Authoritative Servers respond improperly).
How do I disable DNS Cookies on my server entirely?
We don’t recommend doing this, except in extremis. Most of the time, if you are running your servers correctly and professionally, the cause of your problems will lie elsewhere and should be addressed by other server administrators. However, it is possible to disable the use of DNS COOKIE on your server entirely if you really need too (we hope that this is a temporary measure only):
What tools are available for testing for and confirming DNS Cookies support?
You can test whether your servers fully support EDNS via: https://ednscomp.isc.org/
By default, (new) dig will send requests with a DNS COOKIE option present. You can disable this with dig +nocookie.
On receiving a BADCOOKIE response, dig will automatically retry with the correct cookie. You can disable this with with dig +nobadcookie.
You can see the DNS COOKIE values being used by named to talk to servers in the named_dump.db file in the “Address database dump” sections.
© 2001-2018 Internet Systems Consortium
For assistance with problems and questions for which you have not been able to find an answer in our Knowledge Base, we recommend searching our community mailing list archives and/or posting your question there (you will need to register there first for your posts to be accepted). The bind-users and the dhcp-users lists particularly have a long-standing and active membership.
ISC relies on the financial support of the community to fund the development of its open source software products. If you would like to support future product evolution and maintenance as well having peace of mind knowing that our team of experts are poised to provide you with individual technical assistance whenever you call upon them, then please consider our Professional Subscription Support services – details can be found on our main website.
The rfc 7873
Internet Engineering Task Force (IETF) D. Eastlake 3rd Request for Comments: 7873 Huawei Category: Standards Track M. Andrews ISSN: 2070-1721 ISC May 2016
Domain Name System (DNS) Cookies
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 (Network Address Translation - Protocol Translation), 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 Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7873. Eastlake & Andrews Standards Track [Page 1]
RFC 7873 DNS Cookies May 2016 Copyright Notice 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 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Eastlake & Andrews Standards Track [Page 2]
RFC 7873 DNS Cookies May 2016 Eastlake & Andrews Standards Track [Page 3]
RFC 7873 DNS Cookies May 2016
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 Cookie 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. Bypassing 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. Bypassing the weak protection provided by DNS Cookies requires such an attacker to guess a 64-bit pseudorandom "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 Cookie mechanism is designed to work in the presence of NAT and NAT-PT (Network Address Translation - Protocol Translation) boxes, and guidance is provided herein on supporting the DNS Cookie 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 option. Section 5 provides a protocol description. Eastlake & Andrews Standards Track [Page 4]
RFC 7873 DNS Cookies May 2016 Section 6 discusses some NAT considerations and anycast-related DNS Cookies design considerations. Section 7 discusses incremental deployment considerations. Sections 8 and 9 describe IANA considerations and security considerations, respectively.
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 that is learned or derived by a host and that may be discarded when indicated by the policies of that host but can be re-instantiated later if needed. For example, it could be discarded after a period of time or when storage for caching such data becomes full. If operations that require soft state continue after the information has been discarded, the information will be automatically regenerated, 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.
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. Eastlake & Andrews Standards Track [Page 5]
RFC 7873 DNS Cookies May 2016
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 the networks from which the requests originate or taking other actions for those networks.
2.1.1. DNS Amplification Attacks
A request with a forged source 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 Cookie mechanism can severely limit the traffic amplification obtained by requests from an attacker that is 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.
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; 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. Eastlake & Andrews Standards Track [Page 6]
RFC 7873 DNS Cookies May 2016 The computational or communications burden caused by such requests may not depend on a forged source IP 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. The 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 diversions 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.
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. Eastlake & Andrews Standards Track [Page 7]
RFC 7873 DNS Cookies May 2016
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 Cookie 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 authenticates complete transactions but does not authenticate requests. 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 Cookie mechanism: lightweight message authentication of DNS requests and responses with no requirement for pre-configuration or per-client server-side state.
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. Eastlake & Andrews Standards Track [Page 8]
RFC 7873 DNS Cookies May 2016 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 to such a request, the length is variable -- from 16 to 40 bytes, consisting of an 8-byte Client Cookie followed by the variable-length (8 bytes 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 convenience of implementation. 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OPTION-CODE = 10 | OPTION-LENGTH >= 16, <= 40 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+- Client Cookie (fixed size, 8 bytes) -+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | / Server Cookie (variable size, 8 to 32 bytes) / / / +-+-+-+-... Figure 2: COOKIE Option, Known Server Cookie Eastlake & Andrews Standards Track [Page 9]
RFC 7873 DNS Cookies May 2016
4.1. Client Cookie
The Client Cookie SHOULD be a pseudorandom 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 pseudorandom 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 to (1) track a client if the Client IP Address changes due to privacy mechanisms or (2) 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 pseudorandom 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 pseudorandom 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. Eastlake & Andrews Standards Track [Page 10]
RFC 7873 DNS Cookies May 2016
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 responses is covered in Sections 5.1, 5.2, and 5.3. A de facto extension to QUERY to allow the prefetching of a Server Cookie is specified in Section 5.4. Rollover of the Client Secrets and Server Secrets, and transient retention of the old cookie or secret, are 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 a Request
A DNS client that implements DNS Cookies includes one DNS COOKIE 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 a Request
The Server Cookie, when it occurs in a COOKIE 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, the presence of a COOKIE option is ignored and the server responds as if no COOKIE 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 an OPT RR but the COOKIE option is absent from the OPT RR. (2) A COOKIE option is present but is not a legal length or is otherwise malformed. Eastlake & Andrews Standards Track [Page 11]
RFC 7873 DNS Cookies May 2016 (3) There is a COOKIE option of valid length in the request with no Server Cookie. (4) There is a COOKIE option of valid length in the request with a Server Cookie, but that Server Cookie is invalid. (5) There is a COOKIE option of valid length in the request with a correct Server Cookie. These five possibilities are discussed in the subsections below. In all cases of multiple COOKIE 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 Option
If there is no OPT record or no COOKIE option present in the request, then the server responds to the request as if the server doesn't implement the COOKIE option.
5.2.2. Malformed COOKIE Option
If the COOKIE option is too short to contain a Client Cookie, then FORMERR is generated. If the COOKIE option is longer than that required to hold a COOKIE option with just a Client Cookie (8 bytes) but is shorter than the minimum COOKIE option with both a Client Cookie and a Server Cookie (16 bytes), then FORMERR is generated. If the COOKIE option is longer than the maximum valid COOKIE option (40 bytes), then 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. Eastlake & Andrews Standards Track [Page 12]
RFC 7873 DNS Cookies May 2016 If the server responds choosing (2) or (3) above, it SHALL generate its own COOKIE option containing both the Client Cookie copied from the request and a Server Cookie it has generated, and it will add this COOKIE option to the response's OPT record. 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 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 that it had generated previously. This determination normally involves recalculating 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 could be because + it is too old + a client's IP address or Client Cookie changed, and the DNS server is not aware of the change + an anycast cluster of servers is not consistently configured, or + an attempt to spoof the client has occurred 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. Eastlake & Andrews Standards Track [Page 13]
RFC 7873 DNS Cookies May 2016 The server SHALL process the request and include a COOKIE option in the response by (a) copying the complete COOKIE option from the request or (b) generating a new COOKIE option containing both the Client Cookie copied from the request and a valid Server Cookie it has generated.
5.3. Processing Responses
The Client Cookie, when it occurs in a COOKIE 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 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 option length or an incorrect Client Cookie value. If the client is expecting the response to contain a COOKIE option and it is missing, the response MUST be discarded. If the COOKIE 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 extended RCODE in the reply is BADCOOKIE and the Client Cookie in the reply 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 either the shared secrets or the method for generating secrets in an anycast cluster of servers is 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 (1) (see Section 4.1.2 of [RFC1035]). Eastlake & Andrews Standards Track [Page 14]
RFC 7873 DNS Cookies May 2016 For servers with DNS Cookies enabled, the QUERY opcode behavior is extended to support queries with an empty Question Section (a QDCOUNT of zero (0)), provided that an OPT record is present with a COOKIE option. Such servers will send a reply that has an empty Answer Section and has a COOKIE option containing the Client Cookie 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 that 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.
6. NAT Considerations and Anycast Server Considerations
In the classic Internet, DNS Cookies could simply be a pseudorandom 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] (including, for the purposes of this document, NAT-PT, 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 to which an internal client is being mapped change occasionally, the disruption is little more than when a client rolls over its COOKIE secret. Also, external access to a DNS server behind a NAT box is normally handled by a fixed mapping that forwards externally received DNS requests to a specific host. Eastlake & Andrews Standards Track [Page 15]
RFC 7873 DNS Cookies May 2016 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") behind a NAT box, that 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 option of requests to the server with the forged local IP address of some other host and/or client behind the NAT box. (An attacker's 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. This is why the Server Cookie is specified as a pseudorandom 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, for any particular server, a stable Client Cookie is needed. If, for example, the request ID was included in the calculation of the Client 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 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 that 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 in the way that the similar problem was solved for NATed clients; if the Server Cookie was included in the calculation of the Client Cookie in the same way that 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, either (1) the server clones must all use the same Server Secret or (2) the mechanism that distributes requests to the server clones must cause the requests from a particular client to go to a particular server for a sufficiently long period of time that Eastlake & Andrews Standards Track [Page 16]
RFC 7873 DNS Cookies May 2016 extra requests due to changes in Server Cookies 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).
7. Operational and Deployment Considerations
The DNS Cookie 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 that also implements the DNS Cookie mechanism, these servers and clients get the security benefits of the DNS Cookie mechanism.
7.1. Client and Server Secret Rollover
The longer a secret is used, the higher the probability that it has been compromised. Thus, clients and servers are configured with a lifetime setting for their secret, and they roll over 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, daylight saving 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 Eastlake & Andrews Standards Track [Page 17]
RFC 7873 DNS Cookies May 2016 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 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 than 1 second or more than 300 seconds, with a default configuration of 150 seconds. 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 an old Server Cookie that the server is treating as correct, the server MUST include a new Server Cookie in its response.
It is RECOMMENDED that implementations include counters of the occurrences of the various types of requests and responses described in Section 5.
8. IANA Considerations
IANA has assigned the following DNS EDNS0 option code: Value Name Status Reference -------- ------ -------- --------------- 10 COOKIE Standard RFC 7873 IANA has assigned the following DNS error code as an early allocation per [RFC7120]: RCODE Name Description Reference -------- --------- ------------------------- --------------- 23 BADCOOKIE Bad/missing Server Cookie RFC 7873 Eastlake & Andrews Standards Track [Page 18]
RFC 7873 DNS Cookies May 2016
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 plaintext 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, also called "Wi-Fi Protected Access 2") 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 that 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 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 that 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. Eastlake & Andrews Standards Track [Page 19]
RFC 7873 DNS Cookies May 2016
9.1. Cookie Algorithm Considerations
The cookie computation algorithm for use in DNS Cookies SHOULD be based on a pseudorandom 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 plaintext 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-SHA-256 [RFC6234] truncated to 64 bits, assuming that a DNS processor has adequate computational resources available. DNS implementations or applications that need 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.
10. Implementation Considerations
The DNS COOKIE option specified herein is implemented in BIND 9.10 using an experimental option code. BIND 9.10.3 (and later) use the allocated option code.
11.1. 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>. Eastlake & Andrews Standards Track [Page 20]
RFC 7873 DNS Cookies May 2016 [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>. [RFC7120] Cotton, M., "Early IANA Allocation of Standards Track Code Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January 2014, <http://www.rfc-editor.org/info/rfc7120>.
11.2. Informative References
[FNV] Fowler, G., Noll, L., Vo, K., and D. Eastlake 3rd, "The FNV Non-Cryptographic Hash Algorithm", Work in Progress, draft-eastlake-fnv-10, October 2015. [Kaminsky] Olney, M., Mullen, P., and K. Miklavcic, "Dan Kaminsky's 2008 DNS Vulnerability", 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>. [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>. Eastlake & Andrews Standards Track [Page 21]
RFC 7873 DNS Cookies May 2016 [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>. Eastlake & Andrews Standards Track [Page 22]
RFC 7873 DNS Cookies May 2016
Appendix A. Example Client Cookie Algorithms
A.1. A Simple Algorithm
A simple example method to compute Client Cookies is the FNV64 [FNV] of the Client IP Address, the Server IP Address, and the Client Secret: Client Cookie = FNV64( Client IP Address | Server IP Address | Client Secret ) where "|" indicates concatenation. Some computational resources may be saved by pre-computing FNV64 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 FNV64 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 Appendix A.1. Client Cookie = HMAC-SHA256-64( Client IP Address | Server IP Address, Client Secret )
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 FNV64 [FNV] of the request IP address, the Client Cookie, and the Server Secret. Server Cookie = FNV64( 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 FNV64 through the bytes of the Server Secret and Client Cookie, but this would reduce the strength of the Server Cookie and is NOT RECOMMENDED.) Eastlake & Andrews Standards Track [Page 23]
RFC 7873 DNS Cookies May 2016
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 pseudorandomly with the Server Secret as one of the inputs to the pseudorandom function. Types of additional information that could be stored include a timestamp and/or a nonce. The example below is one variation of 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 part that is hard to guess. In BIND 9.10.3 (and later), its computation can be configured to use AES, HMAC-SHA-1, or, as shown below, HMAC-SHA-256: hash = HMAC-SHA256-64( Server Secret, (Client Cookie | Nonce | Time | Client IP Address) ) where "|" represents concatenation. Eastlake & Andrews Standards Track [Page 24]
RFC 7873 DNS Cookies May 2016 Acknowledgments 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, and Peter Yee Authors' Addresses Donald E. Eastlake 3rd Huawei Technologies 155 Beaver Street Milford, MA 01757 United States Phone: +1-508-333-2270 Email: firstname.lastname@example.org Mark Andrews Internet Systems Consortium 950 Charter Street Redwood City, CA 94063 United States Email: email@example.com