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Internet Engineering Task Force (IETF) C. Contavalli Request for Comments: 7871 W. van der Gaast Category: Informational Google ISSN: 2070-1721 D. Lawrence

                                                   Akamai Technologies
                                                             W. Kumari
                                                              May 2016
                    Client Subnet in DNS Queries


 This document describes an Extension Mechanisms for DNS (EDNS0)
 option that is in active use to carry information about the network
 that originated a DNS query and the network for which the subsequent
 response can be cached.  Since it has some known operational and
 privacy shortcomings, a revision will be worked through the IETF for

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 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).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Contavalli, et al. Informational [Page 1] RFC 7871 Client Subnet in DNS Queries 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
 ( 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.

Contavalli, et al. Informational [Page 2] RFC 7871 Client Subnet in DNS Queries May 2016

Table of Contents

 1. Introduction ....................................................4
 2. Privacy Note ....................................................5
 3. Requirements Notation ...........................................5
 4. Terminology .....................................................6
 5. Overview ........................................................7
 6. Option Format ...................................................8
 7. Protocol Description ............................................9
    7.1. Originating the Option .....................................9
         7.1.1. Recursive Resolvers .................................9
         7.1.2. Stub Resolvers .....................................10
         7.1.3. Forwarding Resolvers ...............................11
    7.2. Generating a Response .....................................11
         7.2.1. Authoritative Nameserver ...........................11
         7.2.2. Intermediate Nameserver ............................13
    7.3. Handling ECS Responses and Caching ........................14
         7.3.1. Caching the Response ...............................15
         7.3.2. Answering from Cache ...............................16
    7.4. Delegations and Negative Answers ..........................17
    7.5. Transitivity ..............................................18
 8. IANA Considerations ............................................18
 9. DNSSEC Considerations ..........................................19
 10. NAT Considerations ............................................19
 11. Security Considerations .......................................20
    11.1. Privacy ..................................................20
    11.2. Birthday Attacks .........................................21
    11.3. Cache Pollution ..........................................22
 12. Sending the Option ............................................23
    12.1. Probing ..................................................23
    12.2. Whitelist ................................................24
 13. Example .......................................................24
 14. References ....................................................26
    14.1. Normative References .....................................26
    14.2. Informative References ...................................27
 Acknowledgements ..................................................28
 Contributors ......................................................29
 Authors' Addresses ................................................30

Contavalli, et al. Informational [Page 3] RFC 7871 Client Subnet in DNS Queries May 2016

1. Introduction

 Many Authoritative Nameservers today return different responses based
 on the perceived topological location of the user.  These servers use
 the IP address of the incoming query to identify that location.
 Since most queries come from Intermediate Recursive Resolvers, the
 source address is that of the Recursive Resolver rather than of the
 query originator.
 Traditionally, and probably still in the majority of instances,
 Recursive Resolvers are reasonably close in the topological sense to
 the Stub Resolvers or Forwarding Resolvers that are the source of
 queries.  For these resolvers, using their own IP address is
 sufficient for Authoritative Nameservers that tailor responses based
 upon location of the querier.
 Increasingly, though, a class of Recursive Resolvers has arisen that
 handles query sources that are often not topologically close.  The
 motivation for having such Centralized Resolvers varies but is
 usually because of some enhanced experience, such as greater cache
 security or applying policies regarding where users may connect.
 (Although political censorship usually comes to mind here, the same
 actions may be used by a parent when setting controls on where a
 minor may connect.)  Similarly, many ISPs and other organizations use
 a Centralized Resolver infrastructure that can be distant from the
 clients the resolvers serve.  These cases all lead to less than
 desirable responses from topology-sensitive Authoritative
 This document defines an EDNS0 [RFC6891] option to convey network
 information that is relevant to the DNS message.  It will carry
 sufficient network information about the originator for the
 Authoritative Nameserver to tailor responses.  It will also provide
 for the Authoritative Nameserver to indicate the scope of network
 addresses for which the tailored answer is intended.  This EDNS0
 option is intended for those Recursive Resolvers and Authoritative
 Nameservers that would benefit from the extension and not for general
 purpose deployment.  This is completely optional and can safely be
 ignored by servers that choose not to implement or enable it.
 This document also includes guidelines on how best to cache those
 results, and it provides recommendations on when this protocol
 extension should be used.
 At least a dozen different client and server implementations have
 been written based on earlier draft versions of this specification.
 The protocol is in active production use today.  While the

Contavalli, et al. Informational [Page 4] RFC 7871 Client Subnet in DNS Queries May 2016

 implementations interoperate, there is varying behavior around edge
 cases that were poorly specified.  Known incompatibilities are
 described in this document, and the authors believe that it is better
 to describe the system as it is working today, even if not everyone
 agrees with the details of the original specification
 ([VANDERGAAST]).  The alternative is an undocumented and proprietary
 A revised proposal to improve upon the minor flaws in this protocol
 will be forthcoming to the IETF.

2. Privacy Note

 If we were just beginning to design this mechanism, and not
 documenting existing protocol, it is unlikely that we would have done
 things exactly this way.
 The IETF is actively working on enhancing DNS privacy
 [DPRIVE_Working_Group] and the reinjection of metadata [METADATA] has
 been identified as a problematic design pattern.
 As noted above however, this document primarily describes existing
 behavior of a deployed method to further the understanding of the
 Internet community.
 We recommend that the feature be turned off by default in all
 nameserver software, and that operators only enable it explicitly in
 those circumstances where it provides a clear benefit for their
 clients.  We also encourage the deployment of means to allow users to
 make use of the opt-out provided.  Finally, we recommend that others
 avoid techniques that may introduce additional metadata in future
 work, as it may damage user trust.
 Regrettably, support for the opt-out provisions of this specification
 are currently limited.  Only one stub resolver, getdns, is known to
 be able to originate queries with anonymity requested, and as yet no
 applications are known to be able to indicate that user preference to
 the stub resolver.

3. Requirements Notation

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in [RFC2119].

Contavalli, et al. Informational [Page 5] RFC 7871 Client Subnet in DNS Queries May 2016

4. Terminology

 ECS:  EDNS Client Subnet.
 Client:  A Stub Resolver, Forwarding Resolver, or Recursive Resolver.
    A client to a Recursive Resolver or a Forwarding Resolver.
 Server:  A Forwarding Resolver, Recursive Resolver, or Authoritative
 Stub Resolver:  A simple DNS protocol implementation on the client
    side as described in [RFC1034], Section 5.3.1.  A client to a
    Recursive Resolver or a Forwarding Resolver.
 Authoritative Nameserver:  A nameserver that has authority over one
    or more DNS zones.  These are normally not contacted by Stub
    Resolver or end user clients directly but by Recursive Resolvers.
    Described in [RFC1035], Section 6.
 Recursive Resolver:  A nameserver that is responsible for resolving
    domain names for clients by following the domain's delegation
    chain.  Recursive Resolvers frequently use caches to be able to
    respond to client queries quickly.  Described in [RFC1035],
    Section 7.
 Forwarding Resolver:  A nameserver that does not do iterative
    resolution itself, but instead passes that responsibility to
    another Recursive Resolver, called a "Forwarder" in [RFC2308],
    Section 1.
 Intermediate Nameserver:  Any nameserver in between the Stub Resolver
    and the Authoritative Nameserver, such as a Recursive Resolver or
    a Forwarding Resolver.
 Centralized Resolvers:  Intermediate Nameservers that serve a
    topologically diverse network address space.
 Tailored Response:  A response from a nameserver that is customized
    for the node that sent the query, often based on performance
    (i.e., lowest latency, least number of hops, topological distance,
 Topologically Close:  Refers to two hosts being close in terms of the
    number of hops or the time it takes for a packet to travel from
    one host to the other.  The concept of topological distance is
    only loosely related to the concept of geographical distance: two

Contavalli, et al. Informational [Page 6] RFC 7871 Client Subnet in DNS Queries May 2016

    geographically close hosts can still be very distant from a
    topological perspective, and two geographically distant hosts can
    be quite close on the network.
 For a more comprehensive treatment of DNS terms, please see

5. Overview

 The general idea of this document is to provide an EDNS0 option to
 allow Recursive Resolvers, if they are willing, to forward details
 about the origin network from which a query is coming when talking to
 other nameservers.
 The format of the edns-client-subnet (ECS) EDNS0 option is described
 in Section 6 and is meant to be added in queries sent by Intermediate
 Nameservers in a way that is transparent to Stub Resolvers and end
 users, as described in Section 7.1.  ECS is only defined for the
 Internet (IN) DNS class.
 As described in Section 7.2, an Authoritative Nameserver could use
 ECS as a hint to the end user's network location and provide a better
 answer.  Its response would also contain an ECS option, clearly
 indicating that the server made use of this information, and that the
 answer is tied to the client's network.
 As described in Section 7.3, Intermediate Nameservers would use this
 information to cache the response.
 Some Intermediate Nameservers may also have to be able to forward ECS
 queries they receive, as described in Section 7.5.
 The mechanisms provided by ECS raise various security-related
 concerns related to cache growth, the ability to spoof EDNS0 options,
 and privacy.  Section 11 explores various mitigation techniques.
 The expectation, however, is that this option will primarily be used
 between Recursive Resolvers and Authoritative Nameservers that are
 sensitive to network location issues.  Most Recursive Resolvers,
 Authoritative Nameservers, and Stub Resolvers will never need to know
 about this option and will continue working as they had been.
 Failure to support this option or its improper handling will, at
 worst, cause suboptimal identification of client network location,
 which is a common occurrence in current Content Delivery Network
 (CDN) setups.

Contavalli, et al. Informational [Page 7] RFC 7871 Client Subnet in DNS Queries May 2016

 Section 7.1 also provides a mechanism for Stub Resolvers to signal
 Recursive Resolvers that they do not want ECS treatment for specific
 Additionally, operators of Intermediate Nameservers with ECS enabled
 are allowed to choose how many bits of the address of received
 queries to forward or to reduce the number of bits forwarded for
 queries already including an ECS option.

6. Option Format

 This protocol uses an EDNS0 [RFC6891] option to include client
 address information in DNS messages.  The option is structured as
              +0 (MSB)                            +1 (LSB)
 0: |                          OPTION-CODE                          |
 2: |                         OPTION-LENGTH                         |
 4: |                            FAMILY                             |
 8: |                           ADDRESS...                          /
 o  (Defined in [RFC6891]) OPTION-CODE, 2 octets, for ECS is 8 (0x00
 o  (Defined in [RFC6891]) OPTION-LENGTH, 2 octets, contains the
    length of the payload (everything after OPTION-LENGTH) in octets.
 o  FAMILY, 2 octets, indicates the family of the address contained in
    the option, using address family codes as assigned by IANA in
    Address Family Numbers [Address_Family_Numbers].
 The format of the address part depends on the value of FAMILY.  This
 document only defines the format for FAMILY 1 (IPv4) and FAMILY 2
 (IPv6), which are as follows:
 o  SOURCE PREFIX-LENGTH, an unsigned octet representing the leftmost
    number of significant bits of ADDRESS to be used for the lookup.
    In responses, it mirrors the same value as in the queries.

Contavalli, et al. Informational [Page 8] RFC 7871 Client Subnet in DNS Queries May 2016

 o  SCOPE PREFIX-LENGTH, an unsigned octet representing the leftmost
    number of significant bits of ADDRESS that the response covers.
    In queries, it MUST be set to 0.
 o  ADDRESS, variable number of octets, contains either an IPv4 or
    IPv6 address, depending on FAMILY, which MUST be truncated to the
    number of bits indicated by the SOURCE PREFIX-LENGTH field,
    padding with 0 bits to pad to the end of the last octet needed.
 o  A server receiving an ECS option that uses either too few or too
    many ADDRESS octets, or that has non-zero ADDRESS bits set beyond
    SOURCE PREFIX-LENGTH, SHOULD return FORMERR to reject the packet,
    as a signal to the software developer making the request to fix
    their implementation.
 All fields are in network byte order ("big-endian", per [RFC1700],
 Data Notation).

7. Protocol Description

7.1. Originating the Option

 The ECS option should generally be added by Recursive Resolvers when
 querying Authoritative Nameservers, as described in Section 12.  The
 option can also be initialized by a Stub Resolver or Forwarding

7.1.1. Recursive Resolvers

 The setup of the ECS option in a Recursive Resolver depends on the
 client query that triggered the resolution process.
 In the usual case, where no ECS option was present in the client
 query, the Recursive Resolver initializes the option by setting
 FAMILY of the client's address.  It then uses the value of its
 maximum cacheable prefix length to set SOURCE PREFIX-LENGTH.  For
 privacy reasons, and because the whole IP address is rarely required
 to determine a tailored response, this length SHOULD be shorter than
 the full address, as described in Section 11.
 If the triggering query included an ECS option itself, it MUST be
 examined for its SOURCE PREFIX-LENGTH.  The Recursive Resolver's
 outgoing query MUST then set SOURCE PREFIX-LENGTH to the shorter of
 the incoming query's SOURCE PREFIX-LENGTH or the server's maximum
 cacheable prefix length.

Contavalli, et al. Informational [Page 9] RFC 7871 Client Subnet in DNS Queries May 2016

 Finally, in both cases, SCOPE PREFIX-LENGTH is set to 0 and ADDRESS
 is then added up to SOURCE PREFIX-LENGTH number of bits, with
 trailing 0 bits added, if needed, to fill the final octet.  The total
 number of octets used MUST only be enough to cover SOURCE PREFIX-
 LENGTH bits, rather than the full width that would normally be used
 by addresses in FAMILY.
 FAMILY and ADDRESS information MAY be used from the ECS option in the
 incoming query.  Passing the existing address data is supportive of
 the Recursive Resolver being used as the target of a Forwarding
 Resolver, but could possibly run into policy problems with regard to
 usage agreements between the Recursive Resolver and Authoritative
 Nameserver.  See Section 12.2 for more discussion on this point.  If
 the Recursive Resolver will not forward FAMILY and ADDRESS data from
 the incoming ECS option, it SHOULD return a REFUSED response.
 Subsequent queries to refresh the data MUST, if unrestricted by an
 incoming SOURCE PREFIX-LENGTH, specify the longest SOURCE PREFIX-
 LENGTH that the Recursive Resolver is willing to cache, even if a
 previous response indicated that a shorter prefix length was

7.1.2. Stub Resolvers

 A Stub Resolver MAY generate DNS queries with an ECS option that sets
 SOURCE PREFIX-LENGTH to limit how network information should be
 revealed.  An Intermediate Nameserver that receives such a query MUST
 NOT make queries that include more bits of client address than in the
 originating query.
 A SOURCE PREFIX-LENGTH value of 0 means that the Recursive Resolver
 MUST NOT add the client's address information to its queries.  The
 subsequent Recursive Resolver query to the Authoritative Nameserver
 will then either not include an ECS option or MAY optionally include
 its own address information, which is what the Authoritative
 Nameserver will almost certainly use to generate any Tailored
 Response in lieu of an option.  This allows the answer to be handled
 by the same caching mechanism as other queries, with an explicit
 indicator of the applicable scope.  Subsequent Stub Resolver queries
 for /0 can then be answered from this cached response.
 A Stub Resolver MUST set SCOPE PREFIX-LENGTH to 0.  It MAY include
 FAMILY and ADDRESS data, but should be prepared to handle a REFUSED
 response if the Intermediate Nameserver that it queries has a policy
 that denies forwarding of ADDRESS.  If there is no ADDRESS set, i.e.,
 SOURCE PREFIX-LENGTH is set to 0, then FAMILY SHOULD be set to the
 transport over which the query is sent.  This is for

Contavalli, et al. Informational [Page 10] RFC 7871 Client Subnet in DNS Queries May 2016

 interoperability; at least one major authoritative server will ignore
 the option if FAMILY is not 1 or 2, even though it is irrelevant if
 there are no ADDRESS bits.

7.1.3. Forwarding Resolvers

 Forwarding Resolvers essentially appear to be Stub Resolvers to
 whatever Recursive Resolver is ultimately handling the query, but
 they look like a Recursive Resolver to their client.  A Forwarding
 Resolver using this option MUST prepare it as described in
 Section 7.1.1, "Recursive Resolvers".  In particular, a Forwarding
 Resolver that implements this protocol MUST honor SOURCE PREFIX-
 LENGTH restrictions indicated in the incoming query from its client.
 See also Section 7.5.
 Since the Recursive Resolver it contacts will treat the Forwarding
 Resolver like a Stub Resolver, the Recursive Resolver's policies
 regarding incoming ADDRESS information will apply in the same way.
 If the Forwarding Resolver receives a REFUSED response when it sends
 a query that includes a non-zero ADDRESS, it MUST retry with no

7.2. Generating a Response

7.2.1. Authoritative Nameserver

 When a query containing an ECS option is received, an Authoritative
 Nameserver supporting ECS MAY use the address information specified
 in the option to generate a tailored response.
 Authoritative Nameservers that have not implemented or enabled
 support for the ECS option ought to safely ignore it within incoming
 queries, per [RFC6891], Section 6.1.2.  Such a server MUST NOT
 include an ECS option within replies to indicate lack of support for
 it.  Implementers of Intermediate Nameservers should be aware,
 however, that some nameservers incorrectly echo back unknown EDNS0
 options.  In this protocol, that should be mostly harmless, as the
 SCOPE PREFIX-LENGTH should come back as 0, thus marking the response
 as covering all networks.
 A query with a wrongly formatted option (e.g., an unknown FAMILY)
 MUST be rejected and a FORMERR response MUST be returned to the
 sender, as described in [RFC6891], "Transport Considerations".
 An Authoritative Nameserver that implements this protocol and
 receives an ECS option MUST include an ECS option in its response to
 indicate that it SHOULD be cached accordingly, regardless of whether
 the client information was needed to formulate an answer.  (Note that

Contavalli, et al. Informational [Page 11] RFC 7871 Client Subnet in DNS Queries May 2016

 the requirement in [RFC6891] to reserve space for the OPT record
 could mean that the Answer section of the response will be truncated
 and fall back to TCP indicated accordingly.)  If an ECS option was
 not included in a query, one MUST NOT be included in the response
 even if the server is providing a Tailored Response -- presumably
 based on the address from which it received the query.
 those in the query.  Echoing back these values helps to mitigate
 certain attack vectors, as described in Section 11.
 SCOPE PREFIX-LENGTH in the response indicates the network for which
 the answer is intended.
 indicates that the provided prefix length was not specific enough to
 select the most appropriate Tailored Response.  Future queries for
 the name within the specified network SHOULD use the longer SCOPE
 PREFIX-LENGTH.  Factors affecting whether the Recursive Resolver
 would use the longer length include the amount of privacy masking the
 operator wants to provide their users, and the additional resource
 implications for the cache.
 Conversely, a shorter SCOPE PREFIX-LENGTH indicates that more bits
 than necessary were provided, and the answer is suitable for a
 broader range of addresses.  This could be as short as 0, to indicate
 that the answer is suitable for all addresses in FAMILY.
 As the logical topology of any part of the network with regard to the
 tailored response can vary, an Authoritative Nameserver may return
 different values of SCOPE PREFIX-LENGTH for different networks.
 Since some queries can result in multiple RRsets being added to the
 response, there is an unfortunate ambiguity from the original
 specification as to how SCOPE PREFIX-LENGTH would apply to each
 individual RRset.  For example, multiple types in response to an ANY
 metaquery could all have different applicable SCOPE PREFIX-LENGTH
 values, but this protocol only has the ability to signal one.  The
 response SHOULD therefore, include the longest relevant PREFIX-LENGTH
 of any RRset in the answer, which could have the unfortunate side
 effect of redundantly caching some data that could be cached more
 broadly.  For the specific case of a Canonical Name (CNAME) chain,
 the Authoritative Nameserver SHOULD only place the initial CNAME
 record in the Answer section, to have it cached unambiguously and
 appropriately.  Most modern Recursive Resolvers restart the query
 with the CNAME, so the remainder of the chain is typically ignored

Contavalli, et al. Informational [Page 12] RFC 7871 Client Subnet in DNS Queries May 2016

 anyway.  For message-focused resolvers, rather than RRset-focused
 ones, this will mean caching the entire CNAME chain at the longest
 PREFIX-LENGTH of any RRset in the chain.
 The specific logic that an Authoritative Nameserver uses to choose a
 tailored response is not in the scope of this document.  Implementers
 are encouraged, however, to carefully consider their selection of
 SCOPE PREFIX-LENGTH for the response in the event that the best
 tailored response cannot be determined, and what the implications
 would be over the life of the TTL.
 Authoritative Nameservers might have situations where one Tailored
 Response is appropriate for a relatively broad address range, such as
 an IPv4 /20, except for some exceptions, such as a few /24 ranges
 within that /20.  Because it can't be guaranteed that queries for all
 longer prefix lengths would arrive before one that would be answered
 by the shorter prefix length, an Authoritative Nameserver MUST NOT
 overlap prefixes.
 When the Authoritative Nameserver has a longer prefix length Tailored
 Response within a shorter prefix length Tailored Response, then
 implementations can either:
 1.  Deaggregate the shorter prefix response into multiple longer
     prefix responses, or
 2.  Alert the operator that the order of queries will determine which
     answers get cached, and either warn and continue or treat this as
     an error and refuse to load the configuration.
 This choice should be documented for the operator, for example, in
 the user manual.
 When deaggregating to correct the overlap, prefix lengths should be
 optimized to use the minimum necessary to cover the address space, in
 order to reduce the overhead that results from having multiple copies
 of the same answer.  As a trivial example, if the Tailored Response
 for 1.2.0/20 is A but there is one exception of 1.2.3/24 for B, then
 the Authoritative Nameserver would need to provide Tailored Responses
 for 1.2.0/23, 1.2.2/24, 1.2.4/22, and 1.2.8/21 all pointing to A, and
 1.2.3/24 to B.

7.2.2. Intermediate Nameserver

 When an Intermediate Nameserver uses ECS, whether it passes an ECS
 option in its own response to its client is predicated on whether the
 client originally included the option.  Because a client that did not
 use an ECS option might not be able to understand it, the server MUST

Contavalli, et al. Informational [Page 13] RFC 7871 Client Subnet in DNS Queries May 2016

 NOT provide one in its response.  If the client query did include the
 option, the server MUST include one in its response, especially as it
 could be talking to a Forwarding Resolver, which would need the
 information for its own caching.
 If an Intermediate Nameserver receives a response that has a longer
 SCOPE PREFIX-LENGTH than SOURCE PREFIX-LENGTH that it provided in its
 query, it SHOULD still provide the result as the answer to the
 triggering client request even if the client is in a different
 address range.  The Intermediate Nameserver MAY instead opt to retry
 with a longer SOURCE PREFIX-LENGTH to get a better reply before
 responding to its client, as long as it does not exceed a SOURCE
 PREFIX-LENGTH specified in the query that triggered resolution, but
 this obviously has implications for the latency of the overall
 The logic for using the cache to determine whether the Intermediate
 Nameserver already knows the response to provide to its client is
 covered in the next section.

7.3. Handling ECS Responses and Caching

 When an Intermediate Nameserver receives a response containing an ECS
 option and without the TC bit set, it SHOULD cache the result based
 on the data in the option.  If the TC bit was set, the Intermediate
 Resolver SHOULD retry the query over TCP to get the complete Answer
 section for caching.
 ADDRESS in the response don't match the non-zero fields in the
 corresponding query, the full response MUST be dropped, as described
 in Section 11.  In a response to a query that specified only SOURCE
 PREFIX-LENGTH for privacy masking, the FAMILY and ADDRESS fields MUST
 contain the appropriate non-zero information that the Authoritative
 Nameserver used to generate the answer, so that it can be cached
 If no ECS option is contained in the response, the Intermediate
 Nameserver SHOULD treat this as being equivalent to having received a
 SCOPE PREFIX-LENGTH of 0, which is an answer suitable for all client
 addresses.  See further discussion on the security implications of
 this in Section 11.
 If a REFUSED response is received from an Authoritative Nameserver,
 an ECS-aware resolver MUST retry the query without ECS to distinguish
 the response from one where the Authoritative Nameserver is not
 responsible for the name, which is a common convention for the
 REFUSED status.  Similarly, a client of a Recursive Resolver SHOULD

Contavalli, et al. Informational [Page 14] RFC 7871 Client Subnet in DNS Queries May 2016

 retry after receiving a REFUSED response because it is not
 sufficiently clear whether the REFUSED response was because of the
 ECS option or some other reason.

7.3.1. Caching the Response

 In the cache, all resource records in the Answer section MUST be tied
 to the network specified in the response.  The appropriate prefix
 length depends on the relationship between SOURCE PREFIX-LENGTH,
 SCOPE PREFIX-LENGTH, and the maximum cacheable prefix length
 configured for the cache.
 SCOPE PREFIX-LENGTH bits of ADDRESS, and then mark the response as
 valid for all addresses that fall within that range.
 Similarly, if SOURCE PREFIX-LENGTH is the maximum configured for the
 cache, store SOURCE PREFIX-LENGTH bits of ADDRESS, and then mark the
 response as valid for all addresses that fall within that range.
 If SOURCE PREFIX-LENGTH is shorter than the configured maximum and
 PREFIX-LENGTH bits of ADDRESS, and then mark the response as valid
 only to answer client queries that specify exactly the same SOURCE
 PREFIX-LENGTH in their own ECS option.
 The handling of DNSSEC-related records in the Answer section was
 unspecified in the original draft version of this document and is
 inconsistently handled in existing implementations.  A Resource
 Record Signature (RRSIG) must obviously be tied to the RRset that it
 signs, but it is RECOMMENDED that all other DNSSEC records be scoped
 at /0.  See Section 9 for more information.
 Note that the Additional and Authority sections from a DNS response
 message are specifically excluded here.  Any records from these
 sections MUST NOT be tied to a network.  See Section 7.4 for more
 Records that are cached as /0 because of a query's SOURCE PREFIX-
 LENGTH of 0 MUST be distinguished from those that are cached as /0
 because of a response's SCOPE PREFIX-LENGTH of 0.  The former should
 only be used for other /0 queries that the Intermediate Resolver
 receives, but the latter is suitable as a response for all networks.

Contavalli, et al. Informational [Page 15] RFC 7871 Client Subnet in DNS Queries May 2016

 Although omitting network-specific caching will significantly
 simplify an implementation, the resulting drop in cache hits is very
 likely to defeat most latency benefits provided by ECS.  Therefore,
 implementing full caching support as described in this section is
 strongly RECOMMENDED.
 Enabling support for ECS in an Intermediate Nameserver will
 significantly increase the size of the cache, reduce the number of
 results that can be served from cache, and increase the load on the
 server.  Implementing the mitigation techniques described in
 Section 11 is strongly recommended.  For cache size issues,
 implementers should consider data storage formats that allow the same
 answer data to be shared among multiple prefixes.

7.3.2. Answering from Cache

 Cache lookups are first done as usual for a DNS query, using the
 query tuple of <name, type, class>.  Then, the appropriate RRset MUST
 be chosen based on the longest prefix matching.  The client address
 to use for comparison will depend on whether the Intermediate
 Nameserver received an ECS option in its client query.
 o  If no ECS option was provided, the client's address is used.
 o  If there was an ECS option specifying SOURCE PREFIX-LENGTH and
    ADDRESS covering the client's address, the client address is used
    but SOURCE PREFIX-LENGTH is initially ignored.  If no covering
    entry is found and SOURCE PREFIX-LENGTH is shorter than the
    configured maximum length allowed for the cache, repeat the cache
    lookup for an entry that exactly matches SOURCE PREFIX-LENGTH.
    These special entries, which do not cover longer prefix lengths,
    occur as described in the previous section.
 o  If there was an ECS option with an ADDRESS, the ADDRESS from it
    MAY be used if the local policy allows.  The policy can vary
    depending on the agreements the operator of the Intermediate
    Nameserver has with Authoritative Nameserver operators; see
    Section 12.2.  If the policy does not allow it, a REFUSED response
    SHOULD be sent.  See Section 7.5 for more information.
 If a matching network is found and the relevant data is unexpired,
 the response is generated as per Section 7.2.
 If no matching network is found, the Intermediate Nameserver MUST
 perform resolution as usual.  This is necessary to avoid Tailored
 Responses in the cache from being returned to the wrong clients, and

Contavalli, et al. Informational [Page 16] RFC 7871 Client Subnet in DNS Queries May 2016

 to avoid a single query coming from a client on a different network
 from polluting the cache with a Tailored Response for all the users
 of that resolver.

7.4. Delegations and Negative Answers

 The prohibition against tying ECS data to records from the Authority
 and Additional sections left an unfortunate ambiguity in the original
 specification, primarily with regard to negative answers.  The
 expectation of the original authors was that ECS would only really be
 used for address requests and the positive result in the response's
 Answer section, which was the use case that was driving the
 definition of the protocol.
 For negative answers, some independent implementations of both
 resolvers and authorities did not see the section restriction as
 necessarily meaning that a given name and type must only have either
 positive ECS-tagged answers or a negative answer.  They support being
 able to tell one part of the network that the data does not exist,
 while telling another part of the network that it does.
 Several other implementations, however, do not support being able to
 mix positive and negative answers; thus, interoperability is a
 problem.  It is RECOMMENDED that no specific behavior regarding
 negative answers be relied upon, but that Authoritative Nameservers
 should conservatively expect that Intermediate Nameservers will treat
 all negative answers as /0; therefore, they SHOULD set SCOPE PREFIX-
 LENGTH accordingly.
 This issue is expected to be revisited in a future revision of the
 protocol, possibly blessing the mixing of positive and negative
 answers.  There are implications for cache data structures that
 developers should consider when writing new ECS code.
 The delegations case is a bit easier to tease out.  In operational
 practice, if an authoritative server is using address information to
 provide customized delegations, it is the resolver that will be using
 the answer for its next iterative query.  Addresses in the Additional
 section SHOULD therefore ignore ECS data, and the Authoritative
 Nameserver SHOULD return a zero SCOPE PREFIX-LENGTH on delegations.
 A Recursive Resolver SHOULD treat a non-zero SCOPE PREFIX LENGTH in a
 delegation as though it were zero.

Contavalli, et al. Informational [Page 17] RFC 7871 Client Subnet in DNS Queries May 2016

7.5. Transitivity

 Generally, ECS options will only be present in DNS messages between a
 Recursive Resolver and an Authoritative Nameserver, i.e., one hop.
 However, in certain configurations, for example, multi-tier
 nameserver setups, it may be necessary to implement transitive
 behavior on Intermediate Nameservers.
 Any Intermediate Nameserver that forwards ECS options received from
 its clients MUST fully implement the caching behavior described in
 Section 7.3.
 An Intermediate Nameserver MAY forward ECS options with address
 information.  This information MAY match the source IP address of the
 incoming query, and MAY have more or fewer address bits than the
 nameserver would normally include in a locally originated ECS option.
 If an Intermediate Nameserver receives a query with SOURCE PREFIX-
 LENGTH set to 0, it MUST NOT include client address information in
 queries made to resolve that client's request (see Section 7.1.2).
 If, for any reason, the Intermediate Nameserver does not want to use
 the information in an ECS option it receives (too little address
 information, network address from a range not authorized to use the
 server, private/unroutable address space, etc.), it SHOULD drop the
 query and return a REFUSED response.  Note again that a query MUST
 NOT be refused solely because it provides 0 address bits.
 Be aware that at least one major existing implementation does not
 return REFUSED and instead just processes the query as though the
 problematic information were not present.  This can lead to anomalous
 situations, such as a response from the Intermediate Nameserver that
 indicates it is tailored for one network (the one passed in the
 original query, since the ADDRESS must match) when actually it is for
 another network (the one which contains the address that the
 Intermediate Nameserver saw as making the query).

8. IANA Considerations

 IANA has assigned option code 8 in the "DNS EDNS0 Option Codes (OPT)"
 registry to edns-client-subnet.
 IANA has updated the reference to refer to this RFC.

Contavalli, et al. Informational [Page 18] RFC 7871 Client Subnet in DNS Queries May 2016

9. DNSSEC Considerations

 The presence or absence of an EDNS0 OPT resource record ([RFC6891])
 containing an ECS option in a DNS query does not change the usage of
 the resource records and mechanisms used to provide data origin
 authentication and data integrity to the DNS, as described in
 [RFC4033], [RFC4034], and [RFC4035].  OPT records are not signed.
 Use of this option, however, does imply increased DNS traffic between
 any given Recursive Resolver and Authoritative Nameserver, which
 could be another barrier to further DNSSEC adoption in this area.
 The initial version of this protocol, against which several
 Authoritative and Recursive Nameserver implementations were written,
 did not discuss the handling of DNSSEC RRs; thus, it is expected that
 there are operational inconsistencies in handling them.
 Given the intention of this document to describe how ECS is currently
 deployed, specifying new requirements for DNSSEC handling is out of
 scope.  However, some recommendations can be made as to what is most
 likely to result in successful interoperation for a DNSSEC-signed ECS
 zone, mainly from the point of view of Authoritative Nameservers.
 Most DNSSEC records SHOULD be scoped at /0, except for the RRSIG
 records, which MUST be tied to the RRset that they sign in a Tailored
 Response.  While it is possible to conceive of a way to get other
 DNSSEC records working in a network-specific way, it has little
 apparent benefit or likelihood of working with deployed validating
 One further implication here is that, despite the discussion about
 negative answers in Section 7.4, scoping NextSECure (NSEC) or NSEC3
 records at /0 per the previous paragraph necessarily implies that
 DNSSEC-signed negative answers must also be network-invariant.

10. NAT Considerations

 Special awareness of ECS in devices that perform Network Address
 Translation (NAT) as described in [RFC2663] is not required; queries
 can be passed through as is.  The client's network address SHOULD NOT
 be added, and existing ECS options, if present, SHOULD NOT be
 modified by NAT devices.
 In large-scale global networks behind a NAT device (but, for example
 with Centralized Resolver infrastructure), an internal Intermediate
 Nameserver might have detailed network layout information, and may

Contavalli, et al. Informational [Page 19] RFC 7871 Client Subnet in DNS Queries May 2016

 know which external subnets are used for egress traffic by each
 internal network.  In such cases, the Intermediate Nameserver MAY use
 that information when originating ECS options.
 In other cases, if a Recursive Resolver knows that it is situated
 behind a NAT device, it SHOULD NOT originate ECS options with their
 external IP address and instead rely on downstream Intermediate
 Nameservers to do so.  It MAY, however, choose to include the option
 with their internal address for the purposes of signaling its own
 Full treatment of special network addresses is beyond the scope of
 this document; handling them will likely differ according to the
 operational environments of each service provider.  As a general
 guideline, if an Authoritative Nameserver on the publicly routed
 Internet receives a query that specifies an ADDRESS in [RFC1918] or
 [RFC4193] private address space, it SHOULD ignore ADDRESS and look up
 its answer based on the address of the Recursive Resolver.  In the
 response, it SHOULD set SCOPE PREFIX-LENGTH to cover all of the
 relevant private space.  For example, a query for ADDRESS
 with a SOURCE PREFIX-LENGTH of 24 would get a returned SCOPE PREFIX-
 LENGTH of 8.  The Intermediate Nameserver MAY elect to cache the
 answer under one entry for special-purpose addresses [RFC6890]; see
 Section 11.3 of this document.

11. Security Considerations

11.1. Privacy

 With the ECS option, the network address of the client that initiated
 the resolution becomes visible to all servers involved in the
 resolution process.  Additionally, it will be visible from any
 network traversed by the DNS packets.
 To protect users' privacy, Recursive Resolvers are strongly
 encouraged to conceal part of the user's IP address by truncating
 IPv4 addresses to 24 bits. 56 bits are recommended for IPv6, based on
 ISPs should have more detailed knowledge of their own networks.  That
 is, they might know that all 24-bit prefixes in a /20 are in the same
 area.  In those cases, for optimal cache utilization and improved
 privacy, the ISP's Recursive Resolver SHOULD truncate IP addresses in
 this /20 to just 20 bits, instead of 24 as recommended above.
 Users who wish their full IP address to be hidden need to configure
 their client software, if possible, to include an ECS option
 specifying the wildcard address (i.e., a SOURCE PREFIX-LENGTH of 0).

Contavalli, et al. Informational [Page 20] RFC 7871 Client Subnet in DNS Queries May 2016

 As described in previous sections, this option will be forwarded
 across all the Recursive Resolvers supporting ECS, which MUST NOT
 modify it to include the network address of the client.
 Note that even without an ECS option, any server queried directly by
 the user will be able to see the full client IP address.  Recursive
 Resolvers or Authoritative Nameservers MAY use the source IP address
 of queries to return a cached entry or to generate a Tailored
 Response that best matches the query.

11.2. Birthday Attacks

 ECS adds information to the DNS query tuple (q-tuple).  This allows
 an attacker to send a caching Intermediate Nameserver multiple
 queries with spoofed IP addresses either in the ECS option or as the
 source IP.  These queries will trigger multiple outgoing queries with
 the same name, type, and class, just with different address
 information in the ECS option.
 With multiple queries for the same name in flight, the attacker has a
 higher chance of success to send a matching response with SCOPE
 PREFIX-LENGTH set to 0 to get it cached for all hosts.
 To counter this, the ECS option in a response packet MUST contain the
 full FAMILY, ADDRESS, and SOURCE PREFIX-LENGTH fields from the
 corresponding query.  Intermediate Nameservers processing a response
 MUST verify that these match, and they SHOULD discard the entire
 response if they do not.
 The requirement to discard is categorized as "SHOULD" instead of
 "MUST" because it stands in opposition to the instruction in
 Section 7.3, which states that a response lacking an ECS option
 should be treated as though it had one of SCOPE PREFIX-LENGTH of 0.
 If that is always true, then an attacker does not need to worry about
 matching the original ECS option data and just needs to flood back
 responses that have no ECS option at all.
 This type of attack could be detected in ongoing operations by
 marking whether the responding nameserver had previously been sending
 ECS options and/or by taking note of an incoming flood of bogus
 responses and flagging the relevant query for re-resolution.  This
 type of detection is more complex than existing nameserver responses
 to spoof floods, and it would also need to be sensitive to a
 nameserver legitimately stopping ECS replies even though it had
 previously given them.

Contavalli, et al. Informational [Page 21] RFC 7871 Client Subnet in DNS Queries May 2016

11.3. Cache Pollution

 It is simple for an arbitrary resolver or client to provide false
 information in the ECS option, or to send UDP packets with forged
 source IP addresses.
 This could be used to:
 o  pollute the cache of Intermediate Resolvers by filling it with
    results that will rarely (if ever) be used.
 o  reverse-engineer the algorithms (or data) used by the
    Authoritative Nameserver to calculate Tailored Responses.
 o  mount a denial-of-service attack against an Intermediate
    Nameserver by forcing it to perform many more recursive queries
    than it would normally do, due to how caching is handled for
    queries containing the ECS option.
 Even without malicious intent, Centralized Resolvers providing
 answers to clients in multiple networks will need to cache different
 responses for different networks, putting more memory pressure on the
 To mitigate those problems:
 o  Recursive Resolvers implementing ECS should only enable it in
    deployments where it is expected to bring clear advantages to the
    end users, such as when expecting clients from a variety of
    networks or from a wide geographical area.  Due to the high cache
    pressure introduced by ECS, the feature SHOULD be disabled in all
    default configurations.
 o  Recursive Resolvers SHOULD limit the number of networks and
    answers they keep in the cache for any given query.
 o  Recursive Resolvers SHOULD limit the total number of different
    networks that they keep in cache.
 o  Recursive Resolvers MUST NOT send an ECS option with SOURCE
    PREFIX-LENGTH providing more bits in ADDRESS than they are willing
    to cache responses for.
 o  Recursive Resolvers should implement algorithms to improve the
    cache hit rate, given the size constraints indicated above.
    Recursive Resolvers MAY, for example, decide to discard more-
    specific cache entries first.

Contavalli, et al. Informational [Page 22] RFC 7871 Client Subnet in DNS Queries May 2016

 o  Authoritative Nameservers and Recursive Resolvers should discard
    ECS options that are either obviously forged or otherwise known to
    be wrong.  They SHOULD at least treat unroutable addresses, such
    as some of the address blocks defined in [RFC6890], as equivalent
    to the Recursive Resolver's own identity.  They SHOULD ignore and
    never forward ECS options specifying other routable addresses that
    are known not to be served by the query source.
 o  The ECS option is just a hint to Authoritative Nameservers for
    customizing results.  They can decide to ignore the content of the
    ECS option based on blacklists or whitelists, rate-limiting
    mechanisms, or any other logic implemented in the software.

12. Sending the Option

 When implementing a Recursive Resolver, there are two strategies on
 deciding when to include an ECS option in a query.  At this stage,
 it's not clear which strategy is best.

12.1. Probing

 A Recursive Resolver can send the ECS option with every outgoing
 query.  However, it is RECOMMENDED that resolvers remember which
 Authoritative Nameservers did not return the option with their
 response and omit client address information from subsequent queries
 to those nameservers.
 Additionally, Recursive Resolvers SHOULD be configured never to send
 the option when querying root, top-level, and effective top-level
 (i.e., "public suffix" [Public_Suffix_List]) domain servers.  These
 domains are delegation-centric and are very unlikely to generate
 different responses based on the address of the client.
 When probing, it is important that several things are probed: support
 for ECS, support for EDNS0, support for EDNS0 options, or possibly an
 unreachable nameserver.  Various implementations are known to drop
 DNS packets with OPT RRs (with or without options), thus several
 probes are required to discover what is supported.
 Probing, if implemented, MUST be repeated periodically, e.g., daily.
 If an Authoritative Nameserver indicates ECS support for one zone, it
 is to be expected that the nameserver supports ECS for all of its
 zones.  Likewise, an Authoritative Nameserver that uses ECS
 information for one of its zones MUST indicate support for the option
 in all of its responses to ECS queries.  If the option is supported
 but not actually used for generating a response, its SCOPE PREFIX-
 LENGTH MUST be set to 0.

Contavalli, et al. Informational [Page 23] RFC 7871 Client Subnet in DNS Queries May 2016

12.2. Whitelist

 As described previously, it is expected that only a few Recursive
 Resolvers will need to use ECS, and that it will generally be enabled
 only if it offers a clear benefit to the users.
 To avoid the complexity of implementing a probing and detection
 mechanism (and the possible query loss/delay that may come with it),
 an implementation could use a whitelist of Authoritative Nameservers
 to send the option to, likely specified by their domain name.
 Implementations MAY also allow additional configuring of this based
 on other criteria, such as zone or query type.  As of the time of
 this writing, at least one implementation makes use of a whitelist.
 An advantage of using a whitelist is that partial client address
 information is only disclosed to nameservers that are known to use
 the information, improving privacy.
 A drawback is scalability.  The operator needs to track which
 Authoritative Nameservers support ECS, making it harder for new
 Authoritative Nameservers to start using the option.
 Similarly, Authoritative Nameservers can also use whitelists to limit
 the feature to only certain clients.  For example, a CDN that does
 not want all of their mapping trivially walked might require a legal
 agreement with the Recursive Resolver operator, to clearly describe
 the acceptable use of the feature.
 The maintenance of access control mechanisms is out of scope for this
 protocol definition.

13. Example

 1.   A Stub Resolver, SR, with the IP address
      2001:0db8:fd13:4231:2112:8a2e:c37b:7334 tries to resolve by forwarding the query to the Recursive
      Resolver, RNS, asking for recursion.
 2.   RNS, supporting ECS, looks up in its cache.  An
      entry is found neither for nor for
 3.   RNS builds a query to send to the root and .com servers.  The
      implementation of RNS provides facilities so that an
      administrator can configure it not to forward ECS in certain
      cases.  In particular, RNS is configured not to include an ECS
      option when talking to Top-Level-Domain or root nameservers, as
      described in Section 7.1.  Thus, no ECS option is added, and
      resolution is performed as usual.

Contavalli, et al. Informational [Page 24] RFC 7871 Client Subnet in DNS Queries May 2016

 4.   RNS now knows the next server to query: the Authoritative
      Nameserver, ANS, responsible for
 5.   RNS prepares a new query for, including an ECS
      option with:
  • OPTION-CODE set to 8.
  • OPTION-LENGTH set to 0x00 0x0b for the following fixed 4

octets plus the 7 octets that will be used for ADDRESS.

  • FAMILY set to 0x00 0x02, as IP is an IPv6 address.
  • SOURCE PREFIX-LENGTH set to 0x38, as RNS is configured to

conceal the last 72 bits of every IPv6 address.

  • SCOPE PREFIX-LENGTH set to 0x00, as specified by this

document for all queries.

  • ADDRESS set to 0x20 0x01 0x0d 0xb8 0xfd 0x13 0x42, providing

only the first 56 bits of the IPv6 address.

 6.   The query is sent.  ANS understands and uses ECS.  It parses the
      ECS option, and generates a Tailored Response.
 7.   Due its internal implementation, ANS finds a response that is
      tailored for the whole /16 of the client that performed the
 8.   ANS adds an ECS option in the response, containing:
  • OPTION-CODE set to 8.
  • OPTION-LENGTH set to 0x00 0x07.
  • FAMILY set to 0x00 0x02.
  • SOURCE PREFIX-LENGTH set to 0x38, copied from the query.
  • SCOPE PREFIX-LENGTH set to 0x30, indicating a /48 network.
  • ADDRESS set to 0x20 0x01 0x0d 0xb8 0xfd 0x13 0x42, copied

from the query.

 9.   RNS receives the response containing an ECS option.  It verifies
      that FAMILY, SOURCE PREFIX-LENGTH, and ADDRESS match the query.
      If not, the message is discarded.

Contavalli, et al. Informational [Page 25] RFC 7871 Client Subnet in DNS Queries May 2016

 10.  The response is interpreted as usual.  Since the response
      contains an ECS option, ADDRESS, SCOPE PREFIX-LENGTH, and FAMILY
      in the response are used to cache the entry.
 11.  RNS sends a response to Stub Resolver, SR, without including an
      ECS option.
 12.  RNS receives another query to resolve  This
      time, a response is cached.  The response, however, is tied to a
      particular network.  If the client's address matches any network
      in the cache, then the response is returned from the cache.
      Otherwise, another query is performed.  If multiple results
      match, the one with the longest SCOPE PREFIX-LENGTH is chosen,
      as per common best-network-match algorithms.

14. References

14.1. Normative References

 [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
            STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
 [RFC1035]  Mockapetris, P., "Domain Names - Implementation and
            Specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
            November 1987, <>.
 [RFC1700]  Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,
            DOI 10.17487/RFC1700, October 1994,
 [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
            and E. Lear, "Address Allocation for Private Internets",
            BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
 [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,

Contavalli, et al. Informational [Page 26] RFC 7871 Client Subnet in DNS Queries May 2016

 [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,
 [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,
 [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
            Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
 [RFC6177]  Narten, T., Huston, G., and L. Roberts, "IPv6 Address
            Assignment to End Sites", BCP 157, RFC 6177,
            DOI 10.17487/RFC6177, March 2011,
 [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
            "Special-Purpose IP Address Registries", BCP 153,
            RFC 6890, DOI 10.17487/RFC6890, April 2013,
 [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
            for DNS (EDNS(0))", STD 75, RFC 6891,
            DOI 10.17487/RFC6891, April 2013,

14.2. Informative References

            IANA, "Address Family Numbers",
            IETF, "PNS PRIVate Exchange (dprive) DPRIVE Working
            Group", 2015,
            Hardie, T., Ed., "Design considerations for Metadata
            Insertion", Work in Progress, draft-hardie-privsec-
            metadata-insertion-02, March 2016.
            "Public Suffix List", <>.

Contavalli, et al. Informational [Page 27] RFC 7871 Client Subnet in DNS Queries May 2016

 [RFC2308]  Andrews, M., "Negative Caching of DNS Queries (DNS
            NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
 [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
            Translator (NAT) Terminology and Considerations",
            RFC 2663, DOI 10.17487/RFC2663, August 1999,
 [RFC7719]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
            Terminology", RFC 7719, DOI 10.17487/RFC7719, December
            2015, <>.
            Contavalli, C., Gaast, W., Leach, S., and E. Lewis,
            "Client Subnet in DNS Requests", Work in Progress,
            draft-vandergaast-edns-client-subnet-02, July 2013.


 The authors wish to thank Darryl Rodden for his work as a co-author,
 and the following people for reviewing this document and for
 providing useful feedback: Paul S. R. Chisholm, B. Narendran,
 Leonidas Kontothanassis, David Presotto, Philip Rowlands, Chris
 Morrow, Kara Moscoe, Alex Nizhner, Warren Kumari, and Richard Rabbat
 from Google; Terry Farmer, Mark Teodoro, Edward Lewis, and Eric
 Burger from Neustar; David Ulevitch and Matthew Dempsky from OpenDNS;
 Patrick W. Gilmore and Steve Hill from Akamai; Colm MacCarthaigh and
 Richard Sheehan from Amazon; Tatuya Jinmei from Infoblox; Andrew
 Sullivan from Dyn; John Dickinson from Sinodun; Mark Delany from
 Apple; Yuri Schaeffer from NLnet Labs; Duane Wessels Verisign;
 Antonio Querubin; Daniel Kahn Gillmor from the ACLU; Evan Hunt and
 Mukund Sivaraman from the Internet Software Consortium; Russ Housley
 from Vigilsec; Stephen Farrell from Trinity College Dublin; Alissa
 Cooper from Cisco; Suzanne Woolf; and all of the other people that
 replied to our emails on various mailing lists.

Contavalli, et al. Informational [Page 28] RFC 7871 Client Subnet in DNS Queries May 2016


 The individuals below contributed significantly to this document.
 Edward Lewis
 12025 Waterfront Drive, Suite 300
 Los Angeles, CA 90094-2536
 United States
 Sean Leach
 P.O. Box 78266
 San Francisco, CA 94107
 United States
 Jason Moreau
 Akamai Technologies
 150 Broadway
 Cambridge, MA 02142-1413
 United States

Contavalli, et al. Informational [Page 29] RFC 7871 Client Subnet in DNS Queries May 2016

Authors' Addresses

 Carlo Contavalli
 1600 Amphitheater Parkway
 Mountain View, CA  94043
 United States
 Wilmer van der Gaast
 Belgrave House, 76 Buckingham Palace Road
 London  SW1W 9TQ
 United Kingdom
 David C Lawrence
 Akamai Technologies
 150 Broadway
 Cambridge, MA  02142-1054
 United States
 Warren Kumari
 1600 Amphitheatre Parkway
 Mountain View, CA  94043
 United States

Contavalli, et al. Informational [Page 30]

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