dnsext                                                     C. Contavalli
Internet-Draft                                          W. van der Gaast
Intended status: Experimental                                     Google
Expires: January 5, 2014                                        S. Leach
                                                                E. Lewis
                                                            July 4, 2013

                     Client Subnet in DNS Requests


   This draft defines an EDNS0 extension to carry information about the
   network that originated a DNS query, and the network for which the
   subsequent reply can be cached.

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   This document describes an experimental EDNS0 option.  The purpose of
   this experiment is to discover if the information carried in an edns-
   client-subnet option is sufficiently helpful to Authoritative
   Nameservers to give better Optimized Replies, and to measure the
   latency improvement from this better reply, taking into account a
   likely lower cache hit rate at (and generally higher load on) the
   Recursive Resolver, and possibly other operational aspects.  Details
   on how this experiment will be carried out can be found in
   Section 12.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 5, 2014.

Copyright Notice

   Copyright (c) 2013 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Notation  . . . . . . . . . . . . . . . . . .  5
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Option Format  . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Protocol Description . . . . . . . . . . . . . . . . . . . . . 11
     5.1.  Originating the Option . . . . . . . . . . . . . . . . . . 11
     5.2.  Generating a Response  . . . . . . . . . . . . . . . . . . 11
     5.3.  Handling edns-client-subnet Replies and Caching  . . . . . 12
     5.4.  Transitivity . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   7.  DNSSEC Considerations  . . . . . . . . . . . . . . . . . . . . 17
   8.  NAT Considerations . . . . . . . . . . . . . . . . . . . . . . 18
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
     9.1.  Privacy  . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.2.  Birthday Attacks . . . . . . . . . . . . . . . . . . . . . 19
     9.3.  Cache Pollution  . . . . . . . . . . . . . . . . . . . . . 20
   10. Sending the Option . . . . . . . . . . . . . . . . . . . . . . 22
     10.1. Probing  . . . . . . . . . . . . . . . . . . . . . . . . . 22
     10.2. Whitelist  . . . . . . . . . . . . . . . . . . . . . . . . 22
   11. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   12. Experiment Details . . . . . . . . . . . . . . . . . . . . . . 26
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   Appendix A.   Document Editing History . . . . . . . . . . . . . . 28
   Appendix A.1. -02  . . . . . . . . . . . . . . . . . . . . . . . . 28
   Appendix A.2. -00  . . . . . . . . . . . . . . . . . . . . . . . . 28
   Appendix A.3. -01  . . . . . . . . . . . . . . . . . . . . . . . . 29
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 30
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 30
     14.2. Informative References . . . . . . . . . . . . . . . . . . 30
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32

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1.  Introduction

   Many Authoritative Nameservers today return different replies 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 forwarders that are the source of queries.  For
   these resolvers, using their own IP address is sufficient for
   authority servers that tailor responses based upon location of the

   Increasingly though a class of Recursive Resolvers has arisen that
   serves query sources without regard to topology.  The motivation for
   a query source to use such a Third-party Resolver 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.)  When using a Third-party Resolver, there can no
   longer be any assumption of close proximity between the originator
   and the recursive resolver, leading to less than optimal replies from
   the authority servers.

   A similar situation exists within some ISPs where the Recursive
   Resolvers are topologically distant from some edges of the ISP
   network, resulting in less than optimal replies from the authority

   This draft defines an EDNS0 option to convey network information that
   is relevant to the message but not otherwise included in the
   datagram.  This will provide the mechanism to carry sufficient
   network information about the originator for the authority server to
   tailor responses.  It also provides for the authority server to
   indicate the scope of network addresses that the tailored answer is
   intended.  This EDNS0 option is intended for those recursive and
   authority servers that would benefit from the extension and not for
   general purpose deployment.  It is completely optional and can safely
   be ignored by servers that choose not to implement it or enable it.

   This draft also includes guidelines on how to best cache those
   results and provides recommendations on when this protocol extension
   should be used.

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1.1.  Requirements Notation

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

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2.  Terminology

   Stub Resolver:  A simple DNS protocol implementation on the client
      side as described in [RFC1034] section 5.3.1.

   Authoritative Nameserver:  A nameserver that has authority over one
      or more DNS zones.  These are normally not contacted by clients
      directly but by Recursive Resolvers.  Described in [RFC1035]
      chapter 6.

   Recursive Resolver:  A nameserver that is responsible for resolving
      domain names for clients by following the domain's delegation
      chain, starting at the root.  Recursive Resolvers frequently use
      caches to be able to respond to client queries quickly.  Described
      in [RFC1035] chapter 7.

   Intermediate Nameserver:  Any nameserver (possibly a Recursive
      Resolver) in between the Stub Resolver and the Authoritative

   Third-party Resolvers:  Recursive Resolvers provided by parties that
      are not Internet Service Providers (ISPs).  These services are
      often offered as substitutes for ISP-run nameservers.

   Optimized reply:  A reply from a nameserver that is optimized for the
      node that sent the request, normally based on performance (i.e.
      lowest latency, least number of hops, topological distance, ...).

   Topologically close:  Refers to two hosts being close in terms of
      number of hops or 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
      geographically close hosts can still be very distant from a
      topological perspective.

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3.  Overview

   The general idea of this document is to provide an EDNS0 option so
   that Recursive Resolvers can, if they are willing to, forward details
   about the network a query is coming from when talking to other

   The format of this option is described in Section 4, and is meant to
   be added in queries sent by Intermediate Nameservers in a way
   transparent to Stub Resolvers and end users, as described in
   Section 5.1.

   As described in Section 5.2, an Authoritative Nameserver could use
   this EDNS0 option as a hint to better locate the network of the end
   user, and provide a better answer.

   Its reply would contain an EDNS0 client-subnet option, clearly
   indicating that (1) the server made use of this information and (2)
   the answer is tied to the network of the client.

   As described in Section 5.3, Intermediate Nameservers would use this
   information to cache the reply.

   Some Intermediate Nameservers may also have to be able to forward
   edns-client-subnet queries they receive.  This is described in
   Section 5.4.

   The mechanisms provided by edns-client-subnet raise various security
   related concerns, related to cache growth, the ability to spoof EDNS0
   options, and privacy.  Section 9 explores various mitigation

   The expectation, however, is that this option will only be enabled
   (and used) by Recursive Resolvers and Authoritative Nameserver that
   incur geolocation issues.

   Most Recursive Resolvers, Authoritative Nameservers and Stub Resolver
   will never know about this option, and keep working as usual.

   Failure to support this option or its improper handling will at worst
   cause sub-optimal geolocation, which is a pretty common occurrence in
   current CDN setups and not a cause of concern.

   Section 5.1 also provides a mechanism for Stub Resolvers to signal
   Recursive Resolvers that they do not want an edns-client-subnet with
   their network to be added.

   Additionally, owners of resolvers with edns-client-subnet enabled are

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   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 edns-client-subnet option.

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4.  Option Format

   This draft uses an EDNS0 ([RFC2671]) option to include client IP
   information in DNS messages.  The option is structured as follows:

                +0 (MSB)                            +1 (LSB)
   0: |                          OPTION-CODE                          |
   2: |                         OPTION-LENGTH                         |
   4: |                            FAMILY                             |
   6: |          SOURCE NETMASK       |        SCOPE NETMASK          |
   7: |                           ADDRESS...                          /

   o  (Defined in [RFC2671]) OPTION-CODE, 2 octets, for edns-client-
      subnet is 8.

   o  (Defined in [RFC2671]) OPTION-LENGTH, 2 octets, contains the
      length of the payload (everything after OPTION-LENGTH) in bytes.

   o  FAMILY, 2 octets, indicates the family of the address contained in
      the option, using address family codes as assigned by IANA in
      IANA-AFI [1].

   The format of the address part depends on the value of FAMILY.  This
   document only defines the format for FAMILY 1 (IP version 4) and 2
   (IP version 6), which are as follows:

   o  SOURCE NETMASK, unsigned byte representing the length of the
      netmask pertaining to the query.  In replies, it mirrors the same
      value as in the requests.

   o  SCOPE NETMASK, unsigned byte representing the length of the
      netmask pertaining to the reply.  In requests, it MUST be set to
      0.  In responses, this may or may not match SOURCE NETMASK.

   o  ADDRESS, variable number of octets, contains either an IPv4 or
      IPv6 address (depending on FAMILY), truncated to the number of
      bits indicated by the SOURCE NETMASK field, with bits set to 0 to
      pad up to the end of the last octet used.

   All fields are in network byte order.  Throughout the document, we
   will often refer to "longer" or "shorter" netmasks, corresponding to
   netmasks that have a "higher" or "lower" value when represented as

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5.  Protocol Description

5.1.  Originating the Option

   The edns-client-subnet option should generally be added by Recursive
   Resolvers when querying other servers, as described in Section 10.

   In this option, the server should include the IP of the client that
   caused the query to be generated, truncated to a number of bits
   specified in the SOURCE NETMASK field.

   The IP of the client can generally be determined by looking at the
   source IP indicated in the IP header of the request.

   A Stub Resolver MAY generate DNS queries with an edns-client-subnet
   option with SOURCE NETMASK set to 0 (i.e. to indicate that
   the Recursive Resolver MUST NOT add address information of the client
   to its queries.  The Stub Resolver may also add non-empty edns-
   client-subnet options to its queries, but Recursive Resolvers are not
   required to accept/use this information.

   For privacy reasons, and because the whole IP address is rarely
   required to determine an optimized reply, the ADDRESS field in the
   option SHOULD be truncated to a certain number of bits, chosen by the
   administrators of the server, as described in Section 9.

5.2.  Generating a Response

   When a query containing an edns-client-subnet option is received, an
   Authoritative Nameserver supporting edns-client-subnet MAY use the
   address information specified in the option in order to generate an
   optimized reply.

   Authoritative servers that have not implemented or enabled support
   for the edns-client-subnet may safely ignore the option within
   incoming queries.  Such a server MUST NOT include an edns-client-
   subnet option within replies, to indicate lack of support for the

   Requests with wrongly formatted options (i.e. wrong size) MUST be
   rejected and a FORMERR response must be returned to the sender, as
   described by [RFC2671], Transport Considerations.

   If the Authoritative Nameserver decides to use information from the
   edns-client-subnet option to calculate a response, it MUST include
   the option in the response to indicate that the information was used
   (and has to be cached accordingly).  If the option was not included
   in a query, it MUST NOT be included in the response.

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   The FAMILY, ADDRESS and SOURCE NETMASK in the response MUST match
   those in the request.  Echoing back the address and netmask helps to
   mitigate certain attack vectors, as described in Section 9.

   The SCOPE NETMASK in the reply indicates the netmask of the network
   that the answer is intended for.

   A SCOPE NETMASK value larger than the SOURCE NETMASK indicates that
   the address and netmask provided in the query was not specific enough
   to select a single, best response, and that an optimal reply would
   require at least SCOPE NETMASK bits of address information.

   Conversely, a shorter SCOPE NETMASK indicates that more bits than
   necessary were provided.

   As not all netblocks are the same size, an Authoritative Nameserver
   may return different values of SCOPE NETMASK for different networks.

   In both cases, the value of the SCOPE NETMASK in the reply has strong
   implications with regard to how the reply will be cached by
   Intermediate Nameservers, as described in Section 5.3.

   If the edns-client-subnet option in the request is not used at all
   (for example if an optimized reply was temporarily unavailable or not
   supported for the requested domain name), a server supporting edns-
   client-subnet MUST indicate that no bits of the ADDRESS in the
   request have been used by specifying a SCOPE NETMASK of 0 (equivalent
   to the networks or ::/0).

   If no optimized answer could be found at all for the FAMILY, ADDRESS
   and SOURCE NETMASK indicated in the query, the Authoritative
   Nameserver SHOULD still return the best result it knows of (i.e. by
   using the query source IP address instead, or a sensible default),
   and indicate that this result should only be cached for the FAMILY,
   ADDRESS and SOURCE NETMASK indicated in the request.  The server will
   indicate this by copying the SOURCE NETMASK into the SCOPE NETMASK

5.3.  Handling edns-client-subnet Replies and Caching

   When an Intermediate Nameserver receives a reply containing an edns-
   client-subnet option, it will return a reply to its client and may
   cache the result.

   If the FAMILY, ADDRESS and SOURCE NETMASK fields in the reply don't
   match the fields in the corresponding request, the full reply MUST be
   dropped, as described in Section 9.

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   In the cache, any resource record in the answer section will be tied
   to the network specified by the FAMILY, ADDRESS and SCOPE NETMASK
   fields, as detailed below.  Note that the additional and authority
   sections from a DNS response message are specifically excluded here.

   If another query is received matching the entry in the cache, the
   resolver will verify that the FAMILY and ADDRESS that represent the
   client match any of the networks in the cache for that entry.

   If the address of the client is within any of the networks in the
   cache, then the cached response MUST be returned as usual.  In case
   the address of the client matches multiple networks in the cache, the
   entry with the highest SCOPE NETMASK value MUST be returned, as with
   most route-matching algorithms.

   If the address of the client does not match any network in the cache,
   then the Recursive Resolver MUST behave as if no match was found and
   perform resolution as usual.  This is necessary to avoid sub-optimal
   replies in the cache from being returned to the wrong clients, and to
   avoid a single request coming from a client on a different network
   from polluting the cache with a sub-optimal reply for all the users
   of that resolver.

   Note that every time a Recursive Resolver queries an Authoritative
   Nameserver by forwarding the edns-client-subnet option that it
   received from another client, a low SOURCE NETMASK in the original
   request could cause a sub-optimal reply to be returned by the
   Authoritative Nameserver.

   To avoid this sub-optimal reply from being served from cache for
   clients for which a better reply would be available, the Recursive
   Resolver MUST check the SCOPE NETMASK that was returned by the
   Authoritative Nameserver:

   o  If the SCOPE NETMASK in the reply is longer than the SOURCE
      NETMASK, it means that the reply might be sub-optimal.  A
      Recursive Resolver MUST return this entry from cache only to
      queries that do not contain or allow a longer SOURCE NETMASK to be

   o  If the SCOPE NETMASK in the reply is shorter or equal to the
      SOURCE NETMASK, the reply is optimal, and SHOULD be returned from
      cache to any client within the network indicated by ADDRESS and

   When another request is performed, the existing entries SHOULD be
   kept in the cache until their TTL expires, as per standard behavior.

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   As another reply is received, the reply will be tied to a different
   network.  The server SHOULD keep in cache both replies, and return
   the most appropriate one depending on the address of the client.

   Although omitting this behaviour will significantly simplify an
   implementation, the resulting drop in cache hits is very likely to
   defeat most latency benefits provided by edns-client-subnet.
   Therefore, when implementing this option for latency purposes,
   implementing full caching support as described in this section is

   Any reply containing an edns-client-subnet option considered invalid
   should be treated as if no edns-client-subnet option was specified at

   Replies coming from servers not supporting edns-client-subnet or
   otherwise not containing an edns-client-subnet option SHOULD be
   considered as containing a SCOPE NETMASK of 0 (e.g., cache the result
   for or ::/0) for all the supported families.

   In any case, the response from the resolver to the client MUST NOT
   contain the edns-client-subnet option if none was present in the
   client's original request.  If the original client request contained
   a valid edns-client-subnet option that was used during recursion, the
   Recursive Resolver MUST include the edns-client-subnet option from
   the Authoritative Nameserver response in the response to the client.

   Enabling support for edns-client-subnet in a recursive resolver 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 9 is strongly recommended.

5.4.  Transitivity

   Generally, edns-client-subnet options will only be present in DNS
   messages between a Recursive Resolver and an Authoritative
   Nameserver, i.e. one hop.  In certain configurations however (for
   example multi-tier nameserver setups), it may be necessary to
   implement transitive behaviour on Intermediate Nameservers.

   It is important that any Intermediate Nameserver that implements
   transitive behaviour (i.e. forward edns-client-subnet options
   received from their clients) MUST fully implement the caching
   behaviour described in Section 5.3.

   Intermediate Nameservers (including Recursive Resolvers) supporting
   edns-client-subnet MUST forward options with SOURCE NETMASK set to 0

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   (i.e. anonymized), such an option MUST NOT be replaced with an option
   with more accurate address information.

   An Intermediate Nameserver MAY also forward edns-client-subnet
   options with actual address information.  This information MAY match
   the source IP address of the incoming query, and MAY have more or
   less address bits than the Nameserver would normally include in a
   locally originated edns-client-subnet option.

   If for any reason the Intermediate Nameserver does not want to use
   the information in an edns-client-subnet option it receives (too
   little address information, network address from an IP range not
   authorized to use the server, private/unroutable address space, ...)
   it SHOULD drop the query and return a REFUSED response.  Note again
   that an edns-client-subnet option with 0 address bits MUST NOT be

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6.  IANA Considerations

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

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7.  DNSSEC Considerations

   The presence or absence of an OPT resource record containing an edns-
   client-subnet option in a DNS query does not change the usage of
   those resource records and mechanisms used to provide data origin
   authentication and data integrity to the DNS, as described in
   [RFC4033], [RFC4034] and [RFC4035].

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8.  NAT Considerations

   Special awareness of edns-client-subnet in devices that perform 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 edns-client-subnet options, if present, SHOULD NOT be
   modified by NAT devices.

   In large-scale global networks behind NAT (but for example with
   centralized DNS infrastructure), an internal Intermediate Nameserver
   may have detailed network layout information, and may 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 edns-client-subnet options.

   In other cases, Recursive Resolvers sited behind NAT SHOULD NOT
   originate edns-client-subnet options with their external IP address,
   and instead rely on downstream Intermediate Nameservers doing so.

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9.  Security Considerations

9.1.  Privacy

   With the edns-client-subnet 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 IP address of the user by
   truncating IPv4 addresses to 24 bits.  No recommendation is provided
   for IPv6 at this time, but IPv6 addresses should be similarly
   truncated in order to not allow to uniquely identify the client.

   ISPs will often have more detailed knowledge of their own networks.
   I.e. they will know if 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 can include an
   edns-client-subnet option specifying the wildcard address
   (i.e.  FAMILY set to 1 (IPv4), SOURCE NETMASK to 0 and no ADDRESS).
   As described in previous sections, this option will be forwarded
   across all the Recursive Resolvers supporting edns-client-subnet,
   which MUST NOT modify it to include the network address of the

   Note that even without edns-client-subnet options, 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 requests to return a cached entry or to generate an
   optimized reply that best matches the request.

9.2.  Birthday Attacks

   edns-client-subnet adds information to the q-tuple.  This allows an
   attacker to send a caching Intermediate Nameserver multiple queries
   with spoofed IP addresses either in the edns-client-subnet option or
   as the source IP.  These queries will trigger multiple outgoing
   queries with the same name, type and class, just different address
   information in the edns-client-subnet option.

   With multiple queries for the same name in flight, the attacker has a
   higher chance of success in sending a matching response (with the
   address to still get it cached for many hosts).

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   To counter this, every edns-client-subnet option in a response packet
   MUST contain the full FAMILY, ADDRESS and SOURCE NETMASK fields from
   the corresponding request.  Intermediate Nameservers processing a
   response MUST verify that these match, and MUST discard the entire
   reply if they do not.

9.3.  Cache Pollution

   It is simple for an arbitrary resolver or client to provide false
   information in the edns-client-subnet 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 caclulate the optimized answer.

   o  mount a DoS attack against an intermediate resolver, by forcing it
      to perform many more recursive queries than it would normally do,
      due to how caching is handled for queries containing the edns-
      client-subnet option.

   Even without malicious intent, Third-party Resolvers providing
   answers to clients in multiple networks will need to cache different
   replies for different networks, putting more pressure on the cache.

   To mitigate those problems:

   o  Recursive Resolvers implementing edns-client-subnet should only
      enable it in deployments where it is expected to bring clear
      advantages to the end users.  For example, when expecting clients
      from a variety of networks or from a wide geographical area.  Due
      to the high cache pressure introduced by edns-client-subnet, the
      feature must be disabled in all default configurations.

   o  Recursive Resolvers should limit the number of networks and
      answers they keep in the cache for a given query.

   o  Recursive Resolvers should limit the number of total different
      networks that they keep in cache.

   o  Recursive Resolvers should never send edns-client-subnet options
      with SOURCE NETMASKs providing more bits in the ADDRESS than they
      are willing to cache responses for.

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   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.

   o  Authoritative Nameservers and Recursive Resolvers should discard
      known to be wrong or known to be forged edns-client-subnet
      options.  They must at least ignore unroutable addresses, such as
      some of the address blocks defined in [RFC5735] and [RFC4193], and
      should ignore and never forward edns-client-subnet options
      specifying networks or addresses that are known not to be served
      by those servers when feasible.

   o  Authoritative Nameservers consider the edns-client-subnet option
      just as a hint to provide better results.  They can decide to
      ignore the content of the edns-client-subnet option based on black
      or white lists, rate limiting mechanisms, or any other logic
      implemented in the software.

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10.  Sending the Option

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

10.1.  Probing

   A Recursive Resolver can send the edns-client-subnet 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 MAY be configured to never send the
   option when querying root and TLD servers, as these are unlikely to
   generate different replies based on the IP of the client.

   When probing, it is important that several things are probed: support
   for edns-client-subnet, 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 (i.e. daily).
   If an Authoritative Nameserver indicates edns-client-subnet support
   for one zone, it is to be expected that the Nameserver supports edns-
   client-subnet for all its zones.  Likewise, an Authoritative
   Nameserver that uses edns-client-subnet information for one of its
   zones, MUST indicate support for the option in all its responses.  If
   the option is supported but not actually used for generating a
   response, its SCOPE NETMASK value SHOULD be set to 0.

10.2.  Whitelist

   As described previously, it is expected that only a few Recursive
   Resolvers will need to use edns-client-subnet, 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 decide to use a statically configured
   whitelist of Authoritative Namesevers to send the option to.
   Implementations MAY also allow additionally configuring this based on
   other criteria (i.e. zone, qtype).

   An additional advantage of using a whitelist is that partial client
   address information is only disclosed to Nameservers that are known

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   to use the information, improving privacy.

   A major drawback is scalability.  The operator needs to track which
   Nameservers support edns-client-subnet, making it harder for new
   Authoritative Nameservers to start using the option.

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11.  Example

   1.   A stub resolver SR with IP address tries to resolve
        www.example.com, by forwarding the query to the Recursive
        Resolver R from IP address IP, asking for recursion.

   2.   R, supporting edns-client-subnet, looks up www.example.com in
        its cache.  An entry is found neither for www.example.com, nor
        for example.com.

   3.   R builds a query to send to the root and .com servers.  The
        implementation of R provides facilities so an administrator can
        configure R not to forward edns-client-subnet in certain cases.
        In particular, R is configured to not include an edns-client-
        subnet option when talking to TLD or root nameservers, as
        described in Section 5.1.  Thus, no edns-client-subnet option is
        added, and resolution is performed as usual.

   4.   R now knows the next server to query: Authoritative Nameserver
        ANS, responsible for example.com.

   5.   R prepares a new query for www.example.com, including an edns-
        client-subnet option with:

        *  OPTION-CODE, set to 8.

        *  OPTION-LENGTH, set to 0x00 0x07.

        *  FAMILY, set to 0x00 0x01 as IP is an IPv4 address.

        *  SOURCE NETMASK, set to 0x18, as R is configured to conceal
           the last 8 bits of every IPv4 address.

        *  SCOPE NETMASK, set to 0x00, as specified by this document for
           all requests.

        *  ADDRESS, set to 0xC0 0x00 0x02, providing only the first 24
           bits of the IPv4 address.

   6.   The query is sent.  Server ANS understands and uses edns-client-
        subnet.  It parses the edns-client-subnet option, and generates
        an optimized reply.

   7.   Due to the internal implementation of the Authoritative
        Nameserver ANS, ANS finds a reply that is optimal for the whole
        /16 of the client that performed the request.

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   8.   The Authoritative Nameserver ANS adds an edns-client-subnet
        option in the reply, containing:

        *  OPTION-CODE, set to 8.

        *  OPTION-LENGTH, set to 0x00 0x07.

        *  FAMILY, set to 0x00 0x01.

        *  SOURCE NETMASK, set to 0x18, copied from the request.

        *  SCOPE NETMASK, set to 0x10, indicating a /16 network.

        *  ADDRESS, set to 0xC0 0x00 0x02, copied from the request.

   9.   The Recursive Resolver R receives the reply containing an edns-
        client-subnet option.  The resolver verifies that FAMILY, SOURCE
        NETMASK, and ADDRESS match the request.  If not, the option is

   10.  The reply is interpreted as usual.  Since the reply contains an
        edns-client-subnet option, the ADDRESS, SCOPE NETMASK, and
        FAMILY in the response are used to cache the entry.

   11.  R sends a response to stub resolver SR, without including an
        edns-client-subnet option.

   12.  R receives another request to resolve www.example.com.  This
        time, a reply is cached.  The reply, however, is tied to a
        particular network.  If the address of the client matches any
        network in the cache, then the reply is returned from the cache.
        Otherwise, another query is performed.  If multiple results
        match, the one with the longest SCOPE NETMASK is chosen, as per
        common best-network match algorithms.

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12.  Experiment Details

   This document describes an experiment to be conducted on the
   Internet.  Participation requires the careful eye on the proposed
   EDNS0 option.  What is described in this document may need to be
   altered or adjusted as experience dictates.

   During the experiment, participants will enable edns-client-subnet
   support on their Nameservers (both Intermediate Nameservers and
   Authoritative Nameservers).

   Intermediate Nameservers will be configured with whitelists so edns-
   client-subnet options will be sent only to Authoritative Nameservers
   participating in the experiment.  Some participants might also choose
   to experiment with the probing behaviour described in Section 10.1.

   Authoritative Nameservers could be configured with a whitelist but
   maintainers are strongly encouraged to accept edns-client-subnet
   options from anywhere to encourage openness. ns1-ns4.google.com are
   configured as such and can be used to test client/resolver

   Pending official code point allocation (as described in Section 6), a
   temporary EDNS0 option code will be in use.  This code is not
   guaranteed to be used in further stages of this effort - assuming the
   are further stages.

   The effects of edns-client-subnet will be measured using data of 24-
   hour periods with and without the option enabled on a Recursive
   Resolver.  Metrics to be compared include:

   o  Cache hit rate on the Recursive Resolver.

   o  Average response time on the Recursive Resolver.

   o  Increase in query rate on both Recursive Resolvers and
      Authoritative Nameservers.

   o  Most importantly, difference in load times as measured by user
      agents (especially web browsers).

   Success of the experiment will depend on the level of improvement in
   the last mentioned metric.  Others are for operational reasons (i.e.
   does one need to provision for extra Nameservers?).

   Possible side-effects from using edns-client-subnet will also be
   investigated.  These may include interoperability problems within the
   DNS and decreased (or possibly increased) ease of troubleshooting.

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13.  Acknowledgements

   The authors wish to thank Darryl Rodden for his work as a co-author
   on previous versions, and the following people for reviewing early
   drafts of 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, Richard Rabbat from Google, Terry Farmer, Mark Teodoro,
   Edward Lewis, Eric Burger from Neustar, David Ulevitch, Matthew
   Dempsky from OpenDNS, Patrick W. Gilmore from Akamai, Colm
   MacCarthaigh, Richard Sheehan and all the other people that replied
   to our emails on various mailing lists.

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Appendix A.  Document Editing History

   [This section should be removed by the RFC editor before publishing]

Appendix A.1.  -02

   o  Added IANA-assigned option code.

Appendix A.2.  -00

   o  Document moved to experimental track, added experiment description
      in header with details in a new section.

   o  Specifically note that edns-client-subnet applies to the answer
      section only.

   o  Warn that caching based on edns-client-subnet is optional but very
      important for performance reasons.

   o  Updated NAT section.

   o  Added recommendation to not use the default /24 recommendation for
      the source netmask field if more detailed information about the
      network is available.

   o  Rewritten problem statement to be more clear about the goal of
      edns-client-subnet and the fact that it's entirely optional.

   o  Wire format changed to include the original address and netmask in
      responses in defence against birthday attacks.

   o  Security considerations now includes a section about birthday

   o  Renamed edns-client-ip in edns-client-subnet, following
      suggestions on the mailing list.

   o  Clarified behavior of resolvers when presented with an invalid
      edns-client-subnet option.

   o  Fully take multi-tier DNS setups in mind and be more clear about
      where the option should be originated.

   o  Added a few definitions in the Terminology section, and a few more
      aesthetic changes in the rest of the document.

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Appendix A.3.  -01

   o  Document version number reset from -02 to -00 due to the rename to

   o  Clarified example (dealing with TLDs, and various minor errors).

   o  Referencing RFC5035 instead of RFC1918.

   o  Added a section on probing (and how it should be done) vs.

   o  Moved description on how to forward edns-client-subnet option in
      dedicated section.

   o  Queries with wrongly formatted edns-client-subnet options should
      now be rejected with FORMERR.

   o  Added an "Overview" section, providing an introduction to the

   o  Intermediate Nameservers can now remove an edns-client-subnet
      option, or reduce the SOURCE NETMASK to increase privacy.

   o  Added a reference to DoS attacks in the Security section.

   o  Don't use "network range", as it seems to have different meaning
      in other contexts, and turned out to be confusing.

   o  Use shorter and longer netmasks, rather than higher or lower.  Add
      a better explanation in the format section.

   o  Minor corrections in various other sections.

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14.  References

14.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, March 2005.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, March 2005.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC5735]  Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses",
              RFC 5735, January 2010.

14.2.  Informative References

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663.

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   [1]  <http://www.iana.org/assignments/address-family-numbers/>

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Authors' Addresses

   Carlo Contavalli
   1600 Amphitheater Parkway
   Mountain View, CA  94043

   Email: ccontavalli@google.com

   Wilmer van der Gaast
   Belgrave House, 76 Buckingham Palace Road
   London  SW1W 9TQ

   Email: wilmer@google.com

   Sean Leach
   21355 Ridgetop Circle
   Dulles, VA  20166

   Email: sleach@verisign.com

   Edward Lewis
   46000 Center Oak Plaza
   Sterling, VA  20166

   Email: ed.lewis@neustar.biz

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