dnsop C. Contavalli
Internet-Draft W. van der Gaast
Intended status: Informational Google
Expires: May 19, 2015 D. Lawrence
Akamai Technologies
W. Kumari
Google
November 15, 2014
Client Subnet in DNS Requests
draft-ietf-dnsop-edns-client-subnet-00
Abstract
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.
IESG Note
[RFC Editor: Please remove this note prior to publication ]
This informational document describes an existing, implemented and
deployed system. A subset of the operators using this is at
http://www.afasterinternet.com/participants.htm . The authors believe
that it is better to document this system (even if not everyone
agrees with the concept) than leave it undocumented and proprietary.
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 May 19, 2015.
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Copyright Notice
Copyright (c) 2014 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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Notation . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Option Format . . . . . . . . . . . . . . . . . . . . . . . . 6
6. Protocol Description . . . . . . . . . . . . . . . . . . . . 7
6.1. Originating the Option . . . . . . . . . . . . . . . . . 7
6.2. Generating a Response . . . . . . . . . . . . . . . . . . 8
6.3. Handling edns-client-subnet Replies and Caching . . . . . 9
6.4. Transitivity . . . . . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. DNSSEC Considerations . . . . . . . . . . . . . . . . . . . . 12
9. NAT Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. Security Considerations . . . . . . . . . . . . . . . . . . . 13
10.1. Privacy . . . . . . . . . . . . . . . . . . . . . . . . 13
10.2. Birthday Attacks . . . . . . . . . . . . . . . . . . . . 14
10.3. Cache Pollution . . . . . . . . . . . . . . . . . . . . 14
11. Sending the Option . . . . . . . . . . . . . . . . . . . . . 16
11.1. Probing . . . . . . . . . . . . . . . . . . . . . . . . 16
11.2. Whitelist . . . . . . . . . . . . . . . . . . . . . . . 16
12. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
13. Contributing Authors . . . . . . . . . . . . . . . . . . . . 19
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
15.1. Normative References . . . . . . . . . . . . . . . . . . 19
15.2. Informative References . . . . . . . . . . . . . . . . . 20
15.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Appendix A. Document History . . . . . . . . . . . . . . . . . . 20
A.1. -00 . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
A.2. -01 . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
A.3. -02 . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
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A.4. -03* . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22
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 network topology 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
querier.
Increasingly, though, a class of Recursive Resolvers has arisen that
handle query sources that are often not topologically close. The
motivation for a user to configure such a Centralized 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.) Similarly, many ISPs and other
organizations use a Centralized Resolver infrastructure that can be
distant from the clients the resolvers serve. The cases all lead to
less than optimal replies from topology-sensitive Authoritative
Nameservers.
This draft 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 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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2. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. 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. 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
Nameserver.
Centralized Resolvers: Recursive Resolvers that serve a
topologically diverse network address space.
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, and two geographically distant hosts can
be quite close on the network.
4. 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
Authoritative Nameservers.
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The format of this option is described in Section 5, 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 6.1.
As described in Section 6.2, 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 also contain an edns-client-subnet option, clearly
indicating that the server made use of this information, and that the
answer is tied to the network of the client.
As described in Section 6.3, 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 6.4.
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 10 explores various mitigation
techniques.
The expectation, however, is that this option will only be used by
Recursive Resolvers and Authoritative Nameservers that incur
geolocation issues.
Most Recursive Resolvers, Authoritative Nameservers and Stub
Resolvers will never 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 location, which is a
common occurrence in current content delivery network (CDN) setups
and not a cause of concern.
Section 6.1 also provides a mechanism for Stub Resolvers to signal
Recursive Resolvers that they do not want edns-client-subnet
treatment for specific requests.
Additionally, operators of Intermediate Nameservers with edns-client-
subnet 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 edns-client-subnet option.
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5. Option Format
This draft uses an EDNS0 [RFC6891]) option to include client address
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 [RFC6891]) OPTION-CODE, 2 octets, for edns-client-
subnet is 8 (0x00 0x08).
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
IANA-AFI [2].
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 octet representing the length of the
netmask pertaining to the query. In replies, it mirrors the same
value as in the requests. It can be set to 0 to disable client-
based lookups, in which case the ADDRESS field MUST be absent.
o SCOPE NETMASK, unsigned octet representing the length of the
netmask pertaining to the reply. In requests, it SHOULD be set to
the longest cacheable length supported by the Intermediate
Nameserver. For backwards compatibiilty with draft versions of
this specification, in requests it MAY be set to 0 to have the
Authoritative Nameserver treat the longest cacheable length as the
SOURCE NETMASK length. In responses, this field is set by the
Authoritative Nameserver to indicate the coverage of the response.
It might or might not match SOURCE NETMASK; it could be shorter or
longer.
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o ADDRESS, variable number of octets, contains either an IPv4 or
IPv6 address, depending on FAMILY, truncated in the request 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. (This need
not be as many octets as a complete address would take.) In the
reply, if the SCOPE NETMASK of the request was 0 then ADDRESS must
contain the same octets as in the request. Otherwise, the bits
for ADDRESS will be significant through the maximum of the SOURCE
NETMASK or SCOPE NETMASK, and 0 filled to the end of an octet.
All fields are in network byte order ("big-endian", per [RFC1700],
Data Notation).
6. Protocol Description
6.1. Originating the Option
The edns-client-subnet option should generally be added by Recursive
Resolvers when querying other servers, as described in Section 11.
In this option, the server should include the IP address of the
client that caused the query to be generated, truncated to the number
of bits specified in the SOURCE NETMASK field.
A Stub Resolver MAY generate DNS queries with an edns-client-subnet
option with SOURCE NETMASK set to 0 (i.e. 0.0.0.0/0) to indicate that
the Recursive Resolver MUST NOT add address information of the client
to its queries. The subsequent Recursive Resolver query to the
Authoritative Nameserver will then either not include an edns-client-
subnet option or MAY optionally include its own address information,
which is what the Authoritative Nameserver will use anyway to
generate the reply in lieu of no option. This allows the answer to
be handled by the same caching mechanism as other requests, with an
explicit indicator of the applicable scope. Subsequent Stub Resolver
requests for /0 can then be answered from this cached response.
The Stub Resolver may also add non-empty edns-client-subnet options
to its queries, but Recursive Resolvers are not required to 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 Intermediate Nameserver, as described in
Section 10.
If the Stub Resolver requests additional privacy via a SOURCE NETMASK
that is shorter than the maximum cacheable SCOPE NETMASK that the
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Recursive Resolver allows, the Recursive Resolver SHOULD issue the
query with its longer SCOPE NETMASK.
6.2. 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 Nameservers that have not implemented or enabled
support for the edns-client-subnet option may safely ignore it within
incoming queries. Per [RFC6891] section 6.1.2, such a server MUST
NOT include an edns-client-subnet option within replies, to indicate
lack of support for the option.
Requests with wrongly formatted options (e.g., wrong size) MUST be
rejected and a FORMERR response MUST be returned to the sender, as
described by [RFC6891], 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 SHOULD be cached accordingly. If the option was not included in
a query, it MUST NOT be included in the response.
The FAMILY and SOURCE NETMASK in the response MUST match those in the
request. The first SOURCE NETMASK bits of the ADDRESS in the
response MUST match those in the request, even if fewer bits were
used to form the response. Echoing back the address and netmask
helps to mitigate certain attack vectors, as described in Section 10.
The SCOPE NETMASK in the reply indicates the netmask of the network
for which the answer is intended.
A SCOPE NETMASK value longer than the SOURCE NETMASK indicates that
the address and netmask provided in the query was not specific enough
to select a single, best response. The ADDRESS MUST be extended to
SCOPE NETMASK significant bits to indicate the network for which it
is optimal, but the Recursive Resolver SHOULD still provide the
result as the answer to the triggering client request even if the
client is in a different address range.
Conversely, a shorter SCOPE NETMASK indicates that more bits than
necessary were provided, and the answer is suitable for a broader
range of addresses.
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If a non-zero SCOPE NETMASK was supplied in the request, the SCOPE
NETMASK of the response MUST be no longer than the SCOPE NETMASK of
the request.
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 6.3.
If the edns-client-subnet option in the request is not used at all, 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 0.0.0.0/0 or ::/0. This
could happen, for example, because the reply is invariant across the
network space. The answer is suitable for all clients for the
duration of its TTL.
The specific logic that an Authoritative Nameserver uses to choose an
optimized reply is not in the scope of this document. Implementers
are encouraged, however, to consider carefully their selection of
SCOPE NETMASK for the reply in the event that an optimal reply cannot
be determined.
6.3. 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 SHOULD
cache the result.
If the FAMILY, SOURCE NETMASK, and SOURCE NETMASK bits of ADDRESS in
the reply don't match the fields in the corresponding request, the
full reply MUST be dropped, as described in Section 10.
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.
Any information cached from these sections MUST NOT be tied to a
network.
If another query is received matching the name and type of an entry
in the cache, the resolver will check whether the FAMILY and ADDRESS
of the client matches one of the networks in the cache for that
entry.
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If the address of the client is within any of the networks in the
cache, then the cached response MUST be returned as usual. If the
address of the client matches multiple networks in the cache, the
entry with the longest 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 suboptimal
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 suboptimal 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 short SOURCE NETMASK in the original
request could cause a suboptimal reply to be returned by the
Authoritative Nameserver.
When the request includes a longer SCOPE NETMASK, the reply returned
may still be cached as optimal for the ADDRESS and SCOPE NETMASK of
the reply. This might still be suboptimal for the original client.
To avoid this suboptimal reply from being served from cache for other
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 suboptimal. A Recursive
Resolver MUST return this entry from cache only to queries that do
not contain or allow a longer SOURCE NETMASK to be forwarded, or
to queries originating from the network covered by the ADDRESS and
SCOPE NETMASK..
o If the SCOPE NETMASK in the reply is shorter than 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
SCOPE NETMASK.
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. Per
standard DNS caching behavior, all records SHOULD be retained until
their TTL expires. Subsequent queries to refresh the data should
always specify the longest SCOPE NETMASK that the Recursive Resolver
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is willing to cache, even if previous responses indicated that a
shorter netmask was the optimal response.
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 edns-client-
subnet. Therefore, when implementing this option for latency
purposes, implementing full caching support as described in this
section is STRONGLY RECOMMENDED.
Any reply containing an edns-client-subnet option considered invalid
should be treated as if no edns-client-subnet option was specified at
all.
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 0.0.0.0/0 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 10 is strongly recommended.
6.4. 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 forwards edns-
client-subnet options received from their clients MUST fully
implement the caching behaviour described in Section 6.3.
Intermediate Nameservers, including Recursive Resolvers, supporting
edns-client-subnet MUST forward options with SOURCE NETMASK set to 0
(i.e., completely anonymized), such an option MUST NOT be replaced
with an option with more accurate address information.
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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 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 an edns-client-subnet option with 0 address bits MUST NOT be
refused.
7. IANA Considerations
IANA has already assigned option code 8 in the "DNS EDNS0 Option
Codes (OPT)" registry to edns-client-subnet.
The IANA is requested to update the reference ("draft-vandergaast-
edns-client-subnet") to refer to this RFC when published.
8. DNSSEC Considerations
The presence or absence of an [RFC6891] EDNS0 OPT resource record
containing an edns-client-subnet 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.
9. NAT Considerations
Special awareness of edns-client-subnet 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 edns-client-subnet options,
if present, SHOULD NOT be modified by NAT devices.
In large-scale global networks behind NAT (but, for example, with a
Centralized Resolver infrastructure), an internal Intermediate
Nameserver might have detailed network layout information, and might
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 a NAT device SHOULD
NOT originate edns-client-subnet options with their IP address, and
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instead rely on downstream Intermediate Nameservers doing so. They
MAY, however, choose to include the option with their internal
address for the purposes of signaling a shorter, more anonymous
SOURCE NETMASK.
If an Authoritative Nameserver on the publicly routed Internet
receives a request 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
reply it SHOULD set SCOPE NETMASK to cover all of the relevant
private space. For example, a request for ADDRESS 10.1.2.0 with a
SOURCE NETMASK of 24 would get a returned SCOPE NETMASK of 8. The
Intermediate Nameserver MAY elect to cache the answer under one entry
for special-purpose addresses [RFC6890]; see Section 10.3.
10. Security Considerations
10.1. 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 unique identification of the client.
When a non-zero SCOPE NETMASK is provided by the Recursive Resolver
that is longer than SOURCE NETMASK, users can often obtain more
optimal mapping if the resolver is well-used. Replies will have
answers optimized up to SCOPE NETMASK bits for a subset of the SOURCE
NETMASK. Subsequent requests within the TTL from clients within the
cached range will be served the optimal answer, while still
preserving privacy of the user.
ISPs will often have more detailed knowledge of their own networks.
That is, 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 0.0.0.0/0
(i.e. FAMILY set to 1 (IPv4), SOURCE NETMASK to 0 and no ADDRESS).
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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
client.
Note that even without an edns-client-subnet 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 requests to return a cached entry or to
generate an optimized reply that best matches the request.
10.2. Birthday Attacks
edns-client-subnet adds information to the DNS question tuple
(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 0.0.0.0/0 to get it cached for many hosts).
To counter this, every edns-client-subnet option in a response packet
MUST contain the FAMILY and SOURCE NETMASK fields from the
corresponding request, along with identical ADDRESS bits for SOURCE
NETMASK length. Intermediate Nameservers processing a response MUST
verify that these match, and MUST discard the entire reply if they do
not.
10.3. 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 calculate the optimized answer.
o mount a denial-of-service attack against an Intermediate
Nameserver, by forcing it to perform many more recursive queries
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than it would normally do, due to how caching is handled for
queries containing the edns-client-subnet option.
Even without malicious intent, Centralized Resolvers providing
answers to clients in multiple networks will need to cache different
replies for different networks, putting more memory 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 SHOULD 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 a SCOPE NETMASK that is longer than they are willing to
cache. Similarly, if using the backwards-compatible SCOPE NETMASK
of 0, the request should not set a SOURCE NETMASK of more bits
than they are willing to cache.
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
edns-client-subnet 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 edns-client-subnet
options specifying other routable addresses that are known not to
be served by the query source.
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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11. 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.
11.1. 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, top-level, and effective top-level domain
servers. These domains are delegation-centric and are very unlikely
to generate different replies based on the address 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.
11.2. 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, such as zone or query type.
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An additional 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 major drawback is scalability. The operator needs to track which
Authoritative Nameservers support edns-client-subnet, making it
harder for new Authoritative Nameservers to start using the option.
12. Example
1. A stub resolver SR with IP address 192.0.2.37 tries to resolve
www.example.com, by forwarding the query to the Recursive
Resolver R from IP address IP, asking for recursion.
2. RNS, 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. RNS builds a query to send to the root and .com servers. The
implementation of R provides facilities so an administrator can
configure RNS not to forward edns-client-subnet in certain
cases. In particular, RNS is configured to not include an edns-
client-subnet option when talking to delegation-centric
nameservers, as described in Section 6.1. Thus, no edns-client-
subnet option is added, and resolution is performed as usual.
4. RNS now knows the next server to query, Authoritative Nameserver
ANS, responsible for example.com.
5. RNS prepares a new query for www.example.com, including an edns-
client-subnet option with:
* OPTION-CODE, set to 0x00 0x08.
* OPTION-LENGTH, set to 0x00 0x07 for the following fixed 4
octets plus the 3 octets that will be used for ADDRESS.
* FAMILY, set to 0x00 0x01 as IP is an IPv4 address.
* SOURCE NETMASK, set to 0x18, as RNS is configured to conceal
the last 8 bits of every IPv4 address.
* SCOPE NETMASK, set to 0x1B, as RNS is willing to cache
answers up to a /27.
* ADDRESS, set to 0xC0 0x00 0x02, providing only the first 24
bits of the IPv4 address.
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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 ANS, it finds an answer
that is optimal for several /27 ranges within the ADDRESS/SOURCE
NETMASK of the request. It chooses one randomly. (Note well,
this is just one example of how ANS could pick a suitable
answer. Other selection methods are possible.)
8. The Authoritative Nameserver ANS adds an edns-client-subnet
option in the reply, containing:
* OPTION-CODE, set to 0x00 0x08.
* OPTION-LENGTH, set to 0x00 0x08 for the following fixed 4
octets plus the 4 octets that will be used for ADDRESS .
* FAMILY, set to 0x00 0x01, the same as the request.
* SOURCE NETMASK, set to 0x18, copied from the request.
* SCOPE NETMASK, set to 0x1B, indicating a /27 network.
* ADDRESS, set to 0xC0 0x00 0x02 0xE0, copied from the request.
9. RNS receives the reply containing an edns-client-subnet option.
The resolver verifies that FAMILY, SOURCE NETMASK, and the
SOURCE NETMASK bits of ADDRESS match the request. If not, the
message is discarded.
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. RNS sends a response to stub resolver SR, without including an
edns-client-subnet option.
12. RNS 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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13. Contributing Authors
The below individuals contributed significantly to the draft. The
RFC Editor prefers a maximum of 5 names on the front page, and so we
have listed additional authors in this section
Edward Lewis
ICANN
12025 Waterfront Drive, Suite 300 Los Angeles, CA 90094-2536 USA
Email: edward.lewis@icann.org
Sean Leach
Fastly
POBox 78266
San Francisco, CA 94107
14. 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 and Jason Moreau from
Akamai, Colm MacCarthaigh, Richard Sheehan and all the other people
that replied to our emails on various mailing lists.
15. References
15.1. 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.
[RFC1700] Reynolds, J. and J. Postel, "Assigned Numbers", RFC 1700,
October 1994.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets", BCP
5, RFC 1918, February 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[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.
[RFC6890] Cotton, M., Vegoda, L., Bonica, R., and B. Haberman,
"Special-Purpose IP Address Registries", BCP 153, RFC
6890, April 2013.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
15.2. Informative References
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
15.3. URIs
[1] http://www.iana.org/assignments/address-family-numbers/
Appendix A. Document History
[RFC Editor: Please delete this section before publication.]
A.1. -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.
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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
attacks.
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.
A.2. -01
o Document version number reset from -02 to -00 due to the rename to
edns-client-subnet.
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.
whitelisting.
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
document.
o Intermediate Nameservers can now remove an edns-client-subnet
option, or reduce the SOURCE NETMASK to increase privacy.
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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.
A.3. -02
o Added IANA-assigned option code.
A.4. -03*
o [*] There was no -03 version of the draft; these changes, however,
were made after -02.
o Allow non-zero SCOPE NETMASK for Recursive Resolvers to indicate
their maximum cacheable mask length, and updated the example
accordingly.
o A note on Authoritative Nameservers receiving requests that
specify private address space.
o A note to always ask for the longest acceptable SCOPE NETMASK,
even if a prior answer indicated that a shorter netmask was
optimal.
o Marked up a couple of references.
o Minor grammatical consistency edits.
Authors' Addresses
Carlo Contavalli
Google
1600 Amphitheater Parkway
Mountain View, CA 94043
US
Email: ccontavalli@google.com
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Wilmer van der Gaast
Google
Belgrave House, 76 Buckingham Palace Road
London SW1W 9TQ
UK
Email: wilmer@google.com
David C Lawrence
Akamai Technologies
8 Cambridge Center
Cambridge, MA 02142
US
Email: tale@akamai.com
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: warren@kumari.net
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