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IPv6 Operations (v6ops)                                    Martin Hunek
Internet-Draft                          Technical University of Liberec
Intended status: Standards Track                         March 11, 2019
Expires: September 11, 2019

                NAT64/DNS64 detection via SRV Records


   This document specifies the way of discovering the NAT64 pools in
   use as well as DNS servers providing DNS64 service to the local
   clients. The discovery is done via SRV records, which also allows
   asignment of priorities to the NAT64 pools as well as DNS64 servers.
   It also allows clients to have diferent DNS providers than NAT64
   provider, while providing a secure way via DNSSEC validation of
   provided SRV records. This way, it provides DNS64 service even in
   case where DNS over HTTPS is used.

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
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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six
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   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 September 11, 2019

Copyright Notice

   Copyright (c) 2019 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
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   warranty as described in the Simplified BSD License.

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Table of Contents

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
    1.1. Requirements Language . . . . . . . . . . . . . . . . . . .  3
   2. Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3. NAT64 service SRV record . . . . . . . . . . . . . . . . . . .  3
   4. DNS64 service SRV record . . . . . . . . . . . . . . . . . . .  4
   5. Node Behavior  . . . . . . . . . . . . . . . . . . . . . . . .  4
    5.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   6. IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  7
   7. Security considerations  . . . . . . . . . . . . . . . . . . .  7
   8. References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
    8.1. Normative References  . . . . . . . . . . . . . . . . . . .  7
    8.2. Informative References  . . . . . . . . . . . . . . . . . .  8

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1. Introduction

   The simultaneous use of NAT64/DNS64 and DNSSEC outlined by
   [RFC7050], does not solve all the aspects of such use. Namely
   [RFC7050] does not allow assignment of NAT64 priorities in case when
   multiple network prefixes are in use. [RFC7050] also doesn't work in
   the case when network operator and DNS operator are not the same
   subject, like in the case when the end node is using some public DNS
   resolvers. This document describes the way how to circumvent that
   limitation while maintaining added security provided by DNSSEC.

1.1. Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   and "OPTIONAL" in this document are to be interpreted as described
   in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in
   all capitals, as shown here.

2. Terminology

   End node: Either DNS stub resolver or the DNS recursive resolver
   serving a local area network or station.

   Pref64::/n: an IPv6 prefix used for IPv6 address synthesis

   Pref64::WKA: an IPv6 address consisting of Pref64::/n and WKA at
   any of the locations allowed by [RFC6052].

   Well-Known IPv4 Address (WKA): an IPv4 address that is well-known
   and present in an A record for the well-known name. Two well-known
   IPv4 addresses are defined for Pref64::/n discovery purposes: and

3. NAT64 service SRV record

   This document specifies two new well-known SRV records. The one for
   NAT64 prefix which validation end node MUST implement:

   nat64. ipv6.Name TTL Class SRV Priority Weight Port Target

   The TTL, Class, Priority and Weight follows the same scheme as
   defined in [RFC2782] and have theirs standard meaning.

   Port: IPv6 as L3 protocol doesn't use port numbers. Because of that
   this field SHOULD be either set to zero, or SHOULD be used to
   indicate length of network prefix mask in both IPv6 and IPv4
   protocol, used NAT64. In such case the port 16b integer MUST be
   constructed by directly appending IPv4 pool prefix mask after the

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   IPv6 prefix mask decadicaly. Usually this would mean 9632 stating
   that IPv6 prefix with mask /96 is translated into single IPv4
   address (/32).

   Target: MUST point to AAAA record formed from Pref64::/n prefix and
   WKA same way as in [RFC7050] (Pref64::WKA). Target MAY also point to
   A record, in which case it SHOULD point to IPv4 address used for
   NAT64 (or base address of the NAT64 IPv4 prefix).

4. DNS64 service SRV record

   The second SRV record is for the discovery of DNS64 service. Support
   of this record is OPTIONAL but end node SHOULD implement it.

   dns64.Protocol.Name TTL Class SRV Priority Weight Port Target

   Record informs about location of DNS64 service. This might be used
   in case that network operator doesn't want to deploy DNS64 in their
   main DNS infrastructure. A DNS64 SRV record follows the rules
   specified by [RFC2782] and does not modify meaning of any field.

   Server provided by this record SHOULD only be used for domain names
   which have returned NODATA for AAAA record.

5. Node Behavior

   In early stage of end node connection to the network - after the end
   node is configured with IP address, the end node MUST get local
   domains used in the network. Method of obtaining such information is
   out of scope of this document, but it might contain one or more
   methods, like the SLAAC-DNSSL [RFC8106], the DHCPv6 - option 24 or
   a manual configuration. In case, when no local domain can be
   discovered, the end node SHOULD continue NAT64/DNS64 detection by
   other means, like [RFC7050].

   After the list of local domains has been established, the end node
   MUST ask for NAT64 SRV record for every domain in the list. Result
   of such queries SHOULD be ordered by following the rules of
   [RFC2782]. In case when multiple records do have a same values of
   both priority and weight, the records SHOULD maintain the same order
   as its domain in the discovered domain list.

   For every domain with NAT64 SRV record the end node SHOULD perform
   query for DNS64 SRV record. If such a record is obtained and the end
   node is not configured to make DNS64 synthesis itself, the end node
   SHOULD use preferred target of DNS64 SRV record to query for FQDN
   without AAAA record - when it received NODATA response.

   If the end node is capable of validation of DNS records via DNSSEC,
   the end node MUST perform validation of NAT64/DNS64 SRV record.
   Default behavior of end node SHOULD be to ignore any NAT64/DNS64 SRV
   records which cannot be validated or did not pass the validation.

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5.1. Example

   The end node is a home router connected to the ISP network in which
   the NAT64/DNS64 is used and the ISP has the following SRV records
   in their zones:
   - nat64.ipv6.example.com. IN SRV 5 10 9632 nat64-pool-1.example.com.
   - nat64-pool-1.example.com. IN AAAA 2001:db8:64:ff9b:1::c000:aa
   - nat64-pool-1.example.com. IN A
   - nat64.ipv6.example.com.
                            IN SRV 10 10 9632 nat64-pool-2.example.com.
   - nat64-pool-2.example.com. IN AAAA 2001:db8:64:ff9b:2::c000:aa
   - nat64-pool-2.example.com. IN A
   - nat64.ipv6.example.net. IN SRV 10 10 9624 nat64-pool.example.net.
   - nat64-pool.example.net. IN AAAA 2001:db8:64:ff9b:abc::c000:aa
   - nat64-pool.example.net. IN A
   - nat64.ipv6.example.invalid.
                            IN SRV 10 10 9624 nat64-pool.example.org.
   - nat64-pool.example.org. IN AAAA 2001:db8:64:ff9b:def::c000:aa
   - nat64-pool.example.org. IN A

   In addition the zones "example.net" and "example.invalid" has got
   DNS64 SRV records:
   - dns64.tcp.example.net. IN SRV 5 10 53 dns64.example.net.
   - dns64.udp.example.net. IN SRV 10 10 53 dns64.example.net.
   - dns64.example.net. IN AAAA 2001:db8::53
   - dns64.udp.example.invalid. IN SRV 10 10 53 dns64.example.org.
   - dns64.example.org. IN AAAA 2001:db8:123::53

   The zones "example.com" and "example.net" are secured and
   successfully validated by the DNSSEC. Domain "example.invalid" is
   either not secured by the DNSSEC or its validation failed. Domain
   "example.org" is DNSSEC secured but does not have any NAT64/DNS64
   SRV records.

   The end node has been supplied with the following list of domains
   1. example.net
   2. example.invalid
   3. example.com
   4. example.org

   The end node would fetch all available SRV records and its A and
   AAAA counterparts and sort it in following order:

   pool                        DNSSEC  priority  reason
   nat64-pool-1.example.com.   yes     5         lowest priority field
   nat64-pool.example.net.     yes     10        discovered first
   nat64-pool-2.example.com.   yes     10        higher priority field
   nat64-pool.example.org.     no      10        no valid DNSSEC chain

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   After sorting, the end node SHOULD graylist any record which cannot
   be validated by the DNSSEC. In this example it would be
   "nat64-pool.example.org." because it has been obtained from insecure
   domain "example.invalid". A such pool SHOULD NOT be used if it is
   not confirmed by other DNSSEC secured record.

   If the end node is capable to act as recursive or caching DNS server
   and it is configured to provide the DNS64 service, it MUST provide
   this service using sorted list of NAT64 pools. For such end node
   a process of the NAT64/DNS64 ends here.

   However, when the end node is not capable of record synthesis or it
   is not configured to provide DNS64 service, it SHOULD perform
   detection of DNS64 by querying for "ipv4only.arpa" like in the
   case of [RFC7050]. If the reply contains a pool listed in the NAT64
   pool list, the corresponding entry is marked as having DNS64
   provided by recursive DNS.

   When the end node supports DNS64 SRV record and there is at least
   one non-graylisted NAT64 pool, which is not reachable by using the
   end node's recursive DNS, the end node MUST make a sorted list of
   DNS64 servers from the DNS64 SRV records. The DNS64 sorted list
   would look like this:

   server              proto  DNSSEC  priority  reason
   dns64.example.net.  tcp    yes     5         lowest priority field
   dns64.example.net.  udp    yes     10        higher priority field
   dns64.example.org.  udp    no      10        no valid DNSSEC chain

   Sorting is done in the same fashion as any other SRV record with the
   same exception of graylisting records without valid DNSSEC chain.
   Those SHOULD NOT be used when not confirmed by DNSSEC validated
   record and SHOULD be kept in the end of the list.

   For example when ISP is providing DNS64 service in their main DNS
   infrastructure only for pools in the domains "example.com" and
   "example.org" and the pool "nat64-pool.example.net" is used only
   with corresponding DNS64 server. The final sorted list of NAT64
   prefixes used by the end node in the ISP network would be:

   pool                       state     priority  reason
   nat64-pool-1.example.com.  active    5         lowest priority field
   nat64-pool-2.example.com.  backup    10        higher priority field
   nat64-pool.example.net.    backup*   10        main DNS has priority
   nat64-pool.example.org.    inactive  10        no valid DNSSEC chain

   As the pool "nat64-pool.example.net" is used only with the server
   "dns64.example.net" this would effectively put this pool to the end
   of the list. Because it would be used only for FQDN for which the
   regular DNS infrastructure returns NODATA.

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   Now the end node has successfully identified NAT64 pools and the
   DNS64 servers in the ISP infrastructure. The discovered prefixes
   SHOULD be considered safe and DNSSEC validation of answers in these
   prefixes MUST be either disabled or performed by validating only
   the suffix.

6. IANA Considerations.

   This document proposes a usage of "ipv6" in Proto field and two
   services "nat64" and "dns64" in Service field of SRV RR

7. Security considerations

   Method proposed by this document relies on security principles based
   on DNSSEC and secure discovery of local domain. In order to be
   secure, the network operator MUST deploy DNSSEC on at least one
   domain (advertised to end node) and establish secure channel to this

8. References

8.1. Normative References

   [RFC2119]  S. Bradner. Key words for use in RFCs to Indicate
              Requirement Levels. RFC 2119. RFC Editor, Mar. 1997, pp.
              1-3. url: https://www.rfc-editor.org/rfc/rfc2119.txt.

   [RFC2782]  A. Gulbrandsen, P. Vixie, and L. Esibov. A DNS RR for
              specifying the location of services (DNS SRV). RFC 2782.
              RFC Editor, Feb. 2000, pp. 1-12.
              url: https://www.rfc-editor.org/rfc/rfc2782.txt.

   [RFC6146]  M. Bagnulo, P. Matthews, and I. van Beijnum. Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers. RFC 6146. RFC Editor, Apr. 2011,
              pp. 1-45.
              url: https://www.rfc-editor.org/rfc/rfc6146.txt.

   [RFC7050]  T. Savolainen, J. Korhonen, and D. Wing. Discovery of the
              IPv6 Prefix Used for IPv6 Address Synthesis. RFC 7050.
              RFC Editor, Nov. 2013, pp. 1-22.
              url: https://www.rfc-editor.org/rfc/rfc7050.txt.

   [RFC8174]  B. Leiba. Ambiguity of Uppercase vs Lowercase in RFC 2119
              Key Words. RFC 8174. RFC Editor, May 2017, pp. 1-4.
              url: https://www.rfc-editor.org/rfc/rfc8174.txt.

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8.2. Informative References

   [RFC6052]  C. Bao et al. IPv6 Addressing of IPv4/IPv6 Translators.
              RFC 6052. RFC Editor, Oct. 2010, pp. 1-18.
              url: https://www.rfc-editor.org/rfc/rfc6052.txt.

   [RFC8106]  J. Jeong et al. IPv6 Router Advertisement Options for DNS
              Configuration. RFC 8106. RFC Editor, Mar. 2017, pp. 1-19.
              url: https://www.rfc-editor.org/rfc/rfc8106.txt.


   This work has been supported by Student Grant Scheme (SGS 2019) at
   Technical University of Liberec.

Authors' Addresses

   Martin Hunek
   Technical University of Liberec
   Studentska 1402/2
   Liberec, 46017 Czech Republic

   phone: +420 485 35 3792
   e-mail: martin.hunek@tul.cz

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