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Versions: 00

Network Working Group                                        M. Bretelle
Internet-Draft                                                  Facebook
Intended status: Standards Track                          March 11, 2019
Expires: September 12, 2019


   Encoding DNS-over-TLS (DoT) Subject Public Key Info (SPKI) in Name
                              Server name
              draft-bretelle-dprive-dot-spki-in-ns-name-00

Abstract

   This document describes a mechanism to exchange the Subject Public
   Key Info (SPKI) ([RFC5280] Section 4.1.2.7) fingerprint associated
   with a DNS-over-TLS (DoT [RFC7858]) authoritative server by encoding
   it as part of its name.  The fingerprint can thereafter be used to
   validate the certificate received from the DoT server as well as
   being able to discover support for DoT on the server.

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 https://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 September 12, 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
   (https://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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Validating a remote DoT server  . . . . . . . . . . . . . . .   3
   4.  Encoding data in a domain name label  . . . . . . . . . . . .   3
     4.1.  Formatting DoT SPKI in name server name.  . . . . . . . .   4
       4.1.1.  Example . . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Handling by the recursive servers . . . . . . . . . . . .   4
       4.2.1.  Servers supporting this specification . . . . . . . .   4
       4.2.2.  Servers not supporting this specification . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   This document describes a mechanism to exchange the Subject Public
   Key Info (SPKI) ([RFC5280] Section 4.1.2.7) fingerprint associated
   with a DNS-over-TLS (DoT [RFC7858]) authoritative server by encoding
   it as part of its name.  The fingerprint can thereafter be used to
   validate the certificate received from the DoT server as well as
   being able to discover support for DoT on the server.

2.  Terminology

   A server that supports DNS-over-TLS is called a "DoT server" to
   differentiate it from a "DNS Server" (one that provides DNS service
   over any other protocol), likewise, a client that supports this
   protocol is called a "DoT client"

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








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3.  Validating a remote DoT server

   While DoT provides protection against eavesdropping and on-path
   tampering of the DNS queries exchanged with an authoritative server,
   a recursive server that is talking to a remote DoT server needs a
   mechanism to authenticate that the name server it is communicating
   with is indeed the one that the authority of the zone manages or has
   delegated responsibility to.

   A common mechanism is to have TLS certificates issued by
   "Certification Authorities" (CAs), those CA public keys are used as
   trust anchors, and through a chain of trust, a leaf TLS certificate
   can be validated.  Any CA is able to issue a certificate for any
   domain, which can have its drawbacks ([RFC6698] Section 1.1).

   Another method is to leverage DANE/TLSA ([RFC6698]), in which case a
   recursive resolver would be provided the certificate or SPKI hash
   over DNS and validate it using DNSSEC ([RFC4033], [RFC4034], and
   [RFC4035]).

   This document describes a mechanism to signal to a recursive resolver
   that DoT is supported by the authoritative name server as well as
   providing a fingerprint of the SPKI to expect from the name server,
   this is done by formatting a special first label for the name
   servers.  Recursive servers that understand the naming convention
   detailed in this document will be able to upgrade their connection to
   the authoritative server to TLS, while the ones that don't will
   transparently use the name servers as a standard UDP/53 and TCP/53
   servers.  This format is heavily inspired from [dnscurve].

4.  Encoding data in a domain name label

   A label is limited to a maximum of 63 octets ([RFC1035]
   Section 2.3.4) and has a limited set of characters that can be used
   ([RFC1035] Section 2.3.1), limiting both the amount of data that can
   be embedded in a label as well as the encoding format.

   The set of character used by Base32 encoding ([RFC4648] Section 6),
   without padding character, is suitable to be used in a label.  Base32
   encodes a 5-bit group into 1 byte which allows to encode up to 39
   bytes within the 63 bytes space of a label.

   floor(63 * 5 / 8)

   While this limits what can be encoded in a label, there is enough
   space to store the hash produced by sha256 which requires 32 bytes,
   leaving 7 bytes to spare.




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4.1.  Formatting DoT SPKI in name server name.

   The formatting of a name server is defined as follow:

   <label> ::= <dot-header> <b32-spki-fingerprint>
   <dot-header> ::= "dot-"
   <b32-spki-fingerprint> ::= base32encode(<spki-fingerprint>)
   <spki-fingerprint> ::= sha256(<spki>)
   <spki> ::= der-encoded binary structure of SubjectPublicKeyInfo

4.1.1.  Example

   For the zone example.com, having 2 name servers, one at IPv4
   192.0.2.1 and one at IPv6 2001:DB8::1, both of them providing DoT
   support and using certificate cert.pem, the "<b32-spki-fingerprint>"
   can be generated using the following command line:

   openssl x509 -in /path/to/cert.pem  -pubkey -noout | \
   openssl pkey -pubin -outform der | \
   openssl dgst -sha256 -binary | \
   base32 | tr -d '=' | tr '[:upper:]' '[:lower:]'
   tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a

   To generate the full label, "dot-" get prefixed to the base32 encoded
   fingerprint.

...
...
;; QUESTION SECTION:
;example.com.      IN      NS

;; AUTHORITY SECTION:
example.com. 3600  IN      NS      dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.a.example.com.
example.com. 3600  IN      NS      dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.b.example.com.

;; ADDITIONAL SECTION:
dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.a.example.com. 3600 IN A 192.0.2.1
dot-tpwxmgqdaurcqxqsckxvdq5sty3opxlgcbjj43kumdq62kpqr72a.b.example.com. 3600 IN AAAA 2001:DB8::1
...
...

4.2.  Handling by the recursive servers

4.2.1.  Servers supporting this specification

   When a recursive server gets the list of authoritative servers
   serving a specific zone, it gets a list of name of hosts.




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   If:

   o  the first label is 56 bytes long

   o  AND the first 4 bytes matches "dot-"

   o  AND the remaining 52 bytes can be base32-decoded

   the recursive server will attempt to connect to the name server using
   TLS over port 853 and validate that the SHA256 hash of the SPKI in
   the certificate provided by the name server matches what was
   previously decoded.

   If the TLS session fail to establish, either unavailability of the
   service on port 853, TLS authentication failure, the behaviour of the
   recursive server depends on whether it is operating in strict or
   opportunistic mode ([I-D.ietf-dprive-dtls-and-tls-profiles]).

   In strict mode, the resolver MUST stop using this authoritative name
   server, and MUST try other servers of the DNS zone.  In opportunistic
   mode, the resolver MUST use the authoritative name server despite the
   failure.  It MAY try other name servers of the zone before, in the
   hope they will accept TLS and be authenticated.

4.2.2.  Servers not supporting this specification

   A server not supporting this specification will be unaware of
   anything special with this name server and consider it like any other
   name servers.

5.  Security Considerations

   TODO Security

6.  IANA Considerations

   TODO: This document requires IANA actions (new RR type).

7.  References

7.1.  Normative References

   [I-D.ietf-dprive-dtls-and-tls-profiles]
              Dickinson, S., Gillmor, D., and T. Reddy, "Usage and
              (D)TLS Profiles for DNS-over-(D)TLS", draft-ietf-dprive-
              dtls-and-tls-profiles-11 (work in progress), September
              2017.




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   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/info/rfc4033>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.




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7.2.  Informative References

   [dnscurve]
              "DNSCurve", n.d., <https://dnscurve.org/>.

Acknowledgments

   TODO acknowledge.

Author's Address

   Emmanuel Bretelle
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