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Versions: (draft-saintandre-xmpp-dna) 00 01 02 03 04 05 06 07 08 09 10 11 RFC 7712

Network Working Group                                     P. Saint-Andre
Internet-Draft                                                      &yet
Intended status: Standards Track                               M. Miller
Expires: August 8, 2014                              Cisco Systems, Inc.
                                                        February 4, 2014


Domain Name Associations (DNA) in the Extensible Messaging and Presence
                            Protocol (XMPP)
                         draft-ietf-xmpp-dna-05

Abstract

   This document improves the security of the Extensible Messaging and
   Presence Protocol (XMPP) in two ways.  First, it specifies how
   "prooftypes" can establish a strong association between a domain name
   and an XML stream.  Second, it describes how to securely delegate a
   source domain to a derived domain, which is especially important in
   virtual hosting environments.

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 August 8, 2014.

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



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Flow Chart  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  A Simple Scenario . . . . . . . . . . . . . . . . . . . . . .   6
   5.  One-Way Authentication  . . . . . . . . . . . . . . . . . . .   7
   6.  Piggybacking  . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Assertion . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Supposition . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Alternative Prooftypes  . . . . . . . . . . . . . . . . . . .  10
     7.1.  DANE  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.2.  POSH  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Secure Delegation and Multi-Tenancy . . . . . . . . . . . . .  12
   9.  Prooftype Model . . . . . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Well-Known URI for xmpp-client Service . . . . . . . . .  13
     10.2.  Well-Known URI for xmpp-server Service . . . . . . . . .  13
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The need to establish a strong association between a domain name and
   an XML stream arises in both client-to-server and server-to-server
   communication using the Extensible Messaging and Presence Protocol
   (XMPP) [RFC6120].  Because XMPP servers are typically identified by
   DNS domain names, a client or peer server needs to verify the
   identity of a server to which it connects.

   To date, such verification has been established based on information
   obtained from the Domain Name System (DNS), the Public Key
   Infrastructure (PKI), or similar sources.  In relation to such
   associations, this document does the following:

   1.  Generalizes the model currently in use so that additional
       prooftypes can be defined

   2.  Provides a basis for modernizing some prooftypes to reflect
       progress in underlying technologies such as DNS Security
       [RFC4033]



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   3.  Describes the flow of operations for establishing a domain name
       association (DNA)

   This document also provides guidelines for secure delegation.  The
   need for secure delegation arises because the process for resolving
   the domain name of an XMPP service into the IP address at which an
   XML stream will be negotiated (defined in [RFC6120]) can involve
   delegation of a source domain (say, example.com) to a derived domain
   (say, hosting.example.net) using technologies such as DNS SRV records
   [RFC2782].  If such delegation is not done in a secure manner, then
   the domain name association cannot be authenticated.

2.  Terminology

   This document inherits XMPP terminology from [RFC6120] and
   [XEP-0220], DNS terminology from [RFC1034], [RFC1035], [RFC2782] and
   [RFC4033], and security terminology from [RFC4949] and [RFC5280].
   The terms "source domain", "derived domain", "reference identity",
   and "presented identity" are used as defined in the "CertID"
   specification [RFC6125].  The terms "permissive federation",
   "verified federation", and "encrypted federation" are derived from
   [XEP-0238], although we substitute the term "authenticated
   federation" for the term "trusted federation" from that document.

3.  Flow Chart

   The following flow chart illustrates the protocol flow for
   establishing domain name associations between Server 1 and Server 2,
   as described in the remaining sections of this document.

                       |
                       |
           (Section 4: A Simple Scenario)
                       |
                       |
                DNS RESOLUTION ETC.
                       |
   +-------------STREAM HEADERS--------------------+
   |                                               |
   |  A: <stream from='a.example' to='b.example'>  |
   |                                               |
   |  B: <stream from='b.example' to='a.example'>  |
   |                                               |
   +-----------------------------------------------+
                       |
   +-------------TLS NEGOTIATION-------------------+
   |                                               |
   |   B: Server Certificate                       |



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   |  [B: Certificate Request]                     |
   |  [A: Client Certificate]                      |
   |                                               |
   +-----------------------------------------------+
                       |
       (A establishes DNA for b.example!)
                       |
   +-------------AUTHENTICATION--------------------+
   |                   |                           |
   |          {client certificate?} ----+          |
   |                   |                |          |
   |                   | yes         no |          |
   |                   v                |          |
   |             SASL EXTERNAL          |          |
   |             (mutual auth!)         |          |
   +------------------------------------|----------+
                                        |
                       +----------------+
                       | B needs to auth A
                       |
           (Section 5: One-Way Authentication)
                       |
                       |
                DNS RESOLUTION ETC.
                       |
   +-------------STREAM HEADERS--------------------+
   |                                               |
   |  B: <stream from='b.example' to='a.example'>  |
   |                                               |
   |  A: <stream from='a.example' to='b.example'>  |
   |                                               |
   +-----------------------------------------------+
                       |
   +-------------TLS NEGOTIATION-------------------+
   |                                               |
   |   A: Server Certificate                       |
   |                                               |
   +-----------------------------------------------+
                       |
       (B establishes DNA for a.example!)
                       |
                       |
         (Section 6.1: Piggybacking Assertion)
                       |
                       |
   +----------DIALBACK IDENTITY ASSERTION----------+
   |                                               |
   |  B: <db:result from='c.example'               |



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   |                to='a.example'/>               |
   |                                               |
   +-----------------------------------------------+
                       |
   +-----------DNA DANCE AS ABOVE------------------+
   |                                               |
   |    DNS RESOLUTION, STREAM HEADERS,            |
   |    TLS NEGOTIATION, AUTHENTICATION            |
   |                                               |
   +-----------------------------------------------+
                       |
   +----------DIALBACK IDENTITY VERIFICATION-------+
   |                                               |
   |  A: <db:result from='a.example'               |
   |                to='c.example'                 |
   |                type='valid'/>                 |
   |                                               |
   +-----------------------------------------------+
                       |
                       |
         (Section 6.2: Piggybacking Supposition)
                       |
                       |
   +-----------SUBSEQUENT CONNECTION---------------+
   |                                               |
   |  B: <stream from='c.example'                  |
   |             to='rooms.a.example'>             |
   |                                               |
   |  A: <stream from='rooms.a.example'            |
   |             to='c.example'>                   |
   |                                               |
   +-----------------------------------------------+
                       |
   +-----------DNA DANCE AS ABOVE------------------+
   |                                               |
   |    DNS RESOLUTION, STREAM HEADERS,            |
   |    TLS NEGOTIATION, AUTHENTICATION            |
   |                                               |
   +-----------------------------------------------+
                       |
   +-----------DIALBACK OPTIMIZATION---------------+
   |                                               |
   |  B: <db:result from='c.example'               |
   |                to='rooms.a.example'/>         |
   |                                               |
   |  B: <db:result from='rooms.a.example'         |
   |                to='c.example'                 |
   |                type='valid'/>                 |



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   |                                               |
   +-----------------------------------------------+

4.  A Simple Scenario

   To illustrate the problem, consider the simplified order of events
   (see [RFC6120] for details) in establishing an XML stream between
   Server 1 (a.example) and Server 2 (b.example):

   1.  Server 1 resolves the DNS domain name b.example.


   2.  Server 1 opens a TCP connection to the resolved IP address.


   3.  Server 1 sends an initial stream header to Server 2, asserting
       that it is a.example:

       <stream:stream from='a.example' to='b.example'>



   4.  Server 2 sends a response stream header to Server 1, asserting
       that it is b.example:

       <stream:stream from='b.example' to='a.example'>



   5.  The servers attempt TLS negotiation, during which Server 2
       (acting as a TLS server) presents a PKIX certificate proving that
       it is b.example and Server 1 (acting as a TLS client) presents a
       PKIX certificate proving that it is a.example.


   6.  Server 1 checks the PKIX certificate that Server 2 provided and
       Server 2 checks the PKIX certificate that Server 1 provided; if
       these proofs are consistent with the XMPP profile of the matching
       rules from [RFC6125], each server accepts that there is a strong
       domain name association between its stream to the other party and
       the DNS domain name of the other party.

   Several simplifying assumptions underlie the happy scenario just
   outlined:

   o  Server 1 presents a PKIX certificate during TLS negotiation, which
      enables the parties to complete mutual authentication.




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   o  There are no additional domains associated with Server 1 and
      Server 2 (say, a subdomain rooms.a.example on Server 1 or a second
      domain c.example on Server 2).

   o  The server administrators are able to obtain PKIX certificates in
      the first place.

   o  The server administrators are running their own XMPP servers,
      rather than using hosting services.

   Let's consider each of these "wrinkles" in turn.

5.  One-Way Authentication

   If Server 1 does not present its PKIX certificate during TLS
   negotiation (perhaps because it wishes to verify the identity of
   Server 2 before presenting its own credentials), Server 2 is unable
   to mutually authenticate Server 1.  Therefore, Server 2 needs to
   negotiate and authenticate a stream to Server 1, just as Server 1 has
   done:

   1.  Server 2 resolves the DNS domain name a.example.


   2.  Server 2 opens a TCP connection to the resolved IP address.


   3.  Server 2 sends an initial stream header to Server 1, asserting
       that it is b.example:

       <stream:stream from='b.example' to='a.example'>



   4.  Server 1 sends a response stream header to Server 2, asserting
       that it is a.example:

       <stream:stream from='a.example' to='b.example'>



   5.  The servers attempt TLS negotiation, during which Server 1
       (acting as a TLS server) presents a PKIX certificate proving that
       it is a.example.


   6.  Server 2 checks the PKIX certificate that Server 1 provided; if
       it is consistent with the XMPP profile [RFC6120] of the matching



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       rules from [RFC6125], Server 2 accepts that there is a strong
       domain name association between its stream to Server 1 and the
       DNS domain name a.example.

   At this point the servers are using two TCP connections instead of
   one, which is somewhat wasteful.  However, there are ways to tie the
   authentication achieved on the second TCP connection to the first TCP
   connection; see [XEP-0288] for further discussion.

6.  Piggybacking

6.1.  Assertion

   Consider the common scenario in which Server 2 hosts not only
   b.example but also a second domain c.example (a "multi-tenanted"
   environment).  If a user of Server 2 associated with c.example wishes
   to communicate with a friend at a.example, Server 2 needs to send
   XMPP stanzas from the domain c.example rather than b.example.
   Although Server 2 could open an new TCP connection and negotiate new
   XML streams for the domain pair of c.example and a.example, that too
   is wasteful.  Server 2 already has a connection to a.example, so how
   can it assert that it would like to add a new domain pair to the
   existing connection?

   The traditional method for doing so is the Server Dialback protocol,
   first specified in [RFC3920] and since moved to [XEP-0220].  Here,
   Server 2 can send a <db:result/> element for the new domain pair over
   the existing stream.

       <db:result from='c.example' to='a.example'>
         some-dialback-key
       </db:result>

   This element functions as Server 2's assertion that it is (also)
   c.example, and thus is functionally equivalent to the 'from' address
   of an initial stream header as previously described.

   In response to this assertion, Server 1 needs to obtain some kind of
   proof that Server 2 really is also c.example.  It can do the same
   thing that it did before:

   1.  Server 1 resolves the DNS domain name c.example.


   2.  Server 1 opens a TCP connection to the resolved IP address (which
       might be the same IP address as for b.example).





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   3.  Server 1 sends an initial stream header to Server 2, asserting
       that it is a.example:

       <stream:stream from='a.example' to='c.example'>



   4.  Server 2 sends a response stream header to Server 1, asserting
       that it is c.example:

       <stream:stream from='c.example' to='a.example'>



   5.  The servers attempt TLS negotiation, during which Server 2
       (acting as a TLS server) presents a PKIX certificate proving that
       it is c.example.


   6.  Server 1 checks the PKIX certificate that Server 2 provided; if
       it is consistent with the XMPP profile [RFC6120] of the matching
       rules from [RFC6125], Server 1 accepts that there is a strong
       domain name association between its stream to Server 2 and the
       DNS domain name c.example.

   Now that Server 1 accepts the domain name association, it informs
   Server 2 of that fact:

       <db:result from='a.example' to='c.example' type='valid'/>

   The parties can then terminate the second connection, since it was
   used only for Server 1 to associate a stream over the same IP:port
   combination with the domain name c.example (the dialback key links
   the original stream to the new association).

6.2.  Supposition

   Piggybacking can also occur in the other direction.  Consider the
   common scenario in which Server 1 provides XMPP services not only for
   a.example but also for a subdomain such as a groupchat service at
   rooms.a.example (see [XEP-0045]).  If a user from c.example at Server
   2 wishes to join a room on the groupchat sevice, Server 2 needs to
   send XMPP stanzas from the domain c.example to the domain
   rooms.a.example rather than a.example.  Therefore, Server 2 needs to
   negotiate and authenticate a stream to rooms.a.example:

   1.  Server 2 resolves the DNS domain name rooms.a.example.




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   2.  Server 2 opens a TCP connection to the resolved IP address.


   3.  Server 2 sends an initial stream header to Server 1 acting as
       rooms.a.example, asserting that it is b.example:

       <stream:stream from='b.example' to='rooms.a.example'>



   4.  Server 1 sends a response stream header to Server 2, asserting
       that it is rooms.a.example:

       <stream:stream from='rooms.a.example' to='b.example'>



   5.  The servers attempt TLS negotiation, during which Server 1
       (acting as a TLS server) presents a PKIX certificate proving that
       it is rooms.a.example.


   6.  Server 2 checks the PKIX certificate that Server 1 provided; if
       it is consistent with the XMPP profile [RFC6120] of the matching
       rules from [RFC6125], Server 2 accepts that there is a strong
       domain name association between its stream to Server 1 and the
       DNS domain name rooms.a.example.

   As before, the parties now have two TCP connections open.  So that
   they can close the now-redundant connection, Server 2 sends a
   dialback key to Server 1 over the new connection.

       <db:result from='c.example' to='rooms.a.example'>
         some-dialback-key
       </db:result>

   Server 1 then informs Server 2 that it accepts the domain name
   association:

       <db:result from='rooms.a.example' to='c.example' type='valid'/>

   Server 2 can now close the connection over which it tested the domain
   name association for rooms.a.example.

7.  Alternative Prooftypes

   The foregoing protocol flows assumed that domain name associations
   were proved using the standard PKI prooftype specified in [RFC6120]:



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   that is, the server's proof consists of a PKIX certificate that is
   checked according to the XMPP profile [RFC6120] of the matching rules
   from [RFC6125], the client's verification material is obtained out of
   band in the form of a trusted root, and secure DNS is not necessary.

   However, sometimes XMPP server administrators are unable or unwilling
   to obtain valid PKIX certificates for their servers.  As one example,
   a certificate authority (CA) might try to send email messages to
   authoritative mailbox names [RFC2142], but the administrator of a
   subsidiary service such as im.cs.podunk.example can't receive email
   sent to mailto:hostmaster@podunk.example.  As another example, a
   hosting provider such as hosting.example.net might not want to take
   on the liability of holding the certificate and private key for a
   tenant such as example.com (or the tenant might not want the hosting
   provider to hold its certificate and private key).  In these
   circumstances, prooftypes other than PKIX are desirable.  As
   described below, two alternatives have been defined so far: DNS-Based
   Authentication of Named Entities (DANE) and and PKIX Over Secure HTTP
   (POSH).

7.1.  DANE

   In the DANE prooftype, the server's proof consists of a PKIX
   certificate that is compared as an exact match or a hash of either
   the SubjectPublicKeyInfo or the full certificate, and the client's
   verification material is obtained via secure DNS.

   The DANE prooftype makes use of the DNS-Based Authentication of Named
   Entities [RFC6698], specifically the use of DANE with DNS SRV records
   [I-D.ietf-dane-srv].  For XMPP purposes, the following rules apply:

   o  If there is no SRV resource record, pursue the fallback methods
      described in [RFC6120].

   o  Use the 'to' address of the initial stream header to determine the
      domain name of the TLS client's reference identifier (since use of
      the TLS Server Name Indication is purely discretionary in XMPP, as
      mentioned in [RFC6120]).

7.2.  POSH

   In the POSH prooftype, the server's proof consists of a PKIX
   certificate that is checked according to the rules from [RFC6120] and
   [RFC6125], the client's verification material is obtained by
   retrieving the PKIK certificate over HTTPS at a well-known URI
   [RFC5785], and secure DNS is not necessary since the HTTPS retrieval
   mechanism relies on the chain of trust from the public key
   infrastructure.



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   POSH is defined in [I-D.ietf-xmpp-posh].  For XMPP purposes, the
   well-known URIs [RFC5785] to be used are:

   o  "/.well-known/posh._xmpp-client._tcp.json" for client-to-server
      connections

   o  "/.well-known/posh._xmpp-server._tcp.json" for server-to-server
      connections

8.  Secure Delegation and Multi-Tenancy

   One common method for deploying XMPP services is multi-tenancy or
   virtual hosting: e.g., the XMPP service for example.com is actually
   hosted at hosting.example.net.  Such an arrangement is relatively
   convenient in XMPP given the use of DNS SRV records [RFC2782], such
   as the following pointer from example.com to hosting.example.net:

   _xmpp-server._tcp.example.com. 0 IN SRV 0 0 5269 hosting.example.net

   Secure connections with multi-tenancy can work using the PKIX
   prooftype on a small scale if the provider itself wishes to host
   several domains (e.g., several related domains such as jabber-
   de.example and jabber-ch.example).  However, in practice the security
   of multi-tenancy has been found to be unwieldy when the provider
   hosts large numbers of XMPP services on behalf of multiple tenants.
   Typically there are two main reasons for this state of affairs: the
   service provider (say, hosting.example.net) wishes to limit its
   liability and therefore does not wish to hold the certificate and
   private key for the tenant (say, example.com) and the tenant wishes
   to improve the security of the service and therefore does not wish to
   share its certificate and private key with service provider.  As a
   result, server-to-server communications to example.com go unencrypted
   or the communications are TLS-encrypted but the certificates are not
   checked (which is functionally equivalent to a connection using an
   anonymous key exchange).  This is also true of client-to-server
   communications, forcing end users to override certificate warnings or
   configure their clients to accept certificates for
   hosting.example.net instead of example.com.  The fundamental problem
   here is that if DNSSEC is not used then the act of delegation via DNS
   SRV records is inherently insecure.

   The specification for use of SRV and MX records with DANE
   [I-D.ietf-dane-srv] explains how to use DNSSEC for secure delegation
   with the DANE prooftype, and the POSH specification
   [I-D.ietf-xmpp-posh] explains how to use HTTPS redirects for secure
   delegation with the POSH prooftype.





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9.  Prooftype Model

   In general, a domain name association (DNA) prooftype conforms to the
   following definition:

   prooftype:  A mechanism for proving an association between a domain
      name and an XML stream, where the mechanism defines (1) the nature
      of the server's proof, (2) the matching rules for comparing the
      client's verification material against the server's proof, (3) how
      the client obtains its verification material, and (4) whether the
      mechanism depends on secure DNS.

   The PKI, DANE, and POSH prooftypes adhere to this model.  In
   addition, other prooftypes are possible (examples might include PGP
   keys rather than PKIX certificates, or a token mechanism such as
   Kerberos or OAuth).

   Some prooftypes depend on (or are enhanced by) secure DNS and thus
   also need to describe how they ensure secure delegation.

10.  IANA Considerations

   The POSH specification [I-D.ietf-xmpp-posh] provides guidelines for
   registering the well-known URIs [RFC5785] of protocols that make use
   of POSH.  This specification registers two such URIs, for which the
   completed registration templates follow.

10.1.  Well-Known URI for xmpp-client Service

   This specification registers the well-known URI "posh._xmpp-
   client._tcp.json" in the Well-Known URI Registry as defined by
   [RFC5785].

   URI suffix: posh._xmpp-client._tcp.json

   Change controller: IETF

   Specification document(s): [[ this document ]]

10.2.  Well-Known URI for xmpp-server Service

   This specification registers the well-known URI "posh._xmpp-
   server._tcp.json" in the Well-Known URI Registry as defined by
   [RFC5785].

   URI suffix: posh._xmpp-server._tcp.json

   Change controller: IETF



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   Specification document(s): [[ this document ]]

11.  Security Considerations

   This document supplements but does not supersede the security
   considerations of [RFC6120] and [RFC6125].  Relevant security
   considerations can also be found in [I-D.ietf-dane-srv] and
   [I-D.ietf-xmpp-posh].

12.  References

12.1.  Normative References

   [I-D.ietf-dane-srv]
              Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
              Based Authentication of Named Entities (DANE) TLSA records
              with SRV and MX records.", draft-ietf-dane-srv-03 (work in
              progress), December 2013.

   [I-D.ietf-xmpp-posh]
              Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP
              (POSH)", draft-ietf-xmpp-posh-00 (work in progress),
              February 2014.

   [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.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

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

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", RFC
              4949, August 2007.

   [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, May 2008.






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   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785, April
              2010.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

   [XEP-0220]
              Miller, J., Saint-Andre, P., and P. Hancke, "Server
              Dialback", XSF XEP 0220, September 2013.

12.2.  Informative References

   [RFC2142]  Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
              FUNCTIONS", RFC 2142, May 1997.

   [RFC3920]  Saint-Andre, P., Ed., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 3920, October 2004.

   [XEP-0045]
              Saint-Andre, P., "Multi-User Chat", XSF XEP 0045, February
              2012.

   [XEP-0238]
              Saint-Andre, P., "XMPP Protocol Flows for Inter-Domain
              Federation", XSF XEP 0238, March 2008.

   [XEP-0288]
              Hancke, P. and D. Cridland, "Bidirectional Server-to-
              Server Connections", XSF XEP 0288, September 2013.

Authors' Addresses

   Peter Saint-Andre
   &yet

   Email: ietf@stpeter.im




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   Matthew Miller
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202
   USA

   Email: mamille2@cisco.com












































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