Network Working Group P. Saint-Andre
Internet-Draft M. Miller
Intended status: Standards Track Cisco Systems, Inc.
Expires: April 24, 2014 October 21, 2013

Domain Name Associations (DNA) in the Extensible Messaging and Presence Protocol (XMPP)
draft-ietf-xmpp-dna-04

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

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This Internet-Draft will expire on April 24, 2014.

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Table of Contents

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]
  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                       |
|  [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'               |
|                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'/>                 |
|                                               |
+-----------------------------------------------+
        

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:

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 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).

  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.

  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]: 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:

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.

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

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.miller-posh] explains how to use HTTPS redirects for secure delegation with the POSH prooftype.

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.miller-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

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.miller-posh].

12. References

12.1. Normative References

[I-D.ietf-dane-srv] Finch, T., "Using DNS-Based Authentication of Named Entities (DANE) TLSA records with SRV and MX records.", Internet-Draft draft-ietf-dane-srv-02, February 2013.
[I-D.miller-posh] Miller, M. and P. Saint-Andre, "PKIX over Secure HTTP (POSH)", Internet-Draft draft-miller-posh-02, September 2013.
[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.
[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., "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 Cisco Systems, Inc. 1899 Wynkoop Street, Suite 600 Denver, CO 80202 USA EMail: psaintan@cisco.com
Matthew Miller Cisco Systems, Inc. 1899 Wynkoop Street, Suite 600 Denver, CO 80202 USA EMail: mamille2@cisco.com