JEP-0178: Best Practices for Use of SASL EXTERNAL

This document specifies best practices for use of the SASL EXTERNAL mechanism within XMPP.

WARNING: This Informational JEP is Experimental. Publication as a Jabber Enhancement Proposal does not imply approval of this proposal by the Jabber Software Foundation. Implementation of the best practice or protocol profile described herein is encouraged in exploratory implementations, although production systems should not deploy implementations of this protocol until it advances to a status of Draft.

JEP Information

Status: Experimental
Type: Informational
Number: 0178
Version: 0.2
Last Updated: 2006-03-09
JIG: Standards JIG
Approving Body: Jabber Council
Dependencies: XMPP Core
Supersedes: None
Superseded By: None
Short Name: N/A
Wiki Page: <http://wiki.jabber.org/index.php/Best Practices for Use of SASL EXTERNAL (JEP-0178)>

Author Information

Peter Saint-Andre

Email: stpeter@jabber.org
JID: stpeter@jabber.org

Peter Millard

See Author Note

Legal Notice

This Jabber Enhancement Proposal is copyright 1999 - 2006 by the Jabber Software Foundation (JSF) and is in full conformance with the JSF's Intellectual Property Rights Policy <http://www.jabber.org/jsf/ipr-policy.shtml>. This material may be distributed only subject to the terms and conditions set forth in the Creative Commons Attribution License (<http://creativecommons.org/licenses/by/2.5/>).

Discussion Venue

The preferred venue for discussion of this document is the Standards-JIG discussion list: <http://mail.jabber.org/mailman/listinfo/standards-jig>.

Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) specifications contributed by the Jabber Software Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this JEP has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.

Conformance Terms

The following keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".

Table of Contents

1. Introduction
2. Client-to-Server Recommendation
3. Server-to-Server Recommendation
4. Use of SASL EXTERNAL Without Certificates
5. Security Considerations
6. IANA Considerations
7. Jabber Registrar Considerations
8. Author Note
Notes
Revision History

1. Introduction

Note: This document describes a protocol or best practice that is intended for addition to the specification that will supersede either RFC 3920 [1] or RFC 3921 [2] within the Internet Standards Process. This document is provided only for the purpose of open community discussion of the potential modification and will be obsoleted as soon as the relevant RFC is published.

RFC 3920 allows the use of any SASL mechanism (see RFC 4422 [3]) in XMPP authentication, including the SASL EXTERNAL mechanism. This document specifies a recommended protocol flow for such use.

Note: This document is provided for discussion purposes in order to clarify the usage of SASL EXTERNAL in XMPP systems. It is not meant to supersede the text in RFC 3920 or RFC 4422. However, it is intended that the recommendations in this document will be folded into rfc3920bis.

2. Client-to-Server Recommendation

The RECOMMENDED protocol flow for client-to-server use of SASL EXTERNAL with end-user certificates is as follows:

  1. Client initiates stream to server.

    <stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:client' 
        to='example.com' 
        version='1.0'>

  2. Server replies with stream header.

    <stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:client' 
        id='c2s_234' 
        from='example.com' 
        version='1.0'>

  3. Server advertises TLS stream feature.

    <stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
    <required/>
  </starttls>
</stream:features>

  4. Client sends STARTTLS command to server.

    <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

  5. Server informs client to proceed.

    <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

  6. Client presents certificate.

  7. Server validates certificate.

    1. If certification authority is not recognized or certificate has been revoked, server closes client's TCP connection.

    2. Else server completes successful TLS negotiation and client initiates a new stream header to server.

      <stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:client' 
        to='example.com' 
        version='1.0'>

  8. Server replies with stream header.

    <stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:client' 
        id='c2s_345' 
        from='example.com' 
        version='1.0'>

  9. Server advertises SASL mechanisms. Because client presented a certificate, server advertises SASL EXTERNAL mechanism.

    <stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>DIGEST-MD5<mechanism>
    <mechanism>EXTERNAL<mechanism>
    <mechanism>ANONYMOUS<mechanism>
  </mechanisms>
</stream:features>

  10. Because client presented a certificate, client SHOULD consider EXTERNAL to be its preferred SASL mechanism. The client SHOULD NOT include an authorization identity (i.e., XML character data for the <auth/> element) since client-to-server authorization in XMPP is handled during resource binding.

    <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='EXTERNAL'/>

  11. Server determines whether to allow authenticatation of user.

    1. If the certificate presented by the client contains only one valid XMPP address [4] that corresponds to a registered account on the server, the server SHOULD allow authentication of that JID.

      <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

    2. If the certificate contains more than one valid XMPP address that corresponds to a registered account on the server (e.g., because the server offers virtual hosting), the server SHOULD allow authentication of the JID whose hostname matches the 'to' address of the stream header sent by the client to the server.

      <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

    3. If the certificate does not contain a valid XMPP address that corresponds to a registered account on the server, the server MAY attempt to determine if there is a registered account associated with the user, for example by performing an LDAP lookup based on the Common Name in the certificate; if such a JID mapping is successful, the server SHOULD allow authentication of that mapped JID.

      <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

    4. If the user cannot be associated with an account registered on the server, the server MUST return a SASL failure of <not-authorized/> and close the stream.

      <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
  <not-authorized/>
</failure>
</stream:stream>

  12. If SASL authentication succeeded, client opens new stream, then client and server proceed with resource binding as described in RFC 3920.

3. Server-to-Server Recommendation

The RECOMMENDED protocol flow for server-to-server use of SASL EXTERNAL with server (domain) certificates is as follows:

  1. Server1 initiates stream to server2.

    <stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:server' 
        from='conference.example.org' 
        to='example.com' 
        version='1.0'>

  2. Server2 replies with stream header.

    <stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:server' 
        id='s2s_234' 
        from='example.com' 
        to='conference.example.org'
        version='1.0'>

  3. Server2 advertises TLS stream feature.

    <stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
    <required/>
  </starttls>
</stream:features>

  4. Server1 sends STARTTLS command to Server2.

    <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

  5. Server2 informs Server1 to proceed.

    <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

  6. Server1 presents certificate.

  7. Server2 validates certificate.

    1. If certificate has been revoked, Server2 closes Server1's TCP connection.

    2. Else Server2 completes successful TLS negotiation and Server1 initiates a new stream header to Server2.

      <stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:server' 
        from='conference.example.org'
        to='example.com' 
        version='1.0'>

  8. Server2 replies with stream header.

    <stream:stream 
        xmlns:stream='http://etherx.jabber.org/streams' 
        xmlns='jabber:server' 
        id='s2s_345' 
        from='example.com' 
        to='conference.example.org'
        version='1.0'>

  9. Server2 advertises SASL mechanisms. Because Server1 presented a certificate, Server2 advertises SASL EXTERNAL mechanism.

    <stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>EXTERNAL<mechanism>
  </mechanisms>
</stream:features>

  10. Because Server1 presented a certificate, Server1 SHOULD consider EXTERNAL to be its preferred SASL mechanism. Server1 SHOULD include an authorization identity (base64-encoded according as described in RFC 3920) as the XML character data of the <auth/> element, which SHOULD be the same as the 'from' address on the stream header.

    <auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl' mechanism='EXTERNAL'>Y29uZmVyZW5jZS5leGFtcGxlLm9yZwo=</auth>

    (In this case, the authorization identity is "conference.example.org".)

  11. Server2 determines if hostname is valid.

    1. If the authorization identity provided by Server1 matches one of the valid XMPP addresses in the certificate or one of the Common Names in the certificates, Server2 SHOULD return success.

      <success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

    2. Else server MUST return a <not-authorized/> failure and close the stream.

      <failure xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
  <not-authorized/>
</failure>
</stream:stream>

4. Use of SASL EXTERNAL Without Certificates

The SASL EXTERNAL mechanism can be used outside the context of X.509 certificates, for example Intern Protocol Security (IPSec) as specified in RFC 4301 [5]. A future version of this specification may document best practices for use of SASL EXTERNAL outside the context of the X.509 infrastructure.

5. Security Considerations

This JEP introduces no security considerations or concerns above and beyond those discussed in RFC 3920.

6. IANA Considerations

This JEP requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].

7. Jabber Registrar Considerations

This JEP requires no interaction with the Jabber Registrar [7].

8. Author Note

Peter Millard, co-author of the initial version of this specification, died on April 26, 2006. The remaining author appreciates his assistance in defining the best practices described herein.

Notes

1. RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://www.ietf.org/rfc/rfc3920.txt>.

2. RFC 3921: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://www.ietf.org/rfc/rfc3921.txt>.

3. RFC 4422: Simple Authentication and Security Layer (SASL) <http://www.ietf.org/rfc/rfc4422.txt>.

4. A valid XMPP address is a JID encapsulated as a subjectAltName with an ASN.1 Object Identifier of "id-on-xmppAddr" (which is equivalent to the dotted display format of "1.3.6.1.5.5.7.8.5").

5. RFC 4301: Security Architecture for the Internet Protocol <http://www.ietf.org/rfc/rfc4301.txt>.

6. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

7. The Jabber Registrar maintains a list of reserved Jabber protocol namespaces as well as registries of parameters used in the context of protocols approved by the Jabber Software Foundation. For further information, see <http://www.jabber.org/registrar/>.

Revision History

Version 0.2 (2006-03-09)

Corrected client-server failure case to place error in SASL flow rather than binding flow; added note about non-X.509 usages.

(psa)

Version 0.1 (2006-03-09)

Initial JEP version.

(psa)

Version 0.0.3 (2006-02-10)

Specified inclusion of authorization identity for server-to-server.

(psa/pgm)

Version 0.0.2 (2006-02-10)

Clarified distinction between authentication and authorization.

(psa/pgm)

Version 0.0.1 (2006-02-09)

First draft.

(psa/pgm)

END