XEP-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 document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards 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.

Document Information

Series: XEP
Number: 0178
Publisher: XMPP Standards Foundation
Status: Experimental
Type: Informational
Version: 0.4
Last Updated: 2006-11-27
Approving Body: XMPP 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Superseded By: None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 (XEP-0178)>

Author Information

Peter Saint-Andre

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

Peter Millard

See Author Note

Legal Notice

This XMPP Extension Protocol is copyright 1999 - 2007 by the XMPP Standards Foundation (XSF) and is in full conformance with the XSF's Intellectual Property Rights Policy <http://www.xmpp.org/extensions/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 discussion list: <http://mail.jabber.org/mailman/listinfo/standards>.

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 XMPP Standards 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 document 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. XMPP Registrar Considerations
8. Author Note
Notes
Revision History

1. Introduction

Note: This document describes a protocol or best practice that is intended for incorporation into the specification that will supersede RFC 3920 [1] within the Internet Standards Process, i.e., rfc3920bis [2]. 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, specifically when use of TLS is required by a deployment. [4]

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 certificate is unacceptable (e.g., because the certificate has been revoked, because the certificate has expired, or because the root certificate was issued by a certification authority that is untrusted), 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 and prefers SASL EXTERNAL mechanism.

    <stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>EXTERNAL<mechanism>
    <mechanism>DIGEST-MD5<mechanism>
    <mechanism>ANONYMOUS<mechanism>
    <required/>
  </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 that corresponds to a registered account on the server, the server SHOULD allow authentication of that JID. For the purpose of client authentication with a server, a valid XMPP address is a JID encapsulated as a subjectAltName entity of type otherName with an ASN.1 Object Identifier of "id-on-xmppAddr" as specified in Section 5.1.1. of RFC 3920.

      <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 is unacceptable (e.g., because the certificate has been revoked, because the certificate has expired, or because the root certificate was issued by a certification authority that is untrusted), 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>
    <required/>
  </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 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, Server2 SHOULD return success. For the purpose of server authentication with another server, a valid XMPP address is a JID encapsulated as a subjectAltName extension of type otherName with an ASN.1 Object Identifier of "id-on-xmppAddr" or a domain name (which MAY include the wildcard character '*') encapsulated as a subjectAltName extension of type dNSName.

      <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 via Internet 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 document introduces no security considerations or concerns above and beyond those discussed in RFC 3920.

6. IANA Considerations

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

7. XMPP Registrar Considerations

This document requires no interaction with the XMPP 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. rfc3921bis: proposed revisions to Extensible Messaging and Presence Protocol (XMPP): Core <http://www.ietf.org/internet-drafts/draft-saintandre-rfc3920bis-00.txt>. (work in progress)

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

4. The protocol flows when TLS is not required are more complicated (e.g., alternate flows involving server dialback) and may be documented in a future version of this document.

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 XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <http://www.xmpp.org/registrar/>.

Revision History

Version 0.4 (2006-11-27)

Modified XMPP address encapsulation methods per rfc3920bis; clarified conditions for certificates to be considered acceptable.

(psa)

Version 0.3 (2006-09-21)

Added TLS and SASL required child elements per rfc3920bis.

(psa)

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 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