XEP-0368: SRV records for XMPP over TLS

NOTICE: This document is currently within Last Call or under consideration by the XMPP Council for advancement to the next stage in the XSF standards process. The Last Call ends on 2017-02-22. Please send your feedback to the standards@xmpp.org discussion list.

Table of Contents

1. Introduction

XMPP Core [1] specifies the use of xmpp-client/xmpp-server SRV records as the method of discovering how to connect to an XMPP server. This XEP extends that to include new xmpps-client/xmpps-server SRV records pointing to direct TLS ports and combine priorities and weights as if they were a single SRV record similar to RFC 6186 [2]. It also provides an easy way for clients to bypass restrictive firewalls that only allow HTTPS, for servers to host multiple protocols/services on a single port, and for servers and clients to take advantage of less round trips and existing direct TLS loadbalancers.

2. Glossary

3. Requirements

The following format for DNS SRV resource records is specified in RFC 2782 [3]:

    _service._proto.name. TTL class SRV priority weight port target.
    

XMPP Core defines SRV records only where 'service' is 'xmpp-client' and 'xmpp-server'. This document specifies to additionally look up records where 'service' is 'xmpps-client' and 'xmpps-server'. This document specifies that the following additional rules apply:

1. Both 'xmpp-' and 'xmpps-' records SHOULD be treated as the same record with regard to connection order as specified by RFC 2782, in that all priorities and weights are mixed. This enables the server operator to decide if they would rather clients connect with STARTTLS or direct TLS. However, clients MAY choose to prefer one type of connection over the other.
2. Where 'service' starts with 'xmpps-' the client or server MUST connect with direct TLS enabled.
3. Where 'service' starts with 'xmpp-' the client or server MUST NOT connect with direct TLS enabled, connection method is unchanged from XMPP Core.
4. TLS certificates MUST be validated the same way as for STARTTLS. (i.e., as specified in XMPP Core).
5. STARTTLS MUST NOT be used over direct TLS connections.
6. Client or server MUST set SNI TLS extension to the JID's domain part.
7. Client or server SHOULD set the ALPN (RFC 7301 [4]) TLS extension.
8. When ALPN is used, the ALPN protocol MUST be 'xmpp-client', where the SRV service is 'xmpps-client'.
9. When ALPN is used, the ALPN protocol MUST be 'xmpp-server', where the SRV service is 'xmpps-server'.

4. Use Cases

For server operators, this provides a way to host multiple services on the same port, especially when SNI or ALPN extensions are used. Servers could even host xmpp-client and xmpp-server services on the same TLS port. For clients, this provides a virtually zero overhead way to bypass restrictive firewalls that only allow HTTP over port 80 and HTTPS over port 443, as XMPP-over-TLS is indistinguishable from HTTP-over-TLS when ALPN is not used. For clients and servers, direct TLS saves round trips vs STARTTLS.

5. Implementation Notes

The only overhead is the single additional SRV lookup. All clients that support STARTTLS already have support for direct TLS.

Server operators should not expect multiplexing (via ALPN) to work in all scenarios and therefore should provide additional SRV record(s) that do not require multiplexing (either standard STARTTLS or dedicated direct XMPP-over-TLS). This is a result of relying on ALPN for multiplexing, where ALPN might not be supported by all devices or may be disabled by a user due to privacy reasons.

6. Security Considerations

Direct TLS provides AT LEAST the same level of security as STARTTLS, and more privacy without ALPN as using STARTTLS leaks that the underlying protocol is XMPP, while any direct TLS stream should be indistinguishable from any other direct TLS stream. Direct TLS provides more security than STARTTLS if RFC 7590 [5] is not followed, as it isn't subject to STARTTLS stripping. All security setup and certificate validation code SHOULD be shared between the STARTTLS and direct TLS logic as well. All SRV-based connection methods are subject to DNS modification/stripping/spoofing of SRV records in the absence of DNSSEC.

7. IANA Considerations

ALPN (RFC 7301) requires registration of new Protocol IDs. This document specifies two Protocol IDs:

Protocol: XMPP jabber:client namespace
Identification Sequence:
   0x78 0x6d 0x70 0x70 0x2d 0x63 0x6c 0x69 0x65 0x6e 0x74 ("xmpp-client")
Reference: [XEP-0368]

Protocol: XMPP jabber:server namespace
Identification Sequence:
   0x78 0x6d 0x70 0x70 0x2d 0x73 0x65 0x72 0x76 0x65 0x72 ("xmpp-server")
Reference: [XEP-0368]

The ALPN registry is currently located here.

Issues with direct TLS ports mentioned in RFC 2595 [6] Section 7 do not apply here for these reasons:

1. URL scheme is unchanged.
2. "Security" is the same between STARTTLS and direct TLS, and post-2014 the general consensus is that no public XMPP server or client should connect without some form of TLS per the ubiquitous encryption manifesto.
3. A "Use TLS when available" security model is possible here because the client could fall back to standard STARTTLS SRV records if the server administrator supplies them.
4. No port numbers are assigned because SRV allows using arbitrary ports at the server administrator's discretion.

8. XMPP Registrar Considerations

There are no XMPP Registrar Considerations.

Appendices

Appendix A: Document Information

Series: XEP
Number: 0368
Publisher: XMPP Standards Foundation
Status: Proposed
Type: Standards Track
Version: 0.1.2
Last Updated: 2017-02-15
Approving Body: XMPP Council
Dependencies: XMPP Core, RFC 2782
Supersedes: None
Superseded By: None
Short Name: NOT_YET_ASSIGNED
Source Control: HTML
This document in other formats: XML  PDF

Appendix B: Author Information

Appendix C: Legal Notices

Appendix D: Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) 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.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.

Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.

Given that this XMPP Extension Protocol normatively references IETF technologies, discussion on the <xsf-ietf@xmpp.org> list might also be appropriate.

Errata can be sent to <editor@xmpp.org>.

Appendix F: Requirements Conformance

The following requirements 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".

Appendix G: Notes

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

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