XEP-0418: DNS Queries over XMPP (DoX)

WARNING: This Standards-Track document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.

Table of Contents

1. Introduction

This document defines a specific protocol, DNS over XMPP (DoX), for sending DNS RFC 1035 [1] queries and getting DNS responses over XMPP Core [2] (and therefore TLS RFC 8446 [3] security for integrity and confidentiality.

The integration with XMPP provides a transport suitable for both existing DNS clients and native XMPP applications seeking access to the DNS.

This protocol is almost identical in scope to DNS Queries over HTTPS (DoH) RFC 8484 [4]

2. Requirements

This specification addresses the following requirements:

1. Sending a DNS query.
2. Responding with a DNS response.

3. Protocol

The DoX protocol is extremely simple:

1. The requesting entity (requestor) sends an IQ-get containing a <dns/> element qualified by the 'urn:xmpp:dox:0' namespace, which contains the DNS query.
2. The resolving entity (resolver) returns either an IQ-result containing a <dns/> element qualified by the 'urn:xmpp:dox:0' namespace, which contains the DNS response (if it supports the namespace) or an IQ-error (if it does not).
3. In both the query and response, the content of the <dns/> element is the DNS on-the-wire format as defined in RFC 1035 [1]. The body MUST be encoded with base64 RFC 4648 [5]. Padding characters for base64 MUST NOT be included.

4. Use Cases

Sending a DNS query is done by sending an <iq/> get over the stream from the requestor to the resolver.

Example 1. Query

<iq from='romeo@montague.lit/home' to='juliet@capulet.lit/chamber'
 id='s2c1' type='get'>
  <dns xmlns='urn:xmpp:dox:0'>vOIBIAABAAAAAAABB2V4YW1wbGUDb3JnAAABAAEAACkQAAAAAAAADAAKAAj5HO5JuEe+mA</dns>
</iq>

If the resolver supports the dns namespace, it MUST return an IQ-result, which contains the DNS response:

Example 2. Response

<iq from='juliet@capulet.lit/chamber' to='romeo@montague.lit/home'
 id='s2c1' type='result'>
  <dns xmlns='urn:xmpp:dox:0'>vOKBoAABAAEAAAABB2V4YW1wbGUDb3JnAAABAAHADAABAAEAAAhjAARduNgiAAApEAAAAAAAAAA</dns>
</iq>

If the resolver does not support the dns namespace, it MUST return a <service-unavailable/> error:

Example 3. DNS Not Supported

<iq from='juliet@capulet.lit/chamber' to='romeo@montague.lit/home' id='s2c1' type='error'>
  <dns xmlns='urn:xmpp:dox:0'>vOIBIAABAAAAAAABB2V4YW1wbGUDb3JnAAABAAEAACkQAAAAAAAADAAKAAj5HO5JuEe+mA</dns>
  <error type='cancel'>
    <service-unavailable xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

The other error conditions defined in RFC 6120 could also be returned if appropriate.

5. Determining Support

If an entity supports the DoX protocol, it MUST report that fact by including a service discovery feature of "urn:xmpp:dox:0" in response to a Service Discovery (XEP-0030) [6] information request:

Example 4. Service Discovery information request

<iq type='get'
    from='juliet@capulet.lit/balcony'
    to='capulet.lit'
    id='disco1'>
  <query xmlns='http://jabber.org/protocol/disco#info'/>
</iq>

Example 5. Service Discovery information response

<iq type='result'
    from='capulet.lit'
    to='juliet@capulet.lit/balcony'
    id='disco1'>
  <query xmlns='http://jabber.org/protocol/disco#info'>
    ...
    <feature var='urn:xmpp:dox:0'/>
    ...
  </query>
</iq>

In order for an application to determine whether an entity supports this protocol, where possible it SHOULD use the dynamic, presence-based profile of service discovery defined in Entity Capabilities (XEP-0115) [7]. However, if an application has not received entity capabilities information from an entity, it SHOULD use explicit service discovery instead.

Support could also be pre-arranged between parties by putting a resolver at a known JID, in which case the requestor can just start sending queries to the resolver

6. Implementation Notes

Some XMPP clients do not respond to IQ stanzas containing unsupported payloads. Although this is in violation of XMPP Core [2], this behavior can result in disconnection of clients that are in fact actively connected to the server.

7. Security Considerations

Running DNS over XMPP relies on the security of the underlying XMPP transport, therefore all queries and responses MUST use TLS or equivalent connection security. This mitigates classic amplification attacks for UDP- based DNS.

Session-level encryption has well-known weaknesses with respect to traffic analysis, which might be particularly acute when dealing with DNS queries. DoX resolvers can also add DNS padding RFC 7830 [8] if the DoX requestor requests it in the DNS query. An experimental effort to offer guidance on choosing the padding length can be found in RFC 8467 [9].

The TLS connection provides transport security for the interaction between the DoX resolver and requestor, but it does not provide the response integrity of DNS data provided by DNSSEC. DNSSEC and DoX are independent and fully compatible protocols, each solving different problems. The use of one does not diminish the need nor the usefulness of the other. It is the choice of a requestor to either perform full DNSSEC validation of answers or to trust the DoX resolver to do DNSSEC validation and inspect the AD (Authentic Data) bit in the returned message to determine whether an answer was authentic or not.

In the absence of DNSSEC information, a DoX resolver can give a requestor invalid data in response to a DNS query. A DoX capable requestor MUST discard any responses not specifically requested, this prohibition does not guarantee protection against invalid data, but it does reduce the risk.

If a server receives a dns request directed to a full JID <localpart@domain.tld/resource> associated with a registered account but there is no connected resource matching the 'to' address, RFC 6120 requires it to reply with a <service-unavailable/> error and set the 'from' address of the IQ-error to the full JID provided in the 'to' address of the dns request. If a connected resource receives a dns request but it does not want to reveal its network availability to the sender for any reason (e.g., because the sender is not authorized to know the connected resource's availability), then it too MUST reply with a <service-unavailable/> error. This consistency between the server response and the resolver response helps to prevent presence leaks.

8. IANA Considerations

No interaction with the Internet Assigned Numbers Authority (IANA) [10] is necessary as a result of this document.

9. XMPP Registrar Considerations

10. XML Schema

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:dox:0'
    xmlns='urn:xmpp:dox:0'
    elementFormDefault='qualified'>

  <xs:annotation>
    <xs:documentation>
      The protocol documented by this schema is defined in
      XEP-XXXX: https://xmpp.org/extensions/inbox/dox.html
    </xs:documentation>
  </xs:annotation>

  <xs:element name='dns' type='base64Binary'/>

</xs:schema>

Appendices

Appendix A: Document Information

Series: XEP
Number: 0418
Publisher: XMPP Standards Foundation
Status: Experimental
Type: Standards Track
Version: 0.1.0
Last Updated: 2019-03-29
Approving Body: XMPP Council
Dependencies: XMPP Core, XEP-0030
Supersedes: None
Superseded By: None
Short Name: dox
Schema: <http://www.xmpp.org/schemas/dox.xsd>
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.

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/

END