RFC 3920 allows the use of any SASL mechanism (see &rfc4422;) in XMPP authentication, including the SASL ANONYMOUS mechanism (see &rfc4505;). 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 ANONYMOUS in XMPP systems. It is not meant to supersede the text in RFC 3920, RFC 4422, or RFC 4505. However, the recommendations in this document may be folded into rfc3920bis.

An XMPP server implementation SHOULD NOT enable the SASL ANONYMOUS mechanism by default, but instead SHOULD force an administrator to explicitly enable support in any given deployment.

An XMPP server SHOULD assign a temporary, unique bare JID &LOCALBARE; to a client that authenticates with SASL ANONYMOUS. Although the method for ensuring the uniqueness of localpart is a matter of implementation, it is RECOMMENDED for the localpart to be a UUID as specified in &rfc4122;.

After a client authenticates using the SASL ANONYMOUS mechanism, it MUST bind a resource; the server SHOULD ignore the resource identifier provided by the client (if any) and instead assign a resource identifier that it generates on behalf of the client.

Because an anonymous user is unknown to the server, the server SHOULD appropriately restrict the user's access in order to limit the possibility of malicious behavior, such as denial of service attacks as described in &xep0205;. The following restrictions are encouraged:

1. The user SHOULD NOT be allowed to initiate communication with entities hosted at remote servers.

2. The user SHOULD NOT be allowed to establish long-term relationships such as presence subscriptions, &xep0045; registrations, or &xep0060; subscriptions; however, if the server allows this, it MUST cancel such relationships when the user's session ends.

3. The user SHOULD NOT be allowed to permanently store information on the server (e.g., via &xep0049;); however, if the server allows this, it MUST remove such information when the user's session ends.

4. The user SHOULD NOT be allowed to send large numbers of XMPP stanzas or otherwise use large amounts of system resources (e.g., by binding multiple resource identifiers or creating multiple &xep0065; sessions).

Although RFC 4505 allows the initiating entity (client) to provide so-called "trace data" when authenticating via SASL ANONYMOUS, it is NOT RECOMMENDED for the client to include trace data as the XML character data of the <auth/> element (instead, the <auth/> element SHOULD be empty). However, if trace data is included, the server MUST NOT use it for any purpose other than tracing (e.g., in server logs).

The RECOMMENDED protocol flow following TLS negotiation (refer to RFC 3920) is as follows:

1. Client initiates stream to server.

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2. Server replies with stream header.

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3. Server advertises stream features.

   DIGEST-MD5 ANONYMOUS ]]>

4. Client requests SASL ANONYMOUS mechanism.

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5. Server sends <success/>.

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6. Client opens new stream.

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7. Server tells client that resource binding is required.

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8. Client requests that server create a resource for it.

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9. Server replies with full JID.

   59BEC12A-9BAB-452B-88F8-D1563F09E549@example.com/2384F02A7E01 ]]>

The security considerations discussed in RFC 3920 and RFC 4505 apply to the use of SASL ANONYMOUS in XMPP; specific suggestions regarding usage restrictions for anonymous users are provided under the Recommendations section of this document.

This document requires no interaction with &IANA;.

This document requires no interaction with the ®ISTRAR;.