XMPP Core  allows XMPP server implementations to support any SASL mechanism (see RFC 4422 ) when authenticating clients. This document provides recommendations regarding use of the SASL ANONYMOUS mechanism (see RFC 4505 ) in XMPP systems.
XMPP server implementations can be deployed in a variety of settings. Although it is difficult to provide recommendations for every kind of XMPP deployment, this document attempts to strike a balance between more and less controlled settings by defining three different deployment types:
An XMPP server implementation SHOULD NOT enable the SASL ANONYMOUS mechanism by default, but instead SHOULD force the administrator of a given service to explicitly enable support in the context of that deployment.
When a client authenticates using SASL ANONYMOUS, an XMPP server SHOULD assign a temporary, unique bare JID <email@example.com> to the client. Although the method for ensuring the uniqueness of the localpart is a matter of implementation, it is RECOMMENDED for the localpart to be a UUID as specified in RFC 4122 .
Although RFC 4505 allows the 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., not use it as the resource identifier of the anonymous user's full JID).
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 Best Practices to Discourage Denial of Service Attacks (XEP-0205) ), especially on public deployments. The following restrictions are encouraged on public deployments. Administrators of private deployments and specialized deployments are advised to take these restrictions into account when configuring their services, but can reasonably relax these restrictions if they have appropriate access controls in place or their deployment requirements cannot be met using the more restrictive profile applied in public deployments.
During resource binding, the server MAY ignore the resource identifier provided by the client (if any) and instead assign a resource identifier that it generates on behalf of the client.
The server SHOULD NOT allow the client to initiate communication with entities hosted at remote servers.
The server MAY allow the client to establish relationships with local services and users; such relationships might include presence subscriptions and roster additions (see XMPP IM ), Multi-User Chat (XEP-0045)  registrations, and Publish-Subscribe (XEP-0060)  subscriptions. (Note that allowing presence subscriptions and roster additions can create a sub-optimal user experience for the added contacts.) However, if the server permits such relationships, it MUST cancel them when the client's session ends.
The server MAY allow the client to store information on the server for the purpsoe of providing an optimal user experience (e.g., storage of client preferences using Private XML Storage (XEP-0049) ). However, if the server allows this, it SHOULD remove such information when the client's session ends.
The server SHOULD NOT allow the client 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 SOCKS5 Bytestreams (XEP-0065)  sessions).
The RECOMMENDED protocol flow following TLS negotiation (refer to RFC 3920) is as follows:
Client initiates stream to server.
Server replies with stream header.
Server advertises stream features.
Client requests SASL ANONYMOUS mechanism.
Server sends <success/>.
Client opens new stream.
Server tells client that resource binding is required.
Client requests that server create a resource for it.
Server replies with full JID.
A server MUST reply to a Service Discovery (XEP-0030)  information ("disco#info") request sent to the bare JID <firstname.lastname@example.org> of the user with an identity of "account/anonymous", as shown in the following example.
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 the Internet Assigned Numbers Authority (IANA) .
This document requires no interaction with the XMPP Registrar .
Thanks to Dave Cridland, Tuomas Koski, Jack Moffitt, Andy Skelton, and Kurt Zeilenga for their feedback.
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This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).
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.
The primary venue for discussion of XMPP Extension Protocols is the <email@example.com> 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 <firstname.lastname@example.org>.
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".
12. 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/>.
13. 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 <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Provided more detailed recommendations regarding usage restrictions for anonymous users, including the concept of different deployment types; added note about the account/anonymous service discovery identity.
Recommended that node identifier be a UUID; recommended that trace data not be included.
Per a vote of the Jabber Council, advanced status to Active.
Initial version; modified flow to remove unecessary challenge.