<abstract>This specification defines the Server Dialback protocol, which is used between XMPP servers to provide identity verification. Server Dialback uses the Domain Name System (DNS) as the basis for verifying identity; the basic approach is that when a receiving server accepts a server-to-server connection from an originating server, it does not process traffic over the connection until it has verified a key with an authoritative server for the domain asserted by the originating server. Although Server Dialback does not provide strong authentication or trusted federation and although it is subject to DNS poisoning attacks, it has effectively prevented most instances of address spoofing on the XMPP network since its development in the year 2000.</abstract>
<remark><p>Corrected several small errors and added sentence about the undesirability of the stream features child element, per list discussion.</p></remark>
<remark><p>Per list discussion, reverted the stream features versioning that was added to version 0.10, thus reverting to the format used in versions 0.5 through 0.9 of the spec; corrected several errors in the examples.</p></remark>
<remark><p>Modified stream features to incorporate versioning, thus removing the need for an <errors/> child element; clarified a few points in the text.</p></remark>
<remark><p>To reduce the possibility of confusion, harmonized the protocol sections so that they show only the first dialback negotiation from Originating Server to Receiving Server.</p></remark>
<remark><p>Per list discussion, restored requirement for Receiving Server to close the stream if the dialback key is invalid, since application of dialback to multiple domain pairs will use dialback packets of type error, not type invalid.</p></remark>
<remark><p>Removed stream feature for advertising mere protocol support, using it only for advertising support for enhanced error handling.</p></remark>
<remark><p>Updated to reflect RFC 6120; mandated support for stream feature in XMPP 1.0 implementations; removed requirement for the Receiving Server to close the stream if the dialback key is invalid, since the stream might be used for multiple domain pairs.</p></remark>
<p>When Jabber technologies were first developed in 1998, they were conceived of as a client-server system similar to email, wherein a client would connect to a server in order to communicate with other clients. Similarly, servers would connect with peer servers to provide inter-domain communication (often called "federation"). In a system that allows federation, it is important for a server to be able to determine the identity of a peer server; accepting a connection from any peer without determining its identity would result in the use of merely asserted identities and a completely uncontrolled approach to federation, which on the open Internet would rapidly devolve into chaos. Clearly such a state of affairs would be unsustainable for a network protocol aiming for widespread deployment.</p>
<p>Such potential chaos was the state of affairs on the Jabber network during the earliest releases of the original &jabberd; server codebase (up through the 1.0 release in May 2000). Therefore the Jabber developer community designed a protocol ("Server Dialback") for weak identity verification based on the Domain Name System (DNS), built support for that protocol into the jabberd 1.2 server (released in October 2000), and mandated support for that protocol on the emerging Jabber server network.</p>
<p>When the early Jabber protocols were formalized by the XMPP Working Group of the &IETF; in 2002-2004, support for strong identity verification was added. That support takes the form of Transport Layer Security (TLS) for encryption of server-to-server XML streams and the Simple Authentication and Security Layer (SASL) for authentication of such streams, typically using digital certificates issued by trusted root certificate authorities (CAs). However, the Server Dialback protocol is still in wide use, and probably will be for the foreseeable future given the perceived difficulty of obtaining digital certificates issued by common CAs. Therefore it is important to maintain accurate documentation of the Server Dialback protocol. Such documentation was originally provided in &rfc3920;. Although that documentation was removed from &rfc6120;, it is still provided in this specification for the sake of interoperability.</p>
<p>Server Dialback is a method for weak identity verification. Such verification depends on the Domain Name System (DNS) and the use of keys based on a shared secret known to all XMPP servers within a given administrative domain.</p>
<p>Since October 2000, the use of Server Dialback has made it more difficult to spoof the hostnames of servers (and therefore the addresses of sent messages) on the XMPP network. However, Server Dialback does not provide authentication between servers and is not a security mechanism. Domains requiring high security are advised to use TLS and SASL with certificates issued by trusted roots.</p>
<p>Server Dialback is unidirectional, and results in weak identity verification for one XML stream in one direction. Because Server Dialback is not an authentication mechanism, mutual authentication is not possible via dialback. Therefore, Server Dialback needs to be completed in each direction in order to enable bidirectional communication between two domains.</p>
<p>Dialback does not verify that the IP address returned by a DNS lookup of the originating domain is the same as the source IP address of the inbound TCP connection. While this might often be true, not performing this check enables large deployments to separate inbound and outbound message routing.</p>
<li><p>When a peer service does not support XMPP 1.0 as defined in <cite>RFC 3920</cite> or, more generally, does not offer negotiation of TLS.</p></li>
<p>Both of these scenarios result in an untrusted connection. However, depending on local security policies, a server might accept such an untrusted connection if the use of Server Dialback results in weak identity verification.</p>
<p>The basic idea behind Server Dialback is that a receiving server does not accept XMPP traffic from a sending server until it has (a) "called back" the authoritative server for the domain asserted by the sending server and (b) verified that the sending server is truly authorized to generate XMPP traffic for that domain.</p>
<li>After being transferred to the customer service department, you ask if a worker with that particular ID number is authorized to be visiting your house.</li>
<li>The company tells you that the worker is authorized, so you thank them and hang up.</li>
<li>You then reopen the front door and allow the worker to enter your house.</li>
<p>In Server Dialback, the equivalent of the utility company worker is the ORIGINATING SERVER, i.e., the machine that wants to send a message from an entity at the SENDER DOMAIN to an entity at the TARGET DOMAIN and thus is attempting to establish a connection for the DOMAIN PAIR (combination of Sender Domain and Target Domain). The equivalent of the person at the house is the RECEIVING SERVER, i.e., the machine to which the Originating Server has opened a connection for the purpose of sending a message from the Sender Domain to the Target Domain (and thus the machine that is trying to verify that the Originating Server represents the Sender Domain). And the equivalent of the company headquarters is the AUTHORITATIVE SERVER, i.e., the machine that is discovered from a DNS lookup for the Sender Domain; for simple deployments this will be the Originating Server, but it could be a separate machine in the Originating Server's network (where "network" is defined by knowledge of a shared secret for verification of dialback keys).</p>
<p>The basic flow of events in Server Dialback consists of the following four steps:</p>
<li>The Originating Server generates a dialback key and sends that value over its XML stream with the Receiving Server. (If the Originating Server does not yet have an XML stream to the Receiving Server, it will first need to perform a DNS lookup on the Target Domain and thus discover the Receiving Server, open a TCP connection to the discovered IP address and port, and establish an XML stream with the Receiving Server.)</li>
<li>Instead of immediately accepting XML stanzas on the connection from the Originating Server, the Receiving Server sends the same dialback key over its XML stream with the Authoritative Server for verification. (If the Receiving Server does not yet have an XML stream to the Authoritative Server, it will first need to perform a DNS lookup on the Sender Domain and thus discover the Authoritative Server, open a TCP connection to the discovered IP address and port, and establish an XML stream with the Authoritative Server).</li>
<p>After Step 4, the Originating Server is authorized to send stanzas from the Sender Domain to the Target Domain as communicated in the 'to' and 'from' attributes of the dialback negotiation. In addition to a weak identity verification of the Sender Domain, this also ensures that the Receiving Server is accepting stanzas for the Target Domain.</p>
<p>Note: All XML elements qualified by the Server Dialback namespace MUST be prefixed with the namespace prefix for the 'jabber:server:dialback' namespace as advertised on the stream header originally sent by the entity sending the element. <note>RFC 3920 stipulated that "an implementation SHOULD generate only the 'db:' prefix for such elements and MAY accept only the 'db:' prefix." This restriction was included for the sake of backward compatibility with the jabberd 1.x codebase and is no longer necessary.</note></p>
<p>Section 2.1 describes the protocol from the perspective of an active, outbound connection. Section 2.2 describes the protocol from the perspective of an inbound connection. Note that both parts can be implemented, tested, and used separately.</p>
<p>On an outbound connection there are two different tasks that the sending server can perform. The first task is to request authorization to send stanzas from the Sender Domain to the Target Domain, which is described under Section 2.1.1. The second task is to respond to requests on the validity of a given dialback key as described under Section 2.1.2.</p>
<section3topic="Originating Server Generates Outbound Request for Authorization by Receiving Server">
<p>This subsection describes the interaction between the Originating Server and the Receiving Server, from the perspective of the Originating Server.</p>
<p>When the Originating Server has stanzas to send from the Sender Domain to the Target Domain, does not have a verified connection, or is currently attempting to get a verified connection for this domain pair, it sends a new dialback key to the Receiving Server.</p>
<p>This is done by creating a <db:result/> element whose XML character data is the dialback key; the element MUST possess a 'from' attribute whose value is the Sender Domain and MUST possess a 'to' attribute whose value is the Target Domain.</p>
<p>Note: The Receiving Server MAY use any method to determine the validity of the dialback key and the identity of the Originating Sever. The Originating Server MUST NOT make any assumptions about how the Receiving Server verifies the key. This includes the assumption that the key is ever verified by the Receiving Server.</p>
<p>After that, the Originating Server waits for the verification result. If the Originating Server wishes to send any stanzas for this domain pair, it MUST queue them for sending after it has received authorization to send stanzas from the Receiving Server, and MUST NOT attempt to send stanzas until it has received such authorization. The Originating Server MUST NOT attempt to re-verify the domain pair on this TCP connection.</p>
<p>Note: While waiting for the verification result, the Originating Server SHOULD continue to send stanzas for any domain pair that has already been verified on that connection. It MAY send out additional dialback keys for different domain pairs and issue dialback verification requests as described under Section 2.1.2. To avoid Denial-of-Service attacks (&rfc4732;), the Originating Server MAY impose a timeout on key verification.</p>
<p>If the stream or the underlying TCP connection is closed by the remote side while waiting for the verification result, this is to be handled similar to receiving an error as described below.</p>
<p>If the value of the 'type' attribute is "valid", then the connection between the domain pair is considered verified and the Originating Server can send any outbound stanzas it has queued up for routing to the Receiving Server for the domain pair.</p>
<p>If the value of the 'type' attribute is "invalid", then the Receiving Server is reporting that Originating Server's identity (as valid for the Sender Domain) could not be verified by the Authoritative Server. In this case, the Originating Server MUST NOT attempt to send any outbound stanzas it has queued up for routing to the Receiving Server for the domain pair. In addition, the Receiving Server MUST close the stream as described in Section 4.4 of <cite>RFC 6120</cite>.</p>
<p>If the value of the 'type' attribute is "error", this indicates a problem which is not related to the validity of the dialback key provided. The error conditions are explained in detail under <linkurl='#advertisement-errors'>Dialback with Error Handling</link>. Such an error is to be considered non-fatal for the XML stream, but the Originating Server MUST return any queued stanzas to the respective senders at the Sender Domain with a &timeout; stanza error.</p>
<p>This subsection describes the interaction between the Receiving Server and the Authoritative Server, from the perspective of the Receiving Server.</p>
<p>To determine the validity of a dialback key received from the Originating Server, the Receiving Server needs to establish communications with the Authoritative Server. To do so, either it can reuse an existing XML stream or it needs to establish a new connection. To establish a new connection, the Receiving Server performs a DNS lookup on the Sender Domain, thus finding the IP address and port for server-to-server communication at an authoritative machine for the Sender Domain asserted by the Originating Server (here the machine is "authority.sender.tld").</p>
<p>After the XML stream is established from the Receiving Server to the Authoritative Server, the Receiving Server sends a verification request. This is done by creating a <db:verify/> element whose XML character data is the dialback key received from the Originating Server; the element MUST possess a 'from' attribute whose value is the Target Domain, MUST possess a 'to' attribute whose value is the Sender Domain as provided in the 'from' attribute of Step 1, and MUST possess an 'id' attribute whose value is the stream identifier of the Receiving Server's response stream header to the Originating Server. The combination of 'from', 'to', and 'id' attributes makes it possible for the Receiving Server to uniquely identify the TCP connection on which it received the original request in Step 1.</p>
<p>After that, the Receiving Server waits for the verification result. While doing so, it can still use the connection to send any dialback packets or stanzas for domain pairs that have already been validated.</p>
<p>In addition to the values "valid" and "invalid", the 'type' attribute can also have a value of "error"; see <linkurl='#advertisement-errors'>Dialback with Error Handling</link> for a detailed explanation.</p>
<p>Note: If the underlying TCP connection is closed by the remote side while there are pending verification requests, those requests SHOULD be considered failed and therefore be treated like an error response.</p>
<p>After receiving the validation result from the Authoritative Server, the Receiving Server determines the inbound connection that the dialback key was originally received on. This connection is uniquely identified by the combination of the 'from', 'to', and 'id' attributes. If no inbound connection is found that matches this combination, the verification result MAY be dropped silently. If an inbound connection is found, the Receiving Server uses it to communicate the verification result to the Originating Server. A positive result indicates the readiness of the Receiving Server to accept stanzas from the Originating Server for this domain pair.</p>
<p>There are two different tasks on an inbound connection. The first task is to authorize inbound connections, which is described under Section 2.2.1. The second task is to answer requests for the validity of a dialback key, which is described under Section 2.2.2.</p>
<section3topic="Receiving Server Handles Inbound Authorization Request from Originating Server">
<p>This subsection describes the interaction between the Originating Server and the Receiving Server, from the perspective of the Receiving Server (i.e., this section is the mirror image of Section 2.1.1).</p>
<examplecaption="Receiving Server Receives Dialback Key from Originating Server (Step 1)"><