<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 receives a server-to-server connection request from an originating server, it does not accept the request 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>
<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>Unfortunately, that 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 open-source 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 core 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, using digital certificates issued by trusted root certificate authorities (CAs). Documentation of TLS and SASL within XMPP is provided in &xmppcore;. However, the Server Dialback protocol is still in wide use, and probably will be for the foreseeable future given the difficulty (real or perceived) of obtaining digital certificates issued by common CAs. (The &XSF; has worked to make certificates easier to obtain by running the &XMPPICA;.)</p>
<p>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 &rfc3920bis;, it is still provided in this specification for the sake of interoperability.</p>
<p>There are at least four levels of server-to-server federation in Jabber/XMPP networks:</p>
<olstart='1'>
<li><p>Permissive Federation -- a server accepts a connection from any other peer on the network, even without verifiying the identity of the peer based on DNS lookups. The lack of peer verification or authentication means that domains can be spoofed.</p></li>
<li><p>Verified Federation -- a server accepts a connection from a peer only after the identity of the peer has been weakly verified based on information obtained via the Domain Name System (DNS). However, the connection is not encrypted or authenticated. The use of identity verification effectively prevents domain spoofing, but federation requires proper DNS setup and is still subject to DNS poisoning attacks.</p></li>
<li><p>Encrypted Federation -- a server accepts a connection from a peer only if the peer supports Transport Layer Security (TLS) as defined for XMPP in <cite>RFC 3920</cite> and the peer presents a digital certificate. However, the certificate can be self-signed, in which case mutual authentication is typically not possible. Therefore, after STARTTLS negotiation the parties proceed to weakly verify identity based on DNS information as under Verified Federation. This combination results in an encrypted connection with weak identity verification.</p></li>
<li><p>Trusted Federation -- a server accepts a connection from a peer only if the peer supports Transport Layer Security (TLS) and the peer presents a digital certificate issued by a trusted root certificate authority (CA). The list of trusted root CAs is determined by local service policy, as is the level of trust accorded to various types of certificates (e.g., Class 1, Class 2, or Class 3). The use of trusted domain certificates effectively prevents DNS poisoning attacks and results in mutual authentication.</p></li>
<p>This specification documents the technology that enabled the Jabber server network to advance beyond Permissive Federation to Verified Federation. Combined with the use of TLS, Server Dialback can also result in Encrypted Federation. However, Trusted Federation is not possible with Server Dialback.</p>
<p>Note: For detailed examples showing the protocol flows and outcomes of dialback negotiation for a wide variety of federation scenarios, refer to &xep0238;.</p>
<p>Server Dialback is a method for weak identify 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 trust domain (e.g., the trust domain associated with a given second-level DNS domain such as "example.com" and all of its subdomains).</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 uni-directional, 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 must be completed in each direction in order to enable bi-directional communication between two domains.</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>
<li><p>When STARTTLS negotiation succeeds with a peer service but the peer's certificate is not strong enough to result in mutual authentication via SASL (e.g., because the certificate presented by the peer service during TLS negotiation is self-signed and local service policies stipulate that it is preferable to weakly identify the peer service via Server Dialback rather than depend on the self-signed certificate for identity verification).</p></li>
<p>Both of these scenarios result in an untrusted connection (verified federation in the first scenario and encrypted federation in the second scenario). 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>Dialback is not used if SASL is used for server-to-server authentication, since SASL provides strong authentication using certificates, pre-established passwords, or other credentials.</p>
<p>A service cannot begin negotiation of Server Dialback unless its peer advertises support for the Server Dialback protocol. As described under <linkurl='#o2r-processinitial'>Receiving Server Processes Initial Stream Header</link>, a peer advertises support through inclusion of the Server Dialback namespace declaration in its response stream header and (for XMPP 1.0 servers) through inclusion of the Server Dialback stream feature.</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 "called back" the authoritative server for the domain asserted by the sending server, and verified that the sending server is truly authorized to generate XMPP traffic for that domain.</p>
<p>A helpful analogy might be the following telephone scenario:</p>
<li>A representative from your electric utility company knocks on your front door and says he needs to enter your house.</li>
<li>Rather than letting him in, you ask for his employee ID number and politely close the door for a few moments.</li>
<li>You open the phone book, find the authoritative phone number for the utility company's headquarters, and call them on the phone.</li>
<li>After being transferred to the customer service department, you ask if a rep with that particular ID number is authorized to be visiting your house.</li>
<li>The company tells you that the rep is authorized, so you thank them and hang up.</li>
<li>You then reopen the front door and allow the rep to enter your house.</li>
<p>In Server Dialback, the equivalent of the utility company representative is the ORIGINATING SERVER, i.e., the machine that wants to send a message to an entity at a destination domain and thus is attempting to establish a connection between the two servers. 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 the message and thus is trying to authenticate that the Originating Server represents the domain which it claims to be. And the equivalent of the company headquarters is the AUTHORITATIVE SERVER, i.e., the machine that answers to a DNS lookup for the domain asserted by the originating server (which is not necessarily the machine associated with the originating server); for basic environments 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>
<li>The Originating Server performs a DNS lookup on the hostname of the Receiving Server, opens a TCP connection to the discovered IP address and port, and establishes an XML stream with the Receiving Server.</li>
<li>The Receiving Server does not immediately accept the connection but instead performs a DNS lookup on the hostname of the Authoritative Server, opens a TCP connection to the discovered IP address and port, and establishes an XML stream with the Authoritative Server.</li>
<p>Note: In Steps 1 and 3, it is not always necessary to open a new TCP connection and establish a new stream; for details, see the section on <linkurl='#piggybacking'>Reuse of Negotiated Connections ("Piggybacking")</link>.</p>
<li>The Originating Server is "example.org" (there is no IP address associated with this domain since it is merely asserted by the Originating Server).</li>
<li>The Receiving Server is "xmpp.example.com" and the discovered IP address for the XMPP service at that domain is "192.0.2.1" or "192.0.2.2" (see below).</li>
<li>The Authoritative Server is "example.org" and the discovered IP address for the XMPP service at that domain is "192.0.2.23".</li>
<p>Note: Any error that occurs during dialback negotiation MUST be considered a stream error, resulting in termination of the stream and potentially of the underlying TCP connection. The possible error conditions are specified in the protocol description below.</p>
<p>Before opening a TCP connection to the Receiving Server, the Originating Server first needs to determine the appropriate IP address and port at which to connect. This is done by resolving the Receiving Server's hostname ("xmpp.example.com") using the Domain Name System. As described in <cite>XMPP Core</cite>, the Originating Server will first attempt to resolve a TCP service of _xmpp-server for that hostname using DNS SRV records. Here we assume that example.com has the following records in its DNS configuration:</p>
<examplecaption="DNS SRV Record for Receiving Server"><![CDATA[
_xmpp-server._tcp.xmpp.example.com. 86400 IN SRV 10 0 5269 x1.example.com
_xmpp-server._tcp.xmpp.example.com. 86400 IN SRV 20 0 9625 x2.example.com
]]></example>
<p>These records show that server-to-server connections for the XMPP service "xmpp.example.com" are serviced by two machines: x1.example.com at port 5269 and x2.example.com at port 9625.</p>
<p>Based on the SRV record weights, the Originating Server would then attempt to resolve one of these machines further. Here the Originating Server resolves x2.example.com, for which a standard A lookup yields an IP address of "192.0.2.2".</p>
<p>Note: As described in <cite>XMPP Core</cite>, if the Receiving Server does not provide appropriate DNS SRV records then in order to resolve the hostname of the Receiving Server the Originating Server can fall back to normal IPv4/IPv6 address record resolution to determine the IP address and assume a port of 5269 as registered with the IANA.</p>
<section3topic='Originating Server Opens TCP Connection'anchor='o2r-connect'>
<p>Once the Originating Server has resolved "xmpp.example.com" to an IP address of 192.0.2.2 and port of 9625, it opens a TCP connection to that IP and port.</p>
<p>Note: Instead of opening a new TCP connection to the Receiving Server, the Originating Server MAY reuse an existing TCP connnection; for details, see the <linkurl='#piggybacking'>Reuse of Negotiated Connections ("Piggybacking")</link> section of this document.</p>
<section3topic='Originating Server Sends Initial Stream Header'anchor='o2r-sendinitial'>
<p>Once the Originating Server has opened a TCP connection to the resolved IP address and port of the Receiving Server, it sends an initial stream header to the Receiving Server as described in <cite>XMPP Core</cite>.</p>
<p>Until the initial stream has been validated, the Originating Server MUST NOT send any further XML data to the Receiving Server over that stream.</p>
<p>Note: If the Receiving Server receives any XML stanzas from the Originating Server before the initial stream has been validated, the Receiving Server MUST silently drop those stanzas.</p>
<p>If the Receiving Server can successfully process the initial stream header, it MUST return a response stream header to the Originating Server over the same TCP connection.</p>
<p>In addition to the core XMPP rules regarding response stream headers, the following dialback-related rules apply to the response stream header:</p>
<olstart='1'>
<li>If the initial stream header included a Server Dialback namespace declaration and the Receiving Server supports the Server Dialback protocol, the response stream header also MUST include a declaration for the Server Dialback namespace.</li>
<li>If the initial stream header did not include a Server Dialback namespace declaration and the Receiving Server supports the Server Dialback protocol, the response stream header MAY include a Server Dialback namespace declaration.</li>
<li>If the response stream header includes a Server Dialback namespace declaration, the Server Dialback namespace MUST be 'jabber:server:dialback' and the prefix for the Server Dialback namespace SHOULD be 'db:'.</li>
<p>After sending the response stream header, the Receiving Server also sends stream features to the Originating Server. The Receiving Server SHOULD include the dialback feature in its initial stream features advertisement, including an indication of whether Server Dialback negotiation is optional or required.</p>
<li>The Server Dialback namespace name provided by the Originating Server is incorrect.</li>
<li>The Server Dialback namespace prefix provided by the Originating Server is not supported by the Receiving Server (note: an implementation MAY accept only the 'db:' namespace prefix).</li>
<li>The value of the 'to' address provided by the Originating Server does not match a hostname serviced by the Receiving Server.</li>
<p>If the Server Dialback namespace name is incorrect, then the Receiving Server SHOULD generate an <invalid-namespace/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Invalid Namespace"><![CDATA[
R2O: <stream:error>
<invalid-namespace
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
R2O: </stream:stream>
]]></example>
<p>If the Server Dialback namespace prefix is not supported by the Receiving Server, then the Receiving Server SHOULD generate a <bad-namespace-prefix/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Bad Namespace Prefix"><![CDATA[
R2O: <stream:error>
<bad-namespace-prefix
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
R2O: </stream:stream>
]]></example>
<p>If the value of the 'to' address provided by the Originating Server does not match a hostname serviced by the Receiving Server, then the Receiving Server SHOULD generate a <host-unknown/> or <host-gone/> stream error condition (as appropriate) and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Host Unknown"><![CDATA[
R2O: <stream:error>
<host-unknown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
R2O: </stream:stream>
]]></example>
<p>If the Receiving Server does not allow communication with the hostname of the 'from' address provided by the Originating Server, then the Receiving Server SHOULD generate a <not-authorized/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<pclass='box'>Note Well: The foregoing error flows specify that the Receiving Server SHOULD return a stream error. However, depending on local security policies, the Receiving Server MAY silently terminate the XML stream and underlying TCP connection instead of returning a stream error (e.g., to prevent certain denial of service attacks).</p>
<p>If the response stream header can be successfully processed, the Originating Server MUST generate and send a dialback key as described under <linkurl='#key'>Generation and Exchange of Dialback Key</link>.</p>
<li>The Server Dialback namespace name provided by the Receiving Server is incorrect.</li>
<li>The Server Dialback namespace prefix provided by the Receiving Server is not supported by the Originating Server (note: an implementation MAY accept only the 'db:' namespace prefix).</li>
<li>The value of the 'to' address provided by the Receiving Server does not match a hostname serviced by the Originating Server.</li>
<p>If the Server Dialback namespace name is incorrect, then the Originating Server SHOULD generate an <invalid-namespace/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Invalid Namespace"><![CDATA[
O2R: <stream:error>
<invalid-namespace
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
O2R: </stream:stream>
]]></example>
<p>If the Server Dialback namespace prefix is not supported by the Originating Server, then the Originating Server SHOULD generate a <bad-namespace-prefix/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Bad Namespace Prefix"><![CDATA[
O2R: <stream:error>
<bad-namespace-prefix
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
O2R: </stream:stream>
]]></example>
<p>If the value of the 'to' address provided by the Receiving Server does not match a hostname serviced by the Originating Server, then the Originating Server SHOULD generate a <host-unknown/> or <host-gone/> stream error condition (as appropriate) and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Host Unknown"><![CDATA[
O2R: <stream:error>
<host-unknown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
O2R: </stream:stream>
]]></example>
<p>If the Originating Server does not allow communication with the hostname of the 'from' address provided by the Receiving Server, then the Originating Server SHOULD generate a <not-authorized/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<pclass='box'>Note Well: The foregoing error flows specify that the Originating Server SHOULD return a stream error. However, depending on local security policies, the Originating Server MAY silently terminate the XML stream and underlying TCP connection instead of returning a stream error (e.g., to prevent certain denial of service attacks).</p>
<p>If the Receiving Server does not advertise support for Server Dialback via a Server Dialback namespace declaration or stream feature, then the Originating Server's attempt to negotiate Server Dialback also fails.</p>
<pclass='box'>Note Well: In all of the foregoing error cases, the Originating Server SHOULD consider the Server Dialback negotiation attempt to have failed in an unrecoverable fashion and therefore it SHOULD return a &timeout; stanza error to the local entity that generated the stanza that triggered the Server Dialback negotiation attempt in the first place (if any).</p>
<p>Once the Originating Server has established an XML stream with the Receiving Server, it MUST generate a dialback key for verification by the Authoritative Server.</p>
<p>The method for generating (and verifying) the keys used in Server Dialback MUST take into account the following pieces of information:</p>
<ul>
<li>the hostname of the Originating Server</li>
<li>the hostname of the Receiving Server</li>
<li>the Stream ID</li>
<li>a shared secret known by the Authoritative Server's network</li>
</ul>
<p>The stream ID is security-critical in Server Dialback and therefore MUST be both unpredictable and non-repeating (see &rfc4086; for recommendations regarding randomness for security purposes).</p>
<p>It is RECOMMENDED for the dialback key to be the hexadecimal representation of a Keyed-Hash Message Authentication Code (see &nistfips198a;) generated using the SHA256 hashing algorithm (see &nistfips180-2;), as follows.</p>
<p>The shared secret SHOULD either be set up in a configuration option for each host or process within the Authoritative Server's network or generated as a random string when starting each host or process. The secret's length SHOULD be at least 128 bits or 16 characters long.</p>
<p>Once the Originating Server has processed the response stream header from the Receiving Server and has generated a dialback key for verification by the Receiving Server, it MUST then send that key to the Receiving Server. 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 hostname of the Originating Server and MUST possess a 'to' attribute whose value is the hostname of the Receiving Server.</p>
<p>Note: All XML elements qualified by the Server Dialback namespace MUST be prefixed with the namespace prefix advertised on the stream header originally sent by the entity sending the element.</p>
<examplecaption="Originating Server Sends Dialback Key"><![CDATA[
<p>If the dialback key can be successfully processed, the Receiving Server MUST attempt to open a connection to the Authoritative Server and then ask the Authoritative Server to validate the key provided by the Originating Server, as described in under <linkurl='#r2a'>Stream Setup Between Receiving Server and Authoritative Server</link>.</p>
<p>Note: The dialback key is not examined by the Receiving Server, since the key is validated by the Authoritative Server.</p>
<p>There are several reasons why the Receiving Server's processing of the dialback key might fail:</p>
<olstart='1'>
<li>The Server Dialback namespace prefix provided by the Originating Server is not supported by the Receiving Server (note: an implementation MAY accept only the 'db:' namespace prefix).</li>
<li>The value of the 'to' address provided by the Originating Server does not match a hostname serviced by the Receiving Server.</li>
<li>The Receiving Server does not accept communication with the hostname of the 'from' address provided by the Originating Server.</li>
</ol>
<p>These error cases are described more fully in the remainder of this section.</p>
<p>If the Server Dialback namespace prefix is not supported by the Receiving Server, then the Receiving Server SHOULD generate a <bad-namespace-prefix/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<p>If the value of the 'to' address provided by the Originating Server does not match a hostname serviced by the Receiving Server, then the Receiving Server SHOULD generate a <host-unknown/> or <host-gone/> stream error condition (as appropriate) and terminate both the XML stream and the underlying TCP connection.</p>
<p>If the Receiving Server does not allow communication with the hostname of the 'from' address provided by the Originating Server, then the Receiving Server SHOULD generate a <not-authorized/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<pclass='box'>Note Well: In all of the foregoing error cases, the Originating Server SHOULD consider the Server Dialback negotiation attempt to have failed in an unrecoverable fashion and therefore it SHOULD return a &timeout; stanza error to the local entity that generated the stanza that triggered the Server Dialback negotiation attempt in the first place (if any).</p>
<p>Before opening a TCP connection to the Authoritative Server, the Receiving Server first needs to determine the appropriate IP address and port at which to connect. This is done by resolving the Authoritative Server's hostname ("example.org") using the Domain Name System. As described in <cite>XMPP Core</cite>, the Receiving Server will first attempt to resolve a TCP service of _xmpp-server for that hostname using DNS SRV records. Here we assume that example.org has the following records in its DNS configuration:</p>
<examplecaption="DNS SRV Record for Authoritative Server"><![CDATA[
_xmpp-server._tcp.example.org. 86400 IN SRV 10 0 5269 foo.example.org
]]></example>
<p>These records show that server-to-server connections for the XMPP service "example.org" are serviced by the physical machine foo.example.org at port 5269.</p>
<p>The Receiving Server would then resolve that machine to an IP address, in this case "192.0.2.23".</p>
<p>Note: As described in <cite>XMPP Core</cite>, if the Authoritative Server does not provide appropriate DNS SRV records then in order to resolve the hostname of the Authoritative Server the Receiving Server can fall back to normal IPv4/IPv6 address record resolution to determine the IP address and assume a port of 5269 as registered with the IANA.</p>
<section3topic='Receiving Server Opens TCP Connection'anchor='r2a-connect'>
<p>Once the Receiving Server has resolved "example.org" to an IP address of 192.0.2.23 and port of 5269, it opens a TCP connection to that IP and port.</p>
<p>Note: Instead of opening a new TCP connection to the Authoritative Server, the Receiving Server MAY reuse an existing TCP connnection; for details, see the <linkurl='#piggybacking'>Reuse of Negotiated Connections ("Piggybacking")</link> section of this document.</p>
<section3topic='Receiving Server Sends Initial Stream Header'anchor='r2a-sendinitial'>
<p>Once the Receiving Server has opened a TCP connection to the resolved IP address and port of the Authoritative Server, it sends an initial stream header to the Authoritative Server as described in <cite>XMPP Core</cite>.</p>
<p>If the Authoritative Server can successfully process the initial stream header, it MUST return a response stream header to the Receiving Server over the same TCP connection.</p>
<p>In addition to the core XMPP rules regarding response stream headers, the following dialback-related rules apply to the response stream header:</p>
<olstart='1'>
<li>If the initial stream header included a Server Dialback namespace declaration and the Authoritative Server supports the Server Dialback protocol, the response stream header also MUST include a declaration for the Server Dialback namespace.</li>
<li>If the initial stream header did not include a Server Dialback namespace declaration and the Authoritative Server supports the Server Dialback protocol, the response stream header MAY include a Server Dialback namespace declaration.</li>
<li>If the response stream header includes a Server Dialback namespace declaration, the Server Dialback namespace MUST be 'jabber:server:dialback' and the prefix for the Server Dialback namespace SHOULD be 'db:'.</li>
<p>After sending the response stream header, the Authoritative Server also sends stream features to the Receiving Server. The Authoritative Server SHOULD include the dialback feature in its initial stream features advertisement, including an indication of whether Server Dialback negotiation is optional or required.</p>
<li>The Server Dialback namespace name provided by the Receiving Server is incorrect.</li>
<li>The Server Dialback namespace prefix provided by the Receiving Server is not supported by the Authoritative Server (note: an implementation MAY accept only the 'db:' namespace prefix).</li>
<li>The value of the 'to' address provided by the Receiving Server does not match a hostname serviced by the Authoritative Server.</li>
<li>The Authoritative Server does not accept communication with the hostname of the 'from' address provided by the Receiving Server.</li>
</ol>
<p>These error cases are described more fully in the remainder of this section.</p>
<p>If the Server Dialback namespace name is incorrect, then the Authoritative Server SHOULD generate an <invalid-namespace/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Invalid Namespace"><![CDATA[
A2R: <stream:error>
<invalid-namespace
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
A2R: </stream:stream>
]]></example>
<p>If the Server Dialback namespace prefix is not supported by the Authoritative Server, then the Authoritative Server SHOULD generate a <bad-namespace-prefix/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Bad Namespace Prefix"><![CDATA[
A2R: <stream:error>
<bad-namespace-prefix
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
A2R: </stream:stream>
]]></example>
<p>If the value of the 'to' address provided by the Receiving Server does not match a hostname serviced by the Authoritative Server, then the Authoritative Server SHOULD generate a <host-unknown/> or <host-gone/> stream error condition (as appropriate) and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Host Unknown"><![CDATA[
A2R: <stream:error>
<host-unknown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
A2R: </stream:stream>
]]></example>
<p>If the Authoritative Server does not allow communication with the hostname of the 'from' address provided by the Receiving Server, then the Authoritative Server SHOULD generate a <not-authorized/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<pclass='box'>Note Well: The foregoing error flows specify that the Authoritative Server SHOULD return a stream error. However, depending on local security policies, the Authoritative Server MAY silently terminate the XML stream and underlying TCP connection instead of returning a stream error (e.g., to prevent certain denial of service attacks).</p>
<p>If the response stream header can be successfully processed, the Receiving Server MUST send the dialback key it received from the Originating Server as described under <linkurl='#verify'>Exchange of Verification Request between Receiving Server and Authoritative Server</link>.</p>
<li>The Server Dialback namespace name provided by the Authoritative Server is incorrect.</li>
<li>The Server Dialback namespace prefix provided by the Authoritative Server is not supported by the Receiving Server (note: an implementation MAY accept only the 'db:' namespace prefix).</li>
<li>The value of the 'to' address provided by the Authoritative Server does not match a hostname serviced by the Receiving Server.</li>
<li>The Receiving Server does not accept communication with the hostname of the 'from' address provided by the Authoritative Server.</li>
<li>The Authoritative Server does not advertise support for Server Dialback via a Server Dialback namespace declaration or stream feature.</li>
</ol>
<p>These error cases are described more fully in the remainder of this section.</p>
<p>If the Server Dialback namespace name is incorrect, then the Receiving Server SHOULD generate an <invalid-namespace/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Invalid Namespace"><![CDATA[
R2A: <stream:error>
<invalid-namespace
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
R2A: </stream:stream>
]]></example>
<p>If the Server Dialback namespace prefix is not supported by the Receiving Server, then the Receiving Server SHOULD generate a <bad-namespace-prefix/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Bad Namespace Prefix"><![CDATA[
R2A: <stream:error>
<bad-namespace-prefix
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
R2A: </stream:stream>
]]></example>
<p>If the value of the 'to' address provided by the Authoritative Server does not match a hostname serviced by the Receiving Server, then the Receiving Server SHOULD generate a <host-unknown/> or <host-gone/> stream error condition (as appropriate) and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Host Unknown"><![CDATA[
R2A: <stream:error>
<host-unknown
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
R2A: </stream:stream>
]]></example>
<p>If the Receiving Server does not allow communication with the hostname of the 'from' address provided by the Authoritative Server, then the Receiving Server SHOULD generate a <not-authorized/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<pclass='box'>Note Well: The foregoing error flows specify that the Receiving Server SHOULD return a stream error. However, depending on local security policies, the Receiving Server MAY silently terminate the XML stream and underlying TCP connection instead of returning a stream error (e.g., to prevent certain denial of service attacks).</p>
<p>If the Authoritative Server does not advertise support for Server Dialback via a Server Dialback namespace declaration or stream feature, then the Receiving Server's attempt to request verification of the Originating Server's dialback key also fails.</p>
<p>Note: In all of the foregoing error cases, the Receiving Server SHOULD consider the Server Dialback negotiation attempt to have failed in an unrecoverable fashion and therefore it SHOULD return a &remoteconnection; stream error to the Originating Server.</p>
<section2topic='Exchange of Verification Request'anchor='verify'>
<section3topic='Receiving Server Sends Verification Request'anchor='verify-send'>
<p>Once the Receiving Server has established an XML stream with the Authoritative Server, it MUST send to the Authoritative Server the dialback key it received from the Originating Server. This is done by creating a <db:verify/> element whose XML character data is the dialback key; the element MUST possess a 'from' attribute whose value is the hostname of the Receiving Server, MUST possess a 'to' attribute whose value is the hostname of the Originating Server, and MUST possess an 'id' attribute whose value is the stream identifier from the Receiving Server's response stream header to the Originating Server.</p>
<p>Note: All XML elements qualified by the Server Dialback namespace MUST be prefixed with the namespace prefix advertised on the stream header originally sent by the entity sending the element.</p>
<examplecaption="Receiving Server Sends Verification Request"><![CDATA[
<p>If the verification request can be successfully processed, the Authoritative Server MUST validate the dialback key it received from the Receiving Server as described under <linkurl='#validate'>Validation of Dialback Key by Authoritative Server</link>.</p>
<li>The value of the 'to' address provided by the Receiving Server does not match a hostname serviced by the Authoritative Server's network.</li>
<li>The value of the 'from' address provided by the Receiving Server does not match the hostname sent by the Receiving Server in the 'from' address of the initial stream header it sent to the Authoritative Server.</li>
</ol>
<p>These error cases are described more fully in the remainder of this section.</p>
<p>If the value of the 'to' address provided by the Receiving Server does not match a hostname serviced by the Authoritative Server's network, then the Authoritative Server MUST generate a <host-unknown/> or <host-gone/> stream error condition (as appropriate) and terminate both the XML stream and the underlying TCP connection.</p>
<p>If the value of the 'from' address provided by the Receiving Server does not match the hostname sent by the Receiving Server in the 'from' address of the initial stream header it sent to the Authoritative Server, then the Authoritative Server MUST generate an <invalid-from/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<p>Note: In all of the foregoing error cases, the Receiving Server SHOULD consider the Server Dialback negotiation attempt to have failed in an unrecoverable fashion and therefore it SHOULD return a &remoteconnection; stream error to the Originating Server.</p>
<p>The key shall be considered <strong>valid</strong> if the Authoritative Server determines that the key matches the output it would have produced using its key generation algorithm with the inputs specified in the XML attributes of the verification request along with its shared secret.</p>
<p>The key shall be considered <strong>invalid</strong> if the Authoritative Server determines that the key does not match the output it would have produced using its key generation algorithm with the inputs specified in the XML attributes of the verification request along with its shared secret.</p>
<section3topic='Authoritative Server Sends Validation Result'anchor='validate-send'>
<p>Once the Authoritative Server determines whether the key is valid or invalid, it MUST inform the Receiving Server of its determination. This is done by creating a <db:verify/> element whose XML character data is the dialback key; the element MUST possess a 'from' attribute whose value is the hostname of the Originating Server, MUST possess a 'to' attribute whose value is the hostname of the Receiving Server, MUST possess an 'id' attribute whose value is the stream identifier from the Receiving Server's response stream header to the Originating Server as communicated in the verification request, and MUST possess a 'type' attribute whose value is either "valid" or "invalid".</p>
<p>If the validation result can be successfully processed, the Receiving Server MUST inform the Originating Server of the Server Dialback results described under <linkurl='#result'>Communication of Result from Receiving Server to Originating Server</link>. The Receiving Server then SHOULD also terminate the XML stream and the underlying TCP connection between the Receiving Server and the Authoritative Server.</p>
<li>The value of the 'id' attribute does not match that provided by the Receiving Server in the verification request.</li>
<li>The value of the 'from' address does not match the hostname represented by the Originating Server in the 'from' address of the initial stream header it sent to the Receiving Server.</li>
<li>The value of the 'to' address does not match a hostname serviced by the Receiving Server.</li>
</ol>
<p>These error cases are described more fully in the remainder of this section.</p>
<p>If the value of the 'id' attribute does not match that provided by the Receiving Server in the verification request, then the Receiving Server MUST generate an <invalid-id/> stream error condition and terminate both the XML stream and the underlying TCP connection between it and the Authoritative Server.</p>
<examplecaption="Invalid ID"><![CDATA[
R2A: <stream:error>
<invalid-id
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
R2A: </stream:stream>
]]></example>
<p>The value of the 'from' address does not match the hostname represented by the Originating Server in the 'from' address of the initial stream header it sent to the Receiving Server, then the Receiving Server MUST generate an <invalid-from/> stream error condition and terminate both the XML stream and the underlying TCP connection.</p>
<examplecaption="Invalid From"><![CDATA[
R2A: <stream:error>
<invalid-from
xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
R2A: </stream:stream>
]]></example>
<p>If the value of the 'to' address does not match a hostname serviced by the Receiving Server, then the Receiving Server MUST generate a <host-unknown/> or <host-gone/> stream error condition (as appropriate) and terminate both the XML stream and the underlying TCP connection.</p>
<p>Note: In all of the foregoing error cases, the Receiving Server SHOULD consider the Server Dialback negotiation attempt to have failed in an unrecoverable fashion and therefore it SHOULD return a &remoteconnection; stream error to the Originating Server.</p>
<section2topic='Communication and Handling of Verification Result'anchor='result'>
<section3topic='Receiving Server Communicates Verification Result'anchor='result-communicate'>
<p>Once the Receiving Server successfully processes the validation result it received from the Authoritative Server, it informs the Originating Server of the result. 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 hostname of the Receiving Server, MUST possess a 'to' attribute whose value is the hostname of the Originating Server, MUST possess an 'id' attribute whose value is the stream identifier from the Receiving Server's response stream header to the Originating Server, and MUST possess a 'type' attribute whose value is either "valid" or "invalid".</p>
<examplecaption="Receiving Server Sends Verification Result"><