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0.13rc4

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Peter Saint-Andre 2012-08-08 10:08:27 -06:00
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@ -7,7 +7,7 @@
<xep> <xep>
<header> <header>
<title>Server Dialback</title> <title>Server Dialback</title>
<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 initiating 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> <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 initiating 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. Additionally, the protocol is used to negotitate whether the receiving server is accepting stanzas for the target domain. Although Server Dialback does not provide strong authentication and it is subject to DNS poisoning attacks, it has effectively prevented address spoofing on the XMPP network since its development in the year 2000.</abstract>
&LEGALNOTICE; &LEGALNOTICE;
<number>0220</number> <number>0220</number>
<status>Experimental</status> <status>Experimental</status>
@ -29,9 +29,9 @@
<jid>fippo@psyced.org</jid> <jid>fippo@psyced.org</jid>
</author> </author>
<revision> <revision>
<version>0.13rc3</version> <version>0.13rc4</version>
<initials>ph/psa</initials> <initials>ph/psa</initials>
<date>2012-07-10</date> <date>2012-07-26</date>
<remark> <remark>
<ul> <ul>
<li>Allowed same SRV target in multiplexing business</li> <li>Allowed same SRV target in multiplexing business</li>
@ -40,8 +40,8 @@
<li>Added note about using certificates for "dialback-without-dial-back".</li> <li>Added note about using certificates for "dialback-without-dial-back".</li>
<li>Added note about not forwarding invalid.</li> <li>Added note about not forwarding invalid.</li>
<li>Added another figure for the examples, reordered examples accordingly.</li> <li>Added another figure for the examples, reordered examples accordingly.</li>
<li>Notes about verify-only connections.</li> <li>Notes about verify-only connections.</li>
<li>Added note about bouncing stanzas with an &lt;internal-server-error/&gt; stanza error in section 2.1.1</li> <li>Added note about bouncing stanzas with an &lt;internal-server-error/&gt; stanza error in section 2.1.1</li>
<li>Clarified explanations thoughout the document.</li> <li>Clarified explanations thoughout the document.</li>
<li>Added terminology section.</li> <li>Added terminology section.</li>
<li>Removed hokey telephone analogy.</li> <li>Removed hokey telephone analogy.</li>
@ -158,18 +158,16 @@
<section1 topic="Introduction" anchor="intro"> <section1 topic="Introduction" anchor="intro">
<section2 topic="Why Dialback?" anchor="intro-why"> <section2 topic="Why Dialback?" anchor="intro-why">
<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>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. Therefore the Jabber developer community designed a protocol called "Server Dialback" for 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>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 called "Server Dialback" for 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>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. The protocol also ensures that the receiving server is accepting stanzas for the target domain.</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> <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 (see &rfc3920;). 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 certification authorities (CAs). However, the Server Dialback protocol is still in wide use. In addition, the slow but steady deployment of the DNS security extensions (DNSSEC) &rfc4033; can provide a stronger basis for using Server Dialback.</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 (see &rfc3920;). 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 certification 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. In addition, the slow but steady deployment of the DNS security extensions (DNSSEC) &rfc4033; can provide a stronger basis for using Server Dialback, as explored in &dna-dns;. Therefore it is important to maintain accurate documentation of the Server Dialback protocol.</p>
</section2> </section2>
<section2 topic="What Dialback Accomplishes" anchor="intro-what"> <section2 topic="What Dialback Accomplishes" anchor="intro-what">
<p>Server Dialback is a method for identity verification: if the dialback negotiation succeeds, the receiving server for an XML stream can associate a pair of domain names with the stream (on the concept of a domain name association, see &dna-framework;); those two domain names are the sender domain asserted by the initiating server and the domain name at the receiving server that the initiating server has indicated it wishes to communicate with.</p> <p>Server Dialback is a method for identity verification: if the dialback negotiation succeeds, the receiving server for an XML stream can associate a pair of domain names with the stream; those two domain names are the sender domain asserted by the initiating server and the domain name at the receiving server that the initiating server has indicated it wishes to communicate with.</p>
<p>The verification accomplished in Server Dialback 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. It is a proof-of-possession protocol in the sense of &rfc4949; which asserts that the initiating server and the authoritative server are associated with each other. The relative strength or weakness of the verificaiton depends in part on the strength or weakness of the process for resolving the domain names of the authoritative server; in particular, if DNSSEC is not used then Server Dialback results in weak identity verification, whereas if DNSSEC is used then Server Dialback can result in fairly strong identity verification.</p> <p>The verification accomplished in Server Dialback 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. It is a proof-of-possession protocol in the sense of &rfc4949; which asserts that the initiating server and the authoritative server are associated with each other. The relative strength or weakness of the verification depends in part on the strength or weakness of the process for resolving the domain names of the authoritative server; in particular, if DNSSEC is not used then Server Dialback results in weak identity verification, whereas if DNSSEC is used then Server Dialback can result in fairly strong identity verification.</p>
<p>Since October 2000, the use of Server Dialback (even absent DNSSEC) has made it more difficult to spoof the hostnames of servers (and therefore the addresses of sent messages) on the XMPP network.</p> <p>Since October 2000, the use of Server Dialback (even absent DNSSEC) has made it difficult to spoof the hostnames of servers (and therefore the addresses of sent messages) on the XMPP network.</p>
<p>Server Dialback is unidirectional, and results in weak verification for one XML stream in one direction. Because mutual authentication is not achieved directly by this protocol, Server Dialback needs to be completed in each direction in order to enable bidirectional communication between two domains.</p> <p>Server Dialback is unidirectional, and results in weak verification for one XML stream in one direction. Because traditionally Server-to-Server connections are used unidirectionally, Server Dialback needs to be completed in each direction in order to enable bidirectional communication between two domains (unless &xep0288; is used).</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>
</section2> </section2>
<section2 topic="Terminology" anchor="intro-terms"> <section2 topic="Terminology" anchor="intro-terms">
@ -185,14 +183,14 @@
</section2> </section2>
<section2 topic="How Dialback Works" anchor="intro-howitworks"> <section2 topic="How Dialback Works" anchor="intro-howitworks">
<p>The basic flow of events in Server Dialback consists of the following four steps:</p> <p>Server Dialback is used when a stanza is to be sent from a Sender Domain must be routed to a Target Domain and there is not yet an established connection between the domains. The basic flow of events in Server Dialback consists of the following four steps:</p>
<ol start='1'> <ol start='1'>
<li><p>The Initiating Server generates a dialback key and sends that value over its XML stream with the Receiving Server. (If the Initiating 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.)</p></li> <li><p>The Initiating Server generates a dialback key and sends that value over its XML stream with the Receiving Server. (If the Initiating 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.)</p></li>
<li><p>Instead of immediately accepting XML stanzas on the connection from the Initiating 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).</p></li> <li><p>Instead of immediately accepting XML stanzas on the connection from the Initiating 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).</p></li>
<li><p>The Authoritative Server informs the Receiving Server whether the key is valid or invalid.</p></li> <li><p>The Authoritative Server informs the Receiving Server whether the key is valid or invalid.</p></li>
<li><p>The Receiving Server informs the Initiating Server whether its identity has been verified or not.</p></li> <li><p>The Receiving Server informs the Initiating Server whether its identity has been verified or not.</p></li>
</ol> </ol>
<p>After Step 4, the Initiating 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 identity verification of the Sender Domain, this also ensures that the Receiving Server is accepting stanzas only for the Target Domain.</p> <p>After Step 4, the Initiating 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 identity verification of the Sender Domain, this also ensures that the Receiving Server is accepting stanzas for the Target Domain.</p>
<p>We can represent the flow of events graphically as follows.</p> <p>We can represent the flow of events graphically as follows.</p>
<code><![CDATA[ <code><![CDATA[
@ -243,14 +241,11 @@ Initiating Receiving
</ul> </ul>
<p>This section can be read in two ways:</p> <p>This section can be read in two ways:</p>
<ol> <ol>
<li><p>To understand the protocol flow of each dialback negotiation, read Section 2.1.1 and Section 2.2.1 (aspects of the dialback negotiation from capulet.lit as Initiating Server to montague.lit as Receiving Server), then Section 2.1.2 and 2.2.2 (aspects of the dialback negotiation from montague.lit as Initiating Server to capulet.lit as Receiving Server).</p></li> <li><p>To understand the overall protocol flow of each dialback negotiation, read Section 2.1.1 and Section 2.2.1 (aspects of the dialback negotiation from capulet.lit as Initiating Server to montague.lit as Receiving Server), then Section 2.1.2 and 2.2.2 (aspects of the dialback negotiation from montague.lit as Initiating Server to capulet.lit as Receiving Server).</p></li>
<li><p>To implement the code for either an outbound connection or an inbound connection, read Section 2.1 (outbound) or Section 2.2 (outbound). Note that both parts can be implemented, tested, and used separately.</p></li> <li><p>To implement the code for either an outbound connection or an inbound connection, read Section 2.1 (outbound) or Section 2.2 (outbound). Note that both parts can be implemented, tested, and used separately.</p></li>
</ol> </ol>
<p>The following figure gives an overview of where each example is embedded in the process and illustrates the changing roles of each server.</p> <p>The following figure gives an overview of where each example is embedded in the process and illustrates the changing roles of each server. FIXME: orchard.capulet.lit and home.montague.lit in headers?</p>
<code><![CDATA[ <code><![CDATA[
#############################################################################
STREAM FROM CAPULET.LIT TO MONTAGUE.LIT
#############################################################################
capulet.lit montague.lit capulet.lit montague.lit
(as Initiating) (as Receiving (as Initiating) (as Receiving
Server) Server) Server) Server)
@ -266,7 +261,6 @@ capulet.lit montague.lit
| stream] | | stream] |
| -----------------> | | -----------------> |
| (ID D60000229F) | | (ID D60000229F) |
| |
| | | |
| send | capulet.lit | send | capulet.lit
| dialback key | (as Authoritative | dialback key | (as Authoritative
@ -299,10 +293,6 @@ capulet.lit montague.lit
| from capulet.lit | | | from capulet.lit | |
| to montague.lit | | | to montague.lit | |
| | | | | |
#############################################################################
STREAM FROM MONTAGUE.LIT TO CAPULET.LIT
#############################################################################
| | |
| montague.lit capulet.lit | montague.lit capulet.lit
| (as Initiating (as Receiving | (as Initiating (as Receiving
| Server) Server) | Server) Server)
@ -313,7 +303,6 @@ capulet.lit montague.lit
| | open new stream] | | | open new stream] |
| | -----------------> | | | -----------------> |
| | (ID 417GAF25) | | | (ID 417GAF25) |
| | |
| | | montague.lit | | | montague.lit
| | send | (as Authoritative | | send | (as Authoritative
| | dialback key | Server) | | dialback key | Server)
@ -323,7 +312,6 @@ capulet.lit montague.lit
| | | connection or | | | | connection or |
| | | open new stream] | | | | open new stream] |
| | | -----------------> | | | | -----------------> |
| | | (ID 417GAF25) |
| | | | | | | |
| | | send | | | | send |
| | | verify request | | | | verify request |
@ -343,10 +331,9 @@ capulet.lit montague.lit
| | from montague.lit | | | | from montague.lit | |
| | to capulet.lit | | | | to capulet.lit | |
]]></code> ]]></code>
<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>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.</note></p>
<section2 topic="Outbound Connection"> <section2 topic="Outbound Connection">
<p>On an outbound connection there are two different tasks:</p> <p>On an outbound connection there are two different tasks:</p>
<ol> <ol>
<li>Request authorization to send stanzas from a Sender Domain to a Target Domain, i.e., act as an Initiating Server in relation to a Receiving Server; this is is described under Section 2.1.1.</li> <li>Request authorization to send stanzas from a Sender Domain to a Target Domain, i.e., act as an Initiating Server in relation to a Receiving Server; this is is described under Section 2.1.1.</li>
@ -355,8 +342,8 @@ capulet.lit montague.lit
<section3 topic="Initiating Server Generates Outbound Request for Authorization by Receiving Server"> <section3 topic="Initiating Server Generates Outbound Request for Authorization by Receiving Server">
<p>This subsection describes the interaction between the server hosting capulet.lit (acting as an Initiating Server) and the server hosting montague.lit (acting as a Receiving Server), from the outbound perspective of the Initiating Server.</p> <p>This subsection describes the interaction between the server hosting capulet.lit (acting as an Initiating Server) and the server hosting montague.lit (acting as a Receiving Server), from the outbound perspective of the Initiating Server.</p>
<p>When the Initiating 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>When the Initiating Server has stanzas to send from the Sender Domain to the Target Domain, does not have a verified connection, is currently not attempting to get a verified connection for this domain pair, it sends a new dialback key to the Receiving Server.</p>
<p>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 Initiating Server performs a DNS lookup on the Target Domain, thus finding the IP address and port for server-to-server communication at an authoritative machine for the Target Domain (here the "home.montague.lit").</p> <p>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 Initiating Server performs a DNS lookup on the Target Domain, thus finding the IP address and port for server-to-server communication at an authoritative machine for the Target Domain (which is assumed to be "home.montague.lit").</p>
<p>After the XML stream is established from the Initiating Server to the Receiving Server, the Initiating Server sends a dialback key to the Receiving Server. This is done by creating a &lt;db:result/&gt; 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>After the XML stream is established from the Initiating Server to the Receiving Server, the Initiating Server sends a dialback key to the Receiving Server. This is done by creating a &lt;db:result/&gt; 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>
<example caption="Initiating Server Sends Dialback Key (Step 1)"><![CDATA[ <example caption="Initiating Server Sends Dialback Key (Step 1)"><![CDATA[
send: <db:result send: <db:result
@ -374,8 +361,8 @@ key = HMAC-SHA256(
= 404fe54e60d0259b2b6d9a620e72990ab3e8fe2faca420653da71eb80597e5a4 = 404fe54e60d0259b2b6d9a620e72990ab3e8fe2faca420653da71eb80597e5a4
</code> </code>
<p>Note: The Receiving Server MAY use any method to determine the validity of the dialback key and the identity of the Initiating Server. The Initiating Server MUST NOT make any assumptions about how the Receiving Server verifies the key. This includes the assumption that the key is even verified by the Receiving Server through communication with the Authoritative Server.</p> <p>Note: The Receiving Server MAY use any method to determine the validity of the dialback key and the identity of the Initiating Server. The Initiating Server MUST NOT make any assumptions about how the Receiving Server verifies the key. This includes the assumption that the key is even verified by the Receiving Server through communication with the Authoritative Server.</p>
<p>After sending the dialback key, the Initiating Server waits for the verification result from the Receiving Server. If the Initiating 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 Initiating Server MUST NOT attempt to re-verify the domain pair on this TCP connection.</p> <p>After sending the dialback key, the Initiating Server waits for the verification result from the Receiving Server. If the Initiating 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. FIXME: change wording, allow re-verify after certain time, not too often? The Initiating Server MUST NOT attempt to re-verify the domain pair on this TCP connection.</p>
<p>Note: While waiting for the verification result, the Initiating 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 Initiating Server MAY impose a timeout on key verification.</p> <p>Note: While waiting for the verification result, the Initiating 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;) or deadlock situations, the Initiating Server MAY impose a timeout on dialback operations, i.e. it should consider dialback operations to be failed when there is no response for a certain amount of time.</p>
<p>If the stream or the underlying TCP connection is closed by the Receiving Server while the Initiating Server is waiting for the verification result, the Initiating Server shall behave as it does when receiving a dialback error as described below.</p> <p>If the stream or the underlying TCP connection is closed by the Receiving Server while the Initiating Server is waiting for the verification result, the Initiating Server shall behave as it does when receiving a dialback error as described below.</p>
<p>After the Receiving Server has verified the request, the Initiating Server receives the verification result in the form of a &lt;db:result/&gt; element, where the 'from' attribute MUST be the Target Domain, the 'to' attribute MUST be the Sender Domain, and the 'type' attribute MUST have a value of "valid" or "invalid" (for the value of "error", see below).</p> <p>After the Receiving Server has verified the request, the Initiating Server receives the verification result in the form of a &lt;db:result/&gt; element, where the 'from' attribute MUST be the Target Domain, the 'to' attribute MUST be the Sender Domain, and the 'type' attribute MUST have a value of "valid" or "invalid" (for the value of "error", see below).</p>
<p>Thus the result is either valid...</p> <p>Thus the result is either valid...</p>
@ -394,8 +381,8 @@ recv: <db:result
]]></example> ]]></example>
<p>Note: There are no examples for Step 2 and Step 3 in this section of the document; see the examples under Sections 2.1.2 and 2.2.2.</p> <p>Note: There are no examples for Step 2 and Step 3 in this section of the document; see the examples under Sections 2.1.2 and 2.2.2.</p>
<p>If the value of the 'type' attribute is "valid", then the connection between the domain pair is considered verified and the Initiating Server can send any outbound stanzas it has queued up for routing to the Receiving Server for the domain pair (i.e., from the Sender Domain to the Target Domain). Naturally, the Initiating Server can also enable or negotiate other stream features at this point, such as &xep0138; and &xep0198;.</p> <p>If the value of the 'type' attribute is "valid", then the connection between the domain pair is considered verified and the Initiating Server can send any outbound stanzas it has queued up for routing to the Receiving Server for the domain pair (i.e., from the Sender Domain to the Target Domain). Naturally, the Initiating Server can also enable or negotiate other stream features at this point, such as &xep0138; and &xep0198;.</p>
<p>If the value of the 'type' attribute is "invalid", then the Receiving Server is reporting that Initiating Server's identity (as valid for the Sender Domain) could not be verified (e.g., by checking it with the Authoritative Server). In this case, the Initiating Server MUST NOT attempt to send any outbound stanzas it has queued up for routing to the Receiving Server for the domain pair but instead MUST return such stanzas to the respective senders at the Sender Domain with an &internalserver; stanza error. Since the Receiving Server will most likely close the stream and the underlying TCP connection if that occurs (see Section 2.2.1), the Initiating Server SHOULD NOT attempt to send further stanzas for other domain pairs that have already been authorized.</p> <p>If the value of the 'type' attribute is "invalid", then the Receiving Server is reporting that Initiating Server's identity (as valid for the Sender Domain) was verified as bogus by the Authoritative Server. In this case, the Initiating Server MUST NOT attempt to send any outbound stanzas it has queued up for routing to the Receiving Server for the domain pair but instead MUST return such stanzas to the respective senders at the Sender Domain with an &internalserver; stanza error. Since the Receiving Server will most likely close the stream and the underlying TCP connection if that occurs (see Section 2.2.1), the Initiating Server SHOULD NOT attempt to send further stanzas for other domain pairs that have already been authorized.</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 <link url='#advertisement-errors'>Dialback with Error Handling</link>. Such an error is to be considered non-fatal for the XML stream, but the Initiating Server MUST return any queued stanzas to the respective senders at the Sender Domain with a &timeout; stanza error.</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 <link url='#advertisement-errors'>Dialback with Error Handling</link>. Such an error is non-fatal for the XML stream, but the Initiating Server MUST return any queued stanzas to the respective senders at the Sender Domain with a &timeout; stanza error.</p>
<example caption="Initiating Server Receives Dialback Error from Receiving Server (Step 4)"><![CDATA[ <example caption="Initiating Server Receives Dialback Error from Receiving Server (Step 4)"><![CDATA[
recv: <db:result recv: <db:result
from='montague.lit' from='montague.lit'
@ -410,7 +397,7 @@ recv: <db:result
<section3 topic="Receiving Server Generates Outbound Request for Verification of Initiating Server by Authoritative Server"> <section3 topic="Receiving Server Generates Outbound Request for Verification of Initiating Server by Authoritative Server">
<p>This subsection describes the interaction between the server hosting capulet.lit (acting as a Receiving Server) and the server hosting montague.lit (acting as an Authoritative Server), from the outbound perspective of the Receiving Server.</p> <p>This subsection describes the interaction between the server hosting capulet.lit (acting as a Receiving Server) and the server hosting montague.lit (acting as an Authoritative Server), from the outbound perspective of the Receiving Server.</p>
<p>To determine the validity of a dialback key received from the Initiating Server, the Receiving Server needs to establish communications with the Authoritative Server. To do so, either it can reuse an existing XML stream or 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 Initiating Server (here the machine is "orchard.capulet.lit").</p> <p>To determine the validity of a dialback key received from the Initiating Server, the Receiving Server needs to establish communications with the Authoritative Server. To do so, it can reuse an existing XML stream or 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 Initiating Server (here the machine is "orchard.capulet.lit").</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 &lt;db:verify/&gt; element whose XML character data is the dialback key received from the Initiating 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 ID of the response stream header sent from the Receiving Server to the Initiating Server (here "417GAF25"). 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 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 &lt;db:verify/&gt; element whose XML character data is the dialback key received from the Initiating 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 ID of the response stream header sent from the Receiving Server to the Initiating Server (here "417GAF25"). 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>Note: An implementation MAY open a separate connection to the Authoritative Server for the sole purpose of doing key verification. Such an implementation SHOULD close the connection immediately after receiving the verification result. Not using TLS or any other stream features can reduce the number of round trips in that case.</p> <p>Note: An implementation MAY open a separate connection to the Authoritative Server for the sole purpose of doing key verification. Such an implementation SHOULD close the connection immediately after receiving the verification result. Not using TLS or any other stream features can reduce the number of round trips in that case.</p>
<example caption="Receiving Server Sends Verification Request to Authoritative Server (Step 2)"><![CDATA[ <example caption="Receiving Server Sends Verification Request to Authoritative Server (Step 2)"><![CDATA[
@ -454,7 +441,7 @@ recv: <db:verify
]]></example> ]]></example>
<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>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 SHOULD be dropped silently. If an inbound connection is found, the Receiving Server uses it to communicate the verification result to the Initiating Server. A positive result indicates the readiness of the Receiving Server to accept stanzas from the Initiating Server for this domain pair.</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 SHOULD be dropped silently. If an inbound connection is found, the Receiving Server uses it to communicate the verification result to the Initiating Server. A positive result indicates the readiness of the Receiving Server to accept stanzas from the Initiating Server for this domain pair.</p>
<p>When receiving a verification result of type "invalid", the Receiving Server MAY choose not to relay this result to the Initiating Server. Instead, it might send a dialback error such as &lt;forbidden/&gt; to the Initiating Server. Compared to sending a result of type "invalid", this behavior will not result in the loss of the whole stream and any previously domain pairs previously negotiated, while at the same time not accepting stanzas from the spoofed domain.</p> <p>When receiving a verification result of type "invalid", the Receiving Server MAY choose not to relay this result to the Initiating Server. Instead, it might send a dialback error such as &lt;forbidden/&gt; to the Initiating Server. Compared to sending a result of type "invalid", this behavior will not result in the loss of the whole stream and any previously domain pairs previously negotiated, while at the same time not accepting stanzas from the spoofed domain. Even when not forwarding the "invalid" result, the incident should be logged.</p>
</section3> </section3>
</section2> </section2>
@ -476,10 +463,9 @@ recv: <db:result
</db:result> </db:result>
]]></example> ]]></example>
<p>This key MUST be verified before the Initiating Server is authorized to send stanzas from the Sender Domain ("capulet.lit") to the Target Domain ("montague.lit"). Note that the verification process might fail prematurely, for example, if the Receiving Server's policy states that connections from the Initiating Server or the Sender Domain are not allowed.</p> <p>This key MUST be verified before the Initiating Server is authorized to send stanzas from the Sender Domain ("capulet.lit") to the Target Domain ("montague.lit"). Note that the verification process might fail prematurely, for example, if the Receiving Server's policy states that connections from the Initiating Server or the Sender Domain are not allowed.</p>
<p>The traditional method for verifying that the Initiating Server is authorized to send stanzas from the Sender Domain is for the Receiving Server to "dial back" the Authoritative Server for the Sender Domain and ask it to validate the dialback key which is contained in the XML character data of the request. However, other methods can be used for verifying the identity of the Initiating Server. For example, if TLS is used the the Receiving Server can attempt to verify the certificate (according to the rules specified in &xep0178; and &rfc6125;, or according to some other method such as the one discussed in &dna-dns;) and then send a dialback result without performing the actual dial-back to the Authoritative Server (this technique is sometimes called "dialback without dial-back") <p>The traditional method for verifying that the Initiating Server is authorized to send stanzas from the Sender Domain is for the Receiving Server to "dial back" the Authoritative Server for the Sender Domain and ask it to validate the dialback key which is contained in the XML character data of the request. However, other methods can be used for verifying the identity of the Initiating Server. For example, if TLS is used the the Receiving Server can attempt to verify the certificate (according to the rules specified in &xep0178; and &rfc6125; and then send a dialback result without performing the actual dial-back to the Authoritative Server. This technique is sometimes called "dialback without dial-back".</p>
.</p>
<p>Note: The Receiving Server MUST continue to accept and process stanzas for already verified domain pairs, and MUST continue to process both &lt;db:result/&gt; and &lt;db:verify/&gt; elements.</p> <p>Note: The Receiving Server MUST continue to accept and process stanzas for already verified domain pairs, and MUST continue to process both &lt;db:result/&gt; and &lt;db:verify/&gt; elements.</p>
<p>If the Target Domain as given in the 'to' attribute of the element is not a configured domain of the Receiving Server, this results in a dialback error. This error, which is explained further under <link url='#advertisement-errors'>Section 2.4.2</link>, is not a stream error and therefore MUST NOT result in closing of the stream as described in Section 4.4 of <cite>RFC 6120</cite>, since the stream might already be used for exchanging XML stanzas for other domain pairs.</p> <p>If the Target Domain as given in the 'to' attribute of the element is not a configured domain of the Receiving Server, this results in a dialback error. This error, which is explained further under <link url='#advertisement-errors'>Section 2.4.2</link>, is not a stream error and therefore MUST NOT result in closing of the stream as described in Section 4.4 of <cite>RFC 6120</cite>, since the stream might already be used to exchange XML stanzas for other domain pairs.</p>
<example caption="Receiving Server Sends Dialback Error to Initiating Server (Step 4)"><![CDATA[ <example caption="Receiving Server Sends Dialback Error to Initiating Server (Step 4)"><![CDATA[
send: <db:result send: <db:result
from='montague.lit' from='montague.lit'
@ -506,7 +492,7 @@ send: <db:result
to='capulet.lit' to='capulet.lit'
type='invalid'/> type='invalid'/>
]]></example> ]]></example>
<p>If the type is "invalid", the Initiating Server is attempting to spoof the Sender Domain. The Receiving Server MUST NOT accept stanzas from the Initiating Server for the Sender Domain, SHOULD log the attempt, and MUST close the XML stream (as described in Section 4.4 of <cite>RFC 6120</cite>).</p> <p>If the type is "invalid", the Initiating Server is attempting to spoof the Sender Domain. The Receiving Server MUST NOT accept stanzas from the Initiating Server for the Sender Domain, SHOULD log the attempt, and MUST close the XML stream.</p>
</section3> </section3>
<section3 topic="Authoritative Server Handles Inbound Verification Request from Receiving Server"> <section3 topic="Authoritative Server Handles Inbound Verification Request from Receiving Server">
@ -535,7 +521,7 @@ send: <db:verify
<code> <code>
key = HMAC-SHA256( key = HMAC-SHA256(
SHA256('d14lb4ck43v3r'), SHA256('d14lb4ck43v3r'),
{ 'capulet.lit', ' ', 'montague.lit', ' ', '417GAF25' } { 'capulet.lit', ' ', 'montague.lit', ' ', '417GAF25' }
) )
= d4afb251ac62eb6fc778dac7ad65c43fdee29c4930ba204d479191566ea99496 = d4afb251ac62eb6fc778dac7ad65c43fdee29c4930ba204d479191566ea99496
</code> </code>
@ -561,7 +547,7 @@ send: <db:verify
</section2> </section2>
<section2 topic="Directionality" anchor='directionality'> <section2 topic="Directionality" anchor='directionality'>
<p>The result of the protocol exchanges shown in the foregoing two sections is that the server hosting montague.lit (acting as a Receiving Server) has verified the identity of the server hosting capulet.lit (acting as an Initiating Server); as a result, the Initiating Server can send, and the Receiving Server can accept, XML stanzas over the stream from capulet.lit to montague.lit. In order for montague.lit to send stanzas to capulet.lit, dialback MUST be completed in the opposite direction as well (i.e., with a reversal of roles so that the server hosting montague.lit would act as an Initiating Server and capulet.lit would act as a Receiving Server).</p> <p>The result of the protocol exchanges shown in the foregoing two sections is that the server hosting montague.lit has verified the identity of the server hosting capulet.lit and vice versa. Since XMPP Server-to-Server connections are unidirectional (unless <cite>XEP-0288</cite> is used), Dialback has been completed in each direction before XML stanzas could be exchanged over the two TCP connections between the servers.</p>
</section2> </section2>
<section2 topic="Advertisement" anchor='advertisement'> <section2 topic="Advertisement" anchor='advertisement'>
@ -596,8 +582,8 @@ send: <db:verify
<section2 topic="Dialback Error Conditions" anchor='errors'> <section2 topic="Dialback Error Conditions" anchor='errors'>
<!-- credits: Matthias in http://mail.jabber.org/pipermail/standards/2007-June/015662.html --> <!-- credits: Matthias in http://mail.jabber.org/pipermail/standards/2007-June/015662.html -->
<p><cite>RFC 3920</cite> introduced stream errors for any errors related to dialback. However, this turned out to be overly aggressive, particularly if the XML stream was used to multiplex stanzas for more than one domain pair (since closing the stream would result in throwing away accumulated dialback state for a potentially large number of domain pairs). Therefore this specification introduces a third value for the 'type' attribute: "error".</p> <p><cite>RFC 3920</cite> introduced stream errors for any errors related to dialback. However, this turned out to be overly aggressive, particularly if the XML stream was used to multiplex stanzas for more than one domain pair (since closing the stream would result in throwing away accumulated dialback state for a potentially large number of domain pairs). Therefore this specification introduces a third value for the 'type' attribute: "error".</p>
<p>This usage of the 'error' value for the 'type' attribute is not fully backward compatible with <cite>RFC 3920</cite>. However, the server that generates the error SHOULD still attempt to send the dialback error instead of terminating the stream, as the worst thing that can happen is that the remote server terminates the stream if it does not understand the error or if it eventually times out the connection. Furthermore, a server SHOULD send these errors only to XMPP 1.0 peers that advertise support for dialback errors as described under Section 2.4.2. Dialback errors are to be considered non-fatal for the XML stream, but the Initiating Server MUST return queued stanzas to the respective senders with a &timeout; stanza error. If an error is encountered in Step 3 of the dialback negotiation, the Receiving Server MUST send a &lt;remote-server-not-found/&gt; error to the Initiating Server.</p> <p>This usage of the 'error' value for the 'type' attribute is not fully backward compatible with <cite>RFC 3920</cite>. However, the server that generates the error SHOULD still attempt to send the dialback error instead of terminating the stream, as the worst thing that can happen is that the remote server terminates the stream if it does not understand the error or if it eventually times out the connection. Dialback errors are to be considered non-fatal for the XML stream, but the Initiating Server MUST return queued stanzas to the respective senders with a &timeout; stanza error. If an error is encountered in Step 3 of the dialback negotiation, the Receiving Server MUST send a &lt;remote-server-not-found/&gt; error to the Initiating Server.</p>
<p>When the &lt;db:verify/&gt; or &lt;db:result/&gt; element is of type "error", the element MUST contain an &lt;error/&gt; element qualified by the Server Dialback namespace, which MUST in turn contain an XML element qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace (i.e., a stanza error condition), in accordance with the following table.</p> <p>FIXME When the &lt;db:verify/&gt; or &lt;db:result/&gt; element is of type "error", the element MUST contain an &lt;error/&gt; element qualified by the Server Dialback namespace, which MUST in turn contain an XML element qualified by the 'urn:ietf:params:xml:ns:xmpp-stanzas' namespace (i.e., a stanza error condition), in accordance with the following table.</p>
<table caption='Dialback error conditions'> <table caption='Dialback error conditions'>
<tr> <tr>
<th>Condition</th> <th>Condition</th>
@ -626,23 +612,23 @@ send: <db:verify
</tr> </tr>
<tr> <tr>
<td>&policy;</td> <td>&policy;</td>
<td>The Receiving Server enforces a policy which mandates usage of TLS before dialback and the Initiating Server sent the dialback request without using TLS.</td> <td>The Receiving Server enforces a policy which mandates usage of TLS before dialback and the Initiating Server sent the dialback request without using TLS.</td>
<td>Step 3 or 4</td> <td>Step 3 or 4</td>
</tr> </tr>
<tr> <tr>
<td>&notauthorized;</td> <td>&notauthorized;</td>
<td>The Receiving Server enforces a policy which requires a valid x509 certificate containing the identity of the Sender Domain for dialback requests and the Initiating Server did not provide a certificate with an identity that matches the Sender Domain.</td> <td>The Receiving Server enforces a policy which requires a valid x509 certificate containing the identity of the Sender Domain for dialback requests and the Initiating Server did not provide a certificate with an identity that matches the Sender Domain.</td>
<td>Step 3</td> <td>Step 3</td>
</tr> </tr>
<tr> <tr>
<td>&forbidden;</td> <td>&forbidden;</td>
<td>The Receiving Server received an "invalid" response when attempting to verify the dialback key with the Authoritative Server.</td> <td>The Receiving Server received an "invalid" response when attempting to verify the dialback key with the Authoritative Server.</td>
<td>Step 4</td> <td>Step 4</td>
</tr> </tr>
<tr> <tr>
<td>&notacceptable;</td> <td>&notacceptable;</td>
<td>The Receiving Server was unable to establish the asserted identity of the Initiating Server.</td> <td>The Receiving Server was unable to establish the asserted identity of the Initiating Server.</td>
<td>Step 4</td> <td>Step 4</td>
</tr> </tr>
</table> </table>
</section2> </section2>
@ -660,9 +646,9 @@ send: <db:verify
<example caption="DNS SRV Record for the montague.lit Zone"><![CDATA[ <example caption="DNS SRV Record for the montague.lit Zone"><![CDATA[
_xmpp-server._tcp.montague.lit. 86400 IN SRV 10 0 5269 home.montague.lit _xmpp-server._tcp.montague.lit. 86400 IN SRV 10 0 5269 home.montague.lit
_xmpp-server._tcp.chat.montague.lit. 86400 IN SRV 10 0 5269 home.montague.lit _xmpp-server._tcp.chat.montague.lit. 86400 IN SRV 10 0 5269 home.montague.lit
home.montague.lit. 86400 IN A 10.44.0.4 home.montague.lit. 86400 IN A 10.44.0.4
]]></example> ]]></example>
<p>Because DNS SRV lookups for both "montague.lit" and "chat.montague.lit" point to the same target ("home.montague.lit") and port (5269), or in absence of SRV records they resolve to the same IP address (10.44.0.4) and port (5269), "capulet.lit" MAY initiate a dialback negotation from "capulet.lit" to "chat.montague.lit" over the same XML stream that is already used to send stanzas from "capulet.lit" to "montague.lit".</p> <p>Because DNS SRV lookups for both "montague.lit" and "chat.montague.lit" point to the same target ("home.montague.lit") or IP address and port (5269), or in absence of SRV records they resolve to the same IP address (10.44.0.4) and port (5269), "capulet.lit" MAY initiate a dialback negotation from "capulet.lit" to "chat.montague.lit" over the same XML stream that is already used to send stanzas from "capulet.lit" to "montague.lit".</p>
<p>&xep0288; extends those rules since any domain that has been used as a source domain can be used as a target domain without further negotiation.</p> <p>&xep0288; extends those rules since any domain that has been used as a source domain can be used as a target domain without further negotiation.</p>
</section3> </section3>
</section2> </section2>
@ -670,7 +656,7 @@ home.montague.lit. 86400 IN A 10.44.0.4
<section1 topic='Security Considerations' anchor='security'> <section1 topic='Security Considerations' anchor='security'>
<p>Server Dialback helps protect against domain spoofing, thus making it more difficult to spoof XML stanzas. Absent the use of DNS security (DNSSEC, &rfc4033;), Server Dialback does not provide a mechanism for authenticating a stream, as is done via TLS and SASL, and results in weak verification of server identities only. Furthermore, if DNSSEC is not used then it is susceptible to DNS poisoning attacks.</p> <p>Server Dialback helps protect against domain spoofing, thus making it more difficult to spoof XML stanzas. Absent the use of DNS security (DNSSEC, &rfc4033;), Server Dialback does not provide a mechanism for authenticating a stream, as is done via TLS and SASL, and results in weak verification of server identities only. Furthermore, if DNSSEC is not used then it is susceptible to DNS poisoning attacks.</p>
<p>If DNSSEC is used, Server Dialback provides stream authentication only (i.e., a strong association between a domain name and an XML stream). However, Server Dialback by itself does not provide confidentiality, data integrity, or stream encryption. Some existing implementations are known to support dialback over TLS, however this specification doe snot define that usage.</p> <p>If DNSSEC is used, Server Dialback provides stream authentication only (i.e., a strong association between a domain name and an XML stream). However, Server Dialback by itself does not provide confidentiality, data integrity, or stream encryption. Some existing implementations are known to support dialback over TLS. Server Dialback is typically carried out the same way as without TLS then, gaining from channel encryption.</p>
</section1> </section1>
<section1 topic='IANA Considerations' anchor='iana'> <section1 topic='IANA Considerations' anchor='iana'>
@ -725,7 +711,7 @@ home.montague.lit. 86400 IN A 10.44.0.4
<xs:attribute name='type' use='optional'> <xs:attribute name='type' use='optional'>
<xs:simpleType> <xs:simpleType>
<xs:restriction base='xs:NCName'> <xs:restriction base='xs:NCName'>
<xs:enumeration value='error'/> <xs:enumeration value='error'/>
<xs:enumeration value='invalid'/> <xs:enumeration value='invalid'/>
<xs:enumeration value='valid'/> <xs:enumeration value='valid'/>
</xs:restriction> </xs:restriction>
@ -744,7 +730,7 @@ home.montague.lit. 86400 IN A 10.44.0.4
<xs:attribute name='type' use='optional'> <xs:attribute name='type' use='optional'>
<xs:simpleType> <xs:simpleType>
<xs:restriction base='xs:NCName'> <xs:restriction base='xs:NCName'>
<xs:enumeration value='error'/> <xs:enumeration value='error'/>
<xs:enumeration value='invalid'/> <xs:enumeration value='invalid'/>
<xs:enumeration value='valid'/> <xs:enumeration value='valid'/>
</xs:restriction> </xs:restriction>