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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix XCLIENT Howto</h1>

<hr>

<h2>Purpose of the XCLIENT extension to SMTP</h2>

<p> When an SMTP server announces support for the XCLIENT command,
an SMTP client may send information that overrides one or more
client-related session attributes. The XCLIENT command targets the
following problems: </p>

<ol>

    <li> <p> Access control tests.  SMTP server access rules are
    difficult to verify when decisions can be triggered only by
    remote clients.  In order to facilitate access rule testing,
    an authorized SMTP client test program needs the ability to
    override the SMTP server's idea of the SMTP client hostname,
    network address, and other client information, for the entire
    duration of an SMTP session.  </p>

    <li> <p> Client software that downloads mail from an up-stream
    mail server and injects it into a local MTA via SMTP. In order
    to take advantage of the local MTA's SMTP server access rules,
    the client software needs the ability to override the SMTP
    server's idea of the remote client name, client address and
    other information.  Such information can typically be extracted
    from the up-stream mail server's Received:  message header. </p>

    <li> <p> Post-filter access control and logging. With
    Internet-&gt;filter-&gt;MTA style content filter applications,
    the filter can be simplified if it can delegate decisions
    concerning mail relay and other access control to the MTA. This
    is especially useful when the filter acts as a transparent
    proxy for SMTP commands.  This requires that the filter can
    override the MTA's idea of the SMTP client hostname, network
    address, and other information. </p>

</ol>

<h2>XCLIENT Command syntax</h2>

<p> An example client-server conversation is given at the end
of this document. </p>

<p> In SMTP server EHLO replies, the keyword associated with this
extension is XCLIENT. It is followed by the names of the attributes
that the XCLIENT implementation supports.  </p>

<p> The XCLIENT command may be sent at any time, except in the
middle of a mail delivery transaction (i.e.  between MAIL and DOT,
or MAIL and RSET).  The XCLIENT command may be pipelined when the
server supports ESMTP command pipelining. To avoid triggering
spamware detectors, the command should be sent at the end of a
command group.  </p>

<p> The syntax of XCLIENT requests is described below.  Upper case
and quoted strings specify terminals, lowercase strings specify
meta terminals, and SP is whitespace.  Although command and attribute
names are shown in upper case, they are in fact case insensitive.
</p>

<blockquote>
<p>
    xclient-command = XCLIENT 1*( SP attribute-name"="attribute-value )
</p>
<p>
    attribute-name = ( NAME | ADDR | PORT | PROTO | HELO | LOGIN | DESTADDR | DESTPORT )
</p>
<p>
    attribute-value = xtext
</p>
</blockquote>

<ul>

    <li> <p> Attribute values are xtext encoded as per <a href="https://tools.ietf.org/html/rfc1891">RFC 1891</a>.
    </p>

    <li> <p> The NAME attribute specifies a remote SMTP client
    hostname (not an SMTP client address), [UNAVAILABLE] when client
    hostname lookup failed due to a permanent error, or [TEMPUNAVAIL]
    when the lookup error condition was transient. </p>

    <li> <p> The ADDR attribute specifies a remote SMTP client
    numerical IPv4 network address, an IPv6 address prefixed with
    IPV6:, or [UNAVAILABLE] when the address information is
    unavailable.  Address information is not enclosed with []. </p>

    <li> <p> The PORT attribute specifies a remote SMTP client TCP
    port number as a decimal number, or [UNAVAILABLE] when the
    information is unavailable.  </p>

    <li> <p> The PROTO attribute specifies either SMTP or ESMTP.
    </p>

    <li> <p> The DESTADDR attribute specifies a local SMTP server
    numerical IPv4 network address, an IPv6 address prefixed with
    IPV6:, or [UNAVAILABLE] when the address information is
    unavailable.  Address information is not enclosed with []. </p>

    <li> <p> The DESTPORT attribute specifies a local SMTP server
    TCP port number as a decimal number, or [UNAVAILABLE] when the
    information is unavailable.  </p>

    <li> <p> The HELO attribute specifies an SMTP HELO parameter
    value, or the value [UNAVAILABLE] when the information is
    unavailable.  </p>

    <li> <p> The LOGIN attribute specifies a SASL login name, or
    the value [UNAVAILABLE] when the information is unavailable.
    </p>

</ul>

<p> Note 1: syntactically valid NAME and HELO attribute-value
elements can be up to 255 characters long. The client must not send
XCLIENT commands that exceed the 512 character limit for SMTP
commands. To avoid exceeding the limit the client should send the
information in multiple XCLIENT commands; for example, send NAME
and ADDR last, after HELO and PROTO. Once ADDR is sent, the client
is usually no longer authorized to send XCLIENT commands. </p>

<p> Note 2: [UNAVAILABLE], [TEMPUNAVAIL] and IPV6:  may be specified
in upper case, lower case or mixed case. </p>

<p> Note 3: Postfix implementations prior to version 2.3 do not
xtext encode attribute values. Servers that wish to interoperate
with these older implementations should be prepared to receive
unencoded information. </p>

<p> Note 4: The PORT attribute is implemented in Postfix 2.5 and
later; the LOGIN attribute in Postfix 2.9 and later. </p>

<h2>XCLIENT Server response</h2>

<p> Upon receipt of a correctly formatted XCLIENT command, the
server resets state to the initial SMTP greeting protocol stage.
Depending on the outcome of optional access decisions, the server
responds with 220 or with a suitable rejection code.

<p> For practical reasons it is not always possible to reset the
complete server state to the initial SMTP greeting protocol stage:
</p>

<ul>

<li> <p> TLS session information may not be reset, because turning off
TLS leaves the connection in an undefined state.  Consequently, the
server may not announce STARTTLS when TLS is already active, and
access decisions may be influenced by client certificate information
that was received prior to the XCLIENT command. </p>

<li> <p> The SMTP server must not reset attributes that were received
with the last XCLIENT command. This includes HELO or PROTO attributes.
</p>

</ul>

<p> NOTE: Postfix implementations prior to version 2.3 do not jump
back to the initial SMTP greeting protocol stage.  These older
implementations will not correctly simulate connection-level access
decisions under some conditions.  </p>

<h2> XCLIENT server reply codes </h2>

<blockquote>

<table border="1" bgcolor="#f0f0ff">

<tr> <th> Code </th> <th> Meaning </th> </tr>

<tr> <td> 220 </td> <td> success  </td> </tr>

<tr> <td> 421 </td> <td> unable to proceed, disconnecting </td> </tr>

<tr> <td> 501 </td> <td> bad command parameter syntax </td> </tr>

<tr> <td> 503 </td> <td> mail transaction in progress </td> </tr>

<tr> <td> 550 </td> <td> insufficient authorization </td> </tr>

<tr> <td> other </td> <td> connection rejected by connection-level
access decision </td> </tr>

</table>

</blockquote>

<h2>XCLIENT Example</h2>

<p> In the example, the client impersonates a mail originating
system by passing all SMTP client information via the XCLIENT
command.  Information sent by the client is shown in bold font.
</p>

<blockquote>
<pre>
220 server.example.com ESMTP Postfix
<b>EHLO client.example.com</b>
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-XCLIENT NAME ADDR PROTO HELO
250 8BITMIME
<b>XCLIENT NAME=spike.porcupine.org ADDR=168.100.189.2</b>
220 server.example.com ESMTP Postfix
<b>EHLO spike.porcupine.org</b>
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-XCLIENT NAME ADDR PROTO HELO
250 8BITMIME
<b>MAIL FROM:&lt;wietse (a] porcupine.org&gt;</b>
250 Ok
<b>RCPT TO:&lt;user (a] example.com&gt;</b>
250 Ok
<b>DATA</b>
354 End data with &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt;
<b>. . .<i>message content</i>. . .</b>
<b>.</b>
250 Ok: queued as 763402AAE6
<b>QUIT</b>
221 Bye
</pre>
</blockquote>

<h2>Security</h2>

<p> The XCLIENT command changes audit trails and/or SMTP client
access permissions. Use of this command must be restricted to
authorized SMTP clients. </p>

<h2>SMTP connection caching</h2>

<p> XCLIENT attributes persist until the end of an SMTP session.
If one session is used to deliver mail on behalf of different SMTP
clients, the XCLIENT attributes need to be reset as appropriate
before each MAIL FROM command. </p>

<h2> References </h2>

<p> Moore, K, "SMTP Service Extension for Delivery Status Notifications",
<a href="https://tools.ietf.org/html/rfc1891">RFC 1891</a>, January 1996. </p>

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