XCLIENT_README.html [plain text]
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<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix XCLIENT Howto</h1>
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<h2>Purpose of the XCLIENT extension to SMTP</h2>
<p> 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->filter->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> Examples of client-server conversations are 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). The
XCLIENT command may be pipelined when the server supports ESMTP
command pipelining. </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 | PROTO | HELO )
</p>
</blockquote>
<ul>
<li> <p> The NAME attribute specifies an 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 an 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 PROTO attribute specifies either SMTP or ESMTP.
</p>
<li> <p> The HELO attribute specifies an SMTP HELO parameter
value, or the value [UNAVAILABLE] when the information is
unavailable. </p>
</ul>
<p> Note 1: syntactically valid NAME and HELO attributes 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. </p>
<p> Note 2: [UNAVAILABLE], [TEMPUNAVAIL] and IPV6: may be specified
in upper case, lower case or mixed case. </p>
<p> The XCLIENT server reply codes are as follows: </p>
<blockquote>
<table border="1" bgcolor="#f0f0ff">
<tr> <th> Code </th> <th> Meaning </th> </tr>
<tr> <td> 250 </td> <td> success </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> 421 </td> <td> unable to proceed, disconnecting </td> </tr>
</table>
</blockquote>
<h2>XCLIENT Examples</h2>
<p> In the first example, the client impersonates a mail originating
system by passing all SMTP session information via XCLIENT commands.
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 PROTO=ESMTP </b>
250 Ok
<b>XCLIENT HELO=spike.porcupine.org</b>
250 Ok
<b>MAIL FROM:<wietse@porcupine.org></b>
250 Ok
<b>RCPT TO:<user@example.com></b>
250 Ok
<b>DATA</b>
354 End data with <CR><LF>.<CR><LF>
<b>. . .<i>message content</i>. . .</b>
<b>.</b>
250 Ok: queued as 763402AAE6
<b>QUIT</b>
221 Bye
</pre>
</blockquote>
<p> In the second example, the client impersonates a mail originating
system by sending the XCLIENT command before the EHLO or HELO command.
This increases the realism of impersonation, but requires that the
client knows ahead of time what XCLIENT options the server supports.
</p>
<blockquote>
<pre>
220 server.example.com ESMTP Postfix
<b>XCLIENT NAME=spike.porcupine.org ADDR=168.100.189.2</b>
250 Ok
<b>HELO spike.porcupine.org</b>
250 server.example.com
<b>MAIL FROM:<wietse@porcupine.org></b>
250 Ok
<b>RCPT TO:<user@example.com></b>
250 Ok
<b>DATA</b>
354 End data with <CR><LF>.<CR><LF>
<b>. . .<i>message content</i>. . .</b>
<b>.</b>
250 Ok: queued as CF1E52AAE7
<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. However, the XCLIENT command should not
override its own access control mechanism. </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>
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