Postfix XCLIENT Howto
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Purpose of the XCLIENT extension to SMTP
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:
1. 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.
2. 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.
3. 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.
XCLIENT Command syntax
An example client-server conversation is given at the end of this document.
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.
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.
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.
xclient-command = XCLIENT 1*( SP attribute-name"="attribute-value )
attribute-name = ( NAME | ADDR | PORT | PROTO | HELO | LOGIN | DESTADDR |
DESTPORT )
attribute-value = xtext
* Attribute values are xtext encoded as per RFC 1891.
* 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.
* 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 [].
* The PORT attribute specifies a remote SMTP client TCP port number as a
decimal number, or [UNAVAILABLE] when the information is unavailable.
* The PROTO attribute specifies either SMTP or ESMTP.
* 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 [].
* The DESTPORT attribute specifies a local SMTP server TCP port number as a
decimal number, or [UNAVAILABLE] when the information is unavailable.
* The HELO attribute specifies an SMTP HELO parameter value, or the value
[UNAVAILABLE] when the information is unavailable.
* The LOGIN attribute specifies a SASL login name, or the value [UNAVAILABLE]
when the information is unavailable.
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.
Note 2: [UNAVAILABLE], [TEMPUNAVAIL] and IPV6: may be specified in upper case,
lower case or mixed case.
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.
Note 4: Some Postfix implementations do not implement the PORT or LOGIN
attributes.
XCLIENT Server response
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.
For practical reasons it is not always possible to reset the complete server
state to the initial SMTP greeting protocol stage:
* 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.
* The SMTP server must not reset attributes that were received with the last
XCLIENT command. This includes HELO or PROTO attributes.
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.
XCLIENT server reply codes
|Code |Meaning |
| | |
|220 |success |
| | |
|421 |unable to proceed, disconnecting |
| | |
|501 |bad command parameter syntax |
| | |
|503 |mail transaction in progress |
| | |
|550 |insufficient authorization |
| | |
|other|connection rejected by connection-level access decision|
| | |
XCLIENT Example
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.
220 server.example.com ESMTP Postfix
EHLO client.example.com
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-XCLIENT NAME ADDR PROTO HELO
250 8BITMIME
XCLIENT NAME=spike.porcupine.org ADDR=168.100.189.2
220 server.example.com ESMTP Postfix
EHLO spike.porcupine.org
250-server.example.com
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-XCLIENT NAME ADDR PROTO HELO
250 8BITMIME
MAIL FROM:<wietse@porcupine.org>
250 Ok
RCPT TO:<user@example.com>
250 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
. . .message content. . .
.
250 Ok: queued as 763402AAE6
QUIT
221 Bye
Security
The XCLIENT command changes audit trails and/or SMTP client access permissions.
Use of this command must be restricted to authorized SMTP clients.
SMTP connection caching
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.
References
Moore, K, "SMTP Service Extension for Delivery Status Notifications", RFC 1891,
January 1996.
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