Postfix XFORWARD Howto

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Purpose of the XFORWARD extension to SMTP

When an SMTP server announces support for the XFORWARD command, an SMTP client
may send information that overrides one or more client-related logging
attributes. The XFORWARD command targets the following problem:

  * Logging after SMTP-based content filter. With the deployment of Internet-
    >MTA1->filter->MTA2 style content filter applications, the logging of
    client and message identifying information changes when MTA1 gives the mail
    to the content filter. To simplify the interpretation of MTA2 logging, it
    would help if MTA1 could forward remote client and/or message identifying
    information through the content filter to MTA2, so that the information
    could be logged as part of mail handling transactions.

This extension is implemented as a separate ESMTP command, and can be used to
transmit client or message attributes incrementally. It is not implemented by
passing additional parameters via the MAIL FROM command, because doing so would
require extending the MAIL FROM command length limit by another 600 or more
characters beyond the space that is already needed to support other extensions
such as AUTH and DSN.

XFORWARD Command syntax

An example of a client-server conversation is given at the end of this
document.

In SMTP server EHLO replies, the keyword associated with this extension is
XFORWARD. The keyword is followed by the names of the attributes that the
XFORWARD implementation supports.

After receiving the server's announcement for XFORWARD support, the client may
send XFORWARD requests at any time except in the middle of a mail delivery
transaction (i.e. between MAIL and RSET or DOT). The command may be pipelined
when the server supports ESMTP command pipelining.

The syntax of XFORWARD 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.

    xforward-command = XFORWARD 1*( SP attribute-name"="attribute-value )

    attribute-name = ( NAME | ADDR | PORT | PROTO | HELO | IDENT | SOURCE )

    attribute-value = xtext

  * Attribute values are xtext encoded as per RFC 1891.

  * The NAME attribute specifies the up-stream hostname, or [UNAVAILABLE] when
    the information is unavailable. The hostname may be a non-DNS hostname.

  * The ADDR attribute specifies the up-stream network address: a 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 an up-stream client TCP port number in
    decimal, or [UNAVAILABLE] when the information is unavailable.

  * The PROTO attribute specifies the mail protocol for receiving mail from the
    up-stream host. This may be an SMTP or non-SMTP protocol name of up to 64
    characters, or [UNAVAILABLE] when the information is unavailable.

  * The HELO attribute specifies the hostname that the up-stream host announced
    itself with (not necessarily via the SMTP HELO command), or [UNAVAILABLE]
    when the information is unavailable. The hostname may be a non-DNS
    hostname.

  * The IDENT attribute specifies a local message identifier on the up-stream
    host, or [UNAVAILABLE] when the information is unavailable. The down-stream
    MTA may log this information together with its own local message identifier
    to facilitate message tracking across MTAs.

  * The SOURCE attribute specifies LOCAL when the message was received from a
    source that is local with respect to the up-stream host (for example, the
    message originated from the up-stream host itself), REMOTE for all other
    mail, or [UNAVAILABLE] when the information is unavailable. The down-stream
    MTA may decide to enable features such as header munging or address
    qualification with mail from local sources but not other sources.

Note 1: an attribute-value element must not be longer than 255 characters
(specific attributes may impose shorter lengths). After xtext decoding,
attribute values must not contain control characters, non-ASCII characters,
whitespace, or other characters that are special in message headers.

Note 2: DNS hostnames can be up to 255 characters long. The XFORWARD client
implementation must not send XFORWARD commands that exceed the 512 character
limit for SMTP commands.

Note 3: [UNAVAILABLE] may be specified in upper case, lower case or mixed case.

Note 4: 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.

XFORWARD Server operation

The server maintains a set of XFORWARD attributes with forwarded information,
in addition the current SMTP session attributes. Normally, all XFORWARD
attributes are in the undefined state, and the server uses the current SMTP
session attributes for logging purposes.

Upon receipt of an initial XFORWARD command, the SMTP server initializes all
XFORWARD attributes to [UNAVAILABLE]. With each valid XFORWARD command, the
server updates XFORWARD attributes with the specified values.

The server must not mix client attributes from XFORWARD with client attributes
from the current SMTP session.

At the end of each MAIL FROM transaction (i.e. RSET or DOT), the server resets
all XFORWARD attributes to the undefined state, and is ready to receive another
initial XFORWARD command.

XFORWARD Server reply codes

                                          
    |Code|Meaning                         |
    |    |                                |
    |250 |success                         |
    |    |                                |
    |421 |unable to proceed, disconnecting|
    |    |                                |
    |501 |bad command parameter syntax    |
    |    |                                |
    |503 |mail transaction in progress    |
    |    |                                |
    |550 |insufficient authorization      |
    |    |                                |

XFORWARD Example

In the following example, 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-XFORWARD NAME ADDR PROTO HELO
    250 8BITMIME
    XFORWARD NAME=spike.porcupine.org ADDR=168.100.189.2 PROTO=ESMTP
    250 Ok
    XFORWARD HELO=spike.porcupine.org
    250 Ok
    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 3CF6B2AAE8
    QUIT
    221 Bye

Security

The XFORWARD command changes audit trails. Use of this command must be
restricted to authorized clients.

SMTP connection caching

SMTP connection caching makes it possible to deliver multiple messages within
the same SMTP session. The XFORWARD attributes are reset after the MAIL FROM
transaction completes (after RSET or DOT), so there is no risk of information
leakage.

References

Moore, K, "SMTP Service Extension for Delivery Status Notifications", RFC 1891,
January 1996.

