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Postfix XFORWARD Howto

Purpose of the XFORWARD extension to SMTP

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 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 in order to support other extensions such as AUTH.

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.

The client may send the XFORWARD request at any time except in the middle of a mail delivery transaction (i.e. between MAIL and 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 | PROTO | HELO )

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, or [UNAVAILABLE] when the information is unavailable. Address information is not enclosed with []. The address may be a non-IP address.
The PROTO attribute specifies the mail protocol for receiving mail from the up-stream host. This may be an SMTP 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.

Note 1: Attribute values must not be longer than 255 characters (specific attributes may impose shorter lengths), must not contain control characters, non-ASCII characters, whitespace, or other characters that are special in message headers. Future attributes that may violate this should use xtext encoding as described in RFC 1891.

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: the XFORWARD server implementation must not mix information from the current SMTP session with forwarded information from an up-stream session.

The XFORWARD server reply codes are as follows:

Code Meaning

250 success

501 bad command parameter syntax

503 mail transaction in progress

421 unable to proceed, disconnecting

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 command completes, so there is no risk of information leakage.