Postfix XFORWARD Extension for Net::SMTP

The postfix-xforward extension adds methods to Ruby’s Net::SMTP library to support the XFORWARD SMTP extension, as defined and supported by the Postfix MTA.

Code that currently uses Net::SMTP as follows:

require 'net/smtp'
to   = '[email protected]'
from = '[email protected]'
msg  = <<EMAIL
To: #{to}
From: #{from}
Subject: Test Message

This is a test message.
EMAIL

Net::SMTP.start('mail.localdomain', 25, 'my-hostname') do |smtp|
  smtp.send_message(msg, from, to)
end

… can now use it this way in order to include XFORWARD attributes:

require 'postfix-xforward'
attr = { 'NAME' => 'actual.domain.tld', 'ADDR' => '123.234.012.123' }
to   = '[email protected]'
from = '[email protected]'
msg  = <<EMAIL
To: #{to}
From: #{from}
Subject: Test Message

This is a test message.
EMAIL

Net::SMTP.start('mail.localdomain', 25, 'my-hostname', nil, nil, nil, attr) do |smtp|
  smtp.send_message(msg, from, to)
end

Details

The XFORWARD extension, supported by the Postfix MTA is very simple really. It allows the SMTP client to include various attributes before the MAIL TO command. These attributes tell the server where the email message really came from.

This is almost always used on special, private instances of an SMTP server where only trusted (usually local) clients can connect. This is usually used in configurations such as the following:

client -> smtp-server-1 -> filter-script -> smtp-server-2 -> nexthop

In these setups, smtp-server-1 and 2 both run under the same Postfix instance in order to provide after-queue content filtering. The smtp-server-1 instance has access to the actual information about the client that is sending the message. It can pass this on, if configured correctly to an external filter-script.

This library makes it possible, using Net::SMTP to write filter-script in Ruby and to be able to pass original client information on when speaking SMTP to smtp-server-2 using XFORWARD.

This has just been thrown together so any comments, fixes, improvemnts, or other contributions are appreciated.

Authors and Credits

Authors: Kendall Gifford