Contents:
Headers
Headers Versus Delivery Agent Flags
Headers Learned So Far
Precedence
Sending Real Mail
Trusted User
Things to Try
In the previous chapter you sent mail to yourself and saw that sendmail
added several problematic header lines to your message.
Also the hub machine added some unsuitable headers.
In this chapter we show how to use the H
configuration command
to add legal headers to a message, the P
configuration
command to set message priorities, and the T
configuration command
to define "trusted" users.
The header configuration command begins with the letter H
.
Like all configuration commands, the H
is the first character
on a line. The H
is
then followed by the name of a header:
Hname
:field
Here, name
is the name of a header, such as Subject
.
The list of all header names that are of interest to sendmail can
be found in
Chapter 35, Headers.
The name is followed by a colon
and then text that is appropriate for the header.
Optional whitespace can surround the colon.
RFC822 (modified by RFC1123) specifies that certain header lines must appear in all email messages. Of those, the two you will first add to the client.cf file are shown in Table 14.1.
Name | Description | When Added |
---|---|---|
From: | Address of the sender | If missing |
Received: | Record of receipt | Always |
Unless otherwise specified (as you will see later), a header that is declared
in the configuration file is added to a mail message only if one doesn't already exist.
The exception to this
rule is the Received:
header. It is
always added to a mail message, even if there is already
one (or more) there.
The From:
header contains the official address of
the sender. Declaring the From:
header is simplicity
itself, because it uses as its field
a single macro
surrounded by angle braces:
HFrom: <$g>
Recall that there are two kinds of macros: those that you define
with the D
command and those that sendmail defines.
The g
macro is one of the latter.
The value sendmail gives the g
macro is the
address of the sender (generated by sendmail) as it
appears on the envelope. That address, when you are the
sender, was generated by the Hubset
rule set:
SHubset # Rewrite the sender for the hub R$- $@ $1@${HUB} user -> user@hub
The sender address (you) is rewritten by this rule so that it
becomes you@mail.us.edu. That is, the hostname and domain
of the hub machine are appended so that the mail appears to
come from the hub. This is the envelope address of the sender,
and that is the address placed into the g
macro. For email sent by
you, the value that is given to $g
(and thus to the From:
header) is:
From: <you@mail.us.edu>
The From:
header is added to an outgoing
mail message only if there is not already a From:
header there.
(Often an MUA will add a From:
header.)
It is placed into
the client.cf file to ensure that no mail leaves the local
machine without this required header.
The Received:
header is special. It is
always added to the header portion of every mail message, even
if one or more of them are already present.
The Received:
header is used to make a record
of each machine that mail has passed through. When sendmail
calculates a hop count (to bounce mail with too many hops), it does
so by counting the Received:
header lines.
The declaration of the Received:
header is a bit
complicated.
A minimal Received:
header declaration looks like this:
HReceived: by $j; $b
The word by
is mandatory. It must be followed by the fully qualified,
official name of the local machine. As you saw earlier when you
ran sendmail with the -d0.1
debugging switch, the
fully qualified name was assigned to the j
macro:
============ SYSTEM IDENTITY (after readcf) ============ (short domain name) $w = here (canonical domain name)$j
= here.us.edu note
The Received:
header definition concludes with a semicolon,
followed by the current date and time as stored in $b
.
The b
macro contains the current date in ARPAnet
format; that is, the day of the week, the month, the day, the time,
the year, and the time zone.
These items in the Received:
header
form the minimum information required in this header.
Add the From:
and Received:
headers to the client.cf
file. The new lines in client.cf will look like this:
O OldStyleHeaders=True O BlankSub=. # Replace unquoted spaces with a dot.# Headers
newHFrom: <$g> # Added only if missing
newHReceived: by $j; $b # Always added
new
Here, they follow the options that were added in the last chapter.
As usual, comments have been included to clarify your intent.
See Chapter 31, Defined Macros,
for a more detailed explanation of the j
and b
macros.
Now send mail to yourself just as you did at the end of the preceding chapter:
%./sendmail -Cclient.cf
you
Subject: testing
To:
you
testing
.
Retrieve the mail that you just sent, save it to a file, and look at what you saved. It will look something like this:
From you@mail.us.edu Fri Dec 13 05:47:47 1996 Return-Path: <you@mail.us.edu> Received: from here.us.edu (you@here.us.edu [123.45.67.8]) by mail.us.edu (8.8.4/8.8.4) with ESMTP id FAA13451 for <you>; Fri, 13 Dec 1996 05:47:46 -0700 Date: Fri, 13 Dec 1996 05:47:46 -0700From: you@mail.us.edu (Your Full Name)
note Message-Id: <199509091247.FAA13451@mail.us.edu>Received: by here.us.edu; Fri, 13 Dec 1996 05:47:44 -0700
note Subject: testing To: you testing
Notice that a new
Received:
header was added. This is the one
just declared in the client.cf file.
The two Received:
headers form a trail that shows who first
received the message and how it was passed from one machine
to the other.
Also notice that the contents of the From:
header have
changed. Something has removed the angle brackets and added
your full name. What happened was this:
On the hub machine the address in the envelope for the sender
is taken from the RCPT message that the local machine sends
during the SMTP conversation. That address is the value of
$g
with angle brackets added.
On the hub machine the address in the From:
header
is compared to the sender envelope address.
The address in the From:
header that client.cf
supplied was the value of $g
surrounded in angle brackets.
Whenever the address in the envelope for the sender and the address
in the From:
header are identical, sendmail
removes the From:
header and creates a new one.
Thus sendmail on a correctly configured hub machine removes the From:
header and creates a new one that includes your full name.
The definition of the From:
header on a hub is
more complex then that in the client.cf file.
One possible definition might look
like this:
From: $g $?x($x)$.
This is just like the local definition, but it has a macro
conditional added. A macro conditional is
simply an if-endif construction, where $?
is the
if and $.
is the endif. The preceding definition, then,
can be broken down like this:
$? if x the macro x contains a value ($x) add this to the definition $. endif
The macro x
contains as its value the full name of
the sender. If that full name is not known, x
has
no value and the full name is omitted.
If $x
does contain a value, the full name is
added in parentheses.
Macro conditionals are described in
Chapter 31.