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Chapter 14 Headers, Precedence, and Trust |
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14.2 Headers Versus Delivery Agent Flags
Some headers should be inserted into a mail message only if
a certain delivery agent is used.
For example, one traditionally should include the
Full-Name:
header when mail is being delivered with UUCP but should not include it
for most other delivery agents.
The mechanism that governs inclusion is a list
of flags that prefix the header definition in the configuration file.
Those flags are composed of a series of one or more letters, all of
which are surrounded with a pair of
?
characters.
H?flags?name: value
When
sendmail
decides whether a header is to be added
to the mail message, it compares the
flags
listed with
the flags of the delivery agent's
F=
equate:
Mhub, P=[IPC], S=Hubset, R=0, F=mDFMuXa, T=DNS/RFC822/SMTP, A=IPC $h
flags
If a given flag (letter) appears in both, the header is added to the mail message. Otherwise, it is not.
Traditionally, for example, the
x
flag is used to indicate
the need for a
Full-Name:
header. But our hub
delivery agent does not have an
x
in its
F=
flags.
Since that is the only delivery agent we use, we need to add
that flag to the
client.cf
file.
14.2.1 The Full-Name: Header
The
Full-Name:
header is used to display the full name
of the sender, as taken from the
gecos
field of the
passwd
(5) file. You saw above how the hub machine tries
to add the sender's full name to the
From:
header.
But since you don't necessarily have control over the hub, you should
add a
Full-Name:
header locally, so that the full name
is displayed even if the hub fails to add it.
The way to declare the
Full-Name:
header is like this:
H?x?Full-Name: $?x$x$. # Add full name if available
First prefix it with the
?x?
flag. This means that
the
Full-Name:
header is added only if the
delivery agent also contains that flag.
The value given to the
Full-Name:
header is just like
the conditional that you saw earlier. If (
$?
) the macro
x
contains a value, use that value (
$x
), and endif (
$.
).
We use this conditional test so that the full name is added only if
it is known.
Next, to make the
Full-Name:
effective, you need to
add an
x
flag to the
hub
delivery agent declaration:
Mhub, P=[IPC], S=Hubset, R=0, F=xmDFMuXa, T=DNS/RFC822/SMTP, A=IPC $h
Now any mail that uses the
hub
delivery agent for a recipient
(in other words, all mail)
will add a
Full-Name:
header to the message if there is not
already one there. If the full name is known (
$x
has a value),
that name follows the
Full-Name:
header on the same line;
otherwise, the header contains only the header name and is omitted entirely.
14.2.2 The Date: Header
The
Date:
header is required in all messages to show the
time and day that the message originated.
It is a good idea to include
?
flags
?
in its definition so that custom delivery agents
that do not need the
Date:
can be designed later.
H?D?Date: $a # Add if F=D
The
$a
is the
origin date
of the mail message
in RFC822 format. That date is set internally by
sendmail
to be correct for inclusion in the
Date:
header.
An
F=D
flag already exists in the
hub
delivery agent:
Mhub, P=[IPC], S=Hubset, R=0, F=xmDFMuXa, T=DNS/RFC822/SMTP, A=IPC $h
That
D
was originally put in this delivery agent definition
with the
Date:
header in mind.
14.2.3 The Message-ID: Header
The
Message-ID:
header is used to uniquely identify
each mail message. It must be inserted into the message when
it is first created (first handled by
sendmail
). The form
of the
Message-ID:
header is very specific:
H?M?Message-Id: <$t.$i@$j> # Add if F=M
Here, a
?M?
prefix is included. The
hub
delivery agent
definition already has the
F=M
flag listed:
Mhub, P=[IPC], S=Hubset, R=0, F=xmDFMuXa, T=DNS/RFC822/SMTP, A=IPC $h
The field following the
Message-ID:
must follow
particular rules. First, it must be surrounded by angle brackets.
Then, what appears between the angle brackets must look like a legal
address:
< address>
The address must be composed of pieces of information that uniquely identify the mail message worldwide. We create that address in a way that is commonly used in most configuration files:
<$t.$i@$j>
$t
is the current date and time represented by an integer.
$i
is the local unique identifier for the queue file for this message
(even if the message isn't queued), and
$j
is your host's
fully qualified domain name. Other information may be used, provided that
the result looks like a legal address and provided that no two identical
identifiers ever go out.
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| 14.1 Headers |
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14.3 Headers Learned So Far |