6.3 The local Delivery Agent

When you ran sendmail, it complained that the local delivery agent definition was missing. To keep sendmail happy, this definition will now be added to the client.cf file. As it happens, it is already in your system sendmail.cf file (if you have one), and you can copy it by typing the following command:

% grep "^Mlocal" /etc/sendmail.cf >> client.cf

Note that the ^M above is actually two characters ^ and M, not a CTRL-M. Now load the client.cf file into your editor. It will look something like this: [5]

[5] If any of the equates shown here are missing, you need to type them in by hand.

# This is a comment
V7      # this is another comment
# Delivery agent definition to forward mail to hub
Mhub, P=[IPC], A=IPC $h
Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10, R=20/40, A=mail -d $u

Right off the bat, you'll notice three new equates [6] that are a bit more complicated than our original hub equates. This new M configuration command declares a symbolic name, such as hub. Here, that name is local - the name that sendmail complained was missing. Although the local definition is important and heavily used in a full-fledged sendmail.cf file, the client.cf file uses it only to keep sendmail from complaining.

[6] You may also find a T= equate. We describe it later in this chapter.

The new delivery agent definition is composed of six parts (each separated from the others by commas), a symbolic name and six equates. The F=, S=, and R= equates are new. You've seen the M, P=, and A= before in the hub definition.

M

All mail delivery agent definitions begin with the M configuration-file command. Like all configuration commands, that M must begin a line:

Mhub,   P=[IPC], A=IPC $h
Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10, R=20/40, A=mail -d $u 

define a delivery agent

The symbolic name for each delivery agent follows the M, with no intervening space. The symbolic names here are hub and local. The delivery agent called hub forwards mail to the central hub machine. The local delivery agent delivers mail to users on the local machine.

P=

The P= equate (for Path) specifies the full pathname of mail delivery programs:

Mhub,   P=[IPC], A=IPC $h
Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10, R=20/40, A=mail -d $u 
        
        full pathname of program

Your program names may differ, but in general, the program for local places a mail message into the local user's mail spool file.

A=

The A= equate (for Argument vector) specifies the command-line arguments to be supplied to each mail delivery program.

Mhub,   P=[IPC], A=IPC $h
Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10, R=20/40, A=mail -d $u 
                                                       
                                               command line (argv)

Notice that local uses the $u macro, whereas hub uses the $h macro. The $u macro contains the name of the recipient (such as bob). The $h macro contains the name of a host (such as here.us.edu). Macros are explained in the next chapter. By convention the A= equate is usually last.

Three parts in the new local definition were not used in the hub definition. They are:

F=

The F= equate (for Flags) specifies certain flags that tell sendmail more about the delivery agent. Each flag is a single character, and each is Boolean - either set (if it is present) or not (if it is absent).

Mhub,   P=[IPC], A=IPC $h
Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10, R=20/40, A=mail -d $u 
                     
                     flags for delivery agent

There are many flags to choose from. They are all described in Chapter 30, Delivery Agents, but we will cover a few of them later in this chapter.

S=

The S= equate (for Sender) specifies which rule set to use in rewriting the sender's address:

Mhub,   P=[IPC], A=IPC $h
Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10, R=20/40, A=mail -d $u 
                                        
                                        sender rule set

Addresses need to be rewritten because different delivery agents require addresses to be in different forms. For example, the [IPC] agent requires the form [email protected], while the uucp agent (if you had need for one) requires the form host!user. Here, the S= says that addresses should be rewritten by using rule set 10. We will cover rule sets later, in Chapter 8, Addresses and Rules.

R=

The R= equate (for Recipient) specifies which rule set to use in rewriting the recipient's address:

Mhub,   P=[IPC], A=IPC $h
Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10, R=20/40, A=mail -d $u 
                                              
                                              recipient rule set

Again, those addresses need to be rewritten because different delivery agents require different forms of addresses. Here, the R= says to use rule set 20 in rewriting an envelope address and rule set 40 in rewriting a header address.

In addition to the three new equates, and beginning with V8.7 sendmail, you may discover another new equate in the local delivery agent definition:

T=

Not shown in our examples is the T= equate (for Type), which lists three fields of information about the delivery agent. You will find this equate in configuration files beginning with V8.7 sendmail.

Mlocal, ..., T=DNS/RFC822/X-Unix

First is the type of MTA used (here DNS because sendmail is using DNS to lookup addresses); then a slash followed by the type of addressing used (here RFC822, but it could also be, for example, X.400); and last a slash followed by the type of error messages produced (here X-Unix which says that the program /bin/mail will produce UNIX errors). The T= equate is used to support Delivery Status Notification (DSN) as described in RFC1891, RFC1892, RFC1893, and RFC1894.