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
M, not a CTRL-M.
Now load the client.cf file into your editor.
It will look something like this:
 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
that are a bit more complicated than our original
M configuration command declares a symbolic name,
Here, that name is
local - the
name that sendmail complained was missing.
local definition is
important and heavily used in a full-fledged sendmail.cf
file, the client.cf file uses it only to keep sendmail
 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.
R= equates are new.
You've seen the
A= before in the
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
M, with no intervening space.
The symbolic names here are
The delivery agent called
hub forwards mail to the central
local delivery agent
delivers mail to users on the local machine.
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
A=IPC $hMlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=10, R=20/40,
A=mail -d $ucommand line (argv)
$u macro, whereas
$h macro. The
$u macro contains the name
of the recipient (such as bob). The
contains the name of a host (such as here.us.edu).
Macros are explained in the next chapter. By convention
A= equate is usually last.
Three parts in the new
local definition were not used in the
hub definition. They are:
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.
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
For example, the
[IPC] agent requires the form
email@example.com, while the
uucp agent (if you had
need for one) requires the form host!user.
S= says that addresses should be rewritten by
using rule set 10.
We will cover rule sets later, in
Chapter 8, Addresses and Rules.
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.
R= says to use rule set 20 in rewriting an
envelope address and rule set 40 in rewriting a header
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:
the type of MTA used (here
DNS because sendmail
is using DNS to lookup addresses);
then a slash followed by the type of addressing
RFC822, but it could also be, for example,
and last a slash followed by the type of error messages produced
X-Unix which says that the program /bin/mail
will produce UNIX errors).
T= equate is used to support
Delivery Status Notification (DSN) as described in
RFC1891, RFC1892, RFC1893, and RFC1894.
Because we won't be covering rule sets for a while, and
to simplify things for now, edit the
client.cf file once again and change the
R= equates in the new
# Mailer to forward all mail to the hub machine Mhub, P=[IPC], A=IPC $h Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn,
S=0, R=0, A=mail -d $u new new
Both equates are changed to zero because there are no rule sets
yet. When an
S= or an
R= equate in a delivery
agent definition is zero or missing, sendmail skips
the delivery-agent-specific part of rule-set processing.
Comments are an important part of every configuration file. They remind you of what you are trying to do now and what you have done in the past. Edit the client.cf file now. Remove two old comments and add a new one:
removed comment V7 removed comment # Delivery agent definition to forward mail to hub Mhub, P=[IPC], A=IPC $h
# Sendmail requires this, but we won't use it.added comment Mlocal, P=/bin/mail, F=lsDFMAw5:/|@rmn, S=0, R=0, A=mail -d $u
We threw away the earlier comments because they were only for demonstration purposes.
/usr/lib/sendmail -d0.15 -Cclient.cf -bt </dev/null
-d0.15 debugging switch tells sendmail
(among other things) to show you how it interpreted your delivery
agent definitions. The
-bt causes sendmail to run in
rule-testing mode so that the delivery agents will be displayed.
Running the above command line produces
output like the following (but with long lines unattractively wrapped at the
assorted other information here and above mailer 0 (prog): P=/bin/sh S=0/0 R=0/0 M=0 U=0:0 F=Dlos L=0 E=\n T=DNS/RFC822/X-U nix A=sh -c $u mailer 1 (*file*): P=[FILE] S=0/0 R=0/0 M=0 U=0:0 F=DEFMPlosu L=0 E=\n T=DNS/RFC8 22/X-Unix A=FILE mailer 2 (*include*): P=/dev/null S=0/0 R=0/0 M=0 U=0:0 F=su L=0 E=\n T=<undefine d>/<undefined>/<undefined> A=INCLUDE mailer 3 (hub): P=[IPC] S=0/0 R=0/0 M=0 U=0:0 F= L=0 E=\n T=<undefined>/<undefine d>/<undefined> A=IPC $h mailer 4 (local): P=/bin/mail S=0/0 R=0/0 M=0 U=0:0 F=/5:@ADFMlmnrsw| L=0 E=\n T= <undefined>/<undefined>/<undefined> A=mail -d $u
This output, in addition to verifying that sendmail properly
interpreted the client.cf file, reveals four equates you
haven't seen before:
We won't explain them here, because you don't need them
for the client.cf file. They are explained in detail in
In the preceding output, also note that there are several equates that
were not included in the original
Mhub delivery agent definition.
definition included only the
Mhub, P=[IPC], A=IPC $h
When sendmail saw this definition, it did not find
specifications for any of the equates other than
Rather than complaining, it gave
E= the value
of the newline character (
T= three instances
<undefined>, and the other equates each a value of zero:
mailer 3 (hub): P=[IPC] S=0/0 R=0/0 M=0 U=0:0 F= L=0 E=\n T=<undefined>/<undefine d>/<undefined> A=IPC $h