4.1.1 Daemon Mode (-bd)

The sendmail program can run as a daemon in the background, listening for incoming mail from other machines. The sendmail program reads its configuration file only once, when it first starts as a daemon. It then continues to run forever, never reading the configuration file again. As a consequence, it will never see any changes to that configuration file.

When you change something in the sendmail.cf configuration file, you always need to kill and restart the sendmail daemon. But before you can kill the daemon, you need to know how to correctly restart it. This information is in the /etc/sendmail.pid file or one of your system rc files.

On a Berkeley UNIX-based system, for example, the daemon is usually started like this:

/usr/lib/sendmail -bd -q1h

The -bd command-line switch specifies daemon mode. The -q switch tells sendmail how often to look in its queue to process pending mail. The -q1h switch says to process the queue at one ( 1 ) hour ( h ) intervals.

The actual command to start the sendmail daemon on your system may be different from what we've shown. If you manage many different brands of systems, you'll need to know how to start the daemon on all of them.

4.1.2 Show Queue Mode (-bp)

The sendmail program can also display the contents of its queue directory. It can do this in two ways: by running as a program named mailq or by being run as sendmail with the -bp command-line switch. Whichever way you run it, the contents of the queue directory are printed. If the queue is empty, sendmail prints the following:

Mail queue is empty

If, on the other hand, there is mail waiting in the queue, the output is far more verbose, possibly containing lines similar to these:

.ps 8
                Mail Queue (1 requests)
-Q-ID-- -Size- --Q-Time--- ------Sender/Recipient------
GAA29775*     702 Thu Mar 12 16:51 <you@here.us.edu>
            Deferred: Host fbi.dc.gov is down
                                  <george@fbi.dc.gov>

Here, the output produced with the -bp switch shows that there is only one mail message in the queue. If there were more, each entry would look pretty much the same as this. Each message results in at least two lines of output.

The first line shows details about the message and the sender. The GAA29775 identifies this message in the queue. The * shows that this message is locked and currently being processed. The 702 is the size of the message body in bytes (the size of the df file as mentioned in Section 3.1.2, "The Queue Directory" ). The date shows when this message was originally queued. The address shown is the name of the sender.

A second line may appear giving a reason for failure (if there was one). A message may be queued intentionally or because it couldn't be delivered.

The third and possibly subsequent lines show the addresses of the recipients.

The output produced by the -bp switch is more fully covered in Chapter 23, The Queue .

4.1.3 Rebuild Aliases Mode (-bi)

Because sendmail may have to search through thousands of names in the aliases file, a version of the file is stored in a separate dbm (3) or db (3) database format file. The use of a database significantly improves lookup speed.

Although sendmail can automatically update the database whenever the aliases file is changed, there will be times when you will want to rebuild it yourself. You do this either by running sendmail using the command newaliases or with the -bi command-line switch. The following two commands do the same thing:

% 


newaliases



% 


/usr/lib/sendmail -bi

There will be a delay while sendmail rebuilds the aliases database; then a summary of what it did is printed:

/etc/aliases: 859 aliases, longest 615 bytes, 28096 bytes total

This line shows that the database was successfully rebuilt. Beginning with V8.6 sendmail , multiple alias files became possible, so each line (and there might be many) begins with the name of an alias file. The information then displayed is the number of aliases processed, the size of the biggest entry to the right of the : in the aliases file, and the total number of bytes entered into the database. Any mistakes in an alias file will also be printed here.

The aliases file and how to manipulate it are covered in Chapter 24, Aliases .

4.1.4 Verify Mode (-bv)

A handy tool for checking aliases is the -bv command-line switch. It causes sendmail to recursively look up an alias and report the ultimate real name that it found.

To illustrate, consider the following aliases file:

animals:      farmanimals,wildanimals


bill-eats:    redmeat


birds:        farmbirds,wildbirds


bob-eats:     seafood,whitemeat


farmanimals:  pig,cow


farmbirds:    chicken,turkey


fish:         cod,tuna


redmeat:      animals


seafood:      fish,shellfish


shellfish:    crab,lobster


ted-eats:     bob-eats,bill-eats


whitemeat:    birds


wildanimals:  deer,boar


wildbirds:    quail

Although you can figure out what the name ted-eats ultimately expands to, it is far easier to have sendmail do it for you. By using sendmail , you have the added advantage of being assured accuracy, which is especially important in large and complex aliases files.

In addition to expanding aliases, the -bv switch performs another important function. It verifies whether or not the expanded aliases are in fact deliverable. Consider the following one-line aliases file:

root:       fred,larry

Assume that the user fred is the system administrator and has an account on the local machine. The user larry , however, has left, and his account has been removed. You can run sendmail with the -bv switch to find out whether both names are valid:

% 

/usr/lib/sendmail -bv root

This tells sendmail to verify the name root from the aliases file. Since larry (one of root 's aliases) doesn't exist, the output produced looks like this:

larry... User unknown
fred... deliverable: mailer local, user fred