10.4.1.1. Building a sendmail.cf with m4 macros

The prototype files that come with the sendmail tar are not "ready to run." They must be edited and then processed by the m4 macro processor to produce the actual configuration files. For example, the tcpproto.mc file contains the following macros:

divert(0)dnl 
VERSIONID(`$Id: ch10_04.htm,v 1.3 2002/04/16 22:37:26 ellie Exp $') 
OSTYPE(`unknown') 
FEATURE(`nouucp', `reject') 
AILER(`local')
AILER(`smtp')

These macros are not sendmail commands; they are input for the m4 macro processor. The few lines shown above are the active lines in the tcpproto.mc file. They are preceded by a section of comments, not shown here, that is discarded by m4 because it follows a divert(-1) command, which diverts the output to the "bit bucket." This section of the file begins with a divert(0) command, which means these commands should be processed and that the results should be directed to standard output.

The dnl command that appears at the end of the divert(0) line is used to prevent unwanted lines from appearing in the output file. dnl deletes everything up to the next newline. It affects the appearance, but not the function, of the output file. dnl can appear at the end of any macro command. It can also be used at the beginning of a line. When it is, the line is treated as a comment.

The VERSIONID macro is used for version control. Usually the value passed in the macro call is a version number in RCS (Release Control System) or SCCS (Source Code Control System) format. This macro is optional, and we can just ignore it.

The OSTYPE macro defines operating system-specific information for the configuration. The cf/ostype directory contains almost 50 predefined operating system macro files. The OSTYPE macro is required and the value passed in the OSTYPE macro call must match the name of one of the files in the directory. Examples of values are bsd4.4, solaris8, and linux.

The FEATURE macro defines optional features to be included in the sendmail.cf file. The nouucp feature in the example shown says that UUCP addresses are not used on this system. The argument reject says that local addresses that use the UUCP bang syntax (i.e., contain an ! in the local part) will be rejected. Recall that in the previous section we identified tcpproto.mc as the prototype file for systems that have no UUCP connections. Another prototype file would have different FEATURE values.

The prototype file ends with the mailer macros. These must be the last macros in the input file. The example shown above specifies the local mailer macro and the SMTP mailer macro.

The MAILER(local) macro includes the local mailer that delivers local mail between users of the system and the prog mailer that sends mail files to programs running on the system. All the generic macro configuration files include the MAILER(local) macro because the local and prog mailers provide essential local mail delivery services.

The MAILER(smtp) macro includes all of the mailers needed to send SMTP mail over a TCP/IP network. The mailers included in this set are:

smtp

This mailer can handle traditional 7-bit ASCII SMTP mail. It is outmoded because most modern mail networks handle a variety of data types.

esmtp

This mailer supports Extended SMTP (ESMTP). It understands the ESMTP protocol extensions and it can deal with the complex message bodies and enhanced data types of MIME mail. This is the default mailer used for SMTP mail.

smtp8

This mailer sends 8-bit data to the remote server, even if the remote server does not indicate that it can support 8-bit data. Normally, a server that supports 8-bit data also supports ESMTP and thus can advertise its support for 8-bit data in the response to the EHLO command. (See Chapter 3, "Network Services" for a description of the SMTP protocol and the EHLO command.) It is possible, however, to have a connection to a remote server that can support 8-bit data but does not support ESMTP. In that rare circumstance, this mailer is available for use.

dsmtp

This mailer allows the destination system to retrieve mail queued on the server. Normally, the source system sends mail to the destination in what might be called a "push" model, where the source pushes mail out to the destination. On demand, SMTP allows the destination to "pull" mail down from the mail server when it is ready to receive the mail. This mailer implements the ETRN command that permits on-demand delivery. (The ETRN protocol command is described in RFC 1985.)

relay

This mailer is used when SMTP mail must be relayed through another mail server. Several different mail relay hosts can be defined.

Every server that is connected to or communicates with the Internet uses the MAILER(smtp) set of mailers, and most systems on isolated networks use these mailers because they use TCP/IP on their enterprise network. Despite the fact that the vast majority of sendmail systems require these mailers, installing them is not the default. To support SMTP mail, you must have the MAILER(smtp) macro in your configuration, which is why it is included in the prototype file.

In addition to these two important sets of mailers, there are nine other sets of mailers available with the MAILER command, all of which are covered in Appendix E, "A sendmail Reference". Most of them are of very little interest for an average configuration. The two sets of mailers included in the tcpproto.mc configuration are the only ones that most administrators ever use.

To create a sample sendmail.cf from the tcpproto.mc prototype file, copy the prototype file to a work file. Edit the work file to change the OSTYPE line from unknown to the correct value for your operating system, e.g., solaris8 or linux. In the example we use sed to change unknown to linux. We store the result in a file we call linux.mc:

# sed 's/unknown/linux/' < tcpproto.mc > linux.mc

Then enter the m4 command:

# m4 ../m4/cf.m4 linux.mc > sendmail.cf

The sendmail.cf file output by the m4 command is in the correct format to be read by the sendmail program. With the exception of how UUCP addresses are handled, the output file produced above is similar to the sample generic-linux.cf configuration file delivered with the sendmail distribution.

OSTYPE is not the only thing in the macro file that can be modified to create a custom configuration. There are a large number of configuration options, all of which are explained in Appendix E, "A sendmail Reference". As an example we modify a few options to create a custom configuration that converts user@host email addresses originating from our computer into firstname.lastname@domain. To do this, we create two new configuration files: a macro file with specific values for the domain that we name wrotethebook.com.m4, and a modified macro control file, linux.mc, that calls the new wrotethebook.com.m4 file.

We create the new macro file wrotethebook.com.m4 and place it in the cf/domain directory. The new file contains the following:

$ cat domain/wrotethebook.com.m4
ASQUERADE_AS(wrotethebook.com) 
FEATURE(masquerade_envelope)
FEATURE(genericstable)

These lines say that we want to hide the real hostname and display the name wrotethebook.com in its place in outbound email addresses. Also, we want to do this on "envelope" addresses as well as message header addresses. The first two lines handle the conversion of the host part of the outbound email address. The last line says that we will use the generic address conversion database, which converts login usernames to any value we wish to convert the user part of the outbound address. We must build the database by creating a text file with the data we want and processing that file through the makemap command that comes with sendmail.

The format of the database can be very simple:

dan Dan.Scribner 
tyler Tyler.McCafferty 
pat Pat.Stover 
willy Bill.Wright
craig Craig.Hunt

Each line in the file has two fields: the first field is the key, which is the login name, and the second field is the user's real first and last names separated by a dot. Fields are separated by spaces. Using this database, a query for dan will return the value Dan.Scribner. A small database such as this one can be easily built by hand. On a system with a large number of existing user accounts, you may want to automate this process by extracting the user's login name and first and last names from the /etc/passwd file. The gcos field of the /etc/passwd file often contains the user's real name.[114]

[114]See Appendix E, "A sendmail Reference" for a sample script that builds the realnames database from /etc/passwd.

Once the data is in a text file, convert it to a database with the makemap command. The makemap command is included in the sendmail distribution. The syntax of the makemap command is:

makemap type name

makemap reads the standard input and writes the database out to a file it creates using the value provided by name as the filename. The type field identifies the database type. The most commonly supported database types for sendmail are dbm, btree, and hash.[115] All of these types can be made with the makemap command.

[115]On Solaris systems, NIS maps and NIS+ tables are built with standard commands that come with the operating system. The syntax for using those maps within sendmail is different (see Table 10-3).

Assume that the data shown above has been put in a file named realnames. The following command converts that file to a database:

# makemap hash genericstable < realnames

makemap reads the text file and produces a database file called genericstable. The database maps login names to real names, e.g., the key willy returns the value Bill.Wright.

Now that we have created the database, we create a new sendmail configuration file to use it. All of the m4 macros related to using the database are in the wrotethebook.com.m4 file. We need to include that file in the configuration. To do that, add a DOMAIN(wrotethebook.com) line to the macro control file (linux.mc) and then process the linux.mc through m4. The following grep command shows what the macros in the file look like after the change:

# grep '^[A-Z]' linux.mc 
VERSIONID(`$Id: ch10_04.htm,v 1.3 2002/04/16 22:37:26 ellie Exp $') 
OSTYPE(`linux') 
DOMAIN(`wrotethebook.com') 
FEATURE(`nouucp', `reject') 
AILER(`local') 
AILER(`smtp')
# m4 ../m4/cf.m4 linux.mc > sendmail.cf

Use a prototype mc file as the starting point of your configuration if you install sendmail from the tar file. To use the latest version of sendmail you must build a compatible sendmail.cf file using the m4 macros. Don't attempt to use an old sendmail.cf file with a new version of sendmail; you'll just cause yourself grief. As you can see from the sample above, m4 configuration files are very short and can be constructed from only a few macros. Use m4 to build a fresh configuration every time you upgrade sendmail.

Conversely, you should not use a sendmail.cf file created from the prototype files found in the sendmail distribution with an old version of sendmail. Features in these files require that you run a compatible version of sendmail, which means it is necessary to recompile sendmail to use the new configuration file.[116] This is not something every system administrator will choose to do, because some systems don't have the correct libraries; others don't even have a C compiler! If you choose not to recompile sendmail, you can use the sample sendmail.cf file provided with your system as a starting point. However, if you have major changes planned for your configuration, it is probably easier to recompile sendmail and build a new configuration with m4 than it is to make major changes directly to the sendmail.cf.

[116]See Appendix E, "A sendmail Reference" for information about compiling sendmail.

In the next part of this chapter, we use one of the sample sendmail.cf files provided with Linux. The specific file we start with is generic-linux.cf found in the cf/cf directory of the sendmail distribution. All of the things we discuss in the remainder of the chapter apply equally well to sendmail.cf files that are produced by m4. The structure of a sendmail.cf file, the commands that it contains, and the tools used to debug it are universal.