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(`@(#)tcpproto.mc	8.5 (Berkeley) 3/23/96')
OSTYPE(unknown)
FEATURE(nouucp)
MAILER(local)
MAILER(smtp)

These macros are not sendmail commands; they are input for the m4 macro processor. The few lines shown above are the important 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 that means these commands should be processed and that the results should be directed to standard output. [7]

[7] The dnl option is used to prevent excessive blank lines from appearing in the output file. It affects the appearance, but not the function, of the output file. dnl can appear at the end of any macro command.

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 just ignore it.

The OSTYPE macro defines operating system-specific information for the sendmail.cf file. The cf/ostype directory contains more than 30 pre-defined 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, solaris2, and linux.

The FEATURE macro defines optional features to be included in the sendmail.cf file. The nouucp feature in the sample shown above says that no special UUCP address processing is to be included in the output file. 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 sample shown above specifies the local mailer macro, which adds the local mailer and the prog mailer to the output, and the smtp mailer macro, which adds mailers for SMTP, Extended SMTP, 8-bit SMTP and relayed mail. All of these mailers are described later in this chapter.

To create a sample sendmail.cf for a Linux system from the tcpproto.mc prototype file, copy the prototype file to a work file. Edit the work file by changing the OSTYPE line from unknown to linux to specify the correct operating system. 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. [8] In fact, the output file produced above is almost identical to the sample linux.smtp.cf configuration file delivered with Linux.

[8] New syntax and functions in the latest version of the sendmail.cf file may not be supported by older versions of the sendmail program.

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. 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 nuts.com.m4 and a modified macro control file, linux.mc, that calls the new nuts.com.m4 file.

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

MASQUERADE_AS(nuts.com)
FEATURE(masquerade_envelope)
FEATURE(genericstable)

These lines say that we want to hide the real hostname and display the name nuts.com in its place in outbound email addresses. Also, we want to do this on "envelope" addresses as well as message header addresses. The last line says that we will use the generic address conversion database, which converts login usernames to any value we wish. 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 V8.

The format of the database can be very simple:

dan [email protected]
tyler [email protected]
pat [email protected]
willy [email protected]
craig [email protected]

Each line in the file has two fields: the first field is the key, which is the login name, and the second field is an email address containing 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 [email protected]. 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. [9] 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 V8 tar file. It requires the ndbm library. The syntax of the makemap command is:

[9] See Appendix E for a sample script that builds the realnames database from /etc/passwd.

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 V8 are dbm, btree, and hash. [10] All of these types can be made with the makemap command.

[10] On Sun OS and Solaris systems, NIS maps and NIS+ tables are built with standard commands that come with those operating systems. The syntax for using those maps within sendmail is different (see Table 10-4). Networking Personal Computers with TCP/IP, by Craig Hunt (O'Reilly & Associates) provides an example of using a NIS map inside of sendmail.

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 [email protected].

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 nuts.com.m4 file. We need to include that file in the configuration. To do that, add a DOMAIN(nuts.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(`@(#)tcpproto.mc	8.5 (Berkeley) 3/23/96')
OSTYPE(linux)
DOMAIN(nuts.com)
FEATURE(nouucp)
MAILER(local)
MAILER(smtp)
# m4 ../m4/cf.m4 linux.mc > sendmail.cf

Use the prototype mc files 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. [11] 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.

[11] See Appendix E 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 linux.smtp.cf found in the /usr/src/sendmail directory on the Slackware 96 version of Linux. All of the things that 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.