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25.2 :include: Mailing Lists

The special notation :include: in the right-hand side of an alias causes sendmail to read its list of recipients from an external file. For that directive to be recognized as special, any address that begins with :include: must select the local delivery agent. This is automatic with most configuration files but not with others. If your configuration file does not automatically recognize the :include: directive, you will need to add a new rule near the end of your rule set 0 (see Section 29.6, "Rule Set 0"). For example,

R:include:$*     $@ $#local $: :include:$1

Beginning with V8.7 sendmail, any delivery agent for which the F=: flag (see Section 30.8.7, F=:) is set can also process :include: files. (Note that eliminating the F=: flag for all delivery agent definitions in your configuration file will disable this feature entirely.)

The :include: directive is used in aliases(5) files like this:

localname:   :include:/path

The expression :include: is literal. It must appear exactly as shown, colons and all, with no space between the colons and the "include." As with any right-hand side of an alias, there may be space between the alias colon and the lead colon of the :include:.

The /path is the full pathname of a file containing a list of recipients. It follows the :include: with intervening space allowed.

The /path should be a full pathname. If it is a relative name (such as ../file), it is relative to the sendmail queue directory. For all but V8 sendmail, the /path must not be quoted. If it is quoted, the quotation marks are interpreted as part of the filename. For V8 sendmail, the /path may be quoted, and the quotation marks are automatically stripped.

If the /path cannot be opened for reading for any reason, sendmail prints the following warning and ignores any recipients that might have been in the file:

:include: path... Cannot open path: reason

Here, reason is "no such file or directory," "permission denied," or something similar. If /path exists and can be read, sendmail reads it one line at a time. Empty lines are ignored. Beginning with V8 sendmail, lines that begin with a # character are also ignored:

# a comment
                   <- empty line is ignored

Each line in the :include: file is treated as a list of one or more recipient addresses. Where there is more than one, each should be separated from the others by commas.

addr2, addr3, addr4

The addresses may themselves be aliases that appear to the left in the aliases file. They may also be user addresses, program names, or filenames. A :include: file may also contain additional :include: lists:

engineers                          <- to an alias
biff, bill@otherhost               <- to two recipients
|"/etc/local/loglists thislist"    <- to a program alias
/usr/local/archive/thislist.hist   <- to a file
:include:/yet/another/file         <- from another file

Beginning with V8.7 sendmail, the TimeOut.fileopen option (see Section 34.8.70, Timeout (r)) controls how long sendmail should wait for the open to complete. This is useful when files are remotely mounted as with NFS. This timeout encompasses both this open and the security checks described next. Note that the NFS file system must be soft mounted (or mounted with the intr option) for this to work.

Beginning with V8, sendmail checks the file for security. If the controlling user is root, all components of the path leading to the file are also checked. [2] If the suid bit of the file is set (telling sendmail to run as owner of the file), sendmail checks to be sure that the file is writable only by the owner. If it is group- or world-writable, sendmail silently ignores that suid bit. When checking components of the path, sendmail will print the following warning if it is running as root and if any component of the path is group- or world-writable:

[2] The sendmail program also performs this check for critical system files, such as its configuration file.

WARNING: writable directory offending component

This process is described in greater detail under the -d44 debugging switch (see Section 37.5.159, -d44.4), which can also be used to observe this process.

After sendmail opens the /path for reading but before it reads the file, it sets the controlling user to be the owner of the file (if one is not already set and provided that file ownership cannot be given away with chown(1)). The controlling user provides the uid and gid identities of the sender when delivering mail from the queue (see Section 23.9.2, C line).

The :include: file can neither deliver through programs nor append to files if any of three situations is true: if the owner of the :include: file has a shell that is not listed in /etc/shells (see Section 22.8.4, "The /etc/shells file"), if the :include: file is world-writable, or if the :include: file is group-writable and the UnsafeGroupWrites option (see Section 34.8.73, UnsafeGroupWrites) is true.

25.2.1 Comments in :include: Lists

IDA and V8 sendmail allow comments in :include: files. Comment lines begin with a # character. If the # doesn't begin the line, it is treated as the beginning of an address, thus allowing valid usernames that begin with a # (such as #1user) to appear first in a line by prefixing them with a space:

# Management               <- a comment
# Staff                    <- a comment
 #1user                    <- an address

Note that since comments and empty lines are ignored by sendmail, they can be used to create attractive, well-documented mailing lists.

Under older versions of sendmail, comments can be emulated through the use of RFC822-style comments:

( comment )

By surrounding the comment in parentheses, you cause sendmail to view it and the parentheses as an RFC-style comment and ignore them:

( Management )
( Staff )

This form of comment works with both the old and new sendmail programs.

25.2.2 Tradeoffs

As has been noted, the aliases file should be writable only by root for security reasons. Therefore, ordinary users, such as nonprivileged department heads, cannot use the aliases file to create and manage mailing lists. Fortunately, :include: files allow ordinary users (or groups of users) to maintain mailing lists. This offloads a great deal of work from the system administrator, who would otherwise have to manage these lists, and gives users a sense of participation in the system.

Unfortunately, reading :include: lists is slower than reading an entry from the aliases database. At busy sites or sites with numerous mail messages addressed to mailing lists, this difference in speed can become significant. Note that the -bv command-line switch (see Section 36.7.13, -bv) can be used with sendmail to time and contrast the two different forms of lists. On the other hand, sometimes rebuilding the aliases(5) database can also be very slow. In such instances the :include: file may be faster, since it doesn't require a rebuild each time it changes.

One possible disadvantage to all types of mailing lists is that they are visible to the outside world. This means that anyone in the world can send mail to a local list that is intended for internal use. Many lists are intended for both internal and external use. One such list might be one for discussion of the O'Reilly Nutshell Handbooks, called, say, nuts@ora.com. Anyone inside ora.com and anyone in the outside world can send mail messages to this list, and those messages will be forwarded to everyone on the nuts mailing list.

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