sendmail, 4th Edition

How smrsh Works

Once smrsh is installed and sendmail is configured to use it, and after its approved program directory is populated, smrsh can begin to do its job. Thereafter, whenever smrsh is called to run a program, smrsh strips the leading path from the program name and looks for that program in its special /usr/adm/sm.bin directory. If the program is not found in that directory, the message bounces. Thus, with the ~/.forward line:

|"/tmp/x.sh"

and if x.sh is not in the /usr/adm/sm.bin directory, smrsh causes the email message to bounce with the following error:

smrsh: /usr/adm/sm.bin/x.sh: not available for sendmail programs

The smrsh program also screens out program lines that contain suspicious characters. Consider:

|"cp /bin/sh /home/george/.x; chmod u+s /home/george/.x"

In this instance, smrsh would reject the command line (and thus bounce the message) because it contained a semicolon character:

smrsh: cannot use ; in command

The smrsh program will reject any command line that contains any of the following special characters as well as the newline (\n) and carriage-return (\r) characters:

`<>;$(  )

Beginning with V8.10, smrsh allows the && and || expressions so that ~/.forward file entries such as the following will work:

|"exec /usr/local/bin/archivemail /usr/local/mailarchive/user || exit 75"

Here, || means that if the archivemail program fails, the shell command will exit with a 75 value. This tells sendmail to defer the message back to its queue, instead of bouncing it.

Note that ...