Command Section
SMRSH(8)                FreeBSD System Manager's Manual               SMRSH(8)

       smrsh - restricted shell for sendmail

       smrsh -c command

       The smrsh program is intended as a replacement for sh for use in the
       ``prog'' mailer in sendmail(8) configuration files.  It sharply limits
       the commands that can be run using the ``|program'' syntax of sendmail
       in order to improve the over all security of your system.  Briefly,
       even if a ``bad guy'' can get sendmail to run a program without going
       through an alias or forward file, smrsh limits the set of programs that
       he or she can execute.

       Briefly, smrsh limits programs to be in a single directory, by default
       /usr/libexec/sm.bin, allowing the system administrator to choose the
       set of acceptable commands, and to the shell builtin commands ``exec'',
       ``exit'', and ``echo''.  It also rejects any commands with the
       characters ``', `<', `>', `;', `$', `(', `)', `\r' (carriage return),
       or `\n' (newline) on the command line to prevent ``end run'' attacks.
       It allows ``||'' and ``&&'' to enable commands like: ``"|exec
       /usr/local/bin/filter || exit 75"''

       Initial pathnames on programs are stripped, so forwarding to
       ``/usr/bin/vacation'', ``/home/server/mydir/bin/vacation'', and
       ``vacation'' all actually forward to ``/usr/libexec/sm.bin/vacation''.

       System administrators should be conservative about populating the
       sm.bin directory.  For example, a reasonable additions is vacation(1),
       and the like.  No matter how brow-beaten you may be, never include any
       shell or shell-like program (such as perl(1)) in the sm.bin directory.
       Note that this does not restrict the use of shell or perl scripts in
       the sm.bin directory (using the ``#!'' syntax); it simply disallows
       execution of arbitrary programs.  Also, including mail filtering
       programs such as procmail(1) is a very bad idea.  procmail(1) allows
       users to run arbitrary programs in their procmailrc(5).

       Compilation should be trivial on most systems.  You may need to use
       -DSMRSH_PATH=\"path\" to adjust the default search path (defaults to
       ``/bin:/usr/bin'') and/or -DSMRSH_CMDDIR=\"dir\" to change the default
       program directory (defaults to ``/usr/libexec/sm.bin'').

       /usr/adm/sm.bin - default directory for restricted programs on most OSs

       /var/adm/sm.bin - directory for restricted programs on HP UX and

       /usr/libexec/sm.bin - directory for restricted programs on FreeBSD (>=
       3.3) and DragonFly BSD


                         $Date: 2013-11-22 20:52:00 $                 SMRSH(8)
Command Section