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

NAME

     smrsh - restricted shell for sendmail

SYNOPSIS

     smrsh -c command

DESCRIPTION

     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

     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'').

FILES

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

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

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

SEE ALSO

     sendmail(8)

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