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SMRSH(8)

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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  charac-
       ters ``', `<', `>', `;', `$', `(', `)', `\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 ``vaca-
       tion'' 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  pro-
       grams 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)