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SMRSH(8)                                                              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/adm/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/ucb/vacation'',                           ``/usr/bin/vacation'',
       ``/home/server/mydir/bin/vacation'', and ``vacation'' all actually for-
       ward to ``/usr/adm/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:/usr/ucb'') and/or -DSMRSH_CMDDIR=\"dir\" to change the
       default program directory (defaults to ``/usr/adm/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: 2004/08/06 03:55:35                  SMRSH(8)