smrsh (ADMN)

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(ADMN) 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 administra-
     tor 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' (new-
     line) 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 forward to ``/usr/adm/sm.bin/vacation''.

     System administrators should be conservative about  populat-
     ing  the  sm.bin directory.  For example, a reasonable addi-
     tions is vacation(C), 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.

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 direc-
     tory (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(ADMN)