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/pkg/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/ucb/vacation'',				``/usr/bin/vacation'',
       ``/home/server/mydir/bin/vacation'', and ``vacation'' all actually for‐
       ward to ``/usr/pkg/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:/usr/ucb'') and/or -DSMRSH_CMDDIR=\"dir\" to change the
       default program directory (defaults to ``/usr/pkg/libexec/sm.bin'').

FILES
       /usr/pkg/libexec/sm.bin - default directory for restricted programs

SEE ALSO
       sendmail(8)

			 $Date: 2004/08/06 03:55:35 $		      SMRSH(8)

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