smrsh man page on FreeBSD

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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/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) and DragonFly BSD

SEE ALSO
       sendmail(8)

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