SMRSH(8) SMRSH(8)
NAME
smrsh - restricted shell for sendmail(1,8)
SYNOPSIS
smrsh -c command
DESCRIPTION
The smrsh program is intended as a replacement for sh for use in(1,8) the
``prog'' mailer in(1,8) sendmail(1,8)(8) configuration files. It sharply limits
the commands that can be run using the ``|program'' syntax of sendmail(1,8)
in(1,8) order to improve the over all security of your system. Briefly,
even if(3,n) a ``bad guy'' can get sendmail(1,8) to run a program without going
through an alias or forward file(1,n), smrsh limits the set(7,n,1 builtins) of programs that
he or she can execute.
Briefly, smrsh limits programs to be in(1,8) a single directory, by default
/usr/adm/sm.bin, allowing the system administrator to choose the set(7,n,1 builtins) of
acceptable commands, and to the shell builtin commands ``exec(3,n,1 builtins)'',
``exit(3,n,1 builtins)'', and ``echo(1,3x,1 builtins)''. 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(3,n,1 builtins)
/usr/local/bin/filter(1,3x,3x curs_util) || exit(3,n,1 builtins) 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(1,8) the sm.bin directory.
Note that this does not restrict the use of shell or perl scripts in(1,8)
the sm.bin directory (using the ``#!'' syntax); it simply disallows
execution of arbitrary programs. Also, including mail(1,8) filtering pro-
grams such as procmail(1) is a very bad idea. procmail(1) allows users(1,5)
to run arbitrary programs in(1,8) 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(1,8)(8)
$Date: 2004/08/06 03:55:35 $ SMRSH(8)