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SMRSH(8) System Manager's Manual 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 al- lows ``||'' and ``&&'' to enable commands like: ``"|exec /usr/lo- cal/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 ex- ecution of arbitrary programs. Also, including mail filtering programs 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 So- laris /usr/libexec/sm.bin - directory for restricted programs on FreeBSD (>= 3.3) and DragonFly BSD SEE ALSO sendmail(8) $Date: 2013-11-22 20:52:00 $ SMRSH(8)