SMRSH(8)                OpenBSD 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 /bin/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(8)
     in order to improve the overall 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 the shell built-in 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/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 di-
     rectory (using the ``#!'' syntax); it simply disallows execution of arbi-
     trary programs.  Also, including mail filtering programs such as procmail
     is a very bad idea.  procmail allows users to run arbitrary programs in
     their procmailrc.

FILES
     /usr/libexec/sm.bin  directory for restricted programs

SEE ALSO
     sendmail(8)

OpenBSD 4.7                     August 8, 2004                               1
