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smrsh - restricted shell for sendmail


See Also


smrsh -c command


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 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/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 directory (using the ‘‘#!’’ syntax); it simply disallows execution 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 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’’).


/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



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