smrsh man page

smrsh — restricted shell for sendmail


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/adm/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/ucb/vacation”, “/usr/bin/vacation”, “/home/server/mydir/bin/vacation”, and “vacation” all actually forward 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 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:/usr/ucb”) and/or -DSMRSH_CMDDIR=\"dir\" to change the default program directory (defaults to “/usr/adm/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

See Also



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