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
/etc/mail/smrsh, 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
``/etc/mail/smrsh/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¶
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
``/etc/mail/smrsh'').
FILES¶
/etc/mail/smrsh - 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(8)