SMRSH

NAME
SYNOPSIS
DESCRIPTION
FILES
SEE ALSO

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/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/procmail -f- /etc/procmailrcs/user || 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/smrsh/vacation’’.

System administrators should be conservative about populating the /etc/smrsh directory. Reasonable additions are vacation(1), procmail(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 /etc/smrsh directory. Note that this does not restrict the use of shell or perl scripts in the /etc/smrsh directory (using the ‘‘#!’’ syntax); it simply disallows execution of arbitrary programs.

FILES

/etc/smrsh − directory for restricted programs

SEE ALSO

sendmail(8)