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Comments and suggestions concerning this book should be mailed to gmourani@videotron.ca © Copyright 1999-2000 Gerhard Mourani and Open Network Architecture ® 266 You will next need to populate the “/etc/smrsh” directory with the programs that are allowable for Sendmail to execute. To prevent duplicate programs and make a nice job, it is better to establish links to the allowable programs from “/etc/smrsh” rather than copy programs to this directory. · To allow the mail program “/bin/mail”, use the following commands: [root@deep /]# cd /etc/smrsh [root@deep smrsh]#  ln -s /bin/mail mail · To allow the procmail program “/usr/bin/procmail”, use the following commands: [root@deep /]# cd /etc/smrsh [root@deep smrsh]#  ln -s /usr/bin/procmail procmail Would allow the mail and procmail programs to be run from a user's “.forward” file or an “aliases” which uses the "program" syntax. Step 3 We can now configure Sendmail to use the restricted shell. The program mailer is defined by a single line in the Sendmail configuration file, “/etc/sendmail.cf”. You must modify this single line “Mprog” definition in the “sendmail.cf” file, by replacing the “/bin/sh” specification with “/usr/sbin/smrsh”. Edit the sendmail.cf file (vi /etc/sendmail.cf) and change the line: For example: Mprog,  P=/bin/sh, F=lsDFMoqeu9, S=10/30, R=20/40, D=$z:/, T=X-Unix, A=sh -c $u Which should be changed to: Mprog,  P=/usr/sbin/smrsh, F=lsDFMoqeu9, S=10/30, R=20/40, D=$z:/, T=X-Unix, A=sh -c $u · Now re-start the sendmail process manually with the following command: [root@deep /]# /etc/rc.d/init.d/sendmail restart NOTE: In our “sendmail.mc” configuration file for the Mail Hub Server above, we are already configured this line “Mprog” to use the restricted shell “/usr/sbin/smrsh” with the m4 macro “FEATURE(`smrsh',`/usr/sbin/smrsh')”, so don’t be surprised if the “/usr/sbin/smrsh” specification is already set in your “/etc/sendmail.cf” file for the Mail Hub relay. Instead use the technique show above for other “/etc/sendmail.cf” files in your network like the one for the nullclient “local or neighbor client and server machines” that use the “/etc/null.mc” macro configuration file to generate the “/etc/sendmail.cf” file. The “/etc/aliases” file Wrongly or carelessly administered aliases file can easily be used to gain privileged status. For example, many vendors ship systems with a “decode” alias in the “/etc/aliases” file. The intention is to provide an easy way for users to transfer binary files using mail. At the sending site the user converts the binary to ASCII with “uuencode”, then mails the result to the “decode” alias at the receiving site. That alias pipes the mail message through the “/usr/bin/uuencode” program, which converts the ASCII back into the original binary file. Remove the “decode” alias line from your “/etc/aliases” file. Similarly, every alias that executes a program that you did not place there yourself and check completely should be questioned and probably removed. Edit the aliases file (vi /etc/aliases) and remove the following lines: # Basic system aliases -- these MUST be present. MAILER-DAEMON:  postmaster