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A Red Hat training course is available for Red Hat Enterprise Linux Shell Commands

Option fields allow access rules to launch shell commands through the following two directives:
  • spawn — Launches a shell command as a child process. This directive can perform tasks like using /usr/sbin/safe_finger to get more information about the requesting client or create special log files using the echo command.
    In the following example, clients attempting to access Telnet services from the domain are quietly logged to a special file:
    in.telnetd : \
    	: spawn /bin/echo `/bin/date` from %h>>/var/log/telnet.log \
    	: allow
  • twist — Replaces the requested service with the specified command. This directive is often used to set up traps for intruders (also called "honey pots"). It can also be used to send messages to connecting clients. The twist directive must occur at the end of the rule line.
    In the following example, clients attempting to access FTP services from the domain are sent a message using the echo command:
    vsftpd : \
    	: twist /bin/echo "421 This domain has been black-listed. Access denied!"
For more information about shell command options, see the hosts_options man page.