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Listing 8-6. The Rules in the First Part of the crond PAM Configuration File auth auth auth sufficient required include pam_rootok.so pam_env.so system-auth

From Listing 8-6, you can see two rules after our first rule, and it has the same servicetype part. This is fine, and you can add rules on a servicetype if need fine control on a specific management task. This feature is called the rule stack, and you can add rules separated by lines. The Linux-PAM will evaluate the rules like a stack starting from the first line. The result of the evaluation will be used when evaluating the second line. Like in a stack, the first line gets popped off and thrown out, except for its result. From this process, Linux-PAM will know if it is going to grant or deny the service to the user, because it will refer to the final result of the popping process. The last line on Listing 8-6 uses the include control value that will include all the lines from the file in the modulepath part that matches the management task for this rule. Once the evaluation reaches this rule, the rule will open the system-auth PAM configuration file and get all the lines of that file that have the auth management task on its servicetype part. Assuming that the file has been loaded and all of the auth management tasks have been retrieved, Listing 8-6 will now become Listing 8-7 in memory. Listing 8-7. The auth Management Tasks Added on the Rules auth auth auth auth auth auth sufficient required required sufficient requisite required pam_rootok.so pam_env.so pam_env.so pam_unix.so nullok try_first_pass pam_succeed_if.so uid >= 500 quiet pam_deny.so

After that, evaluation will continue starting from the third line with the pam_env.so module. Now that you know what a PAM rule is and how Linux PAM evaluates it, you will learn more of the parts of the rule starting with the servicename part.

The servicename part of a rule is the name of the application that will use this rule. Every PAM-enabled application that gets initialized will be assigned a name that can be referenced by PAM. When the PAMenabled application needs to authenticate a user, it will identify itself to your Linux PAM. During identification, Linux PAM will look for the entries in pam.conf for the servicename part that contains the applications name then apply the rules on the application. If there is no pam.conf, the PAM system will look for the file that has the application s name and apply the rules inside the file.

Note: There is a service name called other, and it acts as a catchall for PAM rules or applications that do not

have a PAM configuration file. It denies access to all rules by default. You can check its PAM configuration file on /etc/pam.d named other.

The designers of PAM observed that there is a pattern when an application is authenticating a user and decided to categorize them as management groups. The four management groups that you can use are auth, account, session, and password. The servicetype part will hold the chosen management group that will be used by this rule. The auth management group will authenticate users by making them provide identification information such as usernames and passwords. It is also used to associate the user to a group for restriction to the service such as giving read-only permission. The account management group will authenticate users based on account management. This will allow users on the system at a certain time of the day or specify where a user can log into, such as a specific console or terminal. The session management group controls the session of the user after authentication has been accomplished. The session activities include mounting of directories or changing certain files and recording when the user logged in and logged out. The password management group is used when the user authentication token is being updated, which can happen if the user is changing a password for example.