Книга: Distributed operating systems

10.6.5. ACLs

10.6.5. ACLs

Every resource in DCE can be protected by an ACL, modeled after the one in the POSIX 1003.6 standard. The ACL tells who may access the resource and how. ACLs are managed by ACL managers, which are library procedures incorporated into every server. When a request comes in to the server that controls the ACL, it decrypts the client's PAC to see what his ID and groups are, and based on these, the ACL, and the operation desired, the ACL manager is called to make a decision about whether to grant or deny access. Up to 32 operations per resource are supported.

Most servers use a standard, but less mathematically inclined, ACL manager, however. This one divides the resources into two categories: simple resources, such as files and data base entries, and containers, such as directories and data base tables, that hold simple resources. It distinguishes between users who live in the cell and foreign users, and for each category further subdivides them as owner, group, and other. Thus it is possible to specify that the owner can do anything, the local members of the owner's group can do almost anything, unknown users from other cells can do nothing, and so on.

Seven standard rights are supported: read, write, execute, change-ACL, container-insert, container-delete, and test. The first three are as in UNIX. The change-ACL right grants permission to modify the ACL itself. The two container rights are useful to control who may add or delete files from a directory, for example. Finally, test allows a given value to be compared to the resource without disclosing the resource itself. For example, a password file entry might allow users to ask if a given password matched, but would not expose the password file entry itself. An example ACL is shown in Fig. 10-28.

Fig. 10-28. A sample ACL.

In this example, as in all ACLs, the ACL type is specified. This type effectively partitions ACLs into classes based on the type. The default cell is specified next. After that come permissions for two specific users in the default cell, two specific groups in the default cell, and a specification of what all the other users in that cell may do. Finally, the rights of users and groups in other cells can be specified. If a user who does not fit any of the listed categories attempts an access, it will be denied.

To add new users, delete users, or add, delete, or change permissions, an ACL editor program is supplied. To use this program on an access control list, the user must have permission to change the access control list, indicated by the code c in the example of Fig. 10-28.

Оглавление книги