- Asked 2/13
- 2069 Bhadra · 5 marks
- 2070 Bhadra · 4+4 marks
- Year not given
Protection domain and ACL
Short answer
A protection domain is a set of (object, rights) pairs defining what operations a process may perform. The domain-object matrix is implemented via Access Control Lists (ACLs) stored per object listing allowed domains and rights, or capability lists stored per domain. ACLs make it easy to revoke access to an object but slow to find all objects a user can access; UNIX uses simple rwx bits while Windows NTFS uses full ACLs.
Explain protection domain and access control list (ACL) (domain-object and ACL, and how these mechanisms are implemented for security).
Protection domain
A protection domain is a set of (object, rights) pairs. Each pair names an object (file, printer, memory segment) and the operations (read, write, execute) a process running in that domain may perform on it. At any time, a process runs in exactly one domain, but it may switch domains.
In UNIX, a domain is defined by the user ID and group ID (UID, GID). A process started by user X runs in X's domain. The setuid bit lets a process switch to the domain of the file's owner temporarily.
Domain-object matrix (protection matrix)
Rows are domains, columns are objects, entries are rights.
| Domain | File1 | File2 | Printer |
|---|---|---|---|
| D1 | Read | Read, Write | – |
| D2 | – | Read | |
| D3 | Read, Write, Execute | – | – |
Since the matrix is large and sparse, it is stored by column or by row:
- Access Control List (ACL): storing the matrix by columns. Each object has a list of (domain/user, rights).
- Capability list: storing the matrix by rows. Each domain/process keeps a list of objects it may access with rights.
Access control list (ACL)
Each object carries a list of who may do what.
File1: (Alice: read,write)
(Bob: read)
(Group staff: read)
- When a process asks for an operation, the OS searches the object's ACL for a matching entry.
- Easy to find who can access an object, and to revoke an object's rights (edit one list).
- UNIX uses a short ACL: rwx bits for owner, group, others. Windows NTFS has full ACLs.
- Drawback: finding everything one user can access is slow, and the list search takes time on each access.
Thus the domain/object matrix describes the policy, and ACLs (per object) or capabilities (per process) implement it.
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- Short note: Protection domain · Operating System, chapter 8 · asked 2×
- Trap door; firewalls and ACL · Operating System, chapter 8
- Attack types; security of all components · Operating System, chapter 8
- Media Access Control and significance · Computer Networks and Security, chapter 3
- Concurrency control goals and mechanisms · Distributed System, chapter 8
- Flow control mechanisms in data communication · Telecommunications, chapter 8
- Contactor for motor control and protection · Switchgear and Protection, chapter 4 · asked 3×