Security and File Permission. Harish Kumar.H.C Asst.Professor Department of CSE

Transcription

1 Security and File Permission Harish Kumar.H.C Asst.Professor Department of CSE

2 Introduction The security system in UNIX, like any other operating system, is designed to control the access to resources. Everyone who logs on to the system is called user. User are known to the system by their user-ids. Users created in Unix are not equal. Some users have more capabilities than others. Super users(system administration) have the maximum set of capabilities in the system.

3 Users and Groups Users can be organized into groups. The team(group) members can have easy access to each other s files while still protecting the files from users outside the group. Users can belong to multiple groups. group command to determine a user s group.

4 Security Levels There are three levels of security in UNIX Ø System Ø Directory Ø File System security is controlled by the system administrator. Directory and file securities are controlled by the users who own them.

5 System security System Security controls who is allowed to access the system. Users are created in system password file(/etc/passwd). forouzan : zhptisp8hp : 3652 : 24 : B A Forouzan : /staff/forouzan : /bin/ksh Login Name password userid GroupID User Info Home Directory login shell

6 Permission codes To determine who can access and manipulate a directory or file. Permission codes are divided into three set of codes. The first set contains the permissions for owner of directory/file. The second set contains the group permissions for members in a group. The third set contains the permission for everyone else(others).

7 Directory Level Permissions Read Permission(r) The user can read the contents. The user can look at the filenames inside the directory. Write Permission(w) The user can add or delete files from the directory. Execute Permission(x) Ø Search permission. Ø The user can reference or move to directory.

8 File Level Permission Read permission (r) Grants the capability to read, i.e., view the contents of the file. Write permission (w) Grants the capability to modify, or remove the content of the file. Execute Permission (x) Run executable files(can run a file as a program). To check the permission of file or directory

9 Changing Permission When a file(666) or a directory(777) is created, the system automatically assigns default permissions. The owner of file or directory can change permission. The chmod(change mode) command is used to change the permission of file or directory.

10 Changing Permission

11 Symbolic codes

12 Symbolic codes symbols u g o a = + r w x Meaning User (owner) group others all (user + group +others) Assignment (current permissions for a set are replaced by the new permission) Add permission Remove permission read write Execute

13 Octal codes(read= 4 write = 2 Execute =1) Fastest way to assign permission. Number Octal Permission Representation Ref No Permission Execute permission Write permission Execute and write Permission :1 (execute) + 2 (write) = 3 Read permission Read and execute permission: 4 (read) + 1 (execute) = 5 Read and write permission: 4 (read) + 2 (write) = 6 All permissions: 4 (read) + 2 (write) + 1 (execute) = x -w-wx r-r-x rwrwx

14 User masks umask is a command that determines the settings of a mask that controls how file permissions are set for newly created files and directories. Umask value is a three-digit octal number. The default permission for a newly created file is The default permission for a newly created directory is Each mask digit means remove the corresponding digits from the default permission. Default umask value for the Unix system is 0002.

15 Change Ownership(chown)command The chown command is used to change ownership of files and/or directories. Only the current owner or super user may change the ownership or group.

16 Change Group(chgrp) command To change the group without changing the owner. Only the current owner or super user may change the ownership or group.

17 Thank you