101 4.5 manage file permissions and ownership v3
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This document discusses Linux file permissions and ownership. It covers commands used to manage permissions and ownership, including chmod, chown, chgrp, umask. Key areas covered include permissions for users, groups and others; permission levels for files and directories; and configuring user and group information stored in /etc/passwd, /etc/shadow, and /etc/groups files. The goal is to teach system administrators how to manage access permissions on files and directories to maintain security.
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101 4.5 manage file permissions and ownership v3
- 1. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Linux Essenciais and System Administration
- 2. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Key Knowledge Areas Manage access permissions on regular and special files as well as directories. Use access modes such as suid, sgid and the sticky bit to maintain security. Know how to change the file creation mask. Use the group field to grant file access to group members. Devices, Linux Filesystems, Filesystem Hierarchy Standard Manage file permissions and ownership Terms and Utilities chmod umask chown chgrp 2
- 3. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership Permissions 3 Permissions are for Superuser, User and group
- 4. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership Security levels 4
- 5. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership configuring files 5 • User information is stored in two files: /etc/passwd /etc/shadow • Group information is stored in one file: /etc/group /etc/passwd List of user records, one per line, with columns separated by colons. Format: login:x:userid:groupid:gecos:homedir:shell Ex: root:x:0:0:root:/root:/bin/bash mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash Ex:
- 6. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership configuring files 6 /etc/shadow Similar to passwd colon-separated-column list of records: Format: login:password:password aging fields aging fields track dates for password resets, locks, etc Ex: root:pB8msP1fCbCqc:13904:0:99999:7::: nisburgh:vRoPw6a/jQsp.:14466:0:99999:7::: /etc/groups Same colon-separated-column list of records format Format: groupname:grouppassword:groupid:secondarymembers Group passwords allow temporary access to a group, rarely used, not set up by default Ex: daemon:x:2:root,bin,daemon apache:x:48:jack,nisburgh
- 7. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership manage files with management commands 7 For /etc/passwd shadow and groups While it is possible to edit the three files directly, it’s easier and safer to use: management commands to create, modify and delete users and groups useradd, usermod, userdel, groupadd, groupmod, groupdel Useradd Add a new user to the system Accepts various arguments to control the settings on the user account. Most common is -g to specify primary group of user, and -G to list secondary group memberships. Ex: useradd lisa useradd -g clowns -G trouble,simpson bart Usermod Modify a user’s settings. Ex: usermod -G detention bart userdel Remove a user from the system. Main option is -r, which tells userdel to remove the user’s home and spool directories. Ex: userdel moe
- 8. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership Passwords 8 Passwd Change login password. •Root can change the password for any user on the system •Root can setup password aging, allowing for timed password resets and account disabling •passwd is preferred way to lock user account Ex: passwd -l mary PASSWORD AGING •To set maximum lifetime for a user’s password: passwd -x days login •When user’s password has expired, the number of days it can remain expired before disabling the account completely can be set: passwd -i days login
- 9. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership Permissions 9 Linux supports 3 main types of access on a file: 1.read View the contents 2.write Modify the contents and metadata 3.Execute Run the contents Actually, it’s different for files and directories Files Directories Read View the contents List contents Write Change the contents/metadata Create/delete entries, change metadata Execute Run the contents Operate with directory as CWD Combining these permissions allows for the most common access levels: Read only; Read/Write; Execute; etc
- 10. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership Ownership and Permissions 10 All files are associated with one user and one group (ownership). This creates the foundation for the main security infrastructure in the Linux (Unix). When a process attempts an operation on a file, the user and group of the process (every process is associated with one user and one group) are compared with the user and group of the file, which determines what level of permissions is granted or denied on the file. Every file has 3 levels of permissions: •User •Group •Other When a process seeks access, the process user is compared to the file user - if they match, the process gets the User permissions. Next Group. If no match, Other level access All permission information is summarized with 9 characters: rwxrwxrwx The presence of the letter indicates the permission is granted, a hyphen in it’s place indicates the permission is denied. Read only: r--r--r--
- 11. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership Directory and File permissions 11
- 12. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership groups 12
- 13. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership chown 13
- 14. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership chgrp 14
- 15. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership chmod 15
- 16. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership chmod 16
- 17. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership chmod symbolic codes 17
- 18. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership chmod octal commands 18
- 19. CoreLinuxforRedHatandFedoralearningunderGNUFreeDocumentationLicense-Copyleft(c)AcácioOliveira2012 Everyoneispermittedtocopyanddistributeverbatimcopiesofthislicensedocument,changingisallowed Manage file permissions and ownership umask 19