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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Linux Essenciais and System Administration

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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
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Manage file permissions and ownership
Permissions
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Permissions are for Superuser, User and group

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Manage file permissions and ownership
Security levels
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Manage file permissions and ownership
configuring files
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• 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:

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Manage file permissions and ownership
configuring files
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/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

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Manage file permissions and ownership
manage files with management commands
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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

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Manage file permissions and ownership
Passwords
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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

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Manage file permissions and ownership
Permissions
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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

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Manage file permissions and ownership
Ownership and Permissions
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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--

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Manage file permissions and ownership
Directory and File permissions
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Manage file permissions and ownership
groups
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Manage file permissions and ownership
chown
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Manage file permissions and ownership
chgrp
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Manage file permissions and ownership
chmod
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Manage file permissions and ownership
chmod
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Manage file permissions and ownership
chmod symbolic codes
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Manage file permissions and ownership
chmod octal commands
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Manage file permissions and ownership
umask
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Manage file permissions and ownership
Permissions – /etc/passwd
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Fim de sessão
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