Ch11

Chapter 11:Managing Users

The Complete Guide to Linux System Administration

The Complete Guide to Linux System Administration 2

Objectives

• Create and manage user accounts

• Manage complex file permissions

• Review advanced user security issues


Creating and Managing User Accounts

• Process of creating new user accounts– Add default files to be part of every user’s home

directory to /etc/skel directory– Create home directory for all users– Configure default settings for all users– Create groups– Create user account for each person– Create valid password for each user


Managing User Accounts Graphically

• /etc/passwd file defines user accounts, including:– User name – Comment field – Password – User’s home directory– UID – User’s default shell– GID

• vipw command edits /etc/passwd file

• passwd command defines password


Managing User Accounts Graphically (continued)

• Utilities to create new user accounts– Graphical tools– Command-line utilities

• system-config-users utility– Red Hat graphical user account creation tool– Information from /etc/passwd file shown in window– Can edit each fields– Edit password information stored in /etc/shadow





• User Properties dialog, User Data tab– User name– Full name– Password and confirm password– Home directory– Login shell

• User Properties dialog, Account Info tab– Enable account expiration– Lock local password



• User Properties dialog, Password Info tab– View when user last changed password– Enable password expiration

• Experts recommend changing password every 30 to 60 days

• User Properties dialog, Groups tab – Lists groups from /etc/group file– Each user assigned primary group



• User private group model – Used by Red Hat Linux and Fedora– User’s primary group has same name as user– Contains only user as member

• Create new user– system-config-users utility– Add User button– Enter information on dialog– Utility creates home directory based on user name



• /etc/login.defs file– Stores settings used to create new users– Comments describe settings

• Create new group– Click Add Group button– Enter group name



• Delete user– Select user name on Users tab – Click delete button– Some administrators prefer to permanently

disable, not delete


Creating New Users at the Command Line

• useradd utility– Create new users– Must be logged in as root– Example: useradd -g sales -c “Raley Solomon"

rsolomon– -D option displays the default settings– Edit /etc/default/useradd to change defaults


Creating New Users at the Command Line (continued)

• passwd command– Change user’s password– Lock user account: passwd -l thomas– Unlock account: passwd -u thomas


Creating New Groups

• groupadd command– Preferred method for adding new group– Example: groupadd managers


Modifying User and Group Accounts at the Command Line

• usermod command– Modify user account– Uses same options as useradd command

• groupmod command– Modify group– -g option: change GID– -n option: change name



(continued)• grpck command

– Check integrity of /etc/group and /etc/gshadow files

– Uses no parameters

• chage command– Alter password aging information– View password expiration– Change aging for user– Interactive mode



(continued)• pwconv utility converts older /etc/passwd

password storage to /etc/shadow password storage

• pwunconv utility converts existing /etc/shadow passwords to older /etc/passwd system for compatibility

• pwck command shows formatting errors that make accounts unusable


Automating Home Directory Creation

• Files in /etc/skel automatically copied into each user’s home directory – At time account created– Includes hidden configuration files– Files not added to existing user home directories


Disabling User Accounts

• Temporarily disable user’s account– Change password

• passwd command

– passwd -l [username] to lock account– Edit /etc/shadow file in text editor

• Place asterisk before encrypted password

– Place # at beginning of line for user account in /etc/passwd

• Make line into comment


Disabling User Accounts (continued)

• userdel command– Delete user account permanently– Does not remove user’s home directory or

contents

• groupdel command– Remove group– Be careful deleting groups


Complex File Permissions

• Sticky bit – Directory can only be renamed or unlinked by

• User that owns it

• root

– Often used on directories all users should be able to create files such as /tmp

– To set: chmod a+t /tmp


Complex File Permissions (continued)

• Set user ID permission– SUID– Can be added to file that has execute permission

set– Causes user who executes file to take on file

permissions of owner of file– Very useful for few specialized programs– To set: chmod u+s file



• Permissions not always what they seem at first

• Set group ID permission– SGID– Person who executes program has permissions of

file’s group while executing program– To set: chmod g+s file– Can also be set numerically



• SGID set on directory– Any file created within directory assigned to group

of directory

• Permissions examined in order– Owner first– Group second– Others third– Can deny group permission without users having

other permission


User Security Issues

• Other ways of securing Linux– Additional communications– User security mechanisms


Communicating with Users

• /etc/motd file– Contents displayed just before shell is started

each time user logs in– “Message of the day”– Does not appear unless:

• User opens command line

• Or logs in at text-mode console


Communicating with Users (continued)

• wall command– Write all– Communicate immediate message to all users

who are logged in to system– Displays broadcast message on command line for

all users • Working in text mode

• Or with open terminal window


Communicating with Users (continued)

• fuser command– Learn about what users are doing– View users accessing

• File system

• Serial port

• Network connection

– -k option kills processes


Granting Limited Root Access

• sudo command – Assign root privileges to any user account – User can execute just programs that sudo

configuration specifies

• /etc/sudoers configuration file– Syntax can be very complex– Basic format: user host = command_list


Granting Limited Root Access (continued)

• /etc/sudoers configuration file– Can define aliases for:

• Collection of users

• Collection of hosts

• Collection of programs

• Set of sudo options

• visudo program edits /etc/sudoers file

• System administrator must hand out sudo power carefully


Using Pluggable Authentication Modules

• Pluggable authentication module (PAM)– Architecture and set of libraries – Programmer can create module to perform

specific security-related function– System administrators can select, configure, and

use one or more modules • Control operation of program that is aware of PAM

capabilities

– Configured by default for Red Hat Linux


Using Pluggable Authentication Modules (continued)

• To use PAM, select PAM modules for program

• Configured using:– Single configuration file /etc/pam.conf– Series of configuration files in directory /etc/pam.d– Person compiling software selects which

configuration style is used



• Red Hat Linux and Fedora use directory configuration method

• /etc/pam.d directory contains file with name matching program being configured

• When PAM-compatible program executed, checks to see which applicable modules are configured for given task

• Stacked modules– Multiple modules are listed for module type



• control_flag element – Determines how PAM processes stacked modules– Has two forms


Using Network Information Service

• Network Information Service (NIS) – Lets system administrator manage single set of

configuration files for multiple Linux servers– Sometimes called yellow pages service or yp

• When user logs in, server contacts NIS server to see if user has valid user account


Using Network Information Service (continued)

• NIS+ or NISplus– More recent version of NIS

• authconfig program– Set up system to use NIS server

• /etc/nsswitch.conf file– Instructs various system programs in Linux on

where to look for configuration information– Often includes multiple options


Summary

• User accounts can be managed graphically

• useradd command creates user accounts on the command line

• usermod command modifies existing user accounts

• User accounts can be disabled using various methods


Summary (continued)

• SUID and SGID permissions cause user to assume permissions of owner of executable file when program is executed

• Sticky bit and other special file permissions allow administrators to control shared files

• Administrators can broadcast messages to users

• sudo program lets regular users perform tasks that require root privileges


Summary (continued)

• PAMs provide flexible and powerful way for system administrators to configure exactly how user security is handled

• NIS or NIS+ server– Multiple systems can share user files

Ch11

Education

pluggable

red hat linux

managing user

user properties

complex file

command line

modifying

configuration