Lecture 6 Users Administration

8/6/2019 Lecture 6 Users Administration

http://slidepdf.com/reader/full/lecture-6-users-administration 1/57

Linux Based Networks

User Administration Class Meeting # Slide

Zahid Shafique, Instructor

Linux Based NetworksPunjab University College of Information Technology

Instructor: Zahid Shafique

<[email protected]>






User Administration






User Accounts

Because Linux is a multi-user system, the task of adding and maintaining user accounts is common in Linux system administration.

Upon a successful installation of a Linux distribution, twouser accounts are configured: the root user and a normaluser.

These two user accounts represent the two basic types of users that are configurable with Linux.






The first type

The first type, the root user , is unique for several

reasons.It is the only user account with system wide privileges.

Other accounts can be set up as an exact clone of theroot

user account, but it is strongly discouraged.






System AccountsIn your Linux distribution, you will see a number of

accounts set up, like bin, daemon, adm, Ip, sync,shutdown, mail, operator, and others.

They are called "system accounts" and are used forvarying

purposes, some self-explanatory, some not.

These accounts do not have passwords because theyare not

designed for login.

These special-purpose accounts are also called non-login

accounts.

Do not delete them or some programs will not run.






Default Users






Default Groups






Privileges of a root user

System administration tasks are performed from the root or super user account.

The following list summarizes some of the duties andprivileges of a system administrator:

Has complete access to all files and directories regardless of owner and permissions

Controls user account administrationPerforms system maintenance Halts the system when necessarySets up initial user passwords

Changes passwords when necessary. Installs software on the system






The Second Type

A normal user account, is the type of account that is set

up for each individual user.

This type of account differs only in respect to accessprivileges and home directories.




User Administration Class Meeting # Slide :


Important

It is good system practice to have a separate account foreach individual user. Avoid accounts that are shared, if possible.






Adding New Users• Required steps:

– Edit the /etc/passwd file to define the user's account.– Set an initial password.– Create the user's home directory.

• Extra steps (optional):– Copy default startup files to the user's home directory.– Add the user to the /etc/group file.– Set the user's mail home and establish mail aliases.

– Enter the user in the site-wide user database (NIS).– Configure disk quotas.– Verify that the account is set up correctly.– Record accounting information.






User Accounts Utilities

Managing users and groups has traditionally beentedious,

Red Hat Linux has a few tools and conventions that makeusers and groups easier to manage.

You can use useradd to create a new user from theshell

prompt

The easiest way to manage users and groups isthrough

$ redhat-config-users$ redhat-config-users






Account Setup

Adding and deleting users is simple, but understandingthe files involved with user account management is abit more complex.

The key to managing user accounts is to understand theunderlying configuration files.






Key Configuration File

The key file used in user account setup and configurationis the /etc/passwd file.

This file is a simple ASCII text file.






/etc/passwd File

Upon a successful installation, the contents of the/etc/passwd file resemble the following:

root:x:0:0:root:/root:/bin/bash

bin:x:1:1:bin:/bin:

daemon:x:2:2:daemon:/sbin:

adm:x:3:4:adm:/var/adm:

lp:x:4:7:lp:/var/spool/lpd:

sync:x:5:0:sync:/sbin:/bin/sync

shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown

halt:x:7:0:halt:/sbin:/sbin/haltmail:x:8:12:mail:/var/spool/mail:

news:x:9:13:news:/var/spool/news:

uucp:x:10:14:uucp:/var/spool/uucp:

operator:x:11:0:operator:/root:

games:x:12:100:games:/usr/games:

gopher:x:13:30:gopher:/usr/lib/gopher-data:

ftp:x:14:50:FTP User:/var/ftp:

nobody:x:99:99:Nobody:/:

xfs:x:43:43:X Font Server:/etc/X11/fs:/bin/false

apache:x:48:48:Apache:/var/www:/bin/false

named:x:25:25:Named:/var/named:/bin/false

gdm:x:42:42::/home/gdm:/bin/bash

rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/bin/false

rpc:x:32:32:Portmapper RPC user:/:/bin/false

postgres:x:26:26:PostgreSQL Server:/var/lib/pgsql:/bin/bash

mailnull:x:47:47::/var/spool/mqueue:/dev/null

mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash

nscd:x:28:28:NSCD Daemon:/:/bin/false

pvm:x:24:24::/usr/share/pvm3:/bin/bashsquid:x:23:23::/var/spool/squid:/dev/null






The /etc/passwd file

root:*:0:3:The Big Guy,143 MSB:/:/bin/sh

daemon:*:1:5::/:/bin/sh

farrell:*:100:15:Paul Farrell:/users/research/farrell/:/bin/csh

stacke:*:101:15:Olaf P. Stackelberg:/users/fac/stack:/bin/csh

pwang:*:102:15:Paul S. Wang:/users/research/wang:/usr/local/bin/tcsh

abaqus57:x:53:13:License Manager:/var/log/abaqus57:/usr/bin/csh

matlab:x:54:13:Matlab License Manager:/usr/local/matlab5:/usr/bin/csh

catadm:x:55:13::/usr/local/abaqus:/bin/csh

…

root:*:0:3: The Big Guy,143 MSB:/:/bin/sh

daemon:*:1:5::/:/bin/sh

farrell:*:100:15:Paul Farrell:/users/research/farrell/:/bin/csh

stacke:*:101:15:Olaf P. Stackelberg:/users/fac/stack:/bin/csh

pwang:*:102:15:Paul S. Wang:/users/research/wang:/usr/local/bin/tcsh

abaqus57:x:53:13:License Manager:/var/log/abaqus57:/usr/bin/csh

matlab:x:54:13:Matlab License Manager:/usr/local/matlab5:/usr/bin/csh

catadm:x:55:13::/usr/local/abaqus:/bin/csh

…

login_name:password:user_id:group_id:user

info:home:shell

/etc/passwd

� Each entry in /etc/passwd must be on one line.� Each field within each user's entry is separated from the next by a colon.

Note






The /etc/passwd file contents

• Login name– case sensitive and unique for users.– no more than 8 chars long, may include numbers,

except in position 1. (for some old system only)– /etc/aliases can be used to set the alias of user

name for most mail system like sendmail.– Pseudo-Logins execute the correspondingcommands as their shells. Eg daemon, bin.

• Encrypted password– Look like Xv8Q981g71oKK requ, encrypted by MD5 or

DES algorithms.– If null, no password is required, ie Press Enter directly!

– change user's password with passwd.– /etc/passwd is world readable -> no security.

(refer to the /etc/shadow section)






The /etc/passwd file contents• User ID

– uid is the ID for the user, which must be unique.– UID 0 is reserved for root, and traditionally the lower numberare kept for systems programs.

– It is recommended that human users start at 500.– uid is a value between 0 and 65536.– It is a good idea to avoid reusing UIDs, since this avoids

confusion when backups are restored.• Group ID

– gid is the ID of the group that the user belongs to.– gid is generally a value between 0 an 65536. GID 0 is reserved

for root and 1 is usually reserved for daemon.

– Groups' info are defined in /etc/groups.• User Info (GECOS field)

– No particular format, can be Full name, Address, Contactnumber, etc.

– The finger command expects one to have comma. delimited– Can be modified by the chfn command.

Li B d N t k






The /etc/passwd file contents• Home directory

– Users are placed in their home directory when they log in. Eg./home/staff/zahid.

– Home directories are usually named with the user's login name,and should be read, write and executable by the user.

– If it is missing or invalid, a message such as "no home directory"

will be printed or login disabled or login to the root directory.

# mkdir /home/staff/zahid# chown zahid /home/staff/zahid# chgrp zahid /home/staff/zahid# chmod 700 /home/staff/zahid

# mkdir /home/staff/zahid# chown zahid /home/staff/zahid# chgrp zahid /home/staff/zahid# chmod 700 /home/staff/zahid

Shell Specify a standard shell (sh, csh, bash, ksh, etc.), but can be

a restricted shell, or a program. Can be changed by chsh commands. If the shell field is null, Bourne Shell is used.

Li B d N t k






Important Users• daemon: Owner of Unprivileged Software

– Usually has uid 1.– Controls the files that belong to UNIX system.

• bin: Owner of System Commands– Owns directories that contain the system's

command and most of the executable files.• sys (kmem): Owner of the Kernel and Memory Images– Owns special files such as /dev/kmem (kernel's

address space), /dev/mem (the physical memory of

system), and /dev/swap (image of the system'sswap space).

• nobody: Owner of Nothing– The owner of software that doesn't need or

shouldn't have special permissions, eg fingerd.

Li B d N t k






The /etc/group fileroot:X:0:root

daemon:X:1:

bin:X:2:root,bin,daemon

sys:X:3:root,bin,sys,adm

adm:X:4:root,adm,daemon

…

root:X:0:root

daemon:X:1:

bin:X:2:root,bin,daemon

sys:X:3:root,bin,sys,adm

adm:X:4:root,adm,daemon

…

/etc/group

Each line has, 4 colon separated fields,

Group name password - never used

GID - group ID number list of members

In practice, users are in the group given in the password filwhether or not they are entered in that group in /etc/grou

Li B d N t k S f






The Startup Files• Startup files traditionally start with period (.) and end with rc

(run command). Eg .cshrc, .login, .profile, etc• It is useful to place system defaults setting in these files.• Depends on the shell used, common startup files are:

csh .login Sets the terminal type, environment variables..cshrc Sets command aliases, path, umask value, prompt..logout Execute when users logout.

sh /etc/profile System wide setup procedure at login..profile Similar to .login and .cshrc for csh.

ksh /etc/profile System wide setup procedure at login..profile User-specific setup for ksh at login..kshrc Addition command to the shell when a new ksh

started.

bash /etc/profile System wide setup procedure at login./etc/bashrc System wide setup when bash started..bash_profile User-specific setup for bash at login.bash_login read by bash if .bash_profile does not exist..bashrc read by bash only when a new bash started.

.bash_logout read by bash when the shell is exited.

csh .login Sets the terminal type, environment variables..cshrc Sets command aliases, path, umask value, prompt.

.logout Execute when users logout.

sh /etc/profile System wide setup procedure at login..profile Similar to .login and .cshrc for csh.

ksh /etc/profile System wide setup procedure at login. .profile User-specific setup for ksh at login..kshrc Addition command to the shell when a new ksh

started.

bash /etc/profile System wide setup procedure at login./etc/bashrc System wide setup when bash started. .bash_profile User-specific setup for bash at login.bash_login read by bash if .bash_profile does not exist..bashrc read by bash only when a new bash started..bash_logout read by bash when the shell is exited.

Li B d N t k Z hid Sh fi I t t






/etc/shadow File

bin:*:11458:0:99999:7:::

daemon:*:11458:0:99999:7:::adm:*:11458:0:99999:7:::lp:*:11458:0:99999:7:::sync:*:11458:0:99999:7:::

shutdown:*:11458:0:99999:7:::halt:*:11458:0:99999:7:::mail:*:11458:0:99999:7:::

The /etc/shadow password file looks similar to the

following:t:$1$d5.gDvSX$nyQhvBgUGbw0GcNTxAdKR1:11507:0:99999:7

Li B d N t k Z hid Sh fi I t t






Secure Password Files

• Modern UNIX systems like Linux and Solaris, place thepasswords in a secure file (commonly called a shadowpassword file) or files elsewhere.

• The format and location differs between systems, as doesthe entry in the password section of the normal password

file.• The /etc/shadow file:

– Its access permissions are much more restricted, onlyview by the root.

– Provides also Password Aging, like password expiration.

Lin Based Net orks Z hid Sh fi I t t






Much like the /etc/passwd file, the /etc/shadow fileconsists of single-line entries with colon-delimited fields.

root:$1$d5X$nyQBgUGbw0GcNTxAdKR1:11507:0:99999:7:::

ername:password:lastchg:min:max:warn:inactive:expire:fl

username The user's login name (UID).password An encrypted password for the user, a lock string to indicate that the

login is not accessible, or no string, which shows that there is nopassword for the login.

lastchg The number of days between 1/1/70 and the date that the password waslast modified.

min The minimum number of days required between password changes.max The maximum number of days the password is valid.warn The number of days before password expires that the user is warned.inactive The number of days after password expires until the a/c is permanently

disable. (-1 for disable the feature)expire A date, YYYY-MM-DD, specifying when the user will be disabled. (-1 for

disable)flag Reserved for future use.

Linux Based Networks Zahid Shafique Instructor






Adding Users

User accounts can be added by directly editing the /etc/passwd file or by using a utility such as useradd,available in all Linux distributions.

Default characteristics for each user account are definedby the settings in the /etc/login .defs file, as follows







/etc/login.defs#QMAIL_DIR MaildirMAIL_DIR /var/spool/mail#MAIL_FILE .mail# Password aging controls:# PASS_MAX_DAYS Maximum number of days a password may be used.# PASS_MIN_DAYS Minimum number of days allowed between password changes.# PASS_MIN_LEN Minimum acceptable password length.# PASS_WARN_AGE Number of days warning given before a password expires.#PASS_MAX_DAYS 99999PASS_MIN_DAYS 0PASS_MIN_LEN 5PASS_WARN_AGE 7#

# Min/max values for automatic uid selection in useradd#UID_MIN 500UID_MAX 60000## Min/max values for automatic gid selection in groupadd#GID_MIN 500GID_MAX 60000## If defined, this command is run when removing a user.

# It should remove any at/cron/print jobs etc. owned by# the user to be removed (passed as the first argument).##USERDEL_CMD /usr/sbin/userdel_local

## If useradd should create home directories for users by default# On RH systems, we do. This option is ORed with the -m flag on# useradd command line.#

CREATE_HOME yes







When we create a new home directory

A new directory, /home/username, is created andpopulated with copies of the default files in the /etc/skel and /etc/skel.d directories (in that order).







Managing Users with Command Line Tools

Creating a new user account To create a user from your command line you can runthe useradd command.

This will add a new entry in /etc/passwd and in/etc/shadow if you use shadow passwords using system

defaults.

#useraddnewuser







Creating a new user account When I run the preceding command on my Red Hat

system,

/etc/passwd shows a new line such as the following:

newuser:!!:506:506::/home/newuser:/bin/bash

If you remember the /etc/passwd fields from earlierdiscussion, You will see that the password field (thesecond field) is set to !!.

This means that this password is not set and the usercannot log in just yet.

So you will need to create a password for this user byrunning the passwd command as follows: #passwd newuser







Default Settings

The UID and the GID values will be automaticallyselected by useradd.

The home directory is created in the default top-level home

directory

The login shell is also selected from a system default.

You will learn to set these defaults in a later section.







Overriding System Default

If you would like to override a system default, you canspecify a command line option

To override the default home directory, use the -d newdirectory option

#useradd newuser -d/www/newuser

Note: useradd will create only the final directory and not the entire path.

Linux Based Networks Zahid Shafique, Instructor






Private Group

The useradd that comes with Red Hal Linux creates a

private group for the user with the same name as theusername.







Overriding System Default

You can override the idea of private group by using the -ggroup option For example:

This will make useradd create the new user (mjkabir) withthe default group set to the users.

#useradd mjkabir -g

users






q ,

Assigning user an additional group

You can use the -G comma-separated list of groups option.For example:

Here the new user (mjkabir) will be added

to the wheel and admins groups in the/etc/group file.

#useradd mjkabir -G

wheel,admins






q ,

To Find Out which groups user belong to

You can use the

command to find out which user belongs to what group.

#groupsusername






q ,

Creating a new group

To create a new group, use the groupadd command. Forexample:

#groupadd mygroup






q

To change or set a user's password, use the passwdcommand.For example:

Note: When an ordinary user runs the passwd program,no username argument is required, because it will allowher to change only her own password.

#passwdzahid

CHANGING A PASSWORD






CHANGING THE SHELL

If the default shell is not appropriate for a user, you maychange it to any shell you list in /etc/shells. Use the chshcommand to change a user's shell.For example:

Note that a user can change her own shell using thiscommand as well.

You can also use the usermod command to modify theshell information as follows:

#chshzahid

#usermod -s new shell pathusername






CHANGING THE HOME DIRECTORY

To change the home directory of an existing user, run theusermod command as follows:

For example, you can run the command as follows:

However, if you would like to move the home directorycontents to the new location, use the -m option as follows:

#usermod -d new home directory username

#usermod -d /home2/zahid zahid

#usermod -d /home2/zahid -m zahid






CHANGING A DEFAULT GROUP

To change the default group for a user, usethe usermod command as follows:

For example:

#usermod -g group name or GID

username

#usermod -g 777zahid






CHANGING ACCOUNT EXPIRATION DATE

If you are using shadow passwords, you can changethe expiration date of a user account using theusermod command as follows:

For example:

This command will reset the account expiration date for user zahid to 12/31/2002.

#usermod -e MM /DD/YY username

#usermod -e 12/31/2002 zahid






CHANGING FINGER INFORMATION

You can also change the finger information, such as the full name and

phone numbers.

Run the chfn command to change a user's finger information.

For example:

#chfn zahid





Modifying an existing group

To modify an existing group name or GID, usethe groupmod command. To rename a groupto a new name, use the following syntax:

For example:

Here the existing novices group is renamed toexperts.

#groupmod -n new group current group

#groupmod -n experts novices





Deleting a user account

To delete an existing user use the userdel command. For example:

This deletes a user called snake.

If you would like to remove the user's home directory and all the

contents within the directory, use the -r option.

# userdel snake





Caution!!

userdel will not delete the user if the user is currently logged in.





Disabling a user accountIf you would like to temporarily disable a user account,

you can do one of the following:Use the usermod -s new shell username' commandto change the shell to /bin/false (make sure it is inyour / etc/shells). This will disallow the user fromlogging into the system

If you are using the shadow passwords, you can usethe

usermod -e MM/DD/YY username command to cause

the user account to expire.

If you would like to disable all user account accessfor a

temporary reason, you can create a file called/etc/nolo in with a messa e ex lainin wh ou arc





Creating default user settings

The default settings for creating new users using useradd

come from /etc/default/useradd.

An example /etc/default/useradd file

GROUP=100HOME= /homeINACTIVE= -1EXPIRE=SHELL= /bin/bashSKEL= /etc/skel





GROUP=100

The GROUP=100 line specifics that the default group ID is100. This value is used only when you disable (using the-n option) the default private group (that is the group withthe same name as the new user). You can specify a groupname instead of the numeric value as well. The value youspecify in this line must exist In /etc/group.

Yon can change this value as follows:

#useradd -D -g news group name or GID





HOME=/home

The HOME=/home line specifies the default top-level

home directory for new users.

You can change this value as follows:

#useradd -D -bdirectory





INACTIVE=-1

The INACTIVE=-1 line specifies when (in days) theaccount willbecome inactive after the password expires.

This is useful only if you are using shadow passwords.

The default value of -l states that accounts are neverinactive. You can change this value as follows:

#useradd -D -f number of days





EXPIRE=

The EXPIRE= line specifics when a account should hedisabled.

This is useful only if you are using shadow passwords.

By default, accounts never become disabled.

You can change this value as follows:

#useradd D -eMM /DD/YY





SHELL=/bin/bash

The SHELL=/bin/bash line specifies the default login shellpath. You can change this value as follows:

#useradd D -s/bin/tcsh





SKEL=/etc/skel

The files in this directory are copied to the new homedirectory of a new user account.

Normally, you do not want to change this path to someother directory.





Exercise:




Homework

• Homework this week:

• Next week:

Lecture 6 Users Administration

Documents

Transcript of Lecture 6 Users Administration