帳號管理技巧

建立系統用帳號

CentOS/RedHat)

groupadd -r asterisk
useradd -r -g asterisk -d /var/lib/asterisk -M asterisk

Ubuntu/Debian)

addgroup --system asterisk
adduser --system --ingroup asterisk --home /var/lib/asterisk --no-create-home --shell /bin/bash asterisk

變更帳號為管理者權限

# Debian/Ubuntu
# Add the user into the group sudo
sudo usermod -aG sudo <user-name>
# Verify the user's groups
groups <user-name>

強制修改密碼

強迫使用者在第一次登入後，修改他們的登入密碼

# 先將帳號鎖定
usermod -L <username>

# 強制第一次登入必須修改密碼
# 套用後，原密碼會立即過期，直到完成密碼變更。
chage -d 0 <username>

# 解除帳號鎖定
usermod -U <username>

# 檢查帳號的期限
chage -l <user-name>

帳號使用期限

# 檢查帳號期限
chage -l <user-name>

# 設定有效期限 
chage -M 10 <user-name>             # 10 天後密碼即失效
chage -E "2017-02-20" <user-name>   # 2017-02-20 以後帳號即鎖定
chage -I 10 <user-name>             # 如有設定密碼期限時，當密碼失效起 10 日後自動鎖定帳號 

# 解除期限
chage -E -1 <user-name>       ; 數字 -1 解除期限設定

帳號鎖定與解鎖

# 鎖定帳號
usermod -L <user-name>
passwd -l <user-name>
chage -E 0 <user-name>

# 解鎖帳號
usermod -U <user-name>
passwd -u <user-name>
chage -E <user-name>

# 檢查帳號鎖定狀態
grep <user-name> /etc/shadow

dbtest:!$6$hFCW6eI1$kI9J9QrxCjnpvzFPJnxSpNvQ...  密碼欄有 ! 符號表示鎖定

TIPs:
注意：passwd 雖然可以鎖定帳號，但仍可以用 SSH-Key 登入。

修改既有帳號的設定

# 修改註解
usermod -c "John" john
# 修改 shell
usermod -s "/sbin/nologin" alang
# 修改帳號名稱
usermod -l newuser currentuser

限制某帳號不可遠端登入

但可以由其他允許帳號從遠端登入後，執行 su 切換到該帳號

情境：限制 devrpt 可以從遠端登入，但其他帳號在登入後可以 su 到 devrpt。

方法一: 修改 sshd_config

# Added by Alang
# prevent certain users from using ssh for login
# while retaining the option to 'su username'
#
DenyUsers istdc

方法二: 最快速且容易設定但不適用需要有密碼的帳號

# 刪除 devrpt 的密碼
passwd -d devrpt

方法三: 比較嚴謹的做法

以 CentOS 為例：

1. 編輯 /etc/security/access.conf，加上這幾行

# The line 'cron crond' is required
+:devrpt:cron crond tty1 tty2 tty3 tty4 tty5 tty6
-:devrpt:ALL

TIPs：
內容格式為 permission : username: origins

permission + 允許或 - 拒絕
username 帳號
origins 來源，這可以是 tty 名稱'、主機/網域名稱、IP 。

注意：在此例，必須加上 cron crond 這一行，否則該帳號的 crontab 會無法工作。

2. 對於不同的登入服務，需要修改相應的安全設定檔

telnet : /etc/pam.d/remote (修改後立即生效)
SSH : /etc/pam.d/sshd (修改後需重新載入 SSHD)
Local 本機登入 : /etc/pam.d/login

視需要將以下內容加入其中一項或多項檔案內

# Limited users for remote login via telnet
# Check the file /etc/security/access.conf
account    required     pam_access.so

重建帳號的家目錄

mkhomedir_helper <username>

限制登入後的行為

情境: 帳號執行遠端登入後，只能變更密碼與幾個受限制的指令權限

RedHat-KB: https://access.redhat.com/solutions/65822

# Create the restricted shell
cp /bin/bash /bin/rbash

# Create a directory that is used as the HOME of the user
mkdir /home/dbuser/
mkdir /home/dbuser/bin

# Modify the target user for the shell as restricted shell
usermod -d /home/dbuser -s /bin/rbash siview
# or for new user
useradd -d /home/dbuser -s /bin/rbash siview

If a user uses rbash, the user can not do the following after login:

Changing directories with the |cd| built in.
Setting or unsetting the values of the |SHELL|, |PATH|, |ENV|, or |BASH_ENV| variables.
Specifying command names containing slashes.
Specifying a filename containing a slash as an argument to the |.| built in command.
Importing function definitions from the shell environment at startup.
Parsing the value of |SHELLOPTS| from the shell environment at startup.
Redirecting output using the `|>|', `|>||', `|<>|', `|>&|', `|&>|', and `|>>|' redirection operators.
Using the |exec| built in to replace the shell with another command.
Adding or deleting built in commands with the `|-f|' and `|-d|' options to the |enable| built in.
Specifying the `|-p|' option to the |command| built in.
Turning off restricted mode with `|set +r|' or `|set +o restricted|'.

# Create specific profile for the user
vi /home/dbuser/.bash_profile

.bash_profile:

# cat /home/localuser/.bash_profile  
# .bash_profile  

# Get the aliases and functions  
if [ -f ~/.bashrc ]; then  
. ~/.bashrc  
fi  
# User specific environment and startup programs  
PATH=$HOME/bin  
export PATH

# Create the softlinks of commands which are required for the user
ln -s /bin/date /home/dbuser/bin/
ln -s /bin/ls /home/dbuser/bin/
ln -s /usr/bin/passwd /home/dbuser/bin/

密碼強度

RH-KB: https://access.redhat.com/solutions/66322 (RHEL6)
RH-KB: Set a password policy in Red Hat Enterprise Linux 7 (RHEL7)
How to Set password policy in CentOS or RHEL system
RedHat/CentOS: /usr/share/doc/pam-<version>/txts/README.pam_cracklib
[中文] https://www.lijyyh.com/2012/07/pam-managing-account-security-with-pam.html

預設強度:

difok=N , 預設字元數 5 位數
minlen=N, 最少字元位數，預設是 9。
dcredit=-1, 數字至少 1 位數
ucredit=-1, 大寫字母至少 1 位數
lcredit=-1, 小寫字母至少 1 位數

Edit /etc/pam.d/system-auth , /etc/pam.d/password-auth

CentOS 5/6)

NOTE: CentOS 5 沒有 /etc/pam.d/password-auth , 所以只需要設定 /etc/pam.d/system-auth

# Set password strength
#password    requisite     pam_cracklib.so try_first_pass retry=3 type=
password    requisite     pam_cracklib.so minlen=8 dcredit=-1 ucredit=-1 lcredit=-1

CentOS 7/8)

Edit /etc/security/pwquality.conf

# Set password strength
minlen = 8
dcredit = -1
ucredit = -1
lcredit = -1

預設 root 不會套用密碼強度規則，如果要做限制，編輯 /etc/pam.d/system-auth 與 /etc/pam.d/password-auth ，在 password 這一行加上 enforce_for_root。

# Enforce root for password strength
password    requisite     pam_pwquality.so try_first_pass local_users_only retry=3 authtok_type= enforce_for_root

記住幾代密碼
CentOS 6.x)

# Keep history of passwords used
# Add remember=N
# The last n passwords for each user are saved in /etc/security/opasswd in order to force password change history 
# and keep the user from alternating between the same password too frequently.
#password    sufficient    pam_unix.so sha512 shadow nullok try_first_pass use_authtok
password    sufficient    pam_unix.so sha512 remember=6 shadow nullok try_first_pass use_authtok

CentOS 7/8)

password    requisite     pam_pwquality.so try_first_pass local_users_only retry=3 authtok_type=
# Keep history of passwords used, insert the below line after pam_pwquality.so line
password    requisite     pam_pwhistory.so remember=6 use_authtok

TIP: 歷史密碼會被儲存在 /etc/security/opasswd .

登入失敗後鎖定帳號

~~[RH]~~ ~~What is pam_faillock and how to use it in Red Hat Enterprise Linux?~~ ~~[RH]~~ ~~Lock account after 3 failed attempts.~~ ~~Linux 封鎖、解鎖登入失敗次數過多的帳號 pam_faillock 教學與範例~~

~~CentOS 6.x~~
~~Edit~~ /etc/pam.d/system-auth , /etc/pam.d/password-auth

# for auth
# add the below line BEFORE pam_unix.so
auth required pam_faillock.so preauth silent audit deny=3 unlock_time=600  # insert this

auth        sufficient    pam_unix.so nullok try_first_pass

# add the below line AFTER pam_unix.so
auth [default=die] pam_faillock.so authfail audit deny=3 unlock_time=600  # insert this

# for account
# add the below line BEFORE pam_unix.so
account required pam_faillock.so  # insert this

account     required      pam_unix.so

~~預設不會套用在 root；如果需要限制 root，下面這一行加上~~ even_deny_root ：

auth    required    pam_faillock.so preauth silent audit deny=3 even_deny_root unlock_time=1200 root_unlock_time=600

~~如果要排除特定 user，在第一個~~ pam_faillock.so ~~之前加上這行:~~

auth [success=1 default=ignore] pam_succeed_if.so user in user1:user2:user3

~~如何手動解鎖與檢查被鎖定的帳戶~~

# display the authentication failure for all users 
faillock

# display the authentication failure for the specified user
faillock --user mytest

# unlock the user
faillock --user mytest --reset

~~Tip:~~
~~要確認設定是否有作用，可以監看 log 檔 /var/log/secure，登入錯誤次數達到設定值時，應該要出現下面的訊息。~~

~~Mar 8 15:26:08 centos7 sshd[26995]: pam_faillock(sshd:auth): Consecutive login failures for user i04181 account temporarily locked~~

群組管理

# Create a new group
groupadd <group-name>
addgroup <group-name>

# add a group into an account
usermod -aG mygroup user1
useradd -aG family,friends james

# To change the primary group of the user tom to family
usermod -g family tom

# remove user from a group
gpasswd -d user1 mygroup

# list all users in a group
lid -g mygroup

# list groups
groups

passwd

# displays the status of user account password settings
passwd -S evans
evans PS 2020-09-07 0 99999 7 -1 (Password set, SHA512 crypt.)

# lock the password of a specified account
passwd -l user1

# unlock the password
passwd -u user2

# delete a password for an account
passwd -d user1

# expire a password for an account
passwd -e user2

# This sets the number of days before a password can be changed. 
# By default, a value of zero is set, which indicates that the user may change 
# their password at any time.
# This means user2 cannot change its own password until 10 days have passed.
passwd -n 10 user2

# To confirm the password setting made with the -n option above, run the following command:
# The value of 10 after the date indicates the minimum number of days 
# until the password can be changed.
passwd -S user1
user1 PS 2020-12-04 10 99999 7 -1 (Password set, SHA512 crypt.)

# This means after 90 days, the password is required to be changed.
passwd -x 90 user2

# This means the user will receive warnings that the password will expire 7 days 
# before the expiration.
passwd -w 7 user2

# This means after a user account has had an expired password for 5 days, 
# the user may no longer sign on to the account.
passwd -i 5 user2

# This command will read from the echo command and pass it to the passwd command. 
# So this will set the user1 password to userpasswd1.
echo "userpasswd1"|passwd --stdin user1

批次建立多個帳號

# Step 1 – Create an encrypted password
## perl one liner ##
#perl -e 'print crypt("Your-Clear-Text-Password-Here", "salt"),"\n"'

password="1YelloDog@"
pass=$(perl -e 'print crypt($ARGV[0], "password")' $password)
echo "$pass"

# Step 2 – Shell script to add a user and password on Linux
#!/bin/bash
# Purpose - Script to add a user to Linux system including passsword
# Author - Vivek Gite <www.cyberciti.biz> under GPL v2.0+
# ------------------------------------------------------------------
# Am i Root user?
if [ $(id -u) -eq 0 ]; then
	read -p "Enter username : " username
	read -s -p "Enter password : " password
	egrep "^$username" /etc/passwd >/dev/null
	if [ $? -eq 0 ]; then
		echo "$username exists!"
		exit 1
	else
		pass=$(perl -e 'print crypt($ARGV[0], "password")' $password)
		useradd -m -p "$pass" "$username"
		[ $? -eq 0 ] && echo "User has been added to system!" || echo "Failed to add a user!"
	fi
else
	echo "Only root may add a user to the system."
	exit 2
fi

# Step 3 – Change existing Linux user’s password in one CLI
echo "vivek:password" | chpasswd

# Verify that password has been changed
chage -l vivek

# Step 4 – Create Users and change passwords with passwd on a CentOS/RHEL
echo "YourPassword" | passwd --stdin UserName