Linux system security and Application


As an open source operating system, Linux server is widely used because of its significant advantages of security, efficiency and stability. The following mainly optimizes the security of Linux system from the perspective of account security, system boot and login control
User account is the identity certificate of computer users. Everyone accessing system resources must have an account to log in to the computer. In Linux, a variety of mechanisms are provided to ensure the safe use of user account

Basic safety measures

System account cleaning and locking

In the Linux system, in addition to the various accounts manually created by the user, it also includes a large number of other accounts generated during the installation of the system or program, which are used to maintain the system operation, start or maintain the service process. Generally, login is not allowed, which is called non login user. In order to maintain system security, the login shell of these users is usually / sbin/nologin, which means that terminal login is prohibited and should not be changed manually.
For some accounts that have not been used for a long time, if it is uncertain whether to delete them, they should be locked temporarily (the accounts can be locked and unlocked with usermod and passwd commands)

[root@localhost ~]# usermod -L xx                       #Lock account
[root@localhost ~]# passwd -S xx                       #View account status
ysf LK 2017-12-22 0 99999 7 -1 (The password is locked.)
[root@localhost ~]# usermod -U xx                       #Unlock account
[root@localhost ~]# passwd -S xx
ysf PS 2017-12-22 0 99999 7 -1 (Password set, use SHA512 Encryption.)
[root@localhost ~]# passwd -l xx
 Lock user xx Your password.
passwd: Operation successful
[root@localhost ~]# passwd -u xx
 Unlock user xx Your password.
passwd: Operation successful

If the account in the server has been fixed and will not be changed, you can lock the account configuration file. Use the chattr command to lock and unlock files, and use lsattr to view file locking

[root@localhost ~]# chattr +i /etc/passwd /etc/shadow         #+i. Lock file
[root@localhost ~]# lsattr /etc/passwd /etc/shadow           #View file locking
----i--------e- /etc/passwd
----i--------e- /etc/shadow
[root@localhost ~]# useradd xx            #The file is locked, and users cannot be added or deleted, and the user's password, login shell, host directory and other attributes cannot be changed
useradd: cannot open /etc/passwd
[root@localhost ~]# chattr -i /etc/passwd /etc/shadow        #-i. Unlock file
[root@localhost ~]# useradd xx                  #Create user normally
[root@localhost ~]#

Password security control

In order to reduce the risk of password guessing or brute force cracking, long-term use of the same password should be avoided. The administrator can limit the maximum number of days of validity of the user's password on the server. For users whose password has expired, they are required to reset their password when logging in, otherwise they will refuse to log in

[root@localhost ~]# vim /etc/login.defs                   #Adapt to new users
PASS_MAX_DAYS   30              #The password can be used for up to 30 days and must be changed
PASS_MIN_DAYS   0              #The password must be used for at least 0 days before it can be changed
PASS_MIN_LEN    5               #Acceptable password length       
PASS_WARN_AGE   7               #Warning time before password expiration

[root@localhost ~]# chage -M 30 xx           #For users who exist in, the password expires in 30 days
[root@localhost ~]# chage -d 0 xx          #You must change your password the next time you log in

In bash terminal environment, an idle timeout can be set. When there is no input beyond the specified time, the terminal will be automatically logged off. Controlled by the variable TMOUT, the default unit is

[root@localhost ~]# vim /etc/profile         #For new login users
export TMOUT=600

[root@localhost ~]# TMOUT=600               #Applicable to current user
[root@localhost ~]# unset TMOUT             #If the operation takes a long time to avoid interruption, you can cancel the TMOUT variable

User switch to permission

su switching users

Using the su command, you can switch a specified user and have all the permissions of the user

[root@localhost ~]# su - xx       #root switches to an ordinary user without password authentication
[xx@localhost ~]$ su -      # Ordinary users need to be verified when switching to root, but root does not
[root@localhost ~]#

sudo elevation permission

Although you can switch to the root user by using the su command, you must know the root password. If everyone knows the privileged password, there will be more risks. The sudo command allows ordinary users to have some commands that root users can execute without knowing the privileged password

[xx@localhost ~]$ ifconfig eth0:0          #sudo command not used
SIOCSIFADDR: insufficient privilege
SIOCSIFFLAGS: insufficient privilege
[xx@localhost ~]$ sudo ifconfig eth0:0       #Using the sudo command
......  //Omit some contents
[sudo] password for yangshufan:                             #Verify password
[xx@localhost ~]$ ifconfig eth0:0                   #View the command and execute it successfully
eth0:0    Link encap:Ethernet  HWaddr 00:0C:29:1C:B4:FB  
          inet addr:  Bcast:  Mask:

[xx@localhost ~]$ sudo -l                #See which sudo authorizations you have
[sudo] password for xx:  
user xx You can run the following commands on this host:
    (root) /sbin/ifconfig       
[root@localhost ~]# tail /var/log/sudo           #You can see the user's sudo operation record
Dec 27 07:49:35 : xx : TTY=pts/0 ; PWD=/home/xx ; USER=root ;
    COMMAND=/sbin/ifconfig eth0:0

PAM certification

By default, all users are allowed to use the su command, so they have the opportunity to repeatedly try the login password of other users (root), which brings security risks. You can use pam_wheel authentication module allows only a few users to switch using su command

[root@localhost ~]# vim /etc/pam.d/su
auth      required use_uid      #Remove the comment from this line
[root@localhost ~]# su - xx
[xx@localhost ~]$ su - root           #If you try to switch again, you will be prompted with the wrong password
su: Incorrect password
[xx@localhost ~]$ exit
[root@localhost ~]# gpasswd -a xx wheel / / add an authorized user to the wheel group
Adding user ysf to group wheel
[root@localhost ~]# grep wheel /etc/group / / view wheel group members
[root@localhost ~]# su - xx                      
[ysf@localhost ~]$ su -                           //Try switching again and the switching is successful
[root@localhost ~]#

Keywords: Linux

Added by Texan on Fri, 11 Feb 2022 14:23:43 +0200