Ten Tips for improving Linux System Security <2>

Article Title: improve the security of Linux systems. Linux is a technology channel of the IT lab in China. Includes basic categories such as desktop applications, Linux system management, kernel research, embedded systems, and open source.
　　 2nd tips: Restrict System Access
Before entering the Linux system, all users need to log on, that is, users need to enter the user account and password. Only after they pass system verification can users enter the system.
　　
Like other Unix operating systems, Linux typically stores passwords in the/etc/passwd file after encryption. All users in Linux can read the/etc/passwd file. Although the password stored in the file has been encrypted, it is still not safe. Generally, users can use the ready-made password cracking tool to guess the password. The safer method is to set the shadow file/etc/shadow and only allow users with special permissions to read the file.
　　
In Linux, to use a shadow file, you must recompile all the utilities to support the shadow file. This method is troublesome. A simple method is to use the plug-in verification module (PAM ). Many Linux systems use Linux tool PAM, which is an identity authentication mechanism that can be used to dynamically change the authentication methods and requirements without re-compiling other utilities. This is because PAM uses a closed package to hide all authentication-related logic in the module, so it is the best helper for using shadow files.
　　
In addition, PAM has many security features: it can rewrite the traditional DES encryption method to other more powerful encryption methods to ensure that user passwords are not easily decrypted; it can set the upper limit for each user to use computer resources; it can even set the user's computer time and location.
　　
Linux administrators can install and set PAM in just a few hours to greatly improve the security of the Linux system and block many attacks outside the system.
　　
　　 3rd tips: Keep the latest system core
Because there are many channels for Linux distribution, and there are often updated programs and system patches, We must update the system kernel frequently to enhance system security.
　　
Kernel is the core of the Linux operating system. It is used to load other parts of the operating system and implement the basic functions of the operating system. As Kernel controls various functions of the computer and network, its security is crucial to the security of the entire system.
　　
Many well-known security vulnerabilities exist in earlier versions of Kernel, which are also unstable. Only versions 2.0.x and later are stable and secure, and the running efficiency of the new versions is greatly improved. When setting the Kernel function, you only need to select the necessary functions. Do not accept all functions as per your order. Otherwise, the Kernel will become large, occupying both system resources and leaving hackers with an opportunity.
　　
There are often the latest security patches on the Internet. Linux administrators should be well-informed and often patronize Security newsgroups to check new patches.
　　
　　 4th tips: Check the logon Password
Setting a logon password is a very important security measure. If a user's password is not properly set, it will be easily deciphered, especially for users with super user permissions, if you do not have a good password, it will cause a large security vulnerability to the system.
　　
In a multi-user system, if you force each user to select a password that is difficult to guess, the security of the system will be greatly improved. However, if the passwd program cannot force each user on the machine to use the appropriate password, to ensure the security of the password, you can only rely on the password to crack the program.
　　
In fact, the password cracking program is a tool in the hacker toolbox. it encrypts common passwords or all words in the English dictionary that may be used as passwords into a password code word, then compare it with the/etc/passwd password file or/etc/shadow file in Linux. If there is a matched password, you can obtain the clear code.
　　
Many password cracking programs can be found on the network. The most famous program is crack. You can first execute a password cracking program to find the password that is easy to be cracked by hackers. It is better to correct the password first than to be cracked by hackers.