First understanding of the difference between centos7 and centos6 (kernel, commands, etc.)

Differences and features between CentOS7.2 and CentOS6

The boot of the Linux operating system first starts from the BIOS, and then enters the boot loader, which loads the kernel and initializes the kernel. The last step of kernel initialization is to start the init process with pid 1. This process is the first process of the system. It is responsible for spawning all other user processes. init exists as a daemon process and is the ancestor of all other processes. The init process is very unique and can complete tasks that other processes cannot.

Sysvinit is the system V style init system. Sysvinit uses the term runlevel to define "subscribed operating mode". Sysvinit checks whether there is an entry of'initdefault' in the'/etc/inittab' file. This tells the init system whether there is a default operating mode. Sysvinit uses scripts, file naming rules and soft links to implement different runlevels, and serially start various processes and services.

Systemd is the latest initialization system (init) in the Linux system. Its main design goal is to overcome the inherent shortcomings of sysvinit and improve the system startup speed. systemd and ubuntu's upstart are competitors and are expected to replace UpStart. Systemd's goal is to start as few processes as possible, and to start as many processes in parallel as possible.

The difference in file system, Centos6.x generally uses the ext3\ext4 file system format, while centos7 uses the xfs format by default:
EXT3 supports the largest 16TB file system and the largest 2TB file, Ext4 supports respectively
1 EB (1,048,576TB, 1EB=1024PB, 1PB=1024TB) file system, and 16TB files. Ext3 currently only supports 32,000 subdirectories, while Ext4 supports an unlimited number of subdirectories.

EXT4 is the fourth-generation extended file system (English: Fourth EXtended filesystem, abbreviated as ext4) is the log file system under the Linux system, and is the successor version of the ext3 file system.

The file system capacity of Ext4 reaches 1EB, and the capacity of a single file reaches 16TB, which is a very large number. For general desktops and servers, this may not be important, but for users of large disk arrays, it is very important; (disk structure due to Ext4's inode number limit (32 digits)) About 4 billion files, and the single file size of Ext4 can only support up to 16T (4K block size))

XFS is a 64-bit file system that supports a single file system of up to 8EB minus 1 byte. The actual deployment depends on the maximum block limit of the host operating system. For a 32-bit Linux system, the file and file system size will be limited to 16TB.

2.4 Linux network configuration management

Familiar with the commonly used commands and Linux permissions, then how do you get the Linux system online? What are the commands for managing linux server network?
The Linux server default network card configuration file is under /etc/sysconfig/network-scripts/. The name is generally: ifcfg-eth0 ifcfg-eth1, eth0 represents the first network card, eth1 represents the second network card, and so on. Generally DELL R720 comes standard with 4 gigabit network cards.

First know the difference between centos7 and centos6

Basic management tools for the installation system:
yum install tree net-tools bind-utils tree sysstat vim-en* lrzsz NetworkManager-tui ntp ntpdate iftop tcpdump telnet traceroute -y

Centos7 will find the network card name of ens330, how to change it to eth0

Reference: http://jingyan.baidu.com/album/7f41ecec1b022e593d095c1e.html?picindex=5

vim /etc/sysconfig/network-scripts/ifcfg-ens33 
cd /etc/sysconfig/network-scripts/
mv ifcfg-ens33 ifcfg-eth0

Disable this predictable naming rule and disable the firmware to discover the network card name. /etc/default/grub and add "net.ifnames=0 biosdevname=0"
vim /etc/default/grub

Run the command grub2-mkconfig -o /boot/grub2/grub.cfg to regenerate the GRUB configuration and update the kernel parameters.

grub2-mkconfig -o /boot/grub2/grub.cfg #grug2-mkconfig is a command only used by centos, not in 6.
reboot

The system is optimized

One: Close ipv6 provided by sshd

vim /etc/ssh/sshd_config
# AddressFamily any
AddressFamily inet

Note: (inet is ipv4 only; inet6 is ipv6 only)

Two: turn off fireproof and setlinux

systemctl stop firewalld 
setenforce 0
sed -i 's#SELINUX=enforcing#SELINUX=disabled#g'/etc/selinux/config 

Note: You can study the reason why centos6 is replaced by firewalld in centos7.

firewalld should be an advanced version of iptable (but the two provided write rule management commands are iptables)
FirewallD provides a dynamic firewall management tool that supports network/firewall zones to define network links and interface security levels. It supports IPv4, IPv6 firewall settings and Ethernet bridging, and has runtime configuration and permanent configuration options. It also supports interfaces that allow services or applications to directly add firewall rules. The previous system-config-firewall/lokkit firewall model was static, and each modification required a complete restart of the firewall. This process includes the unloading of the kernel netfilter firewall module and the loading of the modules required for the new configuration. The uninstallation of the module will destroy the state firewall and established connections.

In contrast, the firewall daemon dynamically manages the firewall, and changes can be applied without restarting the entire firewall. Therefore, there is no need to reload all kernel firewall modules. However, to use the firewall daemon, all changes to the firewall must be implemented through the daemon to ensure that the state in the daemon is consistent with the firewall in the kernel. In addition, the firewall daemon cannot parse the firewall rules added by the ip*tables and ebtables command line tools.

The daemon provides information about the currently activated firewall settings through D-BUS, and also accepts changes made using PolicyKit authentication through D-BUS.

By default, iptables is not installed. If you are used to using iptables, you can turn off firewalld and replace it with iptables.
Install iptable service
# yum -y install iptables-services

If you want to save firewall rules, use the following command
# iptables-save > iptables-rules.txt

These rules can be added to the /etc/sysconfig/iptables file, but I often modify these rules according to my preferences

systemctl enable iptables

systemctl enable ip6tables

Stop firewalld

systemctl stop firewalld

Disable firewalld

systemctl disable firewalld

Open iptable service

systemctl start iptables

systemctl start ip6tables

Four: About network services
Old network script (service) and ifcfg file

Starting with Centos7, the network is managed by the NetworkManager service. Compared to the old /etc/init.d/network script, NetworkManager is a dynamic and event-driven network management service. The old /etc/init.d/network and ifup, ifdown, etc. still exist, but are in a standby state, that is, when NetworkManager is running, in most cases these scripts will call NetworkManager to complete network configuration tasks; when NetworkManager is not running, these scripts Just manage the network in accordance with the old tradition.
have to be aware of is:
1 ) Systemctl disable NetworkManager.service is not recommended
2 ) Because the old network script is not compatible with the new configuration item name IPADDR0/PREFIX0/GATEWAY0 in the ifcfg-* file
3 ) Unless the 0 at the back is removed, the network card cannot be started after booting

Command difference between CentOS 6 and CentOS 7

(1) Desktop system
[ CentOS6] GNOME 2.x
[ CentOS7] GNOME 3.x（GNOME Shell）

(2) File system
[ CentOS6] ext4
[ CentOS7] xfs

(3) Kernel version
[ CentOS6] 2.6.x-x
[ CentOS7] 3.10.x-x

(4) Boot loader
[ CentOS6] GRUB Legacy (+efibootmgr)
[ CentOS7] GRUB2

(5) Firewall
[ CentOS6] iptables
[ CentOS7] firewalld

(6) Default database
[ CentOS6] MySQL
[ CentOS7] MariaDB

(7) File structure
[ CentOS6] /bin, /sbin, /lib, and /lib64 under /
[ CentOS7] /bin, /sbin, /lib, and /lib64 moved to /usr

(8) CPU name
[ CentOS6] /etc/sysconfig/network
[ CentOS7] /etc/hostname

(9) Time synchronization [CentOS6] ntp ntpq -p

[ CentOS7] chrony chronyc sources

(10) Change the time
[ CentOS6]
$ vim /etc/sysconfig/clock
ZONE=”Asia/Tokyo”
UTC=fales
$ sudo ln -s /usr/share/zoneinfo/Asia/Tokyo /etc/localtime

[ CentOS7] timedatectlset−timezoneAsia/Tokyo timedatectl status

vim /etc/sysconfig/i18n LANG=”ja_JP.utf8″ /etc/sysconfig/i18n locale

[ CentOS7] localectlset−localeLANG=jaJP.utf8 localectl status

(12) Service related

1 ) Start stop [CentOS6] serviceservicenamestart service service_name stop

[ CentOS7] systemctlstartservicename systemctl stop service_name

Self-starting
[ CentOS6]
$ chkconfig service_name on/off

[ CentOS7] systemctlenableservicename systemctl disable service_name

Service overview
[ CentOS6]
$ chkconfig –list

[ CentOS7] systemctllist−unit−files systemctl –type service

Forced stop
[ CentOS6]
$ kill -9

[ CentOS7]
$ systemctl kill –signal=9 sshd

(13) The internet

1 ) Network information [CentOS6] netstat netstat -I

[ CentOS7] ipn ip -s l

2 ) IP address MAC address
[ CentOS6]
$ ifconfig -a

[ CentOS7]
$ ip address show

3 ) Route [CentOS6] route−n route -A inet6 -n

[ CentOS7] iprouteshow ip -6 route show

(14) Restart shutdown

1 )shut down
[ CentOS6]
$ shutdown -h now

[ CentOS7] poweroff systemctl poweroff

2 ) Restart [CentOS6] reboot shutdown -r now

[ CentOS7] reboot systemctl reboot

3 ) Single user mode
[ CentOS6]
$ init S

[ CentOS7]
$ systemctl rescue

4 ) Startup mode
[ CentOS6]
[ GUICUI]
$ vim /etc/inittab
id:3:initdefault:
[ CUIGUI]
$ startx

systemctl isolate graphical.target default systemctl set−defaultgraphical.target systemctl set-default multi-user.target current