The function of keepalived is to detect the status of the server. If a web server crashes or the work fails, Keepalived will detect and remove the faulty server from the system, and use other servers to replace the work of the server. When the server is working After the normal operation, Keepalived automatically adds the server to the server group. All these tasks are completed automatically without manual intervention. All that needs to be done manually is to repair the faulty server.
二. 安装Keepalived（http://www.keepalived.org/download.html）
1. Upload or download keepalived to the /usr/local/src directory
2. Unzip the installation
cd /usr/local/src
tar -zxvf keepalived-1.2.18.tar.gz
cd keepalived-1.2.18
./configure --prefix=/usr/local/keepalived
make && make install
3. Install keepalived as a Linux system service (because the default path of keepalived is not installed (default is /usr/local), after the installation is complete, some work needs to be done)
#复制默认配置文件到默认路径
mkdir /etc/keepalived
cp /usr/local/keepalived/etc/keepalived/keepalived.conf /etc/keepalived/
#复制 keepalived 服务脚本到默认的地址
cp /usr/local/keepalived/etc/rc.d/init.d/keepalived /etc/init.d/
cp /usr/local/keepalived/etc/sysconfig/keepalived /etc/sysconfig/
ln -s /usr/local/sbin/keepalived /usr/sbin/
ln -s /usr/local/keepalived/sbin/keepalived /sbin/
4. Set the keepalived service to start at boot
chkconfig keepalived on
5. Modify the Keepalived configuration file
MASTER 节点配置文件（192.168.1.11）
vi /etc/keepalived/keepalived.conf
global_defs {
##keepalived自带的邮件提醒需要开启sendmail服务。建议用独立的监控或第三方SMTP
##标识本节点的字条串，通常为 hostname
router_id 192.168.1.11
}
##keepalived会定时执行脚本并对脚本执行的结果进行分析，动态调整vrrp_instance
的优先级。如果脚本执行结果为0，并且weight配置的值大于0，则优先级相应的增加。
如果脚本执行结果非0，并且weight配置的值小于 0，则优先级相应的减少。其他情况，
维持原本配置的优先级，即配置文件中priority对应的值。
vrrp_script chk_nginx {
script "/etc/keepalived/nginx_check.sh" ## 检测 nginx 状态的脚本路径
interval 2 ## 检测时间间隔
weight -20 ## 如果条件成立，权重-20
}
## 定义虚拟路由，VI_1为虚拟路由的标示符，自己定义名称
vrrp_instance VI_1 {
state MASTER ## 主节点为MASTER，对应的备份节点为BACKUP
interface eth1 ## 绑定虚拟IP的网络接口，与本机IP地址所在的网络接口相同
virtual_router_id 11 ## 虚拟路由的ID号，两个节点设置必须一样,建议用IP最后段
mcast_src_ip 192.168.1.11 ## 本机 IP 地址
priority 100 ## 节点优先级，值范围0-254，MASTER要比BACKUP高
nopreempt ## 优先级高的设置 nopreempt 解决异常恢复后再次抢占的问题
advert_int 1 ## 组播信息发送间隔，两个节点设置必须一样，默认 1s
## 设置验证信息，两个节点必须一致
authentication {
auth_type PASS
auth_pass 1111
}
## 将 track_script 块加入 instance 配置块
track_script {
chk_nginx ## 执行 Nginx 监控的服务
}
## 虚拟 IP 池, 两个节点设置必须一样
virtual_ipaddress {
192.168.1.10 ## 虚拟 ip，可以定义多个
}
}
BACKUP 节点配置文件（192.168.1.12）
vi /etc/keepalived/keepalived.conf
global_defs {
router_id 192.168.1.12
}
vrrp_script chk_nginx {
script "/etc/keepalived/nginx_check.sh"
interval 2
weight -20
}
vrrp_instance VI_1 {
state BACKUP
interface eth1
virtual_router_id 11
mcast_src_ip 192.168.1.12
priority 90
advert_int 1
authentication {
auth_type PASS
auth_pass 1111
}
track_script {
chk_nginx
}
virtual_ipaddress {
192.168.1.10
}
}
6. Write the Nginx status detection script /etc/keepalived/nginx_check.sh
脚本：如果nginx停止运行，尝试启动，如果无法启动则杀死本机的keepalived进程，
keepalied将虚拟ip绑定到 BACKUP 机器上。内容如下：
vi /etc/keepalived/nginx_check.sh
!/bin/bash
A=`ps -C nginx –no-header |wc -l`
if [ $A -eq 0 ];then
/usr/local/nginx/sbin/nginx
sleep 2
if [ `ps -C nginx --no-header |wc -l` -eq 0 ];then
killall keepalived
fi
fi
7. Give the script execution permission
chmod +x /etc/keepalived/nginx_check.sh
8. Start Keepalived
service keepalived start
Starting keepalived: [ OK ]
9. High availability test of Keepalived+Nginx
(1)关闭 192.168.1.11 中的 Nginx，Keepalived会将它重新启动
/usr/local/nginx/sbin/nginx -s stop
(2)关闭 192.168.1.11 中的 Keepalived，VIP 会切换到 192.168.1.12 中
service keepalived stop
(3)重新启动 192.168.1.11 中的 Keepalived，VIP 又会切回到 192.168.1.11 中来
service keepalived start
Attached Keepalived service management command:
Stop: service keepalived stop
Start: service keepalived start
Restart: service keepalived restart
View status: service keepalived status