从阿里镜像站下了centos7：
https://mirrors.aliyun.com/centos/7/isos/x86_64/；
想安装mosquitto，发现有问题，先修改镜像站，备份一下：
sudo cp /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
使用文本编辑器打开 CentOS-Base.repo 文件：
sudo vi /etc/yum.repos.d/CentOS-Base.repo
[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
baseurl=https://mirrors.aliyun.com/centos/$releasever/os/$basearch/
gpgcheck=0
#enabled=1
#gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-7
[extras]
name=CentOS-$releasever - Extras
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras&infra=$infra
baseurl=https://mirrors.aliyun.com/centos/$releasever/extras/$basearch/
gpgcheck=0
[updates]
name=CentOS-$releasever - Updates
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates&infra=$infra
baseurl=https://mirrors.aliyun.com/centos/$releasever/updates/$basearch/
gpgcheck=0
完成更改后，保存并关闭文件。然后，清理 yum 的缓存并更新包列表：
sudo yum clean all
sudo yum makecache
安装 Mosquitto：
sudo yum install -y epel-release
sudo yum install -y mosquitto mosquitto-clients
启动 Mosquitto 服务：
sudo systemctl start mosquitto
系统启动时自动启动
sudo systemctl enable mosquitto
测试：
mosquitto_pub -h localhost -t "test/topic" -m "Hello, MQTT"
mosquitto_sub -h localhost -t "test/topic"
配置防火墙以允许外部连接：
sudo firewall-cmd --permanent --add-port=1883/tcp
sudo firewall-cmd --permanent --add-port=9001/tcp
sudo firewall-cmd --reload
