问题现象：kubeadm 安装k8s集群的时候，下载flannel文件yml文件报错

wget https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kubeflannel.yml

--2021-05-30 16:08:51--  https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kubeflannel.yml
Resolving raw.githubusercontent.com (raw.githubusercontent.com)... 0.0.0.0, ::
Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|0.0.0.0|:443... failed: Connection refused.
Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|::|:443... failed: Connection refused.

原因分析：
raw.githubusercontent.com 的443端口访问失败，应该是ip地址和域名解析无法解析
解决方法：在/etc/hosts 里面加入ip地址和域名关系

# GitHub Start
52.74.223.119 github.com
192.30.253.119 gist.github.com
54.169.195.247 api.github.com
185.199.111.153 assets-cdn.github.com
151.101.64.133 raw.githubusercontent.com
151.101.108.133 user-images.githubusercontent.com
151.101.76.133 gist.githubusercontent.com
151.101.76.133 cloud.githubusercontent.com
151.101.76.133 camo.githubusercontent.com
151.101.76.133 avatars0.githubusercontent.com
151.101.76.133 avatars1.githubusercontent.com
151.101.76.133 avatars2.githubusercontent.com
151.101.76.133 avatars3.githubusercontent.com
151.101.76.133 avatars4.githubusercontent.com
151.101.76.133 avatars5.githubusercontent.com
151.101.76.133 avatars6.githubusercontent.com
151.101.76.133 avatars7.githubusercontent.com
151.101.76.133 avatars8.githubusercontent.com
# GitHub End

再次执行报错：无法建立安全套接字
Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|151.101.64.133|:443… connected.
Unable to establish SSL connection.
不进行验证
wget --no-check-certificate https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml

`[root@master1 .kube]# wget --help|grep no-check
–no-check-certificate don’t validate the server’s certificate.