安装ROS时sudo rosdep init指令报错：
ERROR: cannot download default sources list from:
https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/sources.list.d/20-default.list
Website may be down.
根本原因是无法解析raw.githubusercontent.com
GitHub的raw.githubusercontent.com无法连接问题
https://site.ip138.com/raw.Githubusercontent.com/
输入raw.githubusercontent.com
查询IP地址
修改hosts Ubuntu，CentOS及macOS直接在终端输入
sudo gedit /etc/hosts
添加以下内容保存即可 （IP地址查询后相应修改，可以ping不同IP的延时 选择最佳IP地址）
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
151.101.76.133 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