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.

这是由于raw.githubusercontent.com被墙，程序无法下载相应的文件导致的。解决方法就是使得raw.githubusercontent.com能够被正常访问。这个网站被墙的方式是dns污染，所以我们只要能够找到这个网站的正确ip就可以了。

下面是一组hosts记录

192.30.253.118 gist.github.com
185.199.110.153 github.io

151.101.113.194 github.global.ssl.fastly.net
52.216.227.168 github-cloud.s3.amazonaws.com
52.74.223.119 github.com

199.232.28.133 avatars1.githubusercontent.com
199.232.28.133 avatars2.githubusercontent.com
199.232.28.133 avatars0.githubusercontent.com
199.232.28.133 avatars3.githubusercontent.com
199.232.28.133 raw.githubusercontent.com
199.232.28.133 user-images.githubusercontent.com
199.232.28.133 avatars.githubusercontent.com
199.232.28.133 github.map.fastly.net
199.232.28.133 avatars7.githubusercontent.com

# Amazon AWS Start
54.239.31.69	aws.amazon.com
54.239.30.25	console.aws.amazon.com
54.239.96.90	ap-northeast-1.console.aws.amazon.com
54.240.226.81	ap-southeast-1.console.aws.amazon.com
54.240.193.125	ap-southeast-2.console.aws.amazon.com
54.239.54.102	eu-central-1.console.aws.amazon.com
177.72.244.194	sa-east-1.console.aws.amazon.com
176.32.114.59	eu-west-1.console.aws.amazon.com
54.239.31.128	us-west-1.console.aws.amazon.com
54.240.254.230	us-west-2.console.aws.amazon.com
54.239.38.102	s3-console-us-standard.console.aws.amazon.com
54.231.49.3	s3.amazonaws.com
52.219.0.4	s3-ap-northeast-1.amazonaws.com
54.231.242.170	s3-ap-southeast-1.amazonaws.com
54.231.251.21	s3-ap-southeast-2.amazonaws.com
54.231.193.37	s3-eu-central-1.amazonaws.com
52.218.16.140	s3-eu-west-1.amazonaws.com
52.92.72.2	s3-sa-east-1.amazonaws.com
54.231.236.6	s3-us-west-1.amazonaws.com
54.231.168.160	s3-us-west-2.amazonaws.com
52.216.80.48	github-cloud.s3.amazonaws.com
54.231.40.3	github-com.s3.amazonaws.com
52.216.20.171	github-production-release-asset-2e65be.s3.amazonaws.com
52.216.228.168	github-production-user-asset-6210df.s3.amazonaws.com

添加到自己电脑的hosts文件里面，文件路径为/etc/hosts

此时再次执行 rosdep init
还是可能会失败，多试几次总会下载成功的。