如题，在用gem install rails时一直报这个错误，完整的错误信息如下：

ERROR:  Could not find a valid gem ‘rails‘ (>= 0), here is why:
          Unable to download data from https://rubygems.org/ - Errno::ETIMEDOUT: Connection timed out - connect(2) for "s3.amazonaws.com" port 443 (https://api.rubygems.org/latest_specs.4.8.gz)

后台在Google上找到解决方案：

sudo gem sources -r https://rubygems.org
sudo gem sources -a http://rubygems.org  

貌似意思是将https替换成http，再试了一下，果然可以了，但是安装完后别忘了把它设回来：

sudo gem sources -r http://rubygems.org
sudo gem sources -a https://rubygems.org

解决方案参考来源：

http://stackoverflow.com/a/19179835/1227911

安装ruby on rails一直报Unable to download data from https://rubygems.org/的解决方案