Change the rubygems source list
    $ gem source --add https://mirrors.ustc.edu.cn/rubygems
    $ gem source
    *** CURRENT SOURCES ***

     http://rubygems.org
     https://mirrors.ustc.edu.cn/rubygems/
    $ gem source -r http://rubygems.org/
    $ gem source
      *** CURRENT SOURCES ***
     https://mirrors.ustc.edu.cn/rubygems
    $ gem source -a http://rubygems.org/
    $ gem source
      *** CURRENT SOURCES ***
      https://mirrors.ustc.edu.cn/rubygems
      http://rubygems.org/
Change the apt source list
    $ apt edit-sources