安装mavlink报错：aurllib2.URLError: ＜urlopen error ＜urlopen error [Errno 111] Connection refused＞

在执行源码安装MAVROS的安装mavlink步骤时，报错：

$ rosinstall_generator --rosdistro melodic mavlink | tee /tmp/mavros.rosinstall
Traceback (most recent call last):
  File "/usr/bin/rosinstall_generator", line 11, in <module>
    load_entry_point('rosinstall-generator==0.1.22', 'console_scripts', 'rosinstall_generator')()
  File "/usr/lib/python2.7/dist-packages/rosinstall_generator/cli.py", line 163, in main
    upstream_version_tag=args.upstream, upstream_source_version=args.upstream_development)
  File "/usr/lib/python2.7/dist-packages/rosinstall_generator/generator.py", line 267, in generate_rosinstall
    wet_package_names, unreleased_repo_names = _get_packages_for_repos(distro_name, repo_names, source=upstream_source_version)
  File "/usr/lib/python2.7/dist-packages/rosinstall_generator/generator.py", line 93, in _get_packages_for_repos
    wet_distro = get_wet_distro(distro_name)
  File "/usr/lib/python2.7/dist-packages/rosinstall_generator/generator.py", line 224, in get_wet_distro
    _wet_distro = _get_wet_distro(distro_name)
  File "/usr/lib/python2.7/dist-packages/rosinstall_generator/distro.py", line 45, in get_distro
    index = get_index(get_index_url())
  File "/usr/lib/python2.7/dist-packages/rosdistro/__init__.py", line 104, in get_index
    yaml_str = load_url(url)
  File "/usr/lib/python2.7/dist-packages/rosdistro/loader.py", line 59, in load_url
    raise URLError(str(e) + ' (%s)' % url)
urllib2.URLError: <urlopen error <urlopen error [Errno 111] Connection refused> (https://raw.githubusercontent.com/ros/rosdistro/master/index-v4.yaml)>

原因是：查看/usr/lib/python2.7/dist-packages/rosdistro/init.py文件，里面需要链接到https://raw.githubusercontent.com这个网址，但是网不行啊。所以直接百度搜了怎么访问这个网站。
解决办法是：解决 GitHub 的 raw.githubusercontent.com 无法访问的问题
即：
终端执行：

sudo gedit /etc/hosts  
# 这里没有用终端修改的方式，即sudo vi，而是用gedit打开了文档修改

然后在文档中的任一空白行添加

199.232.68.133  raw.githubusercontent.com

保存后，
在之前创建的mavros工作空间再次执行安装mavlink的指令就ok了，成功的结果：

$ rosinstall_generator --rosdistro melodic mavlink | tee /tmp/mavros.rosinstall
- git:
    local-name: mavlink
    uri: https://github.com/mavlink/mavlink-gbp-release.git
    version: release/melodic/mavlink/2022.3.3-1