If you are using Vagrant behind a proxy server, you should go for installation of vagrant-proxyconf. But how to install this plug-in when you are currently limited by a proxy?

Set your proxy on Windows, open a command prompt and enter:

set HTTP_PROXY=http://user:password@proxy.domain.tld:8080
set HTTPS_PROXY=https://user:password@proxy.domain.tld:8081

Set your proxy on Linux, open a command prompt and enter:

export http_proxy="http://user:password@proxy.domain.tld:8080"
export https_proxy="http://user:password@proxy.domain.tld:8081"

After that you can install vagrant-proxyconf in the same command prompt that you already opened:

vagrant plugin install vagrant-proxyconf

In your Vagrantfile you can now use the following settings for future builds:

config.proxy.http     = "http://user:password@proxy.domain.tld:8080"
config.proxy.https    = "http://user:password@proxy.domain.tld:8081"
config.proxy.no_proxy = "localhost,127.0.0.1"