I started having this problem when running the docker provisioner (Vagrant 1.7.2): Err http://get.docker.io docker/main amd64 Packages Undetermined Error Err http://get.docker.io docker/main i386 Packages Undetermined Error Ign http://get.docker.io docker/main Translation-en_US Ign http://get.docker.io docker/main Translation-en Stderr from the command: stdin: is not a tty W: Failed to fetch http://get.docker.io/ubuntu/dists/docker/main/binary-amd64/Packages Undetermined Error W: Failed to fetch http://get.docker.io/ubuntu/dists/docker/main/binary-i386/Packages Undetermined Error Changing this to https seemed to fix the problem for now.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.