Twin directories of virtualbox-additions:

[DIR]  ftp://64.50.238.52/.1/gentoo-portage/disk/app-emulation/virtualbox-additions/
[DIR]  ftp://64.50.236.52/.1/gentoo-portage/disk/app-emulation/virtualbox-additions/
[DIR]  ftp://ftp.nsysu.edu.tw/Linux/Gentoo/gentoo-portage/app-emulation/virtualbox-additions/
[DIR]  ftp://ftp.twaren.net/pub/Linux/Gentoo/gentoo-portage/app-emulation/virtualbox-additions/