The DISTDIR variable defines the location where Portage will store the downloaded source code archives. Its value defaults to /var/cache/distfiles on new installations. Previously, the default was /usr/portage/distfiles.
This location, which is also often referred to as the distfiles location, will host the source code archives of all software installed (or attempted to install) on the system. This location is not automatically cleaned up, so users should consider using tools such as the eclean-dist command (which comes as part of the app-portage/gentoolkit package) to keep the storage used by this location under control. Read the Eclean article for more details.
Users can set the DISTDIR variable in /etc/portage/make.conf: