Within Gentoo Linux, users already have one "main" package repository, in the past synonymously referred to as the "Portage tree", although this term should be phased out since package managers other than Portage can interact with package repositorys in Gentoo. The main repository contains all the software packages (called ebuilds) maintained by Gentoo developers. Users can add additional repositories to the tree that are "layed over" the main tree - hence the name, overlays.
Since package repositories are nothing more (or less) than a set of files (ebuilds, metadata files, ChangeLog entries ...) these repositories can be pulled in from public repositories (git, cvs, svn ...) or downloaded as tarballs and extracted manually onto the system. It is advised to use managed repositories by trusted third parties; any installed overlay will cause Portage to look through the overlayed files when deciding which software to install. If compromised code is in the overlay, then compromised packages could be installed on the system.
Additional package trees are usually hosted by repositories. These are overlays that are ready to be incorporated on Gentoo systems. The now default approach for handling repositories is through /etc/portage/repos.conf which, like many other Portage related locations, can be a directory as well.
A deprecated, yet still supported method is to use the
PORTDIR_OVERLAY variable inside /etc/portage/make.conf. This variable can point to one or more additional locations on the file system where repositories are available. The use of the /etc/portage/repos.conf/ directory is highly preferred.
Repository definitions inside /etc/portage/repos.conf/ also inform Portage if and how the repository can be updated. With it, calling emerge --sync will automatically update the repositories as well.
Each overlay has its unique priority. This makes sure that in the case of a specific version being found in several overlays, the resolution is unambiguous. Ebuilds from overlays with higher priorities take precedence over ebuilds from overlays with lower priorities.
The list of overlays with their priorities can be obtained through the output of the following command (look for the "Repositories" string):
emerge --info --verbose
The default Gentoo Portage tree will have a priority of -1000. That means that all other overlays generally take precedence as they are assigned a higher priority. That is the default behavior, because overlays are designed to "lay over/on top" of the portage tree.
A number of tools support or integrate with overlays.
The layman application makes it easier to manage and update multiple additional overlays. It is a command-line application through which publicly available overlays can be listed, subscribed to and unsubscribed from, as well as update those repositories.
- When using the make.conf method, layman manages a dedicated configuration file which should be sourced in by make.conf
- When using repos.conf, layman manages the /etc/portage/repos.conf/layman.conf file directly
See Project:Portage/Sync#Operation and man 1 emaint.
eix-sync is a wrapper starting emerge --sync (which in turn starts emaint sync --auto) followed by eix-update. For further details see the Eix article and man 1 eix.
When large overlays are installed, portage may take a long time to perform operations like dependency resolution. This is because overlays do not usually contain a metadata cache.
Generate a local metadata cache by running emerge --regen after syncing the overlays:
Be careful, because emerge --regen takes a lot of time and it's not recommended for rsync users as rsync updates the cache using server-side caches (most of users of portage are rsync users). Rsync users should simply run emerge --sync (or eix-sync) to regenerate the cache. It's probably only users of very large overlays should try emerge --regen.
Masking when using unsafe overlays
When using huge overlays or those with unknown/low quality it is best practice to hardmask the whole overlay.
After that unmask the packages that will be installed.