This options specifies the Unix group containing the Lix build user accounts. In multi-user Lix installations, builds should not be performed by the Lix account since that would allow users to arbitrarily modify the Nix store and database by supplying specially crafted builders; and they cannot be performed by the calling user since that would allow them to influence the build result.

Therefore, if this option is non-empty and specifies a valid group, builds will be performed under the user accounts that are a member of the group specified here (as listed in /etc/group). Those user accounts should not be used for any other purpose!

Lix will never run two builds under the same user account at the same time. This is to prevent an obvious security hole: a malicious user writing a Nix expression that modifies the build result of a legitimate Nix expression being built by another user. Therefore it is good to have as many Lix build user accounts as you can spare. (Remember: uids are cheap.)

The build users should have permission to create files in the Nix store, but not delete them. Therefore, /nix/store should be owned by the Nix account, its group should be the group specified here, and its mode should be 1775.

If the build users group is empty, builds will be performed under the uid of the Lix process (that is, the uid of the caller if both NIX_REMOTE is either empty or auto and the Nix store is owned by that user, or, alternatively, the uid under which the Nix daemon runs if NIX_REMOTE is daemon or if it is auto and the store is not owned by the caller). Obviously, this should not be used with a nix daemon accessible to untrusted clients.

For the avoidance of doubt, explicitly setting this to empty with a Lix daemon running as root means that builds will be executed as root with respect to the rest of the system. We intend to fix this: https://git.lix.systems/lix-project/lix/issues/242