# Multi-User Mode

To allow a Nix store to be shared safely among multiple users, it is important that users are not able to run builders that modify the Nix store or database in arbitrary ways, or that interfere with builds started by other users. If they could do so, they could install a Trojan horse in some package and compromise the accounts of other users.

To prevent this, the Nix store and database are owned by some privileged user (usually `root`) and builders are executed under special user accounts (usually named `nixbld1`, `nixbld2`, etc.). When a unprivileged user runs a Nix command, actions that operate on the Nix store (such as builds) are forwarded to a Nix daemon running under the owner of the Nix store/database that performs the operation.

Multi-user mode has one important limitation: only `root` and a set of trusted users specified in `nix.conf` can specify arbitrary binary caches. So while unprivileged users may install packages from arbitrary Nix expressions, they may not get pre-built binaries.

## Setting up the build users

The build users are the special UIDs under which builds are performed. They should all be members of the build users group `nixbld`. This group should have no other members. The build users should not be members of any other group. On Linux, you can create the group and users as follows:

    $ groupadd -r nixbld
    $ for n in $(seq 1 10); do useradd -c "Nix build user $n" \
        -d /var/empty -g nixbld -G nixbld -M -N -r -s "$(which nologin)" \
        nixbld$n; done

This creates 10 build users. There can never be more concurrent builds than the number of build users, so you may want to increase this if you expect to do many builds at the same time.

## Running the daemon

The Nix daemon should be started as follows (as `root`):

    $ nix-daemon

You’ll want to put that line somewhere in your system’s boot scripts.

To let unprivileged users use the daemon, they should set the `NIX_REMOTE` environment variable to `daemon`. So you should put a line like

    export NIX_REMOTE=daemon

into the users’ login scripts.

## Restricting access

To limit which users can perform Nix operations, you can use the permissions on the directory `/nix/var/nix/daemon-socket`. For instance, if you want to restrict the use of Nix to the members of a group called `nix-users`, do

    $ chgrp nix-users /nix/var/nix/daemon-socket
    $ chmod ug=rwx,o= /nix/var/nix/daemon-socket

This way, users who are not in the `nix-users` group cannot connect to the Unix domain socket `/nix/var/nix/daemon-socket/socket`, so they cannot perform Nix operations.