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
root) and builders are executed under special user
accounts (usually named
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.confcan 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
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
$ 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
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
into the users’ login scripts.
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
$ 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.