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doc/manual/src/installation/multi-user.md

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+# 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.
+
+> **Note**
+> 
+> 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.
+
+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.
+
+The [Nix daemon](#sec-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](#envar-remote) to `daemon`. So you
+should put a line like
+
+    export NIX_REMOTE=daemon
+
+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
+`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.