2020-07-23 00:17:48 +03:00
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# Multi-User Mode
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To allow a Nix store to be shared safely among multiple users, it is
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important that users are not able to run builders that modify the Nix
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store or database in arbitrary ways, or that interfere with builds
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started by other users. If they could do so, they could install a Trojan
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horse in some package and compromise the accounts of other users.
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To prevent this, the Nix store and database are owned by some privileged
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user (usually `root`) and builders are executed under special user
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accounts (usually named `nixbld1`, `nixbld2`, etc.). When a unprivileged
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user runs a Nix command, actions that operate on the Nix store (such as
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builds) are forwarded to a *Nix daemon* running under the owner of the
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Nix store/database that performs the operation.
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> **Note**
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>
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> Multi-user mode has one important limitation: only root and a set of
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> trusted users specified in `nix.conf` can specify arbitrary binary
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> caches. So while unprivileged users may install packages from
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> arbitrary Nix expressions, they may not get pre-built binaries.
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2020-07-23 15:20:54 +03:00
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## Setting up the build users
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2020-07-23 00:17:48 +03:00
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The *build users* are the special UIDs under which builds are performed.
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They should all be members of the *build users group* `nixbld`. This
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group should have no other members. The build users should not be
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members of any other group. On Linux, you can create the group and users
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as follows:
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$ groupadd -r nixbld
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$ for n in $(seq 1 10); do useradd -c "Nix build user $n" \
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-d /var/empty -g nixbld -G nixbld -M -N -r -s "$(which nologin)" \
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nixbld$n; done
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This creates 10 build users. There can never be more concurrent builds
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than the number of build users, so you may want to increase this if you
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expect to do many builds at the same time.
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2020-07-23 15:20:54 +03:00
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## Running the daemon
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2020-07-24 16:46:16 +03:00
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The [Nix daemon](../command-ref/nix-daemon.md) should be started as
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follows (as `root`):
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2020-07-23 00:17:48 +03:00
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$ nix-daemon
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You’ll want to put that line somewhere in your system’s boot scripts.
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To let unprivileged users use the daemon, they should set the
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2020-07-24 16:46:16 +03:00
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[`NIX_REMOTE` environment variable](../command-ref/env-common.md) to
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`daemon`. So you should put a line like
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2020-07-23 00:17:48 +03:00
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export NIX_REMOTE=daemon
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into the users’ login scripts.
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2020-07-23 15:20:54 +03:00
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## Restricting access
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2020-07-23 00:17:48 +03:00
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To limit which users can perform Nix operations, you can use the
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permissions on the directory `/nix/var/nix/daemon-socket`. For instance,
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if you want to restrict the use of Nix to the members of a group called
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`nix-users`, do
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$ chgrp nix-users /nix/var/nix/daemon-socket
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$ chmod ug=rwx,o= /nix/var/nix/daemon-socket
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This way, users who are not in the `nix-users` group cannot connect to
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the Unix domain socket `/nix/var/nix/daemon-socket/socket`, so they
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cannot perform Nix operations.
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