136 lines
4.2 KiB
XML
136 lines
4.2 KiB
XML
<section xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="ssec-multi-user">
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<title>Multi-User Mode</title>
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<para>To allow a Nix store to be shared safely among multiple users,
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it is important that users are not able to run builders that modify
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the Nix store or database in arbitrary ways, or that interfere with
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builds started by other users. If they could do so, they could
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install a Trojan horse in some package and compromise the accounts of
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other users.</para>
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<para>To prevent this, the Nix store and database are owned by some
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privileged user (usually <literal>root</literal>) and builders are
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executed under special user accounts (usually named
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<literal>nixbld1</literal>, <literal>nixbld2</literal>, etc.). When a
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unprivileged user runs a Nix command, actions that operate on the Nix
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store (such as builds) are forwarded to a <emphasis>Nix
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daemon</emphasis> running under the owner of the Nix store/database
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that performs the operation.</para>
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<note><para>Multi-user mode has one important limitation: only
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<systemitem class="username">root</systemitem> and a set of trusted
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users specified in <filename>nix.conf</filename> can specify arbitrary
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binary caches. So while unprivileged users may install packages from
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arbitrary Nix expressions, they may not get pre-built
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binaries.</para></note>
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<simplesect>
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<title>Setting up the build users</title>
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<para>The <emphasis>build users</emphasis> are the special UIDs under
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which builds are performed. They should all be members of the
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<emphasis>build users group</emphasis> <literal>nixbld</literal>.
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This group should have no other members. The build users should not
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be members of any other group. On Linux, you can create the group and
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users as follows:
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<screen>
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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$n; done
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</screen>
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This creates 10 build users. There can never be more concurrent builds
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than the number of build users, so you may want to increase this if
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you expect to do many builds at the same time.</para>
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<para>On macOS, you can create the required group and users by
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running the following script:
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<programlisting>
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#! /bin/bash -e
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dseditgroup -o create nixbld -q
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gid=$(dscl . -read /Groups/nixbld | awk '($1 == "PrimaryGroupID:") {print $2 }')
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echo "created nixbld group with gid $gid"
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for i in $(seq 1 10); do
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user=/Users/nixbld$i
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uid="$((30000 + $i))"
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dscl . create $user
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dscl . create $user RealName "Nix build user $i"
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dscl . create $user PrimaryGroupID "$gid"
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dscl . create $user UserShell /usr/bin/false
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dscl . create $user NFSHomeDirectory /var/empty
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dscl . create $user UniqueID "$uid"
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dseditgroup -o edit -a nixbld$i -t user nixbld
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echo "created nixbld$i user with uid $uid"
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done
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</programlisting>
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</para>
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</simplesect>
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<simplesect>
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<title>Running the daemon</title>
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<para>The <link linkend="sec-nix-daemon">Nix daemon</link> should be
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started as follows (as <literal>root</literal>):
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<screen>
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$ nix-daemon</screen>
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You’ll want to put that line somewhere in your system’s boot
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scripts.</para>
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<para>To let unprivileged users use the daemon, they should set the
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<link linkend="envar-remote"><envar>NIX_REMOTE</envar> environment
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variable</link> to <literal>daemon</literal>. So you should put a
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line like
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<programlisting>
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export NIX_REMOTE=daemon</programlisting>
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into the users’ login scripts.</para>
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</simplesect>
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<simplesect>
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<title>Restricting access</title>
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<para>To limit which users can perform Nix operations, you can use the
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permissions on the directory
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<filename>/nix/var/nix/daemon-socket</filename>. For instance, if you
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want to restrict the use of Nix to the members of a group called
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<literal>nix-users</literal>, do
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<screen>
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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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</screen>
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This way, users who are not in the <literal>nix-users</literal> group
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cannot connect to the Unix domain socket
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<filename>/nix/var/nix/daemon-socket/socket</filename>, so they cannot
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perform Nix operations.</para>
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</simplesect>
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</section>
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