<refentry xmlns="http://docbook.org/ns/docbook"
xmlns:xlink="http://www.w3.org/1999/xlink"
xmlns:xi="http://www.w3.org/2001/XInclude"
xml:id="sec-conf-file">
<refmeta>
<refentrytitle>nix.conf</refentrytitle>
<manvolnum>5</manvolnum>
<refmiscinfo class="source">Nix</refmiscinfo>
<refmiscinfo class="version"><xi:include href="../version.txt" parse="text"/></refmiscinfo>
</refmeta>
<refnamediv>
<refname>nix.conf</refname>
<refpurpose>Nix configuration file</refpurpose>
</refnamediv>
<refsection><title>Description</title>
<para>Nix reads settings from two configuration files:</para>
<itemizedlist>
<listitem>
<para>The system-wide configuration file
<filename><replaceable>sysconfdir</replaceable>/nix/nix.conf</filename>
(i.e. <filename>/etc/nix/nix.conf</filename> on most systems), or
<filename>$NIX_CONF_DIR/nix.conf</filename> if
<envar>NIX_CONF_DIR</envar> is set.</para>
</listitem>
<listitem>
<para>The user configuration file
<filename>$XDG_CONFIG_HOME/nix/nix.conf</filename>, or
<filename>~/.config/nix/nix.conf</filename> if
<envar>XDG_CONFIG_HOME</envar> is not set.</para>
</listitem>
</itemizedlist>
<para>The configuration files consist of
<literal><replaceable>name</replaceable> =
<replaceable>value</replaceable></literal> pairs, one per line.
Comments start with a <literal>#</literal> character. Here is an
example configuration file:</para>
<programlisting>
gc-keep-outputs = true # Nice for developers
gc-keep-derivations = true # Idem
env-keep-derivations = false
</programlisting>
<para>You can override settings on the command line using the
<option>--option</option> flag, e.g. <literal>--option gc-keep-outputs
false</literal>.</para>
<para>The following settings are currently available:
<variablelist>
<varlistentry xml:id="conf-gc-keep-outputs"><term><literal>gc-keep-outputs</literal></term>
<listitem><para>If <literal>true</literal>, the garbage collector
will keep the outputs of non-garbage derivations. If
<literal>false</literal> (default), outputs will be deleted unless
they are GC roots themselves (or reachable from other roots).</para>
<para>In general, outputs must be registered as roots separately.
However, even if the output of a derivation is registered as a
root, the collector will still delete store paths that are used
only at build time (e.g., the C compiler, or source tarballs
downloaded from the network). To prevent it from doing so, set
this option to <literal>true</literal>.</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-gc-keep-derivations"><term><literal>gc-keep-derivations</literal></term>
<listitem><para>If <literal>true</literal> (default), the garbage
collector will keep the derivations from which non-garbage store
paths were built. If <literal>false</literal>, they will be
deleted unless explicitly registered as a root (or reachable from
other roots).</para>
<para>Keeping derivation around is useful for querying and
traceability (e.g., it allows you to ask with what dependencies or
options a store path was built), so by default this option is on.
Turn it off to save a bit of disk space (or a lot if
<literal>gc-keep-outputs</literal> is also turned on).</para></listitem>
</varlistentry>
<varlistentry><term><literal>env-keep-derivations</literal></term>
<listitem><para>If <literal>false</literal> (default), derivations
are not stored in Nix user environments. That is, the derivation
any build-time-only dependencies may be garbage-collected.</para>
<para>If <literal>true</literal>, when you add a Nix derivation to
a user environment, the path of the derivation is stored in the
user environment. Thus, the derivation will not be
garbage-collected until the user environment generation is deleted
(<command>nix-env --delete-generations</command>). To prevent
build-time-only dependencies from being collected, you should also
turn on <literal>gc-keep-outputs</literal>.</para>
<para>The difference between this option and
<literal>gc-keep-derivations</literal> is that this one is
“sticky”: it applies to any user environment created while this
option was enabled, while <literal>gc-keep-derivations</literal>
only applies at the moment the garbage collector is
run.</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-build-max-jobs"><term><literal>build-max-jobs</literal></term>
<listitem><para>This option defines the maximum number of jobs
that Nix will try to build in parallel. The default is
<literal>1</literal>. The special value <literal>auto</literal>
causes Nix to use the number of CPUs in your system. It can be
overridden using the <option
linkend='opt-max-jobs'>--max-jobs</option> (<option>-j</option>)
command line switch.</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-build-cores"><term><literal>build-cores</literal></term>
<listitem><para>Sets the value of the
<envar>NIX_BUILD_CORES</envar> environment variable in the
invocation of builders. Builders can use this variable at their
discretion to control the maximum amount of parallelism. For
instance, in Nixpkgs, if the derivation attribute
<varname>enableParallelBuilding</varname> is set to
<literal>true</literal>, the builder passes the
<option>-j<replaceable>N</replaceable></option> flag to GNU Make.
It can be overridden using the <option
linkend='opt-cores'>--cores</option> command line switch and
defaults to <literal>1</literal>. The value <literal>0</literal>
means that the builder should use all available CPU cores in the
system.</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-build-max-silent-time"><term><literal>build-max-silent-time</literal></term>
<listitem>
<para>This option defines the maximum number of seconds that a
builder can go without producing any data on standard output or
standard error. This is useful (for instance in an automated
build system) to catch builds that are stuck in an infinite
loop, or to catch remote builds that are hanging due to network
problems. It can be overridden using the <option
linkend="opt-max-silent-time">--max-silent-time</option> command
line switch.</para>
<para>The value <literal>0</literal> means that there is no
timeout. This is also the default.</para>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-build-timeout"><term><literal>build-timeout</literal></term>
<listitem>
<para>This option defines the maximum number of seconds that a
builder can run. This is useful (for instance in an automated
build system) to catch builds that are stuck in an infinite loop
but keep writing to their standard output or standard error. It
can be overridden using the <option
linkend="opt-timeout">--timeout</option> command line
switch.</para>
<para>The value <literal>0</literal> means that there is no
timeout. This is also the default.</para>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-build-max-log-size"><term><literal>build-max-log-size</literal></term>
<listitem>
<para>This option defines the maximum number of bytes that a
builder can write to its stdout/stderr. If the builder exceeds
this limit, it’s killed. A value of <literal>0</literal> (the
default) means that there is no limit.</para>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-build-users-group"><term><literal>build-users-group</literal></term>
<listitem><para>This options specifies the Unix group containing
the Nix build user accounts. In multi-user Nix installations,
builds should not be performed by the Nix account since that would
allow users to arbitrarily modify the Nix store and database by
supplying specially crafted builders; and they cannot be performed
by the calling user since that would allow him/her to influence
the build result.</para>
<para>Therefore, if this option is non-empty and specifies a valid
group, builds will be performed under the user accounts that are a
member of the group specified here (as listed in
<filename>/etc/group</filename>). Those user accounts should not
be used for any other purpose!</para>
<para>Nix will never run two builds under the same user account at
the same time. This is to prevent an obvious security hole: a
malicious user writing a Nix expression that modifies the build
result of a legitimate Nix expression being built by another user.
Therefore it is good to have as many Nix build user accounts as
you can spare. (Remember: uids are cheap.)</para>
<para>The build users should have permission to create files in
the Nix store, but not delete them. Therefore,
<filename>/nix/store</filename> should be owned by the Nix
account, its group should be the group specified here, and its
mode should be <literal>1775</literal>.</para>
<para>If the build users group is empty, builds will be performed
under the uid of the Nix process (that is, the uid of the caller
if <envar>NIX_REMOTE</envar> is empty, the uid under which the Nix
daemon runs if <envar>NIX_REMOTE</envar> is
<literal>daemon</literal>). Obviously, this should not be used in
multi-user settings with untrusted users.</para>
</listitem>
</varlistentry>
<varlistentry><term><literal>build-use-sandbox</literal></term>
<listitem><para>If set to <literal>true</literal>, builds will be
performed in a <emphasis>sandboxed environment</emphasis>, i.e.,
they’re isolated from the normal file system hierarchy and will
only see their dependencies in the Nix store, the temporary build
directory, private versions of <filename>/proc</filename>,
<filename>/dev</filename>, <filename>/dev/shm</filename> and
<filename>/dev/pts</filename> (on Linux), and the paths configured with the
<link linkend='conf-build-sandbox-paths'><literal>build-sandbox-paths</literal>
option</link>. This is useful to prevent undeclared dependencies
on files in directories such as <filename>/usr/bin</filename>. In
addition, on Linux, builds run in private PID, mount, network, IPC
and UTS namespaces to isolate them from other processes in the
system (except that fixed-output derivations do not run in private
network namespace to ensure they can access the network).</para>
<para>Currently, sandboxing only work on Linux and macOS. The use
of a sandbox requires that Nix is run as root (so you should use
the <link linkend='conf-build-users-group'>“build users”
feature</link> to perform the actual builds under different users
than root).</para>
<para>If this option is set to <literal>relaxed</literal>, then
fixed-output derivations and derivations that have the
<varname>__noChroot</varname> attribute set to
<literal>true</literal> do not run in sandboxes.</para>
<para>The default is <literal>false</literal>.</para>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-build-sandbox-paths">
<term><literal>build-sandbox-paths</literal></term>
<listitem><para>A list of paths bind-mounted into Nix sandbox
environments. You can use the syntax
<literal><replaceable>target</replaceable>=<replaceable>source</replaceable></literal>
to mount a path in a different location in the sandbox; for
instance, <literal>/bin=/nix-bin</literal> will mount the path
<literal>/nix-bin</literal> as <literal>/bin</literal> inside the
sandbox. If <replaceable>source</replaceable> is followed by
<literal>?</literal>, then it is not an error if
<replaceable>source</replaceable> does not exist; for example,
<literal>/dev/nvidiactl?</literal> specifies that
<filename>/dev/nvidiactl</filename> will only be mounted in the
sandbox if it exists in the host filesystem.</para>
<para>Depending on how Nix was built, the default value for this option
may be empty or provide <filename>/bin/sh</filename> as a
bind-mount of <command>bash</command>.</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-build-extra-sandbox-paths">
<term><literal>build-extra-sandbox-paths</literal></term>
<listitem><para>A list of additional paths appended to
<option>build-sandbox-paths</option>. Useful if you want to extend
its default value.</para></listitem>
</varlistentry>
<varlistentry><term><literal>build-use-substitutes</literal></term>
<listitem><para>If set to <literal>true</literal> (default), Nix
will use binary substitutes if available. This option can be
disabled to force building from source.</para></listitem>
</varlistentry>
<varlistentry><term><literal>build-fallback</literal></term>
<listitem><para>If set to <literal>true</literal>, Nix will fall
back to building from source if a binary substitute fails. This
is equivalent to the <option>--fallback</option> flag. The
default is <literal>false</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><literal>build-keep-log</literal></term>
<listitem><para>If set to <literal>true</literal> (the default),
Nix will write the build log of a derivation (i.e. the standard
output and error of its builder) to the directory
<filename>/nix/var/log/nix/drvs</filename>. The build log can be
retrieved using the command <command>nix-store -l
<replaceable>path</replaceable></command>.</para></listitem>
</varlistentry>
<varlistentry><term><literal>build-compress-log</literal></term>
<listitem><para>If set to <literal>true</literal> (the default),
build logs written to <filename>/nix/var/log/nix/drvs</filename>
will be compressed on the fly using bzip2. Otherwise, they will
not be compressed.</para></listitem>
</varlistentry>
<varlistentry><term><literal>substituters</literal></term>
<listitem><para>A list of URLs of substituters, separated by
whitespace. The default is
<literal>https://cache.nixos.org</literal>.</para></listitem>
</varlistentry>
<!--
<varlistentry><term><literal>binary-caches-files</literal></term>
<listitem><para>A list of names of files that will be read to
obtain additional binary cache URLs. The default is
<literal>/nix/var/nix/profiles/per-user/<replaceable>username</replaceable>/channels/binary-caches/*</literal>.
Note that when you’re using the Nix daemon,
<replaceable>username</replaceable> is always equal to
<literal>root</literal>, so Nix will only use the binary caches
provided by the channels installed by root. Do not set this
option to read files created by untrusted users!</para></listitem>
</varlistentry>
-->
<varlistentry><term><literal>trusted-substituters</literal></term>
<listitem><para>A list of URLs of substituters, separated by
whitespace. These are not used by default, but can be enabled by
users of the Nix daemon by specifying <literal>--option
substituters <replaceable>urls</replaceable></literal> on the
command line. Unprivileged users are only allowed to pass a
subset of the URLs listed in <literal>substituters</literal> and
<literal>trusted-substituters</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><literal>extra-substituters</literal></term>
<listitem><para>Additional binary caches appended to those
specified in <option>substituters</option>. When used by
unprivileged users, untrusted substituters (i.e. those not listed
in <option>trusted-substituters</option>) are silently
ignored.</para></listitem>
</varlistentry>
<varlistentry><term><literal>signed-binary-caches</literal></term>
<listitem><para>If set to <literal>*</literal> (the default), Nix
will only download binaries if they are signed using one of the
keys listed in <option>binary-cache-public-keys</option>. Set to
the empty string to disable signature checking.</para></listitem>
</varlistentry>
<varlistentry><term><literal>binary-cache-public-keys</literal></term>
<listitem><para>A whitespace-separated list of public keys
corresponding to the secret keys trusted to sign binary
caches. For example:
<literal>cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
hydra.nixos.org-1:CNHJZBh9K4tP3EKF6FkkgeVYsS3ohTl+oS0Qa8bezVs=</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><literal>http-connections</literal></term>
<listitem><para>The maximum number of parallel TCP connections
used to fetch files from binary caches and by other downloads. It
defaults to 25. 0 means no limit.</para></listitem>
</varlistentry>
<varlistentry><term><literal>netrc-file</literal></term>
<listitem><para>If set to an absolute path to a <filename>netrc</filename>
file, Nix will use the HTTP authentication credentials in this file when
trying to download from a remote host through HTTP or HTTPS. Defaults to
<filename>$NIX_CONF_DIR/netrc</filename>.</para>
<para>The <filename>netrc</filename> file consists of a list of
accounts in the following format:
<screen>
machine <replaceable>my-machine</replaceable>
login <replaceable>my-username</replaceable>
password <replaceable>my-password</replaceable>
</screen>
For the exact syntax, see <link
xlink:href="https://ec.haxx.se/usingcurl-netrc.html">the
<literal>curl</literal> documentation.</link></para></listitem>
</varlistentry>
<varlistentry><term><literal>system</literal></term>
<listitem><para>This option specifies the canonical Nix system
name of the current installation, such as
<literal>i686-linux</literal> or
<literal>x86_64-darwin</literal>. Nix can only build derivations
whose <literal>system</literal> attribute equals the value
specified here. In general, it never makes sense to modify this
value from its default, since you can use it to ‘lie’ about the
platform you are building on (e.g., perform a Mac OS build on a
Linux machine; the result would obviously be wrong). It only
makes sense if the Nix binaries can run on multiple platforms,
e.g., ‘universal binaries’ that run on <literal>x86_64-linux</literal> and
<literal>i686-linux</literal>.</para>
<para>It defaults to the canonical Nix system name detected by
<filename>configure</filename> at build time.</para></listitem>
</varlistentry>
<varlistentry><term><literal>fsync-metadata</literal></term>
<listitem><para>If set to <literal>true</literal>, changes to the
Nix store metadata (in <filename>/nix/var/nix/db</filename>) are
synchronously flushed to disk. This improves robustness in case
of system crashes, but reduces performance. The default is
<literal>true</literal>.</para></listitem>
</varlistentry>
<varlistentry><term><literal>auto-optimise-store</literal></term>
<listitem><para>If set to <literal>true</literal>, Nix
automatically detects files in the store that have identical
contents, and replaces them with hard links to a single copy.
This saves disk space. If set to <literal>false</literal> (the
default), you can still run <command>nix-store
--optimise</command> to get rid of duplicate
files.</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-connect-timeout"><term><literal>connect-timeout</literal></term>
<listitem>
<para>The timeout (in seconds) for establishing connections in
the binary cache substituter. It corresponds to
<command>curl</command>’s <option>--connect-timeout</option>
option.</para>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-trusted-users"><term><literal>trusted-users</literal></term>
<listitem>
<para>A list of names of users (separated by whitespace) that
have additional rights when connecting to the Nix daemon, such
as the ability to specify additional binary caches, or to import
unsigned NARs. You can also specify groups by prefixing them
with <literal>@</literal>; for instance,
<literal>@wheel</literal> means all users in the
<literal>wheel</literal> group. The default is
<literal>root</literal>.</para>
<warning><para>The users listed here have the ability to
compromise the security of a multi-user Nix store. For instance,
they could install Trojan horses subsequently executed by other
users. So you should consider carefully whether to add users to
this list.</para></warning>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-allowed-users"><term><literal>allowed-users</literal></term>
<listitem>
<para>A list of names of users (separated by whitespace) that
are allowed to connect to the Nix daemon. As with the
<option>trusted-users</option> option, you can specify groups by
prefixing them with <literal>@</literal>. Also, you can allow
all users by specifying <literal>*</literal>. The default is
<literal>*</literal>.</para>
<para>Note that trusted users are always allowed to connect.</para>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-restrict-eval"><term><literal>restrict-eval</literal></term>
<listitem>
<para>If set to <literal>true</literal>, the Nix evaluator will
not allow access to any files outside of the Nix search path (as
set via the <envar>NIX_PATH</envar> environment variable or the
<option>-I</option> option). The default is
<literal>false</literal>.</para>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-pre-build-hook"><term><literal>pre-build-hook</literal></term>
<listitem>
<para>If set, the path to a program that can set extra
derivation-specific settings for this system. This is used for settings
that can't be captured by the derivation model itself and are too variable
between different versions of the same system to be hard-coded into nix.
</para>
<para>The hook is passed the derivation path and, if sandboxes are enabled,
the sandbox directory. It can then modify the sandbox and send a series of
commands to modify various settings to stdout. The currently recognized
commands are:</para>
<variablelist>
<varlistentry xml:id="extra-sandbox-paths">
<term><literal>extra-sandbox-paths</literal></term>
<listitem>
<para>Pass a list of files and directories to be included in the
sandbox for this build. One entry per line, terminated by an empty
line. Entries have the same format as
<literal>build-sandbox-paths</literal>.</para>
</listitem>
</varlistentry>
</variablelist>
</listitem>
</varlistentry>
<varlistentry xml:id="conf-build-repeat"><term><literal>build-repeat</literal></term>
<listitem><para>How many times to repeat builds to check whether
they are deterministic. The default value is 0. If the value is
non-zero, every build is repeated the specified number of
times. If the contents of any of the runs differs from the
previous ones, the build is rejected and the resulting store paths
are not registered as “valid” in Nix’s database.</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-sandbox-dev-shm-size"><term><literal>sandbox-dev-shm-size</literal></term>
<listitem><para>This option determines the maximum size of the
<literal>tmpfs</literal> filesystem mounted on
<filename>/dev/shm</filename> in Linux sandboxes. For the format,
see the description of the <option>size</option> option of
<literal>tmpfs</literal> in
<citerefentry><refentrytitle>mount</refentrytitle><manvolnum>8</manvolnum></citerefentry>. The
default is <literal>50%</literal>.</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-allow-import-from-derivation"><term><literal>allow-import-from-derivation</literal></term>
<listitem><para>By default, Nix allows you to <function>import</function> from a derivation,
allowing building at evaluation time. With this option set to false, Nix will throw an error
when evaluating an expression that uses this feature, allowing users to ensure their evaluation
will not require any builds to take place.</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-allow-new-privileges"><term><literal>allow-new-privileges</literal></term>
<listitem><para>(Linux-specific.) By default, builders on Linux
cannot acquire new privileges by calling setuid/setgid programs or
programs that have file capabilities. For example, programs such
as <command>sudo</command> or <command>ping</command> will
fail. (Note that in sandbox builds, no such programs are available
unless you bind-mount them into the sandbox via the
<option>build-sandbox-paths</option> option.) You can allow the
use of such programs by enabling this option. This is impure and
usually undesirable, but may be useful in certain scenarios
(e.g. to spin up containers or set up userspace network interfaces
in tests).</para></listitem>
</varlistentry>
<varlistentry xml:id="conf-hashed-mirrors"><term><literal>hashed-mirrors</literal></term>
<listitem><para>A list of web servers used by
<function>builtins.fetchurl</function> to obtain files by
hash. The default is
<literal>http://tarballs.nixos.org/</literal>. Given a hash type
<replaceable>ht</replaceable> and a base-16 hash
<replaceable>h</replaceable>, Nix will try to download the file
from
<literal>hashed-mirror/<replaceable>ht</replaceable>/<replaceable>h</replaceable></literal>.
This allows files to be downloaded even if they have disappeared
from their original URI. For example, given the default mirror
<literal>http://tarballs.nixos.org/</literal>, when building the derivation
<programlisting>
builtins.fetchurl {
url = https://example.org/foo-1.2.3.tar.xz;
sha256 = "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae";
}
</programlisting>
Nix will attempt to download this file from
<literal>http://tarballs.nixos.org/sha256/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae</literal>
first. If it is not available there, if will try the original URI.</para></listitem>
</varlistentry>
</variablelist>
</para>
</refsection>
</refentry>
