conf-file.xml

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          xmlns:xlink="http://www.w3.org/1999/xlink"
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          xml:id="sec-conf-file"
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<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. Other
files can be included with a line like <literal>include
<replaceable>path</replaceable></literal>, where
<replaceable>path</replaceable> is interpreted relative to the current
conf file and a missing file is an error unless
<literal>!include</literal> is used instead.
Comments start with a <literal>#</literal> character.  Here is an
example configuration file:</para>

<programlisting>
keep-outputs = true       # Nice for developers
keep-derivations = true   # Idem
</programlisting>

<para>You can override settings on the command line using the
<option>--option</option> flag, e.g. <literal>--option keep-outputs
false</literal>.</para>

<para>The following settings are currently available:

<variablelist>


  <varlistentry xml:id="conf-allowed-uris"><term><literal>allowed-uris</literal></term>

    <listitem>

      <para>A list of URI prefixes to which access is allowed in
      restricted evaluation mode. For example, when set to
      <literal>https://github.com/NixOS</literal>, builtin functions
      such as <function>fetchGit</function> are allowed to access
      <literal>https://github.com/NixOS/patchelf.git</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>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-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-auto-optimise-store"><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-builders">
    <term><literal>builders</literal></term>
    <listitem>
      <para>A list of machines on which to perform builds. <phrase
      condition="manual">See <xref linkend="chap-distributed-builds"
      /> for details.</phrase></para>
    </listitem>
  </varlistentry>


  <varlistentry xml:id="conf-builders-use-substitutes"><term><literal>builders-use-substitutes</literal></term>

    <listitem><para>If set to <literal>true</literal>, Nix will instruct
    remote build machines to use their own binary substitutes if available. In
    practical terms, this means that remote hosts will fetch as many build
    dependencies as possible from their own substitutes (e.g, from
    <literal>cache.nixos.org</literal>), instead of waiting for this host to
    upload them all. This can drastically reduce build times if the network
    connection between this computer and the remote build host is slow. Defaults
    to <literal>false</literal>.</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 xml:id="conf-compress-build-log"><term><literal>compress-build-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 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-cores"><term><literal>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>

    <para>See also <xref linkend="chap-tuning-cores-and-jobs" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-diff-hook"><term><literal>diff-hook</literal></term>
  <listitem>
    <para>
      Absolute path to an executable capable of diffing build results.
      The hook executes if <xref linkend="conf-run-diff-hook" /> is
      true, and the output of a build is known to not be the same.
      This program is not executed to determine if two results are the
      same.
    </para>

    <para>
      The diff hook is executed by the same user and group who ran the
      build. However, the diff hook does not have write access to the
      store path just built.
    </para>

    <para>The diff hook program receives three parameters:</para>

    <orderedlist>
      <listitem>
        <para>
          A path to the previous build's results
        </para>
      </listitem>

      <listitem>
        <para>
          A path to the current build's results
        </para>
      </listitem>

      <listitem>
        <para>
          The path to the build's derivation
        </para>
      </listitem>

      <listitem>
        <para>
          The path to the build's scratch directory. This directory
          will exist only if the build was run with
          <option>--keep-failed</option>.
        </para>
      </listitem>
    </orderedlist>

    <para>
      The stderr and stdout output from the diff hook will not be
      displayed to the user. Instead, it will print to the nix-daemon's
      log.
    </para>

    <para>When using the Nix daemon, <literal>diff-hook</literal> must
    be set in the <filename>nix.conf</filename> configuration file, and
    cannot be passed at the command line.
    </para>
  </listitem>
  </varlistentry>

  <varlistentry xml:id="conf-enforce-determinism">
    <term><literal>enforce-determinism</literal></term>

    <listitem><para>See <xref linkend="conf-repeat" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-extra-sandbox-paths">
    <term><literal>extra-sandbox-paths</literal></term>

    <listitem><para>A list of additional paths appended to
    <option>sandbox-paths</option>. Useful if you want to extend
    its default value.</para></listitem>

  </varlistentry>


  <varlistentry xml:id="conf-extra-platforms"><term><literal>extra-platforms</literal></term>

    <listitem><para>Platforms other than the native one which
    this machine is capable of building for. This can be useful for
    supporting additional architectures on compatible machines:
    i686-linux can be built on x86_64-linux machines (and the default
    for this setting reflects this); armv7 is backwards-compatible with
    armv6 and armv5tel; some aarch64 machines can also natively run
    32-bit ARM code; and qemu-user may be used to support non-native
    platforms (though this may be slow and buggy). Most values for this
    are not enabled by default because build systems will often
    misdetect the target platform and generate incompatible code, so you
    may wish to cross-check the results of using this option against
    proper natively-built versions of your
    derivations.</para></listitem>

  </varlistentry>


  <varlistentry xml:id="conf-extra-substituters"><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 xml:id="conf-fallback"><term><literal>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 xml:id="conf-fsync-metadata"><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 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>


  <varlistentry xml:id="conf-http-connections"><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 xml:id="conf-keep-build-log"><term><literal>keep-build-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 xml:id="conf-keep-derivations"><term><literal>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>keep-outputs</literal> is also turned on).</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-keep-env-derivations"><term><literal>keep-env-derivations</literal></term>

    <listitem><para>If <literal>false</literal> (default), derivations
    are not stored in Nix user environments.  That is, the derivations of
    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>keep-outputs</literal>.</para>

    <para>The difference between this option and
    <literal>keep-derivations</literal> is that this one is
    “sticky”: it applies to any user environment created while this
    option was enabled, while <literal>keep-derivations</literal>
    only applies at the moment the garbage collector is
    run.</para></listitem>

  </varlistentry>

  <varlistentry xml:id="conf-keep-outputs"><term><literal>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-max-build-log-size"><term><literal>max-build-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-max-free"><term><literal>max-free</literal></term>

    <listitem><para>When a garbage collection is triggered by the
    <literal>min-free</literal> option, it stops as soon as
    <literal>max-free</literal> bytes are available. The default is
    infinity (i.e. delete all garbage).</para></listitem>

  </varlistentry>

  <varlistentry xml:id="conf-max-jobs"><term><literal>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.  <literal>0</literal>
    is useful when using remote builders to prevent any local builds (except for
    <literal>preferLocalBuild</literal> derivation attribute which executes locally
    regardless).  It can be
    overridden using the <option
    linkend='opt-max-jobs'>--max-jobs</option> (<option>-j</option>)
    command line switch.</para>

    <para>See also <xref linkend="chap-tuning-cores-and-jobs" />.</para>
    </listitem>
  </varlistentry>

  <varlistentry xml:id="conf-max-silent-time"><term><literal>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-min-free"><term><literal>min-free</literal></term>

    <listitem>
      <para>When free disk space in <filename>/nix/store</filename>
      drops below <literal>min-free</literal> during a build, Nix
      performs a garbage-collection until <literal>max-free</literal>
      bytes are available or there is no more garbage.  A value of
      <literal>0</literal> (the default) disables this feature.</para>
    </listitem>

  </varlistentry>

  <varlistentry xml:id="conf-narinfo-cache-negative-ttl"><term><literal>narinfo-cache-negative-ttl</literal></term>

    <listitem>

      <para>The TTL in seconds for negative lookups. If a store path is
      queried from a substituter but was not found, there will be a
      negative lookup cached in the local disk cache database for the
      specified duration.</para>

    </listitem>

  </varlistentry>

  <varlistentry xml:id="conf-narinfo-cache-positive-ttl"><term><literal>narinfo-cache-positive-ttl</literal></term>

    <listitem>

      <para>The TTL in seconds for positive lookups. If a store path is
      queried from a substituter, the result of the query will be cached
      in the local disk cache database including some of the NAR
      metadata. The default TTL is a month, setting a shorter TTL for
      positive lookups can be useful for binary caches that have
      frequent garbage collection, in which case having a more frequent
      cache invalidation would prevent trying to pull the path again and
      failing with a hash mismatch if the build isn't reproducible.
      </para>

    </listitem>

  </varlistentry>

  <varlistentry xml:id="conf-netrc-file"><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>

    <note><para>This must be an absolute path, and <literal>~</literal>
    is not resolved. For example, <filename>~/.netrc</filename> won't
    resolve to your home directory's <filename>.netrc</filename>.</para></note>
    </listitem>

  </varlistentry>


  <varlistentry xml:id="conf-plugin-files">
    <term><literal>plugin-files</literal></term>
    <listitem>
      <para>
        A list of plugin files to be loaded by Nix. Each of these
        files will be dlopened by Nix, allowing them to affect
        execution through static initialization. In particular, these
        plugins may construct static instances of RegisterPrimOp to
        add new primops or constants to the expression language,
        RegisterStoreImplementation to add new store implementations,
        RegisterCommand to add new subcommands to the
        <literal>nix</literal> command, and RegisterSetting to add new
        nix config settings. See the constructors for those types for
        more details.
      </para>
      <para>
        Since these files are loaded into the same address space as
        Nix itself, they must be DSOs compatible with the instance of
        Nix running at the time (i.e. compiled against the same
        headers, not linked to any incompatible libraries). They
        should not be linked to any Nix libs directly, as those will
        be available already at load time.
      </para>
      <para>
        If an entry in the list is a directory, all files in the
        directory are loaded as plugins (non-recursively).
      </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>sandbox-paths</literal>.</para>

          </listitem>

        </varlistentry>
      </variablelist>
    </listitem>

  </varlistentry>

  <varlistentry xml:id="conf-post-build-hook">
    <term><literal>post-build-hook</literal></term>
    <listitem>
      <para>Optional. The path to a program to execute after each build.</para>

      <para>This option is only settable in the global
      <filename>nix.conf</filename>, or on the command line by trusted
      users.</para>

      <para>When using the nix-daemon, the daemon executes the hook as
      <literal>root</literal>. If the nix-daemon is not involved, the
      hook runs as the user executing the nix-build.</para>

      <itemizedlist>
        <listitem><para>The hook executes after an evaluation-time build.</para></listitem>
        <listitem><para>The hook does not execute on substituted paths.</para></listitem>
        <listitem><para>The hook's output always goes to the user's terminal.</para></listitem>
        <listitem><para>If the hook fails, the build succeeds but no further builds execute.</para></listitem>
        <listitem><para>The hook executes synchronously, and blocks other builds from progressing while it runs.</para></listitem>
      </itemizedlist>

      <para>The program executes with no arguments. The program's environment
      contains the following environment variables:</para>

      <variablelist>
        <varlistentry>
          <term><envar>DRV_PATH</envar></term>
          <listitem>
            <para>The derivation for the built paths.</para>
            <para>Example:
            <literal>/nix/store/5nihn1a7pa8b25l9zafqaqibznlvvp3f-bash-4.4-p23.drv</literal>
            </para>
          </listitem>
        </varlistentry>

        <varlistentry>
          <term><envar>OUT_PATHS</envar></term>
          <listitem>
            <para>Output paths of the built derivation, separated by a space character.</para>
            <para>Example:
            <literal>/nix/store/zf5lbh336mnzf1nlswdn11g4n2m8zh3g-bash-4.4-p23-dev
            /nix/store/rjxwxwv1fpn9wa2x5ssk5phzwlcv4mna-bash-4.4-p23-doc
            /nix/store/6bqvbzjkcp9695dq0dpl5y43nvy37pq1-bash-4.4-p23-info
            /nix/store/r7fng3kk3vlpdlh2idnrbn37vh4imlj2-bash-4.4-p23-man
            /nix/store/xfghy8ixrhz3kyy6p724iv3cxji088dx-bash-4.4-p23</literal>.
            </para>
          </listitem>
        </varlistentry>
      </variablelist>

      <para>See <xref linkend="chap-post-build-hook" /> for an example
      implementation.</para>

    </listitem>
  </varlistentry>

  <varlistentry xml:id="conf-repeat"><term><literal>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 and <xref linkend="conf-enforce-determinism" /> is
    true, 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-require-sigs"><term><literal>require-sigs</literal></term>

    <listitem><para>If set to <literal>true</literal> (the default),
    any non-content-addressed path added or copied to the Nix store
    (e.g. when substituting from a binary cache) must have a valid
    signature, that is, be signed using one of the keys listed in
    <option>trusted-public-keys</option> or
    <option>secret-key-files</option>. Set to <literal>false</literal>
    to disable signature checking.</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), or to URIs outside of
      <option>allowed-uri</option>. The default is
      <literal>false</literal>.</para>

    </listitem>

  </varlistentry>

  <varlistentry xml:id="conf-run-diff-hook"><term><literal>run-diff-hook</literal></term>
  <listitem>
    <para>
      If true, enable the execution of <xref linkend="conf-diff-hook" />.
    </para>

    <para>
      When using the Nix daemon, <literal>run-diff-hook</literal> must
      be set in the <filename>nix.conf</filename> configuration file,
      and cannot be passed at the command line.
    </para>
  </listitem>
  </varlistentry>

  <varlistentry xml:id="conf-sandbox"><term><literal>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-sandbox-paths'><literal>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>true</literal> on Linux and
    <literal>false</literal> on all other platforms.</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-sandbox-paths">
    <term><literal>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-secret-key-files"><term><literal>secret-key-files</literal></term>

    <listitem><para>A whitespace-separated list of files containing
    secret (private) keys. These are used to sign locally-built
    paths. They can be generated using <command>nix-store
    --generate-binary-cache-key</command>. The corresponding public
    key can be distributed to other users, who can add it to
    <option>trusted-public-keys</option> in their
    <filename>nix.conf</filename>.</para></listitem>

  </varlistentry>


  <varlistentry xml:id="conf-show-trace"><term><literal>show-trace</literal></term>

    <listitem><para>Causes Nix to print out a stack trace in case of Nix
    expression evaluation errors.</para></listitem>

  </varlistentry>


  <varlistentry xml:id="conf-substitute"><term><literal>substitute</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 xml:id="conf-stalled-download-timeout"><term><literal>stalled-download-timeout</literal></term>
    <listitem>
      <para>The timeout (in seconds) for receiving data from servers
      during download. Nix cancels idle downloads after this timeout's
      duration.</para>
    </listitem>
  </varlistentry>

  <varlistentry xml:id="conf-substituters"><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 xml:id="conf-system"><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 xml:id="conf-system-features"><term><literal>system-features</literal></term>

    <listitem><para>A set of system “features” supported by this
    machine, e.g. <literal>kvm</literal>. Derivations can express a
    dependency on such features through the derivation attribute
    <varname>requiredSystemFeatures</varname>. For example, the
    attribute

<programlisting>
requiredSystemFeatures = [ "kvm" ];
</programlisting>

    ensures that the derivation can only be built on a machine with
    the <literal>kvm</literal> feature.</para>

    <para>This setting by default includes <literal>kvm</literal> if
    <filename>/dev/kvm</filename> is accessible, and the
    pseudo-features <literal>nixos-test</literal>,
    <literal>benchmark</literal> and <literal>big-parallel</literal>
    that are used in Nixpkgs to route builds to specific
    machines.</para>

    </listitem>

  </varlistentry>

  <varlistentry xml:id="conf-tarball-ttl"><term><literal>tarball-ttl</literal></term>

    <listitem>
      <para>Default: <literal>3600</literal> seconds.</para>

      <para>The number of seconds a downloaded tarball is considered
      fresh. If the cached tarball is stale, Nix will check whether
      it is still up to date using the ETag header. Nix will download
      a new version if the ETag header is unsupported, or the
      cached ETag doesn't match.
      </para>

      <para>Setting the TTL to <literal>0</literal> forces Nix to always
      check if the tarball is up to date.</para>

      <para>Nix caches tarballs in
      <filename>$XDG_CACHE_HOME/nix/tarballs</filename>.</para>

      <para>Files fetched via <envar>NIX_PATH</envar>,
      <function>fetchGit</function>, <function>fetchMercurial</function>,
      <function>fetchTarball</function>, and <function>fetchurl</function>
      respect this TTL.
      </para>
    </listitem>
  </varlistentry>

  <varlistentry xml:id="conf-timeout"><term><literal>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-trace-function-calls"><term><literal>trace-function-calls</literal></term>

    <listitem>

      <para>Default: <literal>false</literal>.</para>

      <para>If set to <literal>true</literal>, the Nix evaluator will
      trace every function call. Nix will print a log message at the
      "vomit" level for every function entrance and function exit.</para>

      <informalexample><screen>
function-trace entered undefined position at 1565795816999559622
function-trace exited undefined position at 1565795816999581277
function-trace entered /nix/store/.../example.nix:226:41 at 1565795253249935150
function-trace exited /nix/store/.../example.nix:226:41 at 1565795253249941684
</screen></informalexample>

      <para>The <literal>undefined position</literal> means the function
      call is a builtin.</para>

      <para>Use the <literal>contrib/stack-collapse.py</literal> script
      distributed with the Nix source code to convert the trace logs
      in to a format suitable for <command>flamegraph.pl</command>.</para>

    </listitem>

  </varlistentry>

  <varlistentry xml:id="conf-trusted-public-keys"><term><literal>trusted-public-keys</literal></term>

    <listitem><para>A whitespace-separated list of public keys. When
    paths are copied from another Nix store (such as a binary cache),
    they must be signed with one of these keys. For example:
    <literal>cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
    hydra.nixos.org-1:CNHJZBh9K4tP3EKF6FkkgeVYsS3ohTl+oS0Qa8bezVs=</literal>.</para></listitem>

  </varlistentry>

  <varlistentry xml:id="conf-trusted-substituters"><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 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>Adding a user to <option>trusted-users</option>
      is essentially equivalent to giving that user root access to the
      system. For example, the user can set
      <option>sandbox-paths</option> and thereby obtain read access to
      directories that are otherwise inacessible to
      them.</para></warning>

    </listitem>

  </varlistentry>

</variablelist>
</para>

<refsection>
  <title>Deprecated Settings</title>

<para>

<variablelist>

  <varlistentry xml:id="conf-binary-caches">
    <term><literal>binary-caches</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>binary-caches</literal> is now an alias to
    <xref linkend="conf-substituters" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-binary-cache-public-keys">
    <term><literal>binary-cache-public-keys</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>binary-cache-public-keys</literal> is now an alias to
    <xref linkend="conf-trusted-public-keys" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-compress-log">
    <term><literal>build-compress-log</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-compress-log</literal> is now an alias to
    <xref linkend="conf-compress-build-log" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-cores">
    <term><literal>build-cores</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-cores</literal> is now an alias to
    <xref linkend="conf-cores" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-extra-chroot-dirs">
    <term><literal>build-extra-chroot-dirs</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-extra-chroot-dirs</literal> is now an alias to
    <xref linkend="conf-extra-sandbox-paths" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-extra-sandbox-paths">
    <term><literal>build-extra-sandbox-paths</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-extra-sandbox-paths</literal> is now an alias to
    <xref linkend="conf-extra-sandbox-paths" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-fallback">
    <term><literal>build-fallback</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-fallback</literal> is now an alias to
    <xref linkend="conf-fallback" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-max-jobs">
    <term><literal>build-max-jobs</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-max-jobs</literal> is now an alias to
    <xref linkend="conf-max-jobs" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-max-log-size">
    <term><literal>build-max-log-size</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-max-log-size</literal> is now an alias to
    <xref linkend="conf-max-build-log-size" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-max-silent-time">
    <term><literal>build-max-silent-time</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-max-silent-time</literal> is now an alias to
    <xref linkend="conf-max-silent-time" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-repeat">
    <term><literal>build-repeat</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-repeat</literal> is now an alias to
    <xref linkend="conf-repeat" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-timeout">
    <term><literal>build-timeout</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-timeout</literal> is now an alias to
    <xref linkend="conf-timeout" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-use-chroot">
    <term><literal>build-use-chroot</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-use-chroot</literal> is now an alias to
    <xref linkend="conf-sandbox" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-use-sandbox">
    <term><literal>build-use-sandbox</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-use-sandbox</literal> is now an alias to
    <xref linkend="conf-sandbox" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-build-use-substitutes">
    <term><literal>build-use-substitutes</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>build-use-substitutes</literal> is now an alias to
    <xref linkend="conf-substitute" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-gc-keep-derivations">
    <term><literal>gc-keep-derivations</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>gc-keep-derivations</literal> is now an alias to
    <xref linkend="conf-keep-derivations" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-gc-keep-outputs">
    <term><literal>gc-keep-outputs</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>gc-keep-outputs</literal> is now an alias to
    <xref linkend="conf-keep-outputs" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-env-keep-derivations">
    <term><literal>env-keep-derivations</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>env-keep-derivations</literal> is now an alias to
    <xref linkend="conf-keep-env-derivations" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-extra-binary-caches">
    <term><literal>extra-binary-caches</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>extra-binary-caches</literal> is now an alias to
    <xref linkend="conf-extra-substituters" />.</para></listitem>
  </varlistentry>

  <varlistentry xml:id="conf-trusted-binary-caches">
    <term><literal>trusted-binary-caches</literal></term>

    <listitem><para><emphasis>Deprecated:</emphasis>
    <literal>trusted-binary-caches</literal> is now an alias to
    <xref linkend="conf-trusted-substituters" />.</para></listitem>
  </varlistentry>
</variablelist>
</para>
</refsection>

</refsection>

</refentry>