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nix.conf(5) File Formats Manual nix.conf(5)

Name
nix.conf - Nix configuration file

Description
Nix supports a variety of configuration settings, which are read from
configuration files or taken as command line flags.

Configuration file
By default Nix reads settings from the following places, in that order:

1. The system-wide configuration file sysconfdir/nix/nix.conf (i.e.
/etc/nix/nix.conf on most systems), or $NIX_CONF_DIR/nix.conf if
NIX_CONF_DIR <./env-common.md#env-NIX_CONF_DIR> is set.

Values loaded in this file are not forwarded to the Nix daemon. The
client assumes that the daemon has already loaded them.

2. If NIX_USER_CONF_FILES <./env-common.md#env-NIX_USER_CONF_FILES> is
set, then each path separated by : will be loaded in reverse order.

Otherwise it will look for nix/nix.conf files in XDG_CONFIG_DIRS and
XDG_CONFIG_HOME <./env-common.md#env-XDG_CONFIG_HOME>. If unset,
XDG_CONFIG_DIRS defaults to /etc/xdg, and XDG_CONFIG_HOME defaults
to $HOME/.config as per XDG Base Directory Specification
<https://specifications.freedesktop.org/basedir-spec/basedir-spec-
latest.html>.

3. If NIX_CONFIG <./env-common.md#env-NIX_CONFIG> is set, its contents
are treated as the contents of a configuration file.

File format
Configuration files consist of name = value pairs, one per line. Com-
ments start with a # character.

Example:

keep-outputs = true # Nice for developers
keep-derivations = true # Idem

Other files can be included with a line like include <path>, where
<path> is interpreted relative to the current configuration file. A
missing file is an error unless !include is used instead.

A configuration setting usually overrides any previous value. However,
for settings that take a list of items, you can prefix the name of the
setting by extra- to append to the previous value.

For instance,

substituters = a b
extra-substituters = c d

defines the substituters setting to be a b c d.

Unknown option names are not an error, and are simply ignored with a
warning.

Command line flags
Configuration options can be set on the command line, overriding the
values set in the configuration file <#configuration-file>:

• Every configuration setting has corresponding command line flag
(e.g. --max-jobs 16). Boolean settings do not need an argument, and
can be explicitly disabled with the no- prefix (e.g. --keep-failed
and --no-keep-failed).

Unknown option names are invalid flags (unless there is already a
flag with that name), and are rejected with an error.

• The flag --option <name> <value> is interpreted exactly like a
<name> = <value> in a setting file.

Unknown option names are ignored with a warning.

The extra- prefix is supported for settings that take a list of items
(e.g. --extra-trusted users alice or --option extra-trusted-users al-
ice).

Integer settings
Settings that have an integer type support the suffixes K, M, G and T.
These cause the specified value to be multiplied by 2^10, 2^20, 2^30
and 2^40, respectively. For instance, --min-free 1M is equivalent to
--min-free 1048576.

Available settings
• abort-on-warn <#conf-abort-on-warn>

If set to true, builtins.warn
<@docroot@/language/builtins.md#builtins-warn> throws an error when
logging a warning.

This will give you a stack trace that leads to the location of the
warning.

This is useful for finding information about warnings in third-party
Nix code when you can not start the interactive debugger, such as
when Nix is called from a non-interactive script. See debugger-on-
warn <#conf-debugger-on-warn>.

Currently, a stack trace can only be produced when the debugger is
enabled, or when evaluation is aborted.

This option can be enabled by setting NIX_ABORT_ON_WARN=1 in the en-
vironment.

Default: false

• accept-flake-config <#conf-accept-flake-config>

Warning

This setting is part of an experimental feature <@doc-
root@/development/experimental-features.md>.

To change this setting, make sure the flakes experimental
feature <@docroot@/development/experimental-features.md#xp-
feature-flakes> is enabled. For example, include the follow-
ing in nix.conf <@docroot@/command-ref/conf-file.md>:

extra-experimental-features = flakes
accept-flake-config = ...

Whether to accept Nix configuration settings from a flake without
prompting.

Default: false

• access-tokens <#conf-access-tokens>

Access tokens used to access protected GitHub, GitLab, or other lo-
cations requiring token-based authentication.

Access tokens are specified as a string made up of space-separated
host=token values. The specific token used is selected by matching
the host portion against the "host" specification of the input. The
host portion may contain a path element which matches against the
prefix URL for the input. (eg: github.com/org=token). The actual use
of the token value is determined by the type of resource being ac-
cessed:

• Github: the token value is the OAUTH-TOKEN string obtained as the
Personal Access Token from the Github server (see
<https://docs.github.com/en/developers/apps/building-oauth-
apps/authorizing-oauth-apps>).

• Gitlab: the token value is either the OAuth2 token or the Per-
sonal Access Token (these are different types tokens for gitlab,
see <https://docs.gitlab.com/12.10/ee/api/README.html#authentica-
tion>). The token value should be type:tokenstring where type is
either OAuth2 or PAT to indicate which type of token is being
specified.

Example ~/.config/nix/nix.conf:

access-tokens = github.com=23ac...b289 gitlab.mycompany.com=PAT:A123Bp_Cd..EfG gitlab.com=OAuth2:1jklw3jk

Example ~/code/flake.nix:

input.foo = {
type = "gitlab";
host = "gitlab.mycompany.com";
owner = "mycompany";
repo = "pro";
};

This example specifies three tokens, one each for accessing
github.com, gitlab.mycompany.com, and gitlab.com.

The input.foo uses the "gitlab" fetcher, which might requires
specifying the token type along with the token value.

Default: empty

• allow-dirty <#conf-allow-dirty>

Whether to allow dirty Git/Mercurial trees.

Default: true

• allow-dirty-locks <#conf-allow-dirty-locks>

Warning

This setting is part of an experimental feature <@doc-
root@/development/experimental-features.md>.

extra-experimental-features = flakes
allow-dirty-locks = ...

Whether to allow dirty inputs (such as dirty Git workdirs) to be
locked via their NAR hash. This is generally bad practice since Nix
has no way to obtain such inputs if they are subsequently modified.
Therefore lock files with dirty locks should generally only be used
for local testing, and should not be pushed to other users.

Default: false

• allow-import-from-derivation <#conf-allow-import-from-derivation>

By default, Nix allows Import from Derivation <@docroot@/lan-
guage/import-from-derivation.md>.

With this option set to false, Nix throws an error when evaluating
an expression that uses this feature, even when the required store
object is readily available. This ensures that evaluation doesn't
require any builds to take place, regardless of the state of the
store.

Default: true

• allow-symlinked-store <#conf-allow-symlinked-store>

If set to true, Nix stops complaining if the store directory (typi-
cally /nix/store) contains symlink components.

This risks making some builds "impure" because builders sometimes
"canonicalise" paths by resolving all symlink components. Problems
occur if those builds are then deployed to machines where /nix/store
resolves to a different location from that of the build machine. You
can enable this setting if you are sure you're not going to do that.

Default: false

• allow-unsafe-native-code-during-evaluation <#conf-allow-unsafe-
native-code-during-evaluation>

Enable built-in functions that allow executing native code.

In particular, this adds:

• builtins.importNative path symbol

Opens dynamic shared object (DSO) at path, loads the function with
the symbol name symbol from it and runs it. The loaded function
must have the following signature: cpp extern "C" typedef void
(*ValueInitialiser) (EvalState & state, Value & v);

The Nix C++ API documentation
<@docroot@/development/documentation.md#api-documentation> has more
details on evaluator internals.

• builtins.exec arguments

Execute a program, where arguments are specified as a list of
strings, and parse its output as a Nix expression.

Default: false

• allowed-impure-host-deps <#conf-allowed-impure-host-deps>

Which prefixes to allow derivations to ask for access to (primarily
for Darwin).

Default: empty

• allowed-uris <#conf-allowed-uris>

A list of URI prefixes to which access is allowed in restricted
evaluation mode. For example, when set to https://github.com/NixOS,
builtin functions such as fetchGit are allowed to access
https://github.com/NixOS/patchelf.git.

Access is granted when

• the URI is equal to the prefix,
• or the URI is a subpath of the prefix,
• or the prefix is a URI scheme ended by a colon : and the URI has
the same scheme.

Default: empty

• allowed-users <#conf-allowed-users>

A list user names, separated by whitespace. These users are allowed
to connect to the Nix daemon.

You can specify groups by prefixing names with @. For instance,
@wheel means all users in the wheel group. Also, you can allow all
users by specifying *.

Note

Trusted users (set in trusted-users <#conf-trusted-users>)
can always connect to the Nix daemon.

Default: *

• always-allow-substitutes <#conf-always-allow-substitutes>

If set to true, Nix ignores the allowSubstitutes
<@docroot@/language/advanced-attributes.md> attribute in derivations
and always attempt to use available substituters <#conf-substi-
tuters>.

Default: false

• auto-allocate-uids <#conf-auto-allocate-uids>

Warning

This setting is part of an experimental feature <@doc-
root@/development/experimental-features.md>.

To change this setting, make sure the auto-allocate-uids ex-
perimental feature <@docroot@/development/experimental-
features.md#xp-feature-auto-allocate-uids> is enabled. For
example, include the following in nix.conf <@docroot@/com-
mand-ref/conf-file.md>:

extra-experimental-features = auto-allocate-uids
auto-allocate-uids = ...

Whether to select UIDs for builds automatically, instead of using
the users in build-users-group.

UIDs are allocated starting at 872415232 (0x34000000) on Linux and
56930 on macOS.

Default: false

• auto-optimise-store <#conf-auto-optimise-store>

If set to true, Nix automatically detects files in the store that
have identical contents, and replaces them with hard links to a sin-
gle copy. This saves disk space. If set to false (the default), you
can still run nix-store --optimise to get rid of duplicate files.

Default: false

• bash-prompt <#conf-bash-prompt>

The bash prompt (PS1) in nix develop shells.

Default: empty

• bash-prompt-prefix <#conf-bash-prompt-prefix>

Prefix prepended to the PS1 environment variable in nix develop
shells.

Default: empty

• bash-prompt-suffix <#conf-bash-prompt-suffix>

Suffix appended to the PS1 environment variable in nix develop
shells.

Default: empty

• build-dir <#conf-build-dir>

Override the build-dir store setting for all stores that have this
setting.

Default: ``

• build-hook <#conf-build-hook>

The path to the helper program that executes remote builds.

Nix communicates with the build hook over stdio using a custom pro-
tocol to request builds that cannot be performed directly by the Nix
daemon. The default value is the internal Nix binary that imple-
ments remote building.

Important

Change this setting only if you really know what youre doing.

Default: nix __build-remote

• build-poll-interval <#conf-build-poll-interval>

How often (in seconds) to poll for locks.

Default: 5

• build-users-group <#conf-build-users-group>

This options specifies the Unix group containing the Nix build user
accounts. In multi-user Nix installations, builds should not be per-
formed by the Nix account since that would allow users to arbitrar-
ily 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.

Therefore, if this option is non-empty and specifies a valid group,
builds are performed under the user accounts that are a member of
the group specified here (as listed in /etc/group). Those user ac-
counts should not be used for any other purpose!

Nix never runs 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 legiti-
mate Nix expression being built by another user. Therefore it is
good to have as many Nix build user accounts as you can spare. (Re-
member: uids are cheap.)

The build users should have permission to create files in the Nix
store, but not delete them. Therefore, /nix/store should be owned by
the Nix account, its group should be the group specified here, and
its mode should be 1775.

If the build users group is empty, builds areperformed under the uid
of the Nix process (that is, the uid of the caller if NIX_REMOTE is
empty, the uid under which the Nix daemon runs if NIX_REMOTE is dae-
mon). Obviously, this should not be used with a nix daemon accessi-
ble to untrusted clients.

Defaults to nixbld when running as root, empty otherwise.

Default: machine-specific

• builders <#conf-builders>

A semicolon- or newline-separated list of build machines.

In addition to the usual ways of setting configuration options
<@docroot@/command-ref/conf-file.md>, the value can be read from a
file by prefixing its absolute path with @.

Example

This is the default setting:

builders = @/etc/nix/machines

Each machine specification consists of the following elements, sepa-
rated by spaces. Only the first element is required. To leave a
field at its default, set it to -.

1. The URI of the remote store in the format ssh://[username@]host-
name[:port].

Example

ssh://nix@mac

For backward compatibility, ssh:// may be omitted. The hostname
may be an alias defined in ~/.ssh/config.

2. A comma-separated list of Nix system types <@docroot@/develop-
ment/building.md#system-type>. If omitted, this defaults to the
local platform type.

Example

aarch64-darwin

It is possible for a machine to support multiple platform types.

Example

i686-linux,x86_64-linux

3. The SSH identity file to be used to log in to the remote machine.
If omitted, SSH uses its regular identities.

Example

/home/user/.ssh/id_mac

4. The maximum number of builds that Nix executes in parallel on the
machine. Typically this should be equal to the number of CPU
cores.

5. The speed factor, indicating the relative speed of the machine as
a positive integer. If there are multiple machines of the right
type, Nix prefers the fastest, taking load into account.

6. A comma-separated list of supported system features <#conf-sys-
tem-features>.

A machine is only used to build a derivation if all the features
in the derivation's requiredSystemFeatures
<@docroot@/language/advanced-attributes.html#adv-attr-
requiredSystemFeatures> attribute are supported by that machine.

7. A comma-separated list of required system features <#conf-system-
features>.

A machine is only used to build a derivation if all of the ma-
chines required features appear in the derivations requiredSys-
temFeatures <@docroot@/language/advanced-attributes.html#adv-
attr-requiredSystemFeatures> attribute.

8. The (base64-encoded) public host key of the remote machine. If
omitted, SSH uses its regular known_hosts file.

The value for this field can be obtained via base64 -w0.

Example

Multiple builders specified on the command line:

--builders 'ssh://mac x86_64-darwin ; ssh://beastie x86_64-freebsd'

Example

This specifies several machines that can perform i686-linux
builds:

nix@scratchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy 8 1 kvm
nix@itchy.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy 8 2
nix@poochie.labs.cs.uu.nl i686-linux /home/nix/.ssh/id_scratchy 1 2 kvm benchmark

However, poochie only builds derivations that have the at-
tribute

requiredSystemFeatures = [ "benchmark" ];

requiredSystemFeatures = [ "benchmark" "kvm" ];

itchy cannot do builds that require kvm, but scratchy does
support such builds. For regular builds, itchy is preferred
over scratchy because it has a higher speed factor.

For Nix to use substituters, the calling user must be in the
trusted-users <#conf-trusted-users> list.

Note

A build machine must be accessible via SSH and have Nix in-
stalled. nix must be available in $PATH for the user con-
necting over SSH.

Warning

If you are building via the Nix daemon (default), the Nix
daemon user account on the local machine (that is, root) re-
quires access to a user account on the remote machine (not
necessarily root).

If you cant or dont want to configure root to be able to ac-
cess the remote machine, set store <#conf-store> to any local
store <@docroot@/store/types/local-store.html>, e.g. by pass-
ing --store /tmp to the command on the local machine.

To build only on remote machines and disable local builds, set max-
jobs <#conf-max-jobs> to 0.

If you want the remote machines to use substituters, set builders-
use-substitutes <#conf-builders-use-substitutes> to true.

Default: machine-specific

• builders-use-substitutes <#conf-builders-use-substitutes>

If set to true, Nix instructs remote build machines <#conf-builders>
to use their own substituters <#conf-substituters> if available.

It means that remote build hosts fetches as many dependencies as
possible from their own substituters (e.g, from cache.nixos.org) in-
stead of waiting for the local machine to upload them all. This can
drastically reduce build times if the network connection between the
local machine and the remote build host is slow.

Default: false

• commit-lock-file-summary <#conf-commit-lock-file-summary>

Warning

This setting is part of an experimental feature <@doc-
root@/development/experimental-features.md>.

extra-experimental-features = flakes
commit-lock-file-summary = ...

The commit summary to use when committing changed flake lock files.
If empty, the summary is generated based on the action performed.

Default: empty

Deprecated alias: commit-lockfile-summary

• compress-build-log <#conf-compress-build-log>

If set to true (the default), build logs written to
/nix/var/log/nix/drvs are compressed on the fly using bzip2. Other-
wise, they are not compressed.

Default: true

Deprecated alias: build-compress-log

• connect-timeout <#conf-connect-timeout>

The timeout (in seconds) for establishing connections in the binary
cache substituter. It corresponds to curls --connect-timeout option.
A value of 0 means no limit.

Default: 15

• cores <#conf-cores>

Sets the value of the NIX_BUILD_CORES environment variable in the
invocation of the builder executable
<@docroot@/language/derivations.md#builder-execution> of a deriva-
tion. The builder executable can use this variable to control its
own maximum amount of parallelism.

For instance, in Nixpkgs, if the attribute enableParallelBuilding
for the mkDerivation build helper is set to true, it passes the
-j${NIX_BUILD_CORES} flag to GNU Make.

If set to 0, nix will detect the number of CPU cores and pass this
number via NIXBUILDCORES.

Note

The number of parallel local Nix build jobs is independently
controlled with the max-jobs <#conf-max-jobs> setting.

Default: 0

Deprecated alias: build-cores

• debugger-on-trace <#conf-debugger-on-trace>

If set to true and the --debugger flag is given, the following func-
tions enter the debugger like builtins.break <@docroot@/lan-
guage/builtins.md#builtins-break>:

• builtins.trace <@docroot@/language/builtins.md#builtins-trace>
• builtins.traceVerbose <@docroot@/language/builtins.md#builtins-
traceVerbose> if trace-verbose <#conf-trace-verbose> is set to
true.
• builtins.warn <@docroot@/language/builtins.md#builtins-warn>

This is useful for debugging warnings in third-party Nix code.

Default: false

• debugger-on-warn <#conf-debugger-on-warn>

If set to true and the --debugger flag is given, builtins.warn
<@docroot@/language/builtins.md#builtins-warn> will enter the debug-
ger like builtins.break <@docroot@/language/builtins.md#builtins-
break>.

This is useful for debugging warnings in third-party Nix code.

Use debugger-on-trace <#conf-debugger-on-trace> to also enter the
debugger on legacy warnings that are logged with builtins.trace
<@docroot@/language/builtins.md#builtins-trace>.

Default: false

• diff-hook <#conf-diff-hook>

Absolute path to an executable capable of diffing build results. The
hook is executed if run-diff-hook is true, and the output of a build
is known to not be the same. This program is not executed to deter-
mine if two results are the same.

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.

The diff hook program receives three parameters:

1. A path to the previous build's results

2. A path to the current build's results

3. The path to the build's derivation

4. The path to the build's scratch directory. This directory exists
only if the build was run with --keep-failed.

The stderr and stdout output from the diff hook isn't displayed to
the user. Instead, it print to the nix-daemon's log.

When using the Nix daemon, diff-hook must be set in the nix.conf
configuration file, and cannot be passed at the command line.

Default: ``

• download-attempts <#conf-download-attempts>

The number of times Nix attempts to download a file before giving
up.

Default: 5

• download-buffer-size <#conf-download-buffer-size>

The size of Nix's internal download buffer in bytes during curl
transfers. If data is not processed quickly enough to exceed the
size of this buffer, downloads may stall. The default is 67108864
(64 MiB).

Default: 67108864

• download-speed <#conf-download-speed>

Specify the maximum transfer rate in kilobytes per second you want
Nix to use for downloads.

Default: 0

• eval-attrset-update-layer-rhs-threshold <#conf-eval-attrset-update-
layer-rhs-threshold>

Tunes the maximum size of an attribute set that, when used as a
right operand in an attribute set update expression <@docroot@/lan-
guage/operators.md#update>, uses a more space-efficient linked-list
representation of attribute sets.

Setting this to larger values generally leads to less memory alloca-
tions, but may lead to worse evaluation performance.

A value of 0 disables this optimization completely.

This is an advanced performance tuning option and typically should
not be changed. The default value is chosen to balance performance
and memory usage. On 32 bit systems where memory is scarce, the de-
fault is a large value to reduce the amount of allocations.

Default: 16

• eval-cache <#conf-eval-cache>

Whether to use the flake evaluation cache. Certain commands won't
have to evaluate when invoked for the second time with a particular
version of a flake. Intermediate results are not cached.

Default: true

• eval-profile-file <#conf-eval-profile-file>

Specifies the file where evaluation profile <#conf-eval-profiler> is
saved.

Default: nix.profile

• eval-profiler <#conf-eval-profiler>

Enables evaluation profiling. The following modes are supported:

• flamegraph stack sampling profiler. Outputs folded format, one
line per stack (suitable for flamegraph.pl and compatible tools).

Use eval-profile-file <#conf-eval-profile-file> to specify where the
profile is saved.

See Using the eval-profiler <@docroot@/advanced-topics/eval-pro-
filer.md>.

Default: disabled

• eval-profiler-frequency <#conf-eval-profiler-frequency>

Specifies the sampling rate in hertz for sampling evaluation profil-
ers. Use 0 to sample the stack after each function call. See eval-
profiler <#conf-eval-profiler>.

Default: 99

• eval-system <#conf-eval-system>

This option defines builtins.currentSystem
<@docroot@/language/builtins.md#builtins-currentSystem> in the Nix
language if it is set as a non-empty string. Otherwise, if it is
defined as the empty string (the default), the value of the system
<#conf-system> configuration setting is used instead.

Unlike system, this setting does not change what kind of derivations
can be built locally. This is useful for evaluating Nix code on one
system to produce derivations to be built on another type of system.

Default: empty

• experimental-features <#conf-experimental-features>

Experimental features that are enabled.

Example:

experimental-features = nix-command flakes

The following experimental features are available:

Experimental features are further documented in the manual
<@docroot@/development/experimental-features.md>.

Default: empty

• external-builders <#conf-external-builders>

Helper programs that execute derivations.

The program is passed a JSON document that describes the build envi-
ronment as the final argument. The JSON document looks like this:

{ "args": [ "-e", "/nix/store/vj1c3wf9-source-stdenv.sh",
"/nix/store/shkw4qm9-default-builder.sh" ], "builder":
"/nix/store/s1qkj0ph-bash-5.2p37/bin/bash", "env": { "HOME": "/home-
less-shelter", "builder": "/nix/store/s1qkj0ph-
bash-5.2p37/bin/bash", "nativeBuildInputs":
"/nix/store/l31j72f1-version-check-hook", "out":
"/nix/store/2yx2prgx-hello-2.12.2" }, "inputPaths": [
"/nix/store/14dciax3-glibc-2.32-54-dev", "/nix/store/1azs5s8z-get-
text-0.21", ], "outputs": { "out": "/nix/store/2yx2prgx-
hello-2.12.2" }, "realStoreDir": "/nix/store", "storeDir":
"/nix/store", "system": "aarch64-linux", "tmpDir": "/pri-
vate/tmp/nix-build-hello-2.12.2.drv-0/build", "tmpDirInSandbox":
"/build", "topTmpDir": "/private/tmp/nix-build-hello-2.12.2.drv-0",
"version": 1 }

Default: empty

• extra-platforms <#conf-extra-platforms>

System types of executables that can be run on this machine.

Nix only builds a given store derivation
<@docroot@/glossary.md#gloss-store-derivation> locally when its sys-
tem attribute equals any of the values specified here or in the sys-
tem option <#conf-system>.

Setting this can be useful to build derivations locally on compati-
ble machines:

• i686-linux executables can be run on x86_64-linux machines (set
by default)
• x86_64-darwin executables can be run on macOS aarch64-darwin with
Rosetta 2 (set by default where applicable)
• armv6 and armv5tel executables can be run on armv7
• some aarch64 machines can also natively run 32-bit ARM code
• qemu-user may be used to support non-native platforms (though
this may be slow and buggy)

Build systems usually detect the target platform to be the current
physical system and therefore produce machine code incompatible with
what may be intended in the derivation. You should design your de-
rivation's builder accordingly and cross-check the results when us-
ing this option against natively-built versions of your derivation.

Default: machine-specific

• fallback <#conf-fallback>

If set to true, Nix falls back to building from source if a binary
substitute fails. This is equivalent to the --fallback flag. The de-
fault is false.

Default: false

Deprecated alias: build-fallback

• flake-registry <#conf-flake-registry>

Warning

This setting is part of an experimental feature <@doc-
root@/development/experimental-features.md>.

extra-experimental-features = flakes
flake-registry = ...

Path or URI of the global flake registry.

When empty, disables the global flake registry.

Default: https://channels.nixos.org/flake-registry.json

• fsync-metadata <#conf-fsync-metadata>

If set to true, changes to the Nix store metadata (in
/nix/var/nix/db) are synchronously flushed to disk. This improves
robustness in case of system crashes, but reduces performance. The
default is true.

Default: true

• fsync-store-paths <#conf-fsync-store-paths>

Whether to call fsync() on store paths before registering them, to
flush them to disk. This improves robustness in case of system
crashes, but reduces performance. The default is false.

Default: false

• gc-reserved-space <#conf-gc-reserved-space>

Amount of reserved disk space for the garbage collector.

Default: 8388608

• hashed-mirrors <#conf-hashed-mirrors>

A list of web servers used by builtins.fetchurl to obtain files by
hash. Given a hash algorithm ha and a base-16 hash h, Nix tries to
download the file from hashed-mirror/ha/h. This allows files to be
downloaded even if they have disappeared from their original URI.
For example, given an example mirror http://tarballs.nixos.org/,
when building the derivation

builtins.fetchurl {
url = "https://example.org/foo-1.2.3.tar.xz";
sha256 = "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae";
}

Nix will attempt to download this file from http://tar-
balls.nixos.org/sha256/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae
first. If it is not available there, it tries the original URI.

Default: empty

• http-connections <#conf-http-connections>

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.

Default: 25

Deprecated alias: binary-caches-parallel-connections

• http2 <#conf-http2>

Whether to enable HTTP/2 support.

Default: true

• id-count <#conf-id-count>

The number of UIDs/GIDs to use for dynamic ID allocation.

Default: 128

• ignore-try <#conf-ignore-try>

If set to true, ignore exceptions inside `tryEval' calls when evalu-
ating Nix expressions in debug mode (using the -debugger flag). By
default the debugger pauses on all exceptions.

Default: false

• impersonate-linux-26 <#conf-impersonate-linux-26>

Whether to impersonate a Linux 2.6 machine on newer kernels.

Default: false

Deprecated alias: build-impersonate-linux-26

• impure-env <#conf-impure-env>

Warning

This setting is part of an experimental feature <@doc-
root@/development/experimental-features.md>.

To change this setting, make sure the configurable-impure-env
experimental feature <@docroot@/development/experimental-
features.md#xp-feature-configurable-impure-env> is enabled.
For example, include the following in nix.conf <@doc-
root@/command-ref/conf-file.md>:

extra-experimental-features = configurable-impure-env
impure-env = ...

A list of items, each in the format of:

• name=value: Set environment variable name to value.

If the user is trusted (see trusted-users option), when building a
fixed-output derivation, environment variables set in this option is
passed to the builder if they are listed in impureEnvVars <@doc-
root@/language/advanced-attributes.md#adv-attr-impureEnvVars>.

This option is useful for, e.g., setting https_proxy for fixed-out-
put derivations and in a multi-user Nix installation, or setting
private access tokens when fetching a private repository.

Default: empty

• json-log-path <#conf-json-log-path>

A file or unix socket to which JSON records of Nix's log output are
written, in the same format as --log-format internal-json (without
the @nix prefixes on each line). Concurrent writes to the same file
by multiple Nix processes are not supported and may result in inter-
leaved or corrupted log records.

Default: empty

• keep-build-log <#conf-keep-build-log>

If set to true (the default), Nix writes the build log of a deriva-
tion (i.e. the standard output and error of its builder) to the di-
rectory /nix/var/log/nix/drvs. The build log can be retrieved using
the command nix-store -l path.

Default: true

Deprecated alias: build-keep-log

• keep-derivations <#conf-keep-derivations>

If true (default), the garbage collector keeps the derivations from
which non-garbage store paths were built. If false, they are deleted
unless explicitly registered as a root (or reachable from other
roots).

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 keep-outputs is also turned
on).

Default: true

Deprecated alias: gc-keep-derivations

• keep-env-derivations <#conf-keep-env-derivations>

If false (default), derivations are not stored in Nix user environ-
ments. That is, the derivations of any build-time-only dependencies
may be garbage-collected.

If true, when you add a Nix derivation to a user environment, the
path of the derivation is stored in the user environment. Thus, the
derivation isn't garbage-collected until the user environment gener-
ation is deleted (nix-env --delete-generations). To prevent build-
time-only dependencies from being collected, you should also turn on
keep-outputs.

The difference between this option and keep-derivations is that this
one is sticky: it applies to any user environment created while this
option was enabled, while keep-derivations only applies at the mo-
ment the garbage collector is run.

Default: false

Deprecated alias: env-keep-derivations

• keep-failed <#conf-keep-failed>

Whether to keep temporary directories of failed builds.

Default: false

• keep-going <#conf-keep-going>

Whether to keep building derivations when another build fails.

Default: false

• keep-outputs <#conf-keep-outputs>

If true, the garbage collector keeps the outputs of non-garbage de-
rivations. If false (default), outputs are deleted unless they are
GC roots themselves (or reachable from other roots).

In general, outputs must be registered as roots separately. However,
even if the output of a derivation is registered as a root, the col-
lector still deletes store paths that are used only at build time
(e.g., the C compiler, or source tarballs downloaded from the net-
work). To prevent it from doing so, set this option to true.

Default: false

Deprecated alias: gc-keep-outputs

• log-lines <#conf-log-lines>

The number of lines of the tail of the log to show if a build fails.

Default: 25

• max-build-log-size <#conf-max-build-log-size>

This option defines the maximum number of bytes that a builder can
write to its stdout/stderr. If the builder exceeds this limit, its
killed. A value of 0 (the default) means that there is no limit.

Default: 0

Deprecated alias: build-max-log-size

• max-call-depth <#conf-max-call-depth>

The maximum function call depth to allow before erroring.

Default: 10000

• max-free <#conf-max-free>

When a garbage collection is triggered by the min-free option, it
stops as soon as max-free bytes are available. The default is infin-
ity (i.e. delete all garbage).

Default: 9223372036854775807

• max-jobs <#conf-max-jobs>

Maximum number of jobs that Nix tries to build locally in parallel.

The special value auto causes Nix to use the number of CPUs in your
system. Use 0 to disable local builds and directly use the remote
machines specified in builders <#conf-builders>. This doesn't af-
fect derivations that have preferLocalBuild = true <@docroot@/lan-
guage/advanced-attributes.md#adv-attr-preferLocalBuild>, which are
always built locally.

Note

The number of CPU cores to use for each build job is indepen-
dently determined by the cores <#conf-cores> setting.

The setting can be overridden using the --max-jobs (-j) command line
switch.

Default: 1

Deprecated alias: build-max-jobs

• max-silent-time <#conf-max-silent-time>

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 --max-silent-time command line switch.

The value 0 means that there is no timeout. This is also the de-
fault.

Default: 0

Deprecated alias: build-max-silent-time

• max-substitution-jobs <#conf-max-substitution-jobs>

This option defines the maximum number of substitution jobs that Nix
tries to run in parallel. The default is 16. The minimum value one
can choose is 1 and lower values are interpreted as 1.

Default: 16

Deprecated alias: substitution-max-jobs

• min-free <#conf-min-free>

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

Default: 0

• min-free-check-interval <#conf-min-free-check-interval>

Number of seconds between checking free disk space.

Default: 5

• nar-buffer-size <#conf-nar-buffer-size>

Maximum size of NARs before spilling them to disk.

Default: 33554432

• narinfo-cache-negative-ttl <#conf-narinfo-cache-negative-ttl>

The TTL in seconds for negative lookups. If a store path is queried
from a substituter <#conf-substituters> but was not found, a nega-
tive lookup is cached in the local disk cache database for the spec-
ified duration.

Set to 0 to force updating the lookup cache.

To wipe the lookup cache completely:

$ rm $HOME/.cache/nix/binary-cache-v*.sqlite*
# rm /root/.cache/nix/binary-cache-v*.sqlite*

Default: 3600

• narinfo-cache-positive-ttl <#conf-narinfo-cache-positive-ttl>

The TTL in seconds for positive lookups. If a store path is queried
from a substituter, the result of the query is 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.

Default: 2592000

• netrc-file <#conf-netrc-file>

If set to an absolute path to a netrc file, Nix uses the HTTP au-
thentication credentials in this file when trying to download from a
remote host through HTTP or HTTPS. Defaults to $NIX_CONF_DIR/netrc.

The netrc file consists of a list of accounts in the following for-
mat:

machine my-machine login my-username password my-password

For the exact syntax, see the curl documentation
<https://ec.haxx.se/usingcurl-netrc.html>.

Note

This must be an absolute path, and ~ is not resolved. For ex-
ample, ~/.netrc won't resolve to your home directory's
.netrc.

Default: /tmp/dummy/netrc

• nix-path <#conf-nix-path>

List of search paths to use for lookup path
<@docroot@/language/constructs/lookup-path.md> resolution. This
setting determines the value of builtins.nixPath
<@docroot@/language/builtins.md#builtins-nixPath> and can be used
with builtins.findFile <@docroot@/language/builtins.md#builtins-
findFile>.

• The configuration setting is overridden by the NIX_PATH
<@docroot@/command-ref/env-common.md#env-NIX_PATH> environment
variable.
• NIX_PATH is overridden by specifying the setting as the command
line flag <@docroot@/command-ref/conf-file.md#command-line-flags>
--nix-path.
• Any current value is extended by the -I option
<@docroot@/command-ref/opt-common.md#opt-I> or --extra-nix-path.

If the respective paths are accessible, the default values are:

• $HOME/.nix-defexpr/channels

The user channel link <@docroot@/command-ref/files/default-nix-ex-
pression.md#user-channel-link>, pointing to the current state of
channels <@docroot@/command-ref/files/channels.md> for the current
user.

• nixpkgs=$NIX_STATE_DIR/profiles/per-user/root/channels/nixpkgs

The current state of the nixpkgs channel for the root user.

• $NIX_STATE_DIR/profiles/per-user/root/channels

The current state of all channels for the root user.

These files are set up by the Nix installer <@docroot@/installa-
tion/installing-binary.md>. See NIX_STATE_DIR <@docroot@/command-
ref/env-common.md#env-NIX_STATE_DIR> for details on the environment
variable.

Note

If restricted evaluation <@docroot@/command-ref/conf-
file.md#conf-restrict-eval> is enabled, the default value is
empty.

If pure evaluation <#conf-pure-eval> is enabled,
builtins.nixPath always evaluates to the empty list [ ].

Default: machine-specific

• nix-shell-always-looks-for-shell-nix <#conf-nix-shell-always-looks-
for-shell-nix>

Before Nix 2.24, nix-shell <@docroot@/command-ref/nix-shell.md>
would only look at shell.nix if it was in the working directory -
when no file was specified.

Since Nix 2.24, nix-shell always looks for a shell.nix, whether
that's in the working directory, or in a directory that was passed
as an argument.

You may set this to false to temporarily revert to the behavior of
Nix 2.23 and older.

Using this setting is not recommended. It will be deprecated and
removed.

Default: true

• nix-shell-shebang-arguments-relative-to-script <#conf-nix-shell-
shebang-arguments-relative-to-script>

Before Nix 2.24, relative file path expressions in arguments in a
nix-shell shebang were resolved relative to the working directory.

Since Nix 2.24, nix-shell resolves these paths in a manner that is
relative to the base directory <@docroot@/glossary.md#gloss-base-di-
rectory>, defined as the script's directory.

You may set this to false to temporarily revert to the behavior of
Nix 2.23 and older.

Using this setting is not recommended. It will be deprecated and
removed.

Default: true

• plugin-files <#conf-plugin-files>

A list of plugin files to be loaded by Nix. Each of these files is
dlopened by Nix. If they contain the symbol nix_plugin_entry(), this
symbol is called. Alternatively, they can 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
nix command, and RegisterSetting to add new nix config settings. See
the constructors for those types for more details.

Warning! These APIs are inherently unstable and may change from re-
lease to release.

Since these files are loaded into the same address space as Nix it-
self, 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
libraries directly, as those are already available at load time.

If an entry in the list is a directory, all files in the directory
are loaded as plugins (non-recursively).

Default: empty

• post-build-hook <#conf-post-build-hook>

Optional. The path to a program to execute after each build.

This option is only settable in the global nix.conf, or on the com-
mand line by trusted users.

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

• The hook executes after an evaluation-time build.

• The hook does not execute on substituted paths.

• The hook's output always goes to the user's terminal.

• If the hook fails, the build succeeds but no further builds exe-
cute.

• The hook executes synchronously, and blocks other builds from
progressing while it runs.

The program executes with no arguments. The program's environment
contains the following environment variables:

• DRV_PATH The derivation for the built paths.

Example: /nix/store/5nihn1a7pa8b25l9zafqaqibznlvvp3f-
bash-4.4-p23.drv

• OUT_PATHS Output paths of the built derivation, separated by a
space character.

Example: /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.

Default: empty

• pre-build-hook <#conf-pre-build-hook>

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.

The hook is passed the derivation path and, if sandboxes are en-
abled, 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:

• extra-sandbox-paths
Pass a list of files and directories to be included in the sand-
box for this build. One entry per line, terminated by an empty
line. Entries have the same format as sandbox-paths.

Default: empty

• preallocate-contents <#conf-preallocate-contents>

Whether to preallocate files when writing objects with known size.

Default: false

• print-missing <#conf-print-missing>

Whether to print what paths need to be built or downloaded.

Default: true

• pure-eval <#conf-pure-eval>

Pure evaluation mode ensures that the result of Nix expressions is
fully determined by explicitly declared inputs, and not influenced
by external state:

• Restrict file system and network access to files specified by
cryptographic hash
• Disable impure constants:
• builtins.currentSystem <@docroot@/language/builtins.md#builtins-
currentSystem>
• builtins.currentTime <@docroot@/language/builtins.md#builtins-
currentTime>
• builtins.nixPath <@docroot@/language/builtins.md#builtins-
nixPath>
• builtins.storePath <@docroot@/language/builtins.md#builtins-
storePath>

Default: false

• require-drop-supplementary-groups <#conf-require-drop-supplementary-
groups>

Following the principle of least privilege, Nix attempts to drop
supplementary groups when building with sandboxing.

However this can fail under some circumstances. For example, if the
user lacks the CAP_SETGID capability. Search setgroups(2) for EPERM
to find more detailed information on this.

If you encounter such a failure, setting this option to false en-
ables you to ignore it and continue. But before doing so, you
should consider the security implications carefully. Not dropping
supplementary groups means the build sandbox is less restricted than
intended.

This option defaults to true when the user is root (since root usu-
ally has permissions to call setgroups) and false otherwise.

Default: true

• require-sigs <#conf-require-sigs>

If set to true (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 signature by a trusted key. A trusted key is one
listed in trusted-public-keys, or a public key counterpart to a pri-
vate key stored in a file listed in secret-key-files.

Set to false to disable signature checking and trust all non-con-
tent-addressed paths unconditionally.

(Content-addressed paths are inherently trustworthy and thus unaf-
fected by this configuration option.)

Default: true

• restrict-eval <#conf-restrict-eval>

If set to true, the Nix evaluator doesn't allow access to any files
outside of builtins.nixPath <@docroot@/lan-
guage/builtins.md#builtins-nixPath>, or to URIs outside of allowed-
uris <@docroot@/command-ref/conf-file.md#conf-allowed-uris>.

Default: false

• run-diff-hook <#conf-run-diff-hook>

If true, enable the execution of the diff-hook program.

When using the Nix daemon, run-diff-hook must be set in the nix.conf
configuration file, and cannot be passed at the command line.

Default: false

• sandbox <#conf-sandbox>

If set to true, builds are performed in a sandboxed environment,
i.e., theyre isolated from the normal file system hierarchy and only
see their dependencies in the Nix store, the temporary build direc-
tory, private versions of /proc, /dev, /dev/shm and /dev/pts (on
Linux), and the paths configured with the sandbox-paths option. This
is useful to prevent undeclared dependencies on files in directories
such as /usr/bin. 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 net-
work).

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
build users feature to perform the actual builds under different
users than root).

If this option is set to relaxed, then fixed-output derivations and
derivations that have the __noChroot attribute set to true do not
run in sandboxes.

The default is true on Linux and false on all other platforms.

Default: false

Deprecated alias: build-use-chroot, build-use-sandbox

• sandbox-build-dir <#conf-sandbox-build-dir>

Linux only

The build directory inside the sandbox.

This directory is backed by build-dir <#conf-build-dir> on the host.

Default: /build

• sandbox-fallback <#conf-sandbox-fallback>

Whether to disable sandboxing when the kernel doesn't allow it.

Default: true

• sandbox-paths <#conf-sandbox-paths>

A list of paths bind-mounted into Nix sandbox environments. You can
use the syntax target=source to mount a path in a different location
in the sandbox; for instance, /bin=/nix-bin mounts the path /nix-bin
as /bin inside the sandbox. If source is followed by ?, then it is
not an error if source does not exist; for example, /dev/nvidiactl?
specifies that /dev/nvidiactl only be mounted in the sandbox if it
exists in the host filesystem.

If the source is in the Nix store, then its closure is added to the
sandbox as well.

Depending on how Nix was built, the default value for this option
may be empty or provide /bin/sh as a bind-mount of bash.

Default: empty

Deprecated alias: build-chroot-dirs, build-sandbox-paths

• secret-key-files <#conf-secret-key-files>

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

Default: empty

• show-trace <#conf-show-trace>

Whether Nix should print out a stack trace in case of Nix expression
evaluation errors.

Default: false

• ssl-cert-file <#conf-ssl-cert-file>

The path of a file containing CA certificates used to authenticate
https:// downloads. Nix by default uses the first of the following
files that exists:

1. /etc/ssl/certs/ca-certificates.crt
2. /nix/var/nix/profiles/default/etc/ssl/certs/ca-bundle.crt

The path can be overridden by the following environment variables,
in order of precedence:

1. NIX_SSL_CERT_FILE
2. SSL_CERT_FILE

Default: machine-specific

• stalled-download-timeout <#conf-stalled-download-timeout>

The timeout (in seconds) for receiving data from servers during
download. Nix cancels idle downloads after this timeout's duration.

Default: 300

• start-id <#conf-start-id>

The first UID and GID to use for dynamic ID allocation.

Default: 56930

• store <#conf-store>

The URL of the Nix store <@docroot@/store/types/index.md#store-url-
format> to use for most operations. See the Store Types
<@docroot@/store/types/index.md> section of the manual for supported
store types and settings.

Default: auto

• substitute <#conf-substitute>

If set to true (default), Nix uses binary substitutes if available.
This option can be disabled to force building from source.

Default: true

Deprecated alias: build-use-substitutes

• substituters <#conf-substituters>

A list of URLs of Nix stores <@docroot@/store/types/index.md#store-
url-format> to be used as substituters, separated by whitespace. A
substituter is an additional store <@docroot@/glossary.md#gloss-
store> from which Nix can obtain store objects
<@docroot@/store/store-object.md> instead of building them.

Substituters are tried based on their priority value, which each
substituter can set independently. Lower value means higher prior-
ity. The default is https://cache.nixos.org, which has a priority
of 40.

At least one of the following conditions must be met for Nix to use
a substituter:

• The substituter is in the trusted-substituters <#conf-trusted-
substituters> list
• The user calling Nix is in the trusted-users <#conf-trusted-
users> list

In addition, each store path should be trusted as described in
trusted-public-keys <#conf-trusted-public-keys>

Default: https://cache.nixos.org/

Deprecated alias: binary-caches

• sync-before-registering <#conf-sync-before-registering>

Whether to call sync() before registering a path as valid.

Default: false

• system <#conf-system>

The system type of the current Nix installation. Nix only builds a
given store derivation <@docroot@/glossary.md#gloss-store-
derivation> locally when its system attribute equals any of the val-
ues specified here or in extra-platforms <#conf-extra-platforms>.

The default value is set when Nix itself is compiled for the system
it will run on. The following system types are widely used, as Nix
is actively supported on these platforms:

• x86_64-linux
• x86_64-darwin
• i686-linux
• aarch64-linux
• aarch64-darwin
• armv6l-linux
• armv7l-linux

In general, you do not have to modify this setting. While you can
force Nix to run a Darwin-specific builder executable on a Linux ma-
chine, the result would obviously be wrong.

This value is available in the Nix language as builtins.currentSys-
tem <@docroot@/language/builtins.md#builtins-currentSystem> if the
eval-system <#conf-eval-system> configuration option is set as the
empty string.

Default: x86_64-freebsd

• system-features <#conf-system-features>

A set of system features supported by this machine.

This complements the system <#conf-system> and extra-platforms
<#conf-extra-platforms> configuration options and the corresponding
system <@docroot@/language/derivations.md#attr-system> attribute on
derivations.

A derivation can require system features in the requiredSystemFea-
tures attribute <@docroot@/language/advanced-attributes.md#adv-attr-
requiredSystemFeatures>, and the machine to build the derivation
must have them.

System features are user-defined, but Nix sets the following de-
faults:

• apple-virt

Included on Darwin if virtualization is available.

• kvm

Included on Linux if /dev/kvm is accessible.

• nixos-test, benchmark, big-parallel

These historical pseudo-features are always enabled for backwards
compatibility, as they are used in Nixpkgs to route Hydra builds to
specific machines.

• ca-derivations

Included by default if the ca-derivations experimental feature
<@docroot@/development/experimental-features.md#xp-feature-ca-
derivations> is enabled.

This system feature is implicitly required by derivations with the
__contentAddressed attribute <@docroot@/language/advanced-attrib-
utes.md#adv-attr-__contentAddressed>.

• recursive-nix

Included by default if the recursive-nix experimental feature
<@docroot@/development/experimental-features.md#xp-feature-
recursive-nix> is enabled.

• uid-range

On Linux, Nix can run builds in a user namespace where they run as
root (UID 0) and have 65,536 UIDs available. This is primarily use-
ful for running containers such as systemd-nspawn inside a Nix
build. For an example, see [tests/systemd-nspawn/nix][nspawn].

[nspawn]:
<https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-
nspawn.nix>

Included by default on Linux if the auto-allocate-uids <#conf-auto-
allocate-uids> setting is enabled.

Default: machine-specific

• tarball-ttl <#conf-tarball-ttl>

The number of seconds a downloaded tarball is considered fresh. If
the cached tarball is stale, Nix checks whether it is still up to
date using the ETag header. Nix downloads a new version if the ETag
header is unsupported, or the cached ETag doesn't match.

Setting the TTL to 0 forces Nix to always check if the tarball is up
to date.

Nix caches tarballs in $XDG_CACHE_HOME/nix/tarballs.

Files fetched via NIX_PATH, fetchGit, fetchMercurial, fetchTarball,
and fetchurl respect this TTL.

Default: 3600

• timeout <#conf-timeout>

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 --timeout command line switch.

The value 0 means that there is no timeout. This is also the de-
fault.

Default: 0

Deprecated alias: build-timeout

• trace-function-calls <#conf-trace-function-calls>

If set to true, the Nix evaluator traces every function call. Nix
prints a log message at the "vomit" level for every function en-
trance and function exit.

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/.../exam-
ple.nix:226:41 at 1565795253249941684

The undefined position means the function call is a builtin.

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

Default: false

• trace-import-from-derivation <#conf-trace-import-from-derivation>

By default, Nix allows Import from Derivation <@docroot@/lan-
guage/import-from-derivation.md>.

When this setting is true, Nix logs a warning indicating that it
performed such an import. This option has no effect if allow-im-
port-from-derivation is disabled.

Default: false

• trace-verbose <#conf-trace-verbose>

Whether builtins.traceVerbose should trace its first argument when
evaluated.

Default: false

• trust-tarballs-from-git-forges <#conf-trust-tarballs-from-git-
forges>

If enabled (the default), Nix considers tarballs from GitHub and
similar Git forges to be locked if a Git revision is specified, e.g.
github:NixOS/patchelf/7c2f768bf9601268a4e71c2ebe91e2011918a70f.
This requires Nix to trust that the provider returns the correct
contents for the specified Git revision.

If disabled, such tarballs are only considered locked if a narHash
attribute is specified, e.g.
github:NixOS/patchelf/7c2f768bf9601268a4e71c2ebe91e2011918a70f?narHash=sha256-PPX-
qKY2hJng4DBVE0I4xshv/vGLUskL7jl53roB8UdU%3D.

Default: true

• trusted-public-keys <#conf-trusted-public-keys>

A whitespace-separated list of public keys.

At least one of the following condition must be met for Nix to ac-
cept copying a store object from another Nix store (such as a sub-
stituter <#conf-substituters>):

• the store object has been signed using a key in the trusted keys
list
• the require-sigs <#conf-require-sigs> option has been set to
false
• the store URL is configured with trusted=true
• the store object is content-addressed
<@docroot@/glossary.md#gloss-content-addressed-store-object>

Default: cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcD-
ShjY=

Deprecated alias: binary-cache-public-keys

• trusted-substituters <#conf-trusted-substituters>

A list of Nix store URLs <@docroot@/store/types/index.md#store-url-
format>, separated by whitespace. These are not used by default,
but users of the Nix daemon can enable them by specifying substi-
tuters <#conf-substituters>.

Unprivileged users (those set in only allowed-users <#conf-allowed-
users> but not trusted-users <#conf-trusted-users>) can pass as sub-
stituters only those URLs listed in trusted-substituters.

Default: empty

Deprecated alias: trusted-binary-caches

• trusted-users <#conf-trusted-users>

A list of user names, separated by whitespace. These users will
have additional rights when connecting to the Nix daemon, such as
the ability to specify additional substituters <#conf-substituters>,
or to import unsigned realisations or unsigned input-addressed store
objects.

You can also specify groups by prefixing names with @. For in-
stance, @wheel means all users in the wheel group.

Warning

Adding a user to trusted-users is essentially equivalent to
giving that user root access to the system. For example, the
user can access or replace store path contents that are crit-
ical for system security.

Default: root

• upgrade-nix-store-path-url <#conf-upgrade-nix-store-path-url>

Used by nix upgrade-nix, the URL of the file that contains the store
paths of the latest Nix release.

Default: https://github.com/NixOS/nixpkgs/raw/master/nixos/mod-
ules/installer/tools/nix-fallback-paths.nix

• use-case-hack <#conf-use-case-hack>

Whether to enable a macOS-specific hack for dealing with file name
case collisions.

Default: false

• use-registries <#conf-use-registries>

Warning

This setting is part of an experimental feature <@doc-
root@/development/experimental-features.md>.

extra-experimental-features = flakes
use-registries = ...

Whether to use flake registries to resolve flake references.

Default: true

• use-sqlite-wal <#conf-use-sqlite-wal>

Whether SQLite should use WAL mode.

Default: true

• use-xdg-base-directories <#conf-use-xdg-base-directories>

If set to true, Nix conforms to the XDG Base Directory Specification
<https://specifications.freedesktop.org/basedir-spec/basedir-spec-
latest.html> for files in $HOME. The environment variables used to
implement this are documented in the Environment Variables section
<@docroot@/command-ref/env-common.md>.

Warning This changes the location of some well-known symlinks
that Nix creates, which might break tools that rely on the
old, non-XDG-conformant locations.

If you already have Nix installed and are using profiles
<@docroot@/package-management/profiles.md> or channels <@doc-
root@/command-ref/nix-channel.md>, you should migrate manually when
you enable this option. If $XDG_STATE_HOME is not set, use
$HOME/.local/state/nix instead of $XDG_STATE_HOME/nix. This can be
achieved with the following shell commands:

nix_state_home=${XDG_STATE_HOME-$HOME/.local/state}/nix
mkdir -p $nix_state_home
mv $HOME/.nix-profile $nix_state_home/profile
mv $HOME/.nix-defexpr $nix_state_home/defexpr
mv $HOME/.nix-channels $nix_state_home/channels

Default: false

• user-agent-suffix <#conf-user-agent-suffix>

String appended to the user agent in HTTP requests.

Default: empty

• warn-dirty <#conf-warn-dirty>

Whether to warn about dirty Git/Mercurial trees.

Default: true

• warn-large-path-threshold <#conf-warn-large-path-threshold>

Warn when copying a path larger than this number of bytes to the Nix
store (as determined by its NAR serialisation). Default is 0, which
disables the warning. Set it to 1 to warn on all paths.

Default: 0

• warn-short-path-literals <#conf-warn-short-path-literals>

If set to true, the Nix evaluator will warn when encountering rela-
tive path literals that don't start with ./ or ../.

For example, with this setting enabled, foo/bar would emit a warning
suggesting to use ./foo/bar instead.

This is useful for improving code readability and making path liter-
als more explicit.

Default: false

nix.conf(5)

Name | Description | Available settings

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=nix.conf&sektion=5&manpath=FreeBSD+Ports+15.0.quarterly>

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