Provided by: nix-bin_2.26.3+dfsg-1ubuntu4_amd64 
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 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 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. If unset,
XDG_CONFIG_DIRS defaults to /etc/xdg, and XDG_CONFIG_HOME defaults to $HOME/.config as per XDG Base
Directory Specification.
3. If 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. Comments 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:
• 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 alice).
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
If set to true, builtins.warn will throw 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.
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 environment.
Default: false
• accept-flake-config
Warning
This setting is part of an experimental feature.
To change this setting, make sure the flakes experimental feature is enabled. For example,
include the following in nix.conf:
extra-experimental-features = flakes
accept-flake-config = ...
Whether to accept Nix configuration settings from a flake without prompting.
Default: false
• access-tokens
Access tokens used to access protected GitHub, GitLab, or other locations 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
actual use of the token value is determined by the type of resource being accessed:
• 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 Personal Access Token (these are
different types tokens for gitlab, see
https://docs.gitlab.com/12.10/ee/api/README.html#authentication). 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
Whether to allow dirty Git/Mercurial trees.
Default: true
• allow-dirty-locks
Warning
This setting is part of an experimental feature.
To change this setting, make sure the flakes experimental feature is enabled. For example,
include the following in nix.conf:
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
By default, Nix allows Import from Derivation.
With this option set to false, Nix will throw an error when evaluating an expression that uses this
feature, even when the required store object is readily available. This ensures that evaluation will
not require any builds to take place, regardless of the state of the store.
Default: true
• allow-new-privileges
(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 sudo or ping will
fail. (Note that in sandbox builds, no such programs are available unless you bind-mount them into the
sandbox via the sandbox-paths 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).
Default: false
• allow-symlinked-store
If set to true, Nix will stop complaining if the store directory (typically /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
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 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
Which prefixes to allow derivations to ask for access to (primarily for Darwin).
Default: empty
• 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
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) can always connect to the Nix daemon.
Default: *
• always-allow-substitutes
If set to true, Nix will ignore the allowSubstitutes attribute in derivations and always attempt to
use available substituters.
Default: false
• auto-allocate-uids
Warning
This setting is part of an experimental feature.
To change this setting, make sure the auto-allocate-uids experimental feature is enabled. For
example, include the following in nix.conf:
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
If set to true, 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 false (the default),
you can still run nix-store --optimise to get rid of duplicate files.
Default: false
• bash-prompt
The bash prompt (PS1) in nix develop shells.
Default: empty
• bash-prompt-prefix
Prefix prepended to the PS1 environment variable in nix develop shells.
Default: empty
• bash-prompt-suffix
Suffix appended to the PS1 environment variable in nix develop shells.
Default: empty
• build-dir
The directory on the host, in which derivations’ temporary build directories are created.
If not set, Nix will use the system temporary directory indicated by the TMPDIR environment variable.
Note that builds are often performed by the Nix daemon, so its TMPDIR is used, and not that of the Nix
command line interface.
This is also the location where --keep-failed leaves its files.
If Nix runs without sandbox, or if the platform does not support sandboxing with bind mounts (e.g.
macOS), then the builder’s environment will contain this directory, instead of the virtual location
sandbox-build-dir.
Default: ``
• build-hook
The path to the helper program that executes remote builds.
Nix communicates with the build hook over stdio using a custom protocol to request builds that cannot
be performed directly by the Nix daemon. The default value is the internal Nix binary that implements
remote building.
Important
Change this setting only if you really know what you’re doing.
Default: nix __build-remote
• build-poll-interval
How often (in seconds) to poll for locks.
Default: 5
• build-users-group
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.
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 /etc/group). Those user
accounts should not be used for any other purpose!
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.)
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 will be performed 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
daemon). Obviously, this should not be used with a nix daemon accessible to untrusted clients.
Defaults to nixbld when running as root, empty otherwise.
Default: machine-specific
• builders
A semicolon- or newline-separated list of build machines.
In addition to the usual ways of setting configuration options, 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, separated 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@]hostname.
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. 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 will use its
regular identities.
Example
/home/user/.ssh/id_mac
4. The maximum number of builds that Nix will execute 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 will prefer the fastest, taking load into account.
6. A comma-separated list of supported system features.
A machine will only be used to build a derivation if all the features in the derivation’s
requiredSystemFeatures attribute are supported by that machine.
7. A comma-separated list of required system features.
A machine will only be used to build a derivation if all of the machine’s required features appear
in the derivation’s requiredSystemFeatures attribute.
8. The (base64-encoded) public host key of the remote machine. If omitted, SSH will use 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 will only build derivations that have the attribute
requiredSystemFeatures = [ "benchmark" ];
or
requiredSystemFeatures = [ "benchmark" "kvm" ];
itchy cannot do builds that require kvm, but scratchy does support such builds. For regular
builds, itchy will be preferred over scratchy because it has a higher speed factor.
For Nix to use substituters, the calling user must be in the trusted-users list.
Note
A build machine must be accessible via SSH and have Nix installed. nix must be available in
$PATH for the user connecting over SSH.
Warning
If you are building via the Nix daemon (default), the Nix daemon user account on the local
machine (that is, root) requires access to a user account on the remote machine (not
necessarily root).
If you can’t or don’t want to configure root to be able to access the remote machine, set store
to any local store, e.g. by passing --store /tmp to the command on the local machine.
To build only on remote machines and disable local builds, set max-jobs to 0.
If you want the remote machines to use substituters, set builders-use-substitutes to true.
Default: machine-specific
• builders-use-substitutes
If set to true, Nix will instruct remote build machines to use their own substituters if available.
It means that remote build hosts will fetch as many dependencies as possible from their own
substituters (e.g, from cache.nixos.org) instead 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
Warning
This setting is part of an experimental feature.
To change this setting, make sure the flakes experimental feature is enabled. For example,
include the following in nix.conf:
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
If set to true (the default), build logs written to /nix/var/log/nix/drvs will be compressed on the
fly using bzip2. Otherwise, they will not be compressed.
Default: true
Deprecated alias: build-compress-log
• connect-timeout
The timeout (in seconds) for establishing connections in the binary cache substituter. It corresponds
to curl’s --connect-timeout option. A value of 0 means no limit.
Default: 0
• cores
Sets the value of the NIX_BUILD_CORES environment variable in the invocation of the builder executable
of a derivation. 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 will pass the -j${NIX_BUILD_CORES} flag to GNU Make.
The value 0 means that the builder should use all available CPU cores in the system.
Note
The number of parallel local Nix build jobs is independently controlled with the max-jobs
setting.
Default: machine-specific
Deprecated alias: build-cores
• debugger-on-trace
If set to true and the --debugger flag is given, the following functions will enter the debugger like
builtins.break.
• builtins.trace
• builtins.traceVerbose if trace-verbose is set to true.
• builtins.warn
This is useful for debugging warnings in third-party Nix code.
Default: false
• debugger-on-warn
If set to true and the --debugger flag is given, builtins.warn will enter the debugger like
builtins.break.
This is useful for debugging warnings in third-party Nix code.
Use debugger-on-trace to also enter the debugger on legacy warnings that are logged with
builtins.trace.
Default: false
• 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
determine 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 will exist only if the build was run with
--keep-failed.
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.
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
How often Nix will attempt to download a file before giving up.
Default: 5
• 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
Specify the maximum transfer rate in kilobytes per second you want Nix to use for downloads.
Default: 0
• 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-system
This option defines 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 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
Experimental features that are enabled.
Example:
experimental-features = nix-command flakes
The following experimental features are available:
{{#include experimental-features-shortlist.md}}
Experimental features are further documented in the manual.
Default: empty
• extra-platforms
System types of executables that can be run on this machine.
Nix will only build a given derivation locally when its system attribute equals any of the values
specified here or in the system option.
Setting this can be useful to build derivations locally on compatible 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 will 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
derivation’s builder accordingly and cross-check the results when using this option against natively-
built versions of your derivation.
Default: machine-specific
• fallback
If set to true, Nix will fall back to building from source if a binary substitute fails. This is
equivalent to the --fallback flag. The default is false.
Default: false
Deprecated alias: build-fallback
• filter-syscalls
Whether to prevent certain dangerous system calls, such as creation of setuid/setgid files or adding
ACLs or extended attributes. Only disable this if you’re aware of the security implications.
Default: true
• flake-registry
Warning
This setting is part of an experimental feature.
To change this setting, make sure the flakes experimental feature is enabled. For example,
include the following in nix.conf:
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
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
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
Amount of reserved disk space for the garbage collector.
Default: 8388608
• 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 will try 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://tarballs.nixos.org/sha256/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae
first. If it is not available there, if will try the original URI.
Default: empty
• 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
Whether to enable HTTP/2 support.
Default: true
• id-count
The number of UIDs/GIDs to use for dynamic ID allocation.
Default: 8388608
• ignore-try
If set to true, ignore exceptions inside ‘tryEval’ calls when evaluating nix expressions in debug mode
(using the –debugger flag). By default the debugger will pause on all exceptions.
Default: false
• ignored-acls
A list of ACLs that should be ignored, normally Nix attempts to remove all ACLs from files and
directories in the Nix store, but some ACLs like security.selinux or system.nfs4_acl can’t be removed
even by root. Therefore it’s best to just ignore them.
Default: security.csm security.selinux system.nfs4_acl
• impersonate-linux-26
Whether to impersonate a Linux 2.6 machine on newer kernels.
Default: false
Deprecated alias: build-impersonate-linux-26
• impure-env
Warning
This setting is part of an experimental feature.
To change this setting, make sure the configurable-impure-env experimental feature is enabled.
For example, include the following in nix.conf:
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 will be passed to the builder if they are listed in
impureEnvVars.
This option is useful for, e.g., setting https_proxy for fixed-output derivations and in a multi-user
Nix installation, or setting private access tokens when fetching a private repository.
Default: empty
• keep-build-log
If set to true (the default), Nix will write the build log of a derivation (i.e. the standard output
and error of its builder) to the directory /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
If true (default), the garbage collector will keep the derivations from which non-garbage store paths
were built. If false, they will be 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
If false (default), derivations are not stored in Nix user environments. 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 will not be garbage-collected until the user environment
generation 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
moment the garbage collector is run.
Default: false
Deprecated alias: env-keep-derivations
• keep-failed
Whether to keep temporary directories of failed builds.
Default: false
• keep-going
Whether to keep building derivations when another build fails.
Default: false
• keep-outputs
If true, the garbage collector will keep the outputs of non-garbage derivations. If false (default),
outputs will be 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 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 true.
Default: false
Deprecated alias: gc-keep-outputs
• log-lines
The number of lines of the tail of the log to show if a build fails.
Default: 25
• 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, it’s killed. A value of 0 (the default) means that there is no limit.
Default: 0
Deprecated alias: build-max-log-size
• max-call-depth
The maximum function call depth to allow before erroring.
Default: 10000
• 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 infinity (i.e. delete all garbage).
Default: 9223372036854775807
• max-jobs
Maximum number of jobs that Nix will try 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. This will not affect derivations
that have preferLocalBuild = true, which are always built locally.
Note
The number of CPU cores to use for each build job is independently determined by the 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
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 default.
Default: 0
Deprecated alias: build-max-silent-time
• max-substitution-jobs
This option defines the maximum number of substitution jobs that Nix will try to run in parallel. The
default is 16. The minimum value one can choose is 1 and lower values will be interpreted as 1.
Default: 16
Deprecated alias: substitution-max-jobs
• 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
Number of seconds between checking free disk space.
Default: 5
• nar-buffer-size
Maximum size of NARs before spilling them to disk.
Default: 33554432
• narinfo-cache-negative-ttl
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.
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
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.
Default: 2592000
• netrc-file
If set to an absolute path to a netrc 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
$NIX_CONF_DIR/netrc.
The netrc file consists of a list of accounts in the following format:
machine my-machine login my-username password my-password
For the exact syntax, see the curl documentation.
Note
This must be an absolute path, and ~ is not resolved. For example, ~/.netrc won’t resolve to
your home directory’s .netrc.
Default: /dummy/netrc
• nix-path
List of search paths to use for lookup path resolution. This setting determines the value of
builtins.nixPath and can be used with builtins.findFile.
• The configuration setting is overridden by the NIX_PATH environment variable.
• NIX_PATH is overridden by specifying the setting as the command line flag --nix-path.
• Any current value is extended by the -I option or --extra-nix-path.
If the respective paths are accessible, the default values are:
• $HOME/.nix-defexpr/channels
The user channel link, pointing to the current state of channels for the current user.
• nixpkgs=$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. See NIX_STATE_DIR for details on the environment
variable.
Note
If restricted evaluation is enabled, the default value is empty.
If pure evaluation is enabled, builtins.nixPath always evaluates to the empty list [ ].
Default: machine-specific
• nix-shell-always-looks-for-shell-nix
Before Nix 2.24, nix-shell 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
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,
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
A list of plugin files to be loaded by Nix. Each of these files will be dlopened by Nix. If they
contain the symbol nix_plugin_entry(), this symbol will be 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 release to release.
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.
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
Optional. The path to a program to execute after each build.
This option is only settable in the global nix.conf, or on the command 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 execute.
• 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
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 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:
• extra-sandbox-paths
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 sandbox-paths.
Default: empty
• preallocate-contents
Whether to preallocate files when writing objects with known size.
Default: false
• print-missing
Whether to print what paths need to be built or downloaded.
Default: true
• 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
• builtins.currentTime
• builtins.nixPath
• builtins.storePath
Default: false
• require-drop-supplementary-groups
Following the principle of least privilege, Nix will attempt 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 will let you ignore it and continue.
But before doing so, you should consider the security implications carefully. Not dropping
supplementary groups means the build sandbox will be less restricted than intended.
This option defaults to true when the user is root (since root usually has permissions to call
setgroups) and false otherwise.
Default: false
• 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 private key stored in a file listed in
secret-key-files.
Set to false to disable signature checking and trust all non-content-addressed paths unconditionally.
(Content-addressed paths are inherently trustworthy and thus unaffected by this configuration option.)
Default: true
• restrict-eval
If set to true, the Nix evaluator will not allow access to any files outside of builtins.nixPath, or
to URIs outside of allowed-uris.
Default: false
• 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
If set to true, builds will be performed in a sandboxed environment, 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 /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 network).
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: true
Deprecated alias: build-use-chroot, build-use-sandbox
• sandbox-build-dir
Linux only
The build directory inside the sandbox.
This directory is backed by build-dir on the host.
Default: /build
• sandbox-dev-shm-size
Linux only
This option determines the maximum size of the tmpfs filesystem mounted on /dev/shm in Linux
sandboxes. For the format, see the description of the size option of tmpfs in mount(8). The default is
50%.
Default: 50%
• sandbox-fallback
Whether to disable sandboxing when the kernel doesn’t allow it.
Default: true
• 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 will mount 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 will only be mounted in the
sandbox if it exists in the host filesystem.
If the source is in the Nix store, then its closure will be 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
A whitespace-separated list of files containing secret (private) keys. These are used to sign locally-
built paths. They can be generated 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
Whether Nix should print out a stack trace in case of Nix expression evaluation errors.
Default: false
• ssl-cert-file
The path of a file containing CA certificates used to authenticate https:// downloads. Nix by default
will use 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: /etc/ssl/certs/ca-certificates.crt
• 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
The first UID and GID to use for dynamic ID allocation.
Default: 872415232
• store
The URL of the Nix store to use for most operations. See the Store Types section of the manual for
supported store types and settings.
Default: auto
• substitute
If set to true (default), Nix will use binary substitutes if available. This option can be disabled to
force building from source.
Default: true
Deprecated alias: build-use-substitutes
• substituters
A list of URLs of Nix stores to be used as substituters, separated by whitespace. A substituter is an
additional store from which Nix can obtain store objects instead of building them.
Substituters are tried based on their priority value, which each substituter can set independently.
Lower value means higher priority. 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 list
• The user calling Nix is in the trusted-users list
In addition, each store path should be trusted as described in trusted-public-keys
Default: https://cache.nixos.org/
Deprecated alias: binary-caches
• sync-before-registering
Whether to call sync() before registering a path as valid.
Default: false
• system
The system type of the current Nix installation. Nix will only build a given derivation locally when
its system attribute equals any of the values specified here or in 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 machine, the result would obviously be wrong.
This value is available in the Nix language as builtins.currentSystem if the eval-system configuration
option is set as the empty string.
Default: x86_64-linux
• system-features
A set of system “features” supported by this machine.
This complements the system and extra-platforms configuration options and the corresponding system
attribute on derivations.
A derivation can require system features in the requiredSystemFeatures attribute, and the machine to
build the derivation must have them.
System features are user-defined, but Nix sets the following defaults:
• 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 is enabled.
This system feature is implicitly required by derivations with the __contentAddressed attribute.
• recursive-nix
Included by default if the recursive-nix experimental feature 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 useful 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 setting is enabled.
Default: machine-specific
• tarball-ttl
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.
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
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 default.
Default: 0
Deprecated alias: build-timeout
• trace-function-calls
If set to true, 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.
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
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-verbose
Whether builtins.traceVerbose should trace its first argument when evaluated.
Default: false
• trust-tarballs-from-git-forges
If enabled (the default), Nix will consider 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 will return 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-PPXqKY2hJng4DBVE0I4xshv/vGLUskL7jl53roB8UdU%3D.
Default: true
• trusted-public-keys
A whitespace-separated list of public keys.
At least one of the following condition must be met for Nix to accept copying a store object from
another Nix store (such as a substituter):
• the store object has been signed using a key in the trusted keys list
• the require-sigs option has been set to false
• the store URL is configured with trusted=true
• the store object is content-addressed
Default: cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
Deprecated alias: binary-cache-public-keys
• trusted-substituters
A list of Nix store URLs, separated by whitespace. These are not used by default, but users of the
Nix daemon can enable them by specifying substituters.
Unprivileged users (those set in only allowed-users but not trusted-users) can pass as substituters
only those URLs listed in trusted-substituters.
Default: empty
Deprecated alias: trusted-binary-caches
• 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, or to import
unsigned realisations or unsigned input-addressed store objects.
You can also specify groups by prefixing names with @. For instance, @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 critical for
system security.
Default: root
• 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/modules/installer/tools/nix-fallback-
paths.nix
• use-case-hack
Whether to enable a macOS-specific hack for dealing with file name case collisions.
Default: false
• use-cgroups
Whether to execute builds inside cgroups. This is only supported on Linux.
Cgroups are required and enabled automatically for derivations that require the uid-range system
feature.
Default: false
• use-registries
Warning
This setting is part of an experimental feature.
To change this setting, make sure the flakes experimental feature is enabled. For example,
include the following in nix.conf:
extra-experimental-features = flakes
use-registries = ...
Whether to use flake registries to resolve flake references.
Default: true
• use-sqlite-wal
Whether SQLite should use WAL mode.
Default: true
• use-xdg-base-directories
If set to true, Nix will conform to the XDG Base Directory Specification for files in $HOME. The
environment variables used to implement this are documented in the Environment Variables section.
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.
In particular, the following locations change:
┌──────────────────────────────────┬─────────────────────────────────────────────┐
│ Old │ New │
├──────────────────────────────────┼─────────────────────────────────────────────┤
│ ~/.nix-profile │ $XDG_STATE_HOME/nix/profile │
├──────────────────────────────────┼─────────────────────────────────────────────┤
│ ~/.nix-defexpr │ $XDG_STATE_HOME/nix/defexpr │
├──────────────────────────────────┼─────────────────────────────────────────────┤
│ ~/.nix-channels │ $XDG_STATE_HOME/nix/channels │
└──────────────────────────────────┴─────────────────────────────────────────────┘
If you already have Nix installed and are using profiles or channels, 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
String appended to the user agent in HTTP requests.
Default: empty
• warn-dirty
Whether to warn about dirty Git/Mercurial trees.
Default: true
• 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
nix.conf(5)