nix.conf - Nix configuration file


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.


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).

Available settings

  • accept-flake-config


    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 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

    • Gitlab: the token value is either the OAuth2 token or the Personal Access Token (these are different types tokens for gitlab, see 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 =

    Example ~/code/flake.nix: = {
      type = "gitlab";
      host = "";
      owner = "mycompany";
      repo = "pro";

    This example specifies three tokens, one each for accessing,, and

    The 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-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

    Whether builtin functions that allow executing native code should be enabled.

    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, builtin functions such as fetchGit are allowed to access

    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 *.


    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


    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-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.


    Change this setting only if you really know what you’re doing.

    Default: empty

  • 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 @.


    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.



      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.



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



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



    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.


    Multiple builders specified on the command line:

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


    This specifies several machines that can perform i686-linux builds: i686-linux /home/nix/.ssh/id_scratchy 8 1 kvm    i686-linux /home/nix/.ssh/id_scratchy 8 2  i686-linux /home/nix/.ssh/id_scratchy 1 2 kvm benchmark

    However, poochie will only build derivations that have the attribute

    requiredSystemFeatures = [ "benchmark" ];


    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.


    A build machine must be accessible via SSH and have Nix installed. nix must be available in $PATH for the user connecting over SSH.


    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 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-lockfile-summary


    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-lockfile-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

  • 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.


    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, builtins.trace will enter the debugger like builtins.break.

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

    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-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.

    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.


    experimental-features = nix-command flakes

    The following experimental features are available:

    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


    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.


  • 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

  • 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, when building the derivation

    builtins.fetchurl {
      url = "";
      sha256 = "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae";

    Nix will attempt to download this file from 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


    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: -1

  • 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.


    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.


    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 default value is


    It can be overridden with the NIX_PATH environment variable or the -I command line option.


    If pure evaluation is enabled, nixPath evaluates to the empty list [ ].

    Default: machine-specific

  • plugin-files

    A list of plugin files to be loaded by Nix. Each of these files will be dlopened by Nix, allowing them to affect execution through static initialization. In particular, these plugins may construct static instances of RegisterPrimOp to add new primops or constants to the expression language, RegisterStoreImplementation to add new store implementations, RegisterCommand to add new subcommands to the 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:

    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

    The build directory inside the sandbox.

    Default: /build

  • sandbox-dev-shm-size

    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:


    Default: empty

  • 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, which has a priority of 40.

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

    In addition, each store path should be trusted as described in trusted-public-keys


    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.

      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/ script distributed with the Nix source code to convert the trace logs in to a format suitable for

    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 object is content-addressed


    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 NARs.

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


    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.


  • use-case-hack

    Whether to enable a Darwin-specific hack for dealing with file name 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


    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:


    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:

    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