nix.conf - Lix Reference Manual

# Name

`nix.conf` - Lix configuration file

# Description

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

## Configuration file

By default Lix 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>`. A missing file is an error unless `!include` is used instead.

Paths in `include`s and option values are interpreted relative to the current configuration file. In user configuration files, paths starting with `~/` are tilde expanded (by replacing the tilde by the value of `$HOME`).

Only user configuration files (like `$XDG_CONFIG_HOME/nix/nix.conf` or the files listed in `$NIX_USER_CONF_FILES`) can use tilde paths relative to your home directory. Configuration listed in the `$NIX_CONFIG` environment variable may not use relative paths.

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`

  Whether to accept Lix configuration from the `nixConfig` attribute of a flake. Doing so as a trusted user allows Nix flakes to gain root access on your machine if they set one of the several trusted-user-only settings that execute commands as root.

  If set to `true`, such configuration will be accepted without asking; this is almost always a very bad idea. Setting this to `ask` will prompt the user each time whether to allow a certain configuration option set this way, and offer to optionally remember their choice. When set to `false`, the configuration will be automatically declined.

  See multi-user installations for more details on the Lix security model.

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `flakes`, is enabled. For example, include the following in `nix.conf`:

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

  Default: `ask`

- `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-import-from-derivation`

  By default, Lix allows you to `import` from a derivation, allowing building at evaluation time. With this option set to false, Lix will throw an error when evaluating an expression that uses this feature, allowing users to ensure their evaluation will not require any builds to take place.

  Default: `true`

- `allow-symlinked-store`

  If set to `true`, Lix 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

    Runs function with symbol from a dynamic shared object (DSO) at path. This may be used to add new builtins to the Nix language. The procedure must have the following signature:

        extern "C" typedef void (*ValueInitialiser) (EvalState & state, Value & v);

  - `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`.

  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`, Lix will ignore the `allowSubstitutes` attribute in derivations and always attempt to use available substituters. For more information on `allowSubstitutes`, see the manual chapter on advanced attributes.

  Default: `false`

- `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.

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `auto-allocate-uids`, is enabled. For example, include the following in `nix.conf`:

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

  Default: `false`

- `auto-optimise-store`

  If set to `true`, Lix 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.

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `nix-command`, is enabled. For example, include the following in `nix.conf`:

      extra-experimental-features = nix-command
      bash-prompt = ...

  Default: empty

- `bash-prompt-prefix`

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

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `nix-command`, is enabled. For example, include the following in `nix.conf`:

      extra-experimental-features = nix-command
      bash-prompt-prefix = ...

  Default: empty

- `bash-prompt-suffix`

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

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `nix-command`, is enabled. For example, include the following in `nix.conf`:

      extra-experimental-features = nix-command
      bash-prompt-suffix = ...

  Default: empty

- `build-dir`

  The directory on the host, in which derivations' temporary build directories are created.

  If not set, Lix will use the `b` subdirectory of its configured state directory. Lix will create this directory automatically with suitable permissions if it does not exist, otherwise its permissions must allow all users to traverse the directory (i.e. it must have `o+x` set, in unix parlance) for non-sandboxed builds to work correctly.

  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`.

  > Important:

  > `build-dir` must not be set to a world-writable directory. Placing temporary build directories in a world-writable place allows other users to access or modify build data that is currently in use. This alone is merely an impurity, but combined with another factor this has allowed malicious derivations to escape the build sandbox.

  Default: `«nixStateDir»/b`

- `build-hook`

  The path to the helper program that executes remote builds.

  Lix 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 Lix binary that implements remote building.

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

  > Warning This setting is deprecated and will be removed in a future version of Lix.

  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 Lix build user accounts. In multi-user Lix installations, builds should not be performed by the Lix 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 them 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!

  Lix 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 Lix 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 Lix process (that is, the uid of the caller if both `NIX_REMOTE` is either empty or `auto` and the Nix store is owned by that user, or, alternatively, the uid under which the Nix daemon runs if `NIX_REMOTE` is `daemon` or if it is `auto` and the store is not owned by the caller). Obviously, this should not be used with a nix daemon accessible to untrusted clients.

  For the avoidance of doubt, explicitly setting this to empty with a Lix daemon running as root means that builds will be executed as root with respect to the rest of the system. We intend to fix this: https://git.lix.systems/lix-project/lix/issues/242

  Default: running as root: `nixbld`, otherwise: empty

- `builders`

  A semicolon-separated list of build machines. For the exact format and examples, see the manual chapter on remote builds

  Defaults to `@$NIX_CONF_DIR/machines`. The default shown below is only accurate when the value of `NIX_CONF_DIR` has not been overridden at build time or using the environment variable.

  Default: `@/etc/nix/machines`

- `builders-use-substitutes`

  If set to `true`, Lix will instruct remote build machines to use their own binary substitutes if available. In practical terms, this means that remote hosts will fetch as many build dependencies as possible from their own substitutes (e.g, from `cache.nixos.org`), instead of waiting for this host to upload them all. This can drastically reduce build times if the network connection between this computer and the remote build host is slow.

  Default: `false`

- `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.

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `flakes`, is enabled. For example, include the following in `nix.conf`:

      extra-experimental-features = flakes
      commit-lockfile-summary = ...

  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`

- `cores`

  Sets the value of the `NIX_BUILD_CORES` environment variable in the invocation of builders. Builders can use this variable at their discretion to control the maximum amount of parallelism. For instance, in Nixpkgs, if the derivation attribute `enableParallelBuilding` is set to `true`, the builder passes the `-jN` flag to GNU Make. It can be overridden using the `--cores` command line switch and defaults to `1`. The value `0` means that the builder should use all available CPU cores in the system.

  Default: machine-specific

  Deprecated alias: `build-cores`

- `darwin-log-sandbox-violations`

  Whether to log Darwin sandbox access violations to the system log.

  > Note This setting is only available on Darwin systems.

  Default: `false`

- `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`

- `deprecated-features`

  Deprecated features that are allowed.

  Example:

      deprecated-features = url-literals

  The following deprecated feature features can be re-activated:

  - `ancient-let`
  - `cr-line-endings`
  - `nul-bytes`
  - `rec-set-overrides`
  - `shadow-internal-symbols`
  - `url-literals`

  Deprecated features are further documented in the manual.

  Default: empty

- `diff-hook`

  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: empty

- `download-attempts`

  How often Lix will attempt to download a file before giving up.

  Default: `5`

- `download-speed`

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

  Default: `0`

- `enable-core-dumps`

  If set to `false` (the default), `RLIMIT_CORE` has a soft limit of zero. If set to `true`, the soft limit is infinite.

  The hard limit is always infinite.

  Default: `false`

- `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.

  Example:

      experimental-features = nix-command flakes

  The following experimental features are available:

  - `auto-allocate-uids`
  - `cgroups`
  - `coerce-integers`
  - `daemon-trust-override`
  - `fetch-closure`
  - `flake-self-attrs`
  - `flakes`
  - `lix-custom-sub-commands`
  - `nix-command`
  - `pipe-operator`
  - `read-only-local-store`
  - `repl-automation`

  Experimental features are further documented in the manual.

  Default: empty

- `extra-platforms`

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

  Lix 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`, Lix 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`

- `flake-registry`

  Path or URI of the global flake registry.

  URIs are deprecated. When set to 'vendored', defaults to a vendored copy of https://channels.nixos.org/flake-registry.json.

  When empty, disables the global flake registry.

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `flakes`, is enabled. For example, include the following in `nix.conf`:

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

  Default: `vendored`

- `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 type ht and a base-16 hash h, Lix will try to download the file from hashed-mirror/ht/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";
      }

  Lix 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`

- `http3`

  Whether to enable HTTP/3 support.

  Default: `false`

- `id-count`

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

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `auto-allocate-uids`, is enabled. For example, include the following in `nix.conf`:

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

  Default: Linux: `8388608`, other platforms: `128`

- `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: `true`

- `ignored-acls`

  A list of ACLs that should be ignored, normally Lix 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.

  > Note This setting is only available on Linux systems.

  Default: `security.csm security.selinux system.nfs4_acl`

- `impersonate-linux-26`

  Whether to impersonate a Linux 2.6 machine on newer kernels.

  > Note This setting is only available on Linux systems.

  Default: `false`

  Deprecated alias: `build-impersonate-linux-26`

- `initial-connect-timeout`

  The timeout for the first attempt to establish connections for file transfers such as tarball fetches or binary cache substitutions in seconds.

  Lix increases the timeout per failed attempt via exponential backoff. For attempt `i` (starting at `0`) the timeout is determined by

      timeout := min(max_connect_timeout, initial_connect_timeout * 2^i)

  The value is capped by the option `max-connect-timeout`.

  The option `download-attempts` controls how many attempts to download a file there are before giving up.

  Default: `5`

- `keep-build-log`

  If set to `true` (the default), Lix 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-connect-timeout`

  The maximum timeout for establishing connections for file transfers such as tarball fetches or binary cache substitutions in seconds. This is the maximum value Lix sets for `curl`'s `--connect-timeout` option.

  See `initial-connect-timeout` for further information.

  Default: `300`

  Deprecated alias: `connect-timeout`

- `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: infinity

- `max-jobs`

  This option defines the maximum number of jobs that Lix will try to build in parallel. The default is `1`. The special value `auto` causes Lix to use the number of CPUs in your system. `0` is useful when using remote builders to prevent any local builds (except for `preferLocalBuild` derivation attribute which executes locally regardless). It 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, Lix 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.

  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, Lix will use the HTTP authentication credentials in this file when trying to download from a remote host through HTTP or HTTPS.

  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`.

  Defaults to `$NIX_CONF_DIR/netrc`. The default shown below is only accurate when the value of `NIX_CONF_DIR` has not been overridden at build time or using the environment variable.

  Default: `/etc/nix/netrc`

- `nix-path`

  List of directories to be searched for `<...>` file references

  In particular, outside of pure evaluation mode, this determines the value of `builtins.nixPath`.

  Default: machine-specific

- `pasta-path`

  If set to an absolute path, enables fully sandboxing fixed-output derivations, by using `pasta` to pass network traffic between the private network namespace. This allows for greater levels of isolation of builds to the host.

  Default: empty

- `plugin-files`

  A list of plugin files to be loaded by Lix.

  Each of these files will be `dlopen`ed by Lix, allowing them to affect execution by registering various entities in Lix. After the plugins are loaded, they will have the function within them with signature `extern "C" void nix_plugin_entry(void)` called if it is defined.

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

  FIXME(jade): We should provide a `nix_plugin_finalize()` that gets called at some point in teardown for use cases like nix-otel which need to be able to cleanup, flush things to network, etc, on exit without having to do that from life-after-main().

  In particular, these plugins may:

  - Construct static instances of `RegisterPrimOp` to add new primops or constants to the expression language (FIXME: will be replaced with an explicit function).
  - Add new store implementations with `StoreImplementations::add`.
  - Construct static instances of `RegisterCommand` to add new subcommands to the `nix` command (FIXME: will be replaced with an explicit function).
  - Construct static instances of `Setting` to add new Lix settings (FIXME: will be replaced with an explicit function).

  See the documentation for those symbols for more details. Note all the FIXMEs above: Lix is removing its usages of static initializers, see https://git.lix.systems/lix-project/lix/issues/359.

  Warning! These APIs are inherently unstable and may change in minor versions. It's recommended to, if you are relying on Lix's unstable C++ API, develop against Lix `main`, run `main` yourself, and be active in Lix development.

  Since these files are loaded into the same address space as Lix itself, they must be DSOs compatible with the instance of Lix running at the time (i.e. compiled against the same headers, not linked to any incompatible libraries, produced by the same nixpkgs).

  It's recommended that this setting not be used in `nix.conf` since it is almost always the case that there are multiple versions of the Nix implementation on a machine. In particular, CppNix (still true in 2.26 as of this writing) considers plugin load failure to be a hard error unlike Lix (since pre-2.90), which means that putting `plugin-files` in `nix.conf` causes random `nix` execution failures. Prefer instead to wrap the `nix` command using either the `NIX_CONFIG` environment variable or `--option plugin-files`.

  Plugins should not be linked to any Lix libs directly, as those will be available already at load time (FIXME: is it an actual problem if they are, assuming that there are not version mismatches?).

  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.

  At the time of running the hook, the derivation may or may not exist on disk (and, e.g. won't exist in the case of many remote builds).

  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:

  - File system and network access is restricted to accesses to immutable data only:

    - Path literals relative to the home directory like `~/lix` are rejected at parse time.

    - Access to absolute paths that did not result from Nix language evaluation is rejected when such paths are given as parameters to builtins like, for example, `builtins.readFile`.

      Access is nonetheless allowed to (absolute) paths in the Nix store that are returned by builtins like `builtins.filterSource`, `builtins.fetchTarball` and similar.

    - Impure fetches such as not specifying a commit ID for `builtins.fetchGit` or not specifying a hash for `builtins.fetchTarball` are rejected.

    - In flakes, access to relative paths outside of the root of the flake's source tree (often, a git repository) is rejected.

  - The evaluator ignores `NIX_PATH`, `-I` and the `nix-path` setting. Thus, `builtins.nixPath` is an empty list.
  - The builtins `builtins.currentSystem` and `builtins.currentTime` are absent from `builtins`.
  - `builtins.getEnv` always returns empty string for any variable.
  - `builtins.storePath` throws an error (Lix may change this, tracking issue: https://git.lix.systems/lix-project/lix/issues/402)

  Default: `false`

- `repl-overlays`

  A list of files containing Nix expressions that can be used to add default bindings to `nix repl` sessions.

  Each file is called with three arguments:

  1. An attribute set containing at least a `currentSystem` attribute (this is identical to `builtins.currentSystem`, except that it's available in `pure-eval` mode).
  2. The top-level bindings produced by the previous `repl-overlays` value (or the default top-level bindings).
  3. The final top-level bindings produced by calling all `repl-overlays`.

  For example, the following file would alias `pkgs` to `legacyPackages.${info.currentSystem}` (if that attribute is defined):

      info: final: prev:
      if prev ? legacyPackages
         && prev.legacyPackages ? ${info.currentSystem}
      then
      {
        pkgs = prev.legacyPackages.${info.currentSystem};
      }
      else
      { }

  Here's a more elaborate `repl-overlay`, which provides the following variables:

  - The original, unmodified variables are aliased to `original`.
  - `legacyPackages.${system}` (if it exists) or `packages.${system}` (otherwise) is aliased to `pkgs`.
  - All attribute set variables with a `${system}` attribute are abbreviated in the same manner; e.g. `devShells.${system}` is shortened to `devShells`.

  For example, the following attribute set:

      info: final: attrs: let
        # Equivalent to nixpkgs `lib.optionalAttrs`.
        optionalAttrs = predicate: attrs:
          if predicate
          then attrs
          else {};

        # If `attrs.${oldName}.${info.currentSystem}` exists, alias `${newName}` to
        # it.
        collapseRenamed = oldName: newName:
          optionalAttrs (builtins.hasAttr oldName attrs
            && builtins.hasAttr info.currentSystem attrs.${oldName})
          {
            ${newName} = attrs.${oldName}.${info.currentSystem};
          };

        # Alias `attrs.${oldName}.${info.currentSystem} to `${newName}`.
        collapse = name: collapseRenamed name name;

        # Alias all `attrs` keys with an `${info.currentSystem}` attribute.
        collapseAll =
          builtins.foldl'
          (prev: name: prev // collapse name)
          {}
          (builtins.attrNames attrs);
      in
        # Preserve the original bindings as `original`.
        (optionalAttrs (! attrs ? original)
          {
            original = attrs;
          })
        // (collapseRenamed "packages" "pkgs")
        // (collapseRenamed "legacyPackages" "pkgs")
        // collapseAll

  Default: empty

- `require-drop-supplementary-groups`

  Following the principle of least privilege, Lix 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.

  > Note This setting is only available on Linux systems.

  Default: running as root: `true`, otherwise: `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 the Nix search path (as set via the `NIX_PATH` environment variable or the `-I` option), or to URIs outside of `allowed-uris`. The default is `false`.

  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 Lix 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: Linux: `true`, other platforms: `false`

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

- `sandbox-build-dir`

  The build directory inside the sandbox.

  This directory is backed by `build-dir` on the host.

  > Note This setting is only available on Linux systems.

  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%`.

  > Note This setting is only available on Linux systems.

  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 Lix 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 Lix 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. Lix 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: machine-specific

- `stalled-download-timeout`

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

  Default: `300`

- `start-id`

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

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `auto-allocate-uids`, is enabled. For example, include the following in `nix.conf`:

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

  Default: Linux: `872415232`, other platforms: `56930`

- `store`

  The URL of the Nix store to use for most operations. See `nix help-stores` for supported store types and settings.

  Default: `auto`

- `substitute`

  If set to `true` (default), Lix 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 Lix 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 Lix to use a substituter:

  - The substituter is in the `trusted-substituters` list
  - The user calling Lix 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 Lix installation. Lix 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 Lix itself is compiled for the system it will run on. The following system types are widely used, as Lix 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 Lix 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: machine-specific

- `system-features`

  A set of system “features” supported by this machine, e.g. `kvm`. Derivations can express a dependency on such features through the derivation attribute `requiredSystemFeatures`. For example, the attribute

      requiredSystemFeatures = [ "kvm" ];

  ensures that the derivation can only be built on a machine with the `kvm` feature.

  This setting by default includes `kvm` if `/dev/kvm` is accessible, `apple-virt` if hardware virtualization is available on macOS, and the pseudo-features `nixos-test`, `benchmark` and `big-parallel` that are used in Nixpkgs to route builds to specific machines.

  Default: machine-specific

- `tarball-ttl`

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

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

  Lix caches tarballs in `$XDG_CACHE_HOME/nix/tarballs`.

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

  Default: `3600`

- `temp-dir`

  The directory on the host used as the default temporary directory.

  If not set, Nix will use the system temporary directory indicated by the `TMPDIR` environment variable.

  This will be used for anything that would otherwise fall back to `TMPDIR`, and the inherited `TMPDIR` value will be preserved for child processes to use. If `build-dir` is set, that takes precedence over this where it applies.

  If set, the value must be a path that exists and is accessible to all users.

  Default: empty

- `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`

- `trusted-public-keys`

  A whitespace-separated list of public keys.

  At least one of the following condition must be met for Lix 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 output-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 NARs.

  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`

- `use-case-hack`

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

  Default: Darwin: `true`, other platforms: `false`

- `use-cgroups`

  Whether to execute builds inside cgroups.

  Cgroups are required and enabled automatically for derivations that require the `uid-range` system feature.

  Cgroups requires a cgroup delegation according to https://systemd.io/CGROUP_DELEGATION/, i.e. the Nix process (`nix-daemon` or any single user command) should run in a cgroup tree of a parent cgroup which possess the `user.delegate=1` extended attribute.

  In this scenario, Nix will run builds in a sibling cgroup named `nix-build-uid-<build user uid>`.

  Be aware that this option is incompatible with various `nix-build`, `nix-store`, `nix store`, and `nix verify` sub-commands when using a `local` store which will need the option explicitely turned off with the argument `--no-use-cgroups`.

  > Note This setting is only available on Linux systems.

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `cgroups`, is enabled. For example, include the following in `nix.conf`:

      extra-experimental-features = cgroups
      use-cgroups = ...

  Default: `false`

- `use-registries`

  Whether to use flake registries to resolve flake references.

  > Warning This setting is part of an experimental feature.

  To change this setting, you need to make sure the corresponding experimental feature, `flakes`, is enabled. For example, include the following in `nix.conf`:

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

  Default: `true`

- `use-sqlite-wal`

  Whether SQLite should use WAL mode.

  Default: WSL 1: `false`, other platforms: `true`

- `use-xdg-base-directories`

  If set to `true`, Lix 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 Lix 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 Lix 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`