nix.conf 5 Nix nix.conf Nix configuration file Nix reads settings from two configuration files: 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. The user configuration file $XDG_CONFIG_HOME/nix/nix.conf, or ~/.config/nix/nix.conf if XDG_CONFIG_HOME is not set. The configuration files consist of name = value pairs, one per line. Comments start with a # character. Here is an example configuration file: keep-outputs = true # Nice for developers keep-derivations = true # Idem You can override settings on the command line using the --option flag, e.g. --option keep-outputs false. The following settings are currently available: 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. 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). keep-env-derivations If false (default), derivations are not stored in Nix user environments. That is, the derivation 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. max-jobs This option defines the maximum number of jobs that Nix will try to build in parallel. The default is 1. The special value auto causes Nix to use the number of CPUs in your system. It can be overridden using the --max-jobs (-j) command line switch. 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. 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. 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. 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. 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 in multi-user settings with untrusted users. 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 false. 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. 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. build-extra-sandbox-paths A list of additional paths appended to sandbox-paths. Useful if you want to extend its default value. use-substitutes If set to true (default), Nix will use binary substitutes if available. This option can be disabled to force building from source. 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. 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. 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. substituters A list of URLs of substituters, separated by whitespace. The default is https://cache.nixos.org. trusted-substituters A list of URLs of substituters, separated by whitespace. These are not used by default, but can be enabled by users of the Nix daemon by specifying --option substituters urls on the command line. Unprivileged users are only allowed to pass a subset of the URLs listed in substituters and trusted-substituters. extra-substituters Additional binary caches appended to those specified in substituters. When used by unprivileged users, untrusted substituters (i.e. those not listed in trusted-substituters) are silently ignored. 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 valid signature, that is, be signed using one of the keys listed in trusted-public-keys. Set to false to disable signature checking. trusted-public-keys A whitespace-separated list of public keys. When paths are copied from another Nix store (such as a binary cache), they must be signed with one of these keys. For example: cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= hydra.nixos.org-1:CNHJZBh9K4tP3EKF6FkkgeVYsS3ohTl+oS0Qa8bezVs=. 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. 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. system This option specifies the canonical Nix system name of the current installation, such as i686-linux or x86_64-darwin. Nix can only build derivations whose system attribute equals the value specified here. In general, it never makes sense to modify this value from its default, since you can use it to ‘lie’ about the platform you are building on (e.g., perform a Mac OS build on a Linux machine; the result would obviously be wrong). It only makes sense if the Nix binaries can run on multiple platforms, e.g., ‘universal binaries’ that run on x86_64-linux and i686-linux. It defaults to the canonical Nix system name detected by configure at build time. 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. 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. connect-timeout The timeout (in seconds) for establishing connections in the binary cache substituter. It corresponds to curl’s --connect-timeout option. trusted-users A list of names of users (separated by whitespace) that have additional rights when connecting to the Nix daemon, such as the ability to specify additional binary caches, or to import unsigned NARs. You can also specify groups by prefixing them with @; for instance, @wheel means all users in the wheel group. The default is root. The users listed here have the ability to compromise the security of a multi-user Nix store. For instance, they could install Trojan horses subsequently executed by other users. So you should consider carefully whether to add users to this list. allowed-users A list of names of users (separated by whitespace) that are allowed to connect to the Nix daemon. As with the trusted-users option, you can specify groups by prefixing them with @. Also, you can allow all users by specifying *. The default is *. Note that trusted users are always allowed to connect. 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-uri. The default is false. 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. 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. repeat How many times to repeat builds to check whether they are deterministic. The default value is 0. If the value is non-zero, every build is repeated the specified number of times. If the contents of any of the runs differs from the previous ones, the build is rejected and the resulting store paths are not registered as “valid” in Nix’s database. 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 mount8. The default is 50%. allow-import-from-derivation By default, Nix allows you to import from a derivation, allowing building at evaluation time. With this option set to false, Nix will throw an error when evaluating an expression that uses this feature, allowing users to ensure their evaluation will not require any builds to take place. 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). hashed-mirrors A list of web servers used by builtins.fetchurl to obtain files by hash. The default is http://tarballs.nixos.org/. Given a hash type ht and a base-16 hash h, Nix 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 the default mirror http://tarballs.nixos.org/, when building the derivation builtins.fetchurl { url = https://example.org/foo-1.2.3.tar.xz; sha256 = "2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae"; } Nix will attempt to download this file from http://tarballs.nixos.org/sha256/2c26b46b68ffc68ff99b453c1d30413413422d706483bfa0f98a5e886266e7ae first. If it is not available there, if will try the original URI.