# Advanced Attributes

Derivations can declare some infrequently used optional attributes.

- `allowedReferences`
  The optional attribute `allowedReferences` specifies a list of legal references (dependencies) of the output of the builder. For example,

      allowedReferences = [];

  enforces that the output of a derivation cannot have any runtime dependencies on its inputs. To allow an output to have a runtime dependency on itself, use `"out"` as a list item. This is used in NixOS to check that generated files such as initial ramdisks for booting Linux don’t have accidental dependencies on other paths in the Nix store.

- `allowedRequisites`
  This attribute is similar to `allowedReferences`, but it specifies the legal requisites of the whole closure, so all the dependencies recursively. For example,

      allowedRequisites = [ foobar ];

  enforces that the output of a derivation cannot have any other runtime dependency than `foobar`, and in addition it enforces that `foobar` itself doesn't introduce any other dependency itself.

- `disallowedReferences`
  The optional attribute `disallowedReferences` specifies a list of illegal references (dependencies) of the output of the builder. For example,

      disallowedReferences = [ foo ];

  enforces that the output of a derivation cannot have a direct runtime dependencies on the derivation `foo`.

- `disallowedRequisites`
  This attribute is similar to `disallowedReferences`, but it specifies illegal requisites for the whole closure, so all the dependencies recursively. For example,

      disallowedRequisites = [ foobar ];

  enforces that the output of a derivation cannot have any runtime dependency on `foobar` or any other derivation depending recursively on `foobar`.

- `exportReferencesGraph`
  This attribute allows builders access to the references graph of their inputs. The attribute is a list of inputs in the Nix store whose references graph the builder needs to know. The value of this attribute should be a list of pairs `[ name1 path1 name2 path2 ... ]`. The references graph of each pathN will be stored in a text file nameN in the temporary build directory. The text files have the format used by `nix-store --register-validity` (with the deriver fields left empty). For example, when the following derivation is built:

      derivation {
        ...
        exportReferencesGraph = [ "libfoo-graph" libfoo ];
      };

  the references graph of `libfoo` is placed in the file `libfoo-graph` in the temporary build directory.

  `exportReferencesGraph` is useful for builders that want to do something with the closure of a store path. Examples include the builders in NixOS that generate the initial ramdisk for booting Linux (a `cpio` archive containing the closure of the boot script) and the ISO-9660 image for the installation CD (which is populated with a Nix store containing the closure of a bootable NixOS configuration).

- `impureEnvVars`
  This attribute allows you to specify a list of environment variables that should be passed from the environment of the calling user to the builder. Usually, the environment is cleared completely when the builder is executed, but with this attribute you can allow specific environment variables to be passed unmodified. For example, `fetchurl` in Nixpkgs has the line

      impureEnvVars = [ "http_proxy" "https_proxy" ... ];

  to make it use the proxy server configuration specified by the user in the environment variables `http_proxy` and friends.

  This attribute is only allowed in fixed-output derivations (see below), where impurities such as these are okay since (the hash of) the output is known in advance. It is ignored for all other derivations.

  > Warning

  > `impureEnvVars` implementation takes environment variables from the current builder process. When a daemon is building its environmental variables are used. Without the daemon, the environmental variables come from the environment of the `nix-build`.

- `outputHash`; `outputHashAlgo`; `outputHashMode`
  These attributes declare that the derivation is a so-called fixed-output derivation, which means that a cryptographic hash of the output is already known in advance. When the build of a fixed-output derivation finishes, Nix computes the cryptographic hash of the output and compares it to the hash declared with these attributes. If there is a mismatch, the build fails.

  The rationale for fixed-output derivations is derivations such as those produced by the `fetchurl` function. This function downloads a file from a given URL. To ensure that the downloaded file has not been modified, the caller must also specify a cryptographic hash of the file. For example,

      fetchurl {
        url = "http://ftp.gnu.org/pub/gnu/hello/hello-2.1.1.tar.gz";
        sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
      }

  It sometimes happens that the URL of the file changes, e.g., because servers are reorganised or no longer available. We then must update the call to `fetchurl`, e.g.,

      fetchurl {
        url = "ftp://ftp.nluug.nl/pub/gnu/hello/hello-2.1.1.tar.gz";
        sha256 = "1md7jsfd8pa45z73bz1kszpp01yw6x5ljkjk2hx7wl800any6465";
      }

  If a `fetchurl` derivation was treated like a normal derivation, the output paths of the derivation and all derivations depending on it would change. For instance, if we were to change the URL of the Glibc source distribution in Nixpkgs (a package on which almost all other packages depend) massive rebuilds would be needed. This is unfortunate for a change which we know cannot have a real effect as it propagates upwards through the dependency graph.

  For fixed-output derivations, on the other hand, the name of the output path only depends on the `outputHash*` and `name` attributes, while all other attributes are ignored for the purpose of computing the output path. (The `name` attribute is included because it is part of the path.)

  As an example, here is the (simplified) Nix expression for `fetchurl`:

      { stdenv, curl }: # The curl program is used for downloading.

      { url, sha256 }:

      stdenv.mkDerivation {
        name = baseNameOf (toString url);
        builder = ./builder.sh;
        buildInputs = [ curl ];

        # This is a fixed-output derivation; the output must be a regular
        # file with SHA256 hash sha256.
        outputHashMode = "flat";
        outputHashAlgo = "sha256";
        outputHash = sha256;

        inherit url;
      }

  The `outputHashAlgo` attribute specifies the hash algorithm used to compute the hash. It can currently be `"sha1"`, `"sha256"` or `"sha512"`.

  The `outputHashMode` attribute determines how the hash is computed. It must be one of the following two values:

  - `"flat"`
    The output must be a non-executable regular file. If it isn’t, the build fails. The hash is simply computed over the contents of that file (so it’s equal to what Unix commands like `sha256sum` or `sha1sum` produce).

    This is the default.

  - `"recursive"`
    The hash is computed over the NAR archive dump of the output (i.e., the result of `nix-store --dump`). In this case, the output can be anything, including a directory tree.

  The `outputHash` attribute, finally, must be a string containing the hash in either hexadecimal or base-32 notation. (See the `nix-hash` command for information about converting to and from base-32 notation.)

- `__contentAddressed` If this experimental attribute is set to true, then the derivation outputs will be stored in a content-addressed location rather than the traditional input-addressed one. This only has an effect if the `ca-derivation` experimental feature is enabled.

  Setting this attribute also requires setting `outputHashMode` and `outputHashAlgo` like for fixed-output derivations (see above).

- `passAsFile`
  A list of names of attributes that should be passed via files rather than environment variables. For example, if you have

      passAsFile = ["big"];
      big = "a very long string";

  then when the builder runs, the environment variable `bigPath` will contain the absolute path to a temporary file containing `a very long string`. That is, for any attribute x listed in `passAsFile`, Nix will pass an environment variable `xPath` holding the path of the file containing the value of attribute x. This is useful when you need to pass large strings to a builder, since most operating systems impose a limit on the size of the environment (typically, a few hundred kilobyte).

- `preferLocalBuild`
  If this attribute is set to `true` and distributed building is enabled, then, if possible, the derivation will be built locally instead of forwarded to a remote machine. This is appropriate for trivial builders where the cost of doing a download or remote build would exceed the cost of building locally.

- `allowSubstitutes`
  If this attribute is set to `false`, then Nix will always build this derivation; it will not try to substitute its outputs. This is useful for very trivial derivations (such as `writeText` in Nixpkgs) that are cheaper to build than to substitute from a binary cache.

  > Note

  > You need to have a builder configured which satisfies the derivation’s `system` attribute, since the derivation cannot be substituted. Thus it is usually a good idea to align `system` with `builtins.currentSystem` when setting `allowSubstitutes` to `false`. For most trivial derivations this should be the case.