Registries

Cargo installs crates and fetches dependencies from a "registry". The default registry is crates.io. A registry contains an "index" which contains a searchable list of available crates. A registry may also provide a web API to support publishing new crates directly from Cargo.

Note: If you are interested in mirroring or vendoring an existing registry, take a look at Source Replacement.

To use a registry other than crates.io, the name and index URL of the registry must be added to a .cargo/config file. The registries table has a key for each registry, for example:

[registries]
my-registry = { index = "https://my-intranet:8080/git/index" }

The index key should be a URL to a git repository with the registry's index. A crate can then depend on a crate from another registry by specifying the registry key and a value of the registry's name in that dependency's entry in Cargo.toml:

# Sample Cargo.toml
[package]
name = "my-project"
version = "0.1.0"

[dependencies]
other-crate = { version = "1.0", registry = "my-registry" }

As with most config values, the index may be specified with an environment variable instead of a config file. For example, setting the following environment variable will accomplish the same thing as defining a config file:

CARGO_REGISTRIES_MY_REGISTRY_INDEX=https://my-intranet:8080/git/index

Note: crates.io does not accept packages that depend on crates from other registries.

If the registry supports web API access, then packages can be published directly to the registry from Cargo. Several of Cargo's commands such as cargo publish take a --registry command-line flag to indicate which registry to use. For example, to publish the package in the current directory:

cargo login --registry=my-registry

This only needs to be done once. You must enter the secret API token retrieved from the registry's website. Alternatively the token may be passed directly to the publish command with the --token command-line flag or an environment variable with the name of the registry such as CARGO_REGISTRIES_MY_REGISTRY_TOKEN.
cargo publish --registry=my-registry

Instead of always passing the --registry command-line option, the default registry may be set in .cargo/config with the registry.default key.

Setting the package.publish key in the Cargo.toml manifest restricts which registries the package is allowed to be published to. This is useful to prevent accidentally publishing a closed-source package to crates.io. The value may be a list of registry names, for example:

[package]
# ...
publish = ["my-registry"]

The publish value may also be false to restrict all publishing, which is the same as an empty list.

The authentication information saved by cargo login is stored in the credentials file in the Cargo home directory (default $HOME/.cargo). It has a separate table for each registry, for example:

[registries.my-registry]
token = "854DvwSlUwEHtIo3kWy6x7UCPKHfzCmy"

A minimal registry can be implemented by having a git repository that contains an index, and a server that contains the compressed .crate files created by cargo package. Users won't be able to use Cargo to publish to it, but this may be sufficient for closed environments.

A full-featured registry that supports publishing will additionally need to have a web API service that conforms to the API used by Cargo. The web API is documented below.

At this time, there is no widely used software for running a custom registry. There is interest in documenting projects that implement registry support, or existing package caches that add support for Cargo.

The following defines the format of the index. New features are occasionally added, which are only understood starting with the version of Cargo that introduced them. Older versions of Cargo may not be able to use packages that make use of new features. However, the format for older packages should not change, so older versions of Cargo should be able to use them.

The index is stored in a git repository so that Cargo can efficiently fetch incremental updates to the index. In the root of the repository is a file named config.json which contains JSON information used by Cargo for accessing the registry. This is an example of what the crates.io config file looks like:

{
    "dl": "https://crates.io/api/v1/crates",
    "api": "https://crates.io"
}

The keys are:

dl: This is the URL for downloading crates listed in the index. The value may have the markers {crate} and {version} which are replaced with the name and version of the crate to download. If the markers are not present, then the value /{crate}/{version}/download is appended to the end.
api: This is the base URL for the web API. This key is optional, but if it is not specified, commands such as cargo publish will not work. The web API is described below.

The download endpoint should send the .crate file for the requested package. Cargo supports https, http, and file URLs, HTTP redirects, HTTP1 and HTTP2. The exact specifics of TLS support depend on the platform that Cargo is running on, the version of Cargo, and how it was compiled.

The rest of the index repository contains one file for each package, where the filename is the name of the package in lowercase. Each version of the package has a separate line in the file. The files are organized in a tier of directories:

Packages with 1 character names are placed in a directory named 1.
Packages with 2 character names are placed in a directory named 2.
Packages with 3 character names are placed in the directory 3/{first-character} where {first-character} is the first character of the package name.
All other packages are stored in directories named {first-two}/{second-two} where the top directory is the first two characters of the package name, and the next subdirectory is the third and fourth characters of the package name. For example, cargo would be stored in a file named ca/rg/cargo.

Note: Although the index filenames are in lowercase, the fields that contain package names in Cargo.toml and the index JSON data are case-sensitive and may contain upper and lower case characters.

Registries may want to consider enforcing limitations on package names added to their index. Cargo itself allows names with any alphanumeric, -, or _ character. For example, crates.io imposes relatively strict limitations, such as requiring it to be a valid Rust identifier, only allowing ASCII characters, under a specific length, and rejects reserved names such as Windows special filenames like "nul".

Each line in a package file contains a JSON object that describes a published version of the package. The following is a pretty-printed example with comments explaining the format of the entry.

{
    // The name of the package.
    // This must only contain alphanumeric, `-`, or `_` characters.
    "name": "foo",
    // The version of the package this row is describing.
    // This must be a valid version number according to the Semantic
    // Versioning 2.0.0 spec at https://semver.org/.
    "vers": "0.1.0",
    // Array of direct dependencies of the package.
    "deps": [
        {
            // Name of the dependency.
            // If the dependency is renamed from the original package name,
            // this is the new name. The original package name is stored in
            // the `package` field.
            "name": "rand",
            // The semver requirement for this dependency.
            // This must be a valid version requirement defined at
            // https://github.com/steveklabnik/semver#requirements.
            "req": "^0.6",
            // Array of features (as strings) enabled for this dependency.
            "features": ["i128_support"],
            // Boolean of whether or not this is an optional dependency.
            "optional": false,
            // Boolean of whether or not default features are enabled.
            "default_features": true,
            // The target platform for the dependency.
            // null if not a target dependency.
            // Otherwise, a string such as "cfg(windows)".
            "target": null,
            // The dependency kind.
            // "dev", "build", or "normal".
            // Note: this is a required field, but a small number of entries
            // exist in the crates.io index with either a missing or null
            // `kind` field due to implementation bugs.
            "kind": "normal",
            // The URL of the index of the registry where this dependency is
            // from as a string. If not specified or null, it is assumed the
            // dependency is in the current registry.
            "registry": null,
            // If the dependency is renamed, this is a string of the actual
            // package name. If not specified or null, this dependency is not
            // renamed.
            "package": null,
        }
    ],
    // A SHA256 checksum of the `.crate` file.
    "cksum": "d867001db0e2b6e0496f9fac96930e2d42233ecd3ca0413e0753d4c7695d289c",
    // Set of features defined for the package.
    // Each feature maps to an array of features or dependencies it enables.
    "features": {
        "extras": ["rand/simd_support"]
    },
    // Boolean of whether or not this version has been yanked.
    "yanked": false,
    // The `links` string value from the package's manifest, or null if not
    // specified. This field is optional and defaults to null.
    "links": null
}

The JSON objects should not be modified after they are added except for the yanked field whose value may change at any time.

A registry may host a web API at the location defined in config.json to support any of the actions listed below.

Cargo includes the Authorization header for requests that require authentication. The header value is the API token. The server should respond with a 403 response code if the token is not valid. Users are expected to visit the registry's website to obtain a token, and Cargo can store the token using the cargo login command, or by passing the token on the command-line.

Responses use a 200 response code for both success and errors. Cargo looks at the JSON response to determine if there was success or failure. Failure responses have a JSON object with the following structure:

{
    // Array of errors to display to the user.
    "errors": [
        {
            // The error message as a string.
            "detail": "error message text"
        }
    ]
}

Servers may also respond with a 404 response code to indicate the requested resource is not found (for example, an unknown crate name). However, using a 200 response with an errors object allows a registry to provide a more detailed error message if desired.

For backwards compatibility, servers should ignore any unexpected query parameters or JSON fields. If a JSON field is missing, it should be assumed to be null. The endpoints are versioned with the v1 component of the path, and Cargo is responsible for handling backwards compatibility fallbacks should any be required in the future.

Cargo sets the following headers for all requests:

Content-Type: application/json
Accept: application/json
User-Agent: The Cargo version such as cargo 1.32.0 (8610973aa 2019-01-02). This may be modified by the user in a configuration value. Added in 1.29.

Endpoint: /api/v1/crates/new
Method: PUT
Authorization: Included

The publish endpoint is used to publish a new version of a crate. The server should validate the crate, make it available for download, and add it to the index.

The body of the data sent by Cargo is:

32-bit unsigned little-endian integer of the length of JSON data.
Metadata of the package as a JSON object.
32-bit unsigned little-endian integer of the length of the .crate file.
The .crate file.

The following is a commented example of the JSON object. Some notes of some restrictions imposed by crates.io are included only to illustrate some suggestions on types of validation that may be done, and should not be considered as an exhaustive list of restrictions crates.io imposes.

{
    // The name of the package.
    "name": "foo",
    // The version of the package being published.
    "vers": "0.1.0",
    // Array of direct dependencies of the package.
    "deps": [
        {
            // Name of the dependency.
            // If the dependency is renamed from the original package name,
            // this is the original name. The new package name is stored in
            // the `explicit_name_in_toml` field.
            "name": "rand",
            // The semver requirement for this dependency.
            "version_req": "^0.6",
            // Array of features (as strings) enabled for this dependency.
            "features": ["i128_support"],
            // Boolean of whether or not this is an optional dependency.
            "optional": false,
            // Boolean of whether or not default features are enabled.
            "default_features": true,
            // The target platform for the dependency.
            // null if not a target dependency.
            // Otherwise, a string such as "cfg(windows)".
            "target": null,
            // The dependency kind.
            // "dev", "build", or "normal".
            "kind": "normal",
            // The URL of the index of the registry where this dependency is
            // from as a string. If not specified or null, it is assumed the
            // dependency is in the current registry.
            "registry": null,
            // If the dependency is renamed, this is a string of the new
            // package name. If not specified or null, this dependency is not
            // renamed.
            "explicit_name_in_toml": null,
        }
    ],
    // Set of features defined for the package.
    // Each feature maps to an array of features or dependencies it enables.
    // Cargo does not impose limitations on feature names, but crates.io
    // requires alphanumeric ASCII, `_` or `-` characters.
    "features": {
        "extras": ["rand/simd_support"]
    },
    // List of strings of the authors.
    // May be empty. crates.io requires at least one entry.
    "authors": ["Alice <a@example.com>"],
    // Description field from the manifest.
    // May be null. crates.io requires at least some content.
    "description": null,
    // String of the URL to the website for this package's documentation.
    // May be null.
    "documentation": null,
    // String of the URL to the website for this package's home page.
    // May be null.
    "homepage": null,
    // String of the content of the README file.
    // May be null.
    "readme": null,
    // String of a relative path to a README file in the crate.
    // May be null.
    "readme_file": null,
    // Array of strings of keywords for the package.
    "keywords": [],
    // Array of strings of categories for the package.
    "categories": [],
    // String of the license for the package.
    // May be null. crates.io requires either `license` or `license_file` to be set.
    "license": null,
    // String of a relative path to a license file in the crate.
    // May be null.
    "license_file": null,
    // String of the URL to the website for the source repository of this package.
    // May be null.
    "repository": null,
    // Optional object of "status" badges. Each value is an object of
    // arbitrary string to string mappings.
    // crates.io has special interpretation of the format of the badges.
    "badges": {
        "travis-ci": {
            "branch": "master",
            "repository": "rust-lang/cargo"
        }
    },
    // The `links` string value from the package's manifest, or null if not
    // specified. This field is optional and defaults to null.
    "links": null,
}