#cargo #license #spdx #ci

bin+lib cargo-deny

Cargo plugin to help you manage large dependency graphs

12 unstable releases (3 breaking)

✓ Uses Rust 2018 edition

0.4.2 Dec 2, 2019
0.4.1 Nov 28, 2019
0.3.0 Oct 30, 2019
0.2.6 Jul 27, 2019
0.1.0 May 13, 2019

#11 in Cargo plugins

Download history 19/week @ 2019-08-19 63/week @ 2019-08-26 44/week @ 2019-09-02 30/week @ 2019-09-09 130/week @ 2019-09-16 52/week @ 2019-09-23 36/week @ 2019-09-30 85/week @ 2019-10-07 76/week @ 2019-10-14 106/week @ 2019-10-21 101/week @ 2019-10-28 109/week @ 2019-11-04 56/week @ 2019-11-11 150/week @ 2019-11-18 108/week @ 2019-11-25

358 downloads per month



❌ cargo-deny

Build Status Latest version Docs SPDX Version Contributor Covenant Embark

One of the key selling points of Rust is the ever growing and improving ecosystem of crates available that can be easily added to your project incredibly easily via cargo. This is great! However, the larger the project is and the more dependencies you have, the harder it is to keep track of certain things, especially as a project evolves over time, which is what cargo-deny tries to help you with.

  • Licenses - Configure which license terms you accept
  • Bans - Configure whether particular crates are allowed in your dependency graph


  • cargo deny check <license|all> - verify crate graph only contains acceptable license requirements
  • cargo deny check <ban|all> - verify crate graph doesn't contain certain crates
  • cargo deny list - list all of the licenses for all crates in a project

Licenses - cargo deny check license

One important aspect that one must always keep in mind when using code from other people is what the licensing of that code is and whether it fits the requirements of your project. Luckily, most of the crates in the Rust ecosystem tend to follow the example set forth by Rust itself, namely dual-license MIT OR Apache-2.0, but of course, that is not always the case.

So cargo-deny allows you to ensure that all of your dependencies have license requirements that align with your configuration.


Currently, the precedence for determining whether a particular license is accepted or rejected is as follows:

  1. A license specified in the deny list is always rejected.
  2. A license specified in the allow list is always accepted.
  3. If the license is considered copyleft, the [license.copyleft] configuration determines its status
  4. If the license is OSI Approved or FSF Free/Libre, the [license.allow-osi-fsf-free] configuration determines its status
  5. If the license does not match any of the above criteria, it is implicitly rejected.

The [licenses] section

Contains all of the configuration for cargo deny check license.

The unlicensed field

Determines what happens when a crate has not explicitly specified its license terms, and no license information could be easily detected via LICENSE* files in the crate's source.

  • deny (default) - All unlicensed crates will emit an error and fail the license check
  • allow - All unlicensed crates will show a note, but will not fail the license check
  • warn - All unlicensed crates will show a warning, but will not fail the license check

The allow and deny fields

The licenses that should be allowed or denied. The license must be a valid SPDX v2.1 identifier, which must either be in version 3.6 of the SPDX License List, with an optional exception specified by WITH <exception-id>, or else a user defined license reference denoted by LicenseRef-<idstring> for a license not on the SPDX License List.

NOTE: The same license cannot appear in both the allow and deny lists.

The copyleft field

Determines what happens when a license that is considered copyleft is encountered.

  • warn (default) - Will emit a warning that a copyleft license was detected, but will not fail the license check
  • deny - The license is not accepted if it is copyleft, but might not fail the license check if part of an expression that containe
  • allow - The license is accepted if it is copyleft

The allow-osi-fsf-free field

Determines what happens when licenses aren't explicitly allowed or denied, but are marked as OSI Approved or FSF Free/Libre in version 3.6 of the SPDX License List.

  • both - The license is accepted if it is both OSI approved and FSF Free
  • either - The license is accepted if it is either OSI approved or FSF Free
  • osi-only - The license is accepted if it is OSI approved and not FSF Free
  • fsf-only - The license is accepted if it is FSF Free and not OSI approved
  • neither (default) - No special consideration is given the license

The confidence-threshold field

cargo-deny uses askalono to determine the license of a license file, the confidence threshold value determines if askalono's determination meets your minimum requirements. The higher the value, the more closely the license text must be to the canonical license text of a valid SPDX license file.

0.0 - 1.0 (default 0.8)

The clarify field

In some exceptional cases, the crate does not have easily machine readable license information, and would by default be considered "unlicensed" by cargo-deny. As a (hopefully) temporary patch for using the crate, you can specify a clarification for a crate where you can specify the license expression based on your understanding of the requirements as described by the license holder.

The name field

The name of the crate that you are clarifying

The version field

An optional version constraint specifying the range of crate versions you are clarifying. Defaults to all versions (*).

The expression field

The SPDX license expression you are specifying as the license requirements for the crate in question.

The license-files field

Contains one or more files that will be checked to ensure the license expression still applies to a version of the crate. Each file is a path to the file relative to the crate route, and a hash of the contents to detect changes between versions. This hash is printed out when license files cannot have their license determined with high confidence.

Example config

unlicensed = "deny"
allow-osi-fsf-free = "either"
copyleft = "ignore"
confidence-threshold = 0.92
deny = [
allow = [
    "Apache-2.0 WITH LLVM-exception",

# ring has a rather complicated license file, and unfortunately does not
# provide an SPDX expression in the `license` toml
name = "ring"
# SPDX considers OpenSSL to encompass both the OpenSSL and SSLeay licenses
# https://spdx.org/licenses/OpenSSL.html
# ISC - Both BoringSSL and ring use this for their new files
# MIT - "Files in third_party/ have their own licenses, as described therein. The MIT
# license, for third_party/fiat, which, unlike other third_party directories, is
# compiled into non-test libraries, is included below."
# OpenSSL - Obviously
expression = "ISC AND MIT AND OpenSSL"
license-files = [
    { path = "LICENSE", hash = 0xbd0eed23 },

Crate bans - cargo deny check ban

Use Case - Keeping certain crates out of your dependency graph

Sometimes, certain crates just don't fit in your project, so you have to remove them. However, nothing really stops them from sneaking back in due to small changes, like updating a crate to a new version that happens to add it as a dependency, or an existing dependency just changing what crates are included in the default feature set.

For example, we previously depended on OpenSSL as it is the "default" for many crates that deal with HTTP traffic. This was extremely annoying as it required us to have OpenSSL development libraries installed on Windows, for both individuals and CI. We moved all of our dependencies to use the much more streamlined native-tls and ring crates instead, and now we can make sure that OpenSSL doesn't return from the grave by being pulled in as a default feature of some future HTTP crate we might use.

Use Case - Get a handle on duplicate versions

One thing that is part of the tradeoff of being able to use so many crates, is that they all won't necessarily agree on what versions of a dependency they want to use, and cargo and rust will happily chug along compiling all of them. This is great when just trying out a new dependency as quickly as possible, but it does come with some long term costs. Crate fetch times (and disk space) are increased, but in particular, compile times, and ultimately your binary sizes, also increase. If you are made aware that you depend on multiple versions of the same crate, you at least have an opportunity to decide how you want to handle them.

The [bans] section

Contains all of the configuration for cargo deny check ban

The multiple-versions field

Determines what happens when multiple versions of the same crate are encountered.

  • deny - Will emit an error for each crate with duplicates and fail the check.
  • warn (default) - Prints a warning for each crate with duplicates, but does not fail the check.
  • allow - Ignores duplicate versions of the same crate.

The highlight field

When multiple versions of the same crate are encountered and the multiple-versions is set to warn or deny, using the -g <dir> option will print out a dotgraph of each of the versions and how they were included into the graph. This field determines how the graph is colored to help you quickly spot good candidates for removal or updating.

  • lowest-version - Highlights the path to the lowest duplicate version. Highlighted in red
  • simplest-path - Highlights the path to the duplicate version with the fewest number of total edges to the root of the graph, which will often be the best candidate for removal and/or upgrading. Highlighted in blue.
  • all - Highlights both the lowest-version and simplest-path. If they are the same, they are only highlighted in red.


Crate specifier

The allow, deny, skip, and skip-tree fields all use a crate identifier to specify what crate(s) they want to match against.

{ name = "some-crate-name-here", version = "<= 0.7.0" }

The name field

The name of the crate.

The version field

An optional version constraint specifying the range of crate versions that will match. Defaults to all versions (*).

The allow and deny fields

As with licenses, these determine which specificy crates and version ranges are actually allowed or denied.

The skip field

When denying duplicate versions, it sometimes takes time to update versions in transitive dependencies, or big changes in core often used crates such as winapi and others to ripple through the rest of the ecosystem. In such cases, it can be ok to remove certain versions from consideration so that they won't trigger failures due to multiple versions, and can eventually be removed once all crates have update to the later version(s).

Note entries in the skip field that never match a crate in your graph will have a warning printed that they never matched, allowing you to clean up your configuration as your crate graph changes over time.

The skip-tree field

When dealing with duplicate versions, it's often the case that a particular crate acts as a nexus point for a cascade effect, by either using bleeding edge versions of certain crates while in alpha or beta, or on the opposite end, a crate is using severely outdated dependencies while much of the rest of the ecosystem has moved to more recent versions. In both cases, it can be quite tedious to explicitly skip each transitive dependency pulled in by that crate that clashes with your other dependencies, which is where skip-tree comes in.

skip-tree entries are similar to skip in that they are used to specify a crate name and version range that will be skipped, but they also have an additional depth field that can be used to specify the depth from that root crate that will also be ignored when checking for duplicates. In that sense, a depth of 0 would be functionally the same as specifying the same crate name and version constraint in the skip list instead.

Note that by default, the depth is infinite.

Example Config

multiple-versions = "deny"
deny = [
    # You can never be too sure
    { name = "openssl" },
skip = [
    # askalono 0.3.0 uses an ancient regex version which pulls
    # in other duplicates
    { name = "regex", version = "=0.2.11" },
    { name = "regex-syntax", version = "=0.5.6" },
    { name = "aho-corasick", version = "=0.6.10" },

    # some macro crates use the pre 1.0 syn dependencies
    { name = "syn", version = "<=0.15" },
    { name = "proc-macro2", version = "<=0.4" },
    { name = "quote", version = "<=0.6" },
    { name = "unicode-xid", version = "=0.1" },
skip-tree = [
    # tonic is in alpha right now, and pulls in many alpha versions of tokio/tower
    # crates, so ignore all of them for now until things stabilize
    { name = "tonic", version = "0.1.0-alpha.4" },
    # ignore older rand as many crates still use it instead of the newer 0.7+ version
    { name = "rand", version = "=0.6.5" },

CI Usage

cargo-deny is primarily meant to be used in your CI so it can do automatic verification for all your changes, for an example of this, you can look at the self check job for this repository, which just checks cargo-deny itself using the deny.toml config.

List - cargo deny list

Similarly to cargo-license, print out the licenses and crates that use them.

  • layout = license, format = human (default)


  • layout = crate, format = human


  • layout = license, format = json


  • layout = license, format = tsv



We welcome community contributions to this project.

Please read our Contributor Guide for more information on how to get started.


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


~217K SLoC