34 Working With Licenses

As mentioned in the “Licensing” section in the Yocto Project Overview and Concepts Manual, open source projects are open to the public and they consequently have different licensing structures in place. This section describes the mechanism by which the OpenEmbedded Build System tracks changes to licensing text and covers how to maintain open source license compliance during your project’s lifecycle. The section also describes how to enable commercially licensed recipes, which by default are disabled.

34.1 Tracking License Changes

The license of an upstream project might change in the future. In order to prevent these changes going unnoticed, the LIC_FILES_CHKSUM variable tracks changes to the license text. The checksums are validated at the end of the configure step, and if the checksums do not match, the build will fail.

34.1.1 Specifying the LIC_FILES_CHKSUM Variable

The LIC_FILES_CHKSUM variable contains checksums of the license text in the source code for the recipe. Following is an example of how to specify LIC_FILES_CHKSUM:

LIC_FILES_CHKSUM = "file://COPYING;md5=xxxx \
                    file://licfile1.txt;beginline=5;endline=29;md5=yyyy \
                    file://licfile2.txt;endline=50;md5=zzzz \


  • When using “beginline” and “endline”, realize that line numbering begins with one and not zero. Also, the included lines are inclusive (i.e. lines five through and including 29 in the previous example for licfile1.txt).

  • When a license check fails, the selected license text is included as part of the QA message. Using this output, you can determine the exact start and finish for the needed license text.

The build system uses the S variable as the default directory when searching files listed in LIC_FILES_CHKSUM. The previous example employs the default directory.

Consider this next example:

LIC_FILES_CHKSUM = "file://src/ls.c;beginline=5;endline=16;\
LIC_FILES_CHKSUM = "file://${WORKDIR}/license.html;md5=5c94767cedb5d6987c902ac850ded2c6"

The first line locates a file in ${S}/src/ls.c and isolates lines five through 16 as license text. The second line refers to a file in WORKDIR.

Note that LIC_FILES_CHKSUM variable is mandatory for all recipes, unless the LICENSE variable is set to “CLOSED”.

34.1.2 Explanation of Syntax

As mentioned in the previous section, the LIC_FILES_CHKSUM variable lists all the important files that contain the license text for the source code. It is possible to specify a checksum for an entire file, or a specific section of a file (specified by beginning and ending line numbers with the “beginline” and “endline” parameters, respectively). The latter is useful for source files with a license notice header, README documents, and so forth. If you do not use the “beginline” parameter, then it is assumed that the text begins on the first line of the file. Similarly, if you do not use the “endline” parameter, it is assumed that the license text ends with the last line of the file.

The “md5” parameter stores the md5 checksum of the license text. If the license text changes in any way as compared to this parameter then a mismatch occurs. This mismatch triggers a build failure and notifies the developer. Notification allows the developer to review and address the license text changes. Also note that if a mismatch occurs during the build, the correct md5 checksum is placed in the build log and can be easily copied to the recipe.

There is no limit to how many files you can specify using the LIC_FILES_CHKSUM variable. Generally, however, every project requires a few specifications for license tracking. Many projects have a “COPYING” file that stores the license information for all the source code files. This practice allows you to just track the “COPYING” file as long as it is kept up to date.


  • If you specify an empty or invalid “md5” parameter, BitBake returns an md5 mis-match error and displays the correct “md5” parameter value during the build. The correct parameter is also captured in the build log.

  • If the whole file contains only license text, you do not need to use the “beginline” and “endline” parameters.

34.2 Enabling Commercially Licensed Recipes

By default, the OpenEmbedded build system disables components that have commercial or other special licensing requirements. Such requirements are defined on a recipe-by-recipe basis through the LICENSE_FLAGS variable definition in the affected recipe. For instance, the poky/meta/recipes-multimedia/gstreamer/gst-plugins-ugly recipe contains the following statement:

LICENSE_FLAGS = "commercial"

Here is a slightly more complicated example that contains both an explicit recipe name and version (after variable expansion):

LICENSE_FLAGS = "license_${PN}_${PV}"

In order for a component restricted by a LICENSE_FLAGS definition to be enabled and included in an image, it needs to have a matching entry in the global LICENSE_FLAGS_ACCEPTED variable, which is a variable typically defined in your local.conf file. For example, to enable the poky/meta/recipes-multimedia/gstreamer/gst-plugins-ugly package, you could add either the string “commercial_gst-plugins-ugly” or the more general string “commercial” to LICENSE_FLAGS_ACCEPTED. See the “License Flag Matching” section for a full explanation of how LICENSE_FLAGS matching works. Here is the example:

LICENSE_FLAGS_ACCEPTED = "commercial_gst-plugins-ugly"

Likewise, to additionally enable the package built from the recipe containing LICENSE_FLAGS = "license_${PN}_${PV}", and assuming that the actual recipe name was emgd_1.10.bb, the following string would enable that package as well as the original gst-plugins-ugly package:

LICENSE_FLAGS_ACCEPTED = "commercial_gst-plugins-ugly license_emgd_1.10"

As a convenience, you do not need to specify the complete license string for every package. You can use an abbreviated form, which consists of just the first portion or portions of the license string before the initial underscore character or characters. A partial string will match any license that contains the given string as the first portion of its license. For example, the following value will also match both of the packages previously mentioned as well as any other packages that have licenses starting with “commercial” or “license”:

LICENSE_FLAGS_ACCEPTED = "commercial license"

34.2.1 License Flag Matching

License flag matching allows you to control what recipes the OpenEmbedded build system includes in the build. Fundamentally, the build system attempts to match LICENSE_FLAGS strings found in recipes against strings found in LICENSE_FLAGS_ACCEPTED. A match causes the build system to include a recipe in the build, while failure to find a match causes the build system to exclude a recipe.

In general, license flag matching is simple. However, understanding some concepts will help you correctly and effectively use matching.

Before a flag defined by a particular recipe is tested against the entries of LICENSE_FLAGS_ACCEPTED, the expanded string _${PN} is appended to the flag. This expansion makes each LICENSE_FLAGS value recipe-specific. After expansion, the string is then matched against the entries. Thus, specifying LICENSE_FLAGS = "commercial" in recipe “foo”, for example, results in the string "commercial_foo". And, to create a match, that string must appear among the entries of LICENSE_FLAGS_ACCEPTED.

Judicious use of the LICENSE_FLAGS strings and the contents of the LICENSE_FLAGS_ACCEPTED variable allows you a lot of flexibility for including or excluding recipes based on licensing. For example, you can broaden the matching capabilities by using license flags string subsets in LICENSE_FLAGS_ACCEPTED.


When using a string subset, be sure to use the part of the expanded string that precedes the appended underscore character (e.g. usethispart_1.3, usethispart_1.4, and so forth).

For example, simply specifying the string “commercial” in the LICENSE_FLAGS_ACCEPTED variable matches any expanded LICENSE_FLAGS definition that starts with the string “commercial” such as “commercial_foo” and “commercial_bar”, which are the strings the build system automatically generates for hypothetical recipes named “foo” and “bar” assuming those recipes simply specify the following:

LICENSE_FLAGS = "commercial"

Thus, you can choose to exhaustively enumerate each license flag in the list and allow only specific recipes into the image, or you can use a string subset that causes a broader range of matches to allow a range of recipes into the image.

This scheme works even if the LICENSE_FLAGS string already has _${PN} appended. For example, the build system turns the license flag “commercial_1.2_foo” into “commercial_1.2_foo_foo” and would match both the general “commercial” and the specific “commercial_1.2_foo” strings found in the LICENSE_FLAGS_ACCEPTED variable, as expected.

Here are some other scenarios:

  • You can specify a versioned string in the recipe such as “commercial_foo_1.2” in a “foo” recipe. The build system expands this string to “commercial_foo_1.2_foo”. Combine this license flag with a LICENSE_FLAGS_ACCEPTED variable that has the string “commercial” and you match the flag along with any other flag that starts with the string “commercial”.

  • Under the same circumstances, you can add “commercial_foo” in the LICENSE_FLAGS_ACCEPTED variable and the build system not only matches “commercial_foo_1.2” but also matches any license flag with the string “commercial_foo”, regardless of the version.

  • You can be very specific and use both the package and version parts in the LICENSE_FLAGS_ACCEPTED list (e.g. “commercial_foo_1.2”) to specifically match a versioned recipe.

34.3 Maintaining Open Source License Compliance During Your Product’s Lifecycle

One of the concerns for a development organization using open source software is how to maintain compliance with various open source licensing during the lifecycle of the product. While this section does not provide legal advice or comprehensively cover all scenarios, it does present methods that you can use to assist you in meeting the compliance requirements during a software release.

With hundreds of different open source licenses that the Yocto Project tracks, it is difficult to know the requirements of each and every license. However, the requirements of the major FLOSS licenses can begin to be covered by assuming that there are three main areas of concern:

  • Source code must be provided.

  • License text for the software must be provided.

  • Compilation scripts and modifications to the source code must be provided.

There are other requirements beyond the scope of these three and the methods described in this section (e.g. the mechanism through which source code is distributed).

As different organizations have different ways of releasing software, there can be multiple ways of meeting license obligations. At least, we describe here two methods for achieving compliance:

  • The first method is to use OpenEmbedded’s ability to provide the source code, provide a list of licenses, as well as compilation scripts and source code modifications.

    The remainder of this section describes supported methods to meet the previously mentioned three requirements.

  • The second method is to generate a Software Bill of Materials (SBoM), as described in the “Creating a Software Bill of Materials” section. Not only do you generate SPDX output which can be used meet license compliance requirements (except for sharing the build system and layers sources for the time being), but this output also includes component version and patch information which can be used for vulnerability assessment.

Whatever method you choose, prior to releasing images, sources, and the build system, you should audit all artifacts to ensure completeness.


The Yocto Project generates a license manifest during image creation that is located in ${DEPLOY_DIR}/licenses/image_name-datestamp to assist with any audits.

34.3.1 Providing the Source Code

Compliance activities should begin before you generate the final image. The first thing you should look at is the requirement that tops the list for most compliance groups — providing the source. The Yocto Project has a few ways of meeting this requirement.

One of the easiest ways to meet this requirement is to provide the entire DL_DIR used by the build. This method, however, has a few issues. The most obvious is the size of the directory since it includes all sources used in the build and not just the source used in the released image. It will include toolchain source, and other artifacts, which you would not generally release. However, the more serious issue for most companies is accidental release of proprietary software. The Yocto Project provides an archiver class to help avoid some of these concerns.

Before you employ DL_DIR or the archiver class, you need to decide how you choose to provide source. The source archiver class can generate tarballs and SRPMs and can create them with various levels of compliance in mind.

One way of doing this (but certainly not the only way) is to release just the source as a tarball. You can do this by adding the following to the local.conf file found in the Build Directory:

INHERIT += "archiver"
ARCHIVER_MODE[src] = "original"

During the creation of your image, the source from all recipes that deploy packages to the image is placed within subdirectories of DEPLOY_DIR/sources based on the LICENSE for each recipe. Releasing the entire directory enables you to comply with requirements concerning providing the unmodified source. It is important to note that the size of the directory can get large.

A way to help mitigate the size issue is to only release tarballs for licenses that require the release of source. Let us assume you are only concerned with GPL code as identified by running the following script:

# Script to archive a subset of packages matching specific license(s)
# Source and license files are copied into sub folders of package folder
# Must be run from build folder
mkdir -p $src_release_dir
for a in tmp/deploy/sources/*; do
   for d in $a/*; do
      # Get package name from path
      p=`basename $d`
      # Only archive GPL packages (update *GPL* regex for your license check)
      numfiles=`ls tmp/deploy/licenses/$p/*GPL* 2> /dev/null | wc -l`
      if [ $numfiles -ge 1 ]; then
         echo Archiving $p
         mkdir -p $src_release_dir/$p/source
         cp $d/* $src_release_dir/$p/source 2> /dev/null
         mkdir -p $src_release_dir/$p/license
         cp tmp/deploy/licenses/$p/* $src_release_dir/$p/license 2> /dev/null

At this point, you could create a tarball from the gpl_source_release directory and provide that to the end user. This method would be a step toward achieving compliance with section 3a of GPLv2 and with section 6 of GPLv3.

34.3.2 Providing License Text

One requirement that is often overlooked is inclusion of license text. This requirement also needs to be dealt with prior to generating the final image. Some licenses require the license text to accompany the binary. You can achieve this by adding the following to your local.conf file:


Adding these statements to the configuration file ensures that the licenses collected during package generation are included on your image.


Setting all three variables to “1” results in the image having two copies of the same license file. One copy resides in /usr/share/common-licenses and the other resides in /usr/share/license.

The reason for this behavior is because COPY_LIC_DIRS and COPY_LIC_MANIFEST add a copy of the license when the image is built but do not offer a path for adding licenses for newly installed packages to an image. LICENSE_CREATE_PACKAGE adds a separate package and an upgrade path for adding licenses to an image.

As the source archiver class has already archived the original unmodified source that contains the license files, you would have already met the requirements for inclusion of the license information with source as defined by the GPL and other open source licenses.

34.3.3 Providing Compilation Scripts and Source Code Modifications

At this point, we have addressed all we need to prior to generating the image. The next two requirements are addressed during the final packaging of the release.

By releasing the version of the OpenEmbedded build system and the layers used during the build, you will be providing both compilation scripts and the source code modifications in one step.

If the deployment team has a BSP Layer and a distro layer, and those those layers are used to patch, compile, package, or modify (in any way) any open source software included in your released images, you might be required to release those layers under section 3 of GPLv2 or section 1 of GPLv3. One way of doing that is with a clean checkout of the version of the Yocto Project and layers used during your build. Here is an example:

# We built using the dunfell branch of the poky repo
$ git clone -b dunfell git://git.yoctoproject.org/poky
$ cd poky
# We built using the release_branch for our layers
$ git clone -b release_branch git://git.mycompany.com/meta-my-bsp-layer
$ git clone -b release_branch git://git.mycompany.com/meta-my-software-layer
# clean up the .git repos
$ find . -name ".git" -type d -exec rm -rf {} \;

One thing a development organization might want to consider for end-user convenience is to modify meta-poky/conf/templates/default/bblayers.conf.sample to ensure that when the end user utilizes the released build system to build an image, the development organization’s layers are included in the bblayers.conf file automatically:

# POKY_BBLAYERS_CONF_VERSION is increased each time build/conf/bblayers.conf
# changes incompatibly


  ##OEROOT##/meta \
  ##OEROOT##/meta-poky \
  ##OEROOT##/meta-yocto-bsp \
  ##OEROOT##/meta-mylayer \

Creating and providing an archive of the Metadata layers (recipes, configuration files, and so forth) enables you to meet your requirements to include the scripts to control compilation as well as any modifications to the original source.

34.3.4 Compliance Limitations with Executables Built from Static Libraries

When package A is added to an image via the RDEPENDS or RRECOMMENDS mechanisms as well as explicitly included in the image recipe with IMAGE_INSTALL, and depends on a static linked library recipe B (DEPENDS += "B"), package B will neither appear in the generated license manifest nor in the generated source tarballs. This occurs as the license and archiver classes assume that only packages included via RDEPENDS or RRECOMMENDS end up in the image.

As a result, potential obligations regarding license compliance for package B may not be met.

The Yocto Project doesn’t enable static libraries by default, in part because of this issue. Before a solution to this limitation is found, you need to keep in mind that if your root filesystem is built from static libraries, you will need to manually ensure that your deliveries are compliant with the licenses of these libraries.

34.4 Copying Non Standard Licenses

Some packages, such as the linux-firmware package, have many licenses that are not in any way common. You can avoid adding a lot of these types of common license files, which are only applicable to a specific package, by using the NO_GENERIC_LICENSE variable. Using this variable also avoids QA errors when you use a non-common, non-CLOSED license in a recipe.

Here is an example that uses the LICENSE.Abilis.txt file as the license from the fetched source:

NO_GENERIC_LICENSE[Firmware-Abilis] = "LICENSE.Abilis.txt"