#### Best Known Configuration

## Introduction

Component `<tag>`s are used both by OEMs and by customers to identify a known-working (or commercially supported) set of firmware on the machine.

This allows two things:

- Factory recovery where a system in the field has been upgraded
- Ensuring a consistent set of vendor-tested firmware for a specific workload

The tags are either assigned in the firmware cabinet archive (the `.metainfo.xml` file) or added post-upload on the LVFS and are then included in the public AppStream metadata. A single firmware can be marked with multiple tags, and tags can be duplicated for different firmwares. This would allow an OEM to say this set of firmware has been tested as a set for workload A, and this other set of firmware has been tested for workload B which is fairly typical for enterprise deployments.

## LVFS

The LVFS added support for “vendor-defined” component `<tag>`s in 2021, which are also supported in fwupd since version 1.7.3. These tags are typically used by OEMs to identify a manifest of firmware for a specific machine SKU. This is opt-in for each vendor, and so if you are a vendor reading this and want to use this feature, let us know by opening an issue.

## Client

When provisioning the client machine, we can set the BKC by setting `HostBkc=vendor-2021q1` in `/etc/fwupd/fwupd.conf`.

Any invocation of `fwupdmgr sync` will install or downgrade firmware on all compatible devices (e.g. UEFI, RAID, network adapter, & SAS HBA) to make the system match a compatible set. The `fwupdmgr sync` command will also ensure that firmware is installed that matches the device branch, if the device has one assigned.

Updating or downgrading firmware away from the Best Known Configuration or to different branches is allowed, but the UI shows a warning. Using `fwupdmgr sync` will undo any manual changes and bring the machine back to the BKC.

## Local metadata

To define a locally defined BKC, extra metadata is read from the `/usr/share/fwupd/local.d` and `/var/lib/fwupd/local.d` directories. For instance:

For example:

    <?xml version='1.0' encoding='utf-8'?>
    <components origin="mycompanyname">
      <component merge="append">
        <provides>
          <firmware>3ef35d3b-ceeb-5e27-8c0a-ac25f90367ab</firmware>
          <firmware>2ef35d3b-ceeb-5e27-8c0a-ac25f90367ac</firmware>
          <firmware>1ef35d3b-ceeb-5e27-8c0a-ac25f90367ad</firmware>
        </provides>
        <releases>
          <release version="225.53.1649"/>
          <release version="224.48.1605"/>
        </releases>
        <tags>
          <tag>mycompanyname-2022q1</tag>
        </tags>
      </component>
    </components>

This then appears when getting the releases for that specific GUID:

    fwupdmgr get-releases --json 3ef35d3b-ceeb-5e27-8c0a-ac25f90367ab
    {
      "Releases" : [
        {
          ...
          "Version" : "225.53.1649",
          "Tags" : [
            "mycompanyname-2022q1"
          ],
          ...
        },
        {
          ...
          "Version" : "224.48.1605",
          "Tags" : [
            "mycompanyname-2022q1"
          ],
          ...
        },
        {
          ...
          "Version" : "224.45.1389",
          ...
        }
      ]
    }

..and can be synced on the command line:

    $ fwupdmgr sync
    ╔══════════════════════════════════════════════════════════════════════════════╗
    ║ Downgrade System Firmware from 225.52.1521 to 225.53.1649?                   ║
    ╠══════════════════════════════════════════════════════════════════════════════╣
    ║ This release sets the number of HBAs supported by the system to 1024.        ║
    ╚══════════════════════════════════════════════════════════════════════════════╝
    Perform operation? [Y|n]:

## Vendor Firmware Remotes

Tags can also be included in the `metainfo.xml` files included in `.cab` archives installed into `/usr/share/fwupd/remotes.d/vendor/firmware/` — although in most cases it makes sense to actually decouple the tag assignment from the firmware binary by specifying local metadata.

If the tag is specific to the firmware build, then it can be included directly in the metadata:

    <?xml version="1.0" encoding="UTF-8"?>
    <component type="firmware">
      <id>org.fwupd.myproduct.firmware</id>
      ...
      <tags>
        <tag namespace="lvfs">product-mycompanyname-2022q1</tag>
      </tags>
    </component>

NOTE: the `namespace="lvfs"` is required for fwupd as the `<tag>` section is also used by other software for different purposes. Forgetting the namespace will cause fwupd to ignore the tag!