# Configure Fluent Bit

Fluent Bit uses configuration files to store information about your specified [inputs](https://docs.fluentbit.io/manual/data-pipeline/inputs), [outputs](https://docs.fluentbit.io/manual/data-pipeline/outputs), [filters](https://docs.fluentbit.io/manual/data-pipeline/filters), and more. You can write these configuration files in one of these formats:

* [YAML configuration files](https://docs.fluentbit.io/manual/administration/configuring-fluent-bit/yaml) are the standard configuration format as of Fluent Bit v3.2. They use the `.yaml` file extension.
* [Classic configuration files](https://docs.fluentbit.io/manual/administration/configuring-fluent-bit/classic-mode) will be deprecated at the end of 2026. They use the `.conf` file extension.

## Unit sizes

Some configuration settings in Fluent Bit use standardized unit sizes to define data and storage limits. For example, the `buffer_chunk_size` and `buffer_max_size` parameters for the [Tail](https://docs.fluentbit.io/manual/data-pipeline/inputs/tail) input plugin use unit sizes.

The following table describes the unit sizes you can use and what they mean.

| Suffix               | Description                                                                                                           | Example                           |
| -------------------- | --------------------------------------------------------------------------------------------------------------------- | --------------------------------- |
| *none*               | **Bytes**: If you specify an integer without a unit size, Fluent Bit interprets that value as a bytes representation. | `32000` means 32,000 bytes.       |
| `k`, `kb`, `K`, `KB` | **Kilobytes**: A unit of memory equal to 1,000 bytes.                                                                 | `32k` means 32,000 bytes.         |
| `m`, `mb`, `M`, `MB` | **Megabytes**: A unit of memory equal to 1,000,000 bytes.                                                             | `32m` means 32,000,000 bytes.     |
| `g`, `gb`, `G`, `GB` | **Gigabytes**: A unit of memory equal to 1,000,000,000 bytes.                                                         | `32g` means 32,000,000,000 bytes. |

## Command line interface

Fluent Bit exposes most of its configuration features through the command line interface. Use the `-h` or `--help` flag to see a list of available options.

```shell
# Podman container tooling.
podman run -rm -ti fluent/fluent-bit --help

# Docker container tooling.
docker run --rm -it fluent/fluent-bit --help
```

### Validate configuration with `--dry-run`

Use the `--dry-run` flag to validate a configuration file without starting Fluent Bit:

```shell
fluent-bit --dry-run -c /path/to/fluent-bit.yaml
```

A successful validation prints `configuration test is successful` and exits with code `0`. If validation fails, Fluent Bit exits with a non-zero code and prints the errors to stderr.

As of Fluent Bit 4.2, `--dry-run` performs full property validation in addition to syntax checking. Prior to 4.2, unknown or misspelled plugin property names would only surface as errors at runtime; `--dry-run` now catches them during validation. For example, a configuration with an unknown property on a `dummy` input produces:

```
[error] [config] dummy: unknown configuration property 'invalid_property_that_does_not_exist'.
```