Configure Fluent Bit

Fluent Bit uses configuration files to store information about your specified inputs, outputs, filters, and more. You can write these configuration files in one of these formats:

• YAML configuration files are the standard configuration format as of Fluent Bit v3.2. They use the .yaml file extension.

• Classic configuration files 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 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.

# 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:

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'.