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UDP

The UDP input plugin lets you retrieve structured JSON or raw messages over a UDP network interface (UDP port).

Configuration parameters

The plugin supports the following configuration parameters:

Key
Description
Default

Listen

Listener network interface.

0.0.0.0

Port

UDP port used to listen for connections.

5170

Buffer_Size

Specify the maximum buffer size in KB to receive a JSON message. If not set, the default size will be the value of Chunk_Size.

Chunk_Size (value)

Chunk_Size

The default buffer to store incoming JSON messages. Doesn't allocate the maximum memory allowed; instead it allocates memory when required. The rounds of allocations are set by Chunk_Size in KB.

32

Format

Specify the expected payload format. Supported values: json and none. json expects JSON maps. none splits every record using the defined Separator.

json

Separator

When Format is set to none, Fluent Bit needs a separator string to split the records.

LF or 0x10 (break line)

Source_Address_Key

Specify the key where the source address will be injected.

none

Threaded

Indicates whether to run this input in its own thread.

false

Get started

To receive JSON messages over UDP, you can run the plugin from the command line or through the configuration file.

Command line

From the command line you can let Fluent Bit listen for JSON messages with the following options:

fluent-bit -i udp -o stdout

By default, the service listens on all interfaces (0.0.0.0) using UDP port 5170. Optionally. you can change this directly.

In this example the JSON messages will only arrive through network interface at 192.168.3.2 address and UDP Port 9090.

fluent-bit -i udp -pport=9090 -o stdout

Configuration file

In your main configuration file append the following:

pipeline:
    inputs:
        - name: udp
          listen: 0.0.0.0
          port: 5170
          chunk_size: 32
          buffer_size: 64
          format: json

    outputs:
        - name: stdout
          match: '*'

Testing

When Fluent Bit is running, you can send some messages using netcat:

echo '{"key 1": 123456789, "key 2": "abcdefg"}' | nc -u 127.0.0.1 5170

Run Fluent Bit:

bin/fluent-bit -i udp -o stdout -f 1

You should see the following output:

Fluent Bit v2.x.x
* Copyright (C) 2015-2022 The Fluent Bit Authors
* Fluent Bit is a CNCF sub-project under the umbrella of Fluentd
* https://fluentbit.io

______ _                  _    ______ _ _             ___  _____
|  ___| |                | |   | ___ (_) |           /   ||  _  |
| |_  | |_   _  ___ _ __ | |_  | |_/ /_| |_  __   __/ /| || |/' |
|  _| | | | | |/ _ \ '_ \| __| | ___ \ | __| \ \ / / /_| ||  /| |
| |   | | |_| |  __/ | | | |_  | |_/ / | |_   \ V /\___  |\ |_/ /
\_|   |_|\__,_|\___|_| |_|\__| \____/|_|\__|   \_/     |_(_)___/


[2025/07/01 14:44:47] [ info] [fluent bit] version=4.0.3, commit=f5f5f3c17d, pid=1
[2025/07/01 14:44:47] [ info] [storage] ver=1.5.3, type=memory, sync=normal, checksum=off, max_chunks_up=128
[2025/07/01 14:44:47] [ info] [simd    ] disabled
[2025/07/01 14:44:47] [ info] [cmetrics] version=1.0.3
[2025/07/01 14:44:47] [ info] [ctraces ] version=0.6.6
[2025/07/01 14:44:47] [ info] [input:mem:mem.0] initializing
[2025/07/01 14:44:47] [ info] [input:mem:mem.0] storage_strategy='memory' (memory only)
[2025/07/01 14:44:47] [ info] [sp] stream processor started
[2025/07/01 14:44:47] [ info] [engine] Shutdown Grace Period=5, Shutdown Input Grace Period=2
[2025/07/01 14:44:47] [ info] [output:stdout:stdout.0] worker #0 started
[0] udp.0: [[1689912069.078189000, {}], {"key 1"=>123456789, "key 2"=>"abcdefg"}]

Performance considerations

When receiving payloads in JSON format, there are high performance penalties. Parsing JSON is a very expensive task so you could expect your CPU usage increase under high load environments.

To get faster data ingestion, consider using the option Format none to avoid JSON parsing if not needed.

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