TCP

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

Configuration Parameters

The plugin supports the following configuration parameters:

Key	Description	Default
Listen	Listener network interface.	0.0.0.0
Port	TCP port to listen for connections.	5170
Buffer_Size	Specify the maximum buffer size in KB to receive a JSON message. If not set, the default is the value of Chunk_Size.	Chunk_Size
Chunk_Size	The default buffer to store the incoming JSON messages. It doesn't allocate the maximum memory allowed; instead it allocates memory when required. The rounds of allocations are set by Chunk_Size. If not set, Chunk_Size is equal to 32 (32KB).	32
Format	Specify the expected payload format. Supported values: json and none. When set to json it expects JSON maps. When set to none, every record splits 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 to inject the source address.	none
Threaded	Indicates whether to run this input in its own thread.	false

Get started

To receive JSON messages over TCP, 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 tcp -o stdout

By default the service will listen an all interfaces (0.0.0.0) through TCP port 5170. Optionally you can change this directly:

fluent-bit -i tcp://192.168.3.2:9090 -o stdout

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

Configuration file

In your main configuration file append the following sections:

fluent-bit.yaml

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

    outputs:
        - name: stdout
          match: '*'

fluent-bit.conf

[INPUT]
    Name        tcp
    Listen      0.0.0.0
    Port        5170
    Chunk_Size  32
    Buffer_Size 64
    Format      json

[OUTPUT]
    Name        stdout
    Match       *

Test the configuration

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

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

Run Fluent Bit:

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

You should see the following output:

Fluent Bit v1.x.x
* Copyright (C) 2019-2020 The Fluent Bit Authors
* Copyright (C) 2015-2018 Treasure Data
* 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] tcp.0: [1570115975.581246030, {"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 to use the option Format none to avoid JSON parsing if not needed.