Syslog

The Syslog input plugin lets you collect syslog messages through a Unix socket server (UDP or TCP) or over the network using TCP or UDP.

Configuration parameters

The plugin supports the following configuration parameters:

Key

Description

Default

Mode

Defines transport protocol mode: UDP over Unix socket (unix_udp), TCP over Unix socket (unix_tcp), tcp, or udp

unix_udp

Listen

If Mode is set to tcp or udp, specify the network interface to bind.

0.0.0.0

Port

If Mode is set to tcp or udp, specify the TCP port to listen for incoming connections.

5140

Path

If Mode is set to unix_tcp or unix_udp, set the absolute path to the Unix socket file.

none

Unix_Perm

If Mode is set to unix_tcp or unix_udp, set the permission of the Unix socket file.

0644

Parser

Specify an alternative parser for the message. If Mode is set to tcp or udp then the default parser is syslog-rfc5424. Otherwise, syslog-rfc3164-local is used. If your syslogmessages have fractional seconds set this parser value tosyslog-rfc5424` instead.

none

Buffer_Chunk_Size

By default, the buffer to store the incoming syslog messages. Doesn't allocate the maximum memory allowed, instead it allocates memory when required. The rounds of allocations are set by Buffer_Chunk_Size. There are considerations when using udp or unix_udp mode.

32KB (set in code)

Buffer_Max_Size

Specify the maximum buffer size to receive a syslog message. If not set, the default size is the value of Buffer_Chunk_Size.

none

Receive_Buffer_Size

Specify the maximum socket receive buffer size. If not set, the default value is OS-dependant, but generally too low to accept thousands of syslog messages per second without loss on udp or unix_udp sockets. For Linux, the value is capped by sysctl net.core.rmem_max.

none

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

Considerations

When using the Syslog input plugin, Fluent Bit requires access to the parsers configuration file. The path to this file can be specified with the option -R or through the parsers_file key in the service section.
When using udp or unix_udp, the buffer size to receive messages is configurable only through the buffer_chunk_size option, which defaults to 32kb.

Get started

To receive syslog messages, 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 Forward messages with the following options:

# For YAML configuration
./fluent-bit -R /path/to/parsers.yaml -i syslog -p path=/tmp/in_syslog -o stdout

# For classic configuration.
./fluent-bit -R /path/to/parsers.conf -i syslog -p path=/tmp/in_syslog -o stdout

By default the service will create and listen for Syslog messages on the Unix socket /tmp/in_syslog.

Configuration file

In your main configuration file append the following sections:

service:
    flush: 1
    log_level: info
    parsers_file: parsers.yaml
    
pipeline:
    inputs:
        - name: syslog
          path: /tmp/in_syslog
          buffer_chunk_size: 32000
          buffer_max_size: 64000
          receive_buffer_size: 512000
          
    outputs:
        - name: stdout
          match: '*'

[SERVICE]
    Flush               1
    Log_Level           info
    Parsers_File        parsers.conf

[INPUT]
    Name                syslog
    Path                /tmp/in_syslog
    Buffer_Chunk_Size   32000
    Buffer_Max_Size     64000
    Receive_Buffer_Size 512000

[OUTPUT]
    Name   stdout
    Match  *

Testing

When Fluent Bit is running, you can send some messages using the logger tool:

logger -u /tmp/in_syslog my_ident my_message

Then run Fluent bit using the following command:

# For YAML ocnfiguration.
./fluent-bit -R ../conf/parsers.yaml -i syslog -p path=/tmp/in_syslog -o stdout

# For classic configuration.
./fluent-bit -R ../conf/parsers.conf -i syslog -p path=/tmp/in_syslog -o stdout

You should see the following output:

Fluent Bit v4.0.3
* Copyright (C) 2015-2025 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] syslog.0: [1489047822, {"pri"=>"13", "host"=>"edsiper:", "ident"=>"my_ident", "pid"=>"", "message"=>"my_message"}]

Examples

The following configuration examples cover different use cases to integrate Fluent Bit and make it listen for Syslog messages from your systems.

`rsyslog` to Fluent Bit: Network mode over TCP

Fluent Bit configuration

Put the following content in your configuration file:

service:
    flush: 1
    parsers_file: parsers.yaml
    
pipeline:
    inputs:
        - name: syslog
          parser: syslog-rfc3164
          listen: 0.0.0.0
          port: 5140
          mode: tcp
          
    outputs:
        - name: stdout
          match: '*'

[SERVICE]
    Flush        1
    Parsers_File parsers.conf

[INPUT]
    Name     syslog
    Parser   syslog-rfc3164
    Listen   0.0.0.0
    Port     5140
    Mode     tcp

[OUTPUT]
    Name     stdout
    Match    *

Then, start Fluent Bit.

`rsyslog` configuration

Add a new file to your rsyslog configuration rules called 60-fluent-bit.conf inside the directory /etc/rsyslog.d/ and add the following content:

action(type="omfwd" Target="127.0.0.1" Port="5140" Protocol="tcp")

Then, restart your rsyslog daemon:

sudo service rsyslog restart

`rsyslog` to Fluent Bit: Unix socket mode over UDP

Fluent Bit configuration

Put the following content in your Fluent Bit configuration:

service:
    flush: 1
    parsers_file: parsers.yaml
    
pipeline:
    inputs:
        - name: syslog
          parser: syslog-rfc3164
          path: /tmp/fluent-bit.sock
          mode: unix_udp
          unix_perm: 0644
          
    outputs:
        - name: stdout
          match: '*'

[SERVICE]
    Flush        1
    Parsers_File parsers.conf

[INPUT]
    Name      syslog
    Parser    syslog-rfc3164
    Path      /tmp/fluent-bit.sock
    Mode      unix_udp
    Unix_Perm 0644

[OUTPUT]
    Name      stdout
    Match     *

Then, start Fluent Bit.

`rsyslog` configuration

Add a new file to your rsyslog configuration rules called 60-fluent-bit.conf inside the directory /etc/rsyslog.d/ containing the following content:

$ModLoad omuxsock
$OMUxSockSocket /tmp/fluent-bit.sock
*.* :omuxsock:

Make sure that the socket file is readable by rsyslog by modifying Unix_Perm key.

Last updated 2 days ago

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Configuration parameters

Considerations

Get started

Command line

Configuration file

Testing

Examples

rsyslog to Fluent Bit: Network mode over TCP

Fluent Bit configuration

rsyslog configuration

rsyslog to Fluent Bit: Unix socket mode over UDP

Fluent Bit configuration

rsyslog configuration

`rsyslog` to Fluent Bit: Network mode over TCP

`rsyslog` configuration

`rsyslog` to Fluent Bit: Unix socket mode over UDP

`rsyslog` configuration