GELF is Graylog Extended Log Format. The GELF output plugin allows to send logs in GELF format directly to a Graylog input using TLS, TCP or UDP protocols.
The following instructions assumes that you have a fully operational Graylog server running in your environment.

Configuration Parameters

According to GELF Payload Specification, there are some mandatory and optional fields which are used by Graylog in GELF format. These fields are determined with Gelf\*_Key_ key in this plugin.
Pattern to match which tags of logs to be outputted by this plugin
IP address or hostname of the target Graylog server
The port that your Graylog GELF input is listening on
The protocol to use (tls, tcp or udp)
A short descriptive message (MUST be set in GELF)
Your log timestamp (SHOULD be set in GELF)
Key which its value is used as the name of the host, source or application that sent this message. (MUST be set in GELF)
Key to use as the long message that can i.e. contain a backtrace. (Optional in GELF)
Key to be used as the log level. Its value must be in standard syslog levels (between 0 and 7). (Optional in GELF)
If transport protocol is udp, you can set the size of packets to be sent.
If transport protocol is udp, you can set this if you want your UDP packets to be compressed.


GELF output plugin supports TLS/SSL, for more details about the properties available and general configuration, please refer to the TLS/SSL section.


  • If you're using Fluent Bit to collect Docker logs, note that Docker places your log in JSON under key log. So you can set log as your Gelf_Short_Message_Key to send everything in Docker logs to Graylog. In this case, you need your log value to be a string; so don't parse it using JSON parser.
  • The order of looking up the timestamp in this plugin is as follows:
    1. 1.
      Value of Gelf_Timestamp_Key provided in configuration
    2. 2.
      Value of timestamp key
    3. 3.
      If you're using Docker JSON parser, this parser can parse time and use it as timestamp of message. If all above fail, Fluent Bit tries to get timestamp extracted by your parser.
    4. 4.
      Timestamp does not set by Fluent Bit. In this case, your Graylog server will set it to the current timestamp (now).
  • Your log timestamp has to be in UNIX Epoch Timestamp format. If the Gelf_Timestamp_Key value of your log is not in this format, your Graylog server will ignore it.
  • If you're using Fluent Bit in Kubernetes and you're using Kubernetes Filter Plugin, this plugin adds host value to your log by default, and you don't need to add it by your own.
  • The version of GELF message is also mandatory and Fluent Bit sets it to 1.1 which is the current latest version of GELF.
  • If you use udp as transport protocol and set Compress to true, Fluent Bit compresses your packets in GZIP format, which is the default compression that Graylog offers. This can be used to trade more CPU load for saving network bandwidth.

Configuration File Example

If you're using Fluent Bit for shipping Kubernetes logs, you can use something like this as your configuration file:
Name tail
Tag kube.*
Path /var/log/containers/*.log
Parser docker
DB /var/log/flb_kube.db
Mem_Buf_Limit 5MB
Refresh_Interval 10
Name kubernetes
Match kube.*
Merge_Log_Key log
Merge_Log On
Keep_Log Off
Annotations Off
Labels Off
Name nest
Match *
Operation lift
Nested_under log
Name gelf
Match kube.*
Host <your-graylog-server>
Port 12201
Mode tcp
Gelf_Short_Message_Key data
Name docker
Format json
Time_Key time
Time_Format %Y-%m-%dT%H:%M:%S.%L
Time_Keep Off
By default, GELF tcp uses port 12201 and Docker places your logs in /var/log/containers directory. The logs are placed in value of the log key. For example, this is a log saved by Docker:
{"log":"{\"data\": \"This is an example.\"}","stream":"stderr","time":"2019-07-21T12:45:11.273315023Z"}
If you use Tail Input and use a Parser like the docker parser shown above, it decodes your message and extracts data (and any other present) field. This is how this log in stdout looks like after decoding:
[0] kube.log: [1565770310.000198491, {"log"=>{"data"=>"This is an example."}, "stream"=>"stderr", "time"=>"2019-07-21T12:45:11.273315023Z"}]
Now, this is what happens to this log:
  1. 1.
    Fluent Bit GELF plugin adds "version": "1.1" to it.
  2. 2.
    The Nest Filter, unnests fields inside log key. In our example, it puts data alongside stream and time.
  3. 3.
    We used this data key as Gelf_Short_Message_Key; so GELF plugin changes it to short_message.
  4. 4.
    Kubernetes Filter adds host name.
  5. 5.
    Timestamp is generated.
  6. 6.
    Any custom field (not present in GELF Payload Specification) is prefixed by an underline.
Finally, this is what our Graylog server input sees:
{"version":"1.1", "short_message":"This is an example.", "host": "<Your Node Name>", "_stream":"stderr", "timestamp":1565770310.000199}
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Configuration Parameters
Configuration File Example