GELF

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.

Key	Description	default
Match	Pattern to match which tags of logs to be outputted by this plugin
Host	IP address or hostname of the target Graylog server	127.0.0.1
Port	The port that your Graylog GELF input is listening on	12201
Mode	The protocol to use (tls, tcp or udp)	udp
Gelf_Short_Message_Key	A short descriptive message (MUST be set in GELF)	short_message
Gelf_Timestamp_Key	Your log timestamp (SHOULD be set in GELF)	timestamp
Gelf_Host_Key	Key which its value is used as the name of the host, source or application that sent this message. (MUST be set in GELF)	host
Gelf_Full_Message_Key	Key to use as the long message that can i.e. contain a backtrace. (Optional in GELF)	full_message
Gelf_Level_Key	Key to be used as the log level. Its value must be in standard syslog levels (between 0 and 7). (Optional in GELF)	level
Packet_Size	If transport protocol is udp, you can set the size of packets to be sent.	1420
Compress	If transport protocol is udp, you can set this if you want your UDP packets to be compressed.	true

TLS / SSL

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

Notes

• 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. Value of Gelf_Timestamp_Key provided in configuration

  2. Value of timestamp key

  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. 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:

[INPUT]
    Name                    tail
    Tag                     kube.*
    Path                    /var/log/containers/*.log
    Parser                  docker
    DB                      /var/log/flb_kube.db
    Mem_Buf_Limit           5MB
    Refresh_Interval        10

[FILTER]
    Name                    kubernetes
    Match                   kube.*
    Merge_Log_Key           log
    Merge_Log               On
    Keep_Log                Off
    Annotations             Off
    Labels                  Off

[FILTER]
    Name                    nest
    Match                   *
    Operation               lift
    Nested_under            log

[OUTPUT]
    Name                    gelf
    Match                   kube.*
    Host                    <your-graylog-server>
    Port                    12201
    Mode                    tcp
    Gelf_Short_Message_Key  data

[PARSER]
    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. Fluent Bit GELF plugin adds "version": "1.1" to it.

2. The Nest Filter, unnests fields inside log key. In our example, it puts data alongside stream and time.
3. We used this data key as Gelf_Short_Message_Key; so GELF plugin changes it to short_message.

4. Kubernetes Filter adds host name.
5. Timestamp is generated.

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}