# Kafka destination options The `kafka` destination of AxoSyslog can directly publish log messages to the [Apache Kafka]() message bus, where subscribers can access them. The `kafka` destination has the following options. ## Required options: The following options are required: `kafka-bootstrap-servers()`, `topic()`. Note that to use `kafka`, you must add the following lines to the beginning of your AxoSyslog configuration: ``` @include "scl.conf" ``` ## client-lib-dir() | ---|--- Type: | string Default: | The AxoSyslog module directory: /opt/syslog-ng/lib/syslog-ng/java-modules/ _Description:_ The list of the paths where the required Java classes are located. For example, `class-path("/opt/syslog-ng/lib/syslog-ng/java-modules/:/opt/my-java-libraries/libs/")`. If you set this option multiple times in your AxoSyslog configuration (for example, because you have multiple Java-based destinations), AxoSyslog will merge every available paths to a single list. For the `kafka` destination, include the path to the directory where you copied the required libraries (see [Prerequisites](../../../docs/axosyslog-core/chapter-destinations/configuring-destinations-kafka/destination-kafka-prerequisites/index.md)), for example, `client-lib-dir("/opt/syslog-ng/lib/syslog-ng/java-modules/KafkaDestination.jar:/usr/share/kafka/lib/*.jar")`. ## kafka-bootstrap-servers() | ---|--- Type: | list of hostnames Default: | _Description:_ Specifies the hostname or IP address of the Kafka server. When specifying an IP address, IPv4 (for example, `192.168.0.1`) or IPv6 (for example, `[::1]`) can be used as well. Use a colon (`:`) after the address to specify the port number of the server. When specifying multiple addresses, use a comma to separate the addresses, for example, `kafka-bootstrap-servers("127.0.0.1:2525,remote-server-hostname:6464")` ## frac-digits() | ---|--- Type: | number Default: | 0 _Description:_ The AxoSyslog application can store fractions of a second in the timestamps according to the ISO8601 format. The `frac-digits()` parameter specifies the number of digits stored. The digits storing the fractions are padded by zeros if the original timestamp of the message specifies only seconds. Fractions can always be stored for the time the message was received. Note The AxoSyslog application can add the fractions to non-ISO8601 timestamps as well. Note As AxoSyslog is precise up to the microsecond, when the `frac-digits()` option is set to a value higher than 6, AxoSyslog will truncate the fraction seconds in the timestamps after 6 digits. ## hook-commands() _Description:_ This option makes it possible to execute external programs when the relevant driver is initialized or torn down. The `hook-commands()` can be used with all source and destination drivers with the exception of the `usertty()` and `internal()` drivers. Note The AxoSyslog application must be able to start and restart the external program, and have the necessary permissions to do so. For example, if your host is running AppArmor or SELinux, you might have to modify your AppArmor or SELinux configuration to enable AxoSyslog to execute external applications. ### Using `hook-commands()` when AxoSyslog starts or stops To execute an external program when AxoSyslog starts or stops, use the following options: #### `startup()` Type: | string ---|--- Default: | N/A _Description:_ Defines the external program that is executed as AxoSyslog starts. #### `shutdown()` Type: | string ---|--- Default: | N/A _Description:_ Defines the external program that is executed as AxoSyslog stops. ### Using the hook-commands() when AxoSyslog reloads To execute an external program when the AxoSyslog configuration is initiated or torn down, for example, on startup/shutdown or during a AxoSyslog reload, use the following options: #### `setup()` Type: | string ---|--- Default: | N/A _Description:_ Defines an external program that is executed when the AxoSyslog configuration is initiated, for example, on startup or during a AxoSyslog reload. #### `teardown()` Type: | string ---|--- Default: | N/A _Description:_ Defines an external program that is executed when the AxoSyslog configuration is stopped or torn down, for example, on shutdown or during a AxoSyslog reload. ### Example: Using hook-commands() with a network source In the following example, the `hook-commands()` is used with the `network()` driver and it opens an [iptables]( "https://en.wikipedia.org/wiki/Iptables") port automatically as AxoSyslog is started/stopped. The assumption in this example is that the `LOGCHAIN` chain is part of a larger ruleset that routes traffic to it. Whenever the AxoSyslog created rule is there, packets can flow, otherwise the port is closed. ``` source { network(transport(udp) hook-commands(           startup("iptables -I LOGCHAIN 1 -p udp --dport 514 -j ACCEPT")           shutdown("iptables -D LOGCHAIN 1")         )      ); }; ``` ## jvm-options() | ---|--- Type: | list Default: | N/A _Description:_ Specify the Java Virtual Machine (JVM) settings of your Java destination from the AxoSyslog configuration file. For example: ``` jvm-options("-Xss1M -XX:+TraceClassLoading") ``` You can set this option only as a [global option](../../../docs/axosyslog-core/chapter-global-options/index.md), by adding it to the `options` statement of the `syslog-ng.conf` configuration file. ## on-error() Type: | One of: `drop-message`, `drop-property`, `fallback-to-string`, `silently-drop-message`, `silently-drop-property`, `silently-fallback-to-string` ---|--- Default: | Use the global setting (which defaults to `drop-message`) _Description:_ Controls what happens when type-casting fails and AxoSyslog cannot convert some data to the specified type. By default, AxoSyslog drops the entire message and logs the error. Currently the `value-pairs()` option uses the settings of `on-error()`. * `drop-message`: Drop the entire message and log an error message to the `internal()` source. This is the default behavior of AxoSyslog. * `drop-property`: Omit the affected property (macro, template, or message-field) from the log message and log an error message to the `internal()` source. * `fallback-to-string`: Convert the property to string and log an error message to the `internal()` source. * `silently-drop-message`: Drop the entire message silently, without logging the error. * `silently-drop-property`: Omit the affected property (macro, template, or message-field) silently, without logging the error. * `silently-fallback-to-string`: Convert the property to string silently, without logging the error. ## key() | ---|--- Type: | template Default: | N/A _Description:_ The key of the partition under which the message is published. You can use templates to change the topic dynamically based on the source or the content of the message, for example, `key("${PROGRAM}")`. ## log-fifo-size() | ---|--- Type: | number Default: | Use global setting. _Description:_ The number of messages that the output queue can store. ## properties-file() | ---|--- Type: | string (absolute path) Default: | N/A _Description:_ The absolute path and filename of the Kafka properties file to load. For example, `properties-file("/opt/syslog-ng/etc/kafka_dest.properties")`. The AxoSyslog application reads this file and passes the properties to the Kafka Producer. If a property is defined both in the AxoSyslog configuration file (`syslog-ng.conf`) and in the properties file, then AxoSyslog uses the definition from the AxoSyslog configuration file. The AxoSyslog `kafka` destination supports all properties of the official Kafka producer. For details, see the [Apache Kafka documentation](). The `kafka-bootstrap-servers` option is translated to the `bootstrap.servers` property. For example, the following properties file defines the acknowledgment method and compression: ``` acks=all compression.type=snappy ``` ## retries() | ---|--- Type: | number (of attempts) Default: | 3 _Description:_ If AxoSyslog cannot send a message, it will try again until the number of attempts reaches `retries()`. If the number of attempts reaches `retries()`, AxoSyslog will wait for `time-reopen()` time, then tries sending the message again. ## sync-send() | ---|--- Type: | `true Default: | false _Description:_ When `sync-send` is set to `true`, AxoSyslog sends the message reliably: it sends a message to the Kafka server, then waits for a reply. In case of failure, AxoSyslog repeats sending the message, as set in the `retries()` parameter. If sending the message fails for `retries()` times, AxoSyslog drops the message. This method ensures reliable message transfer, but is very slow. When `sync-send()` is set to `false`, AxoSyslog sends messages asynchronously, and receives the response asynchronously. In case of a problem, AxoSyslog cannot resend the messages. This method is fast, but the transfer is not reliable. Several thousands of messages can be lost before AxoSyslog recognizes the error. ## template() | ---|--- Type: | template or template function Default: | `$ISODATE $HOST $MSGHDR$MSG\\n` _Description:_ The message as published to Apache Kafka. You can use templates and template functions (for example, `format-json()`) to format the message, for example, `template("$(format-json --scope rfc5424 --exclude DATE --key ISODATE)")`. For details on formatting messages in JSON format, see [format-json](../../../docs/axosyslog-core/chapter-manipulating-messages/customizing-message-format/reference-template-functions/index.md). ## throttle() | ---|--- Type: | number Default: | 0 _Description:_ Sets the maximum number of messages sent to the destination per second. Use this output-rate-limiting functionality only when using disk-buffer as well to avoid the risk of losing messages. Specifying `0` or a lower value sets the output limit to unlimited. ## topic() | ---|--- Type: | template Default: | N/A _Description:_ The Kafka topic under which the message is published. You can use templates to change the topic dynamically based on the source or the content of the message, for example, `topic("${HOST}")`. ## time-zone() | ---|--- Type: | name of the timezone, or the timezone offset Default: | unspecified _Description:_ Convert timestamps to the timezone specified by this option. If this option is not set, then the original timezone information in the message is used. Converting the timezone changes the values of all date-related macros derived from the timestamp, for example, `HOUR`. For the complete list of such macros, see [Date-related macros](../../../docs/axosyslog-core/chapter-manipulating-messages/customizing-message-format/date-macros/index.md). The timezone can be specified by using the name, for example, `time-zone("Europe/Budapest")`), or as the timezone offset in +/-HH:MM format, for example, `+01:00`). On Linux and UNIX platforms, the valid timezone names are listed under the `/usr/share/zoneinfo` directory. ## ts-format() | ---|--- Type: | rfc3164, bsd, rfc3339, iso Default: | rfc3164 _Description:_ Override the global timestamp format (set in the global `ts-format()` parameter) for the specific destination. For details, see [ts-format()](../../../docs/axosyslog-core/chapter-global-options/reference-options/index.md). Note This option applies only to file and file-like destinations. Destinations that use specific protocols (for example, `network()`, or `syslog()`) ignore this option. For protocol-like destinations, use a template locally in the destination, or use the [proto-template](../../../docs/axosyslog-core/chapter-global-options/reference-options/index.md) option. Last modified May 8, 2026: [Renames hook-commands snippet (473feff3)]()