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syslog-ng Premium Edition 7.0.17 - Administration Guide

Preface Introduction to syslog-ng The concepts of syslog-ng Installing syslog-ng The syslog-ng PE quick-start guide The syslog-ng PE configuration file Collecting log messages — sources and source drivers
How sources work default-network-drivers: Receive and parse common syslog messages internal: Collecting internal messages file: Collecting messages from text files wildcard-file: Collecting messages from multiple text files linux-audit: Collecting messages from Linux audit logs network: Collecting messages using the RFC3164 protocol (network() driver) office365: Fetching logs from Office 365 osquery: Collect and parse osquery result logs pipe: Collecting messages from named pipes program: Receiving messages from external applications python: writing server-style Python sources python-fetcher: writing fetcher-style Python sources snmptrap: Read Net-SNMP traps sun-streams: Collecting messages on Sun Solaris syslog: Collecting messages using the IETF syslog protocol (syslog() driver) system: Collecting the system-specific log messages of a platform systemd-journal: Collecting messages from the systemd-journal system log storage systemd-syslog: Collecting systemd messages using a socket tcp, tcp6, udp, udp6: Collecting messages from remote hosts using the BSD syslog protocol unix-stream, unix-dgram: Collecting messages from UNIX domain sockets windowsevent: Collecting Windows event logs
Sending and storing log messages — destinations and destination drivers
elasticsearch2: Sending messages directly to Elasticsearch version 2.0 or higher (DEPRECATED) elasticsearch-http: Sending messages to Elasticsearch HTTP Event Collector file: Storing messages in plain-text files hdfs: Storing messages on the Hadoop Distributed File System (HDFS) http: Posting messages over HTTP kafka: Publishing messages to Apache Kafka logstore: Storing messages in encrypted files mongodb: Storing messages in a MongoDB database network: Sending messages to a remote log server using the RFC3164 protocol (network() driver) pipe: Sending messages to named pipes program: Sending messages to external applications python: writing custom Python destinations smtp: Generating SMTP messages (email) from logs splunk-hec: Sending messages to Splunk HTTP Event Collector sql: Storing messages in an SQL database stackdriver: Sending logs to the Google Stackdriver cloud syslog: Sending messages to a remote logserver using the IETF-syslog protocol syslog-ng(): Forward logs to another syslog-ng node tcp, tcp6, udp, udp6: Sending messages to a remote log server using the legacy BSD-syslog protocol (tcp(), udp() drivers) unix-stream, unix-dgram: Sending messages to UNIX domain sockets usertty: Sending messages to a user terminal — usertty() destination Client-side failover
Routing messages: log paths, flags, and filters Global options of syslog-ng PE TLS-encrypted message transfer Advanced Log Transfer Protocol Reliability and minimizing the loss of log messages Manipulating messages parser: Parse and segment structured messages Processing message content with a pattern database Correlating log messages Enriching log messages with external data Monitoring statistics and metrics of syslog-ng Multithreading and scaling in syslog-ng PE Troubleshooting syslog-ng Best practices and examples The syslog-ng manual pages

Kafka destination options

The kafka destination of syslog-ng PE 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 syslog-ng PE configuration:

@module mod-java
@include "scl.conf"
Type: string
Default: The syslog-ng PE 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 syslog-ng PE configuration (for example, because you have multiple Java-based destinations), syslog-ng PE 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), for example, client-lib-dir(/opt/syslog-ng/lib/syslog-ng/java-modules/KafkaDestination.jar:/usr/share/kafka/lib/*.jar).

Type: list of hostnames

Description: Specifies the hostname or IP address of the Kafka server. When specifying an IP address, IPv4 (for example, 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(",remote-server-hostname:6464")

Type: number
Default: 0

Description: The syslog-ng 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 that syslog-ng can add the fractions to non-ISO8601 timestamps as well.

Type: list
Default: N/A

Description: Specify the Java Virtual Machine (JVM) settings of your Java destination from the syslog-ng PE configuration file.

For example:

jvm-options("-Xss1M -XX:+TraceClassLoading")

You can set this option only as a global option, by adding it to the options statement of the syslog-ng configuration file.

Accepted values:



Default: Use the global setting (which defaults to drop-message)

Description: Controls what happens when type-casting fails and syslog-ng PE cannot convert some data to the specified type. By default, syslog-ng PE 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 syslog-ng PE.

  • 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.

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}").

Type: number
Default: Use global setting.

Description: The number of messages that the output queue can store.

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/"). The syslog-ng PE application reads this file and passes the properties to the Kafka Producer. If a property is defined both in the syslog-ng PE configuration file (syslog-ng.conf) and in the properties file, then syslog-ng PE uses the definition from the syslog-ng PE configuration file.

The syslog-ng PE 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:

Type: number (of attempts)
Default: 3

Description: The number of times syslog-ng PE attempts to send a message to this destination. If syslog-ng PE could not send a message, it will try again until the number of attempts reaches retries, then drops the message.

Type: true | false
Default: false

Description: When sync-send is set to true, syslog-ng PE sends the message reliably: it sends a message to the Kafka server, then waits for a reply. In case of failure, syslog-ng PE repeats sending the message, as set in the retries() parameter. If sending the message fails for retries() times, syslog-ng PE drops the message.

This method ensures reliable message transfer, but is very slow.

When sync-send is set to false, syslog-ng PE sends messages asynchronously, and receives the response asynchronously. In case of a problem, syslog-ng PE cannot resend the messages.

This method is fast, but the transfer is not reliable. Several thousands of messages can be lost before syslog-ng PE recognizes the error.

Type: template or template function

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.

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 the disk-buffer option as well to avoid the risk of losing messages. Specifying 0 or a lower value sets the output limit to unlimited.

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}").

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.

The timezone can be specified as using the name of the (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.

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().

logstore: Storing messages in encrypted files

The syslog-ng PE application can store log messages securely in encrypted, compressed and timestamped binary files. Timestamps can be requested from an external Timestamping Authority (TSA).

Logstore files consist of individual chunks, every chunk can be encrypted, compressed, and timestamped separately. Chunks contain compressed log messages and header information needed for retrieving messages from the logstore file.

The syslog-ng PE application generates an SHA-1 hash for every chunk to verify the integrity of the chunk. The hashes of the chunks are chained together to prevent injecting chunks into the logstore file. The syslog-ng PE application can encrypt the logstore using various algorithms, using the aes128 encryption algorithm in CBC mode and the hmac-sha1 hashing (HMAC) algorithm as default. For other algorithms, see cipher() and digest().

The destination filename may include macros which get expanded when the message is written, thus a simple logstore() driver may create several files. For more information on available macros see Macros of syslog-ng PE.

If the expanded filename refers to a directory which does not exist, it will be created depending on the create-dirs() setting (both global and a per destination option).

The logstore() has a single required parameter that specifies the filename that stores the log messages. For the list of available optional parameters, see logstore() destination options.


Hazard of data loss! If your log files are on an NFS-mounted network file system, see NFS file system for log files.

logstore(filename options());
Example: Using the logstore() driver

A simple example saving and compressing log messages.

destination d_logstore { logstore("/var/log/messages.lgs" compress(5) ); };

A more detailed example that encrypts messages, modifies the parameters for closing chunks, and sets file privileges.

destination d_logstore {

The URL to the Timestamping Authority and if needed, the OID of the timestamping policy can be set as global options, or also per logstore destination. The following example specifies the URL and the OID as global options:

options {

When using the logstore() destination, update the configuration of your log rotation program to rotate these files. Otherwise, the log files can become very large.


Since the state of each created file must be tracked by syslog-ng, it consumes some memory for each file. If no new messages are written to a file within 60 seconds (controlled by the time-reap() global option), it is closed, and its state is freed.

Exploiting this, a DoS attack can be mounted against the system. If the number of possible destination files and its needed memory is more than the amount available on the syslog-ng server.

The most suspicious macro is ${PROGRAM}, where the number of possible variations is rather high. Do not use the ${PROGRAM} macro in insecure environments.

Displaying the contents of logstore files

To display the contents of a logstore file, use the lgstool (formerly called logcat) command supplied with syslog-ng, for example, lgstool cat /var/log/messages.lgs. Log messages available in the journal file of the logstore (but not yet written to the logstore file itself) are displayed as well.

To display the contents of encrypted log files, specify the private key of the certificate used to encrypt the file, for example, lgstool cat -k private.key /var/log/messages.lgs. The contents of the file are sent to the standard output, so it is possible to use grep and other tools to find particular log messages, for example, lgstool cat /var/log/messages.lgs |grep For further details, see The logstore tool manual page.


The lgstool utility is available for Microsoft Windows operating systems at the Downloads page.


For files that are in use by syslog-ng, the last chunk that is open cannot be read.

Journal files

The syslog-ng PE application processes log messages into a journal file before writing them to the logstore file. That way logstore files are consistent even if syslog-ng PE crashes unexpectedly, avoiding losing messages. Note that this does not protect against losing messages if the operating system crashes.

A journal file is automatically created for every logstore file that syslog-ng PE opens. A journal file consists of journal blocks that store the log messages. When a journal block fills up with messages, syslog-ng PE writes the entire block into the logstore file and starts to reuse the journal block (one journal block becomes one chunk in the logstore file).

If the messages cannot be written to the logstore file (for example, because the disk becomes inaccessible, or file operations are slow), messages are put to the next journal block (syslog-ng PE uses four blocks by default). When all journal blocks become full, syslog-ng PE will stop processing incoming traffic. syslog-ng PE starts accepting messages to the logstore file again when the first journal block is successfully written to the logstore file. If syslog-ng PE receives a HUP or STOP signal, or no new message arrives into the logstore for the period set in the time-reap() parameter, it writes every journal block to the logstore.

When syslog-ng PE is restarted, it automatically processes the journal files to the logstore files, unless a particular logstore file is not part of configuration of syslog-ng PE. Such orphaned journal files can be processed with the lgstool recover command. For details on processing orphaned journal files, see The logstore tool manual page.

  • If a particular logstore destination receives messages at a constant but very low message rate (for example, a 100-byte message every 30 seconds), messages do not get written to the logstore file for a long time, because the journal block does not get full, and messages are more frequent than the time-reap() time. This becomes a problem when using logrotate to rotate the logstore files, because log messages will not be in the files they are expected. To avoid this situation, either use time-based macros in the filenames of the logstore files, or send a HUP signal to syslog-ng PE right before rotating the logstore files.

  • When every block of a journal becomes full and syslog-ng PE stops processing incoming traffic, it will not read new messages at all until a block is successfully written to the related logstore file. This is in contrast with flow-control, where only messages from the source related to the particular destination are not processed.

  • The messages in the journal file are in plain-text format. They are neither encrypted nor compressed. The journal file has the same permission as the logstore: by default, root privileges are required to access them. Make sure you consider this if you change the permissions of the journal file (owner, group, perm) in the syslog-ng PE configuration file.


Journal files are located in the same folder as the logstore file. The name of the journal file is the same as the logstore file with .jor suffix added. For example, the journal file for messages.lgs is messages.lgs.jor.

The syslog-ng PE application uses a separate journal file for every logstore file. Every journal file is processed by a separate thread. The journal files are mapped into the memory. The journal of an individual logstore file uses up to journal-block-size()*journal-block-count() memory address, which is 4MB by default. However, if you have several logstore files open in parallel (for example, you are collecting log messages from 500 hosts and storing them in separate files for every host, and the hosts are continuously sending messages), the memory requirements for journaling rise quickly (to approximately 2GB for the 500 hosts). To limit the memory use of journals, adjust the logstore-journal-shmem-threshold() global option (by default, it is 512MB).

If the memory required for the journal files exceeds the logstore-journal-shmem-threshold() limit, syslog-ng PE will store only a single journal block of every journal file in the memory, and — if more blocks are needed for a journal — store the additional blocks on the hard disk. Opening new logstore files means allocating memory for one new journal block for every new file. In extreme situations involving large traffic, this can lead to syslog-ng PE consuming the entire memory of the system. Adjust the journal-block-size() and your file-naming conventions as needed to avoid such situations. For details on logstore journals, see Journal files.


If you have a large amount of open logstore files in parrallel (for example, you are using the ${HOST} or ${PROGRAM} macros in your filenames) consider lowering the journal-block-size() to avoid syslog-ng PE consuming the entire memory of the system.

Example: Calculating memory usage of logstore journals

If you are using the default settings (4 journal blocks for every logstore journal, one block is 1MB, logstore-journal-shmem-threshold() is 512MB), this means that syslog-ng PE will allocate 4MB memory for every open logstore file, up to 512MB if you have 128 open logstore files. Opening a new logstore file would require 4 more megabytes of memory for journaling, bringing the total required memory to 516MB, which is above the logstore-journal-shmem-threshold(). In this case, syslog-ng PE switches to storing only a single journal block in the memory, lowering the memory requirements of journaling to 129MB. However, opening more and more logstore files will require more and more memory, and this is not limited, except when syslog-ng PE reaches the maximum number of files that can be open (as set in the --fd-limit command-line option).

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