syslog-ng Open Source Edition 3.16 - Administration Guide

Preface Introduction to syslog-ng The concepts of syslog-ng Installing syslog-ng The syslog-ng OSE quick-start guide The syslog-ng OSE configuration file source: Read, receive, and collect log messages
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 network: Collecting messages using the RFC3164 protocol (network() driver) nodejs: Receiving JSON messages from nodejs applications mbox: Converting local e-mail messages to log messages osquery: Collect and parse osquery result logs pipe: Collecting messages from named pipes pacct: Collecting process accounting logs on Linux program: Receiving messages from external applications 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— OBSOLETE unix-stream, unix-dgram: Collecting messages from UNIX domain sockets stdin: Collecting messages from the standard input stream
destination: Forward, send, and store log messages
amqp: Publishing messages using AMQP elasticsearch: Sending messages directly to Elasticsearch version 1.x elasticsearch2: Sending logs directly to Elasticsearch and Kibana 2.0 or higher file: Storing messages in plain-text files graphite: Sending metrics to Graphite Sending logs to Graylog hdfs: Storing messages on the Hadoop Distributed File System (HDFS) Posting messages over HTTP http: Posting messages over HTTP without Java kafka: Publishing messages to Apache Kafka loggly: Using Loggly logmatic: Using mongodb: Storing messages in a MongoDB database network: Sending messages to a remote log server using the RFC3164 protocol (network() driver) osquery: Sending log messages to osquery's syslog table pipe: Sending messages to named pipes program: Sending messages to external applications pseudofile() redis: Storing name-value pairs in Redis riemann: Monitoring your data with Riemann smtp: Generating SMTP messages (e-mail) from logs Splunk: Sending log messages to Splunk sql: Storing messages in an SQL database stomp: Publishing messages using STOMP syslog: Sending messages to a remote logserver using the IETF-syslog protocol syslog-ng: Forwarding messages and tags 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) Telegram: Sending messages to Telegram unix-stream, unix-dgram: Sending messages to UNIX domain sockets usertty: Sending messages to a user terminal: usertty() destination Write your own custom destination in Java or Python
log: Filter and route log messages using log paths, flags, and filters Global options of syslog-ng OSE TLS-encrypted message transfer template and rewrite: Format, modify, and manipulate log messages parser: Parse and segment structured messages db-parser: Process message content with a pattern database (patterndb) Correlating log messages Enriching log messages with external data Statistics of syslog-ng Multithreading and scaling in syslog-ng OSE Troubleshooting syslog-ng Best practices and examples The syslog-ng manual pages Third-party contributions Creative Commons Attribution Non-commercial No Derivatives (by-nc-nd) License About us

Multithreading concepts of syslog-ng OSE

This section is a brief overview on how syslog-ng OSE works in multithreaded mode. It is mainly for illustration purposes: the concept has been somewhat simplified and may not completely match reality.


The way syslog-ng OSE uses multithreading may change in future releases. The current documentation applies to version 3.16.

syslog-ng OSE always uses multiple threads:

  • A main thread that is always running

  • A number of worker threads that process the messages. You can influence the behavior of worker threads using the threaded() option and the --worker-threads command-line option.

  • Some other, special threads for internal functionalities. For example, certain destinations run in a separate thread, independently of the multithreading (threaded()) and --worker-threads settings of syslog-ng OSE.

The maximum number of worker threads syslog-ng OSE uses is the number of CPUs or cores in the host running syslog-ng OSE (up to 64). You can limit this value using the --worker-threads command-line option that sets the maximum total number of threads syslog-ng OSE can use, including the main syslog-ng OSE thread. However, the --worker-threads option does not affect the supervisor of syslog-ng OSE. The supervisor is a separate process (see The syslog-ng manual page), but certain operating systems might display it as a thread. In addition, certain destinations always run in a separate thread, independently of the multithreading (threaded()) and --worker-threads settings of syslog-ng OSE.

When an event requiring a new thread occurs (for example, syslog-ng OSE receives new messages, or a destination becomes available), syslog-ng OSE tries to start a new thread. If there are no free threads, the task waits until a thread finishes its task and becomes available. There are two types of worker threads:

  • Reader threads read messages from a source (as many as possible, but limited by the log-fetch-limit() and log-iw-size() options. The thread then processes these messages, that is, performs filtering, rewriting and other tasks as necessary, and puts the log message into the queue of the destination. If the destination does not have a queue (for example, usertty), the reader thread sends the message to the destination, without the interaction of a separate writer thread.

  • Writer threads take the messages from the queue of the destination and send them to the destination, that is, write the messages into a file, or send them to the syslog server over the network. The writer thread starts to process messages from the queue only if the destination is writable, and there are enough messages in the queue, as set in the flush-lines() and the flush-timeout() options. Writer threads stop processing messages when the destination becomes unavailable, or there are no more messages in the queue.

Sources and destinations affected by multithreading

The following list describes which sources and destinations can use multiple threads. Changing the --worker-threads command-line option changes the number of threads available to these sources and destinations.

  • The tcp and syslog(tcp) sources can process independent connections in separate threads. The number of independent connections is limited by the max-connections() option of the source. Separate sources are processed by separate thread, for example, if you have two separate tcp sources defined that receive messages on different IP addresses or port, syslog-ng OSE will use separate threads for these sources even if they both have only a single active connection.

  • The udp, file, and pipe sources use a single thread for every source statement.

  • The tcp, syslog, and pipe destinations use a single thread for every destination.

  • The file destination uses a single thread for writing the destination file, but may use a separate thread for each destination file if the filename includes macros.

Sources and destinations not affected by multithreading

The following list describes sources and destinations that use a separate thread even if you disable multithreading in syslog-ng OSE, in addition to the limit set in the --worker-threads command-line option.

  • Every sql destination uses its own thread. These threads are independent from the setting of the --worker-threads command-line option.

  • The java destinations use one thread, even if there are multiple Java-based destinations configured. This thread is independent from the setting of the --worker-threads command-line option.

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Configuring multithreading

Starting with version 3.6, syslog-ng OSE runs in multithreaded mode by default. You can enable multithreading in syslog-ng OSE using the following methods:

  • Globally using the threaded(yes) option.

  • Separately for selected sources or destinations using the flags("threaded") option.

Example: Enabling multithreading

To enable multithreading globally, use the threaded option:

options {
    threaded(yes) ;

To enable multithreading only for a selected source or destination, use the flags("threaded") option:

source s_tcp_syslog {
        flags("syslog-protocol", "threaded")

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Optimizing multithreaded performance

Destinations that have a queue process that queue in a single thread. Multiple sources can send messages to the same queue, so the queue can scale to multiple CPUs. However, when the writer thread writes the queue contents to the destination, it will be single-threaded.

Message parsing, rewrite rules, filters, and other types of message processing is performed by the reader thread in a sequential manner. This means that such operations can scale only if reading messages from the source can be multithreaded. For example, if a tcp source can process messages from different connections (clients) in separate threads. If the source cannot use multiple threads to process the messages, the operations will not scale.

To improve the processing power of syslog-ng OSE and scale to more processors, use the following methods:

  • To improve scaling on the source side, use more sources, for example, more source files, or receive the messages from more parallel connections. For network sources, you can also configure a part of your clients to send the messages to a different port of your syslog-ng server, and use separate source definitions for each port.

  • On the destination side, when writing the log messages to files, use macros in the filename to split the messages to separate files (for example, using the ${HOST} macro). Files with macros in their filenames are processed in separate writer threads.

  • On the destination side, when sending messages to a syslog-ng server, you can use multiple connections to the server if you configure the syslog-ng server to receive messages on multiple ports, and configure separate destinations on the clients to use both ports.

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Troubleshooting syslog-ng

This chapter provides tips and guidelines about troubleshooting problems related to syslog-ng.

  • As a general rule, first try to get logging the messages to a local file. Once this is working, you know that syslog-ng is running correctly and receiving messages, and you can proceed to forwarding the messages to the server.

  • Always check the configuration files for any syntax errors on both the client and the server using the syslog-ng --syntax-only command.

  • If the syslog-ng OSE server does not receive the messages, verify that the IP addresses and ports are correct in your sources and destinations. Also, check that the client and the server uses the same protocol (a common error is to send logs on UDP, but configure the server to receive logs on TCP.

    If the problem persist, use tcpdump or a similar packet sniffer tool on the client to verify that the messages are sent correctly, and on the server to verify that it receives the messages.

  • To find message-routing problems, run syslog-ng OSE with the following command syslog-ng -Fevd. That way syslog-ng OSE will run in the foreground, and display debug messages about the messages that are processed.

  • If syslog-ng is closing the connections for no apparent reason, be sure to check the log messages of syslog-ng. You might also want to run syslog-ng with the --verbose or --debug command-line options for more-detailed log messages. You can enable these messages without restarting syslog-ng using the syslog-ng-ctl verbose --set=on command. For details, see the syslog-ng-ctl man page at The syslog-ng control tool manual page.

  • Build up encrypted connections step-by-step: first create a working unencrypted (for example TCP) connection, then add TLS encryption, and finally client authentication if needed.

  • If you use the same driver and options in the destination of your syslog-ng OSE client and the source of your syslog-ng OSE server, everything should work as expected. Unfortunately there are some other combinations, that seem to be working, but result in losing parts of the messages. For details on the working combinations, see Things to consider when forwarding messages between syslog-ng OSE hosts.

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