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syslog-ng Premium Edition 7.0.19 - 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 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 udp-balancer: Receiving UDP messages at very high rate 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 sentinel: Sending logs to the Microsoft Azure Sentinel cloud 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 Glossary

Multithreading concepts of syslog-ng PE

This section is a brief overview on how syslog-ng PE 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 PE uses multithreading may change in future releases. The current documentation applies to version 7.

syslog-ng PE 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 PE.

The maximum number of worker threads syslog-ng PE uses is the number of CPUs or cores in the host running syslog-ng PE (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 PE can use, including the main syslog-ng PE thread. However, the --worker-threads option does not affect the supervisor of syslog-ng PE. 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 PE.

When an event requiring a new thread occurs (for example, syslog-ng PE receives new messages, or a destination becomes available), syslog-ng PE 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 PE 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 udp-balancer source uses a thread for each listener configured in its listeners() option.

  • 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 PE, in addition to the limit set in the --worker-threads command-line option.

  • The logstore destination uses separate threads for writing the messages from the journal to the logstore files, and also for timestamping. These threads are independent from the setting of 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.

Configuring multithreading

Starting with version 5 F4, syslog-ng PE runs in multithreaded mode by default. You can enable multithreading in syslog-ng PE 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 { network(ip( port(1999) flags("syslog-protocol", "threaded") ); };

Optimizing multithreaded performance


File sources scale based on the number of files that the syslog-ng PE is reading. If there are 10 files all coming to the same source, then that source can use 10 threads, one thread for each file.


When collecting log messages from multiple files, the file source is a wildcard-file() source.

TCP-based network sources scale based on the number of active connections. This means that if there are 10 incoming connections all coming to the same source, then that source can use 10 threads, one thread for each connection.


UDP-based network sources do not scale by themselves because they always use a single thread. If you want to handle a large number of UDP connections, it is best to configure a subset of your clients to send the messages to a different port of your syslog-ng server, and use separate source definitions for each port.

Figure 39: How multithreading works — sources

Message processors

Message processors — such as filters, rewrite rules, and parsers — are executed by the reader thread in a sequential manner.

For example, if you have a log path that defines two sources and a filter, the filter will be performed by the source1 reader thread when log messages come from source1, and by the source2 reader thread when log messages come from source2. This means that if log messages come from both source1 and source2, they will both have a reader thread and that way filtering will be performed simultaneously.


This is not true for PatternDB because it uses message correlation. When using PatternDB, it runs in only one thread at a time, and this significantly decreases performance.


In syslog-ng, every destination has a writer thread. To improve scaling on the destination side, use multiple destinations instead of one.

For example, 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 the clients to use both ports.

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.

Figure 40: How multithreading works — destinations

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 PE 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 persists, 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 PE with the following command syslog-ng -Fevd. That way syslog-ng PE 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 may 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 PE client and the source of your syslog-ng PE server, everything should work as expected. Unfortunately, there are some other combinations, that may seem to work, but result in losing parts of the messages. For details on the working combinations, see Things to consider when forwarding messages between syslog-ng PE hosts.

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