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syslog-ng Premium Edition 6.0.21 - Administration Guide

Preface Chapter 1. Introduction to syslog-ng Chapter 2. The concepts of syslog-ng Chapter 3. Installing syslog-ng Chapter 4. The syslog-ng PE quick-start guide Chapter 5. The syslog-ng PE configuration file Chapter 6. Collecting log messages — sources and source drivers Chapter 7. Sending and storing log messages — destinations and destination drivers Chapter 8. Routing messages: log paths, reliability, and filters Chapter 9. Global options of syslog-ng PE Chapter 10. TLS-encrypted message transfer Chapter 12.  Reliable Log Transfer Protocol™ Chapter 13. Reliability and minimizing the loss of log messages Chapter 14. Manipulating messages Chapter 15. Parsing and segmenting structured messages Chapter 16. Processing message content with a pattern database Chapter 17. Statistics and metrics of syslog-ng Chapter 18. Multithreading and scaling in syslog-ng PE Chapter 19. Troubleshooting syslog-ng Chapter 20. Best practices and examples

Logging with syslog-ng

The syslog-ng application reads incoming messages and forwards them to the selected destinations. The syslog-ng application can receive messages from files, remote hosts, and other sources.

Log messages enter syslog-ng in one of the defined sources, and are sent to one or more destinations.

Sources and destinations are independent objects, log paths define what syslog-ng does with a message, connecting the sources to the destinations. A log path consists of one or more sources and one or more destinations: messages arriving from a source are sent to every destination listed in the log path. A log path defined in syslog-ng is called a log statement.

Optionally, log paths can include filters. Filters are rules that select only certain messages, for example, selecting only messages sent by a specific application. If a log path includes filters, syslog-ng sends only the messages satisfying the filter rules to the destinations set in the log path.

Other optional elements that can appear in log statements are parsers and rewriting rules. Parsers segment messages into different fields to help processing the messages, while rewrite rules modify the messages by adding, replacing, or removing parts of the messages.

Procedure 2.1. The route of a log message in syslog-ng


The following procedure illustrates the route of a log message from its source on the syslog-ng client to its final destination on the central syslog-ng server.

Figure 2.1. The route of a log message

The route of a log message


  1. A device or application sends a log message to a source on the syslog-ng client. For example, an Apache web server running on Linux enters a message into the /var/log/apache file.

  2. The syslog-ng client running on the web server reads the message from its /var/log/apache source.

  3. The syslog-ng client processes the first log statement that includes the /var/log/apache source.

  4. The syslog-ng client performs optional operations (message filtering, parsing, and rewriting) on the message, for example, it compares the message to the filters of the log statement (if any). If the message complies with all filter rules, syslog-ng sends the message to the destinations set in the log statement, for example, to the remote syslog-ng server.


    Message filtering, parsing, and rewriting is performed in the order that the operations appear in the log statement.


    The syslog-ng client sends a message to all matching destinations by default. As a result, a message may be sent to a destination more than once, if the destination is used in multiple log statements. To prevent such situations, use the final flag in the destination statements. For details, see Table 8.1, “Log statement flags”.

  5. The syslog-ng client processes the next log statement that includes the /var/log/apache source, repeating Steps 3-4.

  6. The message sent by the syslog-ng client arrives from a source set in the syslog-ng server.

  7. The syslog-ng server reads the message from its source and processes the first log statement that includes that source.

  8. The syslog-ng server performs optional operations (message filtering, parsing, and rewriting) on the message, for example, it compares the message to the filters of the log statement (if any). If the message complies with all filter rules, syslog-ng sends the message to the destinations set in the log statement.


    Message filtering, parsing, and rewriting is performed in the order that the operations appear in the log statement.

  9. The syslog-ng server processes the next log statement, repeating Steps 7-9.


    The syslog-ng application can stop reading messages from its sources if the destinations cannot process the sent messages. This feature is called flow-control and is detailed in the section called “Managing incoming and outgoing messages with flow-control”.

Modes of operation

The syslog-ng Premium Edition application has three distinct operation scenarios: Client, Server, and Relay. The syslog-ng PE application running on a host determines the mode of operation automatically based on the license and the configuration file.

Client mode

Figure 2.2. Client-mode operation

Client-mode operation

In client mode, syslog-ng collects the local logs generated by the host and forwards them through a network connection to the central syslog-ng server or to a relay. Clients often also log the messages locally into files.

No license file is required to run syslog-ng in client mode.

Relay mode

Figure 2.3. Relay-mode operation

Relay-mode operation

In relay mode, syslog-ng receives logs through the network from syslog-ng clients and forwards them to the central syslog-ng server using a network connection. Relays also log the messages from the relay host into a local file, or forward these messages to the central syslog-ng server.

You cannot use the following destinations in relay mode: mongodb(), pipe(), sql(). The file() and logstore() destinations work only for local messages that are generated on the relay.

No license file is required to run syslog-ng in relay mode.

Server mode

Figure 2.4. Server-mode operation

Server-mode operation

In server mode, syslog-ng acts as a central log-collecting server. It receives messages from syslog-ng clients and relays over the network, and stores them locally in files, or passes them to other applications, for example log analyzers.

Running syslog-ng Premium Edition in server mode requires a license file. The license determines how many individual hosts can connect to the server. For details on how syslog-ng PE calculates the number of hosts, see the section called “Licensing”.

Global objects

The syslog-ng application uses the following objects:

For details on the above objects, see the section called “The configuration syntax in detail”.

Timezones and daylight saving

The syslog-ng application receives the timezone and daylight saving information from the operating system it is installed on. If the operating system handles daylight saving correctly, so does syslog-ng.

The syslog-ng application supports messages originating from different timezones. The original syslog protocol (RFC3164) does not include timezone information, but syslog-ng provides a solution by extending the syslog protocol to include the timezone in the log messages. The syslog-ng application also enables administrators to supply timezone information for legacy devices which do not support the protocol extension.

Procedure 2.2. How syslog-ng PE assigns timezone to the message

When syslog-ng PE receives a message, it assigns timezone information to the message using the following algorithm.

  1. The sender application (for example the syslog-ng client) or host specifies the timezone of the messages. If the incoming message includes a timezone it is associated with the message. Otherwise, the local timezone is assumed.

  2. Specify the time-zone() parameter for the source driver that reads the message. This timezone will be associated with the messages only if no timezone is specified within the message itself. Each source defaults to the value of the recv-time-zone() global option. It is not possible to override only the timezone information of the incoming message, but setting the keep-timestamp() option to no allows syslog-ng PE to replace the full timestamp (timezone included) with the time the message was received.


    When processing a message that does not contain timezone information, the syslog-ng PE application will use the timezone and daylight-saving that was effective when the timestamp was generated. For example, the current time is 2011-03-11 (March 11, 2011) in the EU/Budapest timezone. When daylight-saving is active (summertime), the offset is +02:00. When daylight-saving is inactive (wintertime) the timezone offset is +01:00. If the timestamp of an incoming message is 2011-01-01, the timezone associated with the message will be +01:00, but the timestamp will be converted, because 2011-01-01 meant winter time when daylight saving is not active but the current timezone is +02:00.

  3. Specify the timezone in the destination driver using the time-zone() parameter. Each destination driver might have an associated timezone value: syslog-ng converts message timestamps to this timezone before sending the message to its destination (file or network socket). Each destination defaults to the value of the send-time-zone() global option.


    A message can be sent to multiple destination zones. The syslog-ng application converts the timezone information properly for every individual destination zone.


    If syslog-ng PE sends the message is to the destination using the legacy-syslog protocol (RFC3164) which does not support timezone information in its timestamps, the timezone information cannot be encapsulated into the sent timestamp, so syslog-ng PE will convert the hour:min values based on the explicitly specified timezone.

  4. If the timezone is not specified, local timezone is used.

  5. When macro expansions are used in the destination filenames, the local timezone is used. (Also, if the timestamp of the received message does not contain the year of the message, syslog-ng PE uses the local year.)

A note on timezones and timestamps

If the clients run syslog-ng, then use the ISO timestamp, because it includes timezone information. That way you do not need to adjust the recv-time-zone() parameter of syslog-ng.

If you want syslog-ng to output timestamps in Unix (POSIX) time format, use the S_UNIXTIME and R_UNIXTIME macros. You do not need to change any of the timezone related parameters, because the timestamp information of incoming messages is converted to Unix time internally, and Unix time is a timezone-independent time representation. (Actually, Unix time measures the number of seconds elapsed since midnight of Coordinated Universal Time (UTC) January 1, 1970, but does not count leap seconds.)

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