2.9. Message representation in syslog-ng OSE

When the syslog-ng OSE application receives a message, it automatically parses the message. The syslog-ng OSE application can automatically parse log messages that conform to the RFC3164 (BSD or legacy-syslog) or the RFC5424 (IETF-syslog) message formats. If syslog-ng OSE cannot parse a message, it results in an error.


In case you need to relay messages that cannot be parsed without any modifications or changes, use the flags(no-parse) option in the source definition, and a template containing only the ${MESSAGE} macro in the destination definition.

To parse non-syslog messages, for example, JSON, CSV, or other messages, you can use the built-in parsers of syslog-ng OSE. For details, see Chapter 12, Parsers and segmenting structured messages.

A parsed syslog message has the following parts.

  • Timestamps. Two timestamps are associated with every message: one is the timestamp contained within the message (that is, when the sender sent the message), the other is the time when syslog-ng OSE has actually received the message.

  • Severity. The severity of the message.

  • Facility. The facility that sent the message.

  • Tags. Custom text labels added to the message that are mainly used for filtering. None of the current message transport protocols adds tags to the log messages. Tags can be added to the log message only within syslog-ng OSE. The syslog-ng OSE application automatically adds the id of the source as a tag to the incoming messages. Other tags can be added to the message by the pattern database, or using the tags() option of the source.

  • IP address of the sender. The IP address of the host that sent the message. Note that the IP address of the sender is a hard macro and cannot be modified within syslog-ng OSE but the associated hostname can be modified, for example, using rewrite rules.

  • Hard macros. Hard macros contain data that is directly derived from the log message, for example, the ${MONTH} macro derives its value from the timestamp. The most important consideration with hard macros is that they are read-only, meaning they cannot be modified using rewrite rules or other means.

  • Soft macros. Soft macros (sometimes also called name-value pairs) are either built-in macros automatically generated from the log message (for example, ${HOST}), or custom user-created macros generated by using the syslog-ng pattern database or a CSV-parser. The SDATA fields of RFC5424-formatted log messages become soft macros as well. In contrast with hard macros, soft macros are writable and can be modified within syslog-ng OSE, for example, using rewrite rules.


    It is also possible to set the value of built-in soft macros using parsers, for example, to set the ${HOST} macro from the message using a column of a CSV-parser.

    The data extracted from the log messages using named pattern parsers in the pattern database are also soft macros.


    For the list of hard and soft macros, see Section 11.1.4, Hard vs. soft macros.

Message size and encoding

Internally, syslog-ng OSE represents every message as UTF-8. The maximal length of the log messages is limited by the log-msg-size() option: if a message is longer than this value, syslog-ng OSE truncates the message at the location it reaches the log-msg-size() value, and discards the rest of the message.

When encoding is set in a source (using the encoding() option) and the message is longer (in bytes) than log-msg-size() in UTF-8 representation, syslog-ng OSE splits the message at an undefined location (because the conversion between different encodings is not trivial).