# IETF-syslog messages This section describes the format of a syslog message, according to the [IETF-syslog protocol](). A syslog message consists of the following parts: * `HEADER` (includes the `PRI` as well) * `STRUCTURED-DATA` * `MSG` The following is a sample syslog message (source: [https://tools.ietf.org/html/rfc5424)](): ``` <34>1 2003-10-11T22:14:15.003Z mymachine.example.com su - ID47 - BOM'su root' failed for lonvick on /dev/pts/8 ``` The message corresponds to the following format: ``` VERSION ISOTIMESTAMP HOSTNAME APPLICATION PID MESSAGEID STRUCTURED-DATA MSG ``` * Facility is 4, severity is 2, so PRI is 34. * The VERSION is 1. * The message was created on 11 October 2003 at 10:14:15pm UTC, 3 milliseconds into the next second. * The message originated from a host that identifies itself as “mymachine.example.com”. * The APP-NAME is “su” and the PROCID is unknown. * The MSGID is “ID47”. * The MSG is “‘su root’ failed for lonvick…”, encoded in UTF-8. * In this example, the encoding is defined by the BOM: The byte order mark (BOM) is a Unicode character used to signal the byte-order of the message text. * There is no STRUCTURED-DATA present in the message, this is indicated by “-” in the STRUCTURED-DATA field. The HEADER part of the message must be in plain ASCII format, the parameter values of the STRUCTURED-DATA part must be in UTF-8, while the MSG part should be in UTF-8. The different parts of the message are explained in the following sections. ## The PRI message part The PRI part of the syslog message (known as Priority value) represents the Facility and Severity of the message. Facility represents the part of the system sending the message, while severity marks its importance. The Priority value is calculated by first multiplying the Facility number by 8 and then adding the numerical value of the Severity. The possible facility and severity values are presented below. Note Facility codes may slightly vary between different platforms. The AxoSyslog application accepts facility codes as numerical values as well. Numerical Code | Facility ---|--- 0 | kernel messages 1 | user-level messages 2 | mail system 3 | system daemons 4 | security/authorization messages 5 | messages generated internally by syslogd 6 | line printer subsystem 7 | network news subsystem 8 | UUCP subsystem 9 | clock daemon 10 | security/authorization messages 11 | FTP daemon 12 | NTP subsystem 13 | log audit 14 | log alert 15 | clock daemon 16-23 | locally used facilities (local0-local7) The following table lists the severity values. Numerical Code | Severity ---|--- 0 | Emergency: system is unusable 1 | Alert: action must be taken immediately 2 | Critical: critical conditions 3 | Error: error conditions 4 | Warning: warning conditions 5 | Notice: normal but significant condition 6 | Informational: informational messages 7 | Debug: debug-level messages syslog Message Severities ## The HEADER message part The HEADER part contains the following elements: * _VERSION_ : Version number of the syslog protocol standard. Currently this can only be `1`. * _ISOTIMESTAMP_ : The time when the message was generated in the ISO 8601 compatible standard timestamp format (yyyy-mm-ddThh:mm:ss+-ZONE), for example: `2006-06-13T15:58:00.123+01:00`. * _HOSTNAME_ : The machine that originally sent the message. * _APPLICATION_ : The device or application that generated the message * _PID_ : The process name or process ID of the syslog application that sent the message. It is not necessarily the process ID of the application that generated the message. * _MESSAGEID_ : The ID number of the message. Note The AxoSyslog application supports other timestamp formats as well, like ISO, or the PIX extended format. The timestamp used in the IETF-syslog protocol is derived from RFC3339, which is based on ISO8601. For details, see the `ts-format()` option in [Global options reference](../../../docs/axosyslog-core/chapter-global-options/reference-options/index.md). The AxoSyslog application will truncate the following fields: * If _APP-NAME_ is longer than 48 characters it will be truncated to 48 characters. * If _PROC-ID_ is longer than 128 characters it will be truncated to 128 characters. * If _MSGID_ is longer than 32 characters it will be truncated to 32 characters. * If _HOSTNAME_ is longer than 255 characters it will be truncated to 255 characters. ## The STRUCTURED-DATA message part The STRUCTURED-DATA message part may contain meta-information about the syslog message, or application-specific information such as traffic counters or IP addresses. STRUCTURED-DATA consists of data blocks enclosed in brackets (_[]_). Every block includes the ID of the block, and one or more _name=value_ pairs. The AxoSyslog application automatically parses the STRUCTURED-DATA part of syslog messages, which can be referenced in macros (for details, see [Macros of AxoSyslog](../../../docs/axosyslog-core/chapter-manipulating-messages/customizing-message-format/reference-macros/index.md)). An example STRUCTURED-DATA block looks like: ``` [exampleSDID@0 iut="3" eventSource="Application" eventID="1011"][examplePriority@0 class="high"] ``` ## The MSG message part The MSG part contains the text of the message itself. The encoding of the text must be UTF-8 if the BOM The byte order mark (BOM) is a Unicode character used to signal the byte-order of the message text. character is present in the message. If the message does not contain the BOM character, the encoding is treated as unknown. Usually messages arriving from legacy sources do not include the BOM character. CRLF characters will not be removed from the message. Last modified December 9, 2025: [typo fix (4bcb144)]()