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)


  • MSG

The following is a sample syslog message (source:

   <34>1 2003-10-11T22:14:15.003Z su - ID47 - BOM'su root' failed for lonvick on /dev/pts/8

The message corresponds to the following format:

  • 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 “”.

  • 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.

syslog Message Facilities

Numerical Code



kernel messages


user-level messages


mail system


system daemons


security/authorization messages


messages generated internally by syslogd


line printer subsystem


network news subsystem


UUCP subsystem


clock daemon


security/authorization messages


FTP daemon


NTP subsystem


log audit


log alert


clock daemon


locally used facilities (local0-local7)

The following table lists the severity values.

Numerical CodeSeverity
0Emergency: system is unusable
1Alert: action must be taken immediately
2Critical: critical conditions
3Error: error conditions
4Warning: warning conditions
5Notice: normal but significant condition
6Informational: informational messages
7Debug: 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.

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). 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.