RFC5424: The Syslog Protocol

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Obsoletes:  RFC3164

Network Working Group                                        R. Gerhards
Request for Comments: 5424                                  Adiscon GmbH
Obsoletes: 3164                                               March 2009
Category: Standards Track

                          The Syslog Protocol

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

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RFC 5424                  The Syslog Protocol                 March 2009


   This document describes the syslog protocol, which is used to convey
   event notification messages.  This protocol utilizes a layered
   architecture, which allows the use of any number of transport
   protocols for transmission of syslog messages.  It also provides a
   message format that allows vendor-specific extensions to be provided
   in a structured way.

   This document has been written with the original design goals for
   traditional syslog in mind.  The need for a new layered specification
   has arisen because standardization efforts for reliable and secure
   syslog extensions suffer from the lack of a Standards-Track and
   transport-independent RFC.  Without this document, each other
   standard needs to define its own syslog packet format and transport
   mechanism, which over time will introduce subtle compatibility
   issues.  This document tries to provide a foundation that syslog
   extensions can build on.  This layered architecture approach also
   provides a solid basis that allows code to be written once for each
   syslog feature rather than once for each transport.

   This document obsoletes RFC 3164.

Table of Contents

   1. Introduction ....................................................4
   2. Conventions Used in This Document ...............................4
   3. Definitions .....................................................4
   4. Basic Principles ................................................5
      4.1. Example Deployment Scenarios ...............................6
   5. Transport Layer Protocol ........................................7
      5.1. Minimum Required Transport Mapping .........................7
   6. Syslog Message Format ...........................................8
      6.1. Message Length .............................................9
      6.2. HEADER .....................................................9
           6.2.1. PRI .................................................9
           6.2.2. VERSION ............................................11
           6.2.3. TIMESTAMP ..........................................11
           6.2.4. HOSTNAME ...........................................13
           6.2.5. APP-NAME ...........................................14
           6.2.6. PROCID .............................................14
           6.2.7. MSGID ..............................................14
      6.3. STRUCTURED-DATA ...........................................15
           6.3.1. SD-ELEMENT .........................................15
           6.3.2. SD-ID ..............................................15
           6.3.3. SD-PARAM ...........................................16
           6.3.4. Change Control .....................................17
           6.3.5. Examples ...........................................17

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      6.4. MSG .......................................................18
      6.5. Examples ..................................................19
   7. Structured Data IDs ............................................20
      7.1. timeQuality ...............................................20
           7.1.1. tzKnown ............................................21
           7.1.2. isSynced ...........................................21
           7.1.3. syncAccuracy .......................................21
           7.1.4. Examples ...........................................21
      7.2. origin ....................................................22
           7.2.1. ip .................................................22
           7.2.2. enterpriseId .......................................22
           7.2.3. software ...........................................23
           7.2.4. swVersion ..........................................23
           7.2.5. Example ............................................23
      7.3. meta ......................................................24
           7.3.1. sequenceId .........................................24
           7.3.2. sysUpTime ..........................................24
           7.3.3. language ...........................................24
   8. Security Considerations ........................................24
      8.1. UNICODE ...................................................24
      8.2. Control Characters ........................................25
      8.3. Message Truncation ........................................26
      8.4. Replay ....................................................26
      8.5. Reliable Delivery .........................................26
      8.6. Congestion Control ........................................27
      8.7. Message Integrity .........................................28
      8.8. Message Observation .......................................28
      8.9. Inappropriate Configuration ...............................28
      8.10. Forwarding Loop ..........................................29
      8.11. Load Considerations ......................................29
      8.12. Denial of Service ........................................29
   9. IANA Considerations ............................................30
      9.1. VERSION ...................................................30
      9.2. SD-IDs ....................................................30
   10. Working Group .................................................31
   11. Acknowledgments ...............................................31
   12. References ....................................................32
      12.1. Normative References .....................................32
      12.2. Informative References ...................................33
   Appendix A.  Implementer Guidelines ...............................34
     A.1.  Relationship with BSD Syslog ..............................34
     A.2.  Message Length ............................................35
     A.3.  Severity Values  ..........................................36
     A.4.  TIME-SECFRAC Precision ....................................36
     A.5.  Case Convention for Names  ................................36
     A.6.  Syslog Applications Without Knowledge of Time  ............37
     A.7.  Notes on the timeQuality SD-ID ............................37
     A.8.  UTF-8 Encoding and the BOM ................................37

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1.  Introduction

   This document describes a layered architecture for syslog.  The goal
   of this architecture is to separate message content from message
   transport while enabling easy extensibility for each layer.

   This document describes the standard format for syslog messages and
   outlines the concept of transport mappings.  It also describes
   structured data elements, which can be used to transmit easily
   parseable, structured information, and allows for vendor extensions.

   This document does not describe any storage format for syslog
   messages.  It is beyond of the scope of the syslog protocol and is
   unnecessary for system interoperability.

   This document has been written with the original design goals for
   traditional syslog in mind.  The need for a new layered specification
   has arisen because standardization efforts for reliable and secure
   syslog extensions suffer from the lack of a Standards-Track and
   transport-independent RFC.  Without this document, each other
   standard would need to define its own syslog packet format and
   transport mechanism, which over time will introduce subtle
   compatibility issues.  This document tries to provide a foundation
   that syslog extensions can build on.  This layered architecture
   approach also provides a solid basis that allows code to be written
   once for each syslog feature instead of once for each transport.

   This document obsoletes RFC 3164, which is an Informational document
   describing some implementations found in the field.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Definitions

   Syslog utilizes three layers:

   o  "syslog content" is the management information contained in a
      syslog message.

   o  The "syslog application" layer handles generation, interpretation,
      routing, and storage of syslog messages.

   o  The "syslog transport" layer puts messages on the wire and takes
      them off the wire.

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RFC 5424                  The Syslog Protocol                 March 2009

   Certain types of functions are performed at each conceptual layer:

   o  An "originator" generates syslog content to be carried in a

   o  A "collector" gathers syslog content for further analysis.

   o  A "relay" forwards messages, accepting messages from originators
      or other relays and sending them to collectors or other relays.

   o  A "transport sender" passes syslog messages to a specific
      transport protocol.

   o  A "transport receiver" takes syslog messages from a specific
      transport protocol.

   Diagram 1 shows the different entities separated by layer.

 +---------------------+    +---------------------+
 |  content            |    |  content            |
 |---------------------|    |---------------------|
 |  syslog application |    |  syslog application | (originator,
 |                     |    |                     |  collector, relay)
 |---------------------|    |---------------------|
 |  syslog transport   |    |  syslog transport   | (transport sender,
 |                     |    |                     | (transport receiver)
 +---------------------+    +---------------------+
           ^                          ^
           |                          |

   Diagram 1.  Syslog Layers

4.  Basic Principles

   The following principles apply to syslog communication:

   o  The syslog protocol does not provide acknowledgment of message
      delivery.  Though some transports may provide status information,
      conceptually, syslog is a pure simplex communications protocol.

   o  Originators and relays may be configured to send the same message
      to multiple collectors and relays.

   o  Originator, relay, and collector functionality may reside on the
      same system.

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4.1.  Example Deployment Scenarios

   Sample deployment scenarios are shown in Diagram 2.  Other
   arrangements of these examples are also acceptable.  As noted, in the
   following diagram, relays may send all or some of the messages that
   they receive and also send messages that they generate internally.
   The boxes represent syslog-enabled applications.

            +----------+         +---------+
            +----------+         +---------+

            +----------+         +-----+         +---------+
            +----------+         +-----+         +---------+

            +----------+     +-----+            +-----+     +---------+
            +----------+     +-----+            +-----+     +---------+

            +----------+         +-----+         +---------+
            |          |-+       +-----+         +---------+
            +----------+  \
                           \     +-----+         +---------+
                                 +-----+         +---------+

            +----------+         +---------+
            |          |-+       +---------+
            +----------+  \
                           \     +-----+         +---------+
                                 +-----+         +---------+

            +----------+         +-----+            +---------+
            |          |-+       +-----+        +---|         |
            +----------+  \                    /    +---------+
                           \     +-----+      /

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            +----------+         +-----+                   +---------+
            |          |-+       +-----+                +--|         |
            +----------+  \                            /   +---------+
                           \     +------------+       /
                            \    |+----------+|      /
                             +->-||Relay     ||->---/
                                 |+----------||    /

   Diagram 2.  Some Possible Syslog Deployment Scenarios

5.  Transport Layer Protocol

   This document does not specify any transport layer protocol.
   Instead, it describes the format of a syslog message in a transport
   layer independent way.  Syslog transports are defined in other
   documents.  One such transport is defined in [RFC5426] and is
   consistent with the traditional UDP transport.  This transport is
   needed to maintain interoperability as the UDP transport has
   historically been used for the transmission of syslog messages.

   Any syslog transport protocol MUST NOT deliberately alter the syslog
   message.  If the transport protocol needs to perform temporary
   transformations at the transport sender, these transformations MUST
   be reversed by the transport protocol at the transport receiver so
   that the relay or collector will see an exact copy of the message
   generated by the originator or relay.  Otherwise, end-to-end
   cryptographic verifiers (such as signatures) will be broken.  Of
   course, message alteration might occur due to transmission errors or
   other problems.  Guarding against such alterations is not within the
   scope of this document.

5.1.  Minimum Required Transport Mapping

   All implementations of this specification MUST support a TLS-based
   transport as described in [RFC5425].

   All implementations of this specification SHOULD also support a
   UDP-based transport as described in [RFC5426].

   It is RECOMMENDED that deployments of this specification use the TLS-
   based transport.

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6.  Syslog Message Format

   The syslog message has the following ABNF [RFC5234] definition:


                        SP APP-NAME SP PROCID SP MSGID
      PRI             = "<" PRIVAL ">"
      PRIVAL          = 1*3DIGIT ; range 0 .. 191

      PROCID          = NILVALUE / 1*128PRINTUSASCII
      MSGID           = NILVALUE / 1*32PRINTUSASCII

      DATE-MONTH      = 2DIGIT  ; 01-12
      DATE-MDAY       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on
                                ; month/year
      TIME-HOUR       = 2DIGIT  ; 00-23
      TIME-MINUTE     = 2DIGIT  ; 00-59
      TIME-SECOND     = 2DIGIT  ; 00-59
      TIME-SECFRAC    = "." 1*6DIGIT

      SD-ELEMENT      = "[" SD-ID *(SP SD-PARAM) "]"
      SD-PARAM        = PARAM-NAME "=" %d34 PARAM-VALUE %d34
      SD-ID           = SD-NAME
      PARAM-NAME      = SD-NAME
      PARAM-VALUE     = UTF-8-STRING ; characters '"', '\' and
                                     ; ']' MUST be escaped.
      SD-NAME         = 1*32PRINTUSASCII
                        ; except '=', SP, ']', %d34 (")

      MSG             = MSG-ANY / MSG-UTF8
      MSG-ANY         = *OCTET ; not starting with BOM
      MSG-UTF8        = BOM UTF-8-STRING
      BOM             = %xEF.BB.BF

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      UTF-8-STRING    = *OCTET ; UTF-8 string as specified
                        ; in RFC 3629

      OCTET           = %d00-255
      SP              = %d32
      PRINTUSASCII    = %d33-126
      NONZERO-DIGIT   = %d49-57
      DIGIT           = %d48 / NONZERO-DIGIT
      NILVALUE        = "-"

6.1.  Message Length

   Syslog message size limits are dictated by the syslog transport
   mapping in use.  There is no upper limit per se.  Each transport
   mapping defines the minimum maximum required message length support,
   and the minimum maximum MUST be at least 480 octets in length.

   Any transport receiver MUST be able to accept messages of up to and
   including 480 octets in length.  All transport receiver
   implementations SHOULD be able to accept messages of up to and
   including 2048 octets in length.  Transport receivers MAY receive
   messages larger than 2048 octets in length.  If a transport receiver
   receives a message with a length larger than it supports, the
   transport receiver SHOULD truncate the payload.  Alternatively, it
   MAY discard the message.

   If a transport receiver truncates messages, the truncation MUST occur
   at the end of the message.  After truncation, the message MAY contain
   invalid UTF-8 encoding or invalid STRUCTURED-DATA.  The transport
   receiver MAY discard the message or MAY try to process as much as
   possible in this case.

6.2.  HEADER

   The character set used in the HEADER MUST be seven-bit ASCII in an
   eight-bit field as described in [RFC5234].  These are the ASCII codes
   as defined in "USA Standard Code for Information Interchange"

   The header format is designed to provide some interoperability with
   older BSD-based syslog.  For details on this, see Appendix A.1.

6.2.1.  PRI

   The PRI part MUST have three, four, or five characters and will be
   bound with angle brackets as the first and last characters.  The PRI
   part starts with a leading "<" ('less-than' character, %d60),
   followed by a number, which is followed by a ">" ('greater-than'

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   character, %d62).  The number contained within these angle brackets
   is known as the Priority value (PRIVAL) and represents both the
   Facility and Severity.  The Priority value consists of one, two, or
   three decimal integers (ABNF DIGITS) using values of %d48 (for "0")
   through %d57 (for "9").

   Facility and Severity values are not normative but often used.  They
   are described in the following tables for purely informational
   purposes.  Facility values MUST be in the range of 0 to 23 inclusive.

          Numerical             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 (note 2)
             16             local use 0  (local0)
             17             local use 1  (local1)
             18             local use 2  (local2)
             19             local use 3  (local3)
             20             local use 4  (local4)
             21             local use 5  (local5)
             22             local use 6  (local6)
             23             local use 7  (local7)

              Table 1.  Syslog Message Facilities

   Each message Priority also has a decimal Severity level indicator.
   These are described in the following table along with their numerical
   values.  Severity values MUST be in the range of 0 to 7 inclusive.

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           Numerical         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

              Table 2. Syslog Message Severities

   The Priority value is calculated by first multiplying the Facility
   number by 8 and then adding the numerical value of the Severity.  For
   example, a kernel message (Facility=0) with a Severity of Emergency
   (Severity=0) would have a Priority value of 0.  Also, a "local use 4"
   message (Facility=20) with a Severity of Notice (Severity=5) would
   have a Priority value of 165.  In the PRI of a syslog message, these
   values would be placed between the angle brackets as <0> and <165>
   respectively.  The only time a value of "0" follows the "<" is for
   the Priority value of "0".  Otherwise, leading "0"s MUST NOT be used.

6.2.2.  VERSION

   The VERSION field denotes the version of the syslog protocol
   specification.  The version number MUST be incremented for any new
   syslog protocol specification that changes any part of the HEADER
   format.  Changes include the addition or removal of fields, or a
   change of syntax or semantics of existing fields.  This document uses
   a VERSION value of "1".  The VERSION values are IANA-assigned
   (Section 9.1) via the Standards Action method as described in


   The TIMESTAMP field is a formalized timestamp derived from [RFC3339].

   Whereas [RFC3339] makes allowances for multiple syntaxes, this
   document imposes further restrictions.  The TIMESTAMP value MUST
   follow these restrictions:

   o  The "T" and "Z" characters in this syntax MUST be upper case.

   o  Usage of the "T" character is REQUIRED.

   o  Leap seconds MUST NOT be used.

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   The originator SHOULD include TIME-SECFRAC if its clock accuracy and
   performance permit.  The "timeQuality" SD-ID described in Section 7.1
   allows the originator to specify the accuracy and trustworthiness of
   the timestamp.

   A syslog application MUST use the NILVALUE as TIMESTAMP if the syslog
   application is incapable of obtaining system time.  Examples

   Example 1


   This represents 20 minutes and 50.52 seconds after the 23rd hour of
   12 April 1985 in UTC.

   Example 2


   This represents the same time as in example 1, but expressed in US
   Eastern Standard Time (observing daylight savings time).

   Example 3


   This represents 11 October 2003 at 10:14:15pm, 3 milliseconds into
   the next second.  The timestamp is in UTC.  The timestamp provides
   millisecond resolution.  The creator may have actually had a better
   resolution, but providing just three digits for the fractional part
   of a second does not tell us.

   Example 4


   This represents 24 August 2003 at 05:14:15am, 3 microseconds into the
   next second.  The microsecond resolution is indicated by the
   additional digits in TIME-SECFRAC.  The timestamp indicates that its
   local time is -7 hours from UTC.  This timestamp might be created in
   the US Pacific time zone during daylight savings time.

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   Example 5 - An Invalid TIMESTAMP


   This example is nearly the same as Example 4, but it is specifying
   TIME-SECFRAC in nanoseconds.  This results in TIME-SECFRAC being
   longer than the allowed 6 digits, which invalidates it.

6.2.4.  HOSTNAME

   The HOSTNAME field identifies the machine that originally sent the
   syslog message.

   The HOSTNAME field SHOULD contain the hostname and the domain name of
   the originator in the format specified in STD 13 [RFC1034].  This
   format is called a Fully Qualified Domain Name (FQDN) in this

   In practice, not all syslog applications are able to provide an FQDN.
   As such, other values MAY also be present in HOSTNAME.  This document
   makes provisions for using other values in such situations.  A syslog
   application SHOULD provide the most specific available value first.
   The order of preference for the contents of the HOSTNAME field is as

   1.  FQDN

   2.  Static IP address

   3.  hostname

   4.  Dynamic IP address

   5.  the NILVALUE

   If an IPv4 address is used, it MUST be in the format of the dotted
   decimal notation as used in STD 13 [RFC1035].  If an IPv6 address is
   used, a valid textual representation as described in [RFC4291],
   Section 2.2, MUST be used.

   Syslog applications SHOULD consistently use the same value in the
   HOSTNAME field for as long as possible.

   The NILVALUE SHOULD only be used when the syslog application has no
   way to obtain its real hostname.  This situation is considered highly

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6.2.5.  APP-NAME

   The APP-NAME field SHOULD identify the device or application that
   originated the message.  It is a string without further semantics.
   It is intended for filtering messages on a relay or collector.

   The NILVALUE MAY be used when the syslog application has no idea of
   its APP-NAME or cannot provide that information.  It may be that a
   device is unable to provide that information either because of a
   local policy decision, or because the information is not available,
   or not applicable, on the device.

   This field MAY be operator-assigned.

6.2.6.  PROCID

   PROCID is a value that is included in the message, having no
   interoperable meaning, except that a change in the value indicates
   there has been a discontinuity in syslog reporting.  The field does
   not have any specific syntax or semantics; the value is
   implementation-dependent and/or operator-assigned.  The NILVALUE MAY
   be used when no value is provided.

   The PROCID field is often used to provide the process name or process
   ID associated with a syslog system.  The NILVALUE might be used when
   a process ID is not available.  On an embedded system without any
   operating system process ID, PROCID might be a reboot ID.

   PROCID can enable log analyzers to detect discontinuities in syslog
   reporting by detecting a change in the syslog process ID.  However,
   PROCID is not a reliable identification of a restarted process since
   the restarted syslog process might be assigned the same process ID as
   the previous syslog process.

   PROCID can also be used to identify which messages belong to a group
   of messages.  For example, an SMTP mail transfer agent might put its
   SMTP transaction ID into PROCID, which would allow the collector or
   relay to group messages based on the SMTP transaction.

6.2.7.  MSGID

   The MSGID SHOULD identify the type of message.  For example, a
   firewall might use the MSGID "TCPIN" for incoming TCP traffic and the
   MSGID "TCPOUT" for outgoing TCP traffic.  Messages with the same
   MSGID should reflect events of the same semantics.  The MSGID itself
   is a string without further semantics.  It is intended for filtering
   messages on a relay or collector.

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   The NILVALUE SHOULD be used when the syslog application does not, or
   cannot, provide any value.

   This field MAY be operator-assigned.


   STRUCTURED-DATA provides a mechanism to express information in a well
   defined, easily parseable and interpretable data format.  There are
   multiple usage scenarios.  For example, it may express meta-
   information about the syslog message or application-specific
   information such as traffic counters or IP addresses.

   STRUCTURED-DATA can contain zero, one, or multiple structured data
   elements, which are referred to as "SD-ELEMENT" in this document.

   In case of zero structured data elements, the STRUCTURED-DATA field
   MUST contain the NILVALUE.

   The character set used in STRUCTURED-DATA MUST be seven-bit ASCII in
   an eight-bit field as described in [RFC5234].  These are the ASCII
   codes as defined in "USA Standard Code for Information Interchange"
   [ANSI.X3-4.1968].  An exception is the PARAM-VALUE field (see
   Section 6.3.3), in which UTF-8 encoding MUST be used.

   A collector MAY ignore malformed STRUCTURED-DATA elements.  A relay
   MUST forward malformed STRUCTURED-DATA without any alteration.

6.3.1.  SD-ELEMENT

   An SD-ELEMENT consists of a name and parameter name-value pairs.  The
   name is referred to as SD-ID.  The name-value pairs are referred to
   as "SD-PARAM".

6.3.2.  SD-ID

   SD-IDs are case-sensitive and uniquely identify the type and purpose
   of the SD-ELEMENT.  The same SD-ID MUST NOT exist more than once in a

   There are two formats for SD-ID names:

   o  Names that do not contain an at-sign ("@", ABNF %d64) are reserved
      to be assigned by IETF Review as described in BCP26 [RFC5226].
      Currently, these are the names defined in Section 7.  Names of
      this format are only valid if they are first registered with the
      IANA.  Registered names MUST NOT contain an at-sign ('@', ABNF

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      %d64), an equal-sign ('=', ABNF %d61), a closing brace (']', ABNF
      %d93), a quote-character ('"', ABNF %d34), whitespace, or control
      characters (ASCII code 127 and codes 32 or less).

   o  Anyone can define additional SD-IDs using names in the format
      name@<private enterprise number>, e.g., "ourSDID@32473".  The
      format of the part preceding the at-sign is not specified;
      however, these names MUST be printable US-ASCII strings, and MUST
      NOT contain an at-sign ('@', ABNF %d64), an equal-sign ('=', ABNF
      %d61), a closing brace (']', ABNF %d93), a quote-character ('"',
      ABNF %d34), whitespace, or control characters.  The part following
      the at-sign MUST be a private enterprise number as specified in
      Section 7.2.2.  Please note that throughout this document the
      value of 32473 is used for all private enterprise numbers.  This
      value has been reserved by IANA to be used as an example number in
      documentation.  Implementors will need to use their own private
      enterprise number for the enterpriseId parameter, and when
      creating locally extensible SD-ID names.

6.3.3.  SD-PARAM

   Each SD-PARAM consists of a name, referred to as PARAM-NAME, and a
   value, referred to as PARAM-VALUE.

   PARAM-NAME is case-sensitive.  IANA controls all PARAM-NAMEs, with
   the exception of those in SD-IDs whose names contain an at-sign.  The
   PARAM-NAME scope is within a specific SD-ID.  Thus, equally named
   PARAM-NAME values contained in two different SD-IDs are not the same.

   To support international characters, the PARAM-VALUE field MUST be
   encoded using UTF-8.  A syslog application MAY issue any valid UTF-8
   sequence.  A syslog application MUST accept any valid UTF-8 sequence
   in the "shortest form".  It MUST NOT fail if control characters are
   present in PARAM-VALUE.  The syslog application MAY modify messages
   containing control characters (e.g., by changing an octet with value
   0 (USASCII NUL) to the four characters "#000").  For the reasons
   outlined in UNICODE TR36 [UNICODE-TR36], section 3.1, an originator
   MUST encode messages in the "shortest form" and a collector or relay
   MUST NOT interpret messages in the "non-shortest form".

   Inside PARAM-VALUE, the characters '"' (ABNF %d34), '\' (ABNF %d92),
   and ']' (ABNF %d93) MUST be escaped.  This is necessary to avoid
   parsing errors.  Escaping ']' would not strictly be necessary but is
   REQUIRED by this specification to avoid syslog application
   implementation errors.  Each of these three characters MUST be
   escaped as '\"', '\\', and '\]' respectively.  The backslash is used

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   for control character escaping for consistency with its use for
   escaping in other parts of the syslog message as well as in
   traditional syslog.

   A backslash ('\') followed by none of the three described characters
   is considered an invalid escape sequence.  In this case, the
   backslash MUST be treated as a regular backslash and the following
   character as a regular character.  Thus, the invalid sequence MUST
   not be altered.

   An SD-PARAM MAY be repeated multiple times inside an SD-ELEMENT.

6.3.4.  Change Control

   Once SD-IDs and PARAM-NAMEs are defined, syntax and semantics of
   these objects MUST NOT be altered.  Should a change to an existing
   object be desired, a new SD-ID or PARAM-NAME MUST be created and the
   old one remain unchanged.  OPTIONAL PARAM-NAMEs MAY be added to an
   existing SD-ID.

6.3.5.  Examples

   All examples in this section show only the structured data part of
   the message.  Examples should be considered to be on one line.  They
   are wrapped on multiple lines in this document for readability
   purposes.  A description is given after each example.

   Example 1 - Valid

           [exampleSDID@32473 iut="3" eventSource="Application"

   This example is a structured data element with a non-IANA controlled
   SD-ID of type "exampleSDID@32473", which has three parameters.

   Example 2 - Valid

           [exampleSDID@32473 iut="3" eventSource="Application"
           eventID="1011"][examplePriority@32473 class="high"]

   This is the same example as in 1, but with a second structured data
   element.  Please note that the structured data element immediately
   follows the first one (there is no SP between them).

   Example 3 - Invalid

           [exampleSDID@32473 iut="3" eventSource="Application"
           eventID="1011"] [examplePriority@32473 class="high"]

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   This is nearly the same example as 2, but it has a subtle error --
   there is an SP character between the two structured data elements
   ("]SP[").  This is invalid.  It will cause the STRUCTURED-DATA field
   to end after the first element.  The second element will be
   interpreted as part of the MSG field.

   Example 4 - Invalid

           [ exampleSDID@32473 iut="3" eventSource="Application"
           eventID="1011"][examplePriority@32473 class="high"]

   This example is nearly the same as 2.  It has another subtle error --
   the SP character occurs after the initial bracket.  A structured data
   element SD-ID MUST immediately follow the beginning bracket, so the
   SP character invalidates the STRUCTURED-DATA.  A syslog application
   MAY discard this message.

   Example 5 - Valid

           [sigSig ver="1" rsID="1234" ... signature="..."]

   Example 5 is a valid example.  It shows a hypothetical IANA-assigned
   SD-ID.  The ellipses denote missing content, which has been left out
   of this example for brevity.

6.4.  MSG

   The MSG part contains a free-form message that provides information
   about the event.

   The character set used in MSG SHOULD be UNICODE, encoded using UTF-8
   as specified in [RFC3629].  If the syslog application cannot encode
   the MSG in Unicode, it MAY use any other encoding.

   The syslog application SHOULD avoid octet values below 32 (the
   traditional US-ASCII control character range except DEL).  These
   values are legal, but a syslog application MAY modify these
   characters upon reception.  For example, it might change them into an
   escape sequence (e.g., value 0 may be changed to "\0").  A syslog
   application SHOULD NOT modify any other octet values.

   If a syslog application encodes MSG in UTF-8, the string MUST start
   with the Unicode byte order mask (BOM), which for UTF-8 is ABNF
   %xEF.BB.BF.  The syslog application MUST encode in the "shortest
   form" and MAY use any valid UTF-8 sequence.

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   If a syslog application is processing an MSG starting with a BOM and
   the MSG contains UTF-8 that is not shortest form, the MSG MUST NOT be
   interpreted as being encoded in UTF-8, for the reasons outlined in
   [UNICODE-TR36], Section 3.1.  Guidance about this is given in
   Appendix A.8.

   Also, according to UNICODE TR36 [UNICODE-TR36], a syslog application
   MUST NOT interpret messages in the "non-shortest form".  It MUST NOT
   interpret invalid UTF-8 sequences.

6.5.  Examples

   The following are examples of valid syslog messages.  A description
   of each example can be found below it.  The examples are based on
   similar examples from [RFC3164] and may be familiar to readers.  The
   otherwise-unprintable Unicode BOM is represented as "BOM" in the

   Example 1 - with no STRUCTURED-DATA

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

   In this example, the VERSION is 1 and the Facility has the value of
   4.  The Severity is 2.  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.  The encoding is defined by the BOM.
   There is no STRUCTURED-DATA present in the message; this is indicated
   by "-" in the STRUCTURED-DATA field.

   Example 2 - with no STRUCTURED-DATA

         <165>1 2003-08-24T05:14:15.000003-07:00
         myproc 8710 - - %% It's time to make the do-nuts.

   In this example, the VERSION is again 1.  The Facility is 20, the
   Severity 5.  The message was created on 24 August 2003 at 5:14:15am,
   with a -7 hour offset from UTC, 3 microseconds into the next second.
   The HOSTNAME is "", so the syslog application did not know
   its FQDN and used one of its IPv4 addresses instead.  The APP-NAME is
   "myproc" and the PROCID is "8710" (for example, this could be the
   UNIX PID).  There is no STRUCTURED-DATA present in the message; this
   is indicated by "-" in the STRUCTURED-DATA field.  There is no
   specific MSGID and this is indicated by the "-" in the MSGID field.

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   The message is "%% It's time to make the do-nuts.".  As the Unicode
   BOM is missing, the syslog application does not know the encoding of
   the MSG part.

   Example 3 - with STRUCTURED-DATA

           <165>1 2003-10-11T22:14:15.003Z mymachine.example.com
           evntslog - ID47 [exampleSDID@32473 iut="3" eventSource=
           "Application" eventID="1011"] BOMAn application
           event log entry...

   This example is modeled after Example 1.  However, this time it
   contains STRUCTURED-DATA, a single element with the value
   "[exampleSDID@32473 iut="3" eventSource="Application"
   eventID="1011"]".  The MSG itself is "An application event log
   entry..."  The BOM at the beginning of MSG indicates UTF-8 encoding.

   Example 4 - STRUCTURED-DATA Only

           <165>1 2003-10-11T22:14:15.003Z mymachine.example.com
           evntslog - ID47 [exampleSDID@32473 iut="3" eventSource=
           "Application" eventID="1011"][examplePriority@32473

   This example shows a message with only STRUCTURED-DATA and no MSG
   part.  This is a valid message.

7.  Structured Data IDs

   This section defines the initial IANA-registered SD-IDs.  See
   Section 6.3 for a definition of structured data elements.  All SD-IDs
   defined here are OPTIONAL.

   In some of the following, a maximum length is quantified for the
   parameter values.  In each of those cases, the syslog application
   MUST be prepared to receive the number of defined characters in any
   valid UTF-8 code point.  Since each character may be up to 6 octets,
   it is RECOMMENDED that each syslog application be prepared to receive
   up to 6 octets per character.

7.1.  timeQuality

   The SD-ID "timeQuality" MAY be used by the originator to describe its
   notion of system time.  This SD-ID SHOULD be written if the
   originator is not properly synchronized with a reliable external time
   source or if it does not know whether its time zone information is

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   correct.  The main use of this structured data element is to provide
   some information on the level of trust it has in the TIMESTAMP
   described in Section 6.2.3.  All parameters are OPTIONAL.

7.1.1.  tzKnown

   The "tzKnown" parameter indicates whether the originator knows its
   time zone.  If it does, the value "1" MUST be used.  If the time zone
   information is in doubt, the value "0" MUST be used.  If the
   originator knows its time zone but decides to emit time in UTC, the
   value "1" MUST be used (because the time zone is known).

7.1.2.  isSynced

   The "isSynced" parameter indicates whether the originator is
   synchronized to a reliable external time source, e.g., via NTP.  If
   the originator is time synchronized, the value "1" MUST be used.  If
   not, the value "0" MUST be used.

7.1.3.  syncAccuracy

   The "syncAccuracy" parameter indicates how accurate the originator
   thinks its time synchronization is.  It is an integer describing the
   maximum number of microseconds that its clock may be off between
   synchronization intervals.

   If the value "0" is used for "isSynced", this parameter MUST NOT be
   specified.  If the value "1" is used for "isSynced" but the
   "syncAccuracy" parameter is absent, a collector or relay can assume
   that the time information provided is accurate enough to be
   considered correct.  The "syncAccuracy" parameter MUST be written
   only if the originator actually has knowledge of the reliability of
   the external time source.  In most cases, it will gain this in-depth
   knowledge through operator configuration.

7.1.4.  Examples

   The following is an example of an originator that does not know its
   time zone or whether it is being synchronized:

   [timeQuality tzKnown="0" isSynced="0"]

   With this information, the originator indicates that its time
   information is unreliable.  This may be a hint for the collector or
   relay to use its local time instead of the message-provided TIMESTAMP
   for correlation of multiple messages from different originators.

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   The following is an example of an originator that knows its time zone
   and knows that it is properly synchronized to a reliable external

   [timeQuality tzKnown="1" isSynced="1"]

   The following is an example of an originator that knows both its time
   zone and that it is externally synchronized.  It also knows the
   accuracy of the external synchronization:

   [timeQuality tzKnown="1" isSynced="1" syncAccuracy="60000000"]

   The difference between this and the previous example is that the
   originator expects that its clock will be kept within 60 seconds of
   the official time.  Thus, if the originator reports it is 9:00:00, it
   is no earlier than 8:59:00 and no later then 9:01:00.

7.2.  origin

   The SD-ID "origin" MAY be used to indicate the origin of a syslog
   message.  The following parameters can be used.  All parameters are

   Specifying any of these parameters is primarily an aid to log
   analyzers and similar applications.

7.2.1.  ip

   The "ip" parameter denotes an IP address that the originator knows it
   had at the time of originating the message.  It MUST contain the
   textual representation of an IP address as outlined in Section 6.2.4.

   This parameter can be used to provide identifying information in
   addition to what is present in the HOSTNAME field.  It might be
   especially useful if the host's IP address is included in the message
   while the HOSTNAME field still contains the FQDN.  It is also useful
   for describing all IP addresses of a multihomed host.

   If an originator has multiple IP addresses, it MAY either list one of
   its IP addresses in the "ip" parameter or it MAY include multiple
   "ip" parameters in a single "origin" structured data element.

7.2.2.  enterpriseId

   The "enterpriseId" parameter MUST be a 'SMI Network Management
   Private Enterprise Code', maintained by IANA, whose prefix is
   iso.org.dod.internet.private.enterprise (  The number
   that follows MUST be unique and MUST be registered with IANA as per

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   RFC 2578 [RFC2578].  An enterprise is only authorized to assign
   values within the iso.org.dod.internet.private.enterprise.<private
   enterprise number> subtree assigned by IANA to that enterprise.  The
   enterpriseId MUST contain only a value from the
   iso.org.dod.internet.private.enterprise.<private enterprise number>
   subtree.  In general, only the IANA-assigned private enterprise
   number is needed (a single number).  An enterprise might decide to
   use sub-identifiers below its private enterprise number.  If sub-
   identifiers are used, they MUST be separated by periods and be
   represented as decimal numbers.  An example for that would be
   "32473.1.2".  Please note that the ID "32473.1.2" is just an example
   and MUST NOT be used.  The complete up-to-date list of Private
   Enterprise Numbers (PEN) is maintained by IANA.

   By specifying a private enterprise number, the vendor allows more
   specific processing of the message.

7.2.3.  software

   The "software" parameter uniquely identifies the software that
   generated the message.  If it is used, "enterpriseId" SHOULD also be
   specified, so that a specific vendor's software can be identified.
   The "software" parameter is not the same as the APP-NAME header
   field.  It MUST always contain the name of the generating software,
   whereas APP-NAME can contain anything else, including an operator-
   configured value.

   The "software" parameter is a string.  It MUST NOT be longer than 48

7.2.4.  swVersion

   The "swVersion" parameter uniquely identifies the version of the
   software that generated the message.  If it is used, the "software"
   and "enterpriseId" parameters SHOULD be provided, too.

   The "swVersion" parameter is a string.  It MUST NOT be longer than 32

7.2.5.  Example

   The following is an example with multiple IP addresses:

   [origin ip="" ip=""]

   In this example, the originator indicates that it has two IP
   addresses, one being and the other one being

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7.3.  meta

   The SD-ID "meta" MAY be used to provide meta-information about the
   message.  The following parameters can be used.  All parameters are
   OPTIONAL.  If the "meta" SD-ID is used, at least one parameter SHOULD
   be specified.

7.3.1.  sequenceId

   The "sequenceId" parameter tracks the sequence in which the
   originator submits messages to the syslog transport for sending.  It
   is an integer that MUST be set to 1 when the syslog function is
   started and MUST be increased with every message up to a maximum
   value of 2147483647.  If that value is reached, the next message MUST
   be sent with a sequenceId of 1.

7.3.2.  sysUpTime

   The "sysUpTime" parameter MAY be used to include the SNMP "sysUpTime"
   parameter in the message.  Its syntax and semantics are as defined in

   As syslog does not support the SNMP "INTEGER" syntax directly, the
   value MUST be represented as a decimal integer (no decimal point)
   using only the characters "0", "1", "2", "3", "4", "5", "6", "7",
   "8", and "9".

   Note that the semantics in RFC 3418 are "The time (in hundredths of a
   second) since the network management portion of the system was last
   re-initialized."  This of course relates to the SNMP-related
   management portion of the system, which MAY be different than the
   syslog-related management portion of the system.

7.3.3.  language

   The "language" parameter MAY be specified by the originator to convey
   information about the natural language used inside MSG.  If it is
   specified, it MUST contain a language identifier as defined in BCP 47

8.  Security Considerations


   This document uses UTF-8 encoding for the PARAM-VALUE and MSG fields.
   There are a number of security issues with UNICODE.  Any implementer
   and operator is advised to review UNICODE TR36 [UNICODE-TR36] (UTR36)
   to learn about these issues.  This document guards against the

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   technical issues outlined in UTR36 by REQUIRING "shortest form"
   encoding for syslog applications.  However, the visual spoofing due
   to character confusion still persists.  This document tries to
   minimize the effects of visual spoofing by allowing UNICODE only
   where local script is expected and needed.  In all other fields,
   US-ASCII is REQUIRED.  Also, the PARAM-VALUE and MSG fields should
   not be the primary source for identifying information, further
   reducing the risks associated with visual spoofing.

8.2.  Control Characters

   This document does not impose any mandatory restrictions on the MSG
   or PARAM-VALUE content.  As such, they MAY contain control
   characters, including the NUL character.

   In some programming languages (most notably C and C++), the NUL
   character (ABNF %d00) traditionally has a special significance as
   string terminator.  Most implementations of these languages assume
   that a string will not extend beyond the first NUL character.  This
   is primarily a restriction of the supporting run-time libraries.
   This restriction is often carried over to programs and script
   languages written in those languages.  As such, NUL characters must
   be considered with great care and be properly handled.  An attacker
   may deliberately include NUL characters to hide information after
   them.  Incorrect handling of the NUL character may also invalidate
   cryptographic checksums that are transmitted inside the message.

   Many popular text editors are also written in languages with this
   restriction.  Encoding NUL characters when writing to text files is
   advisable.  If they are stored without encoding, the file can become

   Other control characters may also be problematic.  For example, an
   attacker may deliberately include backspace characters to render
   parts of the log message unreadable.  Similar issues exist for almost
   all control characters.

   Finally, invalid UTF-8 sequences may be used by an attacker to inject
   ASCII control characters.

   This specification permits a syslog application to reformat control
   characters received.  Among others, the security risks associated
   with control characters were an important driving force behind this
   restriction.  Originators are advised that if any encoding other than
   ASCII and UTF8 are used, the receiver may corrupt the message in an
   attempt to filter ASCII control characters.

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8.3.  Message Truncation

   Message truncation can be misused by an attacker to hide vital log
   information.  Messages over the minimum supported size may be
   discarded or truncated by the transport receiver.  As such, vital log
   information may be lost.

   In order to prevent information loss, messages should not be longer
   than the minimum maximum size required by Section 6.1.  For best
   performance and reliability, messages should be as small as possible.
   Important information should be placed as early in the message as
   possible because information at the beginning of the message is less
   likely to be discarded by a size-limited transport receiver.

   An originator should limit the size of any user-supplied data within
   a syslog message.  If it does not, an attacker may provide large data
   in hopes of exploiting a potential weakness.

8.4.  Replay

   There is no mechanism in the syslog protocol to detect message
   replay.  An attacker may record a set of messages that indicate
   normal activity of a machine.  At a later time, that attacker may
   remove that machine from the network and replay the syslog messages
   to the relay or collector.  Even with the TIMESTAMP field in the
   HEADER part, an attacker may record the packets and could simply
   modify them to reflect the current time before retransmitting them.
   The administrators may find nothing unusual in the received messages,
   and their receipt would falsely indicate normal activity of the

   Cryptographically signing messages could prevent the alteration of
   TIMESTAMPs and thus the replay attack.

8.5.  Reliable Delivery

   Because there is no mechanism described within this document to
   ensure delivery, and the underlying transport may be unreliable
   (e.g., UDP), some messages may be lost.  They may either be dropped
   through network congestion, or they may be maliciously intercepted
   and discarded.  The consequences of dropping one or more syslog
   messages cannot be determined.  If the messages are simple status
   updates, then their non-receipt may not be noticed or may cause an
   annoyance for the system operators.  On the other hand, if the
   messages are more critical, then the administrators may not become
   aware of a developing and potentially serious problem.  Messages may
   also be intercepted and discarded by an attacker as a way to hide
   unauthorized activities.

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   It may also be desirable to include rate-limiting features in syslog
   originators and relays.  This can reduce potential congestion
   problems when message bursts happen.

   Reliable delivery may not always be desirable.  Reliable delivery
   means that the syslog originator or relay must block when the relay
   or collector is not able to accept any more messages.  In some
   operating systems, namely Unix/Linux, the syslog originator or relay
   runs inside a high-priority system process (syslogd).  If that
   process blocks, the system at large comes to a stand-still.  The same
   occurs if there is a deadlock situation between syslogd and e.g., the
   DNS server.

   To prevent these problems, reliable delivery can be implemented in a
   way that intentionally discards messages when the syslog application
   would otherwise block.  The advantage of reliable delivery in this
   case is that the syslog originator or relay knowingly discards the
   message and is able to notify the relay or collector about that fact.
   So the relay or collector receives the information that something is
   lost.  With unreliable delivery, the message would simply be lost
   without any indication that loss occurred.

8.6.  Congestion Control

   Because syslog can generate unlimited amounts of data, transferring
   this data over UDP is generally problematic, because UDP lacks
   congestion control mechanisms.  Congestion control mechanisms that
   respond to congestion by reducing traffic rates and establish a
   degree of fairness between flows that share the same path are vital
   to the stable operation of the Internet [RFC2914].  This is why the
   syslog TLS transport is REQUIRED to implement and RECOMMENDED for
   general use.

   The only environments where the syslog UDP transport MAY be used as
   an alternative to the TLS transport are managed networks, where the
   network path has been explicitly provisioned for UDP syslog traffic
   through traffic engineering mechanisms, such as rate limiting or
   capacity reservations.  In all other environments, the TLS transport
   SHOULD be used.

   In any implementation, the situation may arise in which an originator
   or relay would need to block sending messages.  A common case is when
   an internal queue is full.  This might happen due to rate-limiting or
   slow performance of the syslog application.  In any event, it is
   highly RECOMMENDED that no messages be dropped but that they should
   be temporarily stored until they can be transmitted.  However, if
   they must be dropped, it is RECOMMENDED that the originator or relay
   drop messages of lower severity in favor of higher severity messages.

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   Messages with a lower numerical SEVERITY value have a higher
   practical severity than those with a numerically higher value.  In
   that situation, the messages that are to be dropped SHOULD simply be
   discarded.  The syslog application may notify a collector or relay
   about the fact that it has dropped messages.

8.7.  Message Integrity

   Besides being discarded, syslog messages may be damaged in transit,
   or an attacker may maliciously modify them.  In such cases, the
   original contents of the message will not be delivered to the
   collector or relay.  Additionally, if an attacker is positioned
   between the transport sender and transport receiver of syslog
   messages, they may be able to intercept and modify those messages
   while in-transit to hide unauthorized activities.

8.8.  Message Observation

   While there are no strict guidelines pertaining to the MSG format,
   most syslog messages are generated in human-readable form with the
   assumption that capable administrators should be able to read them
   and understand their meaning.  The syslog protocol does not have
   mechanisms to provide confidentiality for the messages in transit.
   In most cases, passing clear-text messages is a benefit to the
   operations staff if they are sniffing the packets from the wire.  The
   operations staff may be able to read the messages and associate them
   with other events seen from other packets crossing the wire to track
   down and correct problems.  Unfortunately, an attacker may also be
   able to observe the human-readable contents of syslog messages.  The
   attacker may then use the knowledge gained from those messages to
   compromise a machine or do other damage.

   Operators are advised to use a secure transport mapping to avoid this

8.9.  Inappropriate Configuration

   Because there is no control information distributed about any
   messages or configurations, it is wholly the responsibility of the
   network administrator to ensure that the messages are actually going
   to the intended recipients.  Cases have been noted where syslog
   applications were inadvertently configured to send syslog messages to
   the wrong relays or collectors.  In many cases, the inadvertent
   relays or collectors may not be configured to receive syslog messages
   and will probably discard them.  In certain other cases, the receipt
   of syslog messages has been known to cause problems for the
   unintended recipient.  If messages are not going to the intended
   recipient, then they cannot be reviewed or processed.

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   Using a reliable transport mapping can help identify some of these
   problems.  For example, it can identify a problem where a message is
   being sent to a system that is not configured to receive messages.
   It cannot identify sending messages to a wrong machine that is
   accepting messages.

8.10.  Forwarding Loop

   As shown in Diagram 2, machines may be configured to relay syslog
   messages to subsequent relays before reaching a collector.  In one
   particular case, an administrator found that he had mistakenly
   configured two relays to forward messages with certain SEVERITY
   values to each other.  When either of these machines either received
   or generated that type of message, it would forward it to the other
   relay.  That relay would, in turn, forward it back.  This cycle did
   cause degradation to the intervening network as well as to the
   processing availability on the two devices.  Network administrators
   must take care not to cause such a death spiral.

8.11.  Load Considerations

   Network administrators must take the time to estimate the appropriate
   capacity of the syslog collector.  An attacker may perform a Denial
   of Service attack by filling the disk of the collector with false
   messages.  Placing the records in a circular file may alleviate this
   but has the consequence of not ensuring that an administrator will be
   able to review the records in the future.  Along this line, a
   transport receiver must have a network interface capable of receiving
   the messages sent to it.

   Administrators and network planners must also critically review the
   network paths between the originators, the relays, and the
   collectors.  Generated syslog messages should not overwhelm any of
   the network links.

   In order to reduce the impact of this issue, using transports with
   guaranteed delivery is recommended.

8.12.  Denial of Service

   As with any system, an attacker may just overwhelm a transport
   receiver by sending more messages to it than can be handled by the
   infrastructure or the device itself.  Implementers should attempt to
   provide features that minimize this threat, such as only accepting
   syslog messages from known IP addresses.

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9.  IANA Considerations


   IANA has created a registry entitled "syslog Version Values" of
   VERSION values as described in Section 6.2.2.  Version numbers MUST
   be incremented for any new syslog protocol specification that changes
   any part of the HEADER.  Changes include addition or removal of
   fields or a change of syntax or semantics of existing fields.

   VERSION numbers must be registered via the Standards Action method as
   described in [RFC5226].  IANA has registered the VERSIONs shown in
   Table 3 below.

       1           Defined in [RFC5424]

        Table 3.  IANA-Registered VERSIONs

9.2.  SD-IDs

   IANA has created a registry entitled "syslog Structured Data ID
   Values" of Structured Data ID (SD-ID) values together with their
   associated PARAM-NAME values as described in Section 7.

   New SD-ID and new PARAM-NAME values must be registered through the
   IETF Review method as described in [RFC5226].

   Once SD-IDs and SD-PARAMs are defined, syntax and semantics of these
   objects MUST NOT be altered.  Should a change to an existing object
   be desired, a new SD-ID or SD-PARAM MUST be created and the old one
   remain unchanged.

   A provision is made here for locally extensible names.  The IANA will
   not register, and will not control names with the at-sign (ABNF %d64)
   in them.

   IANA has registered the SD-IDs and PARAM-NAMEs shown in Table 4

       SD-ID              PARAM-NAME
       timeQuality                           OPTIONAL
                          tzKnown            OPTIONAL
                          isSynced           OPTIONAL
                          syncAccuracy       OPTIONAL

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       origin                                OPTIONAL
                          ip                 OPTIONAL
                          enterpriseId       OPTIONAL
                          software           OPTIONAL
                          swVersion          OPTIONAL

       meta                                  OPTIONAL
                          sequenceId         OPTIONAL
                          sysUpTime          OPTIONAL
                          language           OPTIONAL

          Table 4.  IANA-Registered SD-IDs and their PARAM-NAMEs

10.  Working Group

   The working group can be contacted via the mailing list:


   The current Chairs of the Working Group may be contacted at:

         Chris Lonvick
         Cisco Systems
         EMail: clonvick@cisco.com

         David Harrington
         Huawei Technologies USA
         EMail: dbharrington@comcast.net

11.  Acknowledgments

   The authors wish to thank Chris Lonvick, Jon Callas, Andrew Ross,
   Albert Mietus, Anton Okmianski, Tina Bird, Devin Kowatch, David
   Harrington, Sharon Chisholm, Richard Graveman, Tom Petch, Dado
   Colussi, Clement Mathieu, Didier Dalmasso, and all the other people
   who commented on various versions of this proposal.

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12.  References

12.1.  Normative References

   [ANSI.X3-4.1968]  American National Standards Institute, "USA Code
                     for Information Interchange", ANSI X3.4, 1968.

   [RFC1034]         Mockapetris, P., "Domain names - concepts and
                     facilities", STD 13, RFC 1034, November 1987.

   [RFC1035]         Mockapetris, P., "Domain names - implementation and
                     specification", STD 13, RFC 1035, November 1987.

   [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2578]         McCloghrie, K., Ed., Perkins, D., Ed., and J.
                     Schoenwaelder, Ed., "Structure of Management
                     Information Version 2 (SMIv2)", STD 58, RFC 2578,
                     April 1999.

   [RFC2914]         Floyd, S., "Congestion Control Principles", BCP 41,
                     RFC 2914, September 2000.

   [RFC3339]         Klyne, G., Ed. and C. Newman, "Date and Time on the
                     Internet: Timestamps", RFC 3339, July 2002.

   [RFC3418]         Presuhn, R., "Management Information Base (MIB) for
                     the Simple Network Management Protocol (SNMP)",
                     STD 62, RFC 3418, December 2002.

   [RFC3629]         Yergeau, F., "UTF-8, a transformation format of ISO
                     10646", STD 63, RFC 3629, November 2003.

   [RFC4291]         Hinden, R. and S. Deering, "IP Version 6 Addressing
                     Architecture", RFC 4291, February 2006.

   [RFC4646]         Phillips, A. and M. Davis, "Tags for Identifying
                     Languages", BCP 47, RFC 4646, September 2006.

   [RFC5226]         Narten, T. and H. Alvestrand, "Guidelines for
                     Writing an IANA Considerations Section in RFCs",
                     BCP 26, RFC 5226, May 2008.

   [RFC5234]         Crocker, D. and P. Overell, "Augmented BNF for
                     Syntax Specifications: ABNF", STD 68, RFC 5234,
                     January 2008.

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   [RFC5425]         Fuyou, M., Yuzhi, M., and J. Salowey, "TLS
                     Transport Mapping for Syslog", RFC 5425, March

   [RFC5426]         Okmianski, A., "Transmission of Syslog Messages
                     over UDP", RFC 5426, March 2009.

   [UNICODE-TR36]    Davis, M. and M. Suignard, "UNICODE Security
                     Considerations", July 2005.

12.2.  Informative References

   [RFC3164]         Lonvick, C., "The BSD Syslog Protocol", RFC 3164,
                     August 2001.

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Appendix A.  Implementer Guidelines

   Information in this section is given as an aid to implementers.
   While this information is considered to be helpful, it is not
   normative.  As such, an implementation is NOT REQUIRED to follow it
   in order to claim compliance to this specification.

A.1.  Relationship with BSD Syslog

   While BSD syslog is in widespread use, its format has never been
   formally standardized.  [RFC3164] describes observed formats.  It is
   an Informational RFC, and practice shows that there are many
   different implementations.  Research during creation of this document
   showed that there is very little in common between different syslog
   implementations on different platforms.  The only thing that all of
   them agree upon is that messages start with "<" PRIVAL ">".  Other
   than that, legacy syslog messages are not formatted in a consistent
   way.  Consequently, RFC 3164 describes no specific elements inside a
   syslog message.  It states that any message destined to the syslog
   UDP port must be treated as a syslog message, no matter what its
   format or content is.

   This document retains the PRI value syntax and semantics.  This will
   allow legacy syslog implementations to put messages generated by
   syslog applications compliant to this specification into the right

   Most existing implementations support UDP as the transport protocol
   for syslog.  This specification supports UDP transport, but does not
   recommend it.  Deployment of the required TLS support is recommended.
   Additional transport protocols may be used.

   RFC 3164 describes relay behavior.  This document does not specify
   relay behavior.  This might be done in a separate document.

   The TIMESTAMP described in RFC 3164 offers less precision than the
   timestamp specified in this document.  It also lacks the year and
   time zone information.  If a message formatted according to this
   document needs to be reformatted to be in RFC 3164 format, it is
   suggested that the originator's local time zone be used, and the time
   zone information and the year be dropped.  If an RFC 3164 formatted
   message is received and must be transformed to be compliant to this
   document, the current year should be added and the time zone of the
   relay or collector MAY be used.

   The HOSTNAME in RFC 3164 is less specific, but this format is still
   supported in this document as one of the alternate HOSTNAME

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   The MSG part of the message is described as TAG and CONTENT in RFC
   3164.  In this document, MSG is what was called CONTENT in RFC 3164.
   The TAG is now part of the header, but not as a single field.  The
   TAG has been split into APP-NAME, PROCID, and MSGID.  This does not
   totally resemble the usage of TAG, but provides the same
   functionality for most of the cases.

   In RFC 3164, STRUCTURED-DATA was not described.  If a message
   compliant with this document contains STRUCTURED-DATA and must be
   reformatted according to RFC 3164, the STRUCTURED-DATA simply becomes
   part of the RFC 3164 CONTENT free-form text.

   In general, this document tries to provide an easily parseable header
   with clear field separations, whereas traditional BSD syslog suffers
   from some historically developed, hard to parse field separation

A.2.  Message Length

   Implementers should note the message size limitations outlined in
   Section 6.1 and try to keep the most important data early in the
   message (within the minimum guaranteed length).  This ensures the
   data will be seen by the collector or relay even if a transport
   receiver at a relay on the message path truncates the message.

   The reason syslog transport receivers need only support receiving up
   to and including 480 octets has, among other things, to do with
   difficult delivery problems in a broken network.  Syslog messages may
   use a UDP transport mapping with this 480 octet restriction to avoid
   session overhead and message fragmentation.  In a network with
   problems, the likelihood of getting one single-packet message
   delivered successfully is higher than getting two message fragments
   delivered successfully.  Therefore, using a larger size may prevent
   the operator from getting some critical information about the
   problem, whereas using small messages might get that information to
   the operator.  It is recommended that messages intended for
   troubleshooting purposes should not be larger than 480 octets.  To
   further strengthen this point, it has also been observed that some
   UDP implementations generally do not support message sizes of more
   than 480 octets.  This behavior is very rare and may no longer be an

   There are other use cases where syslog messages are used to transmit
   inherently lengthy information, e.g., audit data.  By not enforcing
   any upper limit on the message size, syslog applications can be
   implemented with any size needed and still be compliant with this
   document.  In such cases, it is the operator's responsibility to

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   ensure that all components in a syslog infrastructure support the
   required message sizes.  Transport mappings may recommend specific
   message size limits that must be implemented to be compliant.

   Implementers are reminded that the message length is specified in
   octets.  There is a potentially large difference between the length
   in characters and the length in octets for UTF-8 strings.

   It must be noted that the IPv6 MTU is about 2.5 times 480.  An
   implementation targeted towards an IPv6-only environment might thus
   assume this as a larger minimum size.

A.3.  Severity Values

   This section describes guidelines for using Severity as outlined in
   Section 6.2.1.

   All implementations should try to assign the most appropriate
   severity to their message.  Most importantly, messages designed to
   enable debugging or testing of software should be assigned Severity
   7.  Severity 0 should be reserved for messages of very high
   importance (like serious hardware failures or imminent power
   failure).  An implementation may use Severities 0 and 7 for other
   purposes if this is configured by the administrator.

   Because severities are very subjective, a relay or collector should
   not assume that all originators have the same definition of severity.

A.4.  TIME-SECFRAC Precision

   The TIMESTAMP described in Section 6.2.3 supports fractional seconds.
   This provides grounds for a very common coding error, where leading
   zeros are removed from the fractional seconds.  For example, the
   TIMESTAMP "2003-10-11T22:13:14.003" may be erroneously written as
   "2003-10-11T22:13:14.3".  This would indicate 300 milliseconds
   instead of the 3 milliseconds actually meant.

A.5.  Case Convention for Names

   Names are used at various places in this document, for example for
   SD-IDs and PARAM-NAMEs.  This document uses "lower camel case"
   consistently.  With that, each name begins with a lower case letter
   and each new embedded word starts with an upper case letter, with no
   hyphen or other delimiter.  An example of this is "timeQuality".

   While an implementation is free to use any other case convention for
   experimental names, it is suggested that the case convention outlined
   above is followed.

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A.6.  Syslog Applications Without Knowledge of Time

   In Section 6.2.3, the NILVALUE has been allowed for usage by
   originators without knowledge of time.  This is done to support a
   special case when a syslog application is not aware of time at all.
   It can be argued whether such a syslog application can actually be
   found in today's IT infrastructure.  However, discussion has
   indicated that those things may exist in practice and as such there
   should be a guideline established for this case.

   However, an implementation SHOULD emit a valid TIMESTAMP if the
   underlying operating system, programming system, and hardware
   supports a clock function.  A proper TIMESTAMP should be emitted even
   if it is difficult to obtain the system time.  The NILVALUE should
   only be used when it is actually impossible to obtain time
   information.  This rule should not be used as an excuse for lazy

A.7.  Notes on the timeQuality SD-ID

   It is recommended that the value of "0" be the default for the
   "tzKnown" (Section 7.1.1) parameter.  It should only be changed to
   "1" after the administrator has specifically configured the time
   zone.  The value "1" may be used as the default if the underlying
   operating system provides accurate time zone information.  It is
   still advised that the administrator consider the correctness of the
   time zone information.

   It is important not to create a false impression of accuracy with the
   timeQuality SD-ID (Section 7.1).  An originator should only indicate
   a given accuracy if it actually knows it is within these bounds.  It
   is generally assumed that the originator gains this in-depth
   knowledge through operator configuration.  By default, an accuracy
   should not be provided.

A.8.  UTF-8 Encoding and the BOM

   This document specifies that SD-PARAMS must always be encoded in
   UTF-8.  Other encodings of the message in the MSG portion, including
   ASCIIPRINT, are not permitted by a device conforming to this
   specification.  There are two cases that need to be addressed here.
   First, a syslog application conforming to this specification may not
   be able to ascertain that the information given to it from an
   originator is encoded in UTF-8.  If it cannot determine that with
   certainty, the syslog application may choose to not incorporate the
   BOM in the MSG.  If the syslog application has a good indication that
   the content of the message is encoded in UTF-8, then it should
   include the BOM.  In the second case, a syslog relay may be

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   forwarding a message from a device that does not conform to this
   specification.  In that case, the device would likely not include the
   BOM unless it has ascertained that the received message was encoded
   in UTF-8.

Author's Address

   Rainer Gerhards
   Adiscon GmbH
   Mozartstrasse 21
   Grossrinderfeld, BW  97950

   EMail: rgerhards@adiscon.com

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