Network Working Group R. Gerhards

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.

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document authors. All rights reserved.

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than English.

Gerhards Standards Track [Page 1]

RFC 5424 The Syslog Protocol March 2009

Abstract

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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RFC 5424 The Syslog Protocol March 2009

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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RFC 5424 The Syslog Protocol March 2009

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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
message.

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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RFC 5424 The Syslog Protocol March 2009

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.

+----------+ +---------+
|Originator|---->----|Collector|
+----------+ +---------+

+----------+ +-----+ +---------+
|Originator|---->----|Relay|---->----|Collector|
+----------+ +-----+ +---------+

+----------+ +-----+ +-----+ +---------+
|Originator|-->--|Relay|-->--..-->--|Relay|-->--|Collector|
+----------+ +-----+ +-----+ +---------+

+----------+ +-----+ +---------+
|Originator|---->----|Relay|---->----|Collector|
| |-+ +-----+ +---------+
+----------+ \
\ +-----+ +---------+
+->--|Relay|---->----|Collector|
+-----+ +---------+

+----------+ +---------+
|Originator|---->----|Collector|
| |-+ +---------+
+----------+ \
\ +-----+ +---------+
+->--|Relay|---->----|Collector|
+-----+ +---------+

+----------+ +-----+ +---------+
|Originator|---->----|Relay|---->-------|Collector|
| |-+ +-----+ +---| |
+----------+ \ / +---------+
\ +-----+ /
+->--|Relay|-->--/
+-----+

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RFC 5424 The Syslog Protocol March 2009

+----------+ +-----+ +---------+
|Originator|---->----|Relay|---->--------------|Collector|
| |-+ +-----+ +--| |
+----------+ \ / +---------+
\ +------------+ /
\ |+----------+| /
+->-||Relay ||->---/
|+----------|| /
||Originator||->-/
|+----------+|
+------------+

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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RFC 5424 The Syslog Protocol March 2009

6. Syslog Message Format

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

SYSLOG-MSG = HEADER SP STRUCTURED-DATA [SP MSG]

HEADER = PRI VERSION SP TIMESTAMP SP HOSTNAME
SP APP-NAME SP PROCID SP MSGID
PRI = "<" PRIVAL ">"
PRIVAL = 1*3DIGIT ; range 0 .. 191
VERSION = NONZERO-DIGIT 0*2DIGIT
HOSTNAME = NILVALUE / 1*255PRINTUSASCII

APP-NAME = NILVALUE / 1*48PRINTUSASCII
PROCID = NILVALUE / 1*128PRINTUSASCII
MSGID = NILVALUE / 1*32PRINTUSASCII

TIMESTAMP = NILVALUE / FULL-DATE "T" FULL-TIME
FULL-DATE = DATE-FULLYEAR "-" DATE-MONTH "-" DATE-MDAY
DATE-FULLYEAR = 4DIGIT
DATE-MONTH = 2DIGIT ; 01-12
DATE-MDAY = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on
; month/year
FULL-TIME = PARTIAL-TIME TIME-OFFSET
PARTIAL-TIME = TIME-HOUR ":" TIME-MINUTE ":" TIME-SECOND
[TIME-SECFRAC]
TIME-HOUR = 2DIGIT ; 00-23
TIME-MINUTE = 2DIGIT ; 00-59
TIME-SECOND = 2DIGIT ; 00-59
TIME-SECFRAC = "." 1*6DIGIT
TIME-OFFSET = "Z" / TIME-NUMOFFSET
TIME-NUMOFFSET = ("+" / "-") TIME-HOUR ":" TIME-MINUTE

STRUCTURED-DATA = NILVALUE / 1*SD-ELEMENT
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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RFC 5424 The Syslog Protocol March 2009

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"
[ANSI.X3-4.1968].

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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RFC 5424 The Syslog Protocol March 2009

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
Code

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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RFC 5424 The Syslog Protocol March 2009

Numerical Severity
Code

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
[RFC5226].

6.2.3. TIMESTAMP

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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RFC 5424 The Syslog Protocol March 2009

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.

6.2.3.1. Examples

Example 1

1985-04-12T23:20:50.52Z

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

Example 2

1985-04-12T19:20:50.52-04:00

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

Example 3

2003-10-11T22:14:15.003Z

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

2003-08-24T05:14:15.000003-07:00

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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RFC 5424 The Syslog Protocol March 2009

Example 5 - An Invalid TIMESTAMP

2003-08-24T05:14:15.000000003-07:00

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
document.

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
follows:

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
unlikely.

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RFC 5424 The Syslog Protocol March 2009

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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RFC 5424 The Syslog Protocol March 2009

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

This field MAY be operator-assigned.

6.3. STRUCTURED-DATA

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
message.

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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RFC 5424 The Syslog Protocol March 2009

%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"
eventID="1011"]

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
examples.

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 2003-08-24T05:14:15.000003-07:00 192.0.2.1
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 "192.0.2.1", 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
class="high"]

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
source:

[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
OPTIONAL.

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 (1.3.6.1.4.1). 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
characters.

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
characters.

7.2.5. Example

The following is an example with multiple IP addresses:

[origin ip="192.0.2.1" ip="192.0.2.129"]

In this example, the originator indicates that it has two IP
addresses, one being 192.0.2.1 and the other one being 192.0.2.129.

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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
[RFC3418].

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
[RFC4646].

8. Security Considerations

8.1. UNICODE

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
unreadable.

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
machine.

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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RFC 5424 The Syslog Protocol March 2009

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
problem.

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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RFC 5424 The Syslog Protocol March 2009

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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RFC 5424 The Syslog Protocol March 2009

9. IANA Considerations

9.1. VERSION

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.

VERSION FORMAT
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
below.

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

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RFC 5424 The Syslog Protocol March 2009

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:

[email protected]

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

Chris Lonvick
Cisco Systems
EMail: [email protected]

David Harrington
Huawei Technologies USA
EMail: [email protected]

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
2009.

[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
bins.

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
representations.

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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
rules.

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
issue.

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
implementations.

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
Germany

EMail: [email protected]

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