syslog Working Group R. Gerhards
Internet-Draft Adiscon GmbH
Expires: January 13, 2006 July 12, 2005
The syslog Protocol
draft-ietf-syslog-protocol-14.txt
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Copyright (C) The Internet Society (2005).
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 spirit of traditional syslog
in mind. The reason for a new layered specification has arisen
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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.
The layered architecture also provides a solid basis that allows code
to be written once instead of multiple times, once for each syslog
feature.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Basic Principles . . . . . . . . . . . . . . . . . . . . . . . 7
4.1 Example Deployment Scenarios . . . . . . . . . . . . . . . 7
5. Transport Layer Protocol . . . . . . . . . . . . . . . . . . . 9
5.1 Minimum Required Transport Mapping . . . . . . . . . . . . 9
6. Required syslog Format . . . . . . . . . . . . . . . . . . . . 10
6.1 Message Length . . . . . . . . . . . . . . . . . . . . . . 11
6.2 HEADER . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2.1 VERSION . . . . . . . . . . . . . . . . . . . . . . . 12
6.2.2 FACILITY . . . . . . . . . . . . . . . . . . . . . . . 12
6.2.3 SEVERITY . . . . . . . . . . . . . . . . . . . . . . . 12
6.2.4 TRUNCATE . . . . . . . . . . . . . . . . . . . . . . . 13
6.2.5 TIMESTAMP . . . . . . . . . . . . . . . . . . . . . . 14
6.2.6 HOSTNAME . . . . . . . . . . . . . . . . . . . . . . . 16
6.2.7 APP-NAME . . . . . . . . . . . . . . . . . . . . . . . 17
6.2.8 PROCID . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2.9 MSGID . . . . . . . . . . . . . . . . . . . . . . . . 17
6.3 STRUCTURED-DATA . . . . . . . . . . . . . . . . . . . . . 17
6.3.1 SD-ELEMENT . . . . . . . . . . . . . . . . . . . . . . 18
6.3.2 SD-ID . . . . . . . . . . . . . . . . . . . . . . . . 18
6.3.3 SD-PARAM . . . . . . . . . . . . . . . . . . . . . . . 18
6.3.4 Change Control . . . . . . . . . . . . . . . . . . . . 19
6.3.5 Examples . . . . . . . . . . . . . . . . . . . . . . . 19
6.4 MSG . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.5 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 20
7. Structured Data IDs . . . . . . . . . . . . . . . . . . . . . 23
7.1 timeQuality . . . . . . . . . . . . . . . . . . . . . . . 23
7.1.1 tzKnown . . . . . . . . . . . . . . . . . . . . . . . 23
7.1.2 isSynced . . . . . . . . . . . . . . . . . . . . . . . 23
7.1.3 syncAccuracy . . . . . . . . . . . . . . . . . . . . . 23
7.1.4 Examples . . . . . . . . . . . . . . . . . . . . . . . 24
7.2 origin . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.2.1 ip . . . . . . . . . . . . . . . . . . . . . . . . . . 24
7.2.2 enterpriseId . . . . . . . . . . . . . . . . . . . . . 25
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7.2.3 software . . . . . . . . . . . . . . . . . . . . . . . 25
7.2.4 swVersion . . . . . . . . . . . . . . . . . . . . . . 25
7.2.5 Example . . . . . . . . . . . . . . . . . . . . . . . 25
7.3 meta . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.3.1 sequenceId . . . . . . . . . . . . . . . . . . . . . . 26
7.3.2 sysUpTime . . . . . . . . . . . . . . . . . . . . . . 26
8. Security Considerations . . . . . . . . . . . . . . . . . . . 27
8.1 Diagnostic Logging . . . . . . . . . . . . . . . . . . . . 27
8.2 Control Characters . . . . . . . . . . . . . . . . . . . . 27
8.3 More than Maximum Message Length . . . . . . . . . . . . . 28
8.4 Message Truncation . . . . . . . . . . . . . . . . . . . . 28
8.5 Replaying . . . . . . . . . . . . . . . . . . . . . . . . 28
8.6 Reliable Delivery . . . . . . . . . . . . . . . . . . . . 29
8.7 Message Integrity . . . . . . . . . . . . . . . . . . . . 29
8.8 Message Observation . . . . . . . . . . . . . . . . . . . 29
8.9 Misconfiguration . . . . . . . . . . . . . . . . . . . . . 30
8.10 Forwarding Loop . . . . . . . . . . . . . . . . . . . . . 30
8.11 Load Considerations . . . . . . . . . . . . . . . . . . . 30
8.12 Denial of Service . . . . . . . . . . . . . . . . . . . . 31
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9.1 VERSION . . . . . . . . . . . . . . . . . . . . . . . . . 32
9.2 SD-IDs . . . . . . . . . . . . . . . . . . . . . . . . . . 32
10. Authors and Working Group Chair . . . . . . . . . . . . . . 34
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 35
12. Notes to the RFC Editor . . . . . . . . . . . . . . . . . . 36
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
13.1 Normative . . . . . . . . . . . . . . . . . . . . . . . . 37
13.2 Informative . . . . . . . . . . . . . . . . . . . . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 38
A. Implementor Guidelines . . . . . . . . . . . . . . . . . . . . 39
A.1 Relationship with BSD Syslog . . . . . . . . . . . . . . . 39
A.2 Message Length . . . . . . . . . . . . . . . . . . . . . . 40
A.3 Message Truncation . . . . . . . . . . . . . . . . . . . . 41
A.4 HEADER Parsing . . . . . . . . . . . . . . . . . . . . . . 41
A.5 SEVERITY Values . . . . . . . . . . . . . . . . . . . . . 42
A.6 TIME-SECFRAC Precision . . . . . . . . . . . . . . . . . . 42
A.7 Case Convention for Names . . . . . . . . . . . . . . . . 43
A.8 Leap Seconds . . . . . . . . . . . . . . . . . . . . . . . 43
A.9 Syslog Senders Without Knowledge of Time . . . . . . . . . 43
A.10 Additional Information on PROCID . . . . . . . . . . . . . 44
A.11 Notes on the timeQuality SD-ID . . . . . . . . . . . . . . 44
A.12 Recommendation for Diagnostic Logging . . . . . . . . . . 45
Intellectual Property and Copyright Statements . . . . . . . . 47
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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
parsable, 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
not necessary for system interoperability.
This document has been written with the spirit of RFC 3164 [14] in
mind. The reason 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. The
layered architecture also provides a solid basis that allows code to
be written once instead of multiple times, once for each syslog
feature.
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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 [5].
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3. Definitions
The following definitions are used in this document:
o An application that can generate a syslog message is called a
"sender".
o An application that can receive a syslog message is called a
"receiver".
o An application that can receive syslog messages and forward them
to another receiver is called a "relay".
o An application that receives messages and does not relay them to
any other receiver is called a "collector".
A single application can have multiple roles at the same time.
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4. Basic Principles
The following principles apply to syslog communication:
o The syslog protocol does not provide for any mechanism of
acknowledgement of message delivery. Though some transports may
provide status information, conceptionally, syslog is a pure
simplex communications protocol.
o Senders send messages to receivers with no knowledge of whether
they are collectors or relays.
o Senders may be configured to send the same message to multiple
receivers.
o Relays may send all or some of the messages that they receive to a
subsequent relay or collector. They may also store or otherwise
locally process some or all messages without forwarding. In the
case where a receiver stores some messages and relays some
messages, it is acting as both a collector and a relay.
o Relays may also generate their own messages and send them on to
subsequent relays or collectors. In that case they are acting as
senders and a relay.
o Sender and receiver may reside on the same or different systems.
4.1 Example Deployment Scenarios
Sample deployment scenarios are shown in Diagram 1. Other
arrangements of these examples are also acceptable. As noted, in the
following diagram, relays may pass along all or some of the messages
that they receive and also pass along messages that they internally
generate. The boxes represent syslog-enabled applications.
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+------+ +---------+
|Sender|---->----|Collector|
+------+ +---------+
+------+ +-----+ +---------+
|Sender|---->----|Relay|---->----|Collector|
+------+ +-----+ +---------+
+------+ +-----+ +-----+ +---------+
|Sender|-->--|Relay|-->--..-->--|Relay|-->--|Collector|
+------+ +-----+ +-----+ +---------+
+------+ +-----+ +---------+
|Sender|---->----|Relay|---->----|Collector|
| |-+ +-----+ +---------+
+------+ \
\ +-----+ +---------+
+->--|Relay|---->----|Collector|
+-----+ +---------+
+------+ +---------+
|Sender|---->----|Collector|
| |-+ +---------+
+------+ \
\ +-----+ +---------+
+->--|Relay|---->----|Collector|
+-----+ +---------+
+------+ +-----+ +---------+
|Sender|---->----|Relay|---->-------|Collector|
| |-+ +-----+ +---| |
+------+ \ / +---------+
\ +-----+ /
+->--|Relay|-->--/
+-----+
+------+ +-----+ +---------+
|Sender|---->----|Relay|---->----------|Collector|
| |-+ +-----+ +--| |
+------+ \ / +---------+
\ +--------+ /
\ |+------+| /
+->-||Relay ||->---/
|+------|| /
||Sender||->-/
|+------+|
+--------+
Diagram 1. Some possible syslog deployment scenarios.
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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. This requires that syslog transports be
defined in other documents. The first transport is defined in [13]
and is consistent with the traditional UDP transport.
Any syslog transport protocol MUST NOT deliberately alter the syslog
message. If the transport protocol needs to perform temporary
transformations, these transformations MUST be reversed by the
transport protocol at the receiver, so that the upper layer will see
an exact copy of the message sent from the originator. Otherwise
cryptographic verifiers (like signatures) will be broken. Of course,
message alteration might occur due to transmission or similar errors.
Guarding against such alterations is not within the scope of this
requirement.
5.1 Minimum Required Transport Mapping
All syslog implementations MUST support a UDP-based transport as
described in [13]. This requirement ensures interoperability between
all systems implementing the protocol described in this document.
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6. Required syslog Format
The syslog message has the following ABNF [7] definition:
SYSLOG-MSG = HEADER SP STRUCTURED-DATA [SP MSG]
HEADER = VERSION SP FACILITY SP SEVERITY SP
TRUNCATE SP TIMESTAMP SP HOSTNAME
SP APP-NAME SP PROCID SP MSGID
VERSION = NONZERO-DIGIT 0*2DIGIT
FACILITY = "0" / (NONZERO-DIGIT 0*9DIGIT)
; range 0..2147483647
SEVERITY = "0" / "1" / "2" / "3" / "4" / "5" /
"6" / "7"
TRUNCATE = 1*2DIGIT
HOSTNAME = 1*255PRINTUSASCII
APP-NAME = 1*48PRINTUSASCII
PROCID = "-" / 1*128PRINTUSASCII
MSGID = "-" / 1*32PRINTUSASCII
TIMESTAMP = 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 ; 00-58, 00-59, 00-60 based on leap
; second rules
TIME-SECFRAC = "." 1*6DIGIT
TIME-OFFSET = "Z" / TIME-NUMOFFSET
TIME-NUMOFFSET = ("+" / "-") TIME-HOUR ":" TIME-MINUTE
STRUCTURED-DATA = 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*32OCTET ; VALID UTF-8 String
; except '=', SP, ']', %d34 (")
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MSG = UTF-8-STRING
UTF-8-STRING = *OCTET ; Any VALID UTF-8 String
OCTET = %d00-255
SP = %d32
PRINTUSASCII = %d33-126
NONZERO-DIGIT = "1" / "2" / "3" / "4" / "5" /
"6" / "7" / "8" / "9"
DIGIT = "0" / NONZERO-DIGIT
6.1 Message Length
A receiver MUST be able to accept messages up to and including 480
octets in length. For interoperability reasons, all receiver
implementations SHOULD be able to accept messages up to and including
2,048 octets in length.
If a receiver receives a message with a length larger than 2,048
octets, or larger than it supports, the receiver MAY discard the
message or truncate the payload.
Receivers SHOULD follow this order of preferrence when it comes to
truncation:
1) No truncation
2) Truncation by dropping SD-ELEMENTs
3) If 2) not sufficient, truncate MSG
When a receiver or initial sender truncates a message, the TRUNCATE
(Section 6.2.4) field MUST be updated. In the case of a receiver,
please note that this will break message integrity checking
mechanisms such as digital signatures. This is irrelevant, as the
truncation itself breaks the signature. So no extra harm is done by
updating the TRUNCATE field.
When the MSG part is truncated, the UTF-8 encoding MUST be kept
valid.
If the last SD-ELEMENT of a message is deleted, the STRUCTURED-DATA
field MUST be changed to "-" to indicate empty STRUCTURED-DATA.
Please note that it is possible that the MSG field is truncated
without dropping any SD-PARAMS. This is the case if a message with
an empty STRUCTURED-DATA field must be truncated.
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6.2 HEADER
The character set used in the HEADER MUST be seven-bit ASCII in an
eight-bit field as described in RFC 2234 [7]. These are the ASCII
codes as defined in "USA Standard Code for Information Interchange"
ANSI.X3-4.1968 [1].
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 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 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 RFC 2434 [9].
6.2.2 FACILITY
FACILITY is an integer in the range from 0 to 2147483647. It can be
used for filtering by the receiver. It is a category, allowing a
coarse grouping of messages. There exist some traditional FACILITY
code semantics for the codes in the range from 0 to 23. These
semantics are not closely followed by all senders, and practice has
shown that common semantics for message categories are hard to
establish. Therefore, no specific semantics for FACILITY codes are
specified or implied in this document.
There is no relationship between MSGID (Section 6.2.9) and FACILITY,
because MSGID identifies a specific message whereas FACILITY
specifies a coarse message group and is expected to be operator
assigned most-often.
6.2.3 SEVERITY
The SEVERITY field is used to indicate the severity that the sender
of a message assigned to it. It contains one of the values shown in
table 1 below.
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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 conditions
6 Informational: informational messages
7 Debug: debug-level messages
Table 1. SEVERITY Values.
6.2.3.1 Relation to Alarm MIB
The Alarm MIB RFC3877 [11] defines ITU perceived severities which are
useful to be able to relate to the syslog severities, particularly in
the case where alarms are being logged. The ITUPerceivedSeverity
corresponds to a syslog SEVERITY as shown in table 2 below.
ITU Perceived Severity syslog SEVERITY
Critical Alert
Major Critical
Minor Error
Warning Warning
Indeterminate Notice
Cleared Notice
Table 2. ITUPerceivedSeverity to syslog SEVERITY mapping.
6.2.4 TRUNCATE
The TRUNCATE field is used to indicate if the message has been
truncated since it was sent or generated by an application. Such a
truncation might happen on the initial sender and any receiver,
including receivers on interim systems (relays). Values in the
TRUNCATE field are made up of bits. Each of this bits has been
assigned a specific value so that there is no doubt about bit
ordering. The values described in table 3 below MUST be used.
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VALUE Meaning
1 all or some SD-ELEMENTs were truncated
2 all or part of MSG was truncated
4 truncation occurred at the receiver
8 truncation occurred at an interim system
16 truncation occurred at the initial sender
Table 3. TRUNCATE values.
The value in the TRUNCATE field is the ASCII representation of these
ORed bits. If the initial sender truncates a message, this MUST be
inidicated by setting the "truncation occured at the initial sender"
bit (value 16). If the truncation occurs while receiving the
message, the "truncation occured at the receiver" (value 4) bit MUST
be set. If the receiver forwards the message to another system, the
value of 4 MUST be changed to "truncation occured at an interim
system" (value 8).
Truncation on the initial sender sounds illogical, but may happen.
On many systems, a syslog library or subsystem is responsible for
actually sending the syslog messages. This library or subsystem is
passed the message to send from another application. That
application might ask the library or subsystem to send a message that
is larger than is supported. One alternative is that a failure
status is passed back to the application and the message is not send.
In other cases, it might be advisable to send the message, in which
case it must be truncated directly at the initial sender. As the
information about this might be helpful for the receiver (e.g.
signatures might be valid, which is not the case in other
truncations), there is a bit that can be used to reflect this
information.
Some examples: If no truncation occured, TRUNCATE MUST have a value
of 0. If SD-ELEMENTs were truncated on the receiver, TRUNCATE MUST
have a value of 5. If they were truncated on the initial sender,
TRUNCATE MUST have the value of 17. If structured data and MSG were
truncated on an interim system, TRUNCATE MUST have the value 11. If
only MSG was truncated on the initial sender, TRUNCATE MUST have the
value 18. If MSG and structured data were truncated on the sender,
an interim system and the receiver, TRUNCATE MUST have the value 31.
Please see Message Length (Section 6.1) for details on truncation.
The TRUNCATE field does not specify how much of the STRUCTURED-DATA
or MSG was truncated. It just indicates that truncation occurred.
6.2.5 TIMESTAMP
The TIMESTAMP field is a formalized timestamp derived from RFC 3339
[8].
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Whereas RFC 3339 [8] makes allowances for multiple syntaxes, this
document imposes further restrictions. The TIMESTAMP 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.
The sender SHOULD include TIME-SECFRAC if its clock accuracy and
performance permit. The "timeQuality" SD-ID described in Section 7.1
allows one to specify accuracy and trustworthiness of the timestamp.
6.2.5.1 Syslog Senders Without Knowledge of Time
A syslog sender incapable of obtaining system time MUST use the
following TIMESTAMP:
2000-01-01T00:00:60Z
This TIMESTAMP is in the past and it shows a time that never existed,
because 1 January 2000 had no leap second. So it can never occur in
a valid syslog message of a time-aware sender. A receiver receiving
this TIMESTAMP MUST treat this value as an undefined date and time.
6.2.5.2 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 the
Eastern US time zone (daylight savings time being observed).
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 by providing just three digits for the fractional
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part of a second, it 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.
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.
6.2.6 HOSTNAME
The HOSTNAME field identifies the machine that originally sent the
syslog message.
The HOSTNAME field SHOULD contain the host name and the domain name
of the originator in the format specified in STD 13 [3]. This format
is called a Fully Qualified Domain Name (FQDN) in this document.
In practice, not all senders are able to provide a FQDN. As such,
other values MAY also be present in HOSTNAME. This protocol makes
provisions for using other values in such situations. A sender
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. "0:0:0:0:0:0:0:0"
If an IPv4 address is used, it MUST be in the format of the dotted
decimal notation as used in STD 13 [4]. If an IPv6 address is used,
a valid textual representation described in RFC 3513 [10], Section
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2.2, MUST be used.
Senders SHOULD consistently use the same value in the HOSTNAME field
for as long as possible. If the sender is multihomed, this value
SHOULD be one of its actual IP addresses. If a sender is running on
a machine that has both statically and dynamically assigned
addresses, then that value SHOULD be from the statically assigned
addresses. As an alternative, the sender MAY use the IP address of
the interface that is used to send the message.
6.2.7 APP-NAME
The APP-NAME field SHOULD identify the device or application that
generated the message. It is a string without further semantics. It
is intended for filtering messages on the receiver.
6.2.8 PROCID
The PROCID field SHOULD be used to provide the sender's process name
or process ID. The field does not have any specific syntax. The
dash ("-") is a reserved PROCID field value that SHOULD be used only
to indicate that the PROCID is not provided.
PROCID is primarily meaningful for analysis tools. Properly used, it
might enable log analyzers to detect which messages were generated by
the same sender process. For example, on a UNIX system the syslog
daemon (syslogd) might emit messages to the log. All messages logged
by the same syslogd process will bear the same PROCID. When the
syslogd is restarted, the PROCID value MAY change. That enables the
analysis script to detect the syslogd restart.
6.2.9 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 the receiver.
The dash ("-") is a reserved MSGID field value that MUST be used only
to indicate that the message has no specific ID.
6.3 STRUCTURED-DATA
STRUCTURED-DATA transports data in a well defined, easily parsable
and interpretable format. There are multiple usage scenarios. For
example, it may transport meta-information about the syslog message
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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
value dash ("-") MUST be used.
The character set used in STRUCTURED-DATA MUST be UNICODE, encoded
using UTF-8 as specified in RFC 3629 [6]. A sender MAY issue any
valid UTF-8 sequence. A receiver MUST accept any valid UTF-8
sequence. It MUST NOT fail if control characters are present in the
STRUCTURED-DATA part.
If STRUCTURED-DATA is malformed, a diagnostic entry SHOULD be logged.
A receiver MAY ignore malformed STRUCTURED-DATA elements.
6.3.1 SD-ELEMENT
A 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. IANA controls all SD-IDs. SD-IDs with the
prefatory string "x-" are set aside for experimental or vendor-
specific use. There is no standardization for SD-IDs with the
prefatory string "x-". The same SD-ID MUST NOT exist more than once
in a message.
6.3.3 SD-PARAM
Each SD-PARAM consist 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 experimental SD-IDs or those with the
prefatory string "x-" (without the quotes). PARAM-NAMEs with the
prefatory string x- are set aside for experimental purposes. They
can be specified in any SD-ID. The PARAM-NAME scope is within a
specific SD-ID. Thus, an equally-named PARAM-NAME contained in two
different SD-IDs is not the same.
Inside PARAM-VALUE, the characters '"', '\' and ']' MUST be escaped.
This is necessary to avoid parsing errors. Escaping ']' would not
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strictly be necessary but is REQUIRED by this specification to avoid
parser implementation errors. Each of these three characters MUST be
escaped as '\"', '\\' and '\]' respectively.
A backslash ('\') followed by none of the three described characters
is considered an invalid escape sequence. Upon reception of such an
invalid escape sequence, the receiver MAY replace the two-character
sequence with only the second character received. Alternatively, it
MAY drop the message. It is RECOMMENDED that the receiver log a
diagnostic message on receipt of a message with an invalid escape
sequence.
A SD-PARAM MAY be repeated multiple times inside a 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.
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 for readability purposes only. A
description is given after each example.
Example 1 - Valid
[x-exampleSDID iut="3" eventSource="Application"
eventID="1011"]
This example is a structured data element with an experimental SD-ID
of type "x-exampleSDID" which has three parameters.
Example 2 - Valid
[x-exampleSDID iut="3" eventSource="Application"
eventID="1011"][x-examplePriority 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).
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Example 3 - Invalid
[x-exampleSDID iut="3" eventSource="Application"
eventID="1011"] [x-examplePriority class="high"]
This is nearly the same example as 2, but it has a subtle error.
Please note that there is a 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
[ x-exampleSDID iut="3" eventSource="Application"
eventID="1011"][x-examplePriority class="high"]
This example again is nearly the same as 2. It has another subtle
error. Please note the SP character after the initial bracket. A
structured data element SD-ID MUST immediately follow the beginning
bracket, so the SP character invalidates the STRUCTURED-DATA. Thus,
the receiver 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. Please note that the ellipses denote missing content, which
has been left out 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 MUST be UNICODE, encoded using UTF-8 as
specified in RFC 3629 [6]. A sender MAY issue any valid UTF-8
sequence. A receiver MUST accept any valid UTF-8 sequence. It MUST
NOT fail if control characters are present in the MSG part.
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 RFC 3164 [14] and may be familiar to readers.
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Example 1
1 888 4 0 2003-10-11T22:14:15.003Z mymachine.example.com
su - ID47 - 'su root' failed for lonvick on /dev/pts/8
In this example, the VERSION is 1 and the FACILITY has the value of
888. The severity is 4 ("Warning" semantics). The message was not
truncated (0). It 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". There is no
STRUCTURED-DATA present in the message, this is indicated by "-" in
the STRUCTURED-DATA field. The MSG is "'su root' failed for
lonvick...".
Example 2
1 20 6 0 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 within the
legacy syslog range (20). The severity is 6 ("Notice" semantics).
It 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 sender 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
specific MSGID and this is indicated by the "-" in the MSGID field.
The message is "%% It's time to make the do-nuts.".
Example 3 - with STRUCTURED-DATA
1 888 4 0 2003-10-11T22:14:15.003Z mymachine.example.com
evntslog - ID47 [x-exampleSDID iut="3" eventSource="Application"
eventID="1011"] An application event log entry...
This example is modeled after example 1. However, this time it
contains STRUCTURED-DATA, a single element with the value
"[x-exampleSDID iut="3" eventSource="Application" eventID="1011"]".
The MSG itself is "An application event log entry..."
Example 4 - STRUCTURED-DATA Only
1 888 4 0 2003-10-11T22:14:15.003Z mymachine.example.com
evntslog - ID47 [x-exampleSDID iut="3" eventSource="Application"
eventID="1011"][x-examplePriority class="high"]
This example shows a message with only STRUCTURED-DATA and no MSG
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part. This is a valid message.
Example 5 - with truncated STRUCTURED-DATA
1 888 4 5 2003-10-11T22:14:15.003Z mymachine.example.com
evntslog - ID47 - An application event log entry...
This example is modeled after example 1. However, this time it
originally contained STRUCTURED-DATA, which has been truncated by a
receiver. This is indicated by the TRUNCATE field, which is now set
to "5".
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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
are OPTIONAL.
7.1 timeQuality
The SD-ID "timeQuality" MAY be used by the original sender to
describe its notion of system time. This SD-ID SHOULD be written if
the sender is not properly synchronized with a reliable external time
source or if it does not know whether or not its time zone
information is 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 6.2.5. All parameters are OPTIONAL.
7.1.1 tzKnown
The "tzKnown" parameter indicates whether the original sender knows
its time zone. If it does so, the value "1" MUST be used. If the
time zone information is in doubt, the value "0" MUST be used. If
the sender 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 original sender is
synchronized to a reliable external time source, e.g., via NTP. If
the original sender 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 original
sender 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 receiver MUST assume that the
time information provided is accurate enough to be considered
correct. The "syncAccuracy" parameter MUST be written only if the
original sender actually has knowledge of the reliability of the
external time source. In practice, in most cases, it will gain this
in-depth knowledge through operator configuration.
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7.1.4 Examples
The following is an example of a system that knows that it knows
neither its time zone nor whether it is being synchronized:
[timeQuality tzKnown="0" isSynced="0"]
With this information, the sender indicates that its time information
is unreliable. This may be a hint for the receiver to use its local
time instead of the message-provided TIMESTAMP for correlation of
multiple messages from different senders.
The following is an example of a system 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 a system 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
sender expects that its clock will be kept within 60 seconds of the
official time. So if the sender 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 sender knows it had
at the time of sending the message. It MUST contain the textual
representation of an IP address as outlined in Section 6.2.6.
This parameter can be used to provide additional identifying
information 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.
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If a sender 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 is unique and may be registered by an on-line form at
<http://www.iana.org/>. Only that number and any-enterprise assigned
ID below it MUST be specified in the "enterpriseId" parameter. If
sub-identifiers are used, they MUST be separated by periods and be
represented as decimal numbers ("9.1.30" and "11.2.3.7.5.12"). The
complete up-to-date list of Enterprise Numbers is maintained by IANA
at <http://www.iana.org/assignments/enterprise-numbers>.
By specifying an enterpriseId, the vendor allows more specific
parsing 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 always contains 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"]
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In this example, the sender indicates that it has two ip addresses,
one being 192.0.2.1 and the other one being 192.0.2.129.
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 allows to track the sequence in which the
sender sent the messages. 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
RFC 3418 [12].
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".
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8. Security Considerations
8.1 Diagnostic Logging
This document recommends that an implementation writes a diagnostic
message to indicate unusual situations or other things noteworthy.
Diagnostic messages are a useful tool in discovering configuration
issues as well as instances of system penetration.
Unfortunately, diagnostic logging can cause issues by itself, for
example, if an attacker tries to create a denial of service condition
by willingly sending malformed messages that will lead to the
creation of diagnostic log entries. Due to sheer volume, the
resulting diagnostic log entries may exhaust system resources, e.g.
processing power, I/O capability, or simply storage space. For
example, an attacker could flood a system with messages generating
diagnostic log entries after he has compromised a system. If the log
entries are stored in a circular buffer, the flood of diagnostic log
entries would eventually overwrite useful previous diagnostics.
Besides this risk, too verbose diagnostic logging can cause the
administrator to turn logging off.
8.2 Control Characters
This document does not impose any restrictions on the MSG or
STRUCTURED-DATA content. As such, they MAY contain control
characters, including the NUL character.
In some programming languages (most notably C and C++), the NUL
(0x00) character traditionally has a special significance as string
terminator. Most, if not all, 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.
Please note that 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 unencoded, the file can potentially
become unreadable.
The same is true for other control characters. For example, an
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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.
8.3 More than Maximum Message Length
The message length MAY exceed the RECOMMENDED maximum value specified
in Section 6. Various problems may result if a sender sends messages
with a greater length. Also, an attacker might deliberately
introduce very large messages. As such, it is vital that each
receiver performs the necessary sanity checks to ensure that it will
gracefully discard or truncate messages of larger sizes than it
supports.
8.4 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 receiver or interim systems. As such,
vital log information may be lost.
In order to prevent information loss, messages should not be longer
then the 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 receiver.
A sender 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.
8.5 Replaying
Messages may be recorded and replayed at a later time. 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 collector. Even
with a 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
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TIMESTAMPs and thus the replay attack.
8.6 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 either not be noticed, or it 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.
It may be desirable to use a transport with guaranteed delivery, to
mitigate congestion.
It may also be desirable to include rate-limiting features in syslog
senders. This can reduce potential congestion problems when message
bursts happen.
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. Additionally, if an attacker is positioned between the
sender and collector 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. Neither the syslog protocol nor the
syslog application 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 off
of 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
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those messages to compromise a machine or do other damage.
8.9 Misconfiguration
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 senders were
inadvertently configured to send syslog messages to the wrong
receivers. In many cases, the inadvertent receivers may not be
configured to receive syslog messages and it 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.
Using a reliable transport mapping can help identify these problems.
8.10 Forwarding Loop
As shown in Diagram 1, 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 receivers. 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 that has the consequence of not ensuring that an administrator
will be able to review the records in the future. Along this line, a
receiver or collector must have a network interface capable of
receiving all messages sent to it.
Administrators and network planners must also critically review the
network paths between the devices, 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
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guaranteed delivery is recommended.
8.12 Denial of Service
As with any system, an attacker may just overwhelm a receiver by
sending more messages to it than can be handled by the infrastructure
or the device itself. Implementors 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
9.1 VERSION
IANA must maintain a registry of VERSION values as described in
Section 6.2.1. 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 RFC 2434 [9]. IANA must register the VERSIONs shown in
table 4 below.
VERSION FORMAT
1 according to this document
Table 4. IANA-registered VERSIONs.
9.2 SD-IDs
IANA must maintain a registry 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
Standards Action method as described in RFC 2434 [9].
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.
SD-IDs and PARAM-NAMEs with the prefatory string "x-" (without the
quotes) are reserved for experimental use. IANA must not register
any SD-ID or PARAM-NAME with the prefatory string "x-" for any other
use.
IANA must register the SD-IDs and PARAM-NAMEs shown in table 5 below.
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SD-ID PARAM-NAME
timeQuality OPTIONAL
tzKnown OPTIONAL
isSynced OPTIONAL
syncAccuracy OPTIONAL
origin OPTIONAL
ip OPTIONAL
enterpriseId OPTIONAL
software OPTIONAL
swVersion OPTIONAL
meta OPTIONAL
sequenceId OPTIONAL
sysUpTime OPTIONAL
x- (prefatory RESERVED for experimental SD-IDs
string)
(any) x- (prefatory reserved for experimental
string) PARAM-NAMEs
Table 5. IANA-registered SD-IDs and their PARAM-NAMEs.
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10. Authors and Working Group Chair
The working group can be contacted via the mailing list:
syslog-sec@employees.org
The current Chair of the Working Group may be contacted at:
Chris Lonvick
Cisco Systems
Email: clonvick@cisco.com
The author of this draft is:
Rainer Gerhards
Email: rgerhards@adiscon.com
Phone: +49-9349-92880
Fax: +49-9349-928820
Adiscon GmbH
Mozartstrasse 21
97950 Grossrinderfeld
Germany
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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 other people who
commented on various versions of this proposal.
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12. Notes to the RFC Editor
This is a note to the RFC editor. This ID is submitted along with ID
draft-ietf-syslog-transport-udp and they cross-reference each other.
When RFC numbers are determined for each of these IDs, replace XXXX
with RFC number and remove this note.
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13. References
13.1 Normative
[1] American National Standards Institute, "USA Code for
Information Interchange", ANSI X3.4, 1968.
[2] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[3] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[4] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
STD 63, RFC 3629, November 2003.
[7] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[8] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[9] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[10] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[11] Chisholm, S. and D. Romascanu, "Alarm Management Information
Base (MIB)", RFC 3877, September 2004.
[12] Presuhn, R., "Management Information Base (MIB) for the Simple
Network Management Protocol (SNMP)", STD 62, RFC 3418,
December 2002.
[13] Okmianski, A., "Transmission of syslog messages over UDP",
RFC XXXX, August 2004.
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13.2 Informative
[14] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001.
[15] Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996.
Author's Address
Rainer Gerhards
Adiscon GmbH
Mozartstrasse 21
Grossrinderfeld, BW 97950
Germany
Email: rgerhards@adiscon.com
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Appendix A. Implementor Guidelines
Information in this section is given as an aid to implementors.
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. In RFC 3164 [14] observed formats were
specified. However, RFC 3164 is an informal document, and practice
shows that there are many different implementations.
Consequently, RFC 3164 mandates 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. However, in almost all cases observed in practice, a BSD
syslog message starts with a priority value, which is a number
between brackets. An example is "<133>". This document uses that
known convention to provide some minimal version detection. It has
deliberately changed the syslog message header so that it will never
contain a less-than sign as the first character of the message. This
has two advantages:
If an older receiver receives a message that does not start with a
less-than sign, it still assumes this is a valid syslog message.
However, it does not try to parse any header fields, at least if it
obeys to the rule outlined in RFC 3164. This prevents the receiver
from parsing the message invalidly. It should be noted, however,
that at least some of the older implementations will experience
problems if the message received is larger than 1024 octets. Most of
the implementations will truncate a message after the first 1024
octets. So it is wise not to send messages larger than 1024 octets
to receivers known to be older.
If a receiver compliant with this document receives a message
generated by a non-compliant, older sender, it notices that the
message does not have a proper header and thus is not formatted
according to this document. This enables the receiver to take
appropriate action. Please also see the description on header
parsing in Appendix A.4 for more information on this scenario.
RFC 3164 mandates UDP as transport protocol for syslog. This
document places no restrictions on the transport.
RFC 3164 specifies relay behavior. This document does not specify
relay behavior. This might be done in a separate document.
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The PRI part in RFC 3164 is split into two fields -- FACILITY and
SEVERITY -- in this document. These new fields support the RFC 3164
values but also allow additional values.
The TIMESTAMP in RFC 3164 offers less precision and lacks the year
and timezone information. If a message formatted according to this
document needs to be reformatted to be RFC 3164 compliant, it is
suggested that the sender's local time zone be used, and the time
zone information and the year be dropped. If a RFC 3164 formatted
message is received and must be transformed to be compliant to this
document, the current year should be added and the receiver's time
zone be assumed.
The HOSTNAME in RFC 3164 is less specific, but this format is still
supported in this document as one of the alternate HOSTNAME
representations.
The MSG part of the message is defined 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 defined. If a message compliant
with this document contains STRUCTURED-DATA and must be reformatted
to be compliant with 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 parsable header
with clear field separations whereas traditional BSD syslog suffers
from some historically developed, hard to parse field separation
rules.
A.2 Message Length
Implementors should note the message size limitations outlined in
Section 6.1 and try to keep the most important parts early in the
message (within the minimum guaranteed length). This ensures they
will be seen by the receiver even if it (or a relay on the message
path) truncates the message.
The reason syslog receivers must 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 and have this 480 restriction to avoid session
overhead and message fragmentation. In a network being
troubleshooted, the likelihood of getting one single-packet message
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delivered successfully is higher than getting two message fragments
delivered successfully. So using a larger size may prevent the
operator from getting some critical information about the problem,
whereas keeping within that limit might get that information to the
operator. As such, 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 then 480 octets.
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 senders and receivers can
be implemented with any size needed and still be compliant with this
document. In such cases, it is the operator's responsibility to
ensure that all components in a syslog infrastructure support the
required message sizes. Transport mappings may recommend specific
message size limits that must be enforced.
Implementors 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 environment only might thus
assume this as a larger minimum size.
A.3 Message Truncation
As outlined in Section 6.1, messages may be subject to truncation.
In this case, the TRUNCATE field must be updated as specified in
Section 6.2.4. Implementors must keep in mind that the TRUNCATE
field is a variable-sized entity. Thus, its size requirement (in
octets) may grow during truncation. For example, if a relay
truncates the MSG part of a message previously not truncated, the
TRUNCATE field value changes from 0 to 10, taking up one additional
octet of space. As such, it is necessary to truncate one additional
octet from the MSG to make room for the now-expanded HEADER.
Similar reformatting may be necessary if a relay is operator-
instructed to perform header modifications, e.g. to change the
FACILITY or SEVERITY of a message before forwarding it.
A.4 HEADER Parsing
This section recommends a message header parsing method based on the
VERSION field described in Section 6.2.1.
The receiver should check the VERSION. If the VERSION is within the
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set of versions supported by the receiver, it should parse the
message according to the correct syslog protocol specification.
If the receiver does not support the specified VERSION, it should log
a diagnostic message. It should not parse beyond the VERSION field.
This is because the header format may have changed in a newer
version. The receiver should not try to process the message, but it
may try this if the administrator has configured the receiver to do
so. In the latter case, the results may be undefined. If the
administrator has configured the receiver to parse a non-supported
version, it should assume that these messages are legacy syslog
messages and parse and process them with respect to RFC 3164 [14].
To be precise, a receiver receiving an unknown VERSION number, or a
message without a valid VERSION, should discard the message by
default. However, the administrator may configure it to not discard
these messages. If that happens, the receiver may parse it according
to RFC 3164 [14]. The administrator may again override this setting
and configure the receiver to parse the messages in any way.
The spirit behind these guidelines is that the administrator may
sometimes need the power to allow overriding of version-specific
parsing, but this should be done in the most secure and reliable way.
Therefore, the receiver should use the appropriate defaults specified
above. This document is specific on this point because it is common
experience that parsing unknown formats often leads to security
issues.
A.5 SEVERITY Values
This section describes guidelines for using SEVERITY as outlined in
Section 6.2.3.
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 receiver should not assume
that all senders have the same definition of severity.
A.6 TIME-SECFRAC Precision
The TIMESTAMP described in Section 6.2.5 supports fractional seconds.
This provides ground for a very common coding error, where leading
zeros are removed from the fractional seconds. For example, the
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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.7 Case Convention for Names
Names are used at various places in this document, for example for
SD-IDs and PARAM-NAMEs. This document uses "camel case"
consistently. With that, each name begins with a lower case letter
and each new word starts with an upper case letter, but 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.
There is one exception from this convention in this document itself.
That is that experimental SD-IDs start with "x-", so they are
hyphenated. While this looks like an inconsistency, it was done
because it is common practice (e.g., in SMTP) to prefix experimental
headers with "x-".
A.8 Leap Seconds
The TIMESTAMP described in Section 6.2.5 permits leap seconds, as
described in RFC 3339 [8].
The value "60" in the TIME-SECOND field is used to indicate a leap
second. This must not be misinterpreted. Implementors are advised
to replace the value "60" if seen in the header, with the value "59"
if it otherwise can not be processed, e.g., stored in a database. It
should not be converted to the first second of the next minute.
Please note that such a conversion, if done on the message text
itself, will cause cryptographic signatures to become invalid. As
such, it is suggested that the adjustment is not performed when the
plain message text is to be stored (e.g., for later verification of
signatures).
A.9 Syslog Senders Without Knowledge of Time
In Section 6.2.5.1, a specific TIMESTAMP for usage by senders without
knowledge of time is defined. This is done to support a special case
when a sender is not aware of time at all. It can be argued whether
such a sender 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.
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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, but doable, to obtain the system time. The
TIMESTAMP described in Section 6.2.5.1 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.
If a receiver receives that special TIMESTAMP, it should know that
the sender has no idea of what the time actually is and act
accordingly.
A.10 Additional Information on PROCID
The objective behind PROCID (Section 6.2.8) is to provide a quick way
to detect a new instance of the sender's syslog process. It must be
noted that this is not a reliable identification as a second sender
process may actually be assigned the same process ID as a previous
one. Properly used, PROCID can be helpful for analysis purposes.
While PROCID is defined to contain the sender's process ID, it is up
to the sender to decide what this ID is. For example, on a general
purpose OS, it might actually be the operating system process ID of
the syslog sender's process. Other syslog senders might decide that
it is more appropriate to put an internal identification into PROCID.
For example, a SMTP MTA might not put the operating system process ID
into PROCID but might prefer to put its SMTP transaction ID into
PROCID. This might be very useful, because it allows the receiver to
group messages based on the SMTP transaction, which could also be
called the SMTP "process" in this case. On an embedded system
without any operating system process ID, PROCID might actually be a
reboot ID, which might be the closest thing to a process ID on this
hypothetical embedded system.
A.11 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 explicitly acknowledge 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). A sender should only indicate a
given accuracy if it actually knows it is within these bounds. It is
generally assumed that the sender gains this in-depth knowledge
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through operator configuration. As such, by default, an accuracy
should not be provided.
A.12 Recommendation for Diagnostic Logging
In Section 8.1, this document describes the need for as well as
potential problems with diagnostic logging. In this section, a real-
world approach to useful diagnostic logging is recommended.
While this document recommends writing meaningful diagnostic logs, it
also recommends allowing an operator to limit the amount of
diagnostic logging. At least, an implementation should differentiate
between critical, informational, and debugging or diagnostic message.
Critical messages should be issued only in real critical states,
e.g., expected or occurring malfunction of the application or parts
of it. A strong indication of an ongoing attack may also be
considered critical. As a guideline, there should be very few
critical messages. Informational messages should be used to indicate
that all conditions are not fully correct, but still within the
bounds of normal processing. A diagnostic message logging the fact
that a malformed message has been received is a good example of this
category. A debug diagnostic message should not be needed during
normal operation, but merely as a tool for setting up or testing a
system (which includes the process of an operator configuring
multiple syslog applications in a complex environment). An
application may decide not to provide any debugging diagnostic
messages.
An administrator should be able to configure the level for which
diagnostic messages will be written. Non-configured diagnostic
messages should not be written but discarded. An implementor may
create as many different levels of diagnostic messages as useful -
the above recommendation is just based on real-world experience of
what is considered useful. Please note that experience shows that
too many levels of diagnostics typically do no good, because the
typical administrator may no longer be able to understand what each
level means.
Even with this categorization, a single diagnostic (or a set of them)
may frequently be generated when a specific condition exists (or a
system is being attacked). It will lead to the security issues
outlined at the beginning of Section 8.1. To solve this, it is
recommended that an implementation be allowed to set a limit of how
many duplicate diagnostic messages will be generated within a limited
amount of time. For example, an administrator should be able to
configure that groups of 50 identical messages are logged within a
specified time period with only a single diagnostic message. All
subsequent identical messages will be discarded until the next time
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interval. It is usually considered good form to generate a
subsequent message identifying the number of duplicate messages that
were discarded. While this causes some information loss, it is
considered a good compromise between avoiding overruns and providing
the most in-depth diagnostic information. An implementation offering
this feature should allow the administrator to configure the number
of duplicate messages as well as the time interval to whatever the
administrator thinks is reasonable. It is up to the implementor what
the term "duplicate" means. Some may decide that only totally
identical (in octet-to-octet comparison) messages are actually
duplicates, whereas others may say that a message that is of
identical type but with just some changed parameter (e.g., changed
remote host address) is also considered to be a duplicate. Both
approaches have their advantages and disadvantages. Probably, it is
best to also leave this configurable and allow the administrator to
set the parameters.
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