syslog Working Group R. Gerhards
Internet-Draft Adiscon GmbH
Expires: March 25, 2005 September 24, 2004
The syslog Protocol
draft-ietf-syslog-protocol-06.txt
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Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document describes the syslog protocol which is used to convey
event notification messages. It describes a layered architecture for
an easily extensible syslog protocol. It also describes the basic
message format and structured elements used to provide
meta-information about the message.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Example Deployment Scenarios . . . . . . . . . . . . . . . 6
4. Transport Layer Protocol . . . . . . . . . . . . . . . . . . . 8
4.1 Minimum Required Transport Mapping . . . . . . . . . . . . 8
5. Required syslog Format . . . . . . . . . . . . . . . . . . . . 9
5.1 Message Length . . . . . . . . . . . . . . . . . . . . . . 10
5.2 HEADER . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.1 VERSION . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.2 FACILITY . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.3 SEVERITY . . . . . . . . . . . . . . . . . . . . . . . 11
5.2.4 TIMESTAMP . . . . . . . . . . . . . . . . . . . . . . 11
5.2.5 HOSTNAME . . . . . . . . . . . . . . . . . . . . . . . 13
5.2.6 SENDER-NAME . . . . . . . . . . . . . . . . . . . . . 13
5.2.7 SENDER-INST . . . . . . . . . . . . . . . . . . . . . 14
5.3 STRUCTURED-DATA . . . . . . . . . . . . . . . . . . . . . 14
5.3.1 STR-DATA-ELT . . . . . . . . . . . . . . . . . . . . . 14
5.3.2 Examples . . . . . . . . . . . . . . . . . . . . . . . 15
5.4 MSG . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.5 Examples . . . . . . . . . . . . . . . . . . . . . . . . . 16
6. Structured Data IDs . . . . . . . . . . . . . . . . . . . . . 19
6.1 time . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.1.1 tzknown . . . . . . . . . . . . . . . . . . . . . . . 19
6.1.2 issynced . . . . . . . . . . . . . . . . . . . . . . . 19
6.1.3 syncaccuracy . . . . . . . . . . . . . . . . . . . . . 19
6.1.4 Examples . . . . . . . . . . . . . . . . . . . . . . . 20
6.2 origin . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.2.1 ip . . . . . . . . . . . . . . . . . . . . . . . . . . 20
6.2.2 enterpriseID . . . . . . . . . . . . . . . . . . . . . 21
6.2.3 software . . . . . . . . . . . . . . . . . . . . . . . 21
6.2.4 sw-version . . . . . . . . . . . . . . . . . . . . . . 21
6.2.5 Example . . . . . . . . . . . . . . . . . . . . . . . 21
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7.1 Diagnostic Logging . . . . . . . . . . . . . . . . . . . . 23
7.2 Control Characters . . . . . . . . . . . . . . . . . . . . 23
7.3 More than Maximum Message Length . . . . . . . . . . . . . 24
7.4 Message Length . . . . . . . . . . . . . . . . . . . . . . 24
7.5 Message Truncation . . . . . . . . . . . . . . . . . . . . 24
7.6 Single Source to a Destination . . . . . . . . . . . . . . 25
7.7 Multiple Sources to a Destination . . . . . . . . . . . . 25
7.8 Multiple Sources to Multiple Destinations . . . . . . . . 25
7.9 Replaying . . . . . . . . . . . . . . . . . . . . . . . . 26
7.10 Reliable Delivery . . . . . . . . . . . . . . . . . . . . 26
7.11 Message Integrity . . . . . . . . . . . . . . . . . . . . 26
7.12 Message Observation . . . . . . . . . . . . . . . . . . . 27
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7.13 Misconfiguration . . . . . . . . . . . . . . . . . . . . . 27
7.14 Forwarding Loop . . . . . . . . . . . . . . . . . . . . . 27
7.15 Load Considerations . . . . . . . . . . . . . . . . . . . 28
7.16 Denial of Service . . . . . . . . . . . . . . . . . . . . 28
7.17 Covert Channels . . . . . . . . . . . . . . . . . . . . . 28
8. Notice to RFC Editor . . . . . . . . . . . . . . . . . . . . . 29
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
9.1 Version . . . . . . . . . . . . . . . . . . . . . . . . . 30
9.2 SD-IDs . . . . . . . . . . . . . . . . . . . . . . . . . . 30
10. Authors and Working Group Chair . . . . . . . . . . . . . . 31
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 32
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
12.1 Normative . . . . . . . . . . . . . . . . . . . . . . . . . 33
12.2 Informative . . . . . . . . . . . . . . . . . . . . . . . . 33
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 34
A. Implementor Guidelines . . . . . . . . . . . . . . . . . . . . 35
A.1 Message Length . . . . . . . . . . . . . . . . . . . . . . 35
A.2 HEADER Parsing . . . . . . . . . . . . . . . . . . . . . . 35
A.3 SEVERITY Values . . . . . . . . . . . . . . . . . . . . . 36
A.4 time-secfrac Precision . . . . . . . . . . . . . . . . . . 36
A.5 Leap Seconds . . . . . . . . . . . . . . . . . . . . . . . 36
A.6 Syslog Senders Without Knowledge of Time . . . . . . . . . 36
A.7 Additional Information on SENDER-INST . . . . . . . . . . 37
A.8 Notes on the time SD-ID . . . . . . . . . . . . . . . . . 37
A.9 Recommendation for Diagnostic Logging . . . . . . . . . . 37
Intellectual Property and Copyright Statements . . . . . . . . 40
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1. Introduction
This document describes a layered architecture for syslog. The goal
of this architecture is to separate functionality into different
layers and thus provide easy extensibility.
This document describes the semantics of the syslog protocol,
outlines the concept of transport mappings and provides a standard
format for all syslog messages. It also describes structured data
elements, which can be used to transmit easy parsable, structured
information.
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2. Conventions Used in This Document
The keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
and "MAY" that appear in this document are to be interpreted as
described in RFC2119 [6].
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3. Definitions
The following definitions will be used in this document:
o An application that can generate a message will be called a
"sender".
o An application that can receive a message will be called a
"receiver".
o An application that can receive the message and forward it to
another receiver will be called a "relay".
o An application that receives the message and does not relay it to
any other receivers will be called a "collector".
Please note that a single application can have multiple roles at the
same time.
The following principles apply to syslog communication:
o Senders send messages blindly. They do not receive any
notification if the recipient received the message nor do they
receive any error notifications. Though some transports may
provide limited status information, conceptionally syslog is pure
simplex communication.
o Senders send messages to relays or collectors with no knowledge of
whether it is a collector or relay.
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
those cases, they are 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 it is acting as a
sender and a relay.
3.1 Example Deployment Scenarios
The following deployment scenarios shown in Diagram 1 are valid while
the first one has been known to be the most prevalent. 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 along with passing along messages that they
internally generate. The boxes represent syslog-enabled
applications.
+------+ +---------+
|Sender|---->----|Collector|
+------+ +---------+
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+------+ +-----+ +---------+
|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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4. 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 will require that syslog transports be
defined in other documents. The first transport is defined in [13]
and is consistent with the traditional UDP transport.
Other transport mappings must ensure that all messages MUST be
transmitted unaltered to the destination. If the mapping needs to
perform temporary transformations, it MUST be guaranteed that the
message received at the final destination is an exact copy of the
message sent from the initial originator. Otherwise cryptographic
verifiers (like signatures) will be broken.
4.1 Minimum Required Transport Mapping
As noted, all implementations MUST have a UDP-based transport as
described in [13]. This is to ensure interoperability between all
systems implementing the protocol described in this document.
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5. Required syslog Format
The syslog message has the following ABNF [8] definition:
; The general syslog message format
SYSLOG-MSG = HEADER SP STRUCTURED-DATA SP MSG
HEADER = VERSION SP SP FACILITY SP SEVERITY SP
TIMESTAMP SP HOSTNAME SP SENDER-NAME SP
SENDER-INST
VERSION = NONZERO-DIGIT 0*2DIGIT
FACILITY = "0" / (NONZERO-DIGIT 0*9DIGIT)
; range 0..2147483648
SEVERITY = "0" / "1" / "2" / "3" / "4" / "5" /
"6" / "7"
HOSTNAME = 1*255PRINTUSASCII ; a FQDN
SENDER-NAME = 1*48VISUAL
SENDER-INST = "-" / 1*16VISUAL
VISUAL = (%d33-57/%d59-126) ; all but SP
TIMESTAMP = full-date "T" full-time
date-fullyear = 4DIGIT
date-month = 2DIGIT ; 01-12
date-mday = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on
; month/year
time-hour = 2DIGIT ; 00-23
time-minute = 2DIGIT ; 00-59
time-second = 2DIGIT ; 00-58, 00-59, 00-60 based on leap
; second rules
time-secfrac = "." 1*6DIGIT
time-offset = "Z" / time-numoffset
time-numoffset = ("+" / "-") time-hour ":" time-minute
partial-time = time-hour ":" time-minute ":" time-second
[time-secfrac]
full-date = date-fullyear "-" date-month "-" date-mday
full-time = partial-time time-offset
STRUCTURED-DATA = *STR-DATA-ELT
STR-DATA-ELT = "[" SD-ID 0*(1*SP SD-PARAM) "]"
SD-PARAM = PARAM-NAME "=" %d34 PARAM-VALUE %d34
SD-ID = SD-NAME
PARAM-NAME = SD-NAME
PARAM-VALUE = UTF-8-STRING
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"
DIGIT = "0" / NONZERO-DIGIT
5.1 Message Length
The maximum length of any syslog message is 16,777,216 octets. Any
receiver receiving a larger message MUST discard the message. A
diagnostic message SHOULD be logged in this case.
A receiver MUST be able to receive messages of a length of 480 octets
or less. A receiver SHOULD be able to receive messages of a length
of 65,535 octets or less. It is RECOMMENDED that receivers have the
ability to receive messages up to the maximum message length.
If a receiver receives messages within the maximum length, but with a
length larger than it handles, the receiver MAY discard or truncate
it.
5.2 HEADER
The code set used in the HEADER MUST be seven-bit ASCII in an
eight-bit field as described in RFC 2234 [8]. These are the ASCII
codes as defined in "USA Standard Code for Information Interchange"
ANSI.X3-4.1968 [1].
If the header is not syntactically correct, the receiver MUST NOT try
to parse some of the header fields in order to guess an
interpretation. It MAY assume it is a RFC 3164 [11] compliant
message and MAY decide to process it as such.
5.2.1 VERSION
The VERSION field denotes the version of the syslog protocol
specification. The version number MUST be incremented for each new
syslog protocol specification that changes the format. The value
specified for the value in this document is version "1". Some
additional information about this is specified in Section 9.
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5.2.2 FACILITY
FACILITY is an integer that can be used for filtering by the
receiver. 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. Therefore, no specific semantics for
FACILITY codes are implied in this document.
In order to avoid confusion with RFC 3164 [11], facility codes below
24 SHOULD NOT be used by a sender. If the sender uses them, their
usage SHOULD be limited to the spirit of the semantics defined in RFC
3164 [11].
5.2.3 SEVERITY
The SEVERITY field is used to indicate the severity that the sender
of a message assigned to it. It contains one of these values:
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
5.2.4 TIMESTAMP
The TIMESTAMP field is a formalized timestamp derived from RFC 3339
[12].
While RFC 3339 [12] makes allowances for multiple syntaxes, this
document REQUIRES a restricted set. The TIMESTAMP MUST follow this
restrictions:
o The "T" and "Z" characters in this syntax MUST be upper case.
o Usage of the "T" character is REQUIRED.
o The sender SHOULD include time-secfrac (fractional seconds) if its
clock accuracy and performance permit.
5.2.4.1 Syslog Senders Without Knowledge of Time
A syslog sender being incapable of obtaining system time MUST use the
following TIMESTAMP:
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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. It can never have existed
in a valid syslog message of a time-aware sender. A receiver
receiving that TIMESTAMP MUST treat it as being well-formed.
5.2.4.2 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
1985-04-12T18:20:50.52-06:00
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
settings, it does not tell us.
Example 4
2003-08-24T05:14:15.000003-09:00
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 -9 hours from UTC. This timestamp might be created in
the US Pacific time zone during daylight savings time.
Example 5 - An Invalid TIMESTAMP
2003-08-24T05:14:15.000000003-09:00
This example nearly the same as Example 4, but it is specifying
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time-secfrac in nanoseconds. This will result in time-secfrac to be
longer than the allowed 6 digits, which invalidates it.
5.2.5 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
will be referred to in this document as FQDN.
If the FQDN is not known to the originator, but it knows its IP
address and knows that address is statically assigned, it SHOULD use
that IP address.
If the sender does not know its IP address or if it is statically
assigned and the FQDN is not known, it SHOULD specify its host name
without domain name.
If the FQDN and the host name are not known, and the IP address is
not statically assigned or it is not known if it is statically
assigned, the sender SHOULD specify the IPv4 or IPv6 address it
knows, even if it may be dynamically assigned.
If the sender does not know any identifying information, it SHOULD
provide the value "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 2373 [9], Section 2
MUST be used.
If a sender has multiple IP addresses, it SHOULD use a consistent
value in the HOSTNAME field. This consistent value MUST be one of
its actual IP addresses. If a sender is running on a machine which
has both statically and dynamically assigned addressed, then that
consistent 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.
5.2.6 SENDER-NAME
The SENDER-NAME 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.
SENDER-NAME is similar to the TAG field described in [11], but
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without the instance description that often could be found in TAG.
5.2.7 SENDER-INST
The SENDER-INST SHOULD identify a specific instance of the sender.
It is RECOMMENDED that SENDER-INST contains the operating system
process ID, together with a thread ID, if these things exist. No
specific format is REQUIRED.
The dash character ("-") is a reserved character that MUST only be
used to indicate an unidentified instance.
5.3 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
or application-specific information such as traffic counters or IP
addresses.
STRUCURED-DATA contains none, one or multiple structured data
elements, which are referred to as "STR-DATA-ELT" in this document.
The code set used in STRUCTURED-DATA must be UNICODE, encoded in
UTF-8 as specified in RFC 2279 [7]. 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.
It is RECOMMNEDED that STRUCTURED-DATA is considered to be
non-existing in such cases. A receiver MAY also discard the message.
5.3.1 STR-DATA-ELT
A STR-DATA-ELT consists of a name and parameter name-value pairs.
The name is referred to as SD-ID. It is case-sensitive and uniquely
identifies the type and purpose of the element. The name-value pairs
are referred to as "SD-PARAM".
5.3.1.1 SD-ID
SD-IDs MUST NOT contain SP or the characters '=', '"', or ']'. IANA
controls ALL SD-IDs without a hyphen ('-') in the second character
position. Experimental or vendor-specific SD-IDs MUST start with
"x-". Values with a hyphen on the second character position and the
first character position not being a lower case "x" are undefined and
SHOULD NOT be used. Receivers MAY accept them.
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If a receiver receives a well-formed but unknown SD-ID, it SHOULD
ignore the element.
5.3.1.2 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 and MUST NOT contain SP or the
characters '=', '"', or ']'.
Inside PARAM-VALUE, the characters '"', '\' and ']' MUST be escaped.
This is necessary to avoid parsing errors. Escaping ']' would not
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 MUST replace the two-character
sequence with only the second character received. It is RECOMMENDED
that the receivers logs a diagnostic in this case.
5.3.2 Examples
All examples in this section only show 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-example-iut iut="3" EventSource="Application"
EventID="1011"]
This example is a structured data element with an experimental SD-ID
of type "x-example-iut" which has three parameters.
Example 2 - Valid
[x-example-iut iut="3" EventSource="Application"
EventID="1011"][x-example-priority 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.
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Example 3 - Invalid
[x-example-iut iut="3" EventSource="Application"
EventID="1011"] [x-example-priority 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-example-iut iut="3" EventSource="Application"
EventID="1011"][x-example-priority 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-I. Please note that the dots denote missing fields, which have
been left out for brevity.
5.4 MSG
The MSG part contains a freeform message that gives some detailed
information of the event.
The code set used in MSG MUST be UNICODE, encoded in UTF-8 as
specified in RFC 2279 [7]. 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.
5.5 Examples
The following are examples of valid syslog messages. A description
of each example can be found below it. The examples base on similar
examples from RFC 3164 [11] and are eventually familiar to readers.
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Example 1
1 888 4 2003-10-11T22:14:15.003Z mymachine.example.com su - '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 message was created on October, 11th 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
SENDER-NAME is "su" and the SENDER-INST is unknown. Note the two SP
characters following SENDER-INST. The second SP character is the
STRUCTURED-DATA delimiter. It tells that no STRUCTURED-DATA is
present in this message. The MSG is "'su root' failed for
lonvick...".
Example 2
1 20 6 2003-08-24T05:14:15.000003-09:00 192.0.2.1
myproc 10 %% It's time to
make the do-nuts. %% Ingredients: Mix=OK, Jelly=OK #
Devices: Mixer=OK, Jelly_Injector=OK, Frier=OK # Transport:
Conveyer1=OK, Conveyer2=OK # %%
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 -9 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 the IPv4
address instead. The SENDER-NAME is "myproc" and the SENDER-INST is
"10". The message is "%% It's time to make the do-nuts......".
Example 3 - with STRUCTURED-DATA
1 888 4 2003-10-11T22:14:15.003Z mymachine.example.com
EvntSLog - [x-example-iut 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-example-iut iut="3" EventSource="Application" EventID="1011"]".
The MSG itself is "An application event log entry..."
Example 4 - STRUCTURED-DATA Only
1 888 4 2003-10-11T22:14:15.003Z mymachine.example.com
EvntSLog - [x-example-iut iut="3" EventSource="Application"
EventID="1011"][x-example-priority class="high"]
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This example shows a message with only STRUCTURED-DATA and no MSG
part. This is a valid case.
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6. Structured Data IDs
This section defines the initial IANA-registered SD-IDs. See Section
5.3 for a definition of structured data elements. All SD-IDs are
optional.
6.1 time
The SD-ID "time" 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 if its time zone information is correct. The main
use of this structured data element is to provide some information on
the level of trust of the TIMESTAMP described in Section 5.2.4.
6.1.1 tzknown
The "tzknown" parameter indicates if 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 UTC, the value "1"
MUST be used (because the time zone is known).
6.1.2 issynced
The "issynced" parameter indicates if 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" SHOULD be
used. If not, the value "0" MUST be used.
6.1.3 syncaccuracy
The "syncaccuracy" parameter indicates how accurate the original
sender thinks the time synchronization it participates in is. It is
an integer describing the maximum number of milliseconds that the
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 SHOULD assume that the
time information provided is accurate enough to be considered
correct. The "syncaccuracy" parameter SHOULD ONLY be written 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 only through operator configuration.
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6.1.4 Examples
The following is an example of a system that knows that it does
neither know its time zone nor if it is being synchronized:
[time tzknown="0" issynced="0"]
With this information, the sender indicates that its time information
cannot be trusted. 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:
[time tzknown="1" issynced="1"]
Note: this case SHOULD be assumed by a receiver if no "time" SD-ID is
provided by the sender.
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:
[time tzknown="1" issynced="1" syncaccuracy="60000"]
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.
6.2 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.
6.2.1 ip
The "ip" parameter denotes the IP address that the sender knows it
had at the time of sending this message. It MUST contain the textual
representation of an IP address as outlined in Section 5.2.5.
If a sender has multiple IP addresses, it MAY either use a single of
its IP addresses in the "ip" parameter or it MAY include multiple
"ip" parameters in a single "origin" structured data element.
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6.2.2 enterpriseID
The "enterpriseID" parameter MUST be an '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
which follows is unique and may be registered by an on-line form at
<http://www.iana.org/>. Only that number MUST be specified in the
"enterpriseID" parameter. 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. This may be of aid to log analyzers and
similar processes.
6.2.3 software
The "software" parameter uniquely identifies the software that
generated this 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 SENDER-NAME header
parameter. It always contains the name of the generating software
while SENDER-NAME can contain anything else, including an
operator-configured value.
Specifying the "software" parameter is an aid to log analyzers and
similar processes.
The "software" parameter is a string. It MUST NOT be longer than 48
characters.
6.2.4 sw-version
The "sw-version" 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.
Specifying the "sw-version" parameter is an aid to log analyzers and
similar processes.
The "sw-version" parameter is a string. It MUST NOT be longer than
32 characters.
6.2.5 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.
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7. Security Considerations
7.1 Diagnostic Logging
This document, in multiple sections, recommends that an
implementation writes a diagnostic message to indicate unusual
situations or other things noteworthy. Diagnostic messages are a
useful tool in finding configuration issues as well as a 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 for example in a circular buffer, the flood of
diagnostic log entries would eventually overwrite useful previous
diagnostics.
Besides this risk, diagnostic message, if they occur too frequently,
can become meaningless. Common practice is to turn off diagnostic
logging if it is too verbose. This potentially removes important
diagnostic information which could aid the operator.
7.2 Control Characters
This document does not impose any restrictions on the MSG content.
As such, MSG 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. This means that NUL characters SHOULD NOT be written to
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a file in an unencoded way - otherwise it would potentially render
the file unreadable.
The same is true for other control characters. For example,
deliberately included backspace characters may be used by an attacker
to render parts of the log message unreadable. Similar approaches
exist for almost all control characters.
Finally, invalid UTF-8 sequences may be used by an attacker to inject
ASCII control characters. This is why invalid UTF-8 sequences are
not allowed and MUST be rejected.
7.3 More than Maximum Message Length
The message length MUST NOT exceed the maximum value outlined in
Section 5. Various problems may result if a sender sends out
messages with a greater length. While this document forbids oversize
messages, an attacker may deliberately introduce them. As such, it
is vital that each receiver performs the necessary sanity checks.
7.4 Message Length
An attacker might deliberately send message with the maximum size.
This could lead to massive resource consumption and potentially
denial of service on the receiver or an interim system. Besides the
DoS itself, this could result in the loss of vital log data. As
such, a DoS attack could be used as a way to hide another attack.
To avoid this problem, the network operator may limit the size of the
received message to some value below the maximum supported by the
protocol. An implementation may also provide a feature where only a
configured number of maximum size messages are allowed and truncation
occurs if these occur too frequently.
7.5 Message Truncation
Messages over the minimum to be supported size may be discarded or
truncated by the receiver or interim systems. As such, vital log
information may be lost. Even messages within that size may be lost
if a non-reliable transport mapping is used.
In order to prevent information loss, messages should be less then
the minimum supported size outlined in Section 5.1. For best
performance and reliability, messages SHOULD be as small as possible.
Important information SHOULD be placed as early in the message as
possible, as the information at the begin of the message is less
likely to be discarded by a size-limited receiver.
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In case an application includes some user-supplied data within a
syslog message, this application should limit the size of this data.
Otherwise, an attacker may provide large data in the hope to exploit
this potential weakness.
7.6 Single Source to a Destination
The syslog messages are usually presented (placed in a file,
displayed on the console, etc.) in the order in which they are
received. This is not always in accordance with the sequence in
which they were generated. As they are transmitted across an IP
network, some out of order receipt should be expected. This may lead
to some confusion as messages may be received that would indicate
that a process has stopped before it was started. This is somewhat
rectified by the TIMESTAMP. However, the accuracy of the TIMESTAMP
may not always be sufficiently enough.
It is desirable to use a transport with guaranteed delivery, if one
is available.
7.7 Multiple Sources to a Destination
In syslog, there is no concept of unified event numbering. Single
senders are free to include a sequence number within the MSG but that
can hardly be coordinated between multiple senders. In such cases,
multiple senders may report that each one is sending message number
one. Again, this may be rectified somewhat by the TIMESTAMP. As has
been noted, however, even messages from a single sender to a single
collector may be received out of order. This situation is compounded
when there are several senders configured to send their syslog
messages to a single collector. Messages from one sender may be
delayed so the collector receives messages from another sender first
even though the messages from the first sender were generated before
the messages from the second. If there is no sufficiently-precise
timestamp or coordinated sequence number, then the messages may be
presented in the order in which they were received which may give an
inaccurate view of the sequence of actual events.
7.8 Multiple Sources to Multiple Destinations
The plethora of configuration options available to the network
administrators may further skew the perception of the order of
events. It is possible to configure a group of senders to send
status messages -or other informative messages- to one collector,
while sending messages of relatively higher importance to another
collector. Additionally, the messages may be sent to different files
on the same collector. If the messages do not contain
sufficiently-precise timestamps from the source, it may be difficult
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to order the messages if they are kept in different places. An
administrator may not be able to determine if a record in one file
occurred before or after a record in a different file. This may be
somewhat alleviated by placing marking messages with a timestamp into
all destination files. If these have coordinated timestamps, then
there will be some indication of the time of receipt of the
individual messages. As such, it is highly recommended to use the
best available precision in the TIMESTAMP and use automatic time
synchronization on each systems (as, for example, can be done via
NTP).
7.9 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
TIMESTAMPs and thus the reply attack.
7.10 Reliable Delivery
As there is no mechanism described within this document to ensure
delivery, and since the underlying transport may be lossey (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 the drop of 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 is RECOMMENDED to use a reliable transport mapping to prevent this
problem.
7.11 Message Integrity
Besides being discarded, syslog messages may be damaged in transit,
or an attacker may maliciously modify them. In such cases, the
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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.
7.12 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 of 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
those messages to compromise a machine or do other damage.
7.13 Misconfiguration
Since 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 recipient. Cases have been noted where senders were
inadvertently configured to send syslog messages to the wrong
receiver. In many cases, the inadvertent receiver 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 guard against these problems.
7.14 Forwarding Loop
As it is shown in Figure 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 Priority
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
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processing availability on the two devices. Network administrators
must take care to not cause such a death spiral.
7.15 Load Considerations
Network administrators must take the time to estimate the appropriate
size 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, it is recommended to use
transports with guaranteed delivery.
7.16 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 receiving syslog
messages from known IP addresses.
7.17 Covert Channels
Nothing in this protocol attempts to eliminate covert channels.
Indeed, the unformatted message syntax in the packets could be very
amenable to sending embedded secret messages. In fact, just about
every aspect of syslog messages lends itself to the conveyance of
covert signals. For example, a collusionist could send odd and even
PRI values to indicate Morse Code dashes and dots.
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8. Notice to 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, these
references will be updated to use the RFC numbers. This section will
be removed at that time.
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9. IANA Considerations
9.1 Version
IANA must maintain a registry of VERSION values as described in
Section 5.2.1.
For this document, IANA must register the VERSION "1". New VERSION
numbers must monotonically increment (the next VERSION will be "2")
and will be registered via the Specification Required method as
described in RFC 2434 [10].
9.2 SD-IDs
IANA must maintain a registry of Structured Data ID (SD-ID) values as
described in Section 6. These are the SD-IDs which do NOT have a
hyphen ("-") in the second character position.
New SD-ID values may be registered through the Specification Required
method as described in RFC 2434 [10].
For this document, IANA must register the SD-IDs "time" and "origin".
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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, David Harrington and all
other people who commented on various versions of this proposal.
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12. References
12.1 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] Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
June 1995.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[7] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[8] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[9] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[10] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[11] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001.
[12] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
[13] Okmianski, A., "Transmission of syslog messages over UDP", RFC
9999, August 2004.
12.2 Informative
[14] Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996.
[15] New, D. and M. Rose, "Reliable Delivery for syslog", RFC 3195,
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November 2001.
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 implement
it in order to claim compliance to this specification.
A.1 Message Length
Implementors should note the message size limitations outlined in
Section 5.1 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.
A.2 HEADER Parsing
The section RECOMMENDS a message header parsing method based on the
VERSION field described in Section 5.2.1.
The receiver MUST check the VERSION. If the VERSION is within the
set of versions supported by the receiver, it MUST 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. It 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 [11]. To be precise,
a receiver receiving an unknown VERSION number, or a message without
a valid VERSION, MUST discard the message by default. However, the
administrator may configure it to not discard these messages. If
that happens, the receiver MUST parse it according to RFC 3164 [11].
The administrator may again override this setting and configure the
receiver to parse the messages in any way. It would be considered
good form if the receiver were to attempt to ensure that no
application reliability issues occur.
The spirit behind these guidelines is that the administrator may
sometime 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 MUST 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
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issues.
A.3 SEVERITY Values
This section describes guidelines for using SEVERITY as outlined in
Section 5.2.3.
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.
Since severities are very subjective, the receiver SHOULD NOT assume
that all senders have the same definition of severity.
A.4 time-secfrac Precision
The TIMESTAMP described in Section 5.2.4 supports fractional seconds.
This provides ground 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 Leap Seconds
The TIMESTAMP described in Section 5.2.4 permits leap seconds, as
described in RFC 3339 [12].
The value "60" in the time-second field is used to indicate a leap
second. This MUST NOT be misinterpreted. Developers and
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 to 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.6 Syslog Senders Without Knowledge of Time
In Section 5.2.4.1, 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
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or not such a sender is something that 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. It may also be assumed that
this class of senders will most probably be found in embedded
devices.
Note well: an implementation MUST emit a valid TIMESTAMP if the
underlying operating system, programming system and hardware supports
the clock function. A proper TIMESTAMP MUST be emitted even if it is
difficult, but doable, to obtain the system time. The TIMESTAMP
described in Section 5.2.4.1 MUST 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 had no idea of what the time actually is and act
accordingly.
A.7 Additional Information on SENDER-INST
The objective behind SENDER-INST (Section 5.2.7) is to provide a
quick way to detect a new instance of the same sender. It must be
noted that this is not reliable as a second incarnation of a
SENDER-INST may actually be able to use the same SENDER-INST value as
the prior one. Properly used, the SENDER-INST can be helpful for
analysis purposes.
A.8 Notes on the time SD-ID
It is RECOMMENDED that the value of "0" be the default for the
"tzknown" (Section 6.1.1) 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 acknowledges the
correctness of the time zone information.
It is important not to create a false impression of accuracy with the
time SD-ID (Section 6.1). A sender MUST 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
through operator configuration. As such, by default, an accuracy
SHOULD NOT be provided.
A.9 Recommendation for Diagnostic Logging
In Section 7.1, this document describes the need as well as potential
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problems of diagnostic logging. In this section, a real-world
approach to useful diagnostic logging is RECOMMENDED.
While this document recommends to write meaningful diagnostic logs,
it also recommends to allow an operator to limit the amount of
diagnostic logging. At least, an implementation SHOULD differentiate
between critical, informational and debugging diagnostic message.
Critical messages should only be issued in real critical states, e.g.
expected or happening 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 indicate all conditions 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 to not provide any debugging
diagnostic messages.
An administrator should be able to configure the level for which
diagnostic messages will be written. Non-configured diagnostic
should not be written but discarded. An implementor may create as
many different levels of diagnostic messages as he see useful - the
above recommendation is just based on real-world experience of what
is considered useful. Please note that experience also 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 this section. 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
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
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
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administrator thinks to be reasonable for his needs. It is up to the
implementor of what the term "duplicate" means. Some may decide that
only totally identical (in byte-to-byte comparison) messages are
actually duplicate, some other may say that a message which 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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