syslog Working Group R. Gerhards
Internet-Draft Adiscon GmbH
Expires: August 16, 2004 February 16, 2004
The syslog Protocol
draft-ietf-syslog-protocol-03.txt
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes the syslog protocol. The syslog protocol has
been used throughout the years to convey event notifications. This
documents 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. Definitions and Architecture . . . . . . . . . . . . . . . . 5
3. Transport Layer Protocol . . . . . . . . . . . . . . . . . . 8
3.1 Minimum Required Transport Mapping . . . . . . . . . . . . . 8
4. Required syslog Format . . . . . . . . . . . . . . . . . . . 9
4.1 HEADER Part . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.1 VERSION . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1.2 enterpriseID . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1.3 FACILITY . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1.4 SEVERITY . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1.5 TIMESTAMP . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1.6 HOSTNAME . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.1.7 TAG . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2 MSG . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5. Structured Data . . . . . . . . . . . . . . . . . . . . . . 19
5.1 Format . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
5.2 MSG with just Structured Data . . . . . . . . . . . . . . . 21
5.3 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6. Multi-Part Messages . . . . . . . . . . . . . . . . . . . . 23
6.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1.1 Original Message . . . . . . . . . . . . . . . . . . . . . . 23
6.1.2 Message Part . . . . . . . . . . . . . . . . . . . . . . . . 23
6.1.3 Message Disassembly and Reassembly . . . . . . . . . . . . . 23
6.1.4 Multi-Part Message Header . . . . . . . . . . . . . . . . . 23
6.1.5 Message Part Message . . . . . . . . . . . . . . . . . . . . 23
6.1.6 Multi-Part Messaging . . . . . . . . . . . . . . . . . . . . 24
6.2 SD-ID msgpart . . . . . . . . . . . . . . . . . . . . . . . 24
6.2.1 msgid . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2.2 partnum . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2.3 partcount . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.3 Cryptographically Signing Multi-Part Messages . . . . . . . 26
6.4 Multi-Part Message Examples . . . . . . . . . . . . . . . . 27
7. Structured Data IDs . . . . . . . . . . . . . . . . . . . . 29
7.1 msgpart . . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.2 time . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.2.1 tzknown . . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.2.2 issynced . . . . . . . . . . . . . . . . . . . . . . . . . . 29
7.2.3 syncaccuracy . . . . . . . . . . . . . . . . . . . . . . . . 30
7.2.4 Example . . . . . . . . . . . . . . . . . . . . . . . . . . 30
7.3 origin . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7.3.1 format . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7.3.2 ip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
7.3.3 Example . . . . . . . . . . . . . . . . . . . . . . . . . . 32
8. Relay Operations . . . . . . . . . . . . . . . . . . . . . . 33
8.1 No Message Modification Allowed . . . . . . . . . . . . . . 33
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8.2 RFC 3164 Messages . . . . . . . . . . . . . . . . . . . . . 33
8.2.1 Reception of RFC 3164 Messages . . . . . . . . . . . . . . . 33
8.2.2 Sending RFC 3164 Messages . . . . . . . . . . . . . . . . . 33
8.3 Creation of Multi-Part Messages . . . . . . . . . . . . . . 33
9. Security Considerations . . . . . . . . . . . . . . . . . . 34
9.1 Diagnostic Logging . . . . . . . . . . . . . . . . . . . . . 34
9.2 Packet Parameters . . . . . . . . . . . . . . . . . . . . . 35
9.3 Single Source to a Destination . . . . . . . . . . . . . . . 36
9.4 Multiple Sources to a Destination . . . . . . . . . . . . . 36
9.5 Multiple Sources to Multiple Destinations . . . . . . . . . 36
9.6 Replaying . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.7 Reliable Delivery . . . . . . . . . . . . . . . . . . . . . 37
9.8 Message Integrity . . . . . . . . . . . . . . . . . . . . . 38
9.9 Message Observation . . . . . . . . . . . . . . . . . . . . 38
9.10 Misconfiguration . . . . . . . . . . . . . . . . . . . . . . 38
9.11 Forwarding Loop . . . . . . . . . . . . . . . . . . . . . . 39
9.12 Load Considerations . . . . . . . . . . . . . . . . . . . . 39
9.13 Denial of Service . . . . . . . . . . . . . . . . . . . . . 39
9.14 Covert Channels . . . . . . . . . . . . . . . . . . . . . . 39
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . 40
11. Authors and Working Group Chair . . . . . . . . . . . . . . 41
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 42
References . . . . . . . . . . . . . . . . . . . . . . . . . 43
Author's Address . . . . . . . . . . . . . . . . . . . . . . 43
Intellectual Property and Copyright Statements . . . . . . . 44
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1. Introduction
This document describes the semantics of the syslog protocol,
outlines transport mappings and provides a standard format for all
syslog messages. It also describes structured data elemtns, that can
be used to precisely define specific message aspects. Many of these
structured data elements carry optional information and are as such
optional themselves.
This document describes a layered architecture for syslog. The goal
of this architecture is to separate the functionality into separate
layers and thus provide easy extensibility.
While REQUIRING A specific format for syslog messages, the document
acknowledges the importance of interoperability with existing syslog
implementations. The informational document RFC 3164 [10] describes
a general format of syslog messages as they have been seen on the
wire. In order to be interoperable with existing implementations,
this document RECOMMENDS a set of mappings between the RFC 3164
format and the format outlined herein. These mappings MAY be done by
relays. It is NOT the intention that an implementation implementing
this document can itself send to a RFC 3164 implementation. Neither
is an implementation of this document expected or required to
directly receive messages from a RFC 3164 implementation. The mapping
rules only apply to relays, which then can actually be used as
gateways between implementations of this document and RFC 3164.
In order to claim compliance with this document, an implementation
MUST at least implement all REQUIRED parts. Optional parts must not
necessarily be implemented. Most importantly, RFC 3164
interoperability is NOT a REQUIRED part of this document.
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2. Definitions and Architecture
The following definitions will be used in this document.
A machine that can generate a message will be called a "device".
A machine that can receive the message and forward it to another
machine will be called a "relay".
A machine that receives the message and does not relay it to any
other machines will be called a "collector". This has been
commonly known as a "syslog server".
Any device or relay will be known as the "sender" when it sends a
message.
Any relay or collector will be known as the "receiver" when it
receives the message.
There are machines that both receive messages and forward them to
another machine AND generate syslog messages themselfs. An example
for this may be an application that operates as a syslog relay as
one service while at the same time running other services. These
services may be monitored by the same application, generating new
syslog messages. Such a machine acts both as a relay AND a device.
This case is specifically mentioned as the role a machine plays
has special significance, for example on formatting. A machine as
described here may thus have two separate configurations for each
of the machine's operations modes.
The architecture of the devices may be summarized as follows:
Senders send messages to relays or collectors with no knowledge of
whether it is a collector or relay.
Senders may be configured to send the same message to multiple
receivers.
Relays may send all or some of the messages that they receive to a
subsequent relay or collector. In the case where they do not
forward all of their messages, they are acting as both a collector
and a relay. In the following diagram, these devices will be
designated as relays.
Relays may also generate their own messages and send them on to
subsequent relays or collectors. In that case it is acting as a
device. These devices will also be designated as a relay in the
following diagram.
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The following architectures 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 above,
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.
+------+ +---------+
|Device|---->----|Collector|
+------+ +---------+
+------+ +-----+ +---------+
|Device|---->----|Relay|---->----|Collector|
+------+ +-----+ +---------+
+------+ +-----+ +-----+ +---------+
|Device|-->--|Relay|-->--..-->--|Relay|-->--|Collector|
+------+ +-----+ +-----+ +---------+
+------+ +-----+ +---------+
|Device|---->----|Relay|---->----|Collector|
| |-\ +-----+ +---------+
+------+ \
\ +-----+ +---------+
\-->--|Relay|---->----|Collector|
+-----+ +---------+
+------+ +---------+
|Device|---->----|Collector|
| |-\ +---------+
+------+ \
\ +-----+ +---------+
\-->--|Relay|---->----|Collector|
+-----+ +---------+
+------+ +-----+ +---------+
|Device|---->----|Relay|---->-------|Collector|
| |-\ +-----+ /--| |
+------+ \ / +---------+
\ +-----+ /
\-->--|Relay|-->--/
+-----+
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+------+ +-----+ +---------+
|Device|---->-----|Relay|---->----------|Collector|
| |-\ +-----+ /--| |
+------+ \ / +---------+
\ +--------+ /
\ |+------+| /
\-->-||Relay ||->---/
|+------|| /
||Device||->-/
|+------+|
+--------+
Diagram 1. Some Possible syslog Architectures
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3. Transport Layer Protocol
This document DOES NOT specify a specific transport layer protocol.
Instead, it describes the format of a syslog message in a transport
layer independent way.
Transport mappings being defined MUST ensure that a message formatted
according to this document can be transmitted unaltered over the
mapping. 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. This is vital because otherwise cryptographic verifiers
(like signatures) would be broken.
3.1 Minimum Required Transport Mapping
To claim compliance with this document, each implementation MUST at
least implement the UDP transport mapping described in Anton
Okmianski "Syslog over UDP" (draft-ietf-syslog-udp-transport-00.txt).
This is to ensure a minimum interoperability between systems
implementing this document.
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4. Required syslog Format
The syslog message has the following ABNF [7] definition:
; The general syslog message format
SYSLOG-MSG = HEADER MSG
HEADER = "V" VERSION SP enterpriseID SP FACILITY SP
SEVERITY SP TIMESTAMP SP HOSTNAME SP TAG SP
VERSION = 1*3DIGIT
enterpriseID = 1*10DIGIT ; range 0..2147483648
FACILITY = 1*10DIGIT ; range 0..2147483648
SEVERITY = "0" / "1" / "2" / "3" / "4" / "5" /
"6" / "7"
HOSTNAME = 1*255PRINTUSASCII ; a FQDN
TAG = static-id [full-dyn-id] [":"] ; 64 chars max
static-id = 1*VISUAL
full-dyn-id = "[" proc-id [thread-sep thread-id] "]"
proc-id = 1*ALFANUM ; recommended: number
thread-sep = VISUAL / %d58 ; recommended: ",", or ':', or '.'
thread-id = 1*ALFANUM ; recommended: number
VISUAL = (%d33-57/%d59-126) ; all but SP and ":"
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
MSG = *OCTECT
; VALID UTF-8 String of PRINTABLE characters
OCTET = %d00..255
LF = %d10
CR = %d13
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SP = %d32
PRINTUSASCII = %d33-126
ALFANUM = %d48..57 / %d65..90 / %d97..122
4.1 HEADER Part
The header MUST contain the token identifying the message as a syslog
message complying with this specification, the version of the
specification it complies to, the enterpriseID of the original
sender, the facility, the severity, the timestamp, the hostname and
the tag. Each of this fields MUST be present and MUST be of correct
syntax. The code 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].
If the header is not syntactically correct, the receiver MUST NOT try
to sub-parse some of the header fields in order to find a "good"
interpretation. However, the receiver MAY assume it is a RFC3164
compliant message and MAY decide to process it as such. In this case,
RFC3164 semantics MUST be used.
As a note to implementors, the "V" token at the very beginning of the
message MAY be used as a rough indication whether or not the message
complies to this document. However, it is not sufficient to assume it
complies to this document just because the first character is a "V".
As written above, the full header MUST be validated to assume this.
4.1.1 VERSION
The Version field denotes the version of the syslog protcol
specification the message is formatted to. It is used to uniquely
identify the message format should later revisions of the syslog
protocol specification change the format.
Note well: this document is the first to specify this format,
including the VERSION in the header. Any previous syslog
specification had a different header. As outlined under HEADER above,
an invalid HEADER will automatically tell the receiver that the
message is NOT compliant to this specification. As such, all version
information is well defined (absence of version information means
legacy syslog by the fact that the header is invalid).
The VERSION MUST be a numerical value. It MUST be one of the IANA
assigned valid VERSION numbers. It starts at 1, which means the
format specified in this document. The VERSION number MUST be
incremented for each new syslog protocol specification that changes
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the format. If MUST NOT be incremented if a new syslog protocol
specification does not change the syntax and semantics of the message
format.
The sender of the syslog message MUST specify the VERSION of the
format that the message was formatted to.
The receiver MUST check the VERSION. If the VERSION is within the set
of format versions supported by the receiver, the receiver MUST parse
the message according to the correct syslog protcol specification. A
receiver MUST NOT parse a previous version with some parsing rules
from a later 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 I MAY try this
if the administrator has configured the receiver to do so. In the
later case, the results may be undefined. If the administrator has
instructed the receiver to parse non-supported version, it SHOULD
assume that these messages are legacy syslog messages and parse and
process them in respect to RFC 3164. Again, the administrator MAY
configure the receiver to use a different algorithm.
To be precise, a receiver receiving an unknown VERSION number, MUST,
by default, ignore it. The administrator may configure it to not
ignore it. Then, the receiver MUST, by default, parse it according to
RFC 3164. The administrator may again override this setting. In this
case, the receiver MAY use whatever method the administor has
choosen. In this case, the receiver MUST ensure that no application
reliability issue occurs. If there is a chance for this, it MUST NOT
allow the administrator to select an insecure mode.
The spirit of this behaviour 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 so specific on the defaults and modes because it is
common experience that parsing unknown formats often leads to
security issues.
4.1.2 enterpriseID
The enterprise ID unquily identifies the vender whom's software or
device created the message. This is to support log-parsers
sub-parsing vendor-specifc information from the message part.
The enterprise ID is an integer. It MUST be the enterprise ID
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assigned by IANA to the vendor whoms software or device created the
syslog message.
4.1.3 FACILITY
The facility is primarily a way to filter messages at the receiver.
It is a numerical value. 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 vendors, softwares and devices.
Therefore, no specifc semantics for facility codes are implied in
this document.
FACILITY is just a sender-supplied numerical identifier that can be
used for filtering by the receiver. The facility in itself does not
have any semantics. Semantics MUST be applied by site configuration
(through the site's administrator).
Any implementation of this document MUST support free configuration
of the FACILITY on the sender.
4.1.4 SEVERITY
The SEVERITY field is used to indicate the severity that the sender
of the message assgined to it. It is a numerical value with just few
values. The traditional syslog severity values are reused, because
they have prooven to be useful and sufficient in reality.
SEVERITY is a numerical field, which MUST contain one of the digits
from 0 to 7. Any other value is invalid and MUST NOT be used.
Each of the numerical codes has been assigned the follwing semantics:
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
All implementations SHOULD try to assign the most appropriate
severity to their message. Most importantly, test aid like messages
or programm debugging information SHOULD be assiged severity 7.
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Severity 0 SHOULD be reserved for high-priviledge core processes of
very high importance (like serious hardware failures or a very soon
power failure). An implementation MAY use severities 0 and 7 for
other purposes if this is configured by the administrator.
In general, a receiver should abide to the fact that severities are
often very subjective. As such, a receiver MUST not assume that all
senders have the same sense of severities.
4.1.5 TIMESTAMP
The TIMESTAMP field is a formalized timestamp as taken from RFC 3339
[11].
Note well: RFC 3339 makes allowances for multiple syntaxes for a
timestamp to be used in various cases. This document mandates a
restricted set of syntaxes. The primary characteristics of TIMESTAMP
used in this document are as follows.
o the "T" and "Z" characters in this syntax MUST be upper case.
o usage of the "T" character is mandatory. It MUST NOT be replaced
by any other character (like a SP character).
o the sender SHOULD include time-secfrac (fractional seconds) if its
clock accuracy and performance permits.
o the entire length of the TIMESTAMP field MUST NOT exceed 32
characters.
Please also note that RFC 3339 permits the value "60" in the second
part to indicate a leap second. This must not be misinterpreted. As a
suggestion for application developer, it is 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
cryptographical signatures to become invalid. As such, it is
suggested that the adjustment is not done when the plain message text
is to be stored (e.g. for later verification of signatures).
Two samples of this format are:
1985-04-12T23:20:50.52Z
1985-04-12T18:20:50.52-06:00
The first represents 20 minutes and 50.52 seconds after the 23rd hour
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of April 12th, 1985 in UTC. The second represents the same time but
expressed in the Eastern US timezone (daylight savings time being
observed).
A single space character MUST follow the TIMESTAMP field.
4.1.5.1 Syslog Senders without knowledge of Time
There is one special case, and this is a syslog sender that is NOT
aware of time at all. It can be argued if such a syslog sender is
something that actually can be found in todays IT infrastructure.
However, discussion has indicated that those things may exist in
reality and as such there should be a guideline what to do in such a
case. The other assumes that those syslog senders will most probably
be found in embeded devices.
Note well: an implementation MUST emit a valid TIMESTAMP if the
underlying operating system, programming system and hardware is
capable of doing so. A proper TIMESTAMP MUST be emited even if it is
hard, but doable, to obtain the system time. The rule outlined here
MUST only be used when there is absolutely no way to obtain time
information from the system environment. This rule MUST NOT be used
as an excuse for lazy implementations.
A syslog sender who has absolutely no way of obtaining system time
from its environment, MUST write the following TIMESTAMP:
2000-01-01T00:00:60Z
This TIMESTAMP is in the past, but more importantly, it shows a time
that never existed, because January, 1st 2000 had no leap second
(note the 60 in the second indicator). As such, this TIMESTAMP can
never exist in a valid syslog message, but it is still syntactially
correct in regard to the ABNF above.
If a syslog receiver receives this TIMESTAMP it MUST treat the
TIMESTAMP to be well-formed but MUST also know that the sender had no
idea of what the time actually is. It is left to the application
devloper what this means for further processing of the message (this
is beyond the scope of this document).
4.1.6 HOSTNAME
The HOSTNAME field contains the original creator of 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 [2]. This format
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will be referred to in this document as HOSTNAME-STD13.
If the HOSTNAME-STD13 is not known to the orginator, it MUST use
either its IPv4 address or its IPv6 address.
If the IPv4 address is used, it MUST be shown as the dotted decimal
notation as used in STD 13 [3], and will be referred to as
HOSTNAME-IPV4. If an IPv6 address is used, any valid textual
representation used in RFC 2373 [8], section 2 MAY be used and will
be referred to as HOSTNAME-IPV6.
If a device 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. As an alternative, it MAY use the IP address of
the interface that is used to send the message.
A single space character MUST follow the HOSTNAME field.
4.1.7 TAG
The TAG is a string of visible (printing) characters excluding SP,
that MUST NOT exceed 64 characters in length. The first occurrence of
a SP (space) will terminate the TAG field, but is not part of it.
Note well: the colon (":") is NOT a special character inside the TAG.
It may occur anywhere within it and may occur muliple times. The TAG
is terminated by the first SP, NOT the colon character.
The TAG is used to denote the sender of the message. It MUST be in
the syntax shown in the ABNF above.
A typical example of a TAG is: (without the quotes)
amavis[13149]:
Another example with a dynamic id may be:
"/path/to/PROGNAME[123,456]:"
Another example (from VMS) is: (without the quotes)
"DKA0:[MYDIR.SUBDIR1.SUBDIR2]MYFILE.TXT;1[123,456]".
Please note that in this example,
"DKA0:[MYDIR.SUBDIR1.SUBDIR2]MYFILE.TXT;1" is the static-id while
"[123,456]" is still the full-dyn-id. This shows that a receiver must
be prepared for special characters like '[' and ':' to be present
inside the static part.
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As a note to implementors: the beginning of the full-dyn-id is not
the first but the LAST occurrence of '[' inside the tag and this ONLY
if the tag ends in either "]" or "]". If these conditions are not
met, the '[' is part of the static-id.
Systems that use both process-ID's and thead-IDs, SHOULD fill both
the proc-id and the thread-part. For other systems it is RECOMMENDED
to use the proc-id only.
No specific format inside the tag is required. However, a sender
SHOULD use a consistent tag value.
4.2 MSG
The MSG part contains the details of the message. This has
traditionally been a freeform message that gives some detailed
information of the event. It MAY also contain structured data as
described in Structured Data (Section 5) below.
The code set used in MSG must be UNICODE. It MUST be encoded in UTF-8
as specified in RFC 2279 [6]. A sender MAY issue any valid UTF-8
sequence. A receiver MUST accept any valid UTF-8 sequence. Most
importantly, it must not fail if control characters are present in
the MSG part.
Note to implementors: the octect value 0 (0x00), the C string
terminator, is a valid character and MAY be present in the MSG part.
The implementor must ensure that reception of 0x00 causes no
malfunction, specifically does not cause message truncation. C
programmers please be aware that this requires proper escaping and/or
special string handling.
Another note to implementors: please keep the presence of control
characters in mind when writing textual log files. For example, LF is
a valid character and may be present in the MSG part. Writing this
plainly to a log file may cause problems with log parsers and other
programs that process the log file. It is good practice to escape
non-printable characters in a consistent way when writing to text
files.
4.3 Examples
The following examples are given.
Example 1
V1 0 888 4 2003-10-11T22:14:15.003Z mymachine.example.com su: 'su
root' failed for lonvick on /dev/pts/8
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In this example, the VERSION is 1 (formatted according this
document), the enterprise ID is 0 (IETF), the FACILITY has the value
of 888 (whatever this means is up to the sender and recipient). The
message was created The timestamp is in UTC. on October, 11th 2003 at
10:14:15pm, 3 milliseconds into the next second. Please note that the
sender had millisecond resolution. The sender may have actually had a
better resolution, but by providing just three digits for the
fractional settings, he does not tell us this. The message orignated
from a host that calls itself "mymachine.example.com". The TAG is
"su:". Note that the colon is part of the tag. The MSG is "'su root
failed for lonvick...". Please note that the SP after the TAG is NOT
part of the MSG - it is the seperator between TAG and MSG.
As a note to implementors: please note that the sender had
millisecond time resolution in this example. A common coding bug is
that leading zeros are not written for fractional seconds. Very
often, the above timestamp is errornously being written as:
"2003-10-11T22:13:14.3". This would indicate 300 milliseconds instead
of the 3 milliseconds that are actually meant. Please make sure that
an implementation handles this correctly.
Example 2
V1 0 20 6 2003-08-24T05:14:15.000003-09:00 10.1.1.1
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 and the enterprise ID 0. The
FACILITY is within the legacy syslog range (20), as such we assume
the user has specifically configure the sender to use this FACILITY.
The severity is 6 ("Notice" semantics). The timestamp now has
microsecond resolution, indicated by the additional digits in the
fractional seconds. The sender indicates that its local time is -9
hours from UTC. Given the date stamp, we can assume the sender is in
the US Pacific time zone during daylight savings time. The HOSTNAME
is "10.1.1.1", so the sender did not know its host- and domainname
and used the V4 IP address instead. The TAG is "myproc[10]:%%" - we
can speculate that the sender actually wanted the tag to be
"myproc[10]:", but because there was no SP following it, the TAG
continues until the first SP. The message is "It's time to make the
do-nuts......".
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Example 3 - An Invalid Message
V1 0 20 6 2003-08-24T05:14:15.000000003-09:00 10.1.1.1
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 # %%
This example just just like Example 2, but this time the sender is
overdoing with the clock resolution. It is supplying nanosecond
resolution. This will result in the TIME-SECFRAC part to be longer
than the allowed 6 digits, which invalidates the header and thus the
message. A receiver MUST NOT try to "fix" this error. It MUST detect
this as an invalid message and SHOULD log a diagnostic entry. If the
receiver is capable of processing legacy syslog messages, it MUST
assume that this message is legacy syslog and act accordingly.
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5. Structured Data
While syslog traditionally contains freeform data, there may be
structured data present in the MSG part of a syslog message.
Structured data are special, well defined data elements designed to
be easily computer-parsable. They may transport meta data for the
syslog protocol as well as application-defined information (like
traffic counters, IP addresses and other well-defined elements).
There is a certain set of structured data that is under IANA control.
These structured data elements are described in this and other RFCs.
A second set of structured data elements is not under IANA-control.
This set MUST be used for experimental or vendor-specific elements.
A syslog message may contain none, one or multiple structured data
elements.
5.1 Format
Structured data can be present anywhere within the MSG part and
follows this ABNF:
; Format of structured data element
STRUCTURED-DATA = "[@#" SD-ID 0*(1*SP SD-PARAM) *SP "]"
SD-ID = SD-ID-IANA / SD-ID-EXPERI
SD-ID-IANA = 1*1ID-CHAR [1*1ID-CHARNOSLASH [1*62ID-CHAR]]
SD-ID-EXPERI = %d120 "-" 1*62ID-CHAR ; "x-" (lower case 'x'!)
ID-CHAR = %d32-33 / %d35-60 / %d62-92 / %d94-126 /
%d128-254
; all US-ASCII but '"' (%d34), '=' (%d61), ']'
; (%d93)
ID-CHARNOSLASH = %d32-33 / %d35-44 / %d46-60 / %d62-92 /
%d94-126 / %d128-254
; same as ID-CHAR but without '-' (%d45)
SD-PARAM = PARAM-ID "=%d34" PARAM-VALUE "%d34"
PARAM-ID = 1*64ID-CHAR
PARAM-VALUE = *(SAFE-CHAR / ESCAPED-CHAR)
SAFE-CHAR = *((%d32-33) / (%d35-46) / (%d48-92) /
(%d94-126) / (%d128-254))
ESCAPED-CHAR = ("\\" / %d47.34 / "]") ; 47.34 is \"
Each structured data element MUST begin with the token "[@#". This
designates it as a special entity. This three-character sequence is
highly likely not to be confused with traditional syslog message
patterns.
The beginning token MUST immediatly be followed by the ID of the
structured data element. No space is allowed between the beginning
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token and the SD-ID. The SD-ID uniquely identifies the type and
purpose of the element.
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 hypen on the second character position and the
first character position not being a lower case "x" are undfined and
SHOULD NOT be used. Receivers MAY accept them.
If a receiver receives a well-formed but unknown SD-ID, the receiver
SHOULD ignore this element. It MUST NOT malfunction because of this
unknown SD-ID.
The SD-ID is followed by none, one or many optional parameter/value
pairs. Each of them MUST start with the parameter name, MUST be
followed by an equal sign and quote sign. There MUST NOT be any space
between the SD-ID, the equal and the quote sign. This is followed by
the parameter value and then another quote sign.
The parameter value may contain any character, but the three special
characters '"', '\' and ']' MUST be escaped. This is neccessary to
avoid parsing errors. Please note that escaping ']' would actually
not be necessary but is required in order to avoid parser
implementation errors. Each of these three characters MUST be escaped
as '\"', '\\' and '\]' respectively. If a receiver receives an
invalid
A backslash ('\') followed by none of the three described characters
is considered an invalid escape sequence. Upon reception of such an
invalid message, the receiver MUST replace the two-character sequence
with just the second character received. It is recommended that the
receivers logs a diagnostic message in this case. The receiver MUST
otherwise ignore the invalid escape sequence.
Parameter/Value pairs MUST be separated by at least one SP character.
The structured data element MUST be terminated by the character ']',
the ending token. This MUST follow the last parameter/value pair.
There SHOULD be no SP in front of the ending token, but there MAY be
one or multiple SP in front of it.
If multiple structured data elements are written, it is RECOMMENDED
that they are all sequentially written and no SP be written between
those elements. However, they MAY occur at any position inside MSG.
The order of structured data elements inside the MSG is irrelevant,
except for IANA-assigned SD-IDs which specifically require a certain
order. The same SD-ID MAY exist more than once inside a MSG if this
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is permitted by the SD-ID type.
5.2 MSG with just Structured Data
Any syslog MSG may contain structured as well as traditional free
form data. The free form (or unstructured) part of the syslog MSG is
obtained by omiting all the structured data elements from the MSG.
The resulting free from part of the MSG may consist purely of one or
more SPs. This is considered as a MSG with just structured data
elements.
As far as this specification goes, there is no implied special action
to be taken on messages without a free form content in the MSG field.
This case is just defined so that it may be used for
implementation-specifc (and probably user-configurable) actions.
5.3 Examples
All examples show the MSG part of the syslog message only. All
examples should be considered to be on one line. They are wrapped on
multiple lines for readabily purposes, only.
Example 1
[@#x-adiscon-iut iut="3" EventSource="Application"
EventID="1011"]This is event 1011
This example is a MSG with an experimental SD-ID of type
"x-adiscon-iut" which has two parameters. This is followed by the
free form text "This is event 1011".
Example 2
[@#x-adiscon-iut iut="3" EventSource="Application"
EventID="1011"]This is event 1011 [@#x-adiscon-priority
class="high"]
This is the same example, but with a second structured data element.
Please note that the structured data element does not immediately
follow the first one. Also note that the free form message is
different from the example 1. It now is "This is event 1011>SP<" -
notice the extra space character at the end.
Example 3
This is event[@#x-adiscon-priority class="high"] 1
[@#x-adiscon-iut iut="3" EventSource="Application
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"EventID="1011"]011<SP>
In this example, <SP> is actually a single space character. Although
all elements are re-odered and the free form message is intermixed
with structured data, it is still exactly the same message as in
example 2. The message formatting shown in example three SHOULD be
avoided by syslog senders. However, receivers MUST accept messages
formatted in that way.
Example 4
[ @#x-adiscon-iut iut="3" EventSource="Application"
EventID="1011"]This is event 1011
Example four looks very much like example one. However, it is totally
different because example four does NOT contain any structured data
element at all. This is because there is a SP between the bracket and
the rest of the beginning token "@#". As such, the three-character
beginning token is not identifiable and not parsed as such. Receivers
receiving this format MUST NOT assume structured data. This is
especially important as legacy syslog data may very well contain a
sequence as shown above which actually is no structured data.
Example 5
[@#sigSig Ver="1" RSID="1234" ... Signature="......"]
Example 5 is not a full example. It shows how a hypothetical IANA
assigned SD-ID may be used inside an otherwise empty message. Please
note that the dots denote missing fields, which have been left out
for brevity.
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6. Multi-Part Messages
Multi-part messages are an optional feature. It may be used if the
amount of syslog data to be transmitted is larger than the maximum
allowed for a single message. Multi-part messages are implemented
using STRUCTURED-DATA elements.
6.1 Definitions
6.1.1 Original Message
Multi-part syslog messaging is described in few terms. First, we have
the "original message". This is the message that the original sender
intended to send. It typically is larger than the syslog-allowed
maximum message size.
6.1.2 Message Part
To allow splitting the original message into multiple parts, the
original message is split into one or many "message parts". Each
"message part" is a part of the original message's message text. If
all message parts are concatenated together, the result is the exact
same original message.
6.1.3 Message Disassembly and Reassembly
The process of concatenating the individual message parts is called
"message reassembly". The process of spliting the orginal message
into multiple message parts is called "message disassembly".
6.1.4 Multi-Part Message Header
Message parts are no well-formed syslog messages in themself. They do
not contain the required message header and they also do not contain
the structured data elements necessary to support multi-part
messaging. These things are called the "multi-part message header".
6.1.5 Message Part Message
To transmit each message part, the multi-part message header MUST be
added by the sender. When it is added, a complete syslog message is
formed. his message is called the "message part message".
During message reassembly, the multi-part message header MUST be
removed to form the original message.
A message part message is a syslog message in its own right. As such,
it itself can be digitally signed. If so, the signature validates
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only the authenticy of the message part message but not necessarily
that of the original message.
6.1.6 Multi-Part Messaging
This whole process described here is called "multi-part messaging".
If multi-part messages are used, special processing needs to take
place. In order to avoid complexity, the receiver MUST reassemble the
orginal message before parsing the message content. This original
message MUST NOT contain the multi-part message structured data
elements.
It is RECOMMENDED that multi-part messages are only used if the full
message does not fit into a single syslog message. If the message
fits, the multi-part messages feature SHOULD NOT be used. However, a
sender may still choose to use it even in this case. Thus, a receiver
MUST accept a multi-part message consisting of just a single message
part message.
6.2 SD-ID msgpart
Multi-part messaging uses the IANA-reserved "msgpart" SD-ID.
The "msgpart" SD-ID is a structured data element with 3 parameters.
It describes a single part of a multi-part message. It MUST begin
immediately after the HEADER of the syslog message. The fragment of
the original message immediatly begins AFTER the closing token (']')
of the msgpart SD-ID. There MUST NOT be any SP or other character
between the closing token and the begin of the actual message
content.
The receiver of a message part message MUST NOT try to parse
structured data elements inside a single message part. This MUST only
be done on the fully re-assembled message. The reason for this is
that a single message part may be missing important tokens that will
lead to misdetection of structured data elements.
The "fragement" SD-ID has three parameters: msgid, partnum,
partcount. Each of them is described in detail in the following
sections.
All message part messages of a single syslog message MUST have the
exact same syslog message header, most importantly the exact same
timestamp. It is RECOMMENDED that a sender implementing multi-part
messaging provides better-than-second time-resolution inside the
TIMESTAMP.
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6.2.1 msgid
The parameter "msgid" is an integer value in the range 0..2147483648.
It MUST uniquely identify a message for a given TIMESTAMP. It SHOULD
at least uniquely identify a message between two reboots of the
syslog sender.
It is RECOMMENDED that an incrementing value is used, which MAY be
reset to 0 at the time of the syslog sender's startup. If the value
is incremented and the maximum value is reached, than it is
RECOMMENDED to reset the msgid to 0.
Two otherwise identical msgid received in different message part
messages with different TIMESTAMP in the header MUST be considered to
be two different msgid.
6.2.2 partnum
The parameter "partnum" is an integer value in the range
1..2147483648. This value MUST start at one for the first message
part message and MUST be incremented by one for each subsequent
message part message.
The "partnum" counter MUST be processed on a per-message basis. That
is, when the next full syslog message is to be sent as a multi-part
message, its first message part message again starts with "partnum"
set to 1.
The "partnum" counter MUST never be greater than "partcount". If it
ever is greater, all message part messages MUST be considered
invalid. It is RECOMMENDED that a diagnostic message is logged in
that case.
If the "partnum" is outside the defined range, all message part
messages MUST be considered invalid. It is RECOMMENDED that a
diagnostic message is logged in that case.
6.2.3 partcount
The parameter "partcount" is an integer value in the range
1..2147483648. It specifies into how many message part messages the
message has been split into.
The "partcount" parameter MUST either remain the same for all message
part messages or it must be increasing. If it is increasing,
"partcount" MUST be higher for all message part messages which have a
higer "partnum" than the message in question. If an implementor
decides to use an increasing "partcont", it MUST NOT be incremented
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(each one being one higher than the previous one). An implementation
MAY increase the "partcount" by any value, as long as the next
message part message has a higher "partcount" than the previous one.
A receiver MUST NOT assume that "partcount" is being incremented. If
a receiver receives a message part message with a lower "partcount"
but a higher "partnum" than any previously received message part
message for this multi-part message, all message part messages MUST
be considered invalid. It is RECOMMENDED that a diagnostic message is
logged in that case.
Note well: A receiver can not assume reliable, in-order delivery of
messages. An exception is if the underlying transport mapping
explicitly gurantees this. The minimum required transport mapping
outlined in Section 3.1 does NOT guarantee this. As such, the
receiver must ensure that the above rules are obyed even when message
part messages are received out of order. This is a situation where
message part messages with a higher partnum and partcount are
received before message part messages with lower partnum and
partcount. This in itself is no violation of the rules stated above
and MUST NOT be detected as malformed just for this reason (of
course, it could be malformed for other reasons).
If the "partcount" is outside the defined range, all message part
messages MUST be considered invalid. It is RECOMMENDED that a
diagnostic message is logged in that case.
6.3 Cryptographically Signing Multi-Part Messages
While the author of this draft does not intend to specify how
messages can be signed, he would like to offer a suggestion on the
implications of multi-part messages.
Multi-part messages need to be transmitted in indvidual parts and
need to be reassmebled to be processed, at least in many cases.
During reassembly, the multi-part message header is stripped from the
message. This poses a problem to cryptographically signing the
messages.
An obvious solution to keeping the message signature intact is that
only the original, full-size message is signed. Then, the individual
message part messages are transmitted without a specifc signature
attached to them. Only the re-assembled message will then be used for
verifying the signature.
This mode will probably be very efficient, as the ultimate goal is to
guarantee the integrity of the original message. Any modification to
the message part messges will either result in a protocol error or a
modification of the signed original message. Both of this will be
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detected by verifying the signature in the original, reassembled,
message.
One problem, however, may be caused to signature verifiers who work
on raw logs. Raw logs will most probably include only the individual
message part messages. If this is an issue, it may be worth thinking
about signing both the original message as well as each individual
message part messages. So the message would effectively be signed
twice and verifiable in each state.
The author of this draft thinks it is NOT advisable to only sign the
individual message part message. While this would guarantee the
authenticy of the individual fragments, no authentic signature could
be provided for the reassembled message. This may cause serious
issues with higher-level signature verifiers.
Again, these are just thoughts about implementing signatures.
Depending on the signature specification used, there may be different
solutions. It is RECOMMENDED that authors of signing specifications
specifically describe how their specification deals with multi-part
messages. Authors of signing specifications MUST NOT prohibit the use
of multi-part messages.
6.4 Multi-Part Message Examples
To conserve some space, we use an abbreviated sample, where not all
data is shown:
Base Example
<34>2004-01-19T22:14:15.002 mymachine mwagent:
[@#x-adiscon-iut iut="3" EventSource="Application"
(some lenghty params) EventID="1011"][@#x-adiscon-priority
class="high"]This is event 1011.
(lengthy data) This is the end.
We assume that the lengthy data is longer than does fit into a single
syslog message. As such, it needs to be transmitted as multi-part
message. To keep it simple, we assume that "(some lengthy paramters)"
and "(lenghty data)" are the lengthy parts and that their length
forces us to create three message part messages.
The initial message part message will just contain structured data,
the second message part message some structured data and some free
form data and the last message part message only free form data. This
is how they look:
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Example Message Part Message One
<34>2004-01-19T22:14:15.002 mymachine mwagent:
[@#fragment msgid="12" partcount="3" partnum="1"]
[@#x-adiscon-iut iut="3" EventSource="Application"
(some lenghty params) EventID="1011"][@#x-adiscon-prior
Example Message Part Message Two
<34>2004-01-19T22:14:15.002 mymachine mwagent:
[@#fragment msgid="12" partnum="2" partcount="3"]
ity class="high"]This is event 1011. (lengthy
data) This i
Example Message Part Message Three
<34>2004-01-19T22:14:15.002 mymachine mwagent:
[@#fragment msgid="12" partnum="3"
partcount="3"]s the end.
There are some things worth noting when looking at the examples:
o The header, and most importantly the TIMESTAMP is the same for all
three messages, even though message part messages two and three
are most probably send at a slightly later time. Please also note
that a TIMESTAMP is used to facilitate msgid uniquenes.
o The value for "msgid", 12, is just taken randomly for this
example.
o The sequence of "partnum" and "partcount" is not fixed - their
order is different in message part message one than in message
part message two and three. This is irrelevant.
o The "x-adiscon-priority" SD-ID is split between message part
messages one and two. This is the reason why parsing structured
data should only be done on the re-assmbled (original) message.
Parsing the message part messages themselfs may seriously confuse
the parser.
o Note how the freeform message part is split between message part
message two and three. In message part message three, it starts
with "]s" to complete the "its" from the original message. Please
note that if in message part message three it had been "] s", this
would have been reassembled to be "it s" (with a SP in between).
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7. Structured Data IDs
This section defines the currently IANA-registred structured data IDs
(SD-IDs). See Section 5 for a definition of structured data elements.
7.1 msgpart
This SD-ID is currently described in Section 6.2.
7.2 time
The SD-ID "time" is used by the original sender to describe its
notation 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. It MAY
be written in any other case. The main use of this structured data
element is to provide some information on how much the TIMESTAMP
described in Section 4.1.5 can be trusted.
7.2.1 tzknown
The "tzknown" parameter indicates if the original sender knows its
timezone, as specified in the TIMESTAMP. If so, the value "1" MUST be
used. If the time zone information is in doubt, the value "0" MUST be
used. Please note that if the sender KNOWS its timezone but decides
to emit UTC, the value "1" should still be used (because the time
zone is known).
It is suggested that an implementation uses "0" be default and
changes to "1" only 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 explictely
acknowledges the correctness of the time zone information.
If a system is properly synchronized to an external time zone, the
value "1" should be used in most cases. However, we known of external
time zone synchronizations that do NOT provide the exact time zone
information, just a precise local time. In such (rare) cases, the
"time" structured data element should indicated a properly synced
time but the absence of time zone information by setting the
"tzknown" value to "0".
7.2.2 issynced
The "issynced" parameter indicates if the original sender is
synchronized to a reliable external time source, e.g. via NTP. If so,
the value "1" MUST be provided. If not, the value "0" MUST be
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provided.
7.2.3 syncaccuracy
The "syncaccuracy" parameter indicates how accurate the original
sender thinks the time synchronization it participates is.
If the value "0" is used for "issynced", this parameter MUST NOT be
written. 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 reliabilty of the
external time source. In reality, in most cases, it does not have
this - then the "syncaccuracy" parameter MUST not be written to
prevent false impressions.
The "syncaccuracy" parameter is an interger describing the maximum
number of milliseconds that the clock may be off between
synchoronization intervals.
7.2.4 Example
The following is an example of a system that knows that it does
neither know its time zone nor is being synchronized:
[@#time tzknown="0" issynced="0"]
With this information, the sender indicates that its time information
can not 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 has knows its time zone
and knows that it is properly syncrhonized to an external source:
[@#time tzknown="1" issynced="1"]
The author considers this to be the typical case. While we do not
know the accuracy of the external time synchronization, the time
stamp should be good enough for all message correlations with other
senders' messages.
Note well: this case SHOULD be assumed by a receiver if not "time"
structured data element is provided by the sender.
The following is an example of a system that knows both its time zone
and is externally synchronized. It also knows the accuracy of the
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external synchronization:
[@#time tzknown="1" issynced="1" syncaccuracy="60000"]
The difference between this and the previous example is that the
device knows that its clock will be kept within 60 seconds (more or
less) of the official time. So if the device reports it is 9:00:00,
it is no earlier than 8:59:00 and no later then 9:01:00.
Knowing the accuracy of the time synchronization can be helpful when
correlating syslog messages.
It is important to not create a false impression of accuracy. 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.
7.3 origin
The SD-ID "origin" is optional. It MAY be used by a sender to
indicate the origin of a syslog message. It has the following
parameters:
7.3.1 format
The "format" parameter is optional. If it is present, it denots the
format that this message was originally been created in. Its value
MUST be the number of the RFC it complies to. If the message complies
to an Internet-Draft format, it must specifiy the full internet draft
name. For example, as of this writing, format may either hold the
string "3164" (RFC 3164) or "draft-ietf-syslog-protocol-03.txt".
7.3.2 ip
The "ip" parameter is optional. If it is present, it denotes the IP
address that the sender knows it had at the time of sending this
message. It must be either the textual representation of an IPv4
address or the textual representation of an IPv6 address as outlined
in Section 4.1.6. A host name or FQDN MUST NOT be used inside the
"ip" parameter.
If a device 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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7.3.3 Example
The following is an example with multiple IP addresses:
[@#origin format="draft-ietf-syslog-protocol-03.txt ip="192.0.2.1"
ip="192.0.2.129"]
This example is wrapped for readability. With it, the sender
indicates that it has formatted the message according to this draft
and it two ip address, one being 192.0.2.1 and the other one being
192.0.2.129.
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8. Relay Operations
8.1 No Message Modification Allowed
A relay MUST NOT modify any well-formed message it receives. This is
even the case if the original sender had no knowledge of its time
zone as outlined in Section 4.1.5.1.
8.2 RFC 3164 Messages
A relay has potentially needs to send and receive RFC 3164 [10] type
messages. As we assume that RFC 3164 will be in wide use for a long
time period after this document has been released, we would like to
provide some guidelines on how to interoperate with RFC 3164 based
syslog.
These guidelines MUST be implemented if an implementation provides a
way to communicate with a RFC 3164 based implementation. However, if
an implementation does NOT provide any means to communicate with a
RFC 3164 based implementation, this section can be ignored (as it
does not apply).
It is the intention of this section to provide clear guidelines on
how this document interoperates with RFC 3164. The rules try to
retain as much information as possible. Most importantly, these rules
ensure that a message compliant to this document can travel via a RFC
3164 relay chain without any information loss IF the final recipient
is an implementation of this document.
8.2.1 Reception of RFC 3164 Messages
8.2.2 Sending RFC 3164 Messages
8.3 Creation of Multi-Part Messages
There is one special case in which a relay MAY decide to modify a
message. If a realy receives a message and knows that the message is
larger than the largest message size supported by the transport
mapping where it is to be sent over, the relay SHUOLD use multi-part
messaging as outlined in Section 6 and thus disassemble the message
into multiple message part messages. Alternatively, it MAY drop that
part of the message that does not fit into the transport mappings
message sice. It MAY also decide to drop the message completely.
In any case, the relay SHOULD log a diagnostic message indicating the
message receive, the action taken and the reasoning for this.
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9. Security Considerations
This section is to be updated once the rest of the document has been
confirmed. The current content is incomplete and potentially not in
sync with the rest of the draft.
9.1 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 very
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 ressources, 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, e.g. 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 to many administrators. Common practice is to
turn off diagnostic logging if it turns out to be too verbose. This
potentially removes the administrator of important diagnostic
information.
While this document recommends to write meaningful diagnostic logs,
the author also recommends to allow an administrator to limit the
amount of diagnostic logging. At least, an implemenation SHOULD
differentiate between critical, informational and debuging 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 can eventually
alse be considered critical. As a guideline, there should be very few
critical message. Informational message 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 administrator configuring multiple syslog
applications in a complex environment). An application may decide NOT
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to provide any debugging diagnostic messages.
An administrator should be able to configure the level for which
diagnostic messages will be written. Non-configured diagnostics
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 not 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 frequnetly 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 allows to set a limit of how many
"same" diagnostic messages will be generated within a limited amount
of time. E.g. an administrator should be able to say that only the
first 50 identical messages are logged within a 30 minute interval.
All subsequent identical messages will be discarded until the next
time interval. 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
identical messages as well as the time interval to whatever the
administrator thinks to be reasonable for his needs. It is up to the
implementor of what the term "identical" means. Some may decide that
only totally identical (in byte-to-byte comparison) messages are
actually identical, 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 identical. Both approaches
have there advantages and disadvantages. Probably, it is best to
leave this, too, configurable and allow the administrator to set the
mode.
This document does NOT require nor enforce the outlined diagnostic
message categorization or the duplicate supression feature. It just
would like to show some real-world solutions, which may be helpful
for implementors. A system MAY claim to be compliant to this document
even if it does not implement anything of the above.
9.2 Packet Parameters
The message length must not exceed the maximum value outlined in
Section 4. Various problems may result if a device sends out
messages with a greater length. To avoid inconsistencies between
different implementations, oversize packets SHOULD be dropped.
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Inconsitencies between different implementations have shown to be a
major security issue in many cases. So there is good reasoning for
this somewhat harsh recommendation.
Similarly, the multi-part messaging feature may be misused to overrun
a receiver or a log analyzer with a gigantic message. Any process
reassembling multi-part messages MUST properly check against the
maximum re-assembled message size it supports. Oversize data SHOULD
be dropped.
9.3 Single Source to a Destination
The syslog records 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 a
messages may be received that would indicate that a process has
stopped before it was started. This may be somewhat rectified if the
originating process had timestamped or numbered each of the messages
before transmission. In this, the sending device should utilize an
authoritative time source. It should be remembered, however, that
not all devices are capable of receiving time updates, and not all
devices can timestamp their messages.
9.4 Multiple Sources to a Destination
In syslog, there is no concept of unified event numbering. Single
devices are free to include a sequence number within the CONTENT but
that can hardly be coordinated between multiple devices. In such
cases, multiple devices may report that each one is sending message
number one. Again, this may be rectified somewhat if the sending
devices utilize a timestamp from an authoritative source in their
messages. As has been noted, however, even messages from a single
device to a single collector may be received out of order. This
situation is compounded when there are several devices configured to
send their syslog messages to a single collector. Messages from one
device may be delayed so the collector receives messages from another
device first even though the messages from the first device were
generated before the messages from the second. If there is no
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.
9.5 Multiple Sources to Multiple Destinations
The plethora of configuration options available to the network
administrators may further skew the perception of the order of
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events. It is possible to configure a group of devices to send the
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 timestamps
from the source, it may be difficult 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.
9.6 Replaying
This needs also to be addressed in each transport mapping. Here is
the general information on the issue, the transport mapping should
address the specifcs for the transport in question.
Without any sequence indication or timestamp, 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.
9.7 Reliable Delivery
This could also be a place to elaborate about the SIMPLEX nature.
As there is no mechanism within either the syslog process or the
protocol to ensure delivery, and since the underlying transport is
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.
RFC 3195 may be used for the reliable delivery of all syslog
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messages.
9.8 Message Integrity
Besides being discarded, syslog messages may be damaged in transit,
or an attacker may maliciously modify them. In the case of a packet
containing a syslog message being damaged, there are various
mechanisms built into the link layer as well as into the IP [9] and
UDP protocols which may detect the damage. An intermediary router
may discard a damaged IP packet [10]. Damage to a UDP packet may be
detected by the receiving UDP module, which may silently discard it.
In any case, 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.
9.9 Message Observation
While there are no strict guidelines pertaining to the event message
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.
9.10 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 devices 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 [13]. If
messages are not going to the intended recipient, then they cannot be
reviewed or processed.
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9.11 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
processing availability on the two devices. Network administrators
must take care to not cause such a death spiral.
9.12 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.
9.13 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.
9.14 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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10. IANA Considerations
This document also upholds the Facilities and Severities listed in
RFC 3164 [10]. Those values range from 0 to 191. This document also
instructs the IANA to reserve all other possible values of the
Severities and Facilities above the value of 191 and to distribute
them via the consensus process as defined in RFC 2434 [9].
IANA must also maintain a registry of SD-ID values.
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11. 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@hq.adiscon.com
Phone: +49-9349-92880
Fax: +49-9349-928820
Adiscon GmbH
Mozartstrasse 21
97950 Grossrinderfeld
Germany
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12. Acknowledgements
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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References
[1] American National Standards Institute, "USA Code for
Information Interchange", ANSI X3.4, 1968.
[2] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[3] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[4] Malkin, G., "Internet Users' Glossary", RFC 1983, August 1996.
[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", RFC
2279, January 1998.
[7] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[8] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[9] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
[10] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001.
[11] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
Author's Address
Rainer Gerhards
Adiscon GmbH
Mozartstrasse 21
Grossrinderfeld, BW 97950
Germany
EMail: rgerhards@hq.adiscon.com
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Gerhards Expires August 16, 2004 [Page 45]
