Network Working Group R. Gerhards
Internet-Draft Adiscon GmbH
Intended status: Standards Track C. Lonvick
Expires: September 9, 2010 Cisco Systems, Inc
March 8, 2010
Transmission of Syslog Messages over TCP
draft-gerhards-syslog-plain-tcp-02.txt
Abstract
There have been many implementations and deployments of legacy syslog
over TCP for many years. That protocol has evolved without being
standardized and has proven to be quite interoperable in practice.
The aim of this specification is to document three things: how to
transmit standardized syslog over TCP, how this has been done for
legacy syslog, and how the new syslog architecture can interoperate
with the legacy deployments.
Status of this Memo
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Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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described in the BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Message Transmission . . . . . . . . . . . . . . . . . . . . . 5
3.1. Session . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Session Initiation . . . . . . . . . . . . . . . . . . . . 6
3.3. Message Transfer . . . . . . . . . . . . . . . . . . . . . 6
3.3.1. Octet-Counting . . . . . . . . . . . . . . . . . . . . 7
3.3.2. Octet-Stuffing . . . . . . . . . . . . . . . . . . . . 7
3.4. Retaining the Original Message . . . . . . . . . . . . . . 8
3.5. Session Closure . . . . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4.1. Sender Authentication and Message Forgery . . . . . . . . 8
4.2. Message Observation . . . . . . . . . . . . . . . . . . . 9
4.3. Replaying . . . . . . . . . . . . . . . . . . . . . . . . 9
4.4. Message Prioritization and Differentiation . . . . . . . . 10
4.5. Denial of Service . . . . . . . . . . . . . . . . . . . . 10
4.6. Reliability . . . . . . . . . . . . . . . . . . . . . . . 10
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. Notes to the RFC Editor and Change Log . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative . . . . . . . . . . . . . . . . . . . . . . . . 11
8.2. Informative . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix A. Applicability to Legacy syslog . . . . . . . . . . . 11
A.1. Method Change . . . . . . . . . . . . . . . . . . . . . . 12
A.2. Octet-Counting . . . . . . . . . . . . . . . . . . . . . . 12
A.3. Octet Stuffing . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The syslog protocol [RFC5424] is a text-based protocol used to convey
event information. Before that standard was produced, syslog
messages were being transmitted over UDP. This was described in the
INFORMATIONAL document [RFC3164]. While there has been no documented
standard for transporting syslog messages over TCP, it is widely used
in practice and has proven to be quite interoperable among the
implementations, with some minor issues in some configurations.
While existing implementations interoperate quite well with each
other, there are some differences in protocol handling. This
document will describe the most commonly used approach and explain
how to interoperate with them in a consistent way.
This specification applies to messages transmitted using the
[RFC5424] format. Diagram 1 shows how this relates to the other
syslog message transport protocols. In this diagram three
originators are seen, labeled A, B, and C, along with one collector.
Originator A is using the TCP transport which is described in this
document. Originator B is using the UDP transport which is described
in [RFC5426]. Originator C is using the TLC transport which is
described in [RFC5425]. The collector is shown with the capability
to accept all three transports.
A discussion of how this may be applied to legacy syslog as described
in [RFC3164] is contained below. Two format options have been
observed with legacy syslog being transported over TCP. These are
called octet-stuffing and octet-counting and are described in Section
3.3. This specification is written this way, with two format
options, in an attempt to ensure that syslog transport receivers can
receive and properly interpret messages sent from legacy syslog
senders.
To ensure interoperability between syslog senders and receivers, the
octet-counting method is REQUIRED and the octet-stuffing method is
NOT RECOMMENDED.
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+---------------------+
| Originator A |
|---------------------|
| syslog application |
| |
|---------------------|
| syslog transport |
| TCP |
+---------------------+
v
|
/ +---------------------+
/ | Originator B |
/ |---------------------|
/ +----------------------+ | syslog application |
/ | Collector | | |
| |----------------------| |---------------------|
| | syslog application | | syslog transport |
| | | | UDP |
| |----------------------| |---------------------|
| | syslog transport | v
| | TCP | TLS | UDP | |
| +----------------------+ |
| ^ ^ ^ |
| | | | |
\ / | \ /
--------- | ------------------
|
|
| +---------------------+
| | Originator C |
| |---------------------|
| | syslog application |
| | |
| |---------------------|
| | syslog transport |
| | TLS |
| +---------------------+
| v
\ /
---------------
Diagram 1. Syslog Layers
There are several advantages to using TCP: flow control, error
recovery, reliability, to name a few. One potential disadvantage is
the buffering mechanism used by TCP. Ordinarily, TCP decides when
enough data has been received from the application to form a segment
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for transmission. This may be adjusted through timers but still,
some application data may wait in a buffer for a relatively long
time. Syslog data is not normally time-sensitive but if this delay
is a concern, the syslog transport sender may utilize the PUSH Flag
to have the sending TCP immediately send all buffered data.
[RFC0793]
It is still RECOMMENDED to use the TLS transport [RFC5424] to convey
syslog messages. This specification is provided to ensure
interoperability for transporting syslog over TCP.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
The terminology defined in Section 3 of [RFC5424] is used throughout
this specification. The reader should be familiar with that to
follow this discussion.
This document also references devices that use syslog transported
over UDP as described in [RFC3164]. Devices that continue to use
that will be described as legacy syslog senders and receivers in this
document.
3. Message Transmission
As described in [RFC5424], syslog is simplex in nature. Traditional
implementations of syslog over TCP do not use any backchannel
mechanism to convey information to the transport sender, and
consequently do not use any application-level acknowledgement for
syslog receiver to sender signaling. Reliability and flow control
are provided by the capabilities of TCP.
3.1. Session
A syslog over TCP session is a TCP connection between a client and a
server. The syslog transport sender is the host that sends the
original SYN. The syslog transport receiver is the device that
receives the original SYN and responds with a SYN+ACK. After
initiation, messages are sent from the transport sender to the
transport receiver. No application-level data is transmitted from
the transport receiver to the transport sender. The roles of
transport sender and receiver are fixed once the session is
established, and they can not be reversed during the session.
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However, there can be multiple sessions between two TCP hosts, and
for each session the role of transport sender and transport receiver
can be different based upon which device initiates the session.
It is valid (but rare) for no messages to be exchanged during a TCP
session.
If an error occurs, the peer detecting the error will gracefully
close the TCP session, but has no means to notify its remote peer
about the state of the peer syslog application.
3.2. Session Initiation
The peer that intends to act as a syslog transport receiver listens
to TCP port <TBD>. The peer that intends to act as the transport
sender initiates a TCP session to the syslog transport receiver as
specified in [RFC0793].
3.3. Message Transfer
During the message transfer phase, the syslog transport sender sends
a stream of messages to the transport receiver. Either of the peers
may initiate session closure at any time as specified in Section 3.5
of [RFC0793]. In practice, this is often seen after a prolonged time
of inactivity.
Syslog messages are sent in sequence within a TCP transport stream.
One message is encapsulated inside a frame. Syslog transport senders
MUST support the octet-counting method and they MAY support the
octet-stuffing method. Syslog transport receivers are REQUIRED to
support the octet-counting method and are RECOMMENDED to support the
octet-stuffing method to promote interoperability with legacy devices
that may only use that framing method. Transport senders do not send
any notice about the format they use to the transport receiver.
However, the format itself enables the transport receiver to detect
which framing is used. The syslog transport sender MUST NOT change
the format after it has sent the first message. If the format needs
to be changed, the TCP session must be concluded and a new session
established.
All syslog messages MUST be sent as TCP "data" as per Transmission
Control Protocol [RFC0793]. The syslog message stream has the
following ABNF [RFC5234] definition:
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TCP-DATA = *SYSLOG-FRAME
SYSLOG-FRAME = MSG-LEN SP SYSLOG-MSG ; Octet-counting method
SYSLOG-FRAME =/ SYSLOG-MSG TRAILER ; Octet-stuffing method
MSG-LEN = NONZERO-DIGIT *DIGIT
SP = %d32
NONZERO-DIGIT = %d49-57
DIGIT = %d48 / NONZERO-DIGIT
TRAILER = LF | APP-DEFINED
LF = %d10
APP-DEFINED = 1*2OCTET
SYSLOG-MSG is defined in the syslog protocol [RFC5424]
3.3.1. Octet-Counting
This mode is somewhat similar to the framing used in [RFC5425]. Here
the message length, in octets, is specified as HEADER, followed by
SYSLOG-MSG and no trailer.
MSG-LEN is the octet count of the SYSLOG-MSG in the SYSLOG-FRAME. A
transport receiver MUST use the message length to delimit a syslog
message. There is no upper limit for a message length per se.
However, in order to establish a baseline for interoperability, this
specification requires that a transport receiver MUST be able to
process messages with a length up to and including 2048 octets.
Transport receivers SHOULD be able to process messages with lengths
up to and including 8192 octets.
A transport receiver MUST assume that octet-counting framing is used
if a syslog frame starts with a digit.
3.3.2. Octet-Stuffing
In octet-stuffing mode, there is no header, but a trailer is appended
after SYSLOG-MSG. For this specification, this character MUST be the
USASCII LF (%d10) character.
A transport receiver MUST accept the USASCII LF character as a
TRAILER. It MAY be configurable to accept other characters. A
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discussion of this is below.
A transport receiver MUST assume that octet-stuffing framing is used
if a syslog frame starts with the USASCII character "<" (%d60).
The octet-stuffing method is NOT RECOMMENDED.
3.4. Retaining the Original Message
In both of these methods, a modification is made to the original
message. This is a temporary transformation performed by the
transport sender. According to Section 5 of [RFC5425], this
temporary transformation MUST be reversed by the transport protocol
at the transport receiver so that the relay or collector will see an
exact copy of the message generated by the originator or relay.
In the octet-counting method, a count and a space character are
prepended to the messages. This is very much like the method
described in [RFC5426]. The count and space character MUST be
removed by the transport receiver after it has validated that the
count is correct.
Similarly, the syslog transport receiver MUST discard the TRAILER as
it accepts the packet in the octet-stuffing method.
3.5. Session Closure
The SYSLOG session is closed when one of the peers decides to do so.
It then initiates a local TCP session closure. It does not notify
its remote peer of its intension to close the session, nor does it
accept any messages that are still in transit.
4. Security Considerations
Using this specification on an unsecured network is NOT RECOMMENDED.
Several syslog security considerations are discussed in [RFC5424]
This section focuses on security considerations specific to the
syslog transport over TCP. Some of the security issues raised in
this section can be mitigated through the use of TLS as defined in
[RFC5425]
4.1. Sender Authentication and Message Forgery
This transport mapping does not provide for strong transport sender
authentication. The receiver of the syslog message will not be able
to ascertain that the message was indeed sent from the reported
sender, or whether the packet was sent from another device. This can
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also lead to a case of mistaken identity if an inappropriately
configured machine sends syslog messages representing itself as
another machine.
This transport mapping does not provide protection against syslog
message forgery. An attacker can transmit syslog messages (either
from the machine from which the messages are purportedly sent or from
any other machine) to a receiver.
In one case, an attacker can hide the true nature of an attack amidst
many other messages. As an example, an attacker can start generating
forged messages indicating a problem on some machine. This can get
the attention of the system administrators, who will spend their time
investigating the alleged problem. During this time, the attacker
could be able to compromise a different machine or a different
process on the same machine.
Additionally, an attacker can generate false syslog messages to give
untrue indications of the status of systems. As an example, an
attacker can stop a critical process on a machine, which could
generate a notification of exit. The attacker can subsequently
generate a forged notification that the process had been restarted.
The system administrators could accept that misinformation and not
verify that the process had indeed not been restarted.
4.2. Message Observation
This transport mapping does not provide confidentiality of the
messages in transit. If syslog messages are in clear text, this is
how they will be transferred. In most cases, passing clear-text,
human-readable messages is a benefit to the administrators.
Unfortunately, an attacker could also be able to observe the human-
readable contents of syslog messages. The attacker could then use
the knowledge gained from these messages to compromise a machine. It
is RECOMMENDED that no sensitive information be transmitted via this
transport mapping or that transmission of such information be
restricted to properly secured networks.
4.3. Replaying
Message forgery and observation can be combined into a replay attack.
An attacker could record a set of messages that indicate normal
activity of a machine. At a later time, an attacker could remove
that machine from the network and replay the syslog messages with new
time stamps. The administrators could find nothing unusual in the
received messages, and their receipt would falsely indicate normal
activity of the machine.
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4.4. Message Prioritization and Differentiation
This transport mapping does not mandate prioritization of syslog
messages either on the wire or when processed on the receiving host
based on their severity. Unless some prioritization is implemented
by sender, receiver, and/or network, the security implication of such
behavior is that the syslog receiver or network devices could get
overwhelmed with low-severity messages and be forced to discard
potentially high-severity messages.
4.5. Denial of Service
An attacker could overwhelm a receiver by sending more messages to it
than could be handled by the infrastructure or the device itself.
Implementers SHOULD attempt to provide features that minimize this
threat, such as optionally restricting reception of messages to a set
of known source IP addresses.
4.6. Reliability
It should be noted that the syslog transport specified in this
document does not use application-layer acknowledgments. TCP uses
retransmissions to provide protection against some forms of data
loss. However, if the TCP connection is broken for some reason (or
closed by the transport receiver), the syslog transport sender cannot
always know what messages were successfully delivered to the syslog
application at the other end.
5. IANA Considerations
IANA is requested to provide a TCP port for this protocol.
After that port has been assigned, this section will be revised to
list that port.
6. Acknowledgments
The authors wish to thank David Harrington and all other people who
commented on various versions of this proposal.
7. Notes to the RFC Editor and Change Log
These are notes to the RFC editor. Please delete this section after
the notes have been followed.
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Please replace the instances of <TBD> the port number assigned by
IANA.
This is version -01 based upon review comments from David Harrington.
8. References
8.1. Normative
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[RFC5425] Miao, F., Ma, Y., and J. Salowey, "Transport Layer
Security (TLS) Transport Mapping for Syslog", RFC 5425,
March 2009.
[RFC5426] Okmianski, A., "Transmission of Syslog Messages over UDP",
RFC 5426, March 2009.
8.2. Informative
[RFC3164] Lonvick, C., "The BSD Syslog Protocol", RFC 3164,
August 2001.
Appendix A. Applicability to Legacy syslog
This is an informative appendix provided to promote interoperability
within the various observed implementations. Even though this
specification does not cover legacy syslog messages, the language
used here will be consistent with [RFC2119] to be clear in this and
to show how the new syslog architecture will interoperate with the
legacy implementations.
Syslog over TCP has been around for a number of years. Just like
legacy syslog over UDP, several different implementations exist. The
older method of octet-stuffing has problems so is NOT RECOMMENDED,
but SHOULD be implemented to ensure interoperability with older
clients or servers that may only use this method. The newer method
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of octet-counting is reliable and, as is consistent with this
specification, SHOULD be implemented. When implementers do implement
both methods, it is RECOMMENDED that the default method be octet-
counting.
A.1. Method Change
It has been observed in legacy implementations that the framing may
change on a frame-by-frame basis. This behavior is NOT RECOMMENDED.
However, for interoperability, a transport receiver wishing to
interoperate with these legacy systems SHOULD be prepared to accept
different framing for each frame received.
A.2. Octet-Counting
This framing allows for the transmission of all characters inside
SYSLOG-MSG. Along these lines, some transport senders have been seen
to use this framing to stack multiple messages within a single TCP
frame by using a USASCII NL (%d10) character to separate them. As an
example, two messages may be placed within a single frame with a NL
character between them. This behavior is NOT RECOMMENDED although it
has been observed. Since it cannot be known in advance if the legacy
transport sender is going to send multiple messages within single
frames, it MUST be assumed that they WILL NOT, and that each frame
using this method contains only one syslog message. Implementers MAY
attempt to address this by looking for TRAILER characters within each
frame to try to separate multiple messages. However, this may lead
to more problems than it resolves.
A transport receiver MUST assume that octet-counting framing is used
if a syslog frame starts with a digit.
A.3. Octet Stuffing
The problem with octet-stuffing framing comes from the use of
[RFC3164] messages. In that, the traditional trailer character is
not escaped within SYSLOG-MSG which causes problems for the receiver.
For example, a message in the style of [RFC3164] containing one or
more LF characters may be misinterpreted as multiple messages by the
transport receiver. There is no method to avoid this problem with
the octet-stuffing framing.
In this legacy implementation, the TRAILER consists of a single
character and most often is the USASCII LF (%d10) character.
However, other characters have also been seen occasionally, with
USACII NUL (%d00) being a prominent example. Some devices also emit
a two-character TRAILER, which is usually CR and LF.
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Transport senders MUST support the option to use the USASCII LF
character. Transport receivers MUST also support this. Transport
senders and receivers MAY also support other characters.
Authors' Addresses
Rainer Gerhards
Adiscon GmbH
Mozartstrasse 21
Grossrinderfeld, BW 97950
Germany
Email: rgerhards@adiscon.com
Chris Lonvick
Cisco Systems, Inc
12515 Research Blvd.
Austin, TX 78759
USA
Email: clonvick@cisco.com
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