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IETF RFC 5426
Transmission of Syslog Messages over UDP
Last modified on Tuesday, March 3rd, 2009
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Network Working Group A. Okmianski
Request for Comments: 5426 Cisco Systems, Inc.
Category: Standards Track March 2009
Transmission of Syslog Messages over UDP
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
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Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
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material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
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not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Abstract
This document describes the transport for syslog messages over UDP/
IPv4 or UDP/IPv6. The syslog protocol layered architecture provides
for support of any number of transport mappings. However, for
interoperability purposes, syslog protocol implementers are required
to support this transport mapping.
Okmianski Standards Track PAGE 1 
RFC 5426 Syslog UDP Transport March 2009
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
3. Transport Protocol ..............................................3
3.1. One Message Per Datagram ...................................3
3.2. Message Size ...............................................3
3.3. Source and Target Ports ....................................4
3.4. Source IP Address ..........................................4
3.5. UDP/IP Structure ...........................................4
3.6. UDP Checksums ..............................................4
4. Reliability Considerations ......................................5
4.1. Lost Datagrams .............................................5
4.2. Message Corruption .........................................5
4.3. Congestion Control .........................................5
4.4. Sequenced Delivery .........................................5
5. Security Considerations .........................................6
5.1. Sender Authentication and Message Forgery ..................6
5.2. Message Observation ........................................7
5.3. Replaying ..................................................7
5.4. Unreliable Delivery ........................................7
5.5. Message Prioritization and Differentiation .................7
5.6. Denial of Service ..........................................8
6. IANA Considerations .............................................8
7. Acknowledgements ................................................8
8. References ......................................................8
8.1. Normative References .......................................8
8.2. Informative References .....................................9
1. Introduction
Informational RFC 3164 [8] describes the syslog protocol as it was
observed in existing implementations. It describes both the format
of syslog messages and a UDP [1] transport. Subsequently, a
Standards-Track syslog protocol has been defined in RFC 5424 [2].
RFC 5424 specifies a layered architecture that provides for support
of any number of transport layer mappings for transmitting syslog
messages. This document describes the UDP transport mapping for the
syslog protocol.
The transport described in this document can be used for transmitting
syslog messages over both IPv4 [3] and IPv6 [4].
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RFC 5426 Syslog UDP Transport March 2009
Network administrators and architects should be aware of the
significant reliability and security issues of this transport, which
stem from the use of UDP. They are documented in this specification.
However, this transport is lightweight and is built upon the existing
popular use of UDP for syslog.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [5].
3. Transport Protocol
3.1. One Message Per Datagram
Each syslog UDP datagram MUST contain only one syslog message, which
MAY be complete or truncated. The message MUST be formatted and
truncated according to RFC 5424 [2]. Additional data MUST NOT be
present in the datagram payload.
3.2. Message Size
This transport mapping supports transmission of syslog messages up to
65535 octets minus the UDP header length. This limit stems from the
maximum supported UDP size of 65535 octets specified in RFC 768 [1].
For IPv4, the maximum payload size is 65535 octets minus the UDP
header and minus the IP header because IPv4 has a 16-bit length field
that also includes the header length.
IPv4 syslog receivers MUST be able to receive datagrams with message
sizes up to and including 480 octets. IPv6 syslog receivers MUST be
able to receive datagrams with message sizes up to and including 1180
octets. All syslog receivers SHOULD be able to receive datagrams
with message sizes of up to and including 2048 octets. The ability
to receive larger messages is encouraged.
The above restrictions and recommendations establish a baseline for
interoperability. The minimum required message size support was
determined based on the minimum MTU size that Internet hosts are
required to support: 576 octets for IPv4 [3] and 1280 octets for IPv6
[4]. Datagrams that conform to these limits have the greatest chance
of being delivered because they do not require fragmentation.
It is RECOMMENDED that syslog senders restrict message sizes such
that IP datagrams do not exceed the smallest MTU of the network in
use. This avoids datagram fragmentation and possible issues
surrounding fragmentation such as incorrect MTU discovery.
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RFC 5426 Syslog UDP Transport March 2009
Fragmentation can be undesirable because it increases the risk of the
message being lost due to loss of just one datagram fragment. Syslog
has no acknowledgement facility, and therefore there is no effective
way to handle retransmission. This makes it impossible for syslog to
utilize packetization layer path MTU discovery [9]. When network MTU
is not known in advance, the safest assumption is to restrict
messages to 480 octets for IPv4 and 1180 octets for IPv6.
3.3. Source and Target Ports
Syslog receivers MUST support accepting syslog datagrams on the well-
known UDP port 514, but MAY be configurable to listen on a different
port. Syslog senders MUST support sending syslog message datagrams
to the UDP port 514, but MAY be configurable to send messages to a
different port. Syslog senders MAY use any source UDP port for
transmitting messages.
3.4. Source IP Address
The source IP address of the UDP datagrams SHOULD NOT be interpreted
as the identifier for the host that originated the syslog message.
The entity sending the syslog message could be merely a relay. The
syslog message itself contains the identifier of the originator of
the message.
3.5. UDP/IP Structure
Each UDP/IP datagram sent by the transport layer MUST completely
adhere to the structure specified in the UDP RFC 768 [1] and either
the IPv4 RFC 791 [3] or IPv6 RFC 2460 [4], depending on which
protocol is used.
3.6. UDP Checksums
Syslog senders MUST NOT disable UDP checksums. IPv4 syslog senders
SHOULD use UDP checksums when sending messages. Note that RFC 2460
[4] mandates the use of UDP checksums when sending UDP datagrams over
IPv6.
Syslog receivers MUST NOT disable UDP checksum checks. IPv4 syslog
receivers SHOULD check UDP checksums and SHOULD accept a syslog
message with a zero checksum. Note that RFC 2460 [4] mandates the
use of checksums for UDP over IPv6.
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RFC 5426 Syslog UDP Transport March 2009
4. Reliability Considerations
The UDP is an unreliable, low-overhead protocol. This section
discusses reliability issues inherent in UDP that implementers and
users should be aware of.
4.1. Lost Datagrams
This transport mapping does not provide any mechanism to detect and
correct loss of datagrams. Datagrams can be lost in transit due to
congestion, corruption, or any other intermittent network problem.
IP fragmentation exacerbates this problem because loss of a single
fragment will result in the entire message being discarded.
4.2. Message Corruption
The UDP/IP datagrams can get corrupted in transit due to software,
hardware, or network errors. This transport mapping specifies use of
UDP checksums to enable corruption detection in addition to checksums
used in IP and Layer 2 protocols. However, checksums do not
guarantee corruption detection, and this transport mapping does not
provide for message acknowledgement or retransmission mechanism.
4.3. Congestion Control
Because syslog can generate unlimited amounts of data, transferring
this data over UDP is generally problematic, because UDP lacks
congestion control mechanisms. Congestion control mechanisms that
respond to congestion by reducing traffic rates and establish a
degree of fairness between flows that share the same path are vital
to the stable operation of the Internet [6]. This is why the syslog
TLS transport [7] is REQUIRED to implement and RECOMMENDED for
general use.
The only environments where the syslog UDP transport MAY be used as
an alternative to the TLS transport are managed networks, where the
network path has been explicitly provisioned for UDP syslog traffic
through traffic engineering mechanisms, such as rate limiting or
capacity reservations. In all other environments, the TLS transport
[7] SHOULD be used.
4.4. Sequenced Delivery
The IP transport used by the UDP does not guarantee that the sequence
of datagram delivery will match the order in which the datagrams were
sent. The time stamp contained within each syslog message can serve
as a rough guide in establishing sequence order. However, it will
not help in cases where multiple messages were generated during the
Okmianski Standards Track PAGE 5
RFC 5426 Syslog UDP Transport March 2009
same time slot, the sender could not generate a time stamp, or
messages originated from different hosts whose clocks were not
synchronized. The order of syslog message arrival via this transport
SHOULD NOT be used as an authoritative guide in establishing an
absolute or relative sequence of events on the syslog sender hosts.
5. Security Considerations
Using this specification on an unsecured network is NOT RECOMMENDED.
Several syslog security considerations are discussed in RFC 5424 [2].
This section focuses on security considerations specific to the
syslog transport over UDP. Some of the security issues raised in
this section can be mitigated through the use of IPsec as defined in
RFC 4301 [10].
5.1. Sender Authentication and Message Forgery
This transport mapping does not provide for strong 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
also lead to a case of mistaken identity if an inappropriately
configured machine sends syslog messages to a receiver 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.
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RFC 5426 Syslog UDP Transport March 2009
5.2. 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.
5.3. 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.
5.4. Unreliable Delivery
As was previously discussed in Section 4, Reliability Considerations,
the UDP transport is not reliable, and packets containing syslog
message datagrams can be lost in transit without any notice. There
can be security consequences to the loss of one or more syslog
messages. Administrators could be unaware of a developing and
potentially serious problem. Messages could also be intercepted and
discarded by an attacker as a way to hide unauthorized activities.
5.5. 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.
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RFC 5426 Syslog UDP Transport March 2009
5.6. 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.
6. IANA Considerations
This transport uses UDP port 514 for syslog, as recorded in the IANA
port-numbers registry.
7. Acknowledgements
The author gratefully acknowledges the contributions of: Chris
Lonvick, Rainer Gerhards, David Harrington, Andrew Ross, Albert
Mietus, Bernie Volz, Mickael Graham, Greg Morris, Alexandra Fedorova,
Devin Kowatch, Richard Graveman, and all others who have commented on
the various versions of this proposal.
8. References
8.1. Normative References
[1] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
1980.
[2] Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.
[3] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981.
[4] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Floyd, S., "Congestion Control Principles", BCP 41, RFC 2914,
September 2000.
[7] Miao, F. and Y. Ma, "TLS Transport Mapping for Syslog", RFC
5425, March 2009.
Okmianski Standards Track PAGE 8
RFC 5426 Syslog UDP Transport March 2009
8.2. Informative References
[8] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001.
[9] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[10] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.
Author's Address
Anton Okmianski
Cisco Systems, Inc.
595 Burrard St., Suite 2123
Vancouver, BC V7X 1J1
Canada
Phone: +1-978-936-1612
EMail: aokmians@cisco.com
Okmianski Standards Track PAGE 9
Transmission of Syslog Messages over UDP
RFC TOTAL SIZE: 19354 bytes
PUBLICATION DATE: Tuesday, March 3rd, 2009
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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