syslog Working Group A. Okmianski
Internet-Draft Cisco Systems, Inc.
Expires: October 30, 2004 May 2004
Transmission of syslog messages over UDP
draft-ietf-syslog-transport-udp-02
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document describes the transport for syslog messages over UDP/
IPv4 or UDP/IPv6. While several transport mappings are envisioned
for the syslog protocol, syslog protocol implementors are required to
support the transport mapping described in this document. This
transport specification overcomes limitations of UDP/IP datagram size
by introducing support for fragmentation of large messages.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Transport Protocol Overview . . . . . . . . . . . . . . . . . 4
2.1 Definitions and Architecture . . . . . . . . . . . . . . . 4
2.2 Required Transport Protocol . . . . . . . . . . . . . . . 5
2.3 Encapsulation Layers . . . . . . . . . . . . . . . . . . . 5
3. Message Format . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Basic Header Format . . . . . . . . . . . . . . . . . . . 6
3.2 Extended Header Format . . . . . . . . . . . . . . . . . . 7
3.2.1 Message Identifier . . . . . . . . . . . . . . . . . . 7
3.2.2 Total Length . . . . . . . . . . . . . . . . . . . . . 8
3.2.3 Fragment Offset . . . . . . . . . . . . . . . . . . . 8
3.2.4 Extended Header Example . . . . . . . . . . . . . . . 8
3.3 Payload . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4 Supported Message Length . . . . . . . . . . . . . . . . . 9
4. UDP/IP Layer Considerations . . . . . . . . . . . . . . . . . 10
4.1 Target Port . . . . . . . . . . . . . . . . . . . . . . . 10
4.2 Source Port . . . . . . . . . . . . . . . . . . . . . . . 10
4.3 Source IP Address . . . . . . . . . . . . . . . . . . . . 10
4.4 UDP/IP Headers . . . . . . . . . . . . . . . . . . . . . . 10
5. Fragmentation and Reassembly . . . . . . . . . . . . . . . . . 11
5.1 Message Fragmentation . . . . . . . . . . . . . . . . . . 11
5.2 Message Reassembly . . . . . . . . . . . . . . . . . . . . 12
5.3 Avoiding Fragmentation . . . . . . . . . . . . . . . . . . 12
6. Reliability Considerations . . . . . . . . . . . . . . . . . . 13
6.1 Lost Datagrams . . . . . . . . . . . . . . . . . . . . . . 13
6.2 Message Corruption and Checksums . . . . . . . . . . . . . 13
6.3 Congestion Control . . . . . . . . . . . . . . . . . . . . 13
6.4 Sequenced Delivery . . . . . . . . . . . . . . . . . . . . 13
6.5 Sender Authentication . . . . . . . . . . . . . . . . . . 14
7. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7.1 Message Authenticity . . . . . . . . . . . . . . . . . . . 14
7.2 Message Forgery . . . . . . . . . . . . . . . . . . . . . 14
7.3 Message Observation . . . . . . . . . . . . . . . . . . . 15
7.4 Replaying . . . . . . . . . . . . . . . . . . . . . . . . 15
7.5 Unreliable Delivery . . . . . . . . . . . . . . . . . . . 15
7.6 Message Prioritization and Differentiation . . . . . . . . 15
7.7 Denial of Service . . . . . . . . . . . . . . . . . . . . 16
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
9. Notice to RFC Editor . . . . . . . . . . . . . . . . . . . . . 16
10. Working Group . . . . . . . . . . . . . . . . . . . . . . . 16
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1 Normative References . . . . . . . . . . . . . . . . . . . . 17
12.2 Informative References . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 18
A. Rational For Transport Message Size Restrictions . . . . . . . 18
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Intellectual Property and Copyright Statements . . . . . . . . 20
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1. Introduction
The original syslog protocol has been described in informational RFC
3164[1] as observed in existing implementations. It describes both
the semantics of the syslog message format as well as a UDP
transport. Subsequently, the syslog protocol has been formally
defined in a standards track RFC-protocol[2].
The RFC-protocol[2] has provided for support of any number of
transport layer protocols for transmitting syslog messages and left
it to subsequent RFCs to specify transport protocols. This standards
track RFC describes the UDP transport for the syslog protocol. This
transport protocol is REQUIRED for all syslog protocol
implementations.
This transport protocol was designed to work on top of UDP [3] over
both IPv4 [4] and IPv6 [5]. This protocol overcomes the data size
restrictions of the UDP protocol by supporting message fragmentation.
Support for fragmentation is only REQUIRED for implementations
wishing to support messages which exceed certain size limits outline
in this specification.
This protocol has significant reliability and security issues
stemming from the use of UDP. They are documented in this
specification. This protocol also does not support acknowledgements
of message receipt by the receiver and does not incorporate any
reliable retransmission mechanism for lost datagrams. However, this
protocol is lightweight and extends on the existing popular use of
UDP for syslog. Network administrators and architects should be
aware of the shortcomings of this protocol and plan accordingly.
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[6]. The
words 'byte' and 'octet' are used interchangeably in this
specification.
2. Transport Protocol Overview
2.1 Definitions and Architecture
The following definitions will be used in this document:
o An application that can generate syslog messages will be referred
to as a "sender";
o An application that can receive syslog messages will be referred
to as a "receiver".
An application can function in dual capacity. For example, a syslog
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relay may receive and forward messages. A single system can host any
number of syslog senders. Only one syslog receiver can be hosted on
a single system using the standard listening port.
2.2 Required Transport Protocol
This document describes the UDP transport layer protocol for the
syslog protocol RFC-protocol[2]. Every syslog sender and receiver
implementation which adheres to the RFC-protocol[2] MUST fully
implement the transport protocol specified in this document. An
implementation does not have to support both IPv4 and IPv6 if it is
designed to be used over only one of these protocols.
2.3 Encapsulation Layers
This syslog transport carries syslog messages as a generic payload
encapsulated with a syslog transport header, UDP header and an IP
header. Below is a summary of syslog UDP/IP packet structure as used
by this transport protocol:
+--------------------------------+
| IPv4 or IPv6 Header |
| (20 or more bytes) |
+--------------------------------+
| UDP Header |
| (8 bytes) |
+--------------------------------+
| Syslog Transport Header |
| (5 or 32 bytes) |
+--------------------------------+
| Syslog Message Payload |
| (1 to 1191 bytes) |
+--------------------------------+
Some syslog messages may be transmitted using one UDP/IP datagram per
message. Syslog protocol [2] allows messages as large as 16777216
bytes, while UDP/IP datagram cannot exceed a total size of 65526
bytes [3] and most existing protocols restrict the size of UDP data
to much less. In order to support transmitting large messages over
UDP/IP, this transport protocol supports fragmentation of large
syslog messages into multiple UDP/IP datagrams for transmission and
reassembly on the receiving end.
Each syslog UDP/IP datagram MUST contain one and only one complete
syslog message or one fragment of a message. Transmitting multiple
messages or multiple fragments of different messages in a single UDP
datagram is not supported by this protocol.
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3. Message Format
The syslog transport message consists of a transport header and a
syslog message payload. The format of the transport header is
different for fragmented and non-fragmented messages. The basic
transport header format is used for non-fragmented messages and the
extended transport header format is used for fragmented messages.
The receiver MUST discard messages with incorrectly formatted
headers.
An ASCII-based encoding was chosen for the syslog transport for
consistency with the RFC-protocol[2]. Syslog transport messages have
the following format in ABNF[7] notation:
SyslogTransportMessage = ( BasicHeader / ExtendedHeader )
SP Payload
BasicHeader = Version SP "0"
Version = %d118 1*3DIGIT ; "v1" in this version
ExtendedHeader = Version SP "1" SP MessageId
SP TotalLength SP FragmentOffset
MessageId = 1*8DIGIT ; 0 to 16777215
TotalLength = 1*8DIGIT ; 1 to 16777216
FragmentOffset = 1*8DIGIT ; 0 to 16777215
Payload = 1*1191OCTET
OCTET = %d00-255
DIGIT = %d48-57
SP = %d32
3.1 Basic Header Format
When no fragmentation is used and the entire syslog message is
transferred as a single UDP/IP datagram, a basic syslog transport
header MUST be used. The version for this protocol is "1". It must
be followed by one space, a "0" to indicate that this is a basic
header and a trailing space. Therefore, the only possible value for
the basic header in this protocol is as follows:
"v1 0 "
Example of a syslog message without the transport header (message is
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wrapped for display):
"v1 888 3 2003-10-11T22:14:15.003Z host.domain.com
dns: configuration error"
Example of the same message with the transport header (message is
wrapped for display):
"v1 0 v1 888 4 2003-10-11T22:14:15.003Z host.domain.com
dns: configuration error"
3.2 Extended Header Format
When a syslog message is fragmented by the sender, multiple UDP
datagrams MUST be used and each datagram MUST contain an extended
syslog transport header. The version for this protocol is "1". The
version field MUST be followed by a single space and a "1" to
indicate that this is an extended header. Thus, an extended header
MUST always begin with "v1 1 ", but MUST also have additional fields
which aid in reassembly.
The MessageId, TotalLength and FragmentOffset fields are used solely
for fragmentation of long messages and reassembly. They MUST NOT be
used for other purposes.
3.2.1 Message Identifier
The MessageId field (along with the source UDP port and the IP
address) is used to identify the message such that fragments of a
single syslog message can be reassembled by the receiver into a
complete message. The MessageId field MUST be a numeric value in the
range of 0 to 16777215. Leading zeros MUST NOT be present in the
MessageId field.
Each syslog sender process MUST choose a random MessageId value
within the supported range for its first message. The sender SHOULD
increment the MessageId by 1 up to 16777215 for each subsequent
message and then continue at 0. Incrementing the value each time
ensures that MessageId is unique and does not repeat over a long
range of values. Using random value for the first MessageId helps
reduce the possibility of potential errors in message reassembly.
Refer to discussion about message reassembly (Section 5.2) for more
details.
All datagrams which represent parts of a given fragmented syslog
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message MUST have the same MessageId value.
3.2.2 Total Length
The TotalLength field MUST be a numeric value in the range of 1 to
16777216. It MUST indicate the length of a complete syslog message
in bytes before it was fragmented and before it was encapsulated with
transport headers. The same TotalLength field value MUST be present
in all UDP datagrams which represent fragments of the same syslog
message. Leading zeros MUST not be present in the TotalLength field.
Note that the TotalLength field is used to identify the total length
of a complete syslog message, which is transmitted using multiple
fragments and multiple datagram packets. The fragment length is not
specified in the transport header because it can be inferred from the
size of the IP packet containing the UDP datagram.
3.2.3 Fragment Offset
The FragmentOffset field MUST be a numeric value in the range of 0 to
16777215. It MUST indicate the byte offset of the fragment data in
the complete syslog message. The offset index starts at 0 for the
first fragment. For example, suppose we want to fragment a 700 byte
syslog message into 480 and 220 byte parts. Then, the FragmentOffset
in the first message will be 0 and in the second - 480. Note that
fragments don't have to be the same size. Leading zeros MUST not be
present in the FragmentOffset field.
3.2.4 Extended Header Example
The following is an example of a syslog message without the transport
header (message is wrapped for display):
"v1 888 4 2003-10-11T22:14:15.003Z host.domain.com
dns: configuration error"
Suppose this message had to be fragmented by transport layer into two
parts at an arbitrary point. This would result in two separate UDP
datagrams being sent - one for each fragment. Below is the content
of each of the syslog transport UDP messages with syslog transport
headers but without UDP/IP headers:
"v1 1 45612221 74 0 v1 888 4 2003-10-11T22:14:15.003Z host.dom"
"v1 1 45612221 74 42 ain.com dns: configuration error"
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In the above example, the leading "v1" is the version of the
transport protocol, "1" indicates that this is an extended header
(fragmentation in use), "45612221" is the MessageId, "74" is the
TotalLength of the message, while "0" and "42" are FragmentOffset
fields. Everything following the FragmentOffset and a space is the
Payload of each respective message.
3.3 Payload
The Payload field of the syslog transport message is an entire syslog
message or one fragment. The maximum Payload size depends on the IP
protocol used and the type header that is used.
Maximum Payload size:
With IPv4 and basic header: 507 bytes
With IPv4 and extended header: 480 bytes
With IPv6 and basic header: 1191 bytes
With IPv6 and extended header: 1164 bytes
The receiver MUST discard messages with Payload sizes exceeding the
above restrictions. The Payload size restrictions above effectively
mean that the largest syslog message that can be sent non-fragmented
is 507 bytes for transport via IPv4 and 1191 bytes for transport via
IPv6.
For a discussion of the rational behind the above size restrictions
please refer to Appendix A.
3.4 Supported Message Length
The maximum syslog message length supported by this protocol is the
maximum value of the TotalLength field, which is 16777216 bytes.
However, not all deployment scenarios for syslog will be on hosts
with hardware capable of supporting this maximum length of messages.
Additionally, extremely large messages may not be needed in many
environments. Therefore, implementations are NOT REQUIRED to support
the maximum message length allowed by this protocol.
All implementations MUST support sending and receiving syslog
messages up to and including the size which does not require
fragmentation (507 bytes for IPv4 and 1191 bytes for IPv6). This
size excludes the overhead of the syslog transport and UDP/IP
headers. Support for larger messages is encouraged. Implementors
SHOULD clearly state the maximum supported message size in
documentation.
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The receiver which receives a message greater than it can handle
SHOULD discard the message. A diagnostic message MAY be logged by
the receiver, but care SHOULD be taken not to expose this behavior as
an additional vulnerability for denial of service attack.
4. UDP/IP Layer Considerations
4.1 Target Port
Syslog receivers MUST support accepting syslog message datagrams on
the well-known UDP port 514. Syslog senders MUST support sending
syslog message datagrams to the UDP port 514.
4.2 Source Port
Syslog senders can use any source UDP port for transmitting messages.
Senders MAY randomly select a source port, but MUST use the port in
an exclusive fashion. No concurrent port reuse on the same host is
allowed.
Since source port is used to identify parts of a fragmented message,
the sender MUST use the same port to send all fragments of a given
message. If due to an error or other condition, the sender is unable
to do that, the sender MAY resend all message fragments and if it
does so, it MUST use the new port and a new MessageId field value.
4.3 Source IP Address
The source IP address of the UDP datagrams is one of the data
elements used to identify parts of a fragmented message. Therefore,
a syslog sender MUST attempt to use the same source IP address to
send all fragments of a given syslog message. If due to an error,
reconfiguration or other condition it is unable to do so, the sender
MAY resend all fragments of the syslog message and, if it does so, it
MUST use the new source IP address and a new MessageId value.
4.4 UDP/IP Headers
Each UDP/IP datagram sent by the transport layer MUST completely
adhere to the structure specified in the UDP RFC 768[3] and either
IPv4 RFC 791[4] or IPv6 RFC 2640[5] depending on which protocol is
used.
Use of UDP checksums was defined as optional in RFC 768[3]. IPv6 has
subsequently made UDP checksums required [5]. Syslog senders MUST
utilize valid UDP checksums when sending messages over IPv6 and
SHOULD do it when sending over IPv4. Syslog receivers MUST check the
checksums whenever they are present and discard messages with
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incorrect checksums. Note that this is typically accomplished by the
UDP layer implementation, and some implementations allow for checksum
checks to be enabled or disabled.
Enabling use of checksums serves as an extra measure of corruption
detection in addition to checksums performed by IP and Layer 2
protocols such as Ethernet. None of the above checksums provide a
complete guarantee of corruption detection. Utilizing checksums on
multiple layers reduces the chance of a corruption error not being
detected.
5. Fragmentation and Reassembly
5.1 Message Fragmentation
The syslog transport layer MUST perform fragmentation if the size of
a given syslog message exceeds the maximum allowed Payload size.
Fragmentation SHOULD NOT be used if message can fit into the maximum
allowed Payload size.
Syslog messages SHOULD be fragmented such that all but last message
utilize the Payload to its maximum capacity. For example, when using
IPv4, a 700 byte syslog message SHOULD be fragmented into 480 and 220
byte parts because the maximum Payload size with IPv4 and extended
header is 480 bytes.
Each message fragment MUST be sent as a separate UDP/IP datagram with
an extended syslog transport header. The sender MUST use the same
MessageId value, TotalLength value, source port and source IP address
for all fragments of a given message. These three field together
uniquely identify fragments belonging to a given message.
On a system with short-lived sender processes, it may be possible
that fragments with the same MessageId value, TotalLength value,
source port and source IP address will get generated in short time
proximity. This can be possible because a new process may re-use the
source port that was freed up by another process that just dies.
Such behavior could confuse the receiver if the datagrams were
received out of order or some datagrams got lost.
In order to reduce the risk of such mistaken identity errors, section
3.2.1 specified that each process must start with a random value for
MessageId field. Given a relatively large range of MessageId values
and the unlikely event of a coincidence of having the same MessageId
and TotalLength values combined with re-used source port and UDP
errors, the window for potential mistaken identity errors during
message reassembly is very small and tolerable. The users take a
greater risk by using this protocol due to general UDP reliability
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issues discussed later in this specification.
5.2 Message Reassembly
The reassembly process uses the source IP address from the IP header,
the source port from the UDP header, the MessageId and TotalLength
field values to identify fragments of a given message. It then uses
data from the TotalLength and FragmentOffset fields to re-assemble
fragments into a complete message. If one of the fragments of the
message is not received, all other fragments of the message SHOULD be
discarded.
Typically, an implementation of fragmentation reassembly involves
allocating a buffer for the message when any fragment with a new
combination of source IP address, source port, MessageId and
TotalLength values is received. A timer is used to expire the
message reassembly and clean the buffer if all fragments are not
received within a certain time period. As each fragment is received,
it is placed into the buffer at the appropriate offset and a check is
performed to determine if all fragments have been received using
additional data structures.
The receiver SHOULD make the timeout interval used for message
reassembly configurable for the administrator. The receiver SHOULD
also be able to limit the total amount of memory used for buffers
such that it does not run out of resources under a simple denial of
service attack involving just one message fragment with a large
TotalLength field value. Degrading the service under heavy load or
attack is better than crashing and potentially making the service
completely unavailable.
The receiver MUST validate the FragmentOffset and fragment length
against the TotalLength of the message to ensure that the fragment
fits into the buffer. This would prevent a typical buffer overflow
exploit by attackers.
5.3 Avoiding Fragmentation
Fragmentation and reassembly of messages incurs substantial
processing overhead on both the sender and the receiver hosts. It
also increases the risk of lost messages due to loss of just one
fragment. It is RECOMMENDED that syslog senders which anticipate
sending messages over this transport protocol attempt to reduce the
number of messages which require fragmentation by only sending
messages which are small enough not to require fragmentation.
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6. Reliability Considerations
The UDP is an unreliable low-overhead protocol. This section
discusses reliability issues inherent to UDP that implementers and
users MUST be aware of.
6.1 Lost Datagrams
This transport protocol does not provide any mechanism to detect and
correct loss of datagrams. Datagrams may be lost in transit due to
congestion, corruption or any other intermittent network problem.
The transport protocol fragmentation and IP fragmentation exacerbate
the problem because loss of a single fragment will result in the
entire message being discarded.
In some circumstances the sender may receive an ICMP error message or
other indication of a transmission problem. If the sender receives a
reasonable indication that some datagram may have been lost, it MAY
retransmit previously sent messages by either retransmitting the
datagram(s) or by transmitting the message with a new MessageId
value.
6.2 Message Corruption and Checksums
The UDP/IP datagrams may get corrupted in transit due to software,
hardware or network errors. This protocol specifies use of UDP
checksums to enable corruption detection in addition to checksums
utilized in IP and Layer 2 layers. However, checksums do not
guarantee corruption detection and this protocol does not provide for
message retransmission when a corrupt message is detected.
6.3 Congestion Control
The UDP does not provide for congestion control. Some systems (hosts
or routers) may generate ICMP source quench error, but they are not
required to do so [9]. Any network host can discard UDP packets when
it is overloaded. Due to lack of congestion control one or multiple
syslog senders can maliciously or inadvertently overload the receiver
or the network infrastructure and cause loss of syslog messages.
6.4 Sequenced Delivery
The IP transport utilized 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 may serve as some guide in establishing sequence order, but
it will not help in cases when multiple messages were generated
during the same time slot or when messages originated from different
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hosts whose clocks are not synchronized. The order of syslog message
arrival via the this syslog transport SHOULD NOT be used as an
authoritative guide in establishing the sequence of events on the
syslog sender hosts.
6.5 Sender Authentication
The UDP syslog transport does not strongly associate the message with
the message sender. The receiver of the syslog message will not be
able to ascertain that the message was indeed sent from the reported
sender, or if the packet was sent from another device.
One possible consequence of this behavior is that a misconfigured
machine may send syslog messages to a receiver representing itself as
another machine. The administrators may not be able to readily
discern that there are two or more machines representing themselves
as the same machine.
7. Security Considerations
Several syslog security considerations have been discussed in
RFC-protocol[2] and in the original RFC 3164[1]. This section
focuses on security considerations specific to the syslog transport
over UDP.
7.1 Message Authenticity
This transport protocol does not strongly authenticate the identity
of the message sender and does not provide any assurance that the
message was not modified in transit. The receiver of the syslog
message will not be able to ascertain that the message was indeed
sent from the reported sender, or if the packet was sent from another
device.
7.2 Message Forgery
Syslog messages can be easily forged. An attacker may 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 may hide the true nature of an attack amidst
many other messages. As an example, an attacker may start generating
forged messages indicating a problem on some machine. This may get
the attention of the system administrators who will spend their time
investigating the alleged problem. During this time, the attacker
may be able to compromise a different machine, or a different process
on the same machine.
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Additionally, an attacker may generate false syslog messages to give
untrue indications of status of systems. As an example, an attacker
may stop a critical process on a machine, which may generate a
notification of exit. The attacker may subsequently generate a
forged notification that the process had been restarted. The system
administrators may accept that misinformation and not verify that the
process had indeed been restarted.
7.3 Message Observation
The transport protocol 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 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. It is RECOMMENDED that no sensitive information
be transmitted via this transport protocol or that transmission of
such information be restricted to properly secured networks.
7.4 Replaying
Message forgery and observation can be combined into a replay attack.
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 with new time stamps. The administrators may find nothing
unusual in the received messages and their receipt would falsely
indicate normal activity of the machine.
7.5 Unreliable Delivery
As was previously discussed in the Reliability Considerations
section, 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 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.
7.6 Message Prioritization and Differentiation
The transport protocol described in this document does not require
prioritization of syslog messages on the wire or when processed on
the receiving host based on their severity. The security implication
of such behavior is that the syslog receiver or network devices may
get overwhelmed with low severity messages and be forced to discard
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potentially high severity messages. High severity messages may
contain an indication of serious security problems, but they will not
get a higher priority. It is difficult to make sure that high
severity messages get higher end-to-end delivery priority, especially
over an unreliable UDP transport which provides no congestion
control.
7.7 Denial of Service
An attacker may overwhelm a receiver by sending more messages to it
than can be handled by the infrastructure or the device itself.
Implementers SHOULD attempt to provide features that minimize this
threat such as only receiving syslog messages from known IP
addresses.
8. IANA Considerations
IANA must reserve UDP port 514 for this transport.
9. Notice to RFC Editor
This is a notice to the RFC editor. This ID is submitted along with
ID draft-ietf-syslog-protocol and they cross-reference each other.
When RFC numbers are determined for each of these IDs, please replace
all references to "RFC-protocol" with the RFC number of
draft-ietf-syslog-protocol ID. Please remove this section after
editing.
10. Working Group
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
11. 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 and all others who have commented on the various
versions of this proposal.
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12. References
12.1 Normative References
[1] Lonvick, C., "The BSD Syslog Protocol", RFC 3164, August 2001.
[2] Gerhards, R., "The syslog Protocol", RFC RFC-protocol.
[3] Postel, J., "User Datagram Protocol", STD 6, RFC 768, August
1980.
[4] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[5] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[7] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[8] Braden, R., "Requirements for Internet Hosts - Communication
Layers", STD 3, RFC 1122, October 1989.
12.2 Informative References
[9] Stevens, W., "TCP/IP Illustrated Volume 1. The Protocols.",
January 1994.
[10] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[11] Hedrick, C., "Routing Information Protocol", RFC 1058, June
1988.
[12] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[13] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC
1350, July 1992.
[14] Kent, C. and J. Mogul, ""Fragmentation Considered Harmful,"
Computer Communications Review, vol.17, no.5, pp.390-401",
August 1987.
[15] Nichols, K., Blake, S., Baker, F. and D. Black, "Definition of
the Differentiated Services Field (DS Field) in the IPv4 and
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IPv6 Headers", RFC 2474, December 1998.
Author's Address
Anton Okmianski
Cisco Systems, Inc.
1414 Massachusetts Ave
Boxborough, MA 01719-2205
USA
Phone: +1-978-936-1612
EMail: aokmians@cisco.com
Appendix A. Rational For Transport Message Size Restrictions
This appendix provides the rational behind the Payload size
restrictions for this protocol. The Payload restrictions outlined in
the specification, ensure that the transport message size does not
exceed 512 bytes (without UDP/IP headers) for transport via IPv4 and
does not exceed 1196 bytes for transport via IPv6. These
restrictions put an upper boundary on the UDP/IP datagram size for
this protocol, which accomplishes two goals.
First, they insure interoperability between various UDP/IP
implementations. Even though the maximum IP datagram size is
specified as 65536 bytes, many UDP/IP implementations have been shown
not to work with large datagram sizes [9]. Many established
UDP-based protocols restrict UDP datagram data size to 512 bytes.
For example, DNS [10] and RIP [11] do that. The DHCPv4 [12]
restricts the size to 512 bytes, but allows sides to agree on a
larger value through the protocol. The TFTP [13] restricts the UDP
data size to 518 bytes, which is slightly larger.
The second reason for datagram size restrictions is that it reduces
the likelihood of the IP-layer datagram fragmentation. Syslog
message can be fragmented on two levels: syslog transport protocol
and IP layer. Since fragmentation has significant overhead for
message reassembly, it is best to avoid double fragmentation. The
likelihood of IP fragmentation can be significantly reduced by
sending IP datagrams in sizes which all hosts must be able to
process.
The minimum MTU of a transport protocol determines the minimum size
of packets that hosts must be able to accept. For IPv4, the minimum
MTU is 576 bytes [4] and for IPv6 - 1280 bytes [5]. In both cases,
the maximum message size fits within the MTU of the transport in all
cases except for when extremely large IP headers are used. IPv4
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header can range from 20 to 60 bytes in length and UDP header is
fixed at 8 bytes. Thus, the message size restrictions ensure that in
all cases except for when the IP header is 56 bytes or greater, the
size of the packet will be within the size of the transport MTU.
For IPv6, the specification provides for the same amount of padding
for UDP/IP headers as was conventionally done for IPv4 in DNS, RIP
and DHCPv4 with an additional padding of extra 20 bytes to
accommodate a larger IPv6 header. This follows the methodology
suggested in the IPv6 specification for calculating upper-layer
payload limits [5].
Path MTU discovery can generally be used to discover the MTU of the
link. Unfortunately, using path MTU discovery with UDP is not a
reliable option because it depends on routers providing ICMP errors
and hosts doing retransmission, which are not done consistently.
Implementors MUST follow the size restrictions outlined above and not
rely on path MTU discovery.
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Okmianski Expires October 30, 2004 [Page 21]
