| TOC |
|
This document describes the transport for syslog messages over TCP/ IPv4 or TCP/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.
There have been many implementations and deployments of traditional 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 traditional syslog, and how the new syslog architecture can interoperate with the traditional deployments.
This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”
The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.
This Internet-Draft will expire on June 3, 2010.
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 (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 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the BSD License.
1.
Introduction
2.
Conventions Used in This Document
3.
Message Transmission
3.1.
Session
3.2.
Session Initiation
3.3.
Message Transfer
3.3.1.
Octet-Stuffing
3.3.2.
Octet-Counting
3.4.
Message Content
3.5.
Session Closure
4.
Applicability to Legacy syslog
4.1.
Method Change
4.2.
Octet-Counting
4.3.
Octet Stuffing
5.
Security Considerations
5.1.
Sender Authentication and Message Forgery
5.2.
Message Observation
5.3.
Replaying
5.4.
Message Prioritization and Differentiation
5.5.
Denial of Service
5.6.
Reliability
6.
Authors
7.
IANA Considerations
8.
Acknowledgments
9.
Notes to the RFC Editor
10.
References
10.1.
Normative
10.2.
Informative
§
Authors' Addresses
| TOC |
The syslog protocol (Gerhards, R., “The Syslog Protocol,” March 2009.) [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] (Lonvick, C., “The BSD Syslog Protocol,” August 2001.). 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] (Gerhards, R., “The Syslog Protocol,” March 2009.) format. A discussion of how this may be applied to [RFC3164] (Lonvick, C., “The BSD Syslog Protocol,” August 2001.) messages is contained below. 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.
It is still RECOMMENDED to use the TLS transport (Gerhards, R., “The Syslog Protocol,” March 2009.) [RFC5424] to convey syslog messages. This specification is provided to ensure interoperability for transporting syslog over TCP.
| TOC |
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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].
The terminology defined in Section 3 of [RFC5424] (Gerhards, R., “The Syslog Protocol,” March 2009.) is used throughout this specification. The reader should be familiar with that to follow this discussion.
| TOC |
As described in [RFC5424] (Gerhards, R., “The Syslog Protocol,” March 2009.), syslog is simplex in nature. Traditional TCP implementations 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 abilities of TCP.
| TOC |
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. 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.
|
+<--------------------------+
| |
V |
+------------------+ |
| session initiate | |
+------------------+ |
| | success |
+------+ | |
|shutdown | +------------+ |
|failure V V | |
| +------------------+ | more |
| | send message | | messages |
| +------------------+ | |
| | | | |
| | +------------+ |
+--------+ | failure |
| | shutdown decision |
V V |
+------------------+ |
| session closure | |
+------------------+ |
| re-init |
+---------------------------+
Diagram 1. Transport Sender State Machine
|
+<--------------------------+
| |
V |
+------------------+ |
|listen for connect| |
+------------------+ |
| | success |
+------+ | |
|shutdown | +------------+ |
|failure V V | |
| +------------------+ | more |
| | receive message | | messages |
| +------------------+ | |
| | | | |
| | +------------+ |
+--------+ | failure |
| | shutdown decision |
V V |
+------------------+ |
| session closure | |
+------------------+ |
| re-init |
+---------------------------+
Diagram 1. Transport Receiver State Machine
A syslog transport sender is a simple state machine as shown in diagram 1. A transport receiver is a simple state machine as shown in diagram 2. It is valid (but rare) for no messages to be exchanged during a TCP session.
Note the absence of any real error processing on the state machine. 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.
| TOC |
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] (Postel, J., “Transmission Control Protocol,” September 1981.).
| TOC |
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] (Postel, J., “Transmission Control Protocol,” September 1981.). In practice, this is often seen after a prolonged time of inactivity.
Syslog messages are sent in sequence within a TCP transport stream. At least one message is encapsulated inside a frame. Transport senders use one of two different framing formats. They MUST support the octect-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.
The syslog message stream has the following ABNF (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.) [RFC5234]
definition:
APPLICATION-DATA = *SYSLOG-FRAME
SYSLOG-FRAME = SYSLOG-MSG TRAILER
SYSLOG-FRAME =/ MSG-LEN SP SYSLOG-MSG
MSG-LEN = NONZERO-DIGIT *DIGIT
SP = %d32
NONZERO-DIGIT = %d49-57
DIGIT = %d48 / NONZERO-DIGIT
TRAILER = LF | APP-DEFINED
LF = %d10
| Figure 1 |
Note that APP-DEFINED is not specified inside the ABNF and is used as a placeholder for slightly different terminators found in practice.
| TOC |
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 that the TRAILER character is a USASCII LF. It MAY be configurable to accept other characters. A discussion of this may be found below.
It is recommended, and is current practice, that a transport receiver assumes that octet-stuffing framing is used if a syslog frame starts with the USASCII character "<" (%d60).
The octect-stuffing method is NOT RECOMMENDED.
| TOC |
This mode is somewhat similar to the framing used in [RFC5425] (Miao, F., Ma, Y., and J. Salowey, “Transport Layer Security (TLS) Transport Mapping for Syslog,” March 2009.). 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.
It is recommended and current practice that a transport receiver assumes that octet counted framing is used if a syslog frame starts with a digit.
| TOC |
SYSLOG-MSG is defined in [RFC5424] (Gerhards, R., “The Syslog Protocol,” March 2009.). Most senders use the format described in [RFC3164] (Lonvick, C., “The BSD Syslog Protocol,” August 2001.) as an alternate format.
The syslog transport receiver MUST discard the TRAILER as it accepts the packet. That is to say that if the TRAILER character is kept with the message, then the message received will not be what was sent, and it will also no longer be compliant with the format specified in [RFC5424] (Gerhards, R., “The Syslog Protocol,” March 2009.)
| TOC |
The SYSLOG session is closed when one of the peers decides to do so. It then initiates a 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.
| TOC |
This is an informative section 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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) 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 traditional syslog, several different implementations exist. The older method of octet-stuffing has problems so implementers are encouraged to not use that mechanism. The newer method of octect-counting is reliable and should be used.
It is recommended and current practice that a transport receiver assumes that octet-counting framing is used if a syslog frame starts with a digit.
| TOC |
It has been observed in legacy implementations that the framing may change on a frame-by-frame basis. That is to say that a transport receiver SHOULD be prepared to accept different framing for each frame received. Devices that wish to interoperate with these legacy systems should be aware of this.
| TOC |
This framing allows for the transmission of all characters inside SYSLOG-MSG. Some transport senders have been seen to use this framing to stack multiple messages within a single TCP frame.
This method is REQUIRED for syslog transport receivers and senders.
| TOC |
The problem with octet-stuffing framing comes from the use of [RFC3164] (Lonvick, C., “The BSD Syslog Protocol,” August 2001.) 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] (Lonvick, C., “The BSD Syslog Protocol,” August 2001.) 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. i
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.
| TOC |
Using this specification on an unsecured network is NOT RECOMMENDED. Several syslog security considerations are discussed in [RFC5424] (Gerhards, R., “The Syslog Protocol,” March 2009.) 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] (Miao, F., Ma, Y., and J. Salowey, “Transport Layer Security (TLS) Transport Mapping for Syslog,” March 2009.)
| TOC |
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.
| TOC |
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.
| TOC |
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.
| TOC |
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.
| TOC |
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.
| TOC |
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.
| TOC |
The authors of this draft are:
Rainer Gerhards
Adiscon GmbH
Mozartstrasse 21
97950 Grossrinderfeld
Germany
Email: rgerhards@adiscon.com
Chris Lonvick
Cisco Systems, Inc.
12515 Research Blvd.
Austin 78759
USA
EMail: clonvick@cisco.com
| TOC |
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.
| TOC |
This document was written using the xml2rfc tool described in RFC2629 (Rose, M., “Writing I-Ds and RFCs using XML,” June 1999.) [RFC2629].
The authors wish to thank and all other people who commented on various versions of this proposal.
| TOC |
These are notes to the RFC editor. Please delete this section after the notes have been followed.
Please replace the instances of <TBD> the port number assigned by IANA.
| TOC |
| TOC |
| [RFC0793] | Postel, J., “Transmission Control Protocol,” STD 7, RFC 793, September 1981 (TXT). |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [RFC5234] | Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” STD 68, RFC 5234, January 2008 (TXT). |
| [RFC5424] | Gerhards, R., “The Syslog Protocol,” RFC 5424, March 2009 (TXT). |
| [RFC5425] | Miao, F., Ma, Y., and J. Salowey, “Transport Layer Security (TLS) Transport Mapping for Syslog,” RFC 5425, March 2009 (TXT). |
| TOC |
| [RFC2629] | Rose, M., “Writing I-Ds and RFCs using XML,” RFC 2629, June 1999 (TXT, HTML, XML). |
| [RFC3164] | Lonvick, C., “The BSD Syslog Protocol,” RFC 3164, August 2001 (TXT). |
| TOC |
| 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 |