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RFC 6587

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Transmission of Syslog Messages over TCP


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Internet Engineering Task Force (IETF)                       R. Gerhards
Request for Comments: 6587                                  Adiscon GmbH
Category: Historic                                            C. Lonvick
ISSN: 2070-1721                                      Cisco Systems, Inc.
                                                              April 2012

                Transmission of Syslog Messages over TCP


   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.
   This memo describes how TCP has been used as a transport for syslog

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for the historical record.

   This document defines a Historic Document for the Internet community.
   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at


   The IESG does not recommend implementing or deploying syslog over
   plain tcp, which is described in this document, because it lacks the
   ability to enable strong security [RFC3365].

   Implementation of the TLS transport [RFC5425] is recommended so that
   appropriate security features are available to operators who want to
   deploy secure syslog.  Similarly, those security features can be
   turned off for those who do not want them.

Page 2 
Copyright Notice

   Copyright (c) 2012 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
   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 Simplified BSD License.

Table of Contents

   1. Introduction ....................................................3
   2. Conventions Used in This Document ...............................5
   3. Message Transmission ............................................5
      3.1. Character Encoding Scheme ..................................5
      3.2. Session ....................................................6
      3.3. Session Initiation .........................................6
      3.4. Message Transfer ...........................................6
           3.4.1. Octet Counting ......................................7
           3.4.2. Non-Transparent-Framing .............................7
           3.4.3. Method Change .......................................8
      3.5. Session Closure ............................................8
   4. Applicability Statement .........................................8
   5. Security Considerations .........................................9
   6. Acknowledgments .................................................9
   7. References .....................................................10
      7.1. Normative References ......................................10
      7.2. Informative References ....................................10

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1.  Introduction

   The Standards-Track documents in the syslog series recommend using
   the syslog protocol [RFC5424] with the TLS transport [RFC5425] for
   all event messages.  The authors of this document wholeheartedly
   support that position and only offer this document to describe what
   has been observed with legacy syslog over TCP, which appears to still
   be widely used.

   Two primary format options have been observed with legacy syslog
   being transported over TCP.  These have been called "non-transparent-
   framing" and "octet-counting".  The non-transparent-framing mechanism
   has some inherent problems.

   Diagram 1 shows how all of these syslog transports relate to each
   other.  In this diagram, three originators are seen, labeled A, B,
   and C, along with one collector.  Originator A is using the TCP
   transport that is described in this document.  Originator B is using
   the UDP transport, which is described in [RFC5426].  Originator C is
   using the TLS transport, which is described in [RFC5425].  The
   collector is shown with the capability to accept all three

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    | Originator A        |
    |  syslog application |
    |                     |
    |  syslog transport   |
    |        TCP          |
             /                            +---------------------+
            /                             | 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

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2.  Conventions Used in This Document

   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 the syslog message
   format as described in [RFC3164].  Devices that continue to use that
   message format (regardless of transport) will be described as "legacy
   syslog devices".  Similarly, devices that use the message format as
   described in [RFC5424] will be described as "standardized syslog

3.  Message Transmission

   Syslog is simplex in nature.  It has been observed that
   implementations of syslog over TCP also do not use any back-channel
   mechanism to convey information to the transport sender and,
   consequently, do not use any application-level acknowledgement for
   syslog signaling from receiver to sender.  Message receipt
   acknowledgement, reliability, and flow control are provided by the
   capabilities of TCP.

3.1.  Character Encoding Scheme

   Syslog over TCP messages contain no indication of the coded character
   set (e.g., [US-ASCII] or [UNICODE] ) or character encoding scheme
   (e.g., so-called "7-bit ASCII" or UTF-8 [RFC3629]) in use.  In these
   messages, the predominant approach has been to include characters
   only from the ASCII repertoire (i.e., %d32 to %d126 inclusive) using
   the "Network Virtual Terminal" (NVT) encoding [RFC5198].

   The message header usually contains characters only from the ASCII
   repertoire, in the NVT encoding.  This has been observed even in
   cases where a different encoding (e.g., UTF-8) has been used for the
   MSG part.  However, characters outside the ASCII range have been seen
   inside the header.  In that case, some syslog applications have been
   known to experience problems processing those messages.

   In some cases, it has been observed that characters outside of the
   ASCII range are often being transformed by receivers in an effort to
   "escape control characters".  Some receiver implementations simply
   drop those characters.  This is considered to be a poor practice, as
   it causes problems with coded character sets other than ASCII and
   character encodings other than NVT, most notably the UTF-8 encoding
   of Unicode.

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   It has also been observed that relays will forward messages using the
   character encoding schemes of messages they receive.  In the case
   where two different senders are using different character encoding
   schemes, the relay will forward each message to a collector in that
   character encoding.  The collector of these messages will have to be
   prepared to receive messages from the same relay with different

3.2.  Session

   Like most other protocols, the syslog transport sender is the TCP
   host that initiates the TCP session.  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
   seem to be fixed once the session is established.

   When it has been observed, if an error occurs that cannot be
   corrected by TCP, the host detecting the error gracefully closes the
   TCP session.  There have been no application-level messages seen that
   were sent to notify the other host about the state of the host syslog

3.3.  Session Initiation

   The TCP host acting as a syslog transport receiver listens to a TCP
   port.  The TCP transport sender initiates a TCP session to the syslog
   transport receiver as specified in [RFC0793].

   This protocol has no standardized port assignment.  In practice,
   network administrators generally choose something that they feel will
   not conflict with anything else active in their networks.  This has
   most often been either TCP/514, which is actually allocated to
   another protocol, or some variant of adding 514 to a multiple of
   1000.  Please see Section 4 for more information.

3.4.  Message Transfer

   Syslog over TCP has been around for a number of years.  Just like
   legacy syslog over UDP, different implementations exist.  The older
   method of non-transparent-framing has problems.  The newer method of
   octet-counting is reliable and has not been seen to cause problems
   noted with the non-transparent-framing method.

   In both of these methods, during the message transfer phase, the
   syslog transport sender sends a stream of messages to the transport
   receiver.  These are sent in sequence and one message is encapsulated

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   inside each TCP frame.  Either of the TCP hosts may initiate session
   closure at any time as specified in Section 3.5 of [RFC0793].  In
   practice, this is often seen after a prolonged period of inactivity.

3.4.1.  Octet Counting

   This framing allows for the transmission of all characters inside a
   syslog message and is similar to the method used in [RFC5425].  A
   transport receiver uses the defined message length to delimit a
   syslog message.  As noted in [RFC3164], the upper limit for a legacy
   syslog message length is 1024 octets.  That length has been expanded
   for standardized syslog.

   It can be assumed that octet-counting framing is used if a syslog
   frame starts with a digit.

   All syslog messages can be considered to be TCP "data" as per the
   Transmission Control Protocol [RFC0793].  The syslog message stream
   has the following ABNF [RFC5234] definition:


       SYSLOG-FRAME = MSG-LEN SP SYSLOG-MSG   ; Octet-counting
                                              ; method


       NONZERO-DIGIT = %d49-57

       SYSLOG-MSG is defined in the syslog protocol [RFC5424] and may
                  also be considered to be the payload in [RFC3164]

   MSG-LEN is the octet count of the SYSLOG-MSG in the SYSLOG-FRAME.

3.4.2.  Non-Transparent-Framing

   The non-transparent-framing method inserts a syslog message into a
   frame and terminates it with a TRAILER character.  The TRAILER has
   usually been a single character and most often is ASCII LF (%d10).
   However, other characters have also been seen, with ASCII NUL (%d00)
   being a prominent example.  Some devices have also been seen to emit
   a two-character TRAILER, which is usually CR and LF.

   The problem with non-transparent-framing comes from the use of a
   TRAILER character.  In that, the traditional TRAILER character is not
   escaped within the message, which causes problems for the receiver.

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   For example, a message in the style of [RFC3164] containing one or
   more LF characters may be misinterpreted as multiple messages by the
   receiving syslog application.

   The ABNF for this is shown here:


       SYSLOG-FRAME = SYSLOG-MSG TRAILER  ; non-transparent-framing
                                          ; method



       SYSLOG-MSG is defined in the syslog protocol [RFC5424] and may
                  also be considered to be the payload in [RFC3164]

   A transport receiver can assume that non-transparent-framing is used
   if a syslog frame starts with the ASCII character "<" (%d60).

3.4.3.  Method Change

   In legacy implementations, it has been observed that the framing may
   change on a frame-by-frame basis.  This is probably not a good idea,
   but it's been seen.

3.5.  Session Closure

   The syslog session is closed when one of the TCP hosts decides to do
   so.  It then initiates a local TCP session closure.  Following TCP
   [RFC0793], it doesn't need to notify the remote TCP host of its
   intention to close the session, nor does it accept any messages that
   are still in transit.

4.  Applicability Statement

   Again it must be emphasized that the Standards-Track documents in the
   syslog series recommend using the TLS transport [RFC5425] to
   transport syslog messages.  This document does not recommend that new
   implementations or deployments use syslog over TCP except for the
   explicit purpose of interoperating with existing deployments.

   One of the major problems with interoperability with this protocol is
   that there is no consistent TCP port assigned.  Most of the
   successful implementations have made the selection of a port a user-
   configurable option.  The most frequently observed port for this has
   been TCP/514, which is actually assigned to the Shell protocol.

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   Operators must carefully select which port to use in their deployment
   and be prepared to encounter different default port assignments in

   There are several advantages to using TCP: flow control, error
   recovery, and reliability, to name a few.  These reasons, and the
   ease of programming, have led people to use this transmission
   protocol to transmit syslog.

   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 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 as described in [RFC0793] to have
   the sending TCP immediately send all buffered data.

5.  Security Considerations

   This protocol makes no meaningful provisions for security.  It lacks
   authentication, integrity checking, and privacy.  It makes no
   provision for flow control or end-to-end confirmation of receipt,
   relying instead on the underlying TCP implementations to approximate
   these functions.  It should not be used if deployment of [RFC5425] on
   the systems in question is feasible.

6.  Acknowledgments

   The authors wish to thank David Harrington, Tom Petch, Richard
   Graveman, and all other people who commented on various versions of
   this proposal.  We would also like to thank Peter Saint-Andre for
   clarifying character encodings.

   The authors would also like to thank Randy Presuhn for being our
   reviewer and document shepherd, and a special thanks to Dan Romascanu
   for his support and guidance.

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7.  References

7.1.  Normative References

   [RFC0793]   Postel, J., "Transmission Control Protocol", STD 7,
               RFC 793, September 1981.

   [RFC3365]   Schiller, J., "Strong Security Requirements for Internet
               Engineering Task Force Standard Protocols", BCP 61,
               RFC 3365, August 2002.

   [RFC5198]   Klensin, J. and M. Padlipsky, "Unicode Format for Network
               Interchange", RFC 5198, March 2008.

   [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.

   [US-ASCII]  ANSI, "Coded Character Set -- 7-bit American Standard
               Code for Information Interchange, ANSI X3.4-1986", 1986.

7.2.  Informative References

   [RFC3164]   Lonvick, C., "The BSD Syslog Protocol", RFC 3164,
               August 2001.

   [RFC3629]   Yergeau, F., "UTF-8, a transformation format of ISO
               10646", STD 63, RFC 3629, November 2003.

   [RFC5426]   Okmianski, A., "Transmission of Syslog Messages over
               UDP", RFC 5426, March 2009.

   [UNICODE]   The Unicode Consortium. The Unicode Standard, Version
               6.0.0, (Mountain View, CA: The Unicode Consortium,
               2011. ISBN 978-1-936213-01-6),

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Authors' Addresses

   Rainer Gerhards
   Adiscon GmbH
   Mozartstrasse 21
   Grossrinderfeld, BW  97950


   Chris Lonvick
   Cisco Systems, Inc.
   12515 Research Blvd.
   Austin, TX  78759