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RFC 6012

Proposed STD
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Datagram Transport Layer Security (DTLS) Transport Mapping for Syslog


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Internet Engineering Task Force (IETF)                        J. Salowey
Request for Comments: 6012                           Cisco Systems, Inc.
Category: Standards Track                                       T. Petch
ISSN: 2070-1721                                 Engineering Networks Ltd
                                                             R. Gerhards
                                                            Adiscon GmbH
                                                                 H. Feng
                                             Huaweisymantec Technologies
                                                            October 2010

 Datagram Transport Layer Security (DTLS) Transport Mapping for Syslog


   This document describes the transport of syslog messages over the
   Datagram Transport Layer Security (DTLS) protocol.  It provides a
   secure transport for syslog messages in cases where a connectionless
   transport is desired.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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

Copyright Notice

   Copyright (c) 2010 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.

Page 2 
   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   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
   outside the IETF Standards Process, and derivative works of it may
   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Security Requirements for Syslog . . . . . . . . . . . . . . .  4
   4.  Using DTLS to Secure Syslog  . . . . . . . . . . . . . . . . .  4
   5.  Protocol Elements  . . . . . . . . . . . . . . . . . . . . . .  5
     5.1.  Transport  . . . . . . . . . . . . . . . . . . . . . . . .  5
     5.2.  Port and Service Code Assignment . . . . . . . . . . . . .  5
     5.3.  Initiation . . . . . . . . . . . . . . . . . . . . . . . .  5
       5.3.1.  Certificate-Based Authentication . . . . . . . . . . .  6
     5.4.  Sending Data . . . . . . . . . . . . . . . . . . . . . . .  6
       5.4.1.  Message Size . . . . . . . . . . . . . . . . . . . . .  7
     5.5.  Closure  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  Congestion Control . . . . . . . . . . . . . . . . . . . . . .  8
   7.  Security Policies  . . . . . . . . . . . . . . . . . . . . . .  8
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
     9.1.  DTLS Renegotiation . . . . . . . . . . . . . . . . . . . .  9
     9.2.  Message Loss . . . . . . . . . . . . . . . . . . . . . . .  9
     9.3.  Private Key Generation . . . . . . . . . . . . . . . . . .  9
     9.4.  Trust Anchor Installation and Storage  . . . . . . . . . .  9
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     11.2. Informative References . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   The syslog protocol [RFC5424] is designed to run over different
   transports for different environments.  This document defines the
   transport of syslog messages over the Datagram Transport Layer
   Security (DTLS) protocol [RFC4347].

   The Datagram Transport Layer Security (DTLS) protocol [RFC4347] is
   designed to meet the requirements of applications that need secure
   datagram transport.  DTLS has been mapped onto different transports,
   including UDP [RFC0768] and the Datagram Congestion Control Protocol
   (DCCP) [RFC4340].  This memo defines both options, namely syslog over
   DTLS over UDP, and syslog over DTLS over DCCP.

2.  Terminology

   The following definitions from [RFC5424] are used in this document:

   o  An "originator" generates syslog content to be carried in a

   o  A "collector" gathers syslog content for further analysis.

   o  A "relay" forwards messages, accepting messages from originators
      or other relays, and sending them to collectors or other relays.

   o  A "transport sender" passes syslog messages to a specific
      transport protocol.

   o  A "transport receiver" takes syslog messages from a specific
      transport protocol.

   This document adds the following definitions:

   o  A "DTLS client" is an application that can initiate a DTLS Client
      Hello to a server.

   o  A "DTLS server" is an application that can receive a DTLS Client
      Hello from a client and reply with a Server Hello.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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3.  Security Requirements for Syslog

   The security requirements for the transport of syslog messages are
   discussed in Section 2 of [RFC5425].  These also apply to this

   The following secondary threat is also considered in this document:

   o  Denial of service is discussed in [RFC5424], which states that an
      attacker may send more messages to a transport receiver than the
      transport receiver could handle.  When using a secure transport
      protocol handshake, an attacker may use a spoofed IP source to
      engage the server in a cryptographic handshake to deliberately
      consume the server's resources.

4.  Using DTLS to Secure Syslog

   DTLS can be used as a secure transport to counter all the primary
   threats to syslog described in [RFC5425]:

   o  Confidentiality to counter disclosure of the message contents.

   o  Integrity checking to counter modifications to a message on a hop-
      by-hop basis.

   o  Server or mutual authentication to counter masquerade.

   In addition, DTLS also provides:

   o  A cookie exchange mechanism during handshake to counter Denial of
      Service attacks.

   o  A sequence number in the header to counter replay attacks.

   Note: This secure transport (i.e., DTLS) only secures syslog
   transport in a hop-by-hop manner, and is not concerned with the
   contents of syslog messages.  In particular, the authenticated
   identity of the transport sender (e.g., subject name in the
   certificate) is not necessarily related to the HOSTNAME field of the
   syslog message.  When authentication of syslog message origin is
   required, [RFC5848] can be used.

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5.  Protocol Elements

5.1.  Transport

   DTLS can run over multiple transports.  Implementations of this
   specification MUST support DTLS over UDP and SHOULD support DTLS over
   DCCP [RFC5238].  Transports such as UDP or DCCP do not provide
   session multiplexing and session demultiplexing.  In such cases, the
   application implementer provides this functionality by mapping a
   unique combination of the remote address, remote port number, local
   address, and local port number to a session.

   Each syslog message is delivered by the DTLS record protocol, which
   assigns a sequence number to each DTLS record.  Although the DTLS
   implementer may adopt a queue mechanism to resolve reordering, it may
   not assure that all the messages are delivered in order when mapping
   on the UDP transport.

   When DTLS runs over an unreliable transport, such as UDP, reliability
   is not provided.  With DTLS, an originator or relay may not realize
   that a collector has gone down or lost its DTLS connection state, so
   messages may be lost.

   Syslog over DTLS over TCP MUST NOT be used.  If a secure transport is
   required with TCP, then the appropriate security mechanism is syslog
   over Transport Layer Security (TLS) as described in [RFC5425].

5.2.  Port and Service Code Assignment

   A syslog transport sender is always a DTLS client, and a transport
   receiver is always a DTLS server.

   The UDP and DCCP port 6514 has been allocated as the default port for
   syslog over DTLS as defined in this document.  The service code SYLG
   (1398361159) has been assigned to syslog.

5.3.  Initiation

   The transport sender initiates a DTLS connection by sending a DTLS
   Client Hello to the transport receiver.  Implementations MUST support
   the denial of service countermeasures defined by DTLS.  When these
   countermeasures are used, the transport receiver responds with a DTLS
   Hello Verify Request containing a cookie.  The transport sender
   responds with a DTLS Client Hello containing the received cookie,
   which initiates the DTLS handshake.  The transport sender MUST NOT
   send any syslog messages before the DTLS handshake has successfully

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   Implementations MUST support DTLS 1.0 [RFC4347] and MUST support the
   mandatory to implement cipher suite, which is
   TLS_RSA_WITH_AES_128_CBC_SHA as specified in [RFC5246].  If
   additional cipher suites are supported, then implementations MUST NOT
   negotiate a cipher suite that employs NULL integrity or
   authentication algorithms.

   Where privacy is REQUIRED, then implementations must either negotiate
   a cipher suite that employs a non-NULL encryption algorithm or else
   achieve privacy by other means, such as a physically secured network.

   However, as [RFC5424], Section 8, points out, "In most cases, passing
   clear-text messages is a benefit to the operations staff if they are
   sniffing the packets from the wire", and so where privacy is not a
   requirement, then it is advantageous to use a NULL encryption

5.3.1.  Certificate-Based Authentication

   The mandatory to implement cipher suites for DTLS use certificates
   [RFC5280] to authenticate peers.  Both the syslog transport sender
   (DTLS client) and the syslog transport receiver (DTLS server) MUST
   implement certificate-based authentication.  This consists of
   validating the certificate and verifying that the peer has the
   corresponding private key.  The latter part is performed by DTLS.  To
   ensure interoperability between clients and servers, the methods for
   certificate validation defined in Sections 4.2.1 and 4.2.2 of
   [RFC5425] SHALL be implemented.

   Both transport receiver and transport sender implementations MUST
   provide means to generate a key pair and self-signed certificate in
   case a key pair and certificate are not available through another

   The transport receiver and transport sender SHOULD provide mechanisms
   to record the certificate or certificate fingerprint used by the
   remote endpoint for the purpose of correlating an identity with the
   sent or received data.

5.4.  Sending Data

   All syslog messages MUST be sent as DTLS "application data".  It is
   possible that multiple syslog messages be contained in one DTLS
   record, or that a syslog message be transferred in multiple DTLS
   records.  The application data is defined with the following ABNF
   [RFC5234] expression:

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      SP = %d32

      NONZERO-DIGIT = %d49-57

      DIGIT = %d48 / NONZERO-DIGIT

   SYSLOG-MSG is defined in the syslog [RFC5424] protocol.

5.4.1.  Message Size

   The message length 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.  As stated in [RFC4347], a DTLS record MUST NOT span
   multiple datagrams.  When mapping onto different transports, DTLS has
   different record size limitations.  For UDP, see Section 3.2 of
   [RFC5426].  For DCCP, the application implementer SHOULD determine
   the maximum record size allowed by the DTLS protocol running over
   DCCP, as specified in [RFC4340].  The message size SHOULD NOT exceed
   the DTLS maximum record size limitation of 2^14 bytes.  To be
   consistent with [RFC5425], in establishing 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.

   See Appendix A.2 of [RFC5424] for implementation guidance on message
   length, including fragmentation.

5.5.  Closure

   A transport sender MUST close the associated DTLS connection if the
   connection is not expected to deliver any syslog messages later.  It
   MUST send a DTLS close_notify alert before closing the connection.  A
   transport sender (DTLS client) MAY choose to not wait for the
   transport receiver's close_notify alert and simply close the DTLS
   connection.  Once the transport receiver gets a close_notify from the
   transport sender, it MUST reply with a close_notify.

   When no data is received from a DTLS connection for a long time
   (where the application decides what "long" means), a transport
   receiver MAY close the connection.  The transport receiver (DTLS

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   server) MUST attempt to initiate an exchange of close_notify alerts
   with the transport sender before closing the connection.  Transport
   receivers that are unprepared to receive any more data MAY close the
   connection after sending the close_notify alert.

   Although closure alerts are a component of TLS and so of DTLS, they,
   like all alerts, are not retransmitted by DTLS and so may be lost
   over an unreliable network.

6.  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 establishing a
   degree of fairness between flows that share the same path are vital
   to the stable operation of the Internet (see [RFC2914] and

   DCCP has congestion control.  If DCCP is available, syslog over DTLS
   over DCCP is RECOMMENDED in preference to syslog over DTLS over UDP.
   Implementations of syslog over DTLS over DCCP MUST support Congestion
   Control Identifier (CCID) 3 and SHOULD support CCID 2 to ensure

   The congestion control considerations from Section 4.3 of [RFC5426]
   also apply to syslog over DTLS over UDP.

7.  Security Policies

   Syslog transport over DTLS has been designed to minimize the security
   and operational differences for environments where both syslog over
   TLS [RFC5425] and syslog over DTLS are supported.  The security
   policies for syslog over DTLS are the same as those described in
   [RFC5425], and all the normative requirements of Section 5 of
   [RFC5425] apply.

8.  IANA Considerations

   IANA has assigned a registered UDP and DCCP port number for syslog
   over DTLS.  The values are the same as for syslog over TLS.  That is,
   the registry has been updated as follows:

      syslog-tls    6514/udp    syslog over DTLS [RFC6012]
      syslog-tls    6514/dccp   syslog over DTLS [RFC6012]

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   IANA has assigned the service code SYLG to syslog for use with DCCP.
   The allocation in the Service Code subregistry of the Datagram
   Congestion Control Protocol (DCCP) Parameters registry is as follows:

      1398361159    SYLG    Syslog Protocol    [RFC6012]

9.  Security Considerations

   The security considerations in [RFC4347], [RFC5246], [RFC5425], and
   [RFC5280] apply to this document.

9.1.  DTLS Renegotiation

   TLS and DTLS renegotiation may be vulnerable to attacks described in
   [RFC5746].  Although RFC 5746 provides a fix for some of the issues,
   renegotiation can still cause problems for applications since
   connection security parameters can change without the application
   knowing it.  Therefore it is RECOMMENDED that renegotiation be
   disabled for syslog over DTLS.  If renegotiation is allowed, then the
   specification in RFC 5746 MUST be followed, and the implementation
   MUST make sure that the connection still has adequate security and
   that any identities extracted from client and server certificates do
   not change during renegotiation.

9.2.  Message Loss

   The transports described in this document are unreliable.  It is
   possible for messages to be lost or removed by an attacker without
   the knowledge of the receiver.  [RFC5424] notes that implementers who
   wish a lossless stream should be using tls/tcp as their transport.
   In addition, the use of signed syslog messages [RFC5848] can also
   provide an indication of message loss.

9.3.  Private Key Generation

   Transport receiver and transport sender implementations often
   generate their own key pairs.  An inadequate random number generator
   (RNG) or an inadequate pseudo-random number generator (PRNG) to
   generate these keys can result in little or no security.  See
   [RFC4086] for random number generation guidance.

9.4.  Trust Anchor Installation and Storage

   Trust anchor installation and storage is critical.  Transmission of a
   trust anchor, especially self-signed certificates used as trust
   anchors, from transport receiver to transport sender for installation
   requires one or more out-of-band steps.  Care must be taken to ensure
   the installed trust anchor is in fact the correct trust anchor.  The

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   fingerprint mechanism mentioned in Section 5.3.1 can be used by the
   transport sender to ensure the transport receiver's self-signed
   certificate is properly installed.  Trust anchor information must be
   securely stored.  Changes to trust anchor information can cause
   acceptance of certificates that should be rejected.

10.  Acknowledgements

   The authors would like to thank Wes Hardaker for his review of this
   proposal and for contributing his valuable suggestions on the use of
   DTLS.  Thanks also to Pasi Eronen, David Harrington, Chris Lonvick,
   Eliot Lear, Anton Okmyanskiy, Juergen Schoenwaelder, Richard
   Graveman, the members of the syslog working group, and the members of
   the IESG for their review, comments, and suggestions.

11.  References

11.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340, March 2006.

   [RFC4347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security", RFC 4347, April 2006.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5238]  Phelan, T., "Datagram Transport Layer Security (DTLS) over
              the Datagram Congestion Control Protocol (DCCP)",
              RFC 5238, May 2008.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5424]  Gerhards, R., "The Syslog Protocol", RFC 5424, March 2009.

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   [RFC5425]  Miao, F., Ma, Y., and J. Salowey, "Transport Layer
              Security (TLS) Transport Mapping for Syslog", RFC 5425,
              March 2009.

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

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              <"Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, February 2010.

11.2.  Informative References

   [RFC2914]  Floyd, S., "Congestion Control Principles", BCP 41,
              RFC 2914, September 2000.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405,
              November 2008.

   [RFC5848]  Kelsey, J., Callas, J., and A. Clemm, "Signed Syslog
              Messages", RFC 5848, May 2010.

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Authors' Addresses

   Joseph Salowey
   Cisco Systems, Inc.
   2901 3rd Ave.
   Seattle, WA  98121


   Tom Petch
   Engineering Networks Ltd
   18 Parkwood Close
   Lymm, Cheshire  WA13 0NQ


   Rainer Gerhards
   Adiscon GmbH
   Mozartstrasse 21
   Grossrinderfeld, BW  97950


   Hongyan Feng
   Huaweisymantec Technologies
   20245 Stevens Creek Blvd.
   Cupertino, CA  95014