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Versions: (draft-ietf-netconf-reverse-ssh) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 RFC 8071

NETCONF Working Group                                          K. Watsen
Internet-Draft                                          Juniper Networks
Updates: 4253 (if approved)                             October 10, 2014
Intended status: Standards Track
Expires: April 13, 2015


                           NETCONF Call Home
                    draft-ietf-netconf-call-home-01

Abstract

   This document presents NETCONF Call Home, which enables a NETCONF
   server to initiate a secure connection to the NETCONF client.
   NETCONF Call Home supports both the SSH and TLS transports, and does
   so in a way that preserves the SSH and TLS roles when compared to
   standard NETCONF over SSH or TLS connections.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 13, 2015.

Copyright Notice

   Copyright (c) 2014 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




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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
     1.3.  Applicability Statement . . . . . . . . . . . . . . . . .   3
     1.4.  Update to RFC 4253  . . . . . . . . . . . . . . . . . . .   4
   2.  The NETCONF Server  . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Protocol Operation  . . . . . . . . . . . . . . . . . . .   4
     2.2.  Configuration Data Model  . . . . . . . . . . . . . . . .   5
   3.  The NETCONF Client  . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Protocol Operation  . . . . . . . . . . . . . . . . . . .   5
     3.2.  Server Identification and Verification  . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  10
     A.1.  00 to 01  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This document presents NETCONF Call Home, which enables a NETCONF
   server to initiate a secure connection to the NETCONF client.
   NETCONF Call Home supports both the SSH and TLS transports, and does
   so in a way that preserves the SSH and TLS roles when compared to
   standard NETCONF over SSH or TLS connections.

   The same technique is used to enabled call home for both the SSH and
   TLS transports.  The technique is to have the NETCONF server initiate
   a TCP connection to the intended NETCONF client.  The NETCONF client
   then uses the established TCP connection to initiate either the SSH
   or TLS protocols.  In this way, the NETCONF server is always the SSH
   or TLS server, regardless if call home is used or not.

   Enabling the NETCONF server to maintain the role of SSH or TLS server
   is both necessary and desirable.  It is necessary for the SSH
   protocol, as SSH channels and subsystems can only be opened on the
   SSH server.  It is desirable for both the SSH and TLS protocols as it
   conveniently leverages infrastructure that may be deployed for host-
   key or certificate verification and user authentication.





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1.1.  Motivation

   Call home is generally useful for both the initial deployment and on-
   going management of networking elements.  Here are some scenarios
   enabled by call home:

   o  The network element may proactively call home after being powered
      on for the first time in order to register itself with its
      management system.

   o  The network element may access the network in a way that
      dynamically assigns it an IP address and it doesn't register its
      assigned IP addressed to a mapping service.

   o  The network element may be configured in "stealth mode" and thus
      doesn't have any open ports for the management system to connect
      to.

   o  The network element may be deployed behind a firewall that doesn't
      allow management access to the internal network.

   o  The network element may be deployed behind a firewall that
      implements network address translation (NAT) for all internal
      network IP addresses, thus complicating the ability for a
      management system to connect to it.

   o  The operator may prefer to have network elements initiate
      management connections believing it is easier to secure one open-
      port in the data center than to have an open port on each network
      element in the network.

   Having call home for NETCONF is particularly useful as NETCONF is the
   recommended protocol for configuration [iesg-statement], which is
   needed for provisioning workflows.

1.2.  Requirements Terminology

   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 [RFC2119].

1.3.  Applicability Statement

   The techniques described in this document are suitable for network
   management scenarios such as the ones described in Section 1.1.
   However, these techniques SHOULD only be used for a NETCONF server to
   initiate a connection to a NETCONF client, as described in this
   document.



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   The reason for this restriction is that different protocols have
   different security assumptions.  The NETCONF transport specifications
   require NETCONF clients and servers to verify the identity of the
   other party before starting the NETCONF protocol (section 2.2 of
   [RFC6241].

   This contrasts with the base SSH and TLS protocols, which do not
   require programmatic verification of the other party (e.g., section
   9.3.4 of [RFC4251] and section 4 of [RFC4252]).  In such
   circumstances, allowing the SSH/TLS server to contact the SSH/TLS
   client would open new vulnerabilities.  Any use of call home with
   SSH/TLS for purposes other than NETCONF will need a thorough,
   contextual security analysis.

1.4.  Update to RFC 4253

   This document updates the SSH Transport Layer Protocol [RFC4253] only
   by removing the "The client initiates the connection" statement made
   in Section 4 (Connection Setup).  This document assumes that the
   reference to "connection" refers to the underlying transport
   connection (e.g., TCP).  Security implications related to this change
   are discussed in Security Considerations (Section 4).

2.  The NETCONF Server

2.1.  Protocol Operation

   o  The NETCONF server initiates a TCP connection to the NETCONF
      client on one of the IANA-assigned ports for NETCONF Call Home
      (YYYY for netconf-ch-ssh and ZZZZ for netconf-ch-tls).

   o  The TCP connection is accepted and a TCP session is established.

   o  Using this TCP session, the NETCONF server immediately starts
      either the SSH-server or the TLS-server protocol, depending on
      which port is connected.  The NETCONF server MUST start the SSH-
      server protocol when port YYYY is connected and the TLS-server
      protocol when port ZZZZ is connected.  The SSH-server and TLS-
      server protocols are described by [RFC4253] and [RFC5246]
      respectively.

   o  The NETCONF protocol proceeds normally for SSH and TLS, as defined
      in [RFC6242] and [RFC5539] respectively.








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2.2.  Configuration Data Model

   How to configure a NETCONF server to initiate a NETCONF Call Home
   connection is outside the scope of this document, as implementations
   can support this protocol using proprietary configuration data
   models.  That said, a YANG [RFC6020] model for configuring NETCONF
   Call Home is provided in [draft-ietf-netconf-server-model].

3.  The NETCONF Client

3.1.  Protocol Operation

   o  The NETCONF client listens for TCP connections on one or both of
      the IANA-assigned ports for NETCONF Call Home (YYYY for netconf-
      ch-ssh and ZZZZ for netconf-ch-tls).

   o  The NETCONF client accepts an incoming TCP connection and a TCP
      session is established.

   o  Using this TCP session, the NETCONF client immediately starts
      either the SSH-client or the TLS-client protocol, depending on
      which port is connected.  The NETCONF client MUST start the SSH-
      client protocol when port YYYY is connected and the TLS-client
      protocol when port ZZZZ is connected.  The SSH-client and TLS-
      client protocols are described by [RFC4253] and [RFC5246]
      respectively.

   o  The NETCONF protocol proceeds normally for SSH and TLS, as is
      defined in [RFC6242] and [RFC5539] respectively.

3.2.  Server Identification and Verification

   Under normal circumstances, a NETCONF client initiates the NETCONF
   connection to the NETCONF server.  This action provides essential
   input to verify the NETCONF server's identity.  For instance, when
   using TLS, the input can be compared to the domain names and IP
   addresses encoded in X.509 certificates.  Similarly, when using SSH,
   the input can be compared to information persisted previously.

   However, when receiving a NETCONF Call Home connection, the NETCONF
   client does not have any context leading it to know the connection is
   from a particular NETCONF server.  Thus the NETCONF client must
   derive the NETCONF server's identity using information provided by
   the network and the NETCONF server itself.  This section describes
   strategies a NETCONF client can use to identify a NETCONF server.

   In addition to identifying a NETCONF server, a NETCONF client must
   also be able to verify the NETCONF server's credentials.  Verifying a



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   NETCONF server's credentials is necessary under normal circumstances
   but, due to call home being commonly used for newly deployed NETCONF
   servers, how to verify its credentials the very first time becomes a
   prominent concern.  Therefore, this section also describes strategies
   a NETCONF client can use to verify a NETCONF server's credentials.

   The first information a NETCONF client learns from a NETCONF Call
   Home connection is the IP address of the NETCONF server, as provided
   by the source address of the TCP connection.  This IP address could
   be used as an identifier directly, but doing so would only work in
   networks that use known static addresses, in which case a standard
   NETCONF connection would have worked just as well.  Due to this
   limited use, it is not recommended to identify a NETCONF server based
   on its source IP address.

   The next information a NETCONF client learns is provided by the
   NETCONF server in the form of a host-key or a certificate, for the
   SSH and TLS protocols respectively.  Without examining the contents
   of the host-key or certificate, it is possible to form an identity
   for the NETCONF server using it (e.g., a fingerprint), since each
   NETCONF server is assumed to have a statistically unique public key,
   even in virtualized environments.  This strategy also provides a
   mechanism to verify the NETCONF server, in that a secure connection
   can only be established with the NETCONF server having the matching
   private key.  This strategy is commonly implemented by SSH clients,
   and could be used equally well by TLS-based clients, such as may be
   required when the NETCONF servers have self-signed certificates.
   This strategy is viable and useful when the NETCONF servers call home
   using either SSH with standard RSA/DSA host-keys, or using TLS with
   self-signed certificates.

   Yet another option for identifying a NETCONF server is for its host
   key or certificate to encode its identity directly (e.g., within the
   "Subject" field).  However, in order to trust the content encoded
   within a host-key or certificate, it must be signed by a certificate
   authority trusted by the NETCONF client.  This strategy's use of PKI
   enables a NETCONF client to transparently authenticate NETCONF
   servers, thus eliminating the need for manual authentication, as
   required by the previously discussed strategies.  Elimination of
   manual steps is needed to achieve scalable solutions, however one can
   claim that this merely pushes equivalent work to provisioning the
   NETCONF servers with signed credentials.  This assessment is accurate
   in general, but not in the case where the manufacturer itself
   provisions the credentials, such as is described by
   [Std-802.1AR-2009].  When NETCONF servers are pre-provisioned this
   way, NETCONF clients can transparently authenticate NETCONF servers
   using just the manufacturer's trust anchor and a list of expected
   NETCONF server identifiers, which could be provided along with



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   shipping information.  This strategy is recommended for all
   deployment scenarios.

   In discussing the use of certificates, it is worth noting that TLS
   uses X.509 certificates by default.  However, to use X.509
   certificates with SSH, both the NETCONF client and server must
   support [RFC6187].

4.  Security Considerations

   The security considerations described throughout [RFC6242] and
   [RFC5539], and by extension [RFC4253] and [RFC5246], apply here as
   well.

   This RFC deviates from standard SSH and TLS usage by having the SSH/
   TLS server initiate the underlying TCP connection.  For SSH,
   [RFC4253] says "the client initiates the connection", whereas for
   TLS, [RFC5246] says it is layered on top of "some reliable transport
   protocol" without further attribution.

   For SSH, not having the SSH client initiate the TCP connection means
   that it does not have a preconceived notion of the SSH server's
   identity, and therefore must dynamically derive one from information
   provided by the network or the SSH server itself.  Security
   Considerations for strategies for this are described in Section 3.2.

   An attacker could DoS the NETCONF client by having it perform
   computationally expensive operations, before deducing that the
   attacker doesn't posses a valid key.  This is no different than any
   secured service and all common precautions apply (e.g., blacklisting
   the source address after a set number of unsuccessful login
   attempts).

5.  IANA Considerations

   This document requests that IANA assigns two TCP port numbers in the
   "Registered Port Numbers" range with the service names "netconf-ch-
   ssh" and "netconf-ch-tls".  These ports will be the default ports for
   NETCONF Call Home protocol when using SSH and TLS respectively.
   Below is the registration template following the rules in [RFC6335].











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   Service Name:           netconf-ch-ssh
   Transport Protocol(s):  TCP
   Assignee:               IESG <iesg@ietf.org>
   Contact:                IETF Chair <chair@ietf.org>
   Description:            NETCONF Call Home (SSH)
   Reference:              RFC XXXX
   Port Number:            YYYY

   Service Name:           netconf-ch-tls
   Transport Protocol(s):  TCP
   Assignee:               IESG <iesg@ietf.org>
   Contact:                IETF Chair <chair@ietf.org>
   Description:            NETCONF Call Home (TLS)
   Reference:              RFC XXXX
   Port Number:            ZZZZ

6.  Acknowledgements

   The author would like to thank for following for lively discussions
   on list and in the halls (ordered by last name): Andy Bierman, Martin
   Bjorklund, Mehmet Ersue, Wes Hardaker, Stephen Hanna, David
   Harrington, Jeffrey Hutzelman, Radek Krejci, Alan Luchuk, Mouse, Russ
   Mundy, Tom Petch, Peter Saint-Andre, Joe Touch, Sean Turner, Bert
   Wijnen.

7.  References

7.1.  Normative References

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

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

   [RFC4252]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, January 2006.

   [RFC4253]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, January 2006.

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

   [RFC5539]  Badra, M., "NETCONF over Transport Layer Security (TLS)",
              RFC 5539, May 2009.





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   [RFC6020]  Bjorklund, M., "YANG - A Data Modeling Language for the
              Network Configuration Protocol (NETCONF)", RFC 6020,
              October 2010.

   [RFC6187]  Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure
              Shell Authentication", RFC 6187, March 2011.

   [RFC6241]  Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
              Bierman, "Network Configuration Protocol (NETCONF)", RFC
              6241, June 2011.

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, June 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165, RFC
              6335, August 2011.

7.2.  Informative References

   [Std-802.1AR-2009]
              IEEE SA-Standards Board, "IEEE Standard for Local and
              metropolitan area networks - Secure Device Identity",
              December 2009, <http://standards.ieee.org/findstds/
              standard/802.1AR-2009.html>.

   [draft-ietf-netconf-server-model]
              Watsen, K. and J. Schoenwaelder, "NETCONF Server
              Configuration Model", 2014, <http://tools.ietf.org/html/
              draft-ietf-netconf-server-model>.

   [iesg-statement]
              "Writable MIB Module IESG Statement", March 2014,
              <https://www.ietf.org/iesg/statement/writable-mib-
              module.html>.














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Appendix A.  Change Log

A.1.  00 to 01

   o  The term "TCP connection" is now used throughout.

   o  The terms "network element" and "management system" are now only
      used in the Motivation section.

   o  Restructured doc a little to create an Introduction section.

   o  Fixed reference in Applicability Statement so it would work
      equally well for SSH and TLS.

   o  Fixed reported odd wording and three references.

Author's Address

   Kent Watsen
   Juniper Networks

   EMail: kwatsen@juniper.net





























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