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NETCONF Call Home and RESTCONF Call Home
draft-ietf-netconf-call-home-02

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 8071.
Author Kent Watsen
Last updated 2014-12-05
Replaces draft-ietf-netconf-reverse-ssh
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draft-ietf-netconf-call-home-02
NETCONF Working Group                                          K. Watsen
Internet-Draft                                          Juniper Networks
Updates: 4253 (if approved)                             December 5, 2014
Intended status: Standards Track
Expires: June 8, 2015

                NETCONF Call Home and RESTCONF Call Home
                    draft-ietf-netconf-call-home-02

Abstract

   This document presents NETCONF Call Home and RESTCONF Call Home,
   which respectively enable a NETCONF/RESTCONF server to initiate a
   secure connection to a NETCONF/RESTCONF client.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on June 8, 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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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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 or RESTCONF Server . . . . . . . . . . . .   4
     2.1.  Protocol Operation  . . . . . . . . . . . . . . . . . . .   4
     2.2.  Configuration Data Model  . . . . . . . . . . . . . . . .   5
   3.  The NETCONF Client or RESTCONF Client . . . . . . . . . . . .   5
     3.1.  Protocol Operation  . . . . . . . . . . . . . . . . . . .   5
     3.2.  Server Identification and Verification  . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  11
     A.1.  00 to 01  . . . . . . . . . . . . . . . . . . . . . . . .  11
     A.2.  01 to 02  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   This document presents NETCONF Call Home and RESTCONF Call Home,
   which respectively enable a NETCONF/RESTCONF server to initiate a
   secure connection to a NETCONF/RESTCONF client.  The NETCONF protocol
   is described in [RFC6241] and the RESTCONF is described in
   [draft-ietf-netconf-restconf].

   Both NETCONF Call Home and RESTCONF Call Home preserve the SSH
   [RFC4253] and TLS [RFC5246] transport roles, as when compared to
   standard NETCONF and RESTCONF connections.  Specifically, regardless
   if call home is used or not, the NETCONF server is always the SSH or
   TLS server, and the RESTCONF server is always the TLS server.

   Ensuring consistency in the SSH and TLS roles is both necessary and
   desirable.  Ensuring consistency is necessary, for the SSH protocol,
   as SSH channels and subsystems can only be opened on the SSH server,
   as is needed to support NETCONF over SSH [RFC6242].  Ensuring
   consistency 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 the NETCONF protocol, and the RESTCONF protocol
   by extension, 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 NETCONF Call Home
   and RESTCONF Call Home, as described in this document.

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   The reason for this restriction is that different protocols have
   different security assumptions.  The NETCONF and RESTCONF protocols
   require clients and servers to verify the identity of the other party
   before starting the NETCONF/RESTCONF protocol (section 2.2 of
   [RFC6241] and section FIXME of [draft-ietf-netconf-restconf]).

   This contrasts with the base SSH and TLS protocols, which do not
   require programmatic verification of the other party (section 9.3.4
   of [RFC4251], section 4 of [RFC4252], and section 7.3 of [RFC5246]).
   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 or RESTCONF 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), which the server initiates in a call home
   connection.  Security implications related to this change are
   discussed in Security Considerations (Section 4).

2.  The NETCONF Server or RESTCONF Server

2.1.  Protocol Operation

   o  The NETCONF/RESTCONF server initiates a TCP connection to the
      NETCONF/RESTCONF client on one of the IANA-assigned ports for call
      home (PORT-X for netconf-ch-ssh, PORT-Y for netconf-ch-tls, or
      PORT-Z for restconf-ch-tls).

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

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

   o  When port PORT-X or PORT-Y is connected, the NETCONF protocol
      proceeds normally for SSH and TLS, as defined in [RFC6242] and
      [RFC5539] respectively.  When port PORT-Z is connected, the
      RESTCONF protocol proceeds normally for TLS, as defined in
      [draft-ietf-netconf-restconf].

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2.2.  Configuration Data Model

   How to configure a NETCONF or RESTCONF server to initiate a 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 both
   NETCONF Call Home and RESTCONF Call Home is provided in
   [draft-ietf-netconf-server-model].

3.  The NETCONF Client or RESTCONF Client

3.1.  Protocol Operation

   o  The NETCONF/RESTCONF client listens for TCP connections on one or
      all of the IANA-assigned ports for NETCONF Call Home (PORT-X for
      netconf-ch-ssh and PORT-Y for netconf-ch-tls) or RESTCONF Call
      Home (PORT-Z for restconf-ch-tls).

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

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

   o  When port PORT-X or PORT-Y is connected, the NETCONF protocol
      proceeds normally for SSH and TLS, as defined in [RFC6242] and
      [RFC5539] respectively.  When port PORT-Z is connected, the
      RESTCONF protocol proceeds normally for TLS, as defined in
      [draft-ietf-netconf-restconf].

3.2.  Server Identification and Verification

   Under normal circumstances, a NETCONF/RESTCONF client initiates the
   connection to the NETCONF/RESTCONF server.  This action provides
   essential input to verify the NETCONF/RESTCONF 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 call home connection, the NETCONF/RESTCONF
   client does not have any context leading it to know the connection is
   from a particular NETCONF/RESTCONF server.  Thus the NETCONF/RESTCONF

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   client must derive the NETCONF/RESTCONF server's identity using
   information provided by the network and the NETCONF/RESTCONF server
   itself.  This section describes strategies a NETCONF/RESTCONF client
   can use to identify a NETCONF/RESTCONF server.

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

   The first information a NETCONF/RESTCONF client learns from a call
   rhHome connection is the IP address of the NETCONF/RESTCONF 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/RESTCONF connection would have worked just as
   well.  Due to this limited use, it is not recommended to identify a
   NETCONF/RESTCONF server based on its source IP address.

   The next information a NETCONF/RESTCONF client learns is provided by
   the NETCONF/RESTCONF 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/RESTCONF server using it directly
   (e.g., a fingerprint), since each NETCONF/RESTCONF server is assumed
   to have a statistically unique public key, even in virtualized
   environments.  This strategy also provides a mechanism to verify the
   NETCONF/RESTCONF server, in that a secure connection can only be
   established with the NETCONF/RESTCONF 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/RESTCONF servers have self-signed
   certificates.  This strategy is viable and useful when the NETCONF/
   RESTCONF 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/RESTCONF 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/RESTCONF client.  This
   strategy's use of PKI enables a NETCONF/RESTCONF client to
   transparently authenticate NETCONF/RESTCONF servers, thus eliminating

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   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/RESTCONF 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/RESTCONF servers are pre-provisioned this way, NETCONF/
   RESTCONF clients can transparently authenticate NETCONF/RESTCONF
   servers using just the manufacturer's trust anchor and a list of
   expected NETCONF/RESTCONF server identifiers, which could be provided
   along with 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], [RFC5246], and
   [draft-ietf-netconf-restconf] 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.

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

   An attacker could DoS the NETCONF/RESTCONF 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).

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5.  IANA Considerations

   This document requests that IANA assigns three TCP port numbers in
   the "Registered Port Numbers" range with the service names "netconf-
   ch-ssh", "netconf-ch-tls", and "restconf-ch-tls".  These ports will
   be the default ports for NETCONF Call Home and RESTCONF Call Home
   protocols.  Below is the registration template following the rules in
   [RFC6335].

   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:            PORT-X

   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:            PORT-Y

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

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

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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.

   [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.

   [draft-ietf-netconf-restconf]
              Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", draft-ieft-netconf-restconf-04 (work in
              progress), 2014.

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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.

A.2.  01 to 02

   o  Added call home support for the RESTCONF protocol.

   o  Fixed paragraph 3 of Security Considerations to equally apply to
      the TLS protocol.

Author's Address

   Kent Watsen
   Juniper Networks

   EMail: kwatsen@juniper.net

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