NETCONF Working Group                                          K. Watsen
Internet-Draft                                          Juniper Networks
Updates: 4253 (if approved)                            February 14,                                   April 2014
Intended status: Standards Track
Expires: August 18, October 03, 2014

                   Reverse Secure Shell (Reverse SSH)
                   draft-ietf-netconf-reverse-ssh-03 SSH for NETCONF Call Home


   This memo document presents a technique for a NETCONF server to initiate a
   SSH connection to a NETCONF client.  This is accomplished by the
   NETCONF client listening on IANA-assigned TCP port YYYY and starting
   the SSH client protocol immediately after accepting a TCP connection
   on it.  This role-reversal is necessary as the NETCONF server must
   also be the SSH Server, server, in order for the NETCONF client to open the IANA-
   IANA-assigned SSH subsystem "netconf".

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
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   This Internet-Draft will expire on August 18, October 03, 2014.

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Table of Contents

   1.  Requirements Terminology  . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Applicability Statement . . . . . . . . . . . . . . . . .   2   3
     2.2.  Update to RFC 4253  . . . . . . . . . . . . . . . . . . .   3
   3.  Benefits to Device Management . . . . . . . . . . . . . . . .   3
   4.  The Reverse SSH Protocol  . . . . . . . . . . . . . . . . . .   4
   5.  SSH Server Identification and Verification  . . . . . . . . .   5
   6.  Device Configuration  . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .   9
     A.1.  02 to  03 to 04  . . . . . . . . . . . . . . . . . . . . . . . .   9
     A.2.  02 to 03  . . . . . . . . . . . . . . . . . . . . . . . .  10
     A.3.  01 to 02  . . . . . . . . . . . . . . . . . . . . . . . .   9
     A.3.  10
     A.4.  00 to 01  . . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Introduction

   This memo document presents a technique for a NETCONF [RFC6241] server to
   initiate a Secure Shell (SSH) [RFC4251] connection to a NETCONF
   client.  This is accomplished by the NETCONF client listening on
   IANA-assigned TCP port YYYY and starting the SSH client protocol
   immediately after accepting a TCP connection on it.  This role-
   reversal is necessary as the NETCONF server must also be the SSH
   server, in order for the NETCONF client to open the IANA-assigned SSH
   subsystem "netconf" [RFC6242].

2.1.  Applicability Statement

   The techniques described in this document are suitable for network
   management scenarios such as the ones described in section 3.
   However, these techniques MUST only be used for a NETCONF server to
   initiate a connection to a NETCONF client, as described in this

   The reason for this restriction is that different protocols have
   different security assumptions.  The NETCONF over SSH specification
   requires NETCONF clients and servers to verify the identity of the
   other party before starting the NETCONF protocol.  This contrasts
   with the base SSH protocol, which does not require programmatic
   verification of the other party.  In such circumstances, allowing the
   SSH server to contact the SSH client would open new vulnerabilities.
   Therefore, any use of Reverse SSH for purposes other than NETCONF
   will need a thorough, contextual security analysis.

2.2.  Update to RFC 4253

   This document updates the SSH Transport Layer Protocol [RFC4253] only
   by removing the restriction in Section 4 (Connection Setup) of
   [RFC4252] that the SSH Client must initiate the transport connection.
   Security implications related to this change are discussed in
   Security Considerations (Section 7).

3.  Benefits to Device Management

   The SSH protocol is nearly ubiquitous for device management, as it is
   the transport for the command-line applications `ssh`, `scp`, and
   `sftp` and is the required transport for the NETCONF protocol
   [RFC6241].  However, all these SSH-based protocols expect the network
   element to be the SSH server.

   Reverse SSH enables the network element to consistently be the SSH
   server regardless of which peer initiates the underlying TCP
   connection.  Maintaining the role of SSH Server server is both necessary and
   desirable.  It is necessary because SSH channels and subsystems can
   only be opened on the SSH Server. server.  It is desirable because it
   conveniently leverages infrastructure that may be deployed for host-
   key verification and user authentication.

   Reverse SSH is useful for both initial deployment and on-going device
   management and may be used to enable any of the following scenarios:

   o  The network element may proactively "call home" after being
      powered on for the first time 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

   o  The network element may be deployed behind a firewall that doesn't
      allow SSH 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.

   One key benefit of using SSH as the transport protocol is its ability
   to multiplex an unspecified number of independently flow-controlled
   TCP sessions [RFC4254].  This is valuable as the network element only
   needs to be configured to initiate a single Reverse SSH connection to
   the management system, regardless the number of TCP-based protocols NETCONF channels the
   management system wishes to support.  For instance, in addition wants to having a SSH channel for NETCONF, the management system may "pin
   up" channels for Syslog, SNMP, or file-transfers. open.

4.  The Reverse SSH Protocol

   The NETCONF server's perspective (e.g., the network element)

   o  The NETCONF server initiates a TCP connection to the NETCONF
      client on the IANA-assigned Reverse SSH port YYYY.

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

   o  Using this TCP connection, the NETCONF server immediately starts
      the SSH Server server protocol.  That is, the next message sent on the
      TCP stream is SSH's Protocol Version Exchange message (section
      4.2, [RFC4253]).

   o  The SSH connection is established.

   The NETCONF client's perspective (e.g., the management system)

   o  The NETCONF client listens for TCP connections on the IANA-
      assigned SSH port YYYY.

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

   o  Using this TCP connection, the NETCONF client immediately starts
      the SSH Client protocol, starting with sending the SSH's Protocol
      Version Exchange message (section 4.2, [RFC4253]).

   o  The SSH connection is established.

5.  SSH Server Identification and Verification

   When the management system accepts a new incoming connection, TCP connection on
   the Reverse SSH port, it
   needs to starts the SSH client protocol.  As the SSH
   client, it MUST authenticate the remote peer.  Ultimately, this entails SSH server, by both identifying the peer
   network element and verifying its SSH host key.

   Due to Reverse SSH having the network element initiate the TCP
   connection, the first data the management system has to MAY identify it
   with is the remote peer using
   the source IP address of the TCP connection.  But this
   approach  However, identifying
   the remote peer using the source IP address of the TCP connection is limited
   NOT RECOMMENDED as it can only works work in networks that use known static

   To support network-elements network elements having dynamically-assigned IP addresses addresses,
   or deployed behind gateways that translate their IP address addresses (e.g.,
   NAT), the management system MAY identify the device using its SSH
   host key.  For instance, a fingerprint of the network element's host
   key could itself be used as an identifier since each device has a
   statistically unique host key.  However, identifying the probability of collision remote peer
   using its host key directly is acceptably low.  But this solution NOT RECOMMENDED as it requires the management system
   host key to be configured with each device's manually verified the first time the network element
   connects and anytime its host key each time it changes. changes thereafter.

   Yet another option for identifying the network element is for its
   host key to encode its the network element's identity, such as if it the
   host key were a certificate.  This option enables the host key to
   change over time, so long as it continues to encode the same
   identity, but brings the next issue of how the mangement element management system can
   verify the network element's host key is authentic.

   The security of SSH is anchored in the ability for the SSH client to
   verify the SSH server's hostkey. host key.  Typically this is done by
   comparing the host key presented by the SSH server with one that was
   previously configured on the SSH client, looking it up in a local
   database using the identity of the SSH client as the lookup key.
   Nothing changes regarding this requirement due to the direction
   reversal of the underlying TCP connection.  To ensure security, the
   management system MUST verify the network element's SSH host key each
   time a SSH session is established.

   However, configuring distinct host keys on the management system
   doesn't scale well, which is an important consideration to a network
   management system.  A more scalable strategy for the management
   system is to have for the network element's manufacturer to sign the network-
   element's host key signed by with a common trusted key, such as a certificate
   authority.  Thus,  Then, when the mangement network-element is deployed, the
   management system only needs to trust a single public key, certificate, which
   vouches for the authenticity of the various network element public host

   Since both the identification and verification issues are addressed
   using certificates, this draft RECOMMENDS network elements use a host
   key that can encode a unique (e.g., its serial number) and be signed
   by a common trust anchor (e.g., a certificate authority).  Examples
   of suitable public host keys are the X.509v3 keys defined in defined
   in [RFC6187].

6.  Device Configuration

   Configuring a device to initiate a Reverse SSH connection is outside
   the scope of this document, but entails it
   knowing setting what IP address it a
   device should connect to and what SSH host-key it should present.  A
   complete YANG module [RFC6020] model to configure Reverse SSH is defined specified
   in [I.D.kwatsen-netconf-server] .  This YANG
   module enables a NETCONF client to generically manage a NETCONF
   server's Reverse SSH configuration.  Key aspects of this YANG module
   include support for more than one application, more than one server
   per application, and a reconnection strategy. [draft-ietf-netconf-server-model]

7.  Security Considerations

   This RFC deviates from standard SSH protocol usage by allowing the
   SSH server to initiate the TCP connection.  This conflicts with
   section 4 of the SSH Transport Layer Protocol RFC [RFC4253], which
   states "The client initiates the connection".  However this statement
   is made without rationalization and it's not clear how it impacts the
   security of the protocol, so this section analyzes the security
   offered by having the client initiate the connection.

   First, assuming the SSH server is not using a public host key
   algorithm that certifies its identity, the security of the protocol
   doesn't seem to be sensitive to which peer initiates the connection.

   That is, it is still the case that reliable distribution of host keys
   (or their fingerprints) should occur prior to first connection and
   that verification for subsequent connections happens by comparing the
   host keys in a locally cached database.  It does not seem to matter
   if the SSH Server's server's host name is derived from user-input or extracted
   from the TCP layer, potentially via a reverse-DNS lookup.  Once the
   host name-to-key association is stored in a local database, no man-
   in-the-middle attack is possible due to the attacker being unable to
   guess the real SSH server's private key (Section 9.3.4 (Man-in-th- (Man-in-the-
   middle) of [RFC4251]).

   That said, this RFC recommends implementations use a public host key
   algorithm that certifies the SSH server's identity.  The identity can
   be any unique identifier, such as a device's serial number or a
   deployment-specific value.  If this recommendation is followed, then
   no information from the TCP layer would be needed to lookup the
   device in a local database and therefore the directionality of the
   TCP layer is clearly inconsequential.

   The SSH protocol negotiates which algorithms it will use during key
   exchange (Section 7.1 (Algorithm Negotiation) in [RFC4253]).  The
   algorithm selected is essentially the first compatible algorithm
   listed by the SSH client that is also listed by the SSH server.  For
   a network management application, there may be a need to advertise a
   large number of algorithms to be compatible with the various devices
   it manages.  The SSH client SHOULD order its list of public host key
   algorithms such that all the certifiable public host key algorithms
   are listed first.  Additionally, when possible, SSH servers SHOULD
   only list certifiable public host key algorithms.  Note that since
   the SSH server would have to be configured to know which IP address
   it needs is to connect to, it is expected that it will also be configured
   to know which host key algorithm to use for the particular
   application, and hence only needs to list just that one public host
   key algorithm.

   This RFC suggests implementations can use a device's serial number as
   a form of identity.  A potential concern with using a serial number
   is that the SSH protocol passes the SSH server's host-key in the
   clear and many times serial numbers encode revealing information
   about the device, such as what kind of device it is and when it was
   manufactured.  While there is little security in trying to hide this
   information from an attacker, it is understood that some deployments
   may want to keep this information private.  If this is a concern,
   deployments MAY consider using instead a hash of the device's serial
   number or an application-specified unique identifier.

   An attacker could DoS the application 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

8.  IANA Considerations

   This document requests that IANA assigns a TCP port number in the
   "Registered Port Numbers" range with the service name "reverse-ssh". "netconf-ssh-
   ch".  This port will be the default port for the Reverse SSH for
   NETCONF Call Home protocol and will be used when the NETCONF server needs
   is to initiate a connection to a NETCONF client using SSH.  Below is
   the registration template following the rules in [RFC6335].

         Service Name:           reverse-ssh           netconf-ssh-ch
         Transport Protocol(s):  TCP
         Assignee:               IESG <>
         Contact:                IETF Chair <>
         Description:            Reverse SSH (call home) for NETCONF Call Home
         Reference:              RFC XXXX
         Port Number:            YYYY

9.  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

10.  References

10.1.  Normative References

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

   [RFC4250]  Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Assigned Numbers ", RFC 4250, December 2005.

   [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.

   [RFC4254]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Connection Protocol ", RFC 4254, January 2006.

   [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, "NETCONF Configuration Protocol", RFC 6241, June

   [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", RFC 6335, August

10.2.  Informative References


              Watsen, K. and J. Schoenwaelder, "A YANG Data Model for
              NETCONF Server Configuration", RFC 6242, June 2011.

Appendix A.  Change Log

A.1.  03 to 04

      Changed title to "Reverse SSH for NETCONF Call Home"

      Removed statement on how other SSH channels might be used for
      other protocols

      Improved language on how the management system, as the SSH client,
      MUST authenticate the SSH server

      Clarified that identifying the network element using source IP
      address is NOT RECOMMENDED
      Clarified that identifying the NE using simple certificate
      comparison is NOT RECOMMENDED

      Device Configuration section now more clearly states that the YANG
      model is out of scope

      Change requested port name to "netconf-ssh-ch"

      General edits for grammer, capitalization, and spellings

A.2.  02 to 03

      Updated Device Configuration section to reference


A.3.  01 to 02

      Added Applicability Statement

      Removed references to ZeroConf / ZeroTouch

      Clarified the protocol section

      Added a section for identification and verification


A.4.  00 to 01

      Removed the hmac-* family of algorithms

Author's Address

   Kent Watsen
   Juniper Networks