NETCONF Working Group                                          K. Watsen
Internet-Draft                                          Juniper Networks
Updates: 4253 (if approved)                                November 2013                            February 14, 2014
Intended status: Standards Track
Expires: May 05, August 18, 2014

                   Reverse Secure Shell (Reverse SSH)


   This memo 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, in order for the NETCONF client to open the 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 May 05, August 18, 2014.

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   Copyright (c) 2013 2014 IETF Trust and the persons identified as the
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Table of Contents

   1.  Requirements Terminology  . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Applicability Statement . . . . . . . . . . . . . . . . .   2
     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 . . . . . . . . . . . . . . . . . . .  12   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13   8
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . .  14 . . . . . . .   9
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  15   9
     A.1.  02 to 03  . . . . . . . . . . . . . . . . . . . . . . . .   9
     A.2.  01 to 02  . . . . . . . . . . . . . . . . . . . . . . . .  15
     A.2.   9
     A.3.  00 to 01  . . . . . . . . . . . . . . . . . . . . . . . .  15  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 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 the
   Security Considerations (Section 7) 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 is both necessary and
   desirable.  It is necessary because SSH channels and subsystems can
   only be opened on the SSH Server.  It is desirable because it
   conveniently leverages infrastructure that may be deployed for host-key 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
   the management system wishes to support.  For instance, in addition
   to having a SSH channel for NETCONF, the management system may "pin
   up" channels for Syslog, SNMP, or file-transfers.

4.  The Reverse SSH Protocol

   The NETCONF server's perspective (e.g. (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 accecpted accepted and a TCP session is established.

   o  Using this TCP connection, the NETCONF server immediately starts
      the SSH 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. (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, it
   needs to authenticate the remote peer.  Ultimately, this comes down
   to entails
   identifying the peer 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 identify it
   with is the source IP address of the TCP connection.  But this
   approach is limited as it only works in networks that only use known
   static addresses.

   To support network-elements having dynamically-assigned IP addresses
   or deployed behind gateways that translate their IP address (e.g. (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 be used as an identifier since the probability of collision
   is acceptibly acceptably low.  But this solution requires the management system
   to be configured with each device's host key each time it changes.

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

   The security of SSH is anchored in the ability for the SSH client to
   verify the SSH server's hostkey.  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 is to have the network
   element's host key signed by a common trusted key, such as a
   certificate authority.  Thus, the mangement system only needs to
   trust a single public key, which vouches for the authenticity of the
   various network element's specific element public key. keys.

   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. (e.g., its serial number) and be signed
   by a common trust anchor (e.g. (e.g., a certificate authority).  Examples
   of suitable public host keys are the X.509v3 keys defined in defined
   in [RFC6187].

6.  Device Configuration

   This section defines

   Configuring a device to initiate a Reverse SSH connection entails it
   knowing what IP address it should connect to and what SSH host-key it
   should present.  A complete YANG [RFC6020] module [RFC6020] to configure
   Reverse SSH
   on the device. is defined 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.

   Configuration Example

   <config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
      <reverse-ssh xmlns="urn:ietf:params:xml:ns:yang:ietf-reverse-ssh">
                  This entry requests the device to periodically
                  connect to the Configuration Manager application
                  This entry requests the device to mantain a
                  persistent connection to the Log Monitoring


   The YANG Module

   module ietf-reverse-ssh {

      namespace "urn:ietf:params:xml:ns:yang:ietf-reverse-ssh";

      prefix "rssh";

      import ietf-inet-types { prefix inet; }

        "IETF NETCONF (Network Configuration Protocol) Working Group";

        "WG Web:   <>
         WG List:  <>

         WG Chair: Bert Wijnen

         WG Chair: Mehmet Ersue

         Editor: Kent Watsen

      revision 2013-06-18 {
         description "Initial conception";
         reference "RFC XXXX: Reverse SSH";
      // RFC Ed.: replace XXXX with actual
      // RFC number and remove this note

      container reverse-ssh {
         container applications {
               "All the application that the device
               initiates Reverse SSH connections to";
            list application {
               key name;
               min-elements 1;
               leaf name {
                  mandatory true;
                  type string {
                     length 1..64;
                    "The name of the application the device is
                     connecting to";
               leaf description {
                  type string;
                   "An optional description for the application";
               container servers {
                     "An ordered listing of the application's
                     servers that the device should attempt
                     connecting to.";
                  list server {
                     key host;
                     min-elements 1;
                     ordered-by user;
                     leaf host {
                        mandatory true;
                        type inet:host;
                           "IP address or domain-name for
                           the server";
                     leaf port {
                        type inet:port-number;
                           "The IP port for this server.
                           The device will use the
                           IANA-assigned port if not
               container host-keys {
                     "An ordered listing of the SSH host keys the
                     device should advertise to the application.";
                  list host-key {
                     key name;
                     min-elements 1;
                     ordered-by user;
                     leaf name {
                        mandatory true;
                        type string {
                           length 1..64;
                          "The name of a host key the device
                           should advertise during the SSH
                           key exchange.";
               container connection-type {
                  description "Indicates the application's
                           preference for how the device's
                           connection is maintained.";
                  choice connection-type {
                     default persistent-connection;
                     case persistent-connection {
                        leaf persistent {
                           type empty;
                     case periodic-connection {
                        container periodic {
                           leaf timeout-mins {
                              type uint8;
                              default 5;
                              units minutes;
                                "The maximum amount of unconnected
                                 time the device will wait until
                                 establishing a connection to the
                                 applications again to send it.
                                 The device may establish a
                                 connection before this time if
                                 it has data it needs to send to
                                 the device.";
                           leaf linger-secs {
                              type uint8;
                              default 30;
                              units seconds;
                                "The amount of time the device should
                                 wait after last receiving data from
                                 or sending data to the device before
                                 closing its connection to the app.";

               container reconnect-strategy {
                  leaf start-with {
                     default first-listed;
                     type enumeration {
                        enum first-listed;
                        enum last-connected;
                  leaf interval-secs {
                     type uint8;
                     units seconds;
                     default 5;
                      "time delay between connection attempts";
                  leaf count-max {
                     type uint8;
                     default 3;
                      "num times try to connect to a server";
               container keep-alive-strategy {
                  leaf interval-secs {
                     type uint8;
                     units seconds;
                     default 15;
                      "Sets a timeout interval in seconds after
                       which if no data has been received from
                       the client, a message will be sent to
                       request a response from the SSH client.
                       A value of '0' indicates that no messages
                       should be sent.";
                  leaf count-max {
                     type uint8;
                     default 3;
                      "Sets the number of keep alive messages
                       that may be sent without receiving any
                       response from the SSH client before
                       assuming the SSH client is no longer
                       alive.  If this threshold is reached
                       the device will disconnect the SSH
                       session.  The keep alive interval timer
                       is reset after each transmission.  Thus,
                       an unresponsive SSH client will be
                       disconnected after approximately
                       'count-max * interval-secs' seconds.";

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 locally cached database.  It does not seem to matter if
   the SSH 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-
   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 (Algortihm Negotition) (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 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 to 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. (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".
   This port will be the default port for the Reverse SSH protocol and
   will be used when the NETCONF server needs to initiate a connection
   to a NETCONF client using SSH.  Below is the registration template
   following the rules in [RFC6335].

            Service Name:           reverse-ssh
            Transport Protocol(s):  TCP
            Assignee:               IESG <>
            Contact:                IETF Chair <>
            Description:            Reverse SSH (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, Mouse, Russ Mundy, Tom Petch, Peter
   Saint-Andre, Joe Touch, Sean Turner, Bert Wijnen.

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, Ed., "The Secure Shell (SSH)
              Protocol Assigned Numbers ", RFC 4250, December 2005.

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

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

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

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

   [RFC6020]  Bjorklund, M., Ed., "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., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A.
              Bierman, Ed., "NETCONF Configuration Protocol", RFC 6241, June

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

   [RFC6335]  Cotton, M., Ed., Eggert, L., Ed., Touch, J., Ed., Westerlund, M., Ed., and S.
              Cheshire, Ed., "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.  02 to 03

      Updated Device Configuration section to reference

A.2.  01 to 02

      Added Applicability Statement

      Removed references to ZeroConf / ZeroTouch

      Clarified the protocol section

      Added a section for identification and verification


A.3.  00 to 01


      Removed the hmac-* family of algorithms

Author's Address

   Kent Watsen
   Juniper Networks