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NETCONF Working Group                                          K. Watsen
Internet-Draft                                           Watsen Networks
Intended status: Standards Track                                   G. Wu
Expires: 11 January 2021                                   Cisco Systems
                                                            10 July 2020


             YANG Groupings for SSH Clients and SSH Servers
                draft-ietf-netconf-ssh-client-server-21

Abstract

   This document defines three YANG modules: the first defines groupings
   for a generic SSH client, the second defines groupings for a generic
   SSH server, and the third defines common identities and groupings
   used by both the client and the server.  It is intended that these
   groupings will be used by applications using the SSH protocol.

Editorial Note (To be removed by RFC Editor)

   This draft contains placeholder values that need to be replaced with
   finalized values at the time of publication.  This note summarizes
   all of the substitutions that are needed.  No other RFC Editor
   instructions are specified elsewhere in this document.

   Artwork in this document contains shorthand references to drafts in
   progress.  Please apply the following replacements:

   *  "AAAA" --> the assigned RFC value for draft-ietf-netconf-crypto-
      types

   *  "BBBB" --> the assigned RFC value for draft-ietf-netconf-trust-
      anchors

   *  "CCCC" --> the assigned RFC value for draft-ietf-netconf-keystore

   *  "DDDD" --> the assigned RFC value for draft-ietf-netconf-tcp-
      client-server

   *  "EEEE" --> the assigned RFC value for this draft

   Artwork in this document contains placeholder values for the date of
   publication of this draft.  Please apply the following replacement:

   *  "2020-07-10" --> the publication date of this draft

   The following Appendix section is to be removed prior to publication:




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   *  Appendix A.  Change Log

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 https://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 11 January 2021.

Copyright Notice

   Copyright (c) 2020 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 (https://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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Relation to other RFCs  . . . . . . . . . . . . . . . . .   4
     1.2.  Specification Language  . . . . . . . . . . . . . . . . .   6
     1.3.  Adherence to the NMDA . . . . . . . . . . . . . . . . . .   6
   2.  The "ietf-ssh-common" Module  . . . . . . . . . . . . . . . .   6
     2.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .   6
     2.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .   9
     2.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .   9
   3.  The "ietf-ssh-client" Module  . . . . . . . . . . . . . . . .  19
     3.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .  19
     3.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  21
     3.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  25
   4.  The "ietf-ssh-server" Module  . . . . . . . . . . . . . . . .  32



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     4.1.  Data Model Overview . . . . . . . . . . . . . . . . . . .  32
     4.2.  Example Usage . . . . . . . . . . . . . . . . . . . . . .  34
     4.3.  YANG Module . . . . . . . . . . . . . . . . . . . . . . .  38
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  47
     5.1.  The "ietf-ssh-common" YANG Module . . . . . . . . . . . .  47
     5.2.  The "ietf-ssh-client" YANG Module . . . . . . . . . . . .  48
     5.3.  The "ietf-ssh-server" YANG Module . . . . . . . . . . . .  49
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  50
     6.1.  The "IETF XML" Registry . . . . . . . . . . . . . . . . .  50
     6.2.  The "YANG Module Names" Registry  . . . . . . . . . . . .  50
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  51
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  52
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  54
     A.1.  00 to 01  . . . . . . . . . . . . . . . . . . . . . . . .  54
     A.2.  01 to 02  . . . . . . . . . . . . . . . . . . . . . . . .  54
     A.3.  02 to 03  . . . . . . . . . . . . . . . . . . . . . . . .  54
     A.4.  03 to 04  . . . . . . . . . . . . . . . . . . . . . . . .  55
     A.5.  04 to 05  . . . . . . . . . . . . . . . . . . . . . . . .  55
     A.6.  05 to 06  . . . . . . . . . . . . . . . . . . . . . . . .  55
     A.7.  06 to 07  . . . . . . . . . . . . . . . . . . . . . . . .  55
     A.8.  07 to 08  . . . . . . . . . . . . . . . . . . . . . . . .  56
     A.9.  08 to 09  . . . . . . . . . . . . . . . . . . . . . . . .  56
     A.10. 09 to 10  . . . . . . . . . . . . . . . . . . . . . . . .  56
     A.11. 10 to 11  . . . . . . . . . . . . . . . . . . . . . . . .  56
     A.12. 11 to 12  . . . . . . . . . . . . . . . . . . . . . . . .  56
     A.13. 12 to 13  . . . . . . . . . . . . . . . . . . . . . . . .  57
     A.14. 13 to 14  . . . . . . . . . . . . . . . . . . . . . . . .  57
     A.15. 14 to 15  . . . . . . . . . . . . . . . . . . . . . . . .  57
     A.16. 15 to 16  . . . . . . . . . . . . . . . . . . . . . . . .  57
     A.17. 16 to 17  . . . . . . . . . . . . . . . . . . . . . . . .  57
     A.18. 17 to 18  . . . . . . . . . . . . . . . . . . . . . . . .  58
     A.19. 18 to 19  . . . . . . . . . . . . . . . . . . . . . . . .  58
     A.20. 19 to 20  . . . . . . . . . . . . . . . . . . . . . . . .  59
     A.21. 20 to 21  . . . . . . . . . . . . . . . . . . . . . . . .  59
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  59
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  59














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1.  Introduction

   This document defines three YANG 1.1 [RFC7950] modules: the first
   defines a grouping for a generic SSH client, the second defines a
   grouping for a generic SSH server, and the third defines identities
   and groupings common to both the client and the server.  It is
   intended that these groupings will be used by applications using the
   SSH protocol [RFC4252], [RFC4253], and [RFC4254].  For instance,
   these groupings could be used to help define the data model for an
   OpenSSH [OPENSSH] server or a NETCONF over SSH [RFC6242] based
   server.

   The client and server YANG modules in this document each define one
   grouping, which is focused on just SSH-specific configuration, and
   specifically avoids any transport-level configuration, such as what
   ports to listen on or connect to.  This affords applications the
   opportunity to define their own strategy for how the underlying TCP
   connection is established.  For instance, applications supporting
   NETCONF Call Home [RFC8071] could use the "ssh-server-grouping"
   grouping for the SSH parts it provides, while adding data nodes for
   the TCP-level call-home configuration.

   The modules defined in this document use groupings defined in
   [I-D.ietf-netconf-keystore] enabling keys to be either locally
   defined or a reference to globally configured values.

   The modules defined in this document optionally support [RFC6187]
   enabling X.509v3 certificate based host keys and public keys.

1.1.  Relation to other RFCs

   This document presents one or more YANG modules [RFC7950] that are
   part of a collection of RFCs that work together to define
   configuration modules for clients and servers of both the NETCONF
   [RFC6241] and RESTCONF [RFC8040] protocols.

   The modules have been defined in a modular fashion to enable their
   use by other efforts, some of which are known to be in progress at
   the time of this writing, with many more expected to be defined in
   time.

   The relationship between the various RFCs in the collection is
   presented in the below diagram.  The labels in the diagram represent
   the primary purpose provided by each RFC.  Links the each RFC are
   provided below the diagram.






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                                  crypto-types
                                    ^      ^
                                   /        \
                                  /          \
                         truststore         keystore
                          ^     ^             ^  ^
                          |     +---------+   |  |
                          |               |   |  |
                          |      +------------+  |
   tcp-client-server      |     /         |      |
      ^    ^        ssh-client-server     |      |
      |    |           ^            tls-client-server
      |    |           |              ^     ^        http-client-server
      |    |           |              |     |                 ^
      |    |           |        +-----+     +---------+       |
      |    |           |        |                     |       |
      |    +-----------|--------|--------------+      |       |
      |                |        |              |      |       |
      +-----------+    |        |              |      |       |
                  |    |        |              |      |       |
                  |    |        |              |      |       |
               netconf-client-server       restconf-client-server

   +=======================+===========================================+
   | Label in Diagram      | Originating RFC                           |
   +=======================+===========================================+
   | crypto-types          | [I-D.ietf-netconf-crypto-types]           |
   +-----------------------+-------------------------------------------+
   | truststore            | [I-D.ietf-netconf-trust-anchors]          |
   +-----------------------+-------------------------------------------+
   | keystore              | [I-D.ietf-netconf-keystore]               |
   +-----------------------+-------------------------------------------+
   | tcp-client-server     | [I-D.ietf-netconf-tcp-client-server]      |
   +-----------------------+-------------------------------------------+
   | ssh-client-server     | [I-D.ietf-netconf-ssh-client-server]      |
   +-----------------------+-------------------------------------------+
   | tls-client-server     | [I-D.ietf-netconf-tls-client-server]      |
   +-----------------------+-------------------------------------------+
   | http-client-server    | [I-D.ietf-netconf-http-client-server]     |
   +-----------------------+-------------------------------------------+
   | netconf-client-server | [I-D.ietf-netconf-netconf-client-server]  |
   +-----------------------+-------------------------------------------+
   |restconf-client-server | [I-D.ietf-netconf-restconf-client-server] |
   +-----------------------+-------------------------------------------+

                       Table 1: Label to RFC Mapping





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1.2.  Specification Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.3.  Adherence to the NMDA

   This document in compliant with the Network Management Datastore
   Architecture (NMDA) [RFC8342].  For instance, as described in
   [I-D.ietf-netconf-trust-anchors] and [I-D.ietf-netconf-keystore],
   trust anchors and keys installed during manufacturing are expected to
   appear in <operational>.

2.  The "ietf-ssh-common" Module

   The SSH common model presented in this section contains identities
   and groupings common to both SSH clients and SSH servers.  The
   "transport-params-grouping" grouping can be used to configure the
   list of SSH transport algorithms permitted by the SSH client or SSH
   server.  The lists of algorithms are ordered such that, if multiple
   algorithms are permitted by the client, the algorithm that appears
   first in its list that is also permitted by the server is used for
   the SSH transport layer connection.  The ability to restrict the
   algorithms allowed is provided in this grouping for SSH clients and
   SSH servers that are capable of doing so and may serve to make SSH
   clients and SSH servers compliant with security policies.

   Features are defined for algorithms that are OPTIONAL or are not
   widely supported by popular implementations.  Note that the list of
   algorithms is not exhaustive.  As well, some algorithms that are
   REQUIRED by [RFC4253] are missing, notably "ssh-dss" and "diffie-
   hellman-group1-sha1" due to their weak security and there being
   alternatives that are widely supported.

2.1.  Data Model Overview

2.1.1.  Features

   The following diagram lists all the "feature" statements defined in
   the "ietf-ssh-common" module:








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   Features:
     +-- ssh-ecc
     +-- ssh-x509-certs
     +-- ssh-dh-group-exchange
     +-- ssh-ctr
     +-- ssh-sha2

2.1.2.  Identities

   The following diagram illustrates the relationship amongst the
   "identity" statements defined in the "ietf-ssh-common" module:

   Identities:
     +-- public-key-alg-base
     |  +-- ssh-dss
     |  +-- ssh-rsa
     |  +-- ecdsa-sha2-nistp256
     |  +-- ecdsa-sha2-nistp384
     |  +-- ecdsa-sha2-nistp521
     |  +-- x509v3-ssh-rsa
     |  +-- x509v3-rsa2048-sha256
     |  +-- x509v3-ecdsa-sha2-nistp256
     |  +-- x509v3-ecdsa-sha2-nistp384
     |  +-- x509v3-ecdsa-sha2-nistp521
     +-- key-exchange-alg-base
     |  +-- diffie-hellman-group14-sha1
     |  +-- diffie-hellman-group-exchange-sha1
     |  +-- diffie-hellman-group-exchange-sha256
     |  +-- ecdh-sha2-nistp256
     |  +-- ecdh-sha2-nistp384
     |  +-- ecdh-sha2-nistp521
     +-- encryption-alg-base
     |  +-- triple-des-cbc
     |  +-- aes128-cbc
     |  +-- aes192-cbc
     |  +-- aes256-cbc
     |  +-- aes128-ctr
     |  +-- aes192-ctr
     |  +-- aes256-ctr
     +-- mac-alg-base
        +-- hmac-sha1
        +-- hmac-sha2-256
        +-- hmac-sha2-512

   Comments:

   *  The diagram shows that there are four base identities.




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   *  These identities are used by this module to define algorithms for
      public-key, key-exchange, encryption, and MACs.
   *  These base identities are "abstract", in the object orientied
      programming sense, in that they only define a "class" of
      algorithms, rather than a specific algorithm.

2.1.3.  Groupings

   The following diagram lists all the "grouping" statements defined in
   the "ietf-ssh-common" module:

   Groupings:
     +-- transport-params-grouping

   Each of these groupings are presented in the following subsections.

2.1.3.1.  The "transport-params-grouping" Grouping

   The following tree diagram [RFC8340] illustrates the "transport-
   params-grouping" grouping:

     grouping transport-params-grouping
       +-- host-key
       |  +-- host-key-alg*   identityref
       +-- key-exchange
       |  +-- key-exchange-alg*   identityref
       +-- encryption
       |  +-- encryption-alg*   identityref
       +-- mac
          +-- mac-alg*   identityref

   Comments:

   *  This grouping is used by both the "ssh-client-grouping" and the
      "ssh-server-grouping" groupings defined in Section 3.1.2.1 and
      Section 4.1.2.1, respectively.

   *  This grouping enables client and server configurations to specify
      the algorithms that are to be used when establishing SSH sessions.

   *  Each list is "ordered-by user".

2.1.4.  Protocol-accessible Nodes

   The "ietf-ssh-common" module does not contain any protocol-accessible
   nodes, but the module needs to be "implemented", as described in
   Section 5.6.5 of [RFC7950], in order for the identities in
   Section 2.1.2 to be defined.



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2.2.  Example Usage

   This following example illustrates how the "transport-params-
   grouping' grouping appears when populated with some data.

   <transport-params
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-common"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">
     <host-key>
       <host-key-alg>algs:x509v3-rsa2048-sha256</host-key-alg>
       <host-key-alg>algs:ssh-rsa</host-key-alg>
     </host-key>
     <key-exchange>
       <key-exchange-alg>
         algs:diffie-hellman-group-exchange-sha256
       </key-exchange-alg>
     </key-exchange>
     <encryption>
       <encryption-alg>algs:aes256-ctr</encryption-alg>
       <encryption-alg>algs:aes192-ctr</encryption-alg>
       <encryption-alg>algs:aes128-ctr</encryption-alg>
       <encryption-alg>algs:aes256-cbc</encryption-alg>
       <encryption-alg>algs:aes192-cbc</encryption-alg>
       <encryption-alg>algs:aes128-cbc</encryption-alg>
     </encryption>
     <mac>
       <mac-alg>algs:hmac-sha2-256</mac-alg>
       <mac-alg>algs:hmac-sha2-512</mac-alg>
     </mac>
   </transport-params>

2.3.  YANG Module

   This YANG module has normative references to [RFC4253], [RFC4344],
   [RFC4419], [RFC5656], [RFC6187], and [RFC6668].

   <CODE BEGINS> file "ietf-ssh-common@2020-07-10.yang"

   module ietf-ssh-common {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-common";
     prefix sshcmn;

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

     contact
       "WG Web:   <http://datatracker.ietf.org/wg/netconf/>



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        WG List:  <mailto:netconf@ietf.org>
        Author:   Kent Watsen <mailto:kent+ietf@watsen.net>
        Author:   Gary Wu <mailto:garywu@cisco.com>";

     description
       "This module defines a common features, identities, and
        groupings for Secure Shell (SSH).

        Copyright (c) 2020 IETF Trust and the persons identified
        as authors of the code. All rights reserved.

        Redistribution and use in source and binary forms, with
        or without modification, is permitted pursuant to, and
        subject to the license terms contained in, the Simplified
        BSD License set forth in Section 4.c of the IETF Trust's
        Legal Provisions Relating to IETF Documents
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC EEEE
        (https://www.rfc-editor.org/info/rfcEEEE); see the RFC
        itself for full legal notices.;

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
        'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
        'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
        are to be interpreted as described in BCP 14 (RFC 2119)
        (RFC 8174) when, and only when, they appear in all
        capitals, as shown here.";

     revision 2020-07-10 {
       description
         "Initial version";
       reference
         "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
     }

     // Features

     feature ssh-ecc {
       description
         "Elliptic Curve Cryptography is supported for SSH.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     feature ssh-x509-certs {
       description



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         "X.509v3 certificates are supported for SSH per RFC 6187.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     feature ssh-dh-group-exchange {
       description
         "Diffie-Hellman Group Exchange is supported for SSH.";
       reference
         "RFC 4419: Diffie-Hellman Group Exchange for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     feature ssh-ctr {
       description
         "SDCTR encryption mode is supported for SSH.";
       reference
         "RFC 4344: The Secure Shell (SSH) Transport Layer
                    Encryption Modes";
     }

     feature ssh-sha2 {
       description
         "The SHA2 family of cryptographic hash functions is
          supported for SSH.";
       reference
         "FIPS PUB 180-4: Secure Hash Standard (SHS)";
     }

     // Identities

     identity public-key-alg-base {
       description
         "Base identity used to identify public key algorithms.";
     }

     identity ssh-dss {
       base public-key-alg-base;
       description
         "Digital Signature Algorithm using SHA-1 as the
          hashing algorithm.";
       reference
         "RFC 4253:
            The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity ssh-rsa {



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       base public-key-alg-base;
       description
         "RSASSA-PKCS1-v1_5 signature scheme using SHA-1 as the
          hashing algorithm.";
       reference
         "RFC 4253:
            The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity ecdsa-sha2-nistp256 {
       if-feature "ssh-ecc and ssh-sha2";
       base public-key-alg-base;
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA) using the
          nistp256 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity ecdsa-sha2-nistp384 {
       if-feature "ssh-ecc and ssh-sha2";
       base public-key-alg-base;
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA) using the
          nistp384 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity ecdsa-sha2-nistp521 {
       if-feature "ssh-ecc and ssh-sha2";
       base public-key-alg-base;
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA) using the
          nistp521 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity x509v3-ssh-rsa {
       if-feature "ssh-x509-certs";
       base public-key-alg-base;
       description
         "RSASSA-PKCS1-v1_5 signature scheme using a public key stored
          in an X.509v3 certificate and using SHA-1 as the hashing



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          algorithm.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity x509v3-rsa2048-sha256 {
       if-feature "ssh-x509-certs and ssh-sha2";
       base public-key-alg-base;
       description
         "RSASSA-PKCS1-v1_5 signature scheme using a public key stored
          in an X.509v3 certificate and using SHA-256 as the hashing
          algorithm.  RSA keys conveyed using this format MUST have a
          modulus of at least 2048 bits.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity x509v3-ecdsa-sha2-nistp256 {
       if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
       base public-key-alg-base;
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA)
          using the nistp256 curve with a public key stored in
          an X.509v3 certificate and using the SHA2 family of
          hashing algorithms.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity x509v3-ecdsa-sha2-nistp384 {
       if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
       base public-key-alg-base;
       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA)
          using the nistp384 curve with a public key stored in
          an X.509v3 certificate and using the SHA2 family of
          hashing algorithms.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity x509v3-ecdsa-sha2-nistp521 {
       if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
       base public-key-alg-base;



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       description
         "Elliptic Curve Digital Signature Algorithm (ECDSA)
          using the nistp521 curve with a public key stored in
          an X.509v3 certificate and using the SHA2 family of
          hashing algorithms.";
       reference
         "RFC 6187: X.509v3 Certificates for Secure Shell
                    Authentication";
     }

     identity key-exchange-alg-base {
       description
         "Base identity used to identify key exchange algorithms.";
     }

     identity diffie-hellman-group14-sha1 {
       base key-exchange-alg-base;
       description
         "Diffie-Hellman key exchange with SHA-1 as HASH and
          Oakley Group 14 (2048-bit MODP Group).";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity diffie-hellman-group-exchange-sha1 {
       if-feature "ssh-dh-group-exchange";
       base key-exchange-alg-base;
       description
         "Diffie-Hellman Group and Key Exchange with SHA-1 as HASH.";
       reference
         "RFC 4419: Diffie-Hellman Group Exchange for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     identity diffie-hellman-group-exchange-sha256 {
       if-feature "ssh-dh-group-exchange and ssh-sha2";
       base key-exchange-alg-base;
       description
         "Diffie-Hellman Group and Key Exchange with SHA-256 as HASH.";
       reference
         "RFC 4419: Diffie-Hellman Group Exchange for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     identity ecdh-sha2-nistp256 {
       if-feature "ssh-ecc and ssh-sha2";
       base key-exchange-alg-base;
       description



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         "Elliptic Curve Diffie-Hellman (ECDH) key exchange using the
          nistp256 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity ecdh-sha2-nistp384 {
       if-feature "ssh-ecc and ssh-sha2";
       base key-exchange-alg-base;
       description
         "Elliptic Curve Diffie-Hellman (ECDH) key exchange using the
          nistp384 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity ecdh-sha2-nistp521 {
       if-feature "ssh-ecc and ssh-sha2";
       base key-exchange-alg-base;
       description
         "Elliptic Curve Diffie-Hellman (ECDH) key exchange using the
          nistp521 curve and the SHA2 family of hashing algorithms.";
       reference
         "RFC 5656: Elliptic Curve Algorithm Integration in the
                    Secure Shell Transport Layer";
     }

     identity encryption-alg-base {
       description
         "Base identity used to identify encryption algorithms.";
     }

     identity triple-des-cbc {
       base encryption-alg-base;
       description
         "Three-key 3DES in CBC mode.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity aes128-cbc {
       base encryption-alg-base;
       description
         "AES in CBC mode, with a 128-bit key.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";



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     }

     identity aes192-cbc {
       base encryption-alg-base;
       description
         "AES in CBC mode, with a 192-bit key.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity aes256-cbc {
       base encryption-alg-base;
       description
         "AES in CBC mode, with a 256-bit key.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity aes128-ctr {
       if-feature "ssh-ctr";
       base encryption-alg-base;
       description
         "AES in SDCTR mode, with 128-bit key.";
       reference
         "RFC 4344: The Secure Shell (SSH) Transport Layer Encryption
                    Modes";
     }

     identity aes192-ctr {
       if-feature "ssh-ctr";
       base encryption-alg-base;
       description
         "AES in SDCTR mode, with 192-bit key.";
       reference
         "RFC 4344: The Secure Shell (SSH) Transport Layer Encryption
                    Modes";
     }

     identity aes256-ctr {
       if-feature "ssh-ctr";
       base encryption-alg-base;
       description
         "AES in SDCTR mode, with 256-bit key.";
       reference
         "RFC 4344: The Secure Shell (SSH) Transport Layer Encryption
            Modes";
     }




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     identity mac-alg-base {
       description
         "Base identity used to identify message authentication
          code (MAC) algorithms.";
     }

     identity hmac-sha1 {
       base mac-alg-base;
       description
         "HMAC-SHA1";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
     }

     identity hmac-sha2-256 {
       if-feature "ssh-sha2";
       base mac-alg-base;
       description
         "HMAC-SHA2-256";
       reference
         "RFC 6668: SHA-2 Data Integrity Verification for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     identity hmac-sha2-512 {
       if-feature "ssh-sha2";
       base mac-alg-base;
       description
         "HMAC-SHA2-512";
       reference
         "RFC 6668: SHA-2 Data Integrity Verification for the
                    Secure Shell (SSH) Transport Layer Protocol";
     }

     // Groupings

     grouping transport-params-grouping {
       description
         "A reusable grouping for SSH transport parameters.";
       reference
         "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
       container host-key {
         description
           "Parameters regarding host key.";
         leaf-list host-key-alg {
           type identityref {
             base public-key-alg-base;
           }



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           ordered-by user;
           description
             "Acceptable host key algorithms in order of descending
              preference.  The configured host key algorithms should
              be compatible with the algorithm used by the configured
              private key.  Please see Section 5 of RFC EEEE for
              valid combinations.

              If this leaf-list is not configured (has zero elements)
              the acceptable host key algorithms are implementation-
              defined.";
           reference
             "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
         }
       }
       container key-exchange {
         description
           "Parameters regarding key exchange.";
         leaf-list key-exchange-alg {
           type identityref {
             base key-exchange-alg-base;
           }
           ordered-by user;
           description
             "Acceptable key exchange algorithms in order of descending
              preference.

              If this leaf-list is not configured (has zero elements)
              the acceptable key exchange algorithms are implementation
              defined.";
         }
       }
       container encryption {
         description
           "Parameters regarding encryption.";
         leaf-list encryption-alg {
           type identityref {
             base encryption-alg-base;
           }
           ordered-by user;
           description
             "Acceptable encryption algorithms in order of descending
              preference.

              If this leaf-list is not configured (has zero elements)
              the acceptable encryption algorithms are implementation
              defined.";
         }



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       }
       container mac {
         description
           "Parameters regarding message authentication code (MAC).";
         leaf-list mac-alg {
           type identityref {
             base mac-alg-base;
           }
           ordered-by user;
           description
             "Acceptable MAC algorithms in order of descending
              preference.

              If this leaf-list is not configured (has zero elements)
              the acceptable MAC algorithms are implementation-
              defined.";
         }
       }
     }
   }

   <CODE ENDS>

3.  The "ietf-ssh-client" Module

3.1.  Data Model Overview

3.1.1.  Features

   The following diagram lists all the "feature" statements defined in
   the "ietf-ssh-client" module:

   Features:
     +-- ssh-client-transport-params-config
     +-- ssh-client-keepalives
     +-- client-identity-password
     +-- client-identity-publickey
     +-- client-identity-hostbased
     +-- client-identity-none

3.1.2.  Groupings

   The following diagram lists all the "grouping" statements defined in
   the "ietf-ssh-client" module:

   Groupings:
     +-- ssh-client-grouping




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   Each of these groupings are presented in the following subsections.

3.1.2.1.  The "ssh-client-grouping" Grouping

   The following tree diagram [RFC8340] illustrates the "ssh-client-
   grouping" grouping:

   =============== NOTE: '\' line wrapping per RFC 8792 ================

     grouping ssh-client-grouping
       +-- client-identity
       |  +-- username?      string
       |  +-- public-key! {client-identity-publickey}?
       |  |  +---u ks:local-or-keystore-asymmetric-key-grouping
       |  +-- password?      string {client-identity-password}?
       |  +-- hostbased! {client-identity-hostbased}?
       |  |  +---u ks:local-or-keystore-asymmetric-key-grouping
       |  +-- none?          empty {client-identity-none}?
       |  +-- certificate! {sshcmn:ssh-x509-certs}?
       |     +---u ks:local-or-keystore-end-entity-cert-with-key-groupi\
   ng
       +-- server-authentication
       |  +-- ssh-host-keys!
       |  |  +---u ts:local-or-truststore-public-keys-grouping
       |  +-- ca-certs! {sshcmn:ssh-x509-certs}?
       |  |  +---u ts:local-or-truststore-certs-grouping
       |  +-- ee-certs! {sshcmn:ssh-x509-certs}?
       |     +---u ts:local-or-truststore-certs-grouping
       +-- transport-params {ssh-client-transport-params-config}?
       |  +---u sshcmn:transport-params-grouping
       +-- keepalives! {ssh-client-keepalives}?
          +-- max-wait?       uint16
          +-- max-attempts?   uint8

   Comments:

   *  The "client-identity" node configures a "username" and
      credentials, each enabled by a "feature" statement defined in
      Section 3.1.1.

   *  The "server-authentication" node configures trust anchors for
      authenticating the SSH server, with each option enabled by a
      "feature" statement.

   *  The "transport-params" node, which must be enabled by a feature,
      configures parameters for the SSH sessions established by this
      configuration.




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   *  The "keepalives" node, which must be enabled by a feature,
      configures a "presence" container for testing the aliveness of the
      SSH server.  The aliveness-test occurs at the SSH protocol layer.

   *  For the referenced grouping statement(s):

      -  The "local-or-keystore-asymmetric-key-grouping" grouping is
         discussed in Section 2.1.3.4 of [I-D.ietf-netconf-keystore].
      -  The "local-or-keystore-end-entity-cert-with-key-grouping"
         grouping is discussed in Section 2.1.3.6 of
         [I-D.ietf-netconf-keystore].
      -  The "local-or-truststore-public-keys-grouping" grouping is
         discussed in Section 2.1.3.2 of
         [I-D.ietf-netconf-trust-anchors].
      -  The "local-or-truststore-certs-grouping" grouping is discussed
         in Section 2.1.3.1 of [I-D.ietf-netconf-trust-anchors].
      -  The "transport-params-grouping" grouping is discussed in
         Section 2.1.3.1 in this document.

3.2.  Example Usage

   This section presents two examples showing the "ssh-client-grouping"
   grouping populated with some data.  These examples are effectively
   the same except the first configures the client identity using a
   local key while the second uses a key configured in a keystore.  Both
   examples are consistent with the examples presented in Section 2 of
   [I-D.ietf-netconf-trust-anchors] and Section 3.2 of
   [I-D.ietf-netconf-keystore].

   The following configuration example uses local-definitions for the
   client identity and server authentication:

   =============== NOTE: '\\' line wrapping per RFC 8792 ===============

   <ssh-client
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-client"
     xmlns:ct="urn:ietf:params:xml:ns:yang:ietf-crypto-types"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">

     <!-- how this client will authenticate itself to the server -->
     <client-identity>
       <username>foobar</username>
       <public-key>
         <local-definition>
           <public-key-format>ct:ssh-public-key-format</public-key-form\
   \at>
           <public-key>base64encodedvalue==</public-key>
           <private-key-format>ct:rsa-private-key-format</private-key-f\



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   \ormat>
           <cleartext-private-key>base64encodedvalue==</cleartext-priva\
   \te-key>
         </local-definition>
       </public-key>
     </client-identity>

     <!-- which host keys will this client trust -->
     <server-authentication>
       <ssh-host-keys>
         <local-definition> <!-- FIXME: float 'local-def' down to each?\
   \ -->
           <!--<ssh-public-key>-->
           <public-key>
             <name>corp-fw1</name>
             <public-key-format>ct:ssh-public-key-format</public-key-fo\
   \rmat>
             <public-key>base64encodedvalue==</public-key>
             <!--
           </ssh-public-key>
           <ssh-public-key>
           -->
           </public-key>
           <public-key>
             <name>corp-fw2</name>
             <public-key-format>ct:ssh-public-key-format</public-key-fo\
   \rmat>
             <public-key>base64encodedvalue==</public-key>
             <!--</ssh-public-key>-->
           </public-key>
         </local-definition>
       </ssh-host-keys>
       <ca-certs>
         <local-definition>
           <certificate>
             <name>Server Cert Issuer #1</name>
             <cert-data>base64encodedvalue==</cert-data>
           </certificate>
           <certificate>
             <name>Server Cert Issuer #2</name>
             <cert-data>base64encodedvalue==</cert-data>
           </certificate>
         </local-definition>
       </ca-certs>
       <ee-certs>
         <local-definition>
           <certificate>
             <name>My Application #1</name>



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             <cert-data>base64encodedvalue==</cert-data>
           </certificate>
           <certificate>
             <name>My Application #2</name>
             <cert-data>base64encodedvalue==</cert-data>
           </certificate>
         </local-definition>
       </ee-certs>
     </server-authentication>

     <keepalives>
       <max-wait>30</max-wait>
       <max-attempts>3</max-attempts>
     </keepalives>

   </ssh-client>

   The following configuration example uses keystore-references for the
   client identity and truststore-references for server authentication:
   from the keystore:































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   =============== NOTE: '\' line wrapping per RFC 8792 ================

   <ssh-client
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-client"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">

     <!-- how this client will authenticate itself to the server -->
     <client-identity>
       <username>foobar</username>
       <!-- can an SSH client have move than one key?
       <public-key>
         <keystore-reference>ssh-rsa-key</keystore-reference>
       </public-key>
       -->
       <certificate>
         <keystore-reference>
           <asymmetric-key>ssh-rsa-key-with-cert</asymmetric-key>
           <certificate>ex-rsa-cert2</certificate>
         </keystore-reference>
       </certificate>
     </client-identity>

     <!-- which host-keys will this client trust -->
     <server-authentication>
       <ssh-host-keys> <!-- FIXME: should 'ts-ref' be to bag or each ke\
   y? -->
         <truststore-reference>trusted-ssh-public-keys</truststore-refe\
   rence>
       </ssh-host-keys>
       <ca-certs> <!-- FIXME: should 'ts-ref' be to bag or each key? -->
         <truststore-reference>trusted-server-ca-certs</truststore-refe\
   rence>
       </ca-certs>
       <ee-certs> <!-- FIXME: should 'ts-ref' be to bag or each key? -->
         <truststore-reference>trusted-server-ee-certs</truststore-refe\
   rence>
       </ee-certs>
     </server-authentication>

     <keepalives>
       <max-wait>30</max-wait>
       <max-attempts>3</max-attempts>
     </keepalives>

   </ssh-client>






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3.3.  YANG Module

   This YANG module has normative references to
   [I-D.ietf-netconf-trust-anchors], and [I-D.ietf-netconf-keystore].

   <CODE BEGINS> file "ietf-ssh-client@2020-07-10.yang"

   module ietf-ssh-client {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-client";
     prefix sshc;

     import ietf-netconf-acm {
       prefix nacm;
       reference
         "RFC 8341: Network Configuration Access Control Model";
     }

     import ietf-crypto-types {
       prefix ct;
       reference
         "RFC AAAA: YANG Data Types and Groupings for Cryptography";
     }

     import ietf-truststore {
       prefix ts;
       reference
         "RFC BBBB: A YANG Data Model for a Truststore";
     }

     import ietf-keystore {
       prefix ks;
       reference
         "RFC CCCC: A YANG Data Model for a Keystore";
     }

     import ietf-ssh-common {
       prefix sshcmn;
       revision-date 2020-07-10; // stable grouping definitions
       reference
         "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
     }

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

     contact
       "WG Web:   <http://datatracker.ietf.org/wg/netconf/>



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        WG List:  <mailto:netconf@ietf.org>
        Author:   Kent Watsen <mailto:kent+ietf@watsen.net>
        Author:   Gary Wu <mailto:garywu@cisco.com>";

     description
       "This module defines reusable groupings for SSH clients that
        can be used as a basis for specific SSH client instances.

        Copyright (c) 2020 IETF Trust and the persons identified
        as authors of the code. All rights reserved.

        Redistribution and use in source and binary forms, with
        or without modification, is permitted pursuant to, and
        subject to the license terms contained in, the Simplified
        BSD License set forth in Section 4.c of the IETF Trust's
        Legal Provisions Relating to IETF Documents
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC EEEE
        (https://www.rfc-editor.org/info/rfcEEEE); see the RFC
        itself for full legal notices.;

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
        'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
        'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
        are to be interpreted as described in BCP 14 (RFC 2119)
        (RFC 8174) when, and only when, they appear in all
        capitals, as shown here.";

     revision 2020-07-10 {
       description
         "Initial version";
       reference
         "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
     }

     // Features

     feature ssh-client-transport-params-config {
       description
         "SSH transport layer parameters are configurable on an SSH
          client.";
     }

     feature ssh-client-keepalives {
       description
         "Per socket SSH keepalive parameters are configurable for
          SSH clients on the server implementing this feature.";



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     }

     feature client-identity-password {
       description
         "Indicates that the 'password' authentication type
          is supported for client identification.";
     }

     feature client-identity-publickey {
       description
         "Indicates that the 'publickey' authentication type
          is supported for client identification.

          The 'publickey' authentication type is required by
          RFC 4252, but common implementations enable it to
          be disabled.";
     }

     feature client-identity-hostbased {
       description
         "Indicates that the 'hostbased' authentication type
          is supported for client identification.";
     }

     feature client-identity-none {
       description
         "Indicates that the 'none' authentication type is
          supported for client identification.";
     }

     // Groupings

     grouping ssh-client-grouping {
       description
         "A reusable grouping for configuring a SSH client without
          any consideration for how an underlying TCP session is
          established.

          Note that this grouping uses fairly typical descendent
          node names such that a stack of 'uses' statements will
          have name conflicts.  It is intended that the consuming
          data model will resolve the issue (e.g., by wrapping
          the 'uses' statement in a container called
          'ssh-client-parameters').  This model purposely does
          not do this itself so as to provide maximum flexibility
          to consuming models.";

       container client-identity {



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         nacm:default-deny-write;
         must
           'public-key or password or hostbased or none or certificate';
         description
           "The credentials that the client may use, pending
            the SSH server's requirements, by the SSH client
            to authenticate to the SSH server.";
         leaf username {
           type string;
           description
             "The username of this user.  This will be the username
              used, for instance, to log into an SSH server.";
         }
         container public-key {
           if-feature client-identity-publickey;
           presence
             "Indicates that publickey-based authentication
              is configured";
           description
             "A locally-defined or referenced asymmetric key
              pair to be used for client identification.";
           reference
             "RFC CCCC: A YANG Data Model for a Keystore";
           uses ks:local-or-keystore-asymmetric-key-grouping {
             refine "local-or-keystore/local/local-definition" {
               must 'public-key-format = "ct:ssh-public-key-format"';
             }
             refine "local-or-keystore/keystore/keystore-reference" {
               must 'deref(.)/../ks:public-key-format'
                    + ' = "ct:ssh-public-key-format"';
             }
           }
         }
         leaf password {
           if-feature client-identity-password;
           nacm:default-deny-all;
           type string;
           description
             "A password to be used for client identification.";
         }
         container hostbased {
           if-feature client-identity-hostbased;
           presence
             "Indicates that hostbased authentication is configured";
           description
             "A locally-defined or referenced asymmetric key
              pair to be used for host identification.";
           reference



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             "RFC CCCC: A YANG Data Model for a Keystore";
           uses ks:local-or-keystore-asymmetric-key-grouping {
             refine "local-or-keystore/local/local-definition" {
               must 'public-key-format = "ct:ssh-public-key-format"';
             }
             refine "local-or-keystore/keystore/keystore-reference" {
               must 'deref(.)/../ks:public-key-format'
                    + ' = "ct:ssh-public-key-format"';
             }
           }
         }
         leaf none {
           if-feature client-identity-none;
           type empty;
           description
             "Indicates that 'none' algorithm is used for client
              identification.";
         }
         container certificate {
           if-feature "sshcmn:ssh-x509-certs";
           presence
             "Indicates that certificate-based authentication
              is configured";
           description
             "A locally-defined or referenced certificate
              to be used for client identification.";
           reference
             "RFC CCCC: A YANG Data Model for a Keystore";
           uses
           ks:local-or-keystore-end-entity-cert-with-key-grouping {
             refine "local-or-keystore/local/local-definition" {
               must
                 'public-key-format'
                 + ' = "ct:subject-public-key-info-format"';
             }
             refine "local-or-keystore/keystore/keystore-reference"
                    + "/asymmetric-key" {
               must 'deref(.)/../ks:public-key-format'
                    + ' = "ct:subject-public-key-info-format"';
             }
           }
         }
       } // container client-identity

       container server-authentication {
         nacm:default-deny-write;
         must 'ssh-host-keys or ca-certs or ee-certs';
         description



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           "Specifies how the SSH client can authenticate SSH servers.
            Any combination of credentials is additive and unordered.";
         container ssh-host-keys {
           presence
             "Indicates that the client can authenticate servers
              using the configured SSH host keys.";
           description
             "A list of SSH host keys used by the SSH client to
              authenticate SSH server host keys.  A server host key
              is authenticated if it is an exact match to a
              configured SSH host key.";
           reference
              "RFC BBBB: A YANG Data Model for a Truststore";
           uses ts:local-or-truststore-public-keys-grouping {
             refine
               "local-or-truststore/local/local-definition/public-key" {
               must 'public-key-format = "ct:ssh-public-key-format"';
             }
             refine
               "local-or-truststore/truststore/truststore-reference" {
               must 'deref(.)/../*/ts:public-key-format'
                     + ' = "ct:ssh-public-key-format"';
             }
           }
         }
         container ca-certs {
           if-feature "sshcmn:ssh-x509-certs";
           presence
             "Indicates that the client can authenticate servers
              using the configured trust anchor certificates.";
           description
             "A set of certificate authority (CA) certificates used by
              the SSH client to authenticate SSH servers.  A server
              is authenticated if its certificate  has a valid chain
              of trust to a configured CA certificate.";
            reference
              "RFC BBBB: A YANG Data Model for a Truststore";
           uses ts:local-or-truststore-certs-grouping;
         }
         container ee-certs {
           if-feature "sshcmn:ssh-x509-certs";
           presence
             "Indicates that the client can authenticate servers
              using the configured end-entity certificates.";
           description
             "A set of end-entity certificates used by the SSH client
              to authenticate SSH servers.  A server is authenticated
              if its certificate is an exact match to a configured



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              end-entity certificate.";
            reference
              "RFC BBBB: A YANG Data Model for a Truststore";
           uses ts:local-or-truststore-certs-grouping;
         }
       } // container server-authentication

       container transport-params {
         nacm:default-deny-write;
         if-feature "ssh-client-transport-params-config";
         description
           "Configurable parameters of the SSH transport layer.";
         uses sshcmn:transport-params-grouping;
       } // container transport-parameters

       container keepalives {
         nacm:default-deny-write;
         if-feature "ssh-client-keepalives";
         presence
           "Indicates that the SSH client proactively tests the
            aliveness of the remote SSH server.";
         description
           "Configures the keep-alive policy, to proactively test
            the aliveness of the SSH server.  An unresponsive TLS
            server is dropped after approximately max-wait *
            max-attempts seconds.  Per Section 4 of RFC 4254,
            the SSH client SHOULD send an SSH_MSG_GLOBAL_REQUEST
            message with a purposely nonexistent 'request name'
            value (e.g., keepalive@ietf.org) and the 'want reply'
            value set to '1'.";
         reference
           "RFC 4254: The Secure Shell (SSH) Connection Protocol";
         leaf max-wait {
           type uint16 {
             range "1..max";
           }
           units "seconds";
           default "30";
           description
             "Sets the amount of time in seconds after which if
              no data has been received from the SSH server, a
              TLS-level message will be sent to test the
              aliveness of the SSH server.";
         }
         leaf max-attempts {
           type uint8;
           default "3";
           description



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             "Sets the maximum number of sequential keep-alive
              messages that can fail to obtain a response from
              the SSH server before assuming the SSH server is
              no longer alive.";
         }
       } // container keepalives
     } // grouping ssh-client-grouping
   } // module ietf-ssh-client

   <CODE ENDS>

4.  The "ietf-ssh-server" Module

4.1.  Data Model Overview

4.1.1.  Features

   The following diagram lists all the "feature" statements defined in
   the "ietf-ssh-server" module:

   Features:
     +-- ssh-server-transport-params-config
     +-- ssh-server-keepalives
     +-- client-auth-config-supported
     +-- client-auth-publickey
     +-- client-auth-password
     +-- client-auth-hostbased
     +-- client-auth-none

4.1.2.  Groupings

   The following diagram lists all the "grouping" statements defined in
   the "ietf-ssh-server" module:

   Groupings:
     +-- ssh-server-grouping

   Each of these groupings are presented in the following subsections.

4.1.2.1.  The "ssh-server-grouping" Grouping

   The following tree diagram [RFC8340] illustrates the "ssh-server-
   grouping" grouping:








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   =============== NOTE: '\' line wrapping per RFC 8792 ================

     grouping ssh-server-grouping
       +-- server-identity
       |  +-- host-key* [name]
       |     +-- name?                string
       |     +-- (host-key-type)
       |        +--:(public-key)
       |        |  +-- public-key
       |        |     +---u ks:local-or-keystore-asymmetric-key-grouping
       |        +--:(certificate)
       |           +-- certificate {sshcmn:ssh-x509-certs}?
       |              +---u ks:local-or-keystore-end-entity-cert-with-k\
   ey-grouping
       +-- client-authentication
       |  +-- supported-authentication-methods
       |  |  +-- publickey?   empty
       |  |  +-- password?    empty {client-auth-password}?
       |  |  +-- hostbased?   empty {client-auth-hostbased}?
       |  |  +-- none?        empty {client-auth-none}?
       |  +-- users {client-auth-config-supported}?
       |  |  +-- user* [name]
       |  |     +-- name?          string
       |  |     +-- public-keys! {client-auth-publickey}?
       |  |     |  +---u ts:local-or-truststore-public-keys-grouping
       |  |     +-- password?      ianach:crypt-hash
       |  |     |       {client-auth-password}?
       |  |     +-- hostbased! {client-auth-hostbased}?
       |  |     |  +---u ts:local-or-truststore-public-keys-grouping
       |  |     +-- none?          empty {client-auth-none}?
       |  +-- ca-certs!
       |  |       {client-auth-config-supported,sshcmn:ssh-x509-certs}?
       |  |  +---u ts:local-or-truststore-certs-grouping
       |  +-- ee-certs!
       |          {client-auth-config-supported,sshcmn:ssh-x509-certs}?
       |     +---u ts:local-or-truststore-certs-grouping
       +-- transport-params {ssh-server-transport-params-config}?
       |  +---u sshcmn:transport-params-grouping
       +-- keepalives! {ssh-server-keepalives}?
          +-- max-wait?       uint16
          +-- max-attempts?   uint8

   Comments:

   *  The "server-identity" node configures identity credentials.  The
      ability to use a certificate is enabled by a "feature".





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   *  The "client-authentication" node configures trust anchors for
      authenticating the SSH client, with each option enabled by a
      "feature" statement.

   *  The "transport-params" node, which must be enabled by a feature,
      configures parameters for the SSH sessions established by this
      configuration.

   *  The "keepalives" node, which must be enabled by a feature,
      configures a "presence" container for testing the aliveness of the
      SSH client.  The aliveness-test occurs at the SSH protocol layer.

   *  For the referenced grouping statement(s):

      -  The "local-or-keystore-asymmetric-key-grouping" grouping is
         discussed in Section 2.1.3.4 of [I-D.ietf-netconf-keystore].
      -  The "local-or-keystore-end-entity-cert-with-key-grouping"
         grouping is discussed in Section 2.1.3.6 of
         [I-D.ietf-netconf-keystore].
      -  The "local-or-truststore-public-keys-grouping" grouping is
         discussed in Section 2.1.3.2 of
         [I-D.ietf-netconf-trust-anchors].
      -  The "local-or-truststore-certs-grouping" grouping is discussed
         in Section 2.1.3.1 of [I-D.ietf-netconf-trust-anchors].
      -  The "transport-params-grouping" grouping is discussed in
         Section 2.1.3.1 in this document.

4.2.  Example Usage

   This section presents two examples showing the "ssh-server-grouping"
   grouping populated with some data.  These examples are effectively
   the same except the first configures the server identity using a
   local key while the second uses a key configured in a keystore.  Both
   examples are consistent with the examples presented in Section 2 of
   [I-D.ietf-netconf-trust-anchors] and Section 3.2 of
   [I-D.ietf-netconf-keystore].

   The following configuration example uses local-definitions for the
   server identity and client authentication:

   =============== NOTE: '\' line wrapping per RFC 8792 ================

   <ssh-server
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-server"
     xmlns:ct="urn:ietf:params:xml:ns:yang:ietf-crypto-types"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">

     <!-- the host-key this SSH server will present -->



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     <server-identity>
       <host-key>
         <name>my-pubkey-based-host-key</name>
         <public-key>
           <local-definition>
             <public-key-format>ct:ssh-public-key-format</public-key-fo\
   rmat>
             <public-key>base64encodedvalue==</public-key>
             <private-key-format>ct:rsa-private-key-format</private-key\
   -format>
             <cleartext-private-key>base64encodedvalue==</cleartext-pri\
   vate-key>
           </local-definition>
         </public-key>
       </host-key>
       <host-key>
         <name>my-cert-based-host-key</name>
         <certificate>
           <local-definition>
             <public-key-format>ct:subject-public-key-info-format</publ\
   ic-key-format>
             <public-key>base64encodedvalue==</public-key>
             <private-key-format>ct:rsa-private-key-format</private-key\
   -format>
             <cleartext-private-key>base64encodedvalue==</cleartext-pri\
   vate-key>
             <cert-data>base64encodedvalue==</cert-data>
           </local-definition>
         </certificate>
       </host-key>
     </server-identity>

     <!-- the client credentials this SSH server will trust -->
     <client-authentication>
       <supported-authentication-methods>
         <publickey/>
       </supported-authentication-methods>
       <users>
         <user>
           <name>mary</name>
           <password>$0$secret</password>
           <public-keys>
             <local-definition>
               <!--<ssh-public-key>-->
               <public-key>
                 <name>User A</name>
                 <public-key-format>ct:ssh-public-key-format</public-ke\
   y-format>



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                 <public-key>base64encodedvalue==</public-key>
                 <!--</ssh-public-key>
               <ssh-public-key>-->
               </public-key>
               <public-key>
                 <name>User B</name>
                 <public-key-format>ct:ssh-public-key-format</public-ke\
   y-format>
                 <public-key>base64encodedvalue==</public-key>
               </public-key>
               <!--</ssh-public-key>-->
             </local-definition>
           </public-keys>
         </user>
       </users>
       <ca-certs>
         <local-definition>
           <certificate>
             <name>Identity Cert Issuer #1</name>
             <cert-data>base64encodedvalue==</cert-data>
           </certificate>
           <certificate>
             <name>Identity Cert Issuer #2</name>
             <cert-data>base64encodedvalue==</cert-data>
           </certificate>
         </local-definition>
       </ca-certs>
       <ee-certs>
         <local-definition>
           <certificate>
             <name>Application #1</name>
             <cert-data>base64encodedvalue==</cert-data>
           </certificate>
           <certificate>
             <name>Application #2</name>
             <cert-data>base64encodedvalue==</cert-data>
           </certificate>
         </local-definition>
       </ee-certs>
     </client-authentication>

     <keepalives>
       <max-wait>30</max-wait>
       <max-attempts>3</max-attempts>
     </keepalives>

   </ssh-server>




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   The following configuration example uses keystore-references for the
   server identity and truststore-references for client authentication:
   from the keystore:

   =============== NOTE: '\' line wrapping per RFC 8792 ================

   <ssh-server
     xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-server"
     xmlns:algs="urn:ietf:params:xml:ns:yang:ietf-ssh-common">

     <!-- the host-key this SSH server will present -->
     <server-identity>
       <host-key>
         <name>my-pubkey-based-host-key</name>
         <public-key>
           <keystore-reference>ssh-rsa-key</keystore-reference>
         </public-key>
       </host-key>
       <host-key>
         <name>my-cert-based-host-key</name>
         <certificate>
           <keystore-reference>
             <asymmetric-key>ssh-rsa-key-with-cert</asymmetric-key>
             <certificate>ex-rsa-cert2</certificate>
           </keystore-reference>
         </certificate>
       </host-key>
     </server-identity>

     <!-- the client credentials this SSH server will trust -->
     <client-authentication>
       <supported-authentication-methods>
         <publickey/>
       </supported-authentication-methods>
       <users>
         <user>
           <name>mary</name>
           <password>$0$secret</password>
           <public-keys>
             <truststore-reference>SSH Public Keys for Application A</t\
   ruststore-reference>
           </public-keys>
         </user>
       </users>
       <ca-certs>
         <truststore-reference>trusted-client-ca-certs</truststore-refe\
   rence>
       </ca-certs>



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       <ee-certs>
         <truststore-reference>trusted-client-ee-certs</truststore-refe\
   rence>
       </ee-certs>
     </client-authentication>

     <keepalives>
       <max-wait>30</max-wait>
       <max-attempts>3</max-attempts>
     </keepalives>

   </ssh-server>

4.3.  YANG Module

   This YANG module has normative references to
   [I-D.ietf-netconf-trust-anchors] and [I-D.ietf-netconf-keystore] and
   informative references to [RFC4253] and [RFC7317].

   <CODE BEGINS> file "ietf-ssh-server@2020-07-10.yang"

   module ietf-ssh-server {
     yang-version 1.1;
     namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-server";
     prefix sshs;

     import iana-crypt-hash {
       prefix ianach;
       reference
         "RFC 7317: A YANG Data Model for System Management";
     }

     import ietf-netconf-acm {
       prefix nacm;
       reference
         "RFC 8341: Network Configuration Access Control Model";
     }

     import ietf-crypto-types {
       prefix ct;
       reference
         "RFC AAAA: YANG Data Types and Groupings for Cryptography";
     }

     import ietf-truststore {
       prefix ts;
       reference
         "RFC BBBB: A YANG Data Model for a Truststore";



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     }

     import ietf-keystore {
       prefix ks;
       reference
         "RFC CCCC: A YANG Data Model for a Keystore";
     }

     import ietf-ssh-common {
       prefix sshcmn;
       revision-date 2020-07-10; // stable grouping definitions
       reference
         "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
     }

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

     contact
       "WG Web:   <http://datatracker.ietf.org/wg/netconf/>
        WG List:  <mailto:netconf@ietf.org>
        Author:   Kent Watsen <mailto:kent+ietf@watsen.net>
        Author:   Gary Wu <mailto:garywu@cisco.com>";

     description
       "This module defines reusable groupings for SSH servers that
        can be used as a basis for specific SSH server instances.

        Copyright (c) 2020 IETF Trust and the persons identified
        as authors of the code. All rights reserved.

        Redistribution and use in source and binary forms, with
        or without modification, is permitted pursuant to, and
        subject to the license terms contained in, the Simplified
        BSD License set forth in Section 4.c of the IETF Trust's
        Legal Provisions Relating to IETF Documents
        (https://trustee.ietf.org/license-info).

        This version of this YANG module is part of RFC EEEE
        (https://www.rfc-editor.org/info/rfcEEEE); see the RFC
        itself for full legal notices.;

        The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
        'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
        'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
        are to be interpreted as described in BCP 14 (RFC 2119)
        (RFC 8174) when, and only when, they appear in all
        capitals, as shown here.";



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     revision 2020-07-10 {
       description
         "Initial version";
       reference
         "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
     }

     // Features

     feature ssh-server-transport-params-config {
       description
         "SSH transport layer parameters are configurable on an SSH
          server.";
     }

     feature ssh-server-keepalives {
       description
         "Per socket SSH keepalive parameters are configurable for
          SSH servers on the server implementing this feature.";
     }

     feature client-auth-config-supported {
       description
         "Indicates that the configuration for how to authenticate
          clients can be configured herein, as opposed to in an
          application specific location.  That is, to support the
          consuming data models that prefer to place client
          authentication with client definitions, rather then
          in a data model principally concerned with configuring
          the transport.";
     }

     feature client-auth-publickey {
       description
         "Indicates that the 'publickey' authentication type
          is supported.

          The 'publickey' authentication type is required by
          RFC 4252, but common implementations enable it to
          be disabled.";
       reference
         "RFC 4252:
           The Secure Shell (SSH) Authentication Protocol";
     }

     feature client-auth-password {
       description
         "Indicates that the 'password' authentication type



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          is supported.";
     }

     feature client-auth-hostbased {
       description
         "Indicates that the 'hostbased' authentication type
          is supported.";
     }

     feature client-auth-none {
       description
         "Indicates that the 'none' authentication type is
          supported.";
     }

     // Groupings

     grouping ssh-server-grouping {
       description
         "A reusable grouping for configuring a SSH server without
          any consideration for how underlying TCP sessions are
          established.

          Note that this grouping uses fairly typical descendent
          node names such that a stack of 'uses' statements will
          have name conflicts.  It is intended that the consuming
          data model will resolve the issue (e.g., by wrapping
          the 'uses' statement in a container called
          'ssh-server-parameters').  This model purposely does
          not do this itself so as to provide maximum flexibility
          to consuming models.";

       container server-identity {
         nacm:default-deny-write;
         description
           "The list of host keys the SSH server will present when
            establishing a SSH connection.";
         list host-key {
           key "name";
           min-elements 1;
           ordered-by user;
           description
             "An ordered list of host keys the SSH server will use to
              construct its ordered list of algorithms, when sending
              its SSH_MSG_KEXINIT message, as defined in Section 7.1
              of RFC 4253.";
           reference
             "RFC 4253: The Secure Shell (SSH) Transport Layer



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                        Protocol";
           leaf name {
             type string;
             description
               "An arbitrary name for this host key";
           }
           choice host-key-type {
             mandatory true;
             description
               "The type of host key being specified";
             container public-key {
               description
                 "A locally-defined or referenced asymmetric key pair
                  to be used for the SSH server's host key.";
               reference
                 "RFC CCCC: A YANG Data Model for a Keystore";
               uses ks:local-or-keystore-asymmetric-key-grouping {
                 refine "local-or-keystore/local/local-definition" {
                   must
                     'public-key-format = "ct:ssh-public-key-format"';
                 }
                 refine "local-or-keystore/keystore/"
                        + "keystore-reference" {
                   must 'deref(.)/../ks:public-key-format'
                        + ' = "ct:ssh-public-key-format"';
                 }
               }
             }
             container certificate {
               if-feature "sshcmn:ssh-x509-certs";
               description
                 "A locally-defined or referenced end-entity
                  certificate to be used for the SSH server's
                  host key.";
               reference
                 "RFC CCCC: A YANG Data Model for a Keystore";
               uses
               ks:local-or-keystore-end-entity-cert-with-key-grouping {
                 refine "local-or-keystore/local/local-definition" {
                   must
                     'public-key-format'
                     + ' = "ct:subject-public-key-info-format"';
                 }
                 refine "local-or-keystore/keystore/keystore-reference"
                        + "/asymmetric-key" {
                   must 'deref(.)/../ks:public-key-format'
                        + ' = "ct:subject-public-key-info-format"';
                 }



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               }
             }
           }
         }
       } // container server-identity

       container client-authentication {
         nacm:default-deny-write;
         description
           "Specifies how the SSH server can authenticate SSH clients.";
         container supported-authentication-methods {
           description
             "Indicates which authentication methods the server
              supports.";
           leaf publickey {
             type empty;
             description
               "Indicates that the 'publickey' method is supported.
                Note that RFC 6187 X.509v3 Certificates for SSH uses
                the 'publickey' method name.";
             reference
               "RFC 4252: The Secure Shell (SSH) Authentication
                          Protocol.
                RFC 6187: X.509v3 Certificates for Secure Shell
                          Authentication.";
           }
           leaf password {
             if-feature client-auth-password;
             type empty;
             description
               "Indicates that the 'password' method is supported.";
             reference
               "RFC 4252: The Secure Shell (SSH) Authentication
                          Protocol.";
           }
           leaf hostbased {
             if-feature client-auth-hostbased;
             type empty;
             description
               "Indicates that the 'hostbased' method is supported.";
             reference
               "RFC 4252: The Secure Shell (SSH) Authentication
                          Protocol.";
           }
           leaf none {
             if-feature client-auth-none;
             type empty;
             description



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               "Indicates that the 'none' method is supported.";
             reference
               "RFC 4252: The Secure Shell (SSH) Authentication
                          Protocol.";
           }
         }

         container users {
           if-feature "client-auth-config-supported";
           description
             "A list of locally configured users.";
           list user {
             key name;
             description
               "The list of local users configured on this device.";
             leaf name {
               type string;
               description
                "The user name string identifying this entry.";
             }
             container public-keys {
               if-feature client-auth-publickey;
               presence
                 "Indicates that the server can authenticate this
                  user using any of the configured SSH public keys.";
               description
                 "A set of SSH public keys may be used by the SSH
                  server to authenticate this user.  A user is
                  authenticated if its public key is an exact
                  match to a configured public key.";
               reference
                 "RFC BBBB: A YANG Data Model for a Truststore";
               uses ts:local-or-truststore-public-keys-grouping {
                 refine "local-or-truststore/local/local-definition"
                        + "/public-key" {
                   must 'public-key-format'
                        + ' = "ct:ssh-public-key-format"';
                 }
                 refine "local-or-truststore/truststore/"
                        + "truststore-reference" {
                   must 'deref(.)/../*/ts:public-key-format'
                        + ' = "ct:ssh-public-key-format"';
                 }
               }
             }
             leaf password {
               if-feature client-auth-password;
               type ianach:crypt-hash;



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               description
                 "The password for this user.";
             }

             container hostbased {
               if-feature client-auth-hostbased;
               presence
                 "Indicates that the server can authenticate this
                  user's 'host' using any of the configured SSH
                  host keys.";
               description
                 "A set of SSH host keys may be used by the SSH
                  server to authenticate this user's host.  A
                  user's host is authenticated if its host key
                  is an exact match to a configured host key.";
               reference
                 "RFC 4253: The Secure Shell (SSH) Transport Layer
                  RFC BBBB: A YANG Data Model for a Truststore";
               uses ts:local-or-truststore-public-keys-grouping {
                 refine "local-or-truststore/local/local-definition"
                        + "/public-key" {
                   must 'public-key-format'
                        + ' = "ct:ssh-public-key-format"';
                 }
                 refine "local-or-truststore/truststore"
                        + "/truststore-reference" {
                   must 'deref(.)/../*/ts:public-key-format'
                        + ' = "ct:ssh-public-key-format"';
                 }
               }
             }
             leaf none {
               if-feature client-auth-none;
               type empty;
               description
                 "Indicates that the 'none' method is supported.";
               reference
                 "RFC 4252: The Secure Shell (SSH) Authentication
                            Protocol.";
             }
           }
         }
         container ca-certs {
           if-feature "client-auth-config-supported";
           if-feature "sshcmn:ssh-x509-certs";
           presence
             "Indicates that the SSH server can authenticate SSH
              clients using configured certificate authority (CA)



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              certificates.";
           description
             "A set of certificate authority (CA) certificates used by
              the SSH server to authenticate SSH client certificates.
              A client certificate is authenticated if it has a valid
              chain of trust to a configured CA certificate.";
           reference
             "RFC BBBB: A YANG Data Model for a Truststore";
           uses ts:local-or-truststore-certs-grouping;
         }
         container ee-certs {
           if-feature "client-auth-config-supported";
           if-feature "sshcmn:ssh-x509-certs";
           presence
             "Indicates that the SSH server can authenticate SSH
              clients using configured end-entity certificates.";
           description
             "A set of client certificates (i.e., end entity
              certificates) used by the SSH server to authenticate
              the certificates presented by SSH clients.  A client
              certificate is authenticated if it is an exact match
              to a configured end-entity certificate.";
           reference
             "RFC BBBB: A YANG Data Model for a Truststore";
           uses ts:local-or-truststore-certs-grouping;
         }
       } // container client-authentication

       container transport-params {
         nacm:default-deny-write;
         if-feature "ssh-server-transport-params-config";
         description
           "Configurable parameters of the SSH transport layer.";
         uses sshcmn:transport-params-grouping;
       } // container transport-params

       container keepalives {
         nacm:default-deny-write;
         if-feature "ssh-server-keepalives";
         presence
           "Indicates that the SSH server proactively tests the
            aliveness of the remote SSH client.";
         description
           "Configures the keep-alive policy, to proactively test
            the aliveness of the SSL client.  An unresponsive SSL
            client is dropped after approximately max-wait *
            max-attempts seconds.  Per Section 4 of RFC 4254,
            the SSH server SHOULD send an SSH_MSG_GLOBAL_REQUEST



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            message with a purposely nonexistent 'request name'
            value (e.g., keepalive@ietf.org) and the 'want reply'
            value set to '1'.";
         reference
           "RFC 4254: The Secure Shell (SSH) Connection Protocol";
         leaf max-wait {
           type uint16 {
             range "1..max";
           }
           units "seconds";
           default "30";
           description
             "Sets the amount of time in seconds after which
              if no data has been received from the SSL client,
              a SSL-level message will be sent to test the
              aliveness of the SSL client.";
         }
         leaf max-attempts {
           type uint8;
           default "3";
           description
             "Sets the maximum number of sequential keep-alive
              messages that can fail to obtain a response from
              the SSL client before assuming the SSL client is
              no longer alive.";
         }
       }
     } // grouping ssh-server-grouping
   } // module ietf-ssh-server

   <CODE ENDS>

5.  Security Considerations

5.1.  The "ietf-ssh-common" YANG Module

   The "ietf-ssh-common" YANG module defines "grouping" statements that
   are designed to be accessed via YANG based management protocols, such
   as NETCONF [RFC6241] and RESTCONF [RFC8040].  Both of these protocols
   have mandatory-to-implement secure transport layers (e.g., SSH, TLS)
   with mutual authentication.

   The NETCONF access control model (NACM) [RFC8341] provides the means
   to restrict access for particular users to a pre-configured subset of
   all available protocol operations and content.






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   Since the module in this document only define groupings, these
   considerations are primarily for the designers of other modules that
   use these groupings.

   None of the readable data nodes defined in this YANG module are
   considered sensitive or vulnerable in network environments.  The NACM
   "default-deny-all" extension has not been set for any data nodes
   defined in this module.

   None of the writable data nodes defined in this YANG module are
   considered sensitive or vulnerable in network environments.  The NACM
   "default-deny-write" extension has not been set for any data nodes
   defined in this module.

   This module does not define any RPCs, actions, or notifications, and
   thus the security consideration for such is not provided here.

5.2.  The "ietf-ssh-client" YANG Module

   The "ietf-ssh-client" YANG module defines "grouping" statements that
   are designed to be accessed via YANG based management protocols, such
   as NETCONF [RFC6241] and RESTCONF [RFC8040].  Both of these protocols
   have mandatory-to-implement secure transport layers (e.g., SSH, TLS)
   with mutual authentication.

   The NETCONF access control model (NACM) [RFC8341] provides the means
   to restrict access for particular users to a pre-configured subset of
   all available protocol operations and content.

   Since the module in this document only define groupings, these
   considerations are primarily for the designers of other modules that
   use these groupings.

   One readable data node defined in this YANG module may be considered
   sensitive or vulnerable in some network environments.  This node is
   as follows:

   *  The "client-identity/password" node:

         The cleartext "password" node defined in the "ssh-client-
         grouping" grouping is additionally sensitive to read operations
         such that, in normal use cases, it should never be returned to
         a client.  For this reason, the NACM extension "default-deny-
         all" has been applied to it.







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      |  Please be aware that this module uses the "key" and "private-
      |  key" nodes from the "ietf-crypto-types" module
      |  [I-D.ietf-netconf-crypto-types], where said nodes have the NACM
      |  extension "default-deny-all" set, thus preventing unrestricted
      |  read-access to the cleartext key values.

   All of the writable data nodes defined by this module may be
   considered sensitive or vulnerable in some network environments.  For
   instance, any modification to a key or reference to a key may
   dramatically alter the implemented security policy.  For this reason,
   the NACM extension "default-deny-write" has been set for all data
   nodes defined in this module.

   This module does not define any RPCs, actions, or notifications, and
   thus the security consideration for such is not provided here.

5.3.  The "ietf-ssh-server" YANG Module

   The "ietf-ssh-server" YANG module defines "grouping" statements that
   are designed to be accessed via YANG based management protocols, such
   as NETCONF [RFC6241] and RESTCONF [RFC8040].  Both of these protocols
   have mandatory-to-implement secure transport layers (e.g., SSH, TLS)
   with mutual authentication.

   The NETCONF access control model (NACM) [RFC8341] provides the means
   to restrict access for particular users to a pre-configured subset of
   all available protocol operations and content.

   Since the module in this document only define groupings, these
   considerations are primarily for the designers of other modules that
   use these groupings.

   None of the readable data nodes defined in this YANG module are
   considered sensitive or vulnerable in network environments.  The NACM
   "default-deny-all" extension has not been set for any data nodes
   defined in this module.

      |  Please be aware that this module uses the "key" and "private-
      |  key" nodes from the "ietf-crypto-types" module
      |  [I-D.ietf-netconf-crypto-types], where said nodes have the NACM
      |  extension "default-deny-all" set, thus preventing unrestricted
      |  read-access to the cleartext key values.

   All of the writable data nodes defined by this module may be
   considered sensitive or vulnerable in some network environments.  For
   instance, the addition or removal of references to keys,
   certificates, trusted anchors, etc., or even the modification of
   transport or keepalive parameters can dramatically alter the



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   implemented security policy.  For this reason, the NACM extension
   "default-deny-write" has been set for all data nodes defined in this
   module.

   This module does not define any RPCs, actions, or notifications, and
   thus the security consideration for such is not provided here.

6.  IANA Considerations

6.1.  The "IETF XML" Registry

   This document registers three URIs in the "ns" subregistry of the
   IETF XML Registry [RFC3688].  Following the format in [RFC3688], the
   following registrations are requested:

      URI: urn:ietf:params:xml:ns:yang:ietf-ssh-common
      Registrant Contact: The NETCONF WG of the IETF.
      XML: N/A, the requested URI is an XML namespace.

      URI: urn:ietf:params:xml:ns:yang:ietf-ssh-client
      Registrant Contact: The NETCONF WG of the IETF.
      XML: N/A, the requested URI is an XML namespace.

      URI: urn:ietf:params:xml:ns:yang:ietf-ssh-server
      Registrant Contact: The NETCONF WG of the IETF.
      XML: N/A, the requested URI is an XML namespace.

6.2.  The "YANG Module Names" Registry

   This document registers three YANG modules in the YANG Module Names
   registry [RFC6020].  Following the format in [RFC6020], the following
   registrations are requested:

      name:         ietf-ssh-common
      namespace:    urn:ietf:params:xml:ns:yang:ietf-ssh-common
      prefix:       sshcmn
      reference:    RFC EEEE

      name:         ietf-ssh-client
      namespace:    urn:ietf:params:xml:ns:yang:ietf-ssh-client
      prefix:       sshc
      reference:    RFC EEEE

      name:         ietf-ssh-server
      namespace:    urn:ietf:params:xml:ns:yang:ietf-ssh-server
      prefix:       sshs
      reference:    RFC EEEE




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7.  References

7.1.  Normative References

   [I-D.ietf-netconf-crypto-types]
              Watsen, K., "Common YANG Data Types for Cryptography",
              Work in Progress, Internet-Draft, draft-ietf-netconf-
              crypto-types-15, 20 May 2020,
              <https://tools.ietf.org/html/draft-ietf-netconf-crypto-
              types-15>.

   [I-D.ietf-netconf-keystore]
              Watsen, K., "A YANG Data Model for a Keystore", Work in
              Progress, Internet-Draft, draft-ietf-netconf-keystore-17,
              20 May 2020, <https://tools.ietf.org/html/draft-ietf-
              netconf-keystore-17>.

   [I-D.ietf-netconf-trust-anchors]
              Watsen, K., "A YANG Data Model for a Truststore", Work in
              Progress, Internet-Draft, draft-ietf-netconf-trust-
              anchors-10, 20 May 2020, <https://tools.ietf.org/html/
              draft-ietf-netconf-trust-anchors-10>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4344]  Bellare, M., Kohno, T., and C. Namprempre, "The Secure
              Shell (SSH) Transport Layer Encryption Modes", RFC 4344,
              DOI 10.17487/RFC4344, January 2006,
              <https://www.rfc-editor.org/info/rfc4344>.

   [RFC4419]  Friedl, M., Provos, N., and W. Simpson, "Diffie-Hellman
              Group Exchange for the Secure Shell (SSH) Transport Layer
              Protocol", RFC 4419, DOI 10.17487/RFC4419, March 2006,
              <https://www.rfc-editor.org/info/rfc4419>.

   [RFC5656]  Stebila, D. and J. Green, "Elliptic Curve Algorithm
              Integration in the Secure Shell Transport Layer",
              RFC 5656, DOI 10.17487/RFC5656, December 2009,
              <https://www.rfc-editor.org/info/rfc5656>.

   [RFC6020]  Bjorklund, M., Ed., "YANG - A Data Modeling Language for
              the Network Configuration Protocol (NETCONF)", RFC 6020,
              DOI 10.17487/RFC6020, October 2010,
              <https://www.rfc-editor.org/info/rfc6020>.




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   [RFC6187]  Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure
              Shell Authentication", RFC 6187, DOI 10.17487/RFC6187,
              March 2011, <https://www.rfc-editor.org/info/rfc6187>.

   [RFC6668]  Bider, D. and M. Baushke, "SHA-2 Data Integrity
              Verification for the Secure Shell (SSH) Transport Layer
              Protocol", RFC 6668, DOI 10.17487/RFC6668, July 2012,
              <https://www.rfc-editor.org/info/rfc6668>.

   [RFC7950]  Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
              RFC 7950, DOI 10.17487/RFC7950, August 2016,
              <https://www.rfc-editor.org/info/rfc7950>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8341]  Bierman, A. and M. Bjorklund, "Network Configuration
              Access Control Model", STD 91, RFC 8341,
              DOI 10.17487/RFC8341, March 2018,
              <https://www.rfc-editor.org/info/rfc8341>.

7.2.  Informative References

   [I-D.ietf-netconf-http-client-server]
              Watsen, K., "YANG Groupings for HTTP Clients and HTTP
              Servers", Work in Progress, Internet-Draft, draft-ietf-
              netconf-http-client-server-03, 20 May 2020,
              <https://tools.ietf.org/html/draft-ietf-netconf-http-
              client-server-03>.

   [I-D.ietf-netconf-netconf-client-server]
              Watsen, K., "NETCONF Client and Server Models", Work in
              Progress, Internet-Draft, draft-ietf-netconf-netconf-
              client-server-19, 20 May 2020,
              <https://tools.ietf.org/html/draft-ietf-netconf-netconf-
              client-server-19>.

   [I-D.ietf-netconf-restconf-client-server]
              Watsen, K., "RESTCONF Client and Server Models", Work in
              Progress, Internet-Draft, draft-ietf-netconf-restconf-
              client-server-19, 20 May 2020,
              <https://tools.ietf.org/html/draft-ietf-netconf-restconf-
              client-server-19>.

   [I-D.ietf-netconf-ssh-client-server]
              Watsen, K. and G. Wu, "YANG Groupings for SSH Clients and
              SSH Servers", Work in Progress, Internet-Draft, draft-



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              ietf-netconf-ssh-client-server-19, 20 May 2020,
              <https://tools.ietf.org/html/draft-ietf-netconf-ssh-
              client-server-19>.

   [I-D.ietf-netconf-tcp-client-server]
              Watsen, K. and M. Scharf, "YANG Groupings for TCP Clients
              and TCP Servers", Work in Progress, Internet-Draft, draft-
              ietf-netconf-tcp-client-server-06, 16 June 2020,
              <https://tools.ietf.org/html/draft-ietf-netconf-tcp-
              client-server-06>.

   [I-D.ietf-netconf-tls-client-server]
              Watsen, K. and G. Wu, "YANG Groupings for TLS Clients and
              TLS Servers", Work in Progress, Internet-Draft, draft-
              ietf-netconf-tls-client-server-19, 20 May 2020,
              <https://tools.ietf.org/html/draft-ietf-netconf-tls-
              client-server-19>.

   [OPENSSH]  Project, T. O., "OpenSSH", 2016, <http://www.openssh.com>.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              DOI 10.17487/RFC3688, January 2004,
              <https://www.rfc-editor.org/info/rfc3688>.

   [RFC4252]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252,
              January 2006, <https://www.rfc-editor.org/info/rfc4252>.

   [RFC4253]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
              January 2006, <https://www.rfc-editor.org/info/rfc4253>.

   [RFC4254]  Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
              Connection Protocol", RFC 4254, DOI 10.17487/RFC4254,
              January 2006, <https://www.rfc-editor.org/info/rfc4254>.

   [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
              and A. Bierman, Ed., "Network Configuration Protocol
              (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
              <https://www.rfc-editor.org/info/rfc6241>.

   [RFC6242]  Wasserman, M., "Using the NETCONF Protocol over Secure
              Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
              <https://www.rfc-editor.org/info/rfc6242>.

   [RFC7317]  Bierman, A. and M. Bjorklund, "A YANG Data Model for
              System Management", RFC 7317, DOI 10.17487/RFC7317, August
              2014, <https://www.rfc-editor.org/info/rfc7317>.



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   [RFC8040]  Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
              Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
              <https://www.rfc-editor.org/info/rfc8040>.

   [RFC8071]  Watsen, K., "NETCONF Call Home and RESTCONF Call Home",
              RFC 8071, DOI 10.17487/RFC8071, February 2017,
              <https://www.rfc-editor.org/info/rfc8071>.

   [RFC8340]  Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
              BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
              <https://www.rfc-editor.org/info/rfc8340>.

   [RFC8342]  Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
              and R. Wilton, "Network Management Datastore Architecture
              (NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
              <https://www.rfc-editor.org/info/rfc8342>.

Appendix A.  Change Log

   This section is to be removed before publishing as an RFC.

A.1.  00 to 01

   *  Noted that '0.0.0.0' and '::' might have special meanings.

   *  Renamed "keychain" to "keystore".

A.2.  01 to 02

   *  Removed the groupings 'listening-ssh-client-grouping' and
      'listening-ssh-server-grouping'.  Now modules only contain the
      transport-independent groupings.

   *  Simplified the "client-auth" part in the ietf-ssh-client module.
      It now inlines what it used to point to keystore for.

   *  Added cipher suites for various algorithms into new 'ietf-ssh-
      common' module.

A.3.  02 to 03

   *  Removed 'RESTRICTED' enum from 'password' leaf type.

   *  Added a 'must' statement to container 'server-auth' asserting that
      at least one of the various auth mechanisms must be specified.

   *  Fixed description statement for leaf 'trusted-ca-certs'.




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A.4.  03 to 04

   *  Change title to "YANG Groupings for SSH Clients and SSH Servers"

   *  Added reference to RFC 6668

   *  Added RFC 8174 to Requirements Language Section.

   *  Enhanced description statement for ietf-ssh-server's "trusted-ca-
      certs" leaf.

   *  Added mandatory true to ietf-ssh-client's "client-auth" 'choice'
      statement.

   *  Changed the YANG prefix for module ietf-ssh-common from 'sshcom'
      to 'sshcmn'.

   *  Removed the compression algorithms as they are not commonly
      configurable in vendors' implementations.

   *  Updating descriptions in transport-params-grouping and the
      servers's usage of it.

   *  Now tree diagrams reference ietf-netmod-yang-tree-diagrams

   *  Updated YANG to use typedefs around leafrefs to common keystore
      paths

   *  Now inlines key and certificates (no longer a leafref to keystore)

A.5.  04 to 05

   *  Merged changes from co-author.

A.6.  05 to 06

   *  Updated to use trust anchors from trust-anchors draft (was
      keystore draft)

   *  Now uses new keystore grouping enabling asymmetric key to be
      either locally defined or a reference to the keystore.

A.7.  06 to 07

   *  factored the ssh-[client|server]-groupings into more reusable
      groupings.





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   *  added if-feature statements for the new "ssh-host-keys" and
      "x509-certificates" features defined in draft-ietf-netconf-trust-
      anchors.

A.8.  07 to 08

   *  Added a number of compatibility matrices to Section 5 (thanks
      Frank!)

   *  Clarified that any configured "host-key-alg" values need to be
      compatible with the configured private key.

A.9.  08 to 09

   *  Updated examples to reflect update to groupings defined in the
      keystore -09 draft.

   *  Add SSH keepalives features and groupings.

   *  Prefixed top-level SSH grouping nodes with 'ssh-' and support
      mashups.

   *  Updated copyright date, boilerplate template, affiliation, and
      folding algorithm.

A.10.  09 to 10

   *  Reformatted the YANG modules.

A.11.  10 to 11

   *  Reformatted lines causing folding to occur.

A.12.  11 to 12

   *  Collapsed all the inner groupings into the top-level grouping.

   *  Added a top-level "demux container" inside the top-level grouping.

   *  Added NACM statements and updated the Security Considerations
      section.

   *  Added "presence" statements on the "keepalive" containers, as was
      needed to address a validation error that appeared after adding
      the "must" statements into the NETCONF/RESTCONF client/server
      modules.





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   *  Updated the boilerplate text in module-level "description"
      statement to match copyeditor convention.

A.13.  12 to 13

   *  Removed the "demux containers", floating the nacm:default-deny-
      write to each descendent node, and adding a note to model
      designers regarding the potential need to add their own demux
      containers.

   *  Fixed a couple references (section 2 --> section 3)

   *  In the server model, replaced <client-cert-auth> with <client-
      authentication> and introduced 'local-or-external' choice.

A.14.  13 to 14

   *  Updated to reflect changes in trust-anchors drafts (e.g., s/trust-
      anchors/truststore/g + s/pinned.//)

A.15.  14 to 15

   *  Updated examples to reflect ietf-crypto-types change (e.g.,
      identities --> enumerations)

   *  Updated "server-authentication" and "client-authentication" nodes
      from being a leaf of type "ts:host-keys-ref" or "ts:certificates-
      ref" to a container that uses "ts:local-or-truststore-host-keys-
      grouping" or "ts:local-or-truststore-certs-grouping".

A.16.  15 to 16

   *  Removed unnecessary if-feature statements in the -client and
      -server modules.

   *  Cleaned up some description statements in the -client and -server
      modules.

   *  Fixed a canonical ordering issue in ietf-ssh-common detected by
      new pyang.

A.17.  16 to 17

   *  Removed choice local-or-external by removing the 'external' case
      and flattening the 'local' case and adding a "client-auth-config-
      supported" feature.

   *  Updated examples to include the "*-key-format" nodes.



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   *  Augmented-in "must" expressions ensuring that locally-defined
      public-key-format are "ct:ssh-public-key-format" (must expr for
      ref'ed keys are TBD).

A.18.  17 to 18

   *  Removed leaf-list 'other' from ietf-ssh-server.

   *  Removed unused 'external-client-auth-supported' feature.

   *  Added features client-auth-password, client-auth-hostbased, and
      client-auth-none.

   *  Renamed 'host-key' to 'public-key' for when refering to
      'publickey' based auth.

   *  Added new feature-protected 'hostbased' and 'none' to the 'user'
      node's config.

   *  Added new feature-protected 'hostbased' and 'none' to the 'client-
      identity' node's config.

   *  Updated examples to reflect new "bag" addition to truststore.

   *  Refined truststore/keystore groupings to ensure the key formats
      "must" be particular values.

   *  Switched to using truststore's new "public-key" bag (instead of
      separate "ssh-public-key" and "raw-public-key" bags.

   *  Updated client/server examples to cover ALL cases (local/ref x
      cert/raw-key/psk).

A.19.  18 to 19

   *  Updated the "keepalives" containers to address Michal Vasko's
      request to align with RFC 8071.

   *  Removed algorithm-mapping tables from the "SSH Common Model"
      section

   *  Removed 'algorithm' node from examples.

   *  Added feature "client-identity-publickey"

   *  Removed "choice auth-type", as auth-types aren't exclusive.

   *  Renamed both "client-certs" and "server-certs" to "ee-certs"



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   *  Switch "must" to assert the public-key-format is "subject-public-
      key-info-format" when certificates are used.

   *  Added a "Note to Reviewers" note to first page.

A.20.  19 to 20

   *  Added a "must 'public-key or password or hostbased or none or
      certificate'" statement to the "user" node in ietf-ssh-client

   *  Expanded "Data Model Overview section(s) [remove "wall" of tree
      diagrams].

   *  Moved the "ietf-ssh-common" module section to proceed the other
      two module sections.

   *  Updated the Security Considerations section.

A.21.  20 to 21

   *  Updated examples to reflect new "cleartext-" prefix in the crypto-
      types draft.

Acknowledgements

   The authors would like to thank for following for lively discussions
   on list and in the halls (ordered by last name): Andy Bierman, Martin
   Bjorklund, Benoit Claise, Mehmet Ersue, Balazs Kovacs, Radek Krejci,
   David Lamparter, Ladislav Lhotka, Alan Luchuk, Tom Petch, Juergen
   Schoenwaelder, Phil Shafer, Sean Turner, Michal Vasko, Bert Wijnen,
   and Liang Xia.

Authors' Addresses

   Kent Watsen
   Watsen Networks

   Email: kent+ietf@watsen.net


   Gary Wu
   Cisco Systems

   Email: garywu@cisco.com







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