NETCONF Working Group K. Watsen
Internet-Draft Juniper Networks
Intended status: Standards Track G. Wu
Expires: September 14, 2017 Cisco Systems
March 13, 2017

SSH Client and Server Models
draft-ietf-netconf-ssh-client-server-02

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 many 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.

This document contains references to other drafts in progress, both in the Normative References section, as well as in body text throughout. Please update the following references to reflect their final RFC assignments:

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

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

The following two Appendix sections are to be removed prior to publication:

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on September 14, 2017.

Copyright Notice

Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.


Table of Contents

1. Introduction

This document defines three YANG [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 (SSH is defined in [RFC4252], [RFC4253], and [RFC4254]). It is intended that these groupings will be used by applications using the SSH protocol. 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 enables 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 grouping for the SSH parts it provides, while adding data nodes for the TCP-level call-home configuration.

1.1. Terminology

The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. Tree Diagrams

A simplified graphical representation of the data models is used in this document. The meaning of the symbols in these diagrams is as follows:

2. The SSH Client Model

The SSH client model presented in this section contains one YANG grouping, to just configure the SSH client omitting, for instance, any configuration for which IP address or port the client should connect to.

This grouping references data nodes defined by the keystore model [I-D.ietf-netconf-keystore]. For instance, a reference to the keystore model is made to indicate which trusted CA certificate a client should use to authenticate X.509v3 certificate based host keys [RFC6187].

2.1. Tree Diagram

The following tree diagram presents the data model for the grouping defined in the ietf-ssh-client module. Please see Section 1.2 for tree diagram notation.

module: ietf-ssh-client
  groupings:
  ssh-client-grouping
      +---- server-auth
      |  +---- trusted-ssh-host-keys?
      |  |       -> /ks:keystore/trusted-host-keys/name
      |  +---- trusted-ca-certs?
      |  |       -> /ks:keystore/trusted-certificates/name
      |  |       {sshcom:ssh-x509-certs}?
      |  +---- trusted-server-certs?
      |          -> /ks:keystore/trusted-certificates/name
      |          {sshcom:ssh-x509-certs}?
      +---- client-auth
      |  +---- username?      string
      |  +---- (auth-type)?
      |     +--:(certificate)
      |     |  +---- certificate?   leafref {sshcom:ssh-x509-certs}?
      |     +--:(public-key)
      |     |  +---- public-key?    -> /ks:keystore/keys/key/name
      |     +--:(password)
      |        +---- password?      union
      +---- transport-params {ssh-client-transport-params-config}?
         +---- host-key
         |  +---- host-key-alg*   identityref
         +---- key-exchange
         |  +---- key-exchange-alg*   identityref
         +---- encryption
         |  +---- encryption-alg*   identityref
         +---- mac
         |  +---- mac-alg*   identityref
         +---- compression
            +---- compression-alg*   identityref

2.2. Example Usage

This section shows how it would appear if the ssh-client-grouping were populated with some data. This example is consistent with the examples presented in Section 2.2 of [I-D.ietf-netconf-keystore].

<!-- hypothetical example, as groupings don't have instance data -->
<ssh-client xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-client">

  <!-- which host-keys will this client trust -->
  <server-auth>
    <trusted-ssh-host-keys>explicitly-trusted-ssh-host-keys</trusted-ssh-host-keys>
  </server-auth>

  <!-- how this client will authenticate itself to the server -->
  <client-auth>
    <username>foobar</username>
    <public-key>ex-rsa-key</public-key>
  </client-auth>

</ssh-client>

2.3. YANG Model

This YANG module has a normative references to [RFC6991] and [I-D.ietf-netconf-keystore].


<CODE BEGINS> file "ietf-ssh-client@2017-03-13.yang"

module ietf-ssh-client {
  yang-version 1.1;

  namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-client";
  prefix "sshc";

  import ietf-ssh-common {
    prefix sshcom;
    revision-date 2017-03-13; // stable grouping definitions
    reference
      "RFC XXXX: SSH Client and Server Models";
  }

  import ietf-netconf-acm {
    prefix nacm;
    reference
      "RFC 6536: Network Configuration Protocol (NETCONF) Access
       Control Model";
  }

  import ietf-keystore {
    prefix ks;
    reference
      "RFC YYYY: Keystore Model";
  }

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

  contact
   "WG Web:   <http://tools.ietf.org/wg/netconf/>
    WG List:  <mailto:netconf@ietf.org>

    Author:   Kent Watsen
              <mailto:kwatsen@juniper.net>

    Author:   Gary Wu
              <mailto:garywu@cisco.com>";


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

    Copyright (c) 2014 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
    (http://trustee.ietf.org/license-info).

    This version of this YANG module is part of RFC XXXX; see
    the RFC itself for full legal notices.";

  revision "2017-03-13" {
    description
     "Initial version";
    reference
     "RFC XXXX: SSH Client and Server Models";
  }

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

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

    container server-auth {
      description
        "Trusted server identities.";

      leaf trusted-ssh-host-keys {
        type leafref {
          path "/ks:keystore/ks:trusted-host-keys/ks:name";
        }
        description
          "A reference to a list of SSH host keys used by the
           SSH client to authenticate SSH server host keys.
           A server host key is authenticate if it is an exact
           match to a configured trusted SSH host key.";
      }

      leaf trusted-ca-certs {
        if-feature sshcom:ssh-x509-certs;
        type leafref {
          path "/ks:keystore/ks:trusted-certificates/ks:name";
        }
        description
          "A reference to a list of certificate authority (CA)
           certificates used by the SSH client to authenticate
           SSH server certificates.";
      }

      leaf trusted-server-certs {
        if-feature sshcom:ssh-x509-certs;
        type leafref {
          path "/ks:keystore/ks:trusted-certificates/ks:name";
        }
        description
          "A reference to a list of server certificates used by
           the SSH client to authenticate SSH server certificates.
           A server certificate is authenticated if it is an
           exact match to a configured trusted server certificate.";
      }
    }

    container client-auth {
      description
        "The credentials used by the 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.";
      }

      choice auth-type {
        description
          "The authentication type.";
        leaf certificate {
          if-feature sshcom:ssh-x509-certs;
          type leafref {
            path "/ks:keystore/ks:keys/ks:key/ks:certificates/"
                 + "ks:certificate/ks:name";
          }
          description
            "A certificates to be used for user authentication.";
        }
        leaf public-key {
          type leafref {
            path "/ks:keystore/ks:keys/ks:key/ks:name";
          }
          description
            "A public keys to be used for user authentication.";
        }
        leaf password {
          nacm:default-deny-all;
          type union {
            type string;
            type enumeration {
              enum "RESTRICTED" {
               description
                "The private key is restricted due to access-control.";
              }
            }
          }
          description
            "A password to be used for user authentication.";
        }
      }
    } // end client-auth

    container transport-params {
      if-feature ssh-client-transport-params-config;
      uses sshcom:transport-params-grouping;
      description
        "Configurable parameters for the SSH transport layer.";
    }

  } // ssh-client-grouping

}



<CODE ENDS>

3. The SSH Server Model

The SSH server model presented in this section contains one YANG grouping, for just the SSH-level configuration omitting, for instance, configuration for which ports to open to listen for connections on.

This grouping references data nodes defined by the keystore model [I-D.ietf-netconf-keystore]. For instance, a reference to the keystore model is made to indicate which host key a server should present.

3.1. Tree Diagram

The following tree diagram presents the data model for the grouping defined in the ietf-ssh-server module. Please see Section 1.2 for tree diagram notation.

module: ietf-ssh-server
  groupings:
  ssh-server-grouping
      +---- host-keys
      |  +---- host-key* [name]
      |     +---- name?          string
      |     +---- (host-key-type)
      |        +--:(public-key)
      |        |  +---- public-key?    -> /ks:keystore/keys/key/name
      |        +--:(certificate)
      |           +---- certificate?   leafref {sshcom:ssh-x509-certs}?
      +---- client-cert-auth {sshcom:ssh-x509-certs}?
      |  +---- trusted-ca-certs?
      |  |       -> /ks:keystore/trusted-certificates/name
      |  +---- trusted-client-certs?
      |          -> /ks:keystore/trusted-certificates/name
      +---- transport-params {ssh-server-transport-params-config}?
         +---- host-key
         |  +---- host-key-alg*   identityref
         +---- key-exchange
         |  +---- key-exchange-alg*   identityref
         +---- encryption
         |  +---- encryption-alg*   identityref
         +---- mac
         |  +---- mac-alg*   identityref
         +---- compression
            +---- compression-alg*   identityref

3.2. Example Usage

This section shows how it would appear if the ssh-server-grouping were populated with some data. This example is consistent with the examples presented in Section 2.2 of [I-D.ietf-netconf-keystore].

<!-- hypothetical example, as groupings don't have instance data -->
<ssh-server xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-server">

  <!-- which host-keys will this SSH server present -->
  <host-keys>
    <host-key>
      <name>deployment-specific-certificate</name>
      <certificate>ex-rsa-cert</certificate>
    </host-key>
  </host-keys>

  <!-- NOTE: password/public-key auth is NOT configured here, -->
  <!-- as it is configured in the ietf-system (RFC 7317)      -->
  <!-- module instead.                                        -->

  <!-- which client-certs will this SSH server trust -->
  <client-cert-auth>
    <trusted-ca-certs>deployment-specific-ca-certs</trusted-ca-certs>
    <trusted-client-certs>explicitly-trusted-client-certs</trusted-client-certs>
  </client-cert-auth>

</ssh-server>

3.3. YANG Model

This YANG module has a normative references to [RFC4253], [RFC6991], and [I-D.ietf-netconf-keystore].


<CODE BEGINS> file "ietf-ssh-server@2017-03-13.yang"

module ietf-ssh-server {
  yang-version 1.1;

  namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-server";
  prefix "sshs";

  import ietf-ssh-common {
    prefix sshcom;
    revision-date 2017-03-13; // stable grouping definitions
    reference
      "RFC XXXX: SSH Client and Server Models";
  }

  import ietf-keystore {
    prefix ks;
    reference
      "RFC YYYY: Keystore Model";
  }

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

  contact
   "WG Web:   <http://tools.ietf.org/wg/netconf/>
    WG List:  <mailto:netconf@ietf.org>

    Author:   Kent Watsen
              <mailto:kwatsen@juniper.net>";


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

    Copyright (c) 2014 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
    (http://trustee.ietf.org/license-info).

    This version of this YANG module is part of RFC XXXX; see
    the RFC itself for full legal notices.";

  revision "2017-03-13" {
    description
     "Initial version";
    reference
     "RFC XXXX: SSH Client and Server Models";
  }

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

  // grouping
  grouping ssh-server-grouping {
    description
      "A reusable grouping for configuring a SSH server without
       any consideration for how underlying TCP sessions are 
       established.";
    container host-keys {
      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 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";
          leaf public-key {
            type leafref {
              path "/ks:keystore/ks:keys/ks:key/ks:name";
            }
            description
              "The public key is actually identified by the name of
               its cooresponding private-key in the keystore.";
          }
          leaf certificate {
            if-feature sshcom:ssh-x509-certs;
            type leafref {
              path "/ks:keystore/ks:keys/ks:key/ks:certificates/"
                   + "ks:certificate/ks:name";
            }
            description
              "The name of a certificate in the keystore.";
          }
        }
      }
    }

    container client-cert-auth {
      if-feature sshcom:ssh-x509-certs;
      description
        "A reference to a list of trusted certificate authority (CA)
         certificates and a reference to a list of trusted client
         certificates.";
      leaf trusted-ca-certs {
        type leafref {
          path "/ks:keystore/ks:trusted-certificates/ks:name";
        }
        description
          "A reference to a list of certificate authority (CA) 
           certificates used by the SSH server to authenticate
           SSH client certificates.";
      }
  
      leaf trusted-client-certs {
        type leafref {
          path "/ks:keystore/ks:trusted-certificates/ks:name";
        }
        description
          "A reference to a list of client certificates used by 
           the SSH server to authenticate SSH client certificates.
           A clients certificate is authenticated if it is an
           exact match to a configured trusted client certificate.";
      }
    }

    container transport-params {
      if-feature ssh-server-transport-params-config;
      uses sshcom:transport-params-grouping;
      description
        "Configurable parameters for the SSH transport layer.";
    }

  } // ssh-server-grouping

}



<CODE ENDS>

4. The SSH Common Model

The SSH common model presented in this section contains identities and groupings common to both SSH clients and SSH servers. The transport-params-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 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.

4.1. Tree Diagram

The following tree diagram presents the data model for the grouping defined in the ietf-ssh-common module. Please see Section 1.2 for tree diagram notation.

module: ietf-ssh-common
  groupings:
  transport-params-grouping
      +---- host-key
      |  +---- host-key-alg*   identityref
      +---- key-exchange
      |  +---- key-exchange-alg*   identityref
      +---- encryption
      |  +---- encryption-alg*   identityref
      +---- mac
      |  +---- mac-alg*   identityref
      +---- compression
         +---- compression-alg*   identityref

4.2. Example Usage

This section shows how it would appear if the transport-params-grouping were populated with some data.

<!-- hypothetical example, as groupings don't have instance data -->
<transport-params xmlns="urn:ietf:params:xml:ns:yang:ietf-ssh-common">

  <host-key>
    <host-key-alg>x509v3-rsa2048-sha256</host-key-alg>
    <host-key-alg>ssh-rsa</host-key-alg>
  </host-key>
  <key-exchange>
    <key-exchange-alg>
      diffie-hellman-group-exchange-sha256
    </key-exchange-alg>
  </key-exchange>
  <encryption>
    <encryption-alg>aes256-ctr</encryption-alg>
    <encryption-alg>aes192-ctr</encryption-alg>
    <encryption-alg>aes128-ctr</encryption-alg>
    <encryption-alg>aes256-cbc</encryption-alg>
    <encryption-alg>aes192-cbc</encryption-alg>
    <encryption-alg>aes128-cbc</encryption-alg>
  </encryption>
  <mac>
    <mac-alg>hmac-sha2-256</mac-alg>
    <mac-alg>hmac-sha2-512</mac-alg>
  </mac>
  <compression>
    <compression-alg>none</compression-alg>
  </compression>

</transport-params>

4.3. YANG Model

This YANG module has a normative references to [RFC4344], [RFC4419], and [RFC5656].


<CODE BEGINS> file "ietf-ssh-common@2017-03-13.yang"

module ietf-ssh-common {
  yang-version 1.1;

  namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-common";
  prefix "sshcom";

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

  contact
   "WG Web:   <http://tools.ietf.org/wg/netconf/>
    WG List:  <mailto:netconf@ietf.org>

    Author:   Kent Watsen
              <mailto:kwatsen@juniper.net>

    Author:   Gary Wu
              <mailto:garywu@cisco.com>";


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

    Copyright (c) 2017 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
    (http://trustee.ietf.org/license-info).

    This version of this YANG module is part of RFC XXXX; see
    the RFC itself for full legal notices.";

  revision "2017-03-13" {
    description
     "Initial version";
    reference
     "RFC XXXX: SSH Client and Server Models";
  }

  // 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
      "X.509v3 certificates are supported for SSH as 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)";
  }

  feature ssh-zlib {
    description
      "ZLIB (LZ77) compression is supported for SSH.";
    reference
      "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
  }

  // 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 {
    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 {
    base public-key-alg-base;
    if-feature "ssh-ecc and ssh-sha2";
    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 {
    base public-key-alg-base;
    if-feature "ssh-ecc and ssh-sha2";
    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 {
    base public-key-alg-base;
    if-feature "ssh-ecc and ssh-sha2";
    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 {
    base public-key-alg-base;
    if-feature ssh-x509-certs;
    description
      "RSASSA-PKCS1-v1_5 signature scheme using a public key stored in
       an X.509v3 certificate and using SHA-1 as the hashing
       algorithm.";
    reference
      "RFC 6187: X.509v3 Certificates for Secure Shell
                 Authentication";
  }

  identity x509v3-rsa2048-sha256 {
    base public-key-alg-base;
    if-feature "ssh-x509-certs and ssh-sha2";
    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 {
    base public-key-alg-base;
    if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
    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 {
    base public-key-alg-base;
    if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
    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 {
    base public-key-alg-base;
    if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
    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 {
    base key-exchange-alg-base;
    if-feature ssh-dh-group-exchange;
    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 {
    base key-exchange-alg-base;
    if-feature "ssh-dh-group-exchange and ssh-sha2";
    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 {
    base key-exchange-alg-base;
    if-feature "ssh-ecc and ssh-sha2";
    description
      "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 {
    base key-exchange-alg-base;
    if-feature "ssh-ecc and ssh-sha2";
    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 {
    base key-exchange-alg-base;
    if-feature "ssh-ecc and ssh-sha2";
    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";
  }

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

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

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

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

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

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

  identity none {
    base compression-alg-base;
    description
      "No compression.";
    reference
      "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
  }

  identity zlib {
    base compression-alg-base;
    if-feature ssh-zlib;
    description
      "ZLIB (LZ77) compression.";
    reference
      "RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
  }

  // groupings
  grouping transport-params-grouping {
    description
      "A reusable grouping for SSH transport parameters.
       For configurable parameters, a zero-element leaf-list of
       algorithms indicates the system default configuration for that
       parameter.";
    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;
        }
        ordered-by user;
        description
          "Host key algorithms in order of descending preference.";
      }
    }
    container key-exchange {
      description
        "Parameters regarding key exchange.";
      leaf-list key-exchange-alg {
        type identityref {
          base key-exchange-alg-base;
        }
        ordered-by user;
        description
          "Key exchange algorithms in order of descending
           preference.";
      }
    }
    container encryption {
      description
        "Parameters regarding encryption.";
      leaf-list encryption-alg {
        type identityref {
          base encryption-alg-base;
        }
        ordered-by user;
        description
          "Encryption algorithms in order of descending preference.";
      }
    }
    container mac {
      description
        "Parameters regarding message authentication code (MAC).";
      leaf-list mac-alg {
        type identityref {
          base mac-alg-base;
        }
        ordered-by user;
        description
          "MAC algorithms in order of descending preference.";
      }
    }
    container compression {
      description
        "Parameters regarding compression.";
      leaf-list compression-alg {
        type identityref {
          base compression-alg-base;
        }
        ordered-by user;
        description
          "Compression algorithms in order of descending preference.";
      }
    }
  }
}

<CODE ENDS>

5. Security Considerations

The YANG module defined in this document is 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) [RFC6536] provides the means to restrict access for particular users to a pre-configured subset of all available protocol operations and content.

There are a number of data nodes defined in this YANG module that are writable/creatable/deletable (i.e., config true, which is the default). These data nodes may be considered sensitive or vulnerable in some network environments. Write operations (e.g., edit-config) to these data nodes without proper protection can have a negative effect on network operations. These are the subtrees and data nodes and their sensitivity/vulnerability:

/:
The entire data tree defined by this module is sensitive to write operations. For instance, the addition or removal of references to keys, certificates, trusted anchors, etc., can dramatically alter the implemented security policy. However, no NACM annotations are applied as the data SHOULD be editable by users other than a designated 'recovery session'.

Some of the readable data nodes in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control read access (e.g., via get, get-config, or notification) to these data nodes. These are the subtrees and data nodes and their sensitivity/vulnerability:

/client-auth/password:
This node is additionally sensitive to read operations such that, in normal use cases, it should never be returned to a client. The best reason for returning this node is to support backup/restore type workflows. This being the case, this node is marked with the NACM value 'default-deny-all'.

Some of the RPC operations in this YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control access to these operations. These are the operations and their sensitivity/vulnerability:

NONE

6. IANA Considerations

6.1. The IETF XML Registry

This document registers three URIs in the IETF XML registry [RFC3688]. Following the format in [RFC3688], the following registrations are requested:

   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.

   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.

6.2. The YANG Module Names Registry

This document registers three YANG modules in the YANG Module Names registry [RFC7950]. Following the format in [RFC7950], the the following registrations are requested:

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

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

   name:         ietf-ssh-common
   namespace:    urn:ietf:params:xml:ns:yang:ietf-ssh-common
   prefix:       sshcom
   reference:    RFC XXXX

7. 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, David Lamparter, Alan Luchuk, Ladislav Lhotka, Radek Krejci, Tom Petch, Juergen Schoenwaelder, Phil Shafer, Sean Turner, Michal Vaško, and Bert Wijnen.

8. References

8.1. Normative References

[I-D.ietf-netconf-keystore] Watsen, K. and G. Wu, "Keystore Model", Internet-Draft draft-ietf-netconf-keystore-00, October 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4344] Bellare, M., Kohno, T. and C. Namprempre, "The Secure Shell (SSH) Transport Layer Encryption Modes", RFC 4344, DOI 10.17487/RFC4344, January 2006.
[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.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer", RFC 5656, DOI 10.17487/RFC5656, December 2009.
[RFC6187] Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure Shell Authentication", RFC 6187, DOI 10.17487/RFC6187, March 2011.
[RFC6536] Bierman, A. and M. Bjorklund, "Network Configuration Protocol (NETCONF) Access Control Model", RFC 6536, DOI 10.17487/RFC6536, March 2012.
[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.
[RFC6991] Schoenwaelder, J., "Common YANG Data Types", RFC 6991, DOI 10.17487/RFC6991, July 2013.
[RFC7950] Bjorklund, M., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016.

8.2. Informative References

, "
[OPENSSH]OpenSSH", 2016.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688, DOI 10.17487/RFC3688, January 2004.
[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006.
[RFC4254] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Connection Protocol", RFC 4254, DOI 10.17487/RFC4254, January 2006.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J. and A. Bierman, "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011.
[RFC8040] Bierman, A., Bjorklund, M. and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017.
[RFC8071] Watsen, K., NETCONF Call Home and RESTCONF Call Home", RFC 8071, DOI 10.17487/RFC8071, February 2017.

Appendix A. Change Log

A.1. server-model-09 to 00

A.2. 00 to 01

A.3. 01 to 02

Appendix B. Open Issues

Please see: https://github.com/netconf-wg/ssh-client-server/issues.

Authors' Addresses

Kent Watsen Juniper Networks EMail: kwatsen@juniper.net
Gary Wu Cisco Systems EMail: garywu@cisco.com