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";
  }

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

  import ietf-ssh-common {
    prefix sshcmn;
    reference
      "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
  }

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

  contact
    "WG Web:   https://datatracker.ietf.org/wg/netconf
     WG List:  NETCONF WG list <mailto:netconf@ietf.org>
     Author:   Kent Watsen <mailto:kent+ietf@watsen.net>";

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

     Copyright (c) 2023 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 Revised
     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 2023-12-28 {
    description
      "Initial version";
    reference
      "RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
  }

  // Features

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

  feature local-users-supported {
    description
      "Indicates that the configuration for users can be
       configured herein, as opposed to in an application
       specific location.";
  }

  feature local-user-auth-publickey {
    if-feature "local-users-supported";
    description
      "Indicates that the 'publickey' authentication type,
       per RFC 4252, is supported for locally-defined users.
       The 'publickey' authentication type is required by
       RFC 4252, but common implementations allow it to
       be disabled.";
    reference
      "RFC 4252:
        The Secure Shell (SSH) Authentication Protocol";
  }

  feature local-user-auth-password {
    if-feature "local-users-supported";
    description
      "Indicates that the 'password' authentication type,
       per RFC 4252, is supported for locally-defined users.";
    reference
      "RFC 4252:
        The Secure Shell (SSH) Authentication Protocol";
  }

  feature local-user-auth-hostbased {
    if-feature "local-users-supported";
    description
      "Indicates that the 'hostbased' authentication type,
       per RFC 4252, is supported for locally-defined users.";
    reference
      "RFC 4252:
        The Secure Shell (SSH) Authentication Protocol";
  }

  feature local-user-auth-none {
    if-feature "local-users-supported";
    description
      "Indicates that the 'none' authentication type, per
       RFC 4252, is supported.  It is NOT RECOMMENDED to
       enable this feature.";
    reference
      "RFC 4252:
        The Secure Shell (SSH) Authentication Protocol";
  }

  // 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 descendant
       node names such that a nesting 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 (see RFC 4251) 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 4251: The Secure Shell (SSH) Protocol Architecture
           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";
          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:inline-or-keystore-asymmetric-key-grouping {
              refine "inline-or-keystore/inline/inline-definition" {
                must 'not(public-key-format) or derived-from-or-self'
                 + '(public-key-format, "ct:ssh-public-key-format")';

              }
              refine "inline-or-keystore/central-keystore/"
                   + "central-keystore-reference" {
                must 'not(deref(.)/../ks:public-key-format) or '
                   + 'derived-from-or-self(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:inline-or-keystore-end-entity-cert-with-key-grouping{
              refine "inline-or-keystore/inline/inline-definition" {
                must 'not(public-key-format) or derived-from-or-self'
                   + '(public-key-format, "ct:subject-public-key-'
                   + 'info-format")';
              }
              refine "inline-or-keystore/central-keystore/"
                   + "central-keystore-reference/asymmetric-key" {
                must 'not(deref(.)/../ks:public-key-format) or '
                   + 'derived-from-or-self(deref(.)/../ks:public-key'
                   + '-format, "ct:subject-public-key-info-format")';
              }
            }
          }
        }
      }
    } // container server-identity

    container client-authentication {
      nacm:default-deny-write;
      description
        "Specifies how the SSH server can be configured to
         authenticate SSH clients.  See RFC 4252 for a general
         discussion about SSH authentication.";
      reference
        "RFC 4252: The Secure Shell (SSH) Transport Layer";
      container users {
        if-feature "local-users-supported";
        description
          "A list of locally configured users.";
        list user {
          key "name";
          description
            "A locally configured user.

             The server SHOULD derive the list of authentication
             'method names' returned to the SSH client from the
             descendant nodes configured herein, per Sections
             5.1 and 5.2 in RFC 4252.

             The authentication methods are unordered.  Clients
             must authenticate to all configured methods.
             Whenever a choice amongst methods arises,
             implementations SHOULD use a default ordering
             that prioritizes automation over human-interaction.";
          leaf name {
            type string;
            description
              "The 'user name' for the SSH client, as defined in
               the SSH_MSG_USERAUTH_REQUEST message in RFC 4253.";
            reference
              "RFC 4253: The Secure Shell (SSH) Transport Layer
                         Protocol";
          }
          container public-keys {
            if-feature "local-user-auth-publickey";
            presence
              "Indicates that public keys have been configured.
               This statement is present so the mandatory descendant
               nodes do not imply that this node must be
               configured.";
            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:inline-or-truststore-public-keys-grouping {
              refine "inline-or-truststore/inline/inline-definition/"
                   + "public-key" {
                must 'derived-from-or-self(public-key-format,'
                   + ' "ct:ssh-public-key-format")';
              }
              refine "inline-or-truststore/central-truststore/"
                   + "central-truststore-reference" {
                must 'not(deref(.)/../ts:public-key/ts:public-key-'
                   + 'format[not(derived-from-or-self(., "ct:ssh-'
                   + 'public-key-format"))])';
              }
            }
          }
          leaf password {
            if-feature "local-user-auth-password";
            type ianach:crypt-hash;
            description
              "The password for this user.";
          }
          container hostbased {
            if-feature "local-user-auth-hostbased";
            presence
              "Indicates that hostbased [RFC4252] keys have been
               configured.  This statement is present so the
               mandatory descendant nodes do not imply that this
               node must be configured.";
            description
              "A set of SSH host keys 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 4252: The Secure Shell (SSH) Transport Layer
               RFC BBBB: A YANG Data Model for a Truststore";
            uses ts:inline-or-truststore-public-keys-grouping {
              refine "inline-or-truststore/inline/inline-definition/"
                   + "public-key" {
                must 'derived-from-or-self(public-key-format,'
                   + ' "ct:ssh-public-key-format")';
              }
              refine "inline-or-truststore/central-truststore/"
                   + "central-truststore-reference" {
                must 'not(deref(.)/../ts:public-key/ts:public-key-'
                   + 'format[not(derived-from-or-self(., "ct:ssh-'
                   + 'public-key-format"))])';
              }
            }
          }
          leaf none {
            if-feature "local-user-auth-none";
            type empty;
            description
              "Indicates that the 'none' method is configured
               for this user.";
            reference
              "RFC 4252: The Secure Shell (SSH) Authentication
                         Protocol.";
          }
        }
      } // users
      container ca-certs {
        if-feature "sshcmn:ssh-x509-certs";
        presence
          "Indicates that CA certificates have been configured.
           This statement is present so the mandatory descendant
           nodes do not imply this node must be configured.";
        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:inline-or-truststore-certs-grouping;
      }
      container ee-certs {
        if-feature "sshcmn:ssh-x509-certs";
        presence
          "Indicates that EE certificates have been configured.
           This statement is present so the mandatory descendant
           nodes do not imply this node must be configured.";
        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:inline-or-truststore-certs-grouping;
      }
    } // container client-authentication

    container transport-params {
      nacm:default-deny-write;
      if-feature "sshcmn:transport-params";
      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 SSH client.  An unresponsive SSH
         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
         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 client,
           a SSH-level message will be sent to test the
           aliveness of the SSH 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 SSH client before assuming the SSH client is
           no longer alive.";
      }
    }
  } // grouping ssh-server-grouping

}