NETCONF Working Group K. Watsen Internet-Draft Watsen Networks Intended status: Standards Track G. Wu Expires: April 20, 2020 Cisco Systems L. Xia Huawei October 18, 2019 YANG Groupings for SSH Clients and SSH Servers draft-ietf-netconf-ssh-client-server-15 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: o I-D.ietf-netconf-trust-anchors o I-D.ietf-netconf-keystore Artwork in this document contains shorthand references to drafts in progress. Please apply the following replacements: o "XXXX" --> the assigned RFC value for this draft o "YYYY" --> the assigned RFC value for I-D.ietf-netconf-trust- anchors o "ZZZZ" --> the assigned RFC value for I-D.ietf-netconf-keystore Artwork in this document contains placeholder values for the date of publication of this draft. Please apply the following replacement: Watsen, et al. Expires April 20, 2020 [Page 1] Internet-Draft Groupings for SSH Clients and Servers October 2019 o "2019-10-18" --> the publication date of this draft The following Appendix section is to be removed prior to publication: o 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 April 20, 2020. Copyright Notice Copyright (c) 2019 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. The SSH Client Model . . . . . . . . . . . . . . . . . . . . 4 3.1. Tree Diagram . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Example Usage . . . . . . . . . . . . . . . . . . . . . . 5 3.3. YANG Module . . . . . . . . . . . . . . . . . . . . . . . 8 4. The SSH Server Model . . . . . . . . . . . . . . . . . . . . 13 4.1. Tree Diagram . . . . . . . . . . . . . . . . . . . . . . 13 Watsen, et al. Expires April 20, 2020 [Page 2] Internet-Draft Groupings for SSH Clients and Servers October 2019 4.2. Example Usage . . . . . . . . . . . . . . . . . . . . . . 15 4.3. YANG Module . . . . . . . . . . . . . . . . . . . . . . . 18 5. The SSH Common Model . . . . . . . . . . . . . . . . . . . . 26 5.1. Tree Diagram . . . . . . . . . . . . . . . . . . . . . . 28 5.2. Example Usage . . . . . . . . . . . . . . . . . . . . . . 28 5.3. YANG Module . . . . . . . . . . . . . . . . . . . . . . . 29 6. Security Considerations . . . . . . . . . . . . . . . . . . . 39 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 40 7.1. The IETF XML Registry . . . . . . . . . . . . . . . . . . 40 7.2. The YANG Module Names Registry . . . . . . . . . . . . . 41 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 41 8.1. Normative References . . . . . . . . . . . . . . . . . . 41 8.2. Informative References . . . . . . . . . . . . . . . . . 43 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 45 A.1. 00 to 01 . . . . . . . . . . . . . . . . . . . . . . . . 45 A.2. 01 to 02 . . . . . . . . . . . . . . . . . . . . . . . . 45 A.3. 02 to 03 . . . . . . . . . . . . . . . . . . . . . . . . 45 A.4. 03 to 04 . . . . . . . . . . . . . . . . . . . . . . . . 45 A.5. 04 to 05 . . . . . . . . . . . . . . . . . . . . . . . . 46 A.6. 05 to 06 . . . . . . . . . . . . . . . . . . . . . . . . 46 A.7. 06 to 07 . . . . . . . . . . . . . . . . . . . . . . . . 46 A.8. 07 to 08 . . . . . . . . . . . . . . . . . . . . . . . . 46 A.9. 08 to 09 . . . . . . . . . . . . . . . . . . . . . . . . 46 A.10. 09 to 10 . . . . . . . . . . . . . . . . . . . . . . . . 47 A.11. 10 to 11 . . . . . . . . . . . . . . . . . . . . . . . . 47 A.12. 11 to 12 . . . . . . . . . . . . . . . . . . . . . . . . 47 A.13. 12 to 13 . . . . . . . . . . . . . . . . . . . . . . . . 47 A.14. 13 to 14 . . . . . . . . . . . . . . . . . . . . . . . . 47 A.15. 14 to 15 . . . . . . . . . . . . . . . . . . . . . . . . 48 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 48 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 48 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 Watsen, et al. Expires April 20, 2020 [Page 3] Internet-Draft Groupings for SSH Clients and Servers October 2019 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. 2. Terminology 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. 3. The SSH Client Model 3.1. Tree Diagram This section provides a tree diagram [RFC8340] for the "ietf-ssh- client" module that does not have groupings expanded. Watsen, et al. Expires April 20, 2020 [Page 4] Internet-Draft Groupings for SSH Clients and Servers October 2019 ========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) =========== module: ietf-ssh-client grouping ssh-client-grouping +-- client-identity | +-- username? string | +-- (auth-type) | +--:(password) | | +-- password? string | +--:(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-key-\ grouping +-- server-authentication | +-- ssh-host-keys! {ts:ssh-host-keys}? | | +---u ts:local-or-truststore-host-keys-grouping | +-- ca-certs! {sshcmn:ssh-x509-certs,ts:x509-certificates}? | | +---u ts:local-or-truststore-certs-grouping | +-- server-certs! | {sshcmn:ssh-x509-certs,ts:x509-certificates}? | +---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 3.2. Example Usage This section presents two examples showing the ssh-client-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 example configures the client identity using a local key: ========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) =========== foobar rsa2048 base64encodedvalue== base64encodedvalue== explicitly-trusted-ssh-host-keys algs:ssh-rsa algs:diffie-hellman-group-exchange-sha256 algs:aes256-ctr algs:aes192-ctr algs:aes128-ctr algs:aes256-cbc algs:aes192-cbc algs:aes128-cbc algs:hmac-sha2-256 algs:hmac-sha2-512 30 3 Watsen, et al. Expires April 20, 2020 [Page 6] Internet-Draft Groupings for SSH Clients and Servers October 2019 The following example configures the client identity using a key from the keystore: ========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) =========== foobar rsa-asymmetric-key explicitly-trusted-ssh-host-keys algs:ssh-rsa algs:diffie-hellman-group-exchange-sha256 algs:aes256-ctr algs:aes192-ctr algs:aes128-ctr algs:aes256-cbc algs:aes192-cbc algs:aes128-cbc algs:hmac-sha2-256 Watsen, et al. Expires April 20, 2020 [Page 7] Internet-Draft Groupings for SSH Clients and Servers October 2019 algs:hmac-sha2-512 30 3 3.3. YANG Module This YANG module has normative references to [I-D.ietf-netconf-trust-anchors], and [I-D.ietf-netconf-keystore]. file "ietf-ssh-client@2019-10-18.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 sshcmn; revision-date 2019-10-18; // stable grouping definitions reference "RFC XXXX: YANG Groupings for SSH Clients and SSH Servers"; } import ietf-truststore { prefix ts; reference "RFC YYYY: A YANG Data Model for a Truststore"; } import ietf-keystore { prefix ks; reference "RFC ZZZZ: A YANG Data Model for a Keystore"; } import ietf-netconf-acm { prefix nacm; reference "RFC 8341: Network Configuration Access Control Model"; } Watsen, et al. Expires April 20, 2020 [Page 8] Internet-Draft Groupings for SSH Clients and Servers October 2019 organization "IETF NETCONF (Network Configuration) Working Group"; contact "WG Web: WG List: Author: Kent Watsen Author: Gary Wu "; description "This module defines reusable groupings for SSH clients that can be used as a basis for specific SSH client instances. Copyright (c) 2019 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 XXXX (https://www.rfc-editor.org/info/rfcXXXX); 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 2019-10-18 { description "Initial version"; reference "RFC XXXX: 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."; } Watsen, et al. Expires April 20, 2020 [Page 9] Internet-Draft Groupings for SSH Clients and Servers October 2019 feature ssh-client-keepalives { description "Per socket SSH keepalive parameters are configurable for SSH clients on the server implementing this feature."; } // 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 { nacm:default-deny-write; 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 { mandatory true; description "The authentication type. What happens when more than one decendent is configured is undefined. FIXME."; leaf password { nacm:default-deny-all; type string; description "A password to be used for client authentication."; } container public-key { uses ks:local-or-keystore-asymmetric-key-grouping; description Watsen, et al. Expires April 20, 2020 [Page 10] Internet-Draft Groupings for SSH Clients and Servers October 2019 "A locally-defined or referenced asymmetric key pair to be used for client authentication."; reference "RFC ZZZZ: YANG Data Model for a Centralized Keystore Mechanism"; } container certificate { if-feature "sshcmn:ssh-x509-certs"; uses ks:local-or-keystore-end-entity-cert-with-key-grouping; description "A locally-defined or referenced certificate to be used for client authentication."; reference "RFC ZZZZ: YANG Data Model for a Centralized Keystore Mechanism"; } } } // container client-identity container server-authentication { nacm:default-deny-write; must 'ssh-host-keys or ca-certs or server-certs'; description "Trusted server identities."; container ssh-host-keys { if-feature "ts: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 YYYY: YANG Data Model for Global Trust Anchors"; uses ts:local-or-truststore-host-keys-grouping; } container ca-certs { if-feature "sshcmn:ssh-x509-certs"; if-feature "ts:x509-certificates"; presence "Indicates that the client can authenticate servers using the configured trust anchor certificates."; description "A list of certificate authority (CA) certificates used by the SSH client to authenticate SSH server certificates. Watsen, et al. Expires April 20, 2020 [Page 11] Internet-Draft Groupings for SSH Clients and Servers October 2019 A server certificate is authenticated if it has a valid chain of trust to a configured CA certificate."; reference "RFC YYYY: YANG Data Model for Global Trust Anchors"; uses ts:local-or-truststore-certs-grouping; } container server-certs { if-feature "sshcmn:ssh-x509-certs"; if-feature "ts:x509-certificates"; presence "Indicates that the client can authenticate servers using the configured server certificates."; description "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 server certificate."; reference "RFC YYYY: YANG Data Model for Global Trust Anchors"; 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 keepalives are enabled."; 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."; leaf max-wait { type uint16 { range "1..max"; } units "seconds"; default "30"; description "Sets the amount of time in seconds after which if Watsen, et al. Expires April 20, 2020 [Page 12] Internet-Draft Groupings for SSH Clients and Servers October 2019 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 "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 } 4. The SSH Server Model 4.1. Tree Diagram This section provides a tree diagram [RFC8340] for the "ietf-ssh- server" module that does not have groupings expanded. Watsen, et al. Expires April 20, 2020 [Page 13] Internet-Draft Groupings for SSH Clients and Servers October 2019 ========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) =========== module: ietf-ssh-server 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 | | +-- passsword? empty | | +-- hostbased? empty | | +-- none? empty | | +-- other* string | +-- (local-or-external) | +--:(local) {local-client-auth-supported}? | | +-- users | | +-- user* [name] | | +-- name? string | | +-- password? ianach:crypt-hash | | +-- host-keys! {ts:ssh-host-keys}? | | | +---u ts:local-or-truststore-host-keys-grouping | | +-- ca-certs! | | | {sshcmn:ssh-x509-certs,ts:x509-certificat\ es}? | | | +---u ts:local-or-truststore-certs-grouping | | +-- client-certs! | | {sshcmn:ssh-x509-certs,ts:x509-certificat\ es}? | | +---u ts:local-or-truststore-certs-grouping | +--:(external) {external-client-auth-supported}? | +-- client-auth-defined-elsewhere? empty +-- transport-params {ssh-server-transport-params-config}? | +---u sshcmn:transport-params-grouping +-- keepalives! {ssh-server-keepalives}? +-- max-wait? uint16 +-- max-attempts? uint8 Watsen, et al. Expires April 20, 2020 [Page 14] Internet-Draft Groupings for SSH Clients and Servers October 2019 4.2. Example Usage This section presents two examples showing the ssh-server-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 example configures the server identity using a local key: ========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) =========== deployment-specific-certificate rsa2048 base64encodedvalue== base64encodedvalue== mary $0$secret explicitly-trusted-ssh-host-keys explicitly-trusted-client-ca-certs explicitly-trusted-client-certs algs:ssh-rsa algs:diffie-hellman-group-exchange-sha256 algs:aes256-ctr algs:aes192-ctr algs:aes128-ctr algs:aes256-cbc algs:aes192-cbc algs:aes128-cbc algs:hmac-sha2-256 algs:hmac-sha2-512 The following example configures the server identity using a key from the keystore: ========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) =========== deployment-specific-certificate Watsen, et al. Expires April 20, 2020 [Page 16] Internet-Draft Groupings for SSH Clients and Servers October 2019 rsa-asymmetric-key mary $0$secret explicitly-trusted-ssh-host-keys explicitly-trusted-client-ca-certs explicitly-trusted-client-certs algs:ssh-rsa algs:diffie-hellman-group-exchange-sha256 algs:aes256-ctr algs:aes192-ctr algs:aes128-ctr algs:aes256-cbc algs:aes192-cbc algs:aes128-cbc Watsen, et al. Expires April 20, 2020 [Page 17] Internet-Draft Groupings for SSH Clients and Servers October 2019 algs:hmac-sha2-256 algs:hmac-sha2-512 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]. file "ietf-ssh-server@2019-10-18.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 sshcmn; revision-date 2019-10-18; // stable grouping definitions reference "RFC XXXX: YANG Groupings for SSH Clients and SSH Servers"; } import ietf-truststore { prefix ts; reference "RFC YYYY: A YANG Data Model for a Truststore"; } import ietf-keystore { prefix ks; reference "RFC ZZZZ: A YANG Data Model for a Keystore"; } import iana-crypt-hash { prefix ianach; reference "RFC 7317: A YANG Data Model for System Management"; } import ietf-netconf-acm { prefix nacm; Watsen, et al. Expires April 20, 2020 [Page 18] Internet-Draft Groupings for SSH Clients and Servers October 2019 reference "RFC 8341: Network Configuration Access Control Model"; } organization "IETF NETCONF (Network Configuration) Working Group"; contact "WG Web: WG List: Author: Kent Watsen Author: Gary Wu "; description "This module defines reusable groupings for SSH servers that can be used as a basis for specific SSH server instances. Copyright (c) 2019 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 XXXX (https://www.rfc-editor.org/info/rfcXXXX); 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 2019-10-18 { description "Initial version"; reference "RFC XXXX: YANG Groupings for SSH Clients and SSH Servers"; } // Features feature ssh-server-transport-params-config { Watsen, et al. Expires April 20, 2020 [Page 19] Internet-Draft Groupings for SSH Clients and Servers October 2019 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 local-client-auth-supported { description "Indicates that the SSH server supports local configuration of client credentials."; } feature external-client-auth-supported { description "Indicates that the SSH server supports external configuration of client credentials."; } // 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"; Watsen, et al. Expires April 20, 2020 [Page 20] Internet-Draft Groupings for SSH Clients and Servers October 2019 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"; container public-key { uses ks:local-or-keystore-asymmetric-key-grouping; description "A locally-defined or referenced asymmetric key pair to be used for the SSH server's host key."; reference "RFC ZZZZ: YANG Data Model for a Centralized Keystore Mechanism"; } container certificate { if-feature "sshcmn:ssh-x509-certs"; uses ks:local-or-keystore-end-entity-cert-with-key-grouping; description "A locally-defined or referenced end-entity certificate to be used for the SSH server's host key."; reference "RFC ZZZZ: YANG Data Model for a Centralized Keystore Mechanism"; } } } } // container server-identity container client-authentication { nacm:default-deny-write; description "Specifies if SSH client authentication is required or Watsen, et al. Expires April 20, 2020 [Page 21] Internet-Draft Groupings for SSH Clients and Servers October 2019 optional, and specifies if the SSH client authentication credentials are configured locally or externally."; 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 passsword { type empty; description "Indicates that the 'password' method is supported."; reference "RFC 4252: The Secure Shell (SSH) Authentication Protocol."; } leaf hostbased { type empty; description "Indicates that the 'hostbased' method is supported."; reference "RFC 4252: The Secure Shell (SSH) Authentication Protocol."; } leaf none { type empty; description "Indicates that the 'none' method is supported."; reference "RFC 4252: The Secure Shell (SSH) Authentication Protocol."; } leaf-list other { type string; description "Indicates a supported method name not defined by RFC 4253."; reference Watsen, et al. Expires April 20, 2020 [Page 22] Internet-Draft Groupings for SSH Clients and Servers October 2019 "RFC 4252: The Secure Shell (SSH) Authentication Protocol."; } } choice local-or-external { mandatory true; description "Indicates if the client credentials are configured locally or externally. Configuring certificates externally enables applications to place client authentication with client definitions, rather then in a data model principally concerned with configuring the transport."; case local { if-feature "local-client-auth-supported"; description "Client credentials are configured locally."; container users { 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."; } leaf password { type ianach:crypt-hash; description "The password for this entry."; } container host-keys { // FIXME: plural too much? if-feature "ts:ssh-host-keys"; presence "Indicates that the server can authenticate this user using the configured SSH host keys."; description "A list of public SSH host keys for this user. A host key is authenticated if it is an exact match to a configured host key."; reference Watsen, et al. Expires April 20, 2020 [Page 23] Internet-Draft Groupings for SSH Clients and Servers October 2019 "RFC 4253: The Secure Shell (SSH) Transport Layer"; uses ts:local-or-truststore-host-keys-grouping; } container ca-certs { // FIXME: plural too much? if-feature "sshcmn:ssh-x509-certs"; if-feature "ts:x509-certificates"; presence "Indicates that the server can authenticate this user using the configured trust anchor certificates."; description "A list of certificate authority (CA) certificates for this user. A client certificate is authenticated if it has a valid chain of trust to a configured CA certificate."; reference "RFC YYYY: YANG Data Model for Global Trust Anchors"; uses ts:local-or-truststore-certs-grouping; } container client-certs { // FIXME: plural too much? if-feature "sshcmn:ssh-x509-certs"; if-feature "ts:x509-certificates"; presence "Indicates that the server can authenticate this user using the configured client certificates."; description "A list of client certificates for this user. A client certificate is authenticated if it is an exact match to a configured client certificate."; reference "RFC YYYY: YANG Data Model for Global Trust Anchors"; uses ts:local-or-truststore-certs-grouping; } } // list user } // container users } // case local case external { if-feature "external-client-auth-supported"; description "Client credentials are configured externally, such as via RADIUS, RFC 7317, or another mechanism."; leaf client-auth-defined-elsewhere { type empty; description "Indicates that client credentials are configured elsewhere."; Watsen, et al. Expires April 20, 2020 [Page 24] Internet-Draft Groupings for SSH Clients and Servers October 2019 } } } // choice local-or-external } // 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 keepalives are enabled."; 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."; 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."; } } // container keepalives } // grouping server-identity-grouping } Watsen, et al. Expires April 20, 2020 [Page 25] Internet-Draft Groupings for SSH Clients and Servers October 2019 5. 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 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. [I-D.ietf-netconf-crypto-types] defines six categories of cryptographic algorithms (hash-algorithm, symmetric-key-encryption- algorithm, mac-algorithm, asymmetric-key-encryption-algorithm, signature-algorithm, key-negotiation-algorithm) and lists several widely accepted algorithms for each of them. The SSH client and server models use one or more of these algorithms. The SSH common model includes four parameters for configuring its permitted SSH algorithms, which are: host-key-alg, key-exchange-alg, encryption-alg and mac-alg. The following tables are provided, in part, to define the subset of algorithms defined in the crypto-types model used by SSH and, in part, to ensure compatibility of configured SSH cryptographic parameters for configuring its permitted SSH algorithms ("sshcmn" representing SSH common model, and "ct" representing crypto-types model which the SSH client/server model is based on): +-------------------------------+-------------------------------+ | sshcmn:host-key-alg | ct:signature-algorithm | +-------------------------------+-------------------------------+ | dsa-sha1 | dsa-sha1 | | rsa-pkcs1-sha1 | rsa-pkcs1-sha1 | | rsa-pkcs1-sha256 | rsa-pkcs1-sha256 | | rsa-pkcs1-sha512 | rsa-pkcs1-sha512 | | ecdsa-secp256r1-sha256 | ecdsa-secp256r1-sha256 | | ecdsa-secp384r1-sha384 | ecdsa-secp384r1-sha384 | | ecdsa-secp521r1-sha512 | ecdsa-secp521r1-sha512 | | x509v3-rsa-pkcs1-sha1 | x509v3-rsa-pkcs1-sha1 | | x509v3-rsa2048-pkcs1-sha256 | x509v3-rsa2048-pkcs1-sha1 | | x509v3-ecdsa-secp256r1-sha256 | x509v3-ecdsa-secp256r1-sha256 | | x509v3-ecdsa-secp384r1-sha384 | x509v3-ecdsa-secp384r1-sha384 | | x509v3-ecdsa-secp521r1-sha512 | x509v3-ecdsa-secp521r1-sha512 | +-------------------------------+-------------------------------+ Table 1 The SSH Host-key-alg Compatibility Matrix Watsen, et al. Expires April 20, 2020 [Page 26] Internet-Draft Groupings for SSH Clients and Servers October 2019 +-------------------------------+-------------------------------+ | sshcmn:key-exchange-alg | ct:key-negotiation-algorithm | +-------------------------------+-------------------------------+ | diffie-hellman-group14-sha1 | diffie-hellman-group14-sha1 | | diffie-hellman-group14-sha256 | diffie-hellman-group14-sha256 | | diffie-hellman-group15-sha512 | diffie-hellman-group15-sha512 | | diffie-hellman-group16-sha512 | diffie-hellman-group16-sha512 | | diffie-hellman-group17-sha512 | diffie-hellman-group17-sha512 | | diffie-hellman-group18-sha512 | diffie-hellman-group18-sha512 | | ecdh-sha2-secp256r1 | ecdh-sha2-secp256r1 | | ecdh-sha2-secp384r1 | ecdh-sha2-secp384r1 | +-------------------------------+-------------------------------+ Table 2 The SSH Key-exchange-alg Compatibility Matrix +-----------------------+---------------------------------------+ | sshcmn:encryption-alg | ct:symmetric-key-encryption-algorithm | +-----------------------+---------------------------------------+ | aes-128-cbc | aes-128-cbc | | aes-192-cbc | aes-192-cbc | | aes-256-cbc | aes-256-cbc | | aes-128-ctr | aes-128-ctr | | aes-192-ctr | aes-192-ctr | | aes-256-ctr | aes-256-ctr | +-----------------------+---------------------------------------+ Table 3 The SSH Encryption-alg Compatibility Matrix +----------------+-------------------+ | sshcmn:mac-alg | ct:mac-algorithm | +----------------+-------------------+ | hmac-sha1 | hmac-sha1 | | hmac-sha1-96 | hmac-sha1-96 | | hmac-sha2-256 | hmac-sha2-256 | | hmac-sha2-512 | hmac-sha2-512 | +----------------+-------------------+ Table 4 The SSH Mac-alg Compatibility Matrix As is seen in the tables above, the names of the "sshcmn" algorithms are all identical to the names of algorithms defined in [I-D.ietf-netconf-crypto-types]. While appearing to be redundant, it is important to realize that not all the algorithms defined in [I-D.ietf-netconf-crypto-types] are supported by SSH. That is, the algorithms supported by SSH are a subset of the algorithms defined in [I-D.ietf-netconf-crypto-types]. The algorithms used by SSH are redefined in this document in order to constrain the algorithms that may be selected to just the ones used by SSH. Watsen, et al. Expires April 20, 2020 [Page 27] Internet-Draft Groupings for SSH Clients and Servers October 2019 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. 5.1. Tree Diagram The following tree diagram [RFC8340] provides an overview of the data model for the "ietf-ssh-common" module. module: ietf-ssh-common grouping transport-params-grouping +-- host-key | +-- host-key-alg* identityref +-- key-exchange | +-- key-exchange-alg* identityref +-- encryption | +-- encryption-alg* identityref +-- mac +-- mac-alg* identityref 5.2. Example Usage This following example illustrates how the transport-params-grouping appears when populated with some data. Watsen, et al. Expires April 20, 2020 [Page 28] Internet-Draft Groupings for SSH Clients and Servers October 2019 algs:x509v3-rsa2048-sha256 algs:ssh-rsa algs:diffie-hellman-group-exchange-sha256 algs:aes256-ctr algs:aes192-ctr algs:aes128-ctr algs:aes256-cbc algs:aes192-cbc algs:aes128-cbc algs:hmac-sha2-256 algs:hmac-sha2-512 5.3. YANG Module This YANG module has normative references to [RFC4253], [RFC4344], [RFC4419], [RFC5656], [RFC6187], and [RFC6668]. file "ietf-ssh-common@2019-10-18.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: WG List: Author: Kent Watsen Author: Gary Wu "; description Watsen, et al. Expires April 20, 2020 [Page 29] Internet-Draft Groupings for SSH Clients and Servers October 2019 "This module defines a common features, identities, and groupings for Secure Shell (SSH). Copyright (c) 2019 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 XXXX (https://www.rfc-editor.org/info/rfcXXXX); 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 2019-10-18 { description "Initial version"; reference "RFC XXXX: 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 "X.509v3 certificates are supported for SSH per RFC 6187."; reference "RFC 6187: X.509v3 Certificates for Secure Shell Authentication"; } Watsen, et al. Expires April 20, 2020 [Page 30] Internet-Draft Groupings for SSH Clients and Servers October 2019 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 { base public-key-alg-base; description "RSASSA-PKCS1-v1_5 signature scheme using SHA-1 as the hashing algorithm."; reference "RFC 4253: Watsen, et al. Expires April 20, 2020 [Page 31] Internet-Draft Groupings for SSH Clients and Servers October 2019 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"; } Watsen, et al. Expires April 20, 2020 [Page 32] Internet-Draft Groupings for SSH Clients and Servers October 2019 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 Watsen, et al. Expires April 20, 2020 [Page 33] Internet-Draft Groupings for SSH Clients and Servers October 2019 "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"; } Watsen, et al. Expires April 20, 2020 [Page 34] Internet-Draft Groupings for SSH Clients and Servers October 2019 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 Watsen, et al. Expires April 20, 2020 [Page 35] Internet-Draft Groupings for SSH Clients and Servers October 2019 "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 { Watsen, et al. Expires April 20, 2020 [Page 36] Internet-Draft Groupings for SSH Clients and Servers October 2019 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"; } // 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; } 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 XXXX for valid combinations. Watsen, et al. Expires April 20, 2020 [Page 37] Internet-Draft Groupings for SSH Clients and Servers October 2019 If this leaf-list is not configured (has zero elements) the acceptable host key algorithms are implementation- defined."; reference "RFC XXXX: 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."; } } container mac { description "Parameters regarding message authentication code (MAC)."; leaf-list mac-alg { type identityref { base mac-alg-base; } Watsen, et al. Expires April 20, 2020 [Page 38] Internet-Draft Groupings for SSH Clients and Servers October 2019 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."; } } } } 6. Security Considerations The YANG modules defined in this document 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 modules in this document only define groupings, these considerations are primarily for the designers of other modules that use these groupings. There are a number of data nodes defined in the YANG modules 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: *: All of the nodes defined by the grouping statement in both the "ietf-ssh-client" and "ietf-ssh-server" modules are sensitive to write operations. 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 implemented security policy. For this reason, all the nodes are protected the NACM extension "default-deny-write". Watsen, et al. Expires April 20, 2020 [Page 39] Internet-Draft Groupings for SSH Clients and Servers October 2019 Some of the readable data nodes in the YANG modules 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: ssh-client-grouping/client-identity/: This subtree in the "ietf- ssh-client" module contains nodes that are additionally sensitive to read operations such that, in normal use cases, they should never be returned to a client. Specifically, the descendent nodes 'password', 'public-key/local-definition/ private-key' and 'certificate/local-definition/private-key'. For this reason, all of these node are protected by the NACM extension "default-deny-all". ssh-server-grouping/server-identity/: This subtree in the "ietf- ssh-server" module contains nodes that are additionally sensitive to read operations such that, in normal use cases, they should never be returned to a client. Specifically, the descendent nodes 'host-key/public-key/local-definition/private- key' and 'host-key/certificate/local-definition/private-key'. For this reason, both of these node are protected by the NACM extension "default-deny-all". Some of the 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: *: The groupings defined in this document include "action" statements that come from groupings defined in [I-D.ietf-netconf-crypto-types]. Please consult that document for the security considerations of the "action" statements defined by the "grouping" statements defined in this document. 7. IANA Considerations 7.1. 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: Watsen, et al. Expires April 20, 2020 [Page 40] Internet-Draft Groupings for SSH Clients and Servers October 2019 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. 7.2. 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-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: sshcmn reference: RFC XXXX 8. References 8.1. Normative References [I-D.ietf-netconf-crypto-types] Watsen, K. and H. Wang, "Common YANG Data Types for Cryptography", draft-ietf-netconf-crypto-types-10 (work in progress), July 2019. [I-D.ietf-netconf-keystore] Watsen, K., "A YANG Data Model for a Keystore", draft- ietf-netconf-keystore-12 (work in progress), July 2019. Watsen, et al. Expires April 20, 2020 [Page 41] Internet-Draft Groupings for SSH Clients and Servers October 2019 [I-D.ietf-netconf-trust-anchors] Watsen, K., "A YANG Data Model for a Truststore", draft- ietf-netconf-trust-anchors-05 (work in progress), June 2019. [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, . [RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010, . [RFC6187] Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure Shell Authentication", RFC 6187, DOI 10.17487/RFC6187, March 2011, . [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, . [RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Watsen, et al. Expires April 20, 2020 [Page 42] Internet-Draft Groupings for SSH Clients and Servers October 2019 [RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration Access Control Model", STD 91, RFC 8341, DOI 10.17487/RFC8341, March 2018, . 8.2. Informative References [OPENSSH] Project, T. O., "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, Ed., "The Secure Shell (SSH) Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252, January 2006, . [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006, . [RFC4254] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Connection Protocol", RFC 4254, DOI 10.17487/RFC4254, January 2006, . [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, . [RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011, . [RFC7317] Bierman, A. and M. Bjorklund, "A YANG Data Model for System Management", RFC 7317, DOI 10.17487/RFC7317, August 2014, . [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, . Watsen, et al. Expires April 20, 2020 [Page 43] Internet-Draft Groupings for SSH Clients and Servers October 2019 [RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams", BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018, . Watsen, et al. Expires April 20, 2020 [Page 44] Internet-Draft Groupings for SSH Clients and Servers October 2019 Appendix A. Change Log A.1. 00 to 01 o Noted that '0.0.0.0' and '::' might have special meanings. o Renamed "keychain" to "keystore". A.2. 01 to 02 o Removed the groupings 'listening-ssh-client-grouping' and 'listening-ssh-server-grouping'. Now modules only contain the transport-independent groupings. o Simplified the "client-auth" part in the ietf-ssh-client module. It now inlines what it used to point to keystore for. o Added cipher suites for various algorithms into new 'ietf-ssh- common' module. A.3. 02 to 03 o Removed 'RESTRICTED' enum from 'password' leaf type. o Added a 'must' statement to container 'server-auth' asserting that at least one of the various auth mechanisms must be specified. o Fixed description statement for leaf 'trusted-ca-certs'. A.4. 03 to 04 o Change title to "YANG Groupings for SSH Clients and SSH Servers" o Added reference to RFC 6668 o Added RFC 8174 to Requirements Language Section. o Enhanced description statement for ietf-ssh-server's "trusted-ca- certs" leaf. o Added mandatory true to ietf-ssh-client's "client-auth" 'choice' statement. o Changed the YANG prefix for module ietf-ssh-common from 'sshcom' to 'sshcmn'. o Removed the compression algorithms as they are not commonly configurable in vendors' implementations. Watsen, et al. Expires April 20, 2020 [Page 45] Internet-Draft Groupings for SSH Clients and Servers October 2019 o Updating descriptions in transport-params-grouping and the servers's usage of it. o Now tree diagrams reference ietf-netmod-yang-tree-diagrams o Updated YANG to use typedefs around leafrefs to common keystore paths o Now inlines key and certificates (no longer a leafref to keystore) A.5. 04 to 05 o Merged changes from co-author. A.6. 05 to 06 o Updated to use trust anchors from trust-anchors draft (was keystore draft) o Now uses new keystore grouping enabling asymmetric key to be either locally defined or a reference to the keystore. A.7. 06 to 07 o factored the ssh-[client|server]-groupings into more reusable groupings. o 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 o Added a number of compatibility matrices to Section 5 (thanks Frank!) o Clarified that any configured "host-key-alg" values need to be compatible with the configured private key. A.9. 08 to 09 o Updated examples to reflect update to groupings defined in the keystore -09 draft. o Add SSH keepalives features and groupings. o Prefixed top-level SSH grouping nodes with 'ssh-' and support mashups. Watsen, et al. Expires April 20, 2020 [Page 46] Internet-Draft Groupings for SSH Clients and Servers October 2019 o Updated copyright date, boilerplate template, affiliation, and folding algorithm. A.10. 09 to 10 o Reformatted the YANG modules. A.11. 10 to 11 o Reformatted lines causing folding to occur. A.12. 11 to 12 o Collapsed all the inner groupings into the top-level grouping. o Added a top-level "demux container" inside the top-level grouping. o Added NACM statements and updated the Security Considerations section. o 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. o Updated the boilerplate text in module-level "description" statement to match copyeditor convention. A.13. 12 to 13 o 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. o Fixed a couple references (section 2 --> section 3) o In the server model, replaced with and introduced 'local-or-external' choice. A.14. 13 to 14 o Updated to reflect changes in trust-anchors drafts (e.g., s/trust- anchors/truststore/g + s/pinned.//) Watsen, et al. Expires April 20, 2020 [Page 47] Internet-Draft Groupings for SSH Clients and Servers October 2019 A.15. 14 to 15 o Updated examples to reflect ietf-crypto-types change (e.g., identities --> enumerations) o 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". 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, David Lamparter, Alan Luchuk, Ladislav Lhotka, Radek Krejci, Tom Petch, Juergen Schoenwaelder, Phil Shafer, Sean Turner, Michal Vasko, and Bert Wijnen. Authors' Addresses Kent Watsen Watsen Networks EMail: kent+ietf@watsen.net Gary Wu Cisco Systems EMail: garywu@cisco.com Liang Xia Huawei EMail: frank.xialiang@huawei.com Watsen, et al. Expires April 20, 2020 [Page 48]