YANG Groupings for SSH Clients and SSH Servers

YANG Groupings for SSH Clients and SSH Servers Watsen Networks

kent+ietf@watsen.net

OPS netconf This document presents three IETF-defined YANG modules and a script used to create four supporting IANA modules. The three IETF modules are ietf-ssh-common, ietf-ssh-client, and ietf-ssh-server. The "ietf-ssh-client" and "ietf-ssh-server" modules are the primary productions of this work, supporting the configuration and monitoring of Secure Shell (SSH) clients and servers. The four IANA modules are iana-ssh-encryption-algs, iana-ssh-key-exchange-algs, iana-ssh-mac-algs, and iana-ssh-public-key-algs. These modules each define YANG enumerations providing support for an IANA-maintained algorithm registry.

Introduction This document presents three IETF-defined YANG modules and a script used to create four supporting IANA modules. The three IETF modules are ietf-ssh-common (), ietf-ssh-client (), and ietf-ssh-server (). The "ietf-ssh-client" and "ietf-ssh-server" modules are the primary productions of this work, supporting the configuration and monitoring of SSH clients and servers. The groupings defined in this document are expected to be used in conjunction with the groupings defined in an underlying transport-level module, such as the groupings defined in . The transport-level data model enables the configuration of transport-level values, such as a remote address, a remote port, a local address, and a local port. The four IANA modules are: iana-ssh-encryption-algs, iana-ssh-key-exchange-algs, iana-ssh-mac-algs, and iana-ssh-public-key-algs. These modules each define YANG enumerations providing support for an IANA-maintained algorithm registry. This document assumes that the four IANA modules exist and presents a script in that IANA may use to generate those YANG modules. This document does not publish the initial versions of these four modules. IANA publishes these modules.

Regarding the Three IETF Modules The three IETF modules define features and groupings to model "generic" SSH clients and SSH servers, where "generic" should be interpreted as "least common denominator" rather than "complete." Support for the basic SSH protocol is afforded by these modules, leaving configuration of advanced features (e.g., multiple channels) to augmentations made by consuming modules. It is intended that the YANG groupings will be used by applications needing to configure SSH client and server protocol stacks. For instance, these groupings are used to help define the data models in , for clients and servers using the Network Configuration Protocol (NETCONF) over SSH . The "ietf-ssh-client" and "ietf-ssh-server" YANG modules each define one grouping, which is focused on just SSH-specific configuration, and specifically avoid any transport-level configuration, such as what ports to listen on or connect to. This affords applications the opportunity to define their own strategy for how the underlying TCP connection is established. For instance, applications supporting NETCONF Call Home 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 optionally support , which describes enabling host keys and public keys based on X.509v3 certificates.

Relation to Other RFCs This document presents three YANG modules that are part of a collection of RFCs that work together to ultimately support the configuration of both the clients and servers of both the NETCONF and RESTCONF protocols. The dependency relationship between the primary YANG groupings defined in the various RFCs is presented in the below diagram. In some cases, a document may define secondary groupings that introduce dependencies not illustrated in the diagram. The labels in the diagram are shorthand names for the defining RFCs. The citation references for shorthand names are provided below the diagram. Please note that the arrows in the diagram point from referencer to referenced. For example, the "crypto-types" RFC does not have any dependencies, whilst the "keystore" RFC depends on the "crypto-types" RFC. Label in Diagram to RFC Mapping

Label in Diagram	Reference
crypto-types
truststore
keystore
tcp-client-server
ssh-client-server	RFC9644
tls-client-server
http-client-server
netconf-client-server
restconf-client-server

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

Adherence to the NMDA This document is compliant with the Network Management Datastore Architecture (NMDA) . For instance, as described in and , trust anchors and keys installed during manufacturing are expected to appear in <operational> () and <system> if implemented.

Conventions Various examples in this document use "BASE64VALUE=" as a placeholder value for binary data that has been base64 encoded (per ). This placeholder value is used because real base64-encoded structures are often many lines long and hence distracting to the example being presented. Various examples in this document use the XML encoding. Other encodings, such as JSON , could alternatively be used. Various examples in this document contain long lines that may be folded, as described in .

The "ietf-ssh-common" Module The SSH common model presented in this section is common to both SSH clients and SSH servers. The "transport-params-grouping" grouping can be used to configure the list of SSH transport algorithms permitted by the SSH client or SSH server. The lists of permitted algorithms are in decreasing order of usage preference. The algorithm that appears first in the client list that also appears in the server list is the one that 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.

Data Model Overview This section provides an overview of the "ietf-ssh-common" module in terms of its features, identities, groupings, and protocol-accessible nodes.

Features The following diagram lists all the "feature" statements defined in the "ietf-ssh-common" module: The diagram above uses syntax that is similar to but not defined in . Please refer to the YANG module for a description of each feature.

Groupings The "ietf-ssh-common" module defines the following "grouping" statement:

transport-params-grouping

This grouping is presented in the following subsection.

The "transport-params-grouping" Grouping The following tree diagram illustrates the "transport-params-grouping" grouping: Comments:

This grouping is used by both the "ssh-client-grouping" and the "ssh-server-grouping" groupings defined in Sections and , respectively.
This grouping enables client and server configurations to specify the algorithms that are to be used when establishing SSH sessions.
Each list is "ordered-by user".

Protocol-Accessible Nodes The following tree diagram lists all the protocol-accessible nodes defined in the "ietf-ssh-common" module without expanding the "grouping" statements: Comments:

Protocol-accessible nodes are those nodes that are accessible when the module is "implemented", as described in .
The protocol-accessible nodes for the "ietf-ssh-common" module are limited to the "supported-algorithms" container, which is constrained by the "algorithm-discovery" feature, and the "generate-asymmetric-key-pair" RPC, which is constrained by the "asymmetric-key-pair-generation" feature.
The "encrypted-by-grouping" grouping is discussed in .
The "asymmetric-key-pair-grouping" grouping is discussed in .

Example Usage The following example illustrates the "transport-params-grouping' grouping when populated with some data.

x509v3-rsa2048-sha256

ssh-rsa

ssh-rsa@openssh.com

diffie-hellman-group-exchange-sha256

aes256-ctr

aes192-ctr

aes128-ctr

aes256-gcm@openssh.com

hmac-sha2-256

hmac-sha2-512

]]> The following example illustrates operational state data indicating the SSH algorithms supported by the server.

aes256-ctr

arcfour256

serpent256-ctr

AEAD_AES_128_GCM

AEAD_AES_256_GCM

aes256-gcm@openssh.com

ecdh-sha2-nistp256

rsa2048-sha256

gss-group14-sha1-nistp256 gss-gex-sha1-nistp256

gss-group14-sha256-1.2.840.10045.3.1.1 curve25519-sha256

hmac-sha2-256

hmac-sha2-512

AEAD_AES_256_GCM

rsa-sha2-256

rsa-sha2-512

spki-sign-rsa

pgp-sign-dss

x509v3-rsa2048-sha256

ecdsa-sha2-nistp256

ecdsa-sha2-1.3.132.0.37 ssh-ed25519

ssh-rsa@openssh.com

]]> The following example illustrates the "generate-asymmetric-key-pair" RPC. REQUEST ecdsa-sha2-nistp256

521

hidden-asymmetric-key

]]> RESPONSE

ct:subject-public-key-info-format BASE64VALUE=

ct:ec-private-key-format BASE64VALUE= ]]>

YANG Module This YANG module has normative references to , , , and . Author: Kent Watsen Author: Gary Wu "; description "This module defines common features and groupings for Secure Shell (SSH). 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. Copyright (c) 2024 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 9644 (https://www.rfc-editor.org/info/rfc9644); see the RFC itself for full legal notices."; revision 2024-10-10 { description "Initial version."; reference "RFC 9644: YANG Groupings for SSH Clients and SSH Servers"; } // Features feature ssh-x509-certs { description "X.509v3 certificates are supported for SSH."; reference "RFC 6187: X.509v3 Certificates for Secure Shell Authentication"; } feature transport-params { description "SSH transport layer parameters are configurable."; } feature asymmetric-key-pair-generation { description "Indicates that the server implements the 'generate-asymmetric-key-pair' RPC."; } feature algorithm-discovery { description "Indicates that the server implements the 'supported-algorithms' container."; } // Typedefs typedef ssh-public-key-algorithm { type union { type sshpka:ssh-public-key-algorithm; type string { length "1..64" { description "Non-IANA-maintained algorithms must include the at sign (@) in them, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers"; } pattern '.*@.*' { description "Non-IANA-maintained algorithms must include the at sign (@) in them, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers"; } } } description "A type that enables the public key algorithm to be either an IANA-maintained public key algorithm in the 'iana-ssh-public-key-algs' YANG module (RFC 9644) or a locally defined algorithm, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers RFC 9644: YANG Groupings for SSH Clients and SSH Servers"; } typedef ssh-key-exchange-algorithm { type union { type sshkea:ssh-key-exchange-algorithm; type string { length "1..64" { description "Non-IANA-maintained algorithms must include the at sign (@) in them, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers"; } pattern '.*@.*' { description "Non-IANA-maintained algorithms must include the at sign (@) in them, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers"; } } } description "A type that enables the key exchange algorithm to be either an IANA-maintained key exchange algorithm in the 'iana-ssh-key-exchange-algs' YANG module (RFC 9644) or a locally defined algorithm, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers RFC 9644: YANG Groupings for SSH Clients and SSH Servers"; } typedef ssh-encryption-algorithm { type union { type sshea:ssh-encryption-algorithm; type string { length "1..64" { description "Non-IANA-maintained algorithms must include the at sign (@) in them, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers"; } pattern '.*@.*' { description "Non-IANA-maintained algorithms must include the at sign (@) in them, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers"; } } } description "A type that enables the encryption algorithm to be either an IANA-maintained encryption algorithm in the 'iana-ssh-encryption-algs' YANG module (RFC 9644) or a locally defined algorithm, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers RFC 9644: YANG Groupings for SSH Clients and SSH Servers"; } typedef ssh-mac-algorithm { type union { type sshma:ssh-mac-algorithm; type string { length "1..64" { description "Non-IANA-maintained algorithms must include the at sign (@) in them, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers"; } pattern '.*@.*' { description "Non-IANA-maintained algorithms must include the at sign (@) in them, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers"; } } } description "A type that enables the message authentication code (MAC) algorithm to be either an IANA-maintained MAC algorithm in the 'iana-ssh-mac-algs' YANG module (RFC 9644) or a locally defined algorithm, per Section 4.6.1 of RFC 4250."; reference "RFC 4250: The Secure Shell (SSH) Protocol Assigned Numbers RFC 9644: YANG Groupings for SSH Clients and SSH Servers"; } // 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 ssh-public-key-algorithm; ordered-by user; description "Acceptable host key algorithms in order of decreasing preference. If this leaf-list is not configured (has zero elements), the acceptable host key algorithms are implementation-defined."; reference "RFC 9644: YANG Groupings for SSH Clients and SSH Servers"; } } container key-exchange { description "Parameters regarding key exchange."; leaf-list key-exchange-alg { type ssh-key-exchange-algorithm; ordered-by user; description "Acceptable key exchange algorithms in order of decreasing 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 ssh-encryption-algorithm; ordered-by user; description "Acceptable encryption algorithms in order of decreasing 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 ssh-mac-algorithm; ordered-by user; description "Acceptable MAC algorithms in order of decreasing preference. If this leaf-list is not configured (has zero elements), the acceptable MAC algorithms are implementation-defined."; } } } // Protocol-accessible Nodes container supported-algorithms { if-feature "algorithm-discovery"; config false; description "Identifies all of the supported algorithms."; container public-key-algorithms { description "A container for a list of public key algorithms supported by the server."; leaf-list supported-algorithm { type ssh-public-key-algorithm; description "A public key algorithm supported by the server."; } } container encryption-algorithms { description "A container for a list of encryption algorithms supported by the server."; leaf-list supported-algorithm { type ssh-encryption-algorithm; description "An encryption algorithm supported by the server."; } } container key-exchange-algorithms { config false; description "A container for a list of key exchange algorithms supported by the server."; leaf-list supported-algorithm { type ssh-key-exchange-algorithm; description "A key exchange algorithm supported by the server."; } } container mac-algorithms { config false; description "A container for a list of MAC algorithms supported by the server."; leaf-list supported-algorithm { type ssh-mac-algorithm; description "A MAC algorithm supported by the server."; } } } rpc generate-asymmetric-key-pair { if-feature "asymmetric-key-pair-generation"; description "Requests the device to generate a public key using the specified key algorithm."; input { leaf algorithm { type ssh-public-key-algorithm; mandatory true; description "The algorithm to be used when generating the key."; } leaf num-bits { type uint16; description "Specifies the number of bits in the key to create. For RSA keys, the minimum size is 1024 bits and the default is 3072 bits. Generally, 3072 bits is considered sufficient. DSA keys must be exactly 1024 bits, as specified by FIPS 186-5. For Elliptic Curve Digital Signature Algorithm (ECDSA) keys, the 'num-bits' value determines the key length by selecting from one of three elliptic curve sizes: 256, 384, or 521 bits. Attempting to use bit lengths other than these three values for ECDSA keys will fail. ECDSA-SK, Ed25519, and Ed25519-SK keys have a fixed length, and thus, the 'num-bits' value is not specified."; reference "FIPS 186-5: Digital Signature Standard (DSS)"; } container private-key-encoding { description "Indicates how the private key is to be encoded."; choice private-key-encoding { mandatory true; description "A choice amongst optional private key handling."; case cleartext { if-feature "ct:cleartext-private-keys"; leaf cleartext { type empty; description "Indicates that the private key is to be returned as a cleartext value."; } } case encrypted { if-feature "ct:encrypted-private-keys"; container encrypted { description "Indicates that the private key is to be encrypted using the specified symmetric or asymmetric key."; uses ks:encrypted-by-grouping; } } case hidden { if-feature "ct:hidden-private-keys"; leaf hidden { type empty; description "Indicates that the private key is to be hidden. Unlike the 'cleartext' and 'encrypt' options, the key returned is a placeholder for an internally stored key. See the 'Support for Built-in Keys' section in RFC 9642 for information about hidden keys. It is expected that the server will instantiate the hidden key in the same location where built-in keys are located. Rather than returning the key, just the key's location is returned in the output."; } } } } } output { choice key-or-hidden { case key { uses ct:asymmetric-key-pair-grouping; } case hidden { leaf location { type instance-identifier; description "The location to where a hidden key was created."; } } description "The output can be either a key (for cleartext and encrypted keys) or the location to where the key was created (for hidden keys)."; } } } // end generate-asymmetric-key-pair } ]]>

The "ietf-ssh-client" Module This section defines a YANG 1.1 module called "ietf-ssh-client". A high-level overview of the module is provided in . Examples illustrating the module's use are provided in ("Example Usage"). The YANG module itself is defined in .

Data Model Overview This section provides an overview of the "ietf-ssh-client" module in terms of its features and groupings.

Features The following diagram lists all the "feature" statements defined in the "ietf-ssh-client" module: The diagram above uses syntax that is similar to but not defined in . Please refer to the YANG module for a description of each feature.

Groupings The "ietf-ssh-client" module defines the following "grouping" statement:

ssh-client-grouping

This grouping is presented in the following subsection.

The "ssh-client-grouping" Grouping The following tree diagram illustrates the "ssh-client-grouping" grouping: Comments:

The "client-identity" node configures a "username" and authentication methods, each enabled by a "feature" statement defined in .
The "server-authentication" node configures trust anchors for authenticating the SSH server, with each option enabled by a "feature" statement.
The "transport-params" node, which must be enabled by a feature, configures parameters for the SSH sessions established by this configuration.
The "keepalives" node, which must be enabled by a feature, configures a "presence" container for testing the aliveness of the SSH server. The aliveness-test occurs at the SSH protocol layer.
For the referenced grouping statements:
- The "inline-or-keystore-asymmetric-key-grouping" grouping is discussed in .
- The "inline-or-keystore-end-entity-cert-with-key-grouping" grouping is discussed in .
- The "inline-or-truststore-public-keys-grouping" grouping is discussed in .
- The "inline-or-truststore-certs-grouping" grouping is discussed in .
- The "transport-params-grouping" grouping is discussed in in this document.

Protocol-Accessible Nodes The "ietf-ssh-client" module defines only "grouping" statements that are used by other modules to instantiate protocol-accessible nodes. Thus, this module, when implemented, does not itself define any protocol-accessible nodes.

Example Usage This section presents two examples showing the "ssh-client-grouping" grouping populated with some data. These examples are effectively the same, except the first configures the client identity using an inlined key, while the second uses a key configured in a keystore. Both examples are consistent with the examples presented in and . The following configuration example uses inline-definitions for the client identity and server authentication:

foobar

ct:rsa-private-key-format BASE64VALUE=

corp-fw1

ct:ssh-public-key-format BASE64VALUE=

corp-fw2

ct:ssh-public-key-format BASE64VALUE=

Server Cert Issuer #1

BASE64VALUE=

Server Cert Issuer #2

BASE64VALUE=

My Application #1

BASE64VALUE=

My Application #2

BASE64VALUE=

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

foobar

ssh-rsa-key

ssh-rsa-key-with-cert

ex-rsa-cert2

trusted-ssh-public-keys

trusted-server-ca-certs

trusted-server-ee-certs

]]>

YANG Module This YANG module has normative references to , , , , , and . Author: Kent Watsen "; description "This module defines a reusable grouping for SSH clients that can be used as a basis for specific SSH client instances. 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. Copyright (c) 2024 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 9644 (https://www.rfc-editor.org/info/rfc9644); see the RFC itself for full legal notices."; revision 2024-10-10 { description "Initial version."; reference "RFC 9644: YANG Groupings for SSH Clients and SSH Servers"; } // Features feature ssh-client-keepalives { description "SSH keepalive parameters are configurable for SSH clients on the server implementing this feature."; } feature client-ident-publickey { description "Indicates that the 'publickey' authentication type, per RFC 4252, is supported for client identification. 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 client-ident-password { description "Indicates that the 'password' authentication type, per RFC 4252, is supported for client identification."; reference "RFC 4252: The Secure Shell (SSH) Authentication Protocol"; } feature client-ident-hostbased { description "Indicates that the 'hostbased' authentication type, per RFC 4252, is supported for client identification."; reference "RFC 4252: The Secure Shell (SSH) Authentication Protocol"; } feature client-ident-none { description "Indicates that the 'none' authentication type, per RFC 4252, is supported for client identification. It is NOT RECOMMENDED to enable this feature."; reference "RFC 4252: The Secure Shell (SSH) Authentication Protocol"; } // Groupings grouping ssh-client-grouping { description "A reusable grouping for configuring an SSH client without any consideration for how an underlying TCP session is 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-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 username and authentication methods for the client. The authentication methods are unordered. Clients may initially send any configured method or, per Section 5.2 of RFC 4252, send the 'none' method to prompt the server to provide a list of productive methods. Whenever a choice amongst methods arises, implementations SHOULD use a default ordering that prioritizes automation over human interaction."; leaf username { type string; description "The username of this user. This will be the username used, for instance, to log into an SSH server."; } container public-key { if-feature "client-ident-publickey"; presence "Indicates that public-key-based authentication has been configured. This statement is present so the mandatory descendant nodes do not imply that this node must be configured."; description "A locally defined or referenced asymmetric key pair to be used for client identification."; reference "RFC 9642: 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 password { if-feature "client-ident-password"; presence "Indicates that password-based authentication has been configured. This statement is present so the mandatory descendant nodes do not imply that this node must be configured."; description "A password to be used to authenticate the client's identity."; uses ct:password-grouping; } container hostbased { if-feature "client-ident-hostbased"; presence "Indicates that host-based authentication is configured. This statement is present so the mandatory descendant nodes do not imply that this node must be configured."; description "A locally defined or referenced asymmetric key pair to be used for host identification."; reference "RFC 9642: 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")'; } } } leaf none { if-feature "client-ident-none"; type empty; description "Indicates that the 'none' algorithm is used for client identification."; } container certificate { if-feature "sshcmn:ssh-x509-certs"; presence "Indicates that certificate-based authentication has been configured. This statement is present so the mandatory descendant nodes do not imply that this node must be configured."; description "A locally defined or referenced certificate to be used for client identification."; reference "RFC 9642: 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 client-identity container server-authentication { nacm:default-deny-write; must 'ssh-host-keys or ca-certs or ee-certs'; description "Specifies how the SSH client can authenticate SSH servers. Any combination of authentication methods is additive and unordered."; container ssh-host-keys { presence "Indicates that the SSH host key have been configured. This statement is present so the mandatory descendant nodes do not imply that this node must be configured."; description "A bag 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 9641: 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"))])'; } } } container ca-certs { if-feature "sshcmn:ssh-x509-certs"; presence "Indicates that the CA certificates 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 Certification Authority (CA) certificates used by the SSH client to authenticate SSH servers. A server is authenticated if its certificate has a valid chain of trust to a configured CA certificate."; reference "RFC 9641: 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 the EE certificates 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 end-entity (EE) certificates used by the SSH client to authenticate SSH servers. A server is authenticated if its certificate is an exact match to a configured end-entity certificate."; reference "RFC 9641: A YANG Data Model for a Truststore"; uses ts:inline-or-truststore-certs-grouping; } } // container server-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-parameters container keepalives { nacm:default-deny-write; if-feature "ssh-client-keepalives"; presence "Indicates that the SSH client proactively tests the aliveness of the remote SSH server."; description "Configures the keepalive policy to proactively test the aliveness of the SSH server. An unresponsive SSH server is dropped after approximately max-wait * max-attempts seconds. Per Section 4 of RFC 4254, the SSH client SHOULD send an SSH_MSG_GLOBAL_REQUEST message with a purposely nonexistent 'request name' value (e.g., keepalive@example.com) 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 an SSH-level message will be sent to test the aliveness of the SSH server if no data has been received from the SSH server."; } leaf max-attempts { type uint8; default "3"; description "Sets the maximum number of sequential keepalive 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 } ]]>

The "ietf-ssh-server" Module This section defines a YANG 1.1 module called "ietf-ssh-server". A high-level overview of the module is provided in . Examples illustrating the module's use are provided in ("Example Usage"). The YANG module itself is defined in .

Data Model Overview This section provides an overview of the "ietf-ssh-server" module in terms of its features and groupings.

Features The following diagram lists all the "feature" statements defined in the "ietf-ssh-server" module: The diagram above uses syntax that is similar to but not defined in . Please refer to the YANG module for a description of each feature.

Groupings The "ietf-ssh-server" module defines the following "grouping" statement:

ssh-server-grouping

This grouping is presented in the following subsection.

The "ssh-server-grouping" Grouping The following tree diagram illustrates the "ssh-server-grouping" grouping: Comments:

The "server-identity" node configures the authentication methods the server can use to identify itself to clients. The ability to use a certificate is enabled by a "feature".
The "client-authentication" node configures trust anchors for authenticating the SSH client, with each option enabled by a "feature" statement.
The "transport-params" node, which must be enabled by a feature, configures parameters for the SSH sessions established by this configuration.
The "keepalives" node, which must be enabled by a feature, configures a "presence" container for testing the aliveness of the SSH client. The aliveness-test occurs at the SSH protocol layer.
For the referenced grouping statements:
- The "inline-or-keystore-asymmetric-key-grouping" grouping is discussed in .
- The "inline-or-keystore-end-entity-cert-with-key-grouping" grouping is discussed in .
- The "inline-or-truststore-public-keys-grouping" grouping is discussed in .
- The "inline-or-truststore-certs-grouping" grouping is discussed in .
- The "transport-params-grouping" grouping is discussed in in this document.

Protocol-Accessible Nodes The "ietf-ssh-server" module defines only "grouping" statements that are used by other modules to instantiate protocol-accessible nodes. Thus, this module, when implemented, does not itself define any protocol-accessible nodes.

Example Usage This section presents two examples showing the "ssh-server-grouping" grouping populated with some data. These examples are effectively the same, except the first configures the server identity using an inlined key, while the second uses a key configured in a keystore. Both examples are consistent with the examples presented in and . The following configuration example uses inline-definitions for the server identity and client authentication:

my-pubkey-based-host-key

ct:rsa-private-key-format BASE64VALUE=

my-cert-based-host-key

ct:rsa-private-key-format BASE64VALUE=

BASE64VALUE=

mary

$0$example-secret

Mary-Key-1

ct:ssh-public-key-format BASE64VALUE=

Mary-Key-2

ct:ssh-public-key-format BASE64VALUE=

Identity Cert Issuer #1

BASE64VALUE=

Identity Cert Issuer #2

BASE64VALUE=

Application #1

BASE64VALUE=

Application #2

BASE64VALUE=

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

my-pubkey-based-host-key

ssh-rsa-key

my-cert-based-host-key

ssh-rsa-key-with-cert

ex-rsa-cert2

mary

$0$example-secret

SSH Public Keys for Applicat\ ion A

trusted-client-ca-certs

trusted-client-ee-certs

]]>

YANG Module This YANG module has normative references to , , , , , , , , , and . Author: Kent Watsen "; description "This module defines a reusable grouping for SSH servers that can be used as a basis for specific SSH server instances. 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. Copyright (c) 2024 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 9644 (https://www.rfc-editor.org/info/rfc9644); see the RFC itself for full legal notices."; revision 2024-10-10 { description "Initial version."; reference "RFC 9644: YANG Groupings for SSH Clients and SSH Servers"; } // Features feature ssh-server-keepalives { description "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 an 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 an 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 9642: 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 9642: 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) Authentication Protocol"; 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 of 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 'username' 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 9641: 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"))])'; } } } container password { description "A password the SSH server may use to authenticate this user. A user is authenticated if the hash of the supplied password matches this value."; leaf hashed-password { if-feature "local-user-auth-password"; type ianach:crypt-hash; description "The password for this user."; } leaf last-modified { type yang:date-and-time; config false; description "Identifies when the password was last set."; } } container hostbased { if-feature "local-user-auth-hostbased"; presence "Indicates that host-based (RFC 4252) 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) Authentication Protocol RFC 9641: 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 Certification 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 9641: 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 9641: 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 keepalive 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@example.com) 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 an SSH-level message will be sent to test the aliveness of the SSH client if no data has been received from the SSH client."; } leaf max-attempts { type uint8; default "3"; description "Sets the maximum number of sequential keepalive 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 } ]]>

Security Considerations The three IETF YANG modules in this document define groupings and will not be deployed as standalone modules. Their security implications may be context-dependent based on their use in other modules. The designers of modules that import these groupings must conduct their own analysis of the security considerations.

Considerations for the "iana-ssh-key-exchange-algs" Module This section is modeled after the template defined in . The "iana-ssh-key-exchange-algs" YANG module defines a data model that is designed to be accessed via YANG-based management protocols, such as NETCONF and RESTCONF . These protocols have mandatory-to-implement secure transport layers (e.g., Secure Shell (SSH) , TLS , and QUIC ) and mandatory-to-implement mutual authentication The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular users to a preconfigured subset of all available protocol operations and content. This YANG module defines YANG enumerations for a public IANA-maintained registry. YANG enumerations are not security-sensitive, as they are statically defined in the publicly accessible YANG module. IANA MAY deprecate and/or obsolete enumerations over time as needed to address security issues found in the algorithms. This module does not define any writable nodes, RPCs, actions, or notifications, and thus, the security considerations for such are not provided here.

Considerations for the "iana-ssh-encryption-algs" Module This section is modeled after the template defined in . The "iana-ssh-encryption-algs" YANG module defines a data model that is designed to be accessed via YANG-based management protocols, such as NETCONF and RESTCONF . These protocols have mandatory-to-implement secure transport layers (e.g., Secure Shell (SSH) , TLS , and QUIC ) and mandatory-to-implement mutual authentication. The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular users to a preconfigured subset of all available protocol operations and content. This YANG module defines YANG enumerations for a public IANA-maintained registry. YANG enumerations are not security-sensitive, as they are statically defined in the publicly accessible YANG module. This module does not define any writable nodes, RPCs, actions, or notifications, and thus, the security considerations for such are not provided here.

Considerations for the "iana-ssh-mac-algs" Module This section is modeled after the template defined in . The "iana-ssh-mac-algs" YANG module defines a data model that is designed to be accessed via YANG-based management protocols, such as NETCONF and RESTCONF . These protocols have mandatory-to-implement secure transport layers (e.g., Secure Shell (SSH) , TLS , and QUIC ) and mandatory-to-implement mutual authentication. The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular users to a preconfigured subset of all available protocol operations and content. This YANG module defines YANG enumerations for a public IANA-maintained registry. YANG enumerations are not security-sensitive, as they are statically defined in the publicly accessible YANG module. This module does not define any writable nodes, RPCs, actions, or notifications, and thus, the security considerations for such are not provided here.

Considerations for the "iana-ssh-public-key-algs" Module This section is modeled after the template defined in . The "iana-ssh-public-key-algs" YANG module defines a data model that is designed to be accessed via YANG-based management protocols, such as NETCONF and RESTCONF . These protocols have mandatory-to-implement secure transport layers (e.g., Secure Shell (SSH) , TLS , and QUIC ) and mandatory-to-implement mutual authentication. The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular users to a preconfigured subset of all available protocol operations and content. This YANG module defines YANG enumerations for a public IANA-maintained registry. YANG enumerations are not security-sensitive, as they are statically defined in the publicly accessible YANG module. This module does not define any writable nodes, RPCs, actions, or notifications, and thus, the security considerations for such are not provided here.

Considerations for the "ietf-ssh-common" YANG Module This section is modeled after the template defined in . The "ietf-ssh-common" YANG module defines a data model that is designed to be accessed via YANG-based management protocols, such as NETCONF and RESTCONF . These protocols have mandatory-to-implement secure transport layers (e.g., Secure Shell (SSH) , TLS , and QUIC ) and mandatory-to-implement mutual authentication. The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular users to a preconfigured subset of all available protocol operations and content. Please be aware that this YANG module uses groupings from other YANG modules that define nodes that may be considered sensitive or vulnerable in network environments. Please review the security considerations for dependent YANG modules for information as to which nodes may be considered sensitive or vulnerable in network environments. None of the readable data nodes defined in this YANG module are considered sensitive or vulnerable in network environments. The NACM "default-deny-all" extension has not been set for any data nodes defined in this module. None of the writable data nodes defined in this YANG module are considered sensitive or vulnerable in network environments. The NACM "default-deny-write" extension has not been set for any data nodes defined in this module. This module defines the "generate-asymmetric-key-pair" RPC, which may, if the "ct:cleartext-private-keys" feature is enabled and the client requests it, return the private clear in cleartext form. It is NOT RECOMMENDED for private keys to pass the server's security perimeter. This module does not define any actions or notifications, and thus, the security considerations for such are not provided here.

Considerations for the "ietf-ssh-client" YANG Module This section is modeled after the template defined in . The "ietf-ssh-client" YANG module defines "grouping" statements that are designed to be accessed via YANG-based management protocols, such as NETCONF and RESTCONF . These protocols have mandatory-to-implement secure transport layers (e.g., Secure Shell (SSH) , TLS , and QUIC ) and mandatory-to-implement mutual authentication. The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular users to a preconfigured subset of all available protocol operations and content. Please be aware that this YANG module uses groupings from other YANG modules that define nodes that may be considered sensitive or vulnerable in network environments. Please review the security considerations for dependent YANG modules for information as to which nodes may be considered sensitive or vulnerable in network environments. One readable data node defined in this YANG module may be considered sensitive or vulnerable in some network environments. This node is as follows:

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

All the writable data nodes defined by this module may be considered sensitive or vulnerable in some network environments. For instance, any modification to a key or reference to a key may dramatically alter the implemented security policy. For this reason, the NACM extension "default-deny-write" has been set for all data nodes defined in this module. This module does not define any RPCs, actions, or notifications, and thus, the security considerations for such are not provided here.

Considerations for the "ietf-ssh-server" YANG Module This section is modeled after the template defined in . The "ietf-ssh-server" YANG module defines "grouping" statements that are designed to be accessed via YANG-based management protocols, such as NETCONF and RESTCONF . These protocols have mandatory-to-implement secure transport layers (e.g., Secure Shell (SSH) , TLS , and QUIC ) and mandatory-to-implement mutual authentication. The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular users to a preconfigured subset of all available protocol operations and content. Please be aware that this YANG module uses groupings from other YANG modules that define nodes that may be considered sensitive or vulnerable in network environments. Please review the security considerations for dependent YANG modules for information as to which nodes may be considered sensitive or vulnerable in network environments. None of the readable data nodes defined in this YANG module are considered sensitive or vulnerable in network environments. The NACM "default-deny-all" extension has not been set for any data nodes defined in this module. All the writable data nodes defined by this module may be considered sensitive or vulnerable in some network environments. For instance, the addition or removal of references to keys, certificates, trusted anchors, etc., or even the modification of transport or keepalive parameters can dramatically alter the implemented security policy. For this reason, the NACM extension "default-deny-write" has been set for all data nodes defined in this module. This module does not define any RPCs, actions, or notifications, and thus, the security considerations for such are not provided here.

IANA Considerations

The IETF XML Registry IANA has registered seven URIs in the "ns" registry of the "IETF XML Registry" as follows.

URI:: urn:ietf:params:xml:ns:yang:iana-ssh-key-exchange-algs
Registrant Contact:: The IESG
XML:: N/A; the requested URI is an XML namespace.

URI:: urn:ietf:params:xml:ns:yang:iana-ssh-encryption-algs
Registrant Contact:: The IESG
XML:: N/A; the requested URI is an XML namespace.

URI:: urn:ietf:params:xml:ns:yang:iana-ssh-mac-algs
Registrant Contact:: The IESG
XML:: N/A; the requested URI is an XML namespace.

URI:: urn:ietf:params:xml:ns:yang:iana-ssh-public-key-algs
Registrant Contact:: The IESG
XML:: N/A; the requested URI is an XML namespace.

URI:: urn:ietf:params:xml:ns:yang:ietf-ssh-common
Registrant Contact:: The IESG
XML:: N/A; the requested URI is an XML namespace.

URI:: urn:ietf:params:xml:ns:yang:ietf-ssh-client
Registrant Contact:: The IESG
XML:: N/A; the requested URI is an XML namespace.

URI:: urn:ietf:params:xml:ns:yang:ietf-ssh-server
Registrant Contact:: The IESG
XML:: N/A; the requested URI is an XML namespace.

The YANG Module Names Registry IANA has registered seven YANG modules in the "YANG Module Names" registry as follows.

Name:: iana-ssh-key-exchange-algs
Namespace:: urn:ietf:params:xml:ns:yang:iana-ssh-key-exchange-algs
Prefix:: sshkea
Reference:: RFC 9644

Name:: iana-ssh-encryption-algs
Namespace:: urn:ietf:params:xml:ns:yang:iana-ssh-encryption-algs
Prefix:: sshea
Reference:: RFC 9644

Name:: iana-ssh-mac-algs
Namespace:: urn:ietf:params:xml:ns:yang:iana-ssh-mac-algs
Prefix:: sshma
Reference:: RFC 9644

Name:: iana-ssh-public-key-algs
Namespace:: urn:ietf:params:xml:ns:yang:iana-ssh-public-key-algs
Prefix:: sshpka
Reference:: RFC 9644

Name:: ietf-ssh-common
Namespace:: urn:ietf:params:xml:ns:yang:ietf-ssh-common
Prefix:: sshcmn
Reference:: RFC 9644

Name:: ietf-ssh-client
Namespace:: urn:ietf:params:xml:ns:yang:ietf-ssh-client
Prefix:: sshc
Reference:: RFC 9644

Name:: ietf-ssh-server
Namespace:: urn:ietf:params:xml:ns:yang:ietf-ssh-server
Prefix:: sshs
Reference:: RFC 9644

Considerations for the "iana-ssh-encryption-algs" Module This section follows the template defined in . This document presents a script (see ) for IANA to use to generate the IANA-maintained "iana-ssh-encryption-algs" YANG module. The most recent version of the YANG module is available in the "YANG Parameters" registry group . IANA has added the following note to the registry:

New values must not be directly added to the "iana-ssh-encryption-algs" YANG module. They must instead be added to the "Encryption Algorithm Names" registry of the "Secure Shell (SSH) Protocol Parameters" registry group .

When a value is added to the "Encryption Algorithm Names" registry, a new "enum" statement must be added to the "iana-ssh-encryption-algs" YANG module. The "enum" statement, and substatements thereof, should be defined as follows:

enum: Replicates a name from the registry.
value: Contains the decimal value of the IANA-assigned value.
status: Include only if a registration has been deprecated or obsoleted. An IANA "Note" containing the word "HISTORIC" maps to YANG status "obsolete". Since the registry is unable to express a "SHOULD NOT" recommendation, there is no mapping to YANG status "deprecated".
description: Contains "Enumeration for the 'foo-bar' algorithm.", where "foo-bar" is a placeholder for the algorithm's name (e.g., "3des-cbc").
reference: Replicates the reference(s) from the registry with the title of the document(s) added.

Unassigned or reserved values are not present in the module. When the "iana-ssh-encryption-algs" YANG module is updated, a new "revision" statement with a unique revision date must be added in front of the existing revision statements. The "revision" must have a "description" statement explaining why the update occurred and must have a "reference" substatement that points to the document defining the registry update that resulted in this change. For instance: IANA has added the following note to the "Encryption Algorithm Names" registry.

When this registry is modified, the YANG module "iana-ssh-encryption-algs" must be updated as defined in RFC 9644.

Considerations for the "iana-ssh-mac-algs" Module This section follows the template defined in . This document presents a script (see ) for IANA to use to generate the IANA-maintained "iana-ssh-mac-algs" YANG module. The most recent version of the YANG module is available in the "YANG Parameters" registry group . IANA has added the following note to the registry:

New values must not be directly added to the "iana-ssh-mac-algs" YANG module. They must instead be added to the "MAC Algorithm Names" registry of the "Secure Shell (SSH) Protocol Parameters" registry group .

When a value is added to the "MAC Algorithm Names" registry, a new "enum" statement must be added to the "iana-ssh-mac-algs" YANG module. The "enum" statement, and substatements thereof, should be defined as follows:

enum: Replicates a name from the registry.
value: Contains the decimal value of the IANA-assigned value.
status: Include only if a registration has been deprecated or obsoleted.
description: Contains "Enumeration for the 'foo-bar' algorithm.", where "foo-bar" is a placeholder for the algorithm's name (e.g., "3des-cbc").
reference: Replicates the reference(s) from the registry with the title of the document(s) added.

Unassigned or reserved values are not present in the module. When the "iana-ssh-mac-algs" YANG module is updated, a new "revision" statement with a unique revision date must be added in front of the existing revision statements. The "revision" must have a "description" statement explaining why the update occurred and must have a "reference" substatement that points to the document defining the registry update that resulted in this change. For instance: IANA has added the following note to the "MAC Algorithm Names" registry.

When this registry is modified, the YANG module "iana-ssh-mac-algs" must be updated as defined in RFC 9644.

Considerations for the "iana-ssh-public-key-algs" Module This section follows the template defined in . This document presents a script (see ) for IANA to use to generate the IANA-maintained "iana-ssh-public-key-algs" YANG module. The most recent version of the YANG module is available in the "YANG Parameters" registry group . IANA has added the following note to the registry:

New values must not be directly added to the "iana-ssh-public-key-algs" YANG module. They must instead be added to the "Public Key Algorithm Names" registry of the "Secure Shell (SSH) Protocol Parameters" registry group .

When a value is added to the "Public Key Algorithm Names" registry, a new "enum" statement must be added to the "iana-ssh-public-key-algs" YANG module. The "enum" statement, and substatements thereof, should be defined as follows:

enum: Replicates a name from the registry.
value: Contains the decimal value of the IANA-assigned value.
status: Include only if a registration has been deprecated or obsoleted.
description: Contains "Enumeration for the 'foo-bar' algorithm.", where "foo-bar" is a placeholder for the algorithm's name (e.g., "3des-cbc").
reference: Replicates the reference(s) from the registry with the title of the document(s) added.

In the case that the algorithm name ends with "-*", the family of enumerations must be added. The family of enum algorithm names are generated by replacing the "*" character with these strings: "nistp256", "nistp384", "nistp521", "1.3.132.0.1", "1.2.840.10045.3.1.1", "1.3.132.0.33", "1.3.132.0.26", "1.3.132.0.27", "1.3.132.0.16", "1.3.132.0.36", "1.3.132.0.37", and "1.3.132.0.38". Unassigned or reserved values are not present in the module. When the "iana-ssh-public-key-algs" YANG module is updated, a new "revision" statement with a unique revision date must be added in front of the existing revision statements. The "revision" must have a "description" statement explaining why the update occurred and must have a "reference" substatement that points to the document defining the registry update that resulted in this change. For instance: IANA has added the following note to the "Public Key Algorithm Names" registry.

When this registry is modified, the YANG module "iana-ssh-public-key-algs" must be updated as defined in RFC 9644.

Considerations for the "iana-ssh-key-exchange-algs" Module This section follows the template defined in . This document presents a script (see ) for IANA to use to generate the IANA-maintained "iana-ssh-key-exchange-algs" YANG module. The most recent version of the YANG module is available in the "YANG Parameters" registry group . IANA has added the following note to the registry:

New values must not be directly added to the "iana-ssh-key-exchange-algs" YANG module. They must instead be added to the "Key Exchange Method Names" registry of the "Secure Shell (SSH) Protocol Parameters" registry group .

When a value is added to the "Key Exchange Method Names" registry, a new "enum" statement must be added to the "iana-ssh-key-exchange-algs" YANG module. The "enum" statement, and substatements thereof, should be defined as follows:

enum: Replicates a name from the registry.
value: Contains the decimal value of the IANA-assigned value.
status: Include only if a registration has been deprecated or obsoleted. An IANA "OK to Implement" containing "SHOULD NOT" maps to YANG status "deprecated". An IANA "OK to Implement" containing "MUST NOT" maps to YANG status "obsolete".
description: Contains "Enumeration for the 'foo-bar' algorithm.", where "foo-bar" is a placeholder for the algorithm's name (e.g., "3des-cbc").
reference: Replicates the reference(s) from the registry with the title of the document(s) added.

In the case that the algorithm name ends with "-*", the family of enumerations must be added. The family of enum algorithm names are generated by replacing the "*" character with these strings: "nistp256", "nistp384", "nistp521", "1.3.132.0.1", "1.2.840.10045.3.1.1", "1.3.132.0.33", "1.3.132.0.26", "1.3.132.0.27", "1.3.132.0.16", "1.3.132.0.36", "1.3.132.0.37", and "1.3.132.0.38". Unassigned or reserved values are not present in the module. When the "iana-ssh-key-exchange-algs" YANG module is updated, a new "revision" statement with a unique revision date must be added in front of the existing revision statements. The "revision" must have a "description" statement explaining why the update occurred, and must have a "reference" substatement that points to the document defining the registry update that resulted in this change. For instance: IANA has added the following note to the "Key Exchange Method Names" registry.

When this registry is modified, the YANG module "iana-ssh-key-exchange-algs" must be updated as defined in RFC 9644.

References Normative References YANG Data Types and Groupings for Cryptography Watsen Networks A YANG Data Model for a Truststore Watsen Networks A YANG Data Model for a Keystore Watsen Networks Informative References YANG Groupings for TCP Clients and TCP Servers Watsen Networks Hochschule Esslingen - University of Applied Sciences YANG Groupings for TLS Clients and TLS Servers Watsen Networks Digital Signature Standard (DSS) NIST Key Exchange Method Names IANA Encryption Algorithm Names IANA MAC Algorithm Names IANA Public Key Algorithm Names IANA YANG Parameters IANA Extensible Markup Language (XML) 1.0 (Fifth Edition)

Script to Generate IANA-Maintained YANG Modules This section is not normative. The Python script contained in this section will create the four IANA-maintained modules that are described (but not contained) in this document. Run the script using the command "python gen-yang-modules.py" to produce four YANG module files in the current directory. Be aware that the script does not attempt to copy the "revision" statements from the previous/current YANG module. Copying the revision statements must be done manually. ', item)) break else: raise Exception("RFC title not found") # Insert a space: "RFCXXXX" --> "RFC XXXX" index = refs.index(ref) refs[index] = "RFC " + ref[3:] elif ref.startswith("FIPS"): # Special case for FIPS, since no bibtex to fetch if ref == "FIPS 46-3" or ref == "FIPS-46-3": titles.append("Data Encryption Standard (DES\ )") else: raise Exception("FIPS ref not found") else: raise Exception("ref not found") # Function used below def write_enumeration(alg): f.write('\n') f.write(f' enum {alg} {{\n') if "HISTORIC" in row["Note"]: f.write(f' status obsolete;\n') elif "OK to Implement" in row: if "MUST NOT" in row["OK to Implement"]: f.write(f' status obsolete;\n') elif "SHOULD NOT" in row["OK to Implement"]: f.write(f' status deprecated;\n') f.write(f' description\n') description = f' "Enumeration for the \'{al\ g}\' algorithm.' if "Section" in row["Note"]: description += " " + row["Note"] description += '";' description = textwrap.fill(description, width=69, s\ ubsequent_indent=" ") f.write(f'{description}\n') f.write(' reference\n') f.write(' "') if row["Reference"] == "": f.write(' Missing in IANA registry.') else: ref_len = len(refs) for i in range(ref_len): ref = refs[i] f.write(f'{ref}:\n') title = " " + titles[i] if i == ref_len - 1: title += '";' title = textwrap.fill(title, width=67, subse\ quent_indent=" ") f.write(f'{title}') if i != ref_len - 1: f.write('\n ') f.write('\n') f.write(' }\n') # Write one or more "enumeration" statements if not row[first_colname].endswith("-*"): # just one enu\ meration # Avoid duplicate entries caused by the "ecdh-sha2-*\ " family expansion if not row[first_colname].startswith("ecdh-sha2-nist\ p"): write_enumeration(row[first_colname]) else: # a family of enumerations curve_ids = [ "nistp256", "nistp384", "nistp521", "1.3.132.0.1", "1.2.840.10045.3.1.1", "1.3.132.0.33", "1.3.132.0.26", "1.3.132.0.27", "1.3.132.0.16", "1.3.132.0.36", "1.3.132.0.37", "1.3.132.0.38", ] for curve_id in curve_ids: write_enumeration(row[first_colname][:-1] + curv\ e_id) def create_module_end(module, f): # Close out the enumeration, typedef, and module f.write(" }\n") f.write(" description\n") f.write(f' "An enumeration for SSH {module["spaced_name"]} \ algorithms.";\n') f.write(" }\n") f.write('\n') f.write('}\n') def create_module(module): # Install cache for 8x speedup requests_cache.install_cache() # Ascertain YANG module's name yang_module_name = "iana-ssh-" + module["hyphenated_name"] + "-a\ lgs.yang" # Create YANG module file with open(yang_module_name, "w") as f: create_module_begin(module, f) create_module_body(module, f) create_module_end(module, f) def main(): for module in MODULES: create_module(module) if __name__ == "__main__": main() ]]>

Acknowledgements The authors would like to thank the following for lively discussions on list and in the halls (ordered by first name): , , , , , , , , , , , , , , , , , , , , , , , , , , , , and .

Contributors Special acknowledgement goes to for his work on the "ietf-ssh-common" module.