NETCONF Working Group K. Watsen
Internet-Draft Watsen Networks
Intended status: Standards Track G. Wu
Expires: November 21, 2020 Cisco Systems
May 20, 2020
YANG Groupings for SSH Clients and SSH Servers
draft-ietf-netconf-ssh-client-server-19
Abstract
This document defines three YANG modules: the first defines groupings
for a generic SSH client, the second defines groupings for a generic
SSH server, and the third defines common identities and groupings
used by both the client and the server. It is intended that these
groupings will be used by applications using the SSH protocol.
Editorial Note (To be removed by RFC Editor)
This draft contains placeholder values that need to be replaced with
finalized values at the time of publication. This note summarizes
all of the substitutions that are needed. No other RFC Editor
instructions are specified elsewhere in this document.
Artwork in this document contains shorthand references to drafts in
progress. Please apply the following replacements:
o "AAAA" --> the assigned RFC value for draft-ietf-netconf-crypto-
types
o "BBBB" --> the assigned RFC value for draft-ietf-netconf-trust-
anchors
o "CCCC" --> the assigned RFC value for draft-ietf-netconf-keystore
o "DDDD" --> the assigned RFC value for draft-ietf-netconf-tcp-
client-server
o "EEEE" --> the assigned RFC value for this draft
Artwork in this document contains placeholder values for the date of
publication of this draft. Please apply the following replacement:
o "2020-05-20" --> the publication date of this draft
The following Appendix section is to be removed prior to publication:
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o Appendix A. Change Log
Note to Reviewers (To be removed by RFC Editor)
This document presents a YANG module or modules that is/are part of a
collection of drafts that work together to produce the ultimate goal
of the NETCONF WG: to define configuration modules for NETCONF client
and servers, and RESTCONF client and servers.
The relationship between the various drafts in the collection is
presented in the below diagram.
crypto-types
^ ^
/ \
/ \
trust-anchors keystore
^ ^ ^ ^
| +---------+ | |
| | | |
| +------------+ |
tcp-client-server | / | |
^ ^ ssh-client-server | |
| | ^ tls-client-server
| | | ^ ^ http-client-server
| | | | | ^
| | | +-----+ +---------+ |
| | | | | |
| +-----------|--------|--------------+ | |
| | | | | |
+-----------+ | | | | |
| | | | | |
| | | | | |
netconf-client-server restconf-client-server
Full draft names and link to drafts:
o draft-ietf-netconf-crypto-types (html [1])
o draft-ietf-netconf-trust-anchors (html [2])
o draft-ietf-netconf-keystore (html [3])
o draft-ietf-netconf-tcp-client-server (html [4])
o draft-ietf-netconf-ssh-client-server (html [5])
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o draft-ietf-netconf-tls-client-server (html [6])
o draft-ietf-netconf-http-client-server (html [7])
o draft-ietf-netconf-netconf-client-server (html [8])
o draft-ietf-netconf-restconf-client-server (html [9])
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 November 21, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. The SSH Client Model . . . . . . . . . . . . . . . . . . . . 5
3.1. Tree Diagram . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Example Usage . . . . . . . . . . . . . . . . . . . . . . 6
3.3. YANG Module . . . . . . . . . . . . . . . . . . . . . . . 10
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4. The SSH Server Model . . . . . . . . . . . . . . . . . . . . 17
4.1. Tree Diagram . . . . . . . . . . . . . . . . . . . . . . 17
4.2. Example Usage . . . . . . . . . . . . . . . . . . . . . . 19
4.3. YANG Module . . . . . . . . . . . . . . . . . . . . . . . 23
5. The SSH Common Model . . . . . . . . . . . . . . . . . . . . 32
5.1. Tree Diagram . . . . . . . . . . . . . . . . . . . . . . 33
5.2. Example Usage . . . . . . . . . . . . . . . . . . . . . . 33
5.3. YANG Module . . . . . . . . . . . . . . . . . . . . . . . 34
6. Security Considerations . . . . . . . . . . . . . . . . . . . 44
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 45
7.1. The IETF XML Registry . . . . . . . . . . . . . . . . . . 45
7.2. The YANG Module Names Registry . . . . . . . . . . . . . 46
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 46
8.1. Normative References . . . . . . . . . . . . . . . . . . 46
8.2. Informative References . . . . . . . . . . . . . . . . . 48
8.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 50
A.1. 00 to 01 . . . . . . . . . . . . . . . . . . . . . . . . 50
A.2. 01 to 02 . . . . . . . . . . . . . . . . . . . . . . . . 50
A.3. 02 to 03 . . . . . . . . . . . . . . . . . . . . . . . . 50
A.4. 03 to 04 . . . . . . . . . . . . . . . . . . . . . . . . 50
A.5. 04 to 05 . . . . . . . . . . . . . . . . . . . . . . . . 51
A.6. 05 to 06 . . . . . . . . . . . . . . . . . . . . . . . . 51
A.7. 06 to 07 . . . . . . . . . . . . . . . . . . . . . . . . 51
A.8. 07 to 08 . . . . . . . . . . . . . . . . . . . . . . . . 51
A.9. 08 to 09 . . . . . . . . . . . . . . . . . . . . . . . . 51
A.10. 09 to 10 . . . . . . . . . . . . . . . . . . . . . . . . 52
A.11. 10 to 11 . . . . . . . . . . . . . . . . . . . . . . . . 52
A.12. 11 to 12 . . . . . . . . . . . . . . . . . . . . . . . . 52
A.13. 12 to 13 . . . . . . . . . . . . . . . . . . . . . . . . 52
A.14. 13 to 14 . . . . . . . . . . . . . . . . . . . . . . . . 52
A.15. 14 to 15 . . . . . . . . . . . . . . . . . . . . . . . . 53
A.16. 15 to 16 . . . . . . . . . . . . . . . . . . . . . . . . 53
A.17. 16 to 17 . . . . . . . . . . . . . . . . . . . . . . . . 53
A.18. 17 to 18 . . . . . . . . . . . . . . . . . . . . . . . . 53
A.19. 18 to 19 . . . . . . . . . . . . . . . . . . . . . . . . 54
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 54
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 55
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
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OpenSSH [OPENSSH] server or a NETCONF over SSH [RFC6242] based
server.
The client and server YANG modules in this document each define one
grouping, which is focused on just SSH-specific configuration, and
specifically avoids any transport-level configuration, such as what
ports to listen on or connect to. This affords applications the
opportunity to define their own strategy for how the underlying TCP
connection is established. For instance, applications supporting
NETCONF Call Home [RFC8071] could use the "ssh-server-grouping"
grouping for the SSH parts it provides, while adding data nodes for
the TCP-level call-home configuration.
The modules defined in this document use groupings defined in
[I-D.ietf-netconf-keystore] enabling keys to be either locally
defined or a reference to globally configured values.
The modules defined in this document optionally support [RFC6187]
enabling X.509v3 certificate based host keys and public keys.
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.
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========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) ===========
module: ietf-ssh-client
grouping ssh-client-grouping
+-- client-identity
| +-- username? string
| +-- public-key! {client-identity-publickey}?
| | +---u ks:local-or-keystore-asymmetric-key-grouping
| +-- password? string {client-identity-password}?
| +-- hostbased! {client-identity-hostbased}?
| | +---u ks:local-or-keystore-asymmetric-key-grouping
| +-- none? empty {client-identity-none}?
| +-- certificate! {sshcmn:ssh-x509-certs}?
| +---u ks:local-or-keystore-end-entity-cert-with-key-groupi\
ng
+-- server-authentication
| +-- ssh-host-keys!
| | +---u ts:local-or-truststore-public-keys-grouping
| +-- ca-certs! {sshcmn:ssh-x509-certs}?
| | +---u ts:local-or-truststore-certs-grouping
| +-- ee-certs! {sshcmn:ssh-x509-certs}?
| +---u ts:local-or-truststore-certs-grouping
+-- transport-params {ssh-client-transport-params-config}?
| +---u sshcmn:transport-params-grouping
+-- keepalives! {ssh-client-keepalives}?
+-- max-wait? uint16
+-- max-attempts? uint8
3.2. Example Usage
This section presents two examples showing the "ssh-client-grouping"
grouping populated with some data. These examples are effectively
the same except the first configures the client identity using a
local key while the second uses a key configured in a keystore. Both
examples are consistent with the examples presented in Section 2 of
[I-D.ietf-netconf-trust-anchors] and Section 3.2 of
[I-D.ietf-netconf-keystore].
The following example configures the client identity using a local
key:
========== NOTE: '\\' line wrapping per BCP XXX (RFC XXXX) ==========
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foobar
ct:ssh-public-key-format
base64encodedvalue==
ct:rsa-private-key-format
base64encodedvalue==
corp-fw1
ct:ssh-public-key-format
base64encodedvalue==
corp-fw2
ct:ssh-public-key-format
base64encodedvalue==
base64encodedvalue==
base64encodedvalue==
base64encodedvalue==
base64encodedvalue==
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
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30
3
The following example configures the client identity using a key from
the keystore:
========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) ===========
foobar
ssh-rsa-key-with-cert
ex-rsa-cert2
trusted-ssh-public-keys
trusted-server-ca-certs
trusted-server-ee-certs
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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
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@2020-05-20.yang"
module ietf-ssh-client {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-client";
prefix sshc;
import ietf-netconf-acm {
prefix nacm;
reference
"RFC 8341: Network Configuration Access Control Model";
}
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import ietf-crypto-types {
prefix ct;
reference
"RFC AAAA: Common YANG Data Types for Cryptography";
}
import ietf-truststore {
prefix ts;
reference
"RFC BBBB: A YANG Data Model for a Truststore";
}
import ietf-keystore {
prefix ks;
reference
"RFC CCCC: A YANG Data Model for a Keystore";
}
import ietf-ssh-common {
prefix sshcmn;
revision-date 2020-05-20; // stable grouping definitions
reference
"RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
}
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) 2020 IETF Trust and the persons identified
as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with
or without modification, is permitted pursuant to, and
subject to the license terms contained in, the Simplified
BSD License set forth in Section 4.c of the IETF Trust's
Legal Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info).
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This version of this YANG module is part of RFC EEEE
(https://www.rfc-editor.org/info/rfcEEEE); see the RFC
itself for full legal notices.;
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
are to be interpreted as described in BCP 14 (RFC 2119)
(RFC 8174) when, and only when, they appear in all
capitals, as shown here.";
revision 2020-05-20 {
description
"Initial version";
reference
"RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
}
// Features
feature ssh-client-transport-params-config {
description
"SSH transport layer parameters are configurable on an SSH
client.";
}
feature ssh-client-keepalives {
description
"Per socket SSH keepalive parameters are configurable for
SSH clients on the server implementing this feature.";
}
feature client-identity-password {
description
"Indicates that the 'password' authentication type
is supported for client identification.";
}
feature client-identity-publickey {
description
"Indicates that the 'publickey' authentication type
is supported for client identification.
The 'publickey' authentication type is required by
RFC 4252, but common implementations enable it to
be disabled.";
}
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feature client-identity-hostbased {
description
"Indicates that the 'hostbased' authentication type
is supported for client identification.";
}
feature client-identity-none {
description
"Indicates that the 'none' authentication type is
supported for client identification.";
}
// Groupings
grouping ssh-client-grouping {
description
"A reusable grouping for configuring a SSH client without
any consideration for how an underlying TCP session is
established.
Note that this grouping uses fairly typical descendent
node names such that a stack of 'uses' statements will
have name conflicts. It is intended that the consuming
data model will resolve the issue (e.g., by wrapping
the 'uses' statement in a container called
'ssh-client-parameters'). This model purposely does
not do this itself so as to provide maximum flexibility
to consuming models.";
container client-identity {
nacm:default-deny-write;
description
"The credentials that the client may use, pending
the SSH server's requirements, by the SSH client
to authenticate to the SSH server.";
leaf username {
type string;
description
"The username of this user. This will be the username
used, for instance, to log into an SSH server.";
}
container public-key {
if-feature client-identity-publickey;
presence
"Indicates that publickey-based authentication
is configured";
description
"A locally-defined or referenced asymmetric key
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pair to be used for client identification.";
reference
"RFC CCCC: A YANG Data Model for a Keystore";
uses ks:local-or-keystore-asymmetric-key-grouping {
refine "local-or-keystore/local/local-definition" {
must 'public-key-format = "ct:ssh-public-key-format"';
}
refine "local-or-keystore/keystore/keystore-reference" {
must 'deref(.)/../ks:public-key-format'
+ ' = "ct:ssh-public-key-format"';
}
}
}
leaf password {
if-feature client-identity-password;
nacm:default-deny-all;
type string;
description
"A password to be used for client identification.";
}
container hostbased {
if-feature client-identity-hostbased;
presence
"Indicates that hostbased authentication is configured";
description
"A locally-defined or referenced asymmetric key
pair to be used for host identification.";
reference
"RFC CCCC: A YANG Data Model for a Keystore";
uses ks:local-or-keystore-asymmetric-key-grouping {
refine "local-or-keystore/local/local-definition" {
must 'public-key-format = "ct:ssh-public-key-format"';
}
refine "local-or-keystore/keystore/keystore-reference" {
must 'deref(.)/../ks:public-key-format'
+ ' = "ct:ssh-public-key-format"';
}
}
}
leaf none {
if-feature client-identity-none;
type empty;
description
"Indicates that 'none' algorithm is used for client
identification.";
}
container certificate {
if-feature "sshcmn:ssh-x509-certs";
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presence
"Indicates that certificate-based authentication
is configured";
description
"A locally-defined or referenced certificate
to be used for client identification.";
reference
"RFC CCCC: A YANG Data Model for a Keystore";
uses
ks:local-or-keystore-end-entity-cert-with-key-grouping {
refine "local-or-keystore/local/local-definition" {
must
'public-key-format'
+ ' = "ct:subject-public-key-info-format"';
}
refine "local-or-keystore/keystore/keystore-reference"
+ "/asymmetric-key" {
must 'deref(.)/../ks:public-key-format'
+ ' = "ct:subject-public-key-info-format"';
}
}
}
} // container client-identity
container server-authentication {
nacm:default-deny-write;
must 'ssh-host-keys or ca-certs or ee-certs';
description
"Specifies how the SSH client can authenticate SSH servers.
Any combination of credentials is additive and unordered.";
container ssh-host-keys {
presence
"Indicates that the client can authenticate servers
using the configured SSH host keys.";
description
"A list of SSH host keys used by the SSH client to
authenticate SSH server host keys. A server host key
is authenticated if it is an exact match to a
configured SSH host key.";
reference
"RFC BBBB: A YANG Data Model for a Truststore";
uses ts:local-or-truststore-public-keys-grouping {
refine
"local-or-truststore/local/local-definition/public-key" {
must 'public-key-format = "ct:ssh-public-key-format"';
}
refine
"local-or-truststore/truststore/truststore-reference" {
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must 'deref(.)/../*/ts:public-key-format'
+ ' = "ct:ssh-public-key-format"';
}
}
}
container ca-certs {
if-feature "sshcmn:ssh-x509-certs";
presence
"Indicates that the client can authenticate servers
using the configured trust anchor certificates.";
description
"A set of certificate authority (CA) certificates used by
the SSH client to authenticate SSH servers. A server
is authenticated if its certificate has a valid chain
of trust to a configured CA certificate.";
reference
"RFC BBBB: A YANG Data Model for a Truststore";
uses ts:local-or-truststore-certs-grouping;
}
container ee-certs {
if-feature "sshcmn:ssh-x509-certs";
presence
"Indicates that the client can authenticate servers
using the configured end-entity certificates.";
description
"A set of end-entity certificates used by the SSH client
to authenticate SSH servers. A server is authenticated
if its certificate is an exact match to a configured
end-entity certificate.";
reference
"RFC BBBB: A YANG Data Model for a Truststore";
uses ts:local-or-truststore-certs-grouping;
}
} // container server-authentication
container transport-params {
nacm:default-deny-write;
if-feature "ssh-client-transport-params-config";
description
"Configurable parameters of the SSH transport layer.";
uses sshcmn:transport-params-grouping;
} // container transport-parameters
container keepalives {
nacm:default-deny-write;
if-feature "ssh-client-keepalives";
presence
"Indicates that the SSH client proactively tests the
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aliveness of the remote SSH server.";
description
"Configures the keep-alive policy, to proactively test
the aliveness of the SSH server. An unresponsive TLS
server is dropped after approximately max-wait *
max-attempts seconds. Per Section 4 of RFC 4254,
the SSH client SHOULD send an SSH_MSG_GLOBAL_REQUEST
message with a purposely nonexistent 'request name'
value (e.g., keepalive@ietf.org) and the 'want reply'
value set to '1'.";
reference
"RFC 4254: The Secure Shell (SSH) Connection Protocol";
leaf max-wait {
type uint16 {
range "1..max";
}
units "seconds";
default "30";
description
"Sets the amount of time in seconds after which if
no data has been received from the SSH server, a
TLS-level message will be sent to test the
aliveness of the SSH server.";
}
leaf max-attempts {
type uint8;
default "3";
description
"Sets the maximum number of sequential keep-alive
messages that can fail to obtain a response from
the SSH server before assuming the SSH server is
no longer alive.";
}
} // container keepalives
} // grouping ssh-client-grouping
} // module ietf-ssh-client
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.
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========== 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
| | +-- password? empty {client-auth-password}?
| | +-- hostbased? empty {client-auth-hostbased}?
| | +-- none? empty {client-auth-none}?
| +-- users {client-auth-config-supported}?
| | +-- user* [name]
| | +-- name? string
| | +-- public-keys! {client-auth-publickey}?
| | | +---u ts:local-or-truststore-public-keys-grouping
| | +-- password? ianach:crypt-hash
| | | {client-auth-password}?
| | +-- hostbased! {client-auth-hostbased}?
| | | +---u ts:local-or-truststore-public-keys-grouping
| | +-- none? empty {client-auth-none}?
| +-- ca-certs!
| | {client-auth-config-supported,sshcmn:ssh-x509-certs}?
| | +---u ts:local-or-truststore-certs-grouping
| +-- ee-certs!
| {client-auth-config-supported,sshcmn:ssh-x509-certs}?
| +---u ts:local-or-truststore-certs-grouping
+-- transport-params {ssh-server-transport-params-config}?
| +---u sshcmn:transport-params-grouping
+-- keepalives! {ssh-server-keepalives}?
+-- max-wait? uint16
+-- max-attempts? uint8
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4.2. Example Usage
This section presents two examples showing the "ssh-server-grouping"
grouping populated with some data. These examples are effectively
the same except the first configures the server identity using a
local key while the second uses a key configured in a keystore. Both
examples are consistent with the examples presented in Section 2 of
[I-D.ietf-netconf-trust-anchors] and Section 3.2 of
[I-D.ietf-netconf-keystore].
The following example configures the server identity using a local
key:
========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) ===========
my-pubkey-based-host-key
ct:ssh-public-key-format
base64encodedvalue==
ct:rsa-private-key-format
base64encodedvalue==
my-cert-based-host-key
ct:subject-public-key-info-format
base64encodedvalue==
ct:rsa-private-key-format
base64encodedvalue==
base64encodedvalue==
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mary
$0$secret
User A
ct:ssh-public-key-format
base64encodedvalue==
User B
ct:ssh-public-key-format
base64encodedvalue==
base64encodedvalue==
base64encodedvalue==
base64encodedvalue==
base64encodedvalue==
base64encodedvalue==
base64encodedvalue==
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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
The following example configures the server identity using a key from
the keystore:
========== NOTE: '\' line wrapping per BCP XXX (RFC XXXX) ===========
my-pubkey-based-host-key
ssh-rsa-key
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my-cert-based-host-key
ssh-rsa-key-with-cert
ex-rsa-cert2
mary
$0$secret
SSH Public Keys for User A
trusted-client-ca-certs
trusted-client-ee-certs
algs:ssh-rsa
algs:diffie-hellman-group-exchange-sha256
algs:aes256-ctr
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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
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@2020-05-20.yang"
module ietf-ssh-server {
yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-ssh-server";
prefix sshs;
import iana-crypt-hash {
prefix ianach;
reference
"RFC 7317: A YANG Data Model for System Management";
}
import ietf-netconf-acm {
prefix nacm;
reference
"RFC 8341: Network Configuration Access Control Model";
}
import ietf-crypto-types {
prefix ct;
reference
"RFC AAAA: Common YANG Data Types for Cryptography";
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}
import ietf-truststore {
prefix ts;
reference
"RFC BBBB: A YANG Data Model for a Truststore";
}
import ietf-keystore {
prefix ks;
reference
"RFC CCCC: A YANG Data Model for a Keystore";
}
import ietf-ssh-common {
prefix sshcmn;
revision-date 2020-05-20; // stable grouping definitions
reference
"RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
}
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) 2020 IETF Trust and the persons identified
as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with
or without modification, is permitted pursuant to, and
subject to the license terms contained in, the Simplified
BSD License set forth in Section 4.c of the IETF Trust's
Legal Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC EEEE
(https://www.rfc-editor.org/info/rfcEEEE); see the RFC
itself for full legal notices.;
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The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
are to be interpreted as described in BCP 14 (RFC 2119)
(RFC 8174) when, and only when, they appear in all
capitals, as shown here.";
revision 2020-05-20 {
description
"Initial version";
reference
"RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
}
// Features
feature ssh-server-transport-params-config {
description
"SSH transport layer parameters are configurable on an SSH
server.";
}
feature ssh-server-keepalives {
description
"Per socket SSH keepalive parameters are configurable for
SSH servers on the server implementing this feature.";
}
feature client-auth-config-supported {
description
"Indicates that the configuration for how to authenticate
clients can be configured herein, as opposed to in an
application specific location. That is, to support the
consuming data models that prefer to place client
authentication with client definitions, rather then
in a data model principally concerned with configuring
the transport.";
}
feature client-auth-publickey {
description
"Indicates that the 'publickey' authentication type
is supported.
The 'publickey' authentication type is required by
RFC 4252, but common implementations enable it to
be disabled.";
reference
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"RFC 4252:
The Secure Shell (SSH) Authentication Protocol";
}
feature client-auth-password {
description
"Indicates that the 'password' authentication type
is supported.";
}
feature client-auth-hostbased {
description
"Indicates that the 'hostbased' authentication type
is supported.";
}
feature client-auth-none {
description
"Indicates that the 'none' authentication type is
supported.";
}
// Groupings
grouping ssh-server-grouping {
description
"A reusable grouping for configuring a SSH server without
any consideration for how underlying TCP sessions are
established.
Note that this grouping uses fairly typical descendent
node names such that a stack of 'uses' statements will
have name conflicts. It is intended that the consuming
data model will resolve the issue (e.g., by wrapping
the 'uses' statement in a container called
'ssh-server-parameters'). This model purposely does
not do this itself so as to provide maximum flexibility
to consuming models.";
container server-identity {
nacm:default-deny-write;
description
"The list of host keys the SSH server will present when
establishing a SSH connection.";
list host-key {
key "name";
min-elements 1;
ordered-by user;
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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 {
description
"A locally-defined or referenced asymmetric key pair
to be used for the SSH server's host key.";
reference
"RFC CCCC: A YANG Data Model for a Keystore";
uses ks:local-or-keystore-asymmetric-key-grouping {
refine "local-or-keystore/local/local-definition" {
must
'public-key-format = "ct:ssh-public-key-format"';
}
refine "local-or-keystore/keystore/"
+ "keystore-reference" {
must 'deref(.)/../ks:public-key-format'
+ ' = "ct:ssh-public-key-format"';
}
}
}
container certificate {
if-feature "sshcmn:ssh-x509-certs";
description
"A locally-defined or referenced end-entity
certificate to be used for the SSH server's
host key.";
reference
"RFC CCCC: A YANG Data Model for a Keystore";
uses
ks:local-or-keystore-end-entity-cert-with-key-grouping {
refine "local-or-keystore/local/local-definition" {
must
'public-key-format'
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+ ' = "ct:subject-public-key-info-format"';
}
refine "local-or-keystore/keystore/keystore-reference"
+ "/asymmetric-key" {
must 'deref(.)/../ks:public-key-format'
+ ' = "ct:subject-public-key-info-format"';
}
}
}
}
}
} // container server-identity
container client-authentication {
nacm:default-deny-write;
description
"Specifies how the SSH server can authenticate SSH clients.";
container supported-authentication-methods {
description
"Indicates which authentication methods the server
supports.";
leaf publickey {
type empty;
description
"Indicates that the 'publickey' method is supported.
Note that RFC 6187 X.509v3 Certificates for SSH uses
the 'publickey' method name.";
reference
"RFC 4252: The Secure Shell (SSH) Authentication
Protocol.
RFC 6187: X.509v3 Certificates for Secure Shell
Authentication.";
}
leaf password {
if-feature client-auth-password;
type empty;
description
"Indicates that the 'password' method is supported.";
reference
"RFC 4252: The Secure Shell (SSH) Authentication
Protocol.";
}
leaf hostbased {
if-feature client-auth-hostbased;
type empty;
description
"Indicates that the 'hostbased' method is supported.";
reference
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"RFC 4252: The Secure Shell (SSH) Authentication
Protocol.";
}
leaf none {
if-feature client-auth-none;
type empty;
description
"Indicates that the 'none' method is supported.";
reference
"RFC 4252: The Secure Shell (SSH) Authentication
Protocol.";
}
}
container users {
if-feature "client-auth-config-supported";
description
"A list of locally configured users.";
list user {
key name;
description
"The list of local users configured on this device.";
leaf name {
type string;
description
"The user name string identifying this entry.";
}
container public-keys {
if-feature client-auth-publickey;
presence
"Indicates that the server can authenticate this
user using any of the configured SSH public keys.";
description
"A set of SSH public keys may be used by the SSH
server to authenticate this user. A user is
authenticated if its public key is an exact
match to a configured public key.";
reference
"RFC BBBB: A YANG Data Model for a Truststore";
uses ts:local-or-truststore-public-keys-grouping {
refine "local-or-truststore/local/local-definition"
+ "/public-key" {
must 'public-key-format'
+ ' = "ct:ssh-public-key-format"';
}
refine "local-or-truststore/truststore/"
+ "truststore-reference" {
must 'deref(.)/../*/ts:public-key-format'
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+ ' = "ct:ssh-public-key-format"';
}
}
}
leaf password {
if-feature client-auth-password;
type ianach:crypt-hash;
description
"The password for this user.";
}
container hostbased {
if-feature client-auth-hostbased;
presence
"Indicates that the server can authenticate this
user's 'host' using any of the configured SSH
host keys.";
description
"A set of SSH host keys may be used by the SSH
server to authenticate this user's host. A
user's host is authenticated if its host key
is an exact match to a configured host key.";
reference
"RFC 4253: The Secure Shell (SSH) Transport Layer
RFC BBBB: A YANG Data Model for a Truststore";
uses ts:local-or-truststore-public-keys-grouping {
refine "local-or-truststore/local/local-definition"
+ "/public-key" {
must 'public-key-format'
+ ' = "ct:ssh-public-key-format"';
}
refine "local-or-truststore/truststore"
+ "/truststore-reference" {
must 'deref(.)/../*/ts:public-key-format'
+ ' = "ct:ssh-public-key-format"';
}
}
}
leaf none {
if-feature client-auth-none;
type empty;
description
"Indicates that the 'none' method is supported.";
reference
"RFC 4252: The Secure Shell (SSH) Authentication
Protocol.";
}
}
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}
container ca-certs {
if-feature "client-auth-config-supported";
if-feature "sshcmn:ssh-x509-certs";
presence
"Indicates that the SSH server can authenticate SSH
clients using configured certificate authority (CA)
certificates.";
description
"A set of certificate authority (CA) certificates used by
the SSH server to authenticate SSH client certificates.
A client certificate is authenticated if it has a valid
chain of trust to a configured CA certificate.";
reference
"RFC BBBB: A YANG Data Model for a Truststore";
uses ts:local-or-truststore-certs-grouping;
}
container ee-certs {
if-feature "client-auth-config-supported";
if-feature "sshcmn:ssh-x509-certs";
presence
"Indicates that the SSH server can authenticate SSH
clients using configured end-entity certificates.";
description
"A set of client certificates (i.e., end entity
certificates) used by the SSH server to authenticate
the certificates presented by SSH clients. A client
certificate is authenticated if it is an exact match
to a configured end-entity certificate.";
reference
"RFC BBBB: A YANG Data Model for a Truststore";
uses ts:local-or-truststore-certs-grouping;
}
} // container client-authentication
container transport-params {
nacm:default-deny-write;
if-feature "ssh-server-transport-params-config";
description
"Configurable parameters of the SSH transport layer.";
uses sshcmn:transport-params-grouping;
} // container transport-params
container keepalives {
nacm:default-deny-write;
if-feature "ssh-server-keepalives";
presence
"Indicates that the SSH server proactively tests the
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aliveness of the remote SSH client.";
description
"Configures the keep-alive policy, to proactively test
the aliveness of the SSL client. An unresponsive SSL
client is dropped after approximately max-wait *
max-attempts seconds. Per Section 4 of RFC 4254,
the SSH server SHOULD send an SSH_MSG_GLOBAL_REQUEST
message with a purposely nonexistent 'request name'
value (e.g., keepalive@ietf.org) and the 'want reply'
value set to '1'.";
reference
"RFC 4254: The Secure Shell (SSH) Connection Protocol";
leaf max-wait {
type uint16 {
range "1..max";
}
units "seconds";
default "30";
description
"Sets the amount of time in seconds after which
if no data has been received from the SSL client,
a SSL-level message will be sent to test the
aliveness of the SSL client.";
}
leaf max-attempts {
type uint8;
default "3";
description
"Sets the maximum number of sequential keep-alive
messages that can fail to obtain a response from
the SSL client before assuming the SSL client is
no longer alive.";
}
}
} // grouping ssh-server-grouping
} // module ietf-ssh-server
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" 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
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the SSH transport layer connection. The ability to restrict the
algorithms allowed is provided in this grouping for SSH clients and
SSH servers that are capable of doing so and may serve to make SSH
clients and SSH servers compliant with security policies.
Features are defined for algorithms that are OPTIONAL or are not
widely supported by popular implementations. Note that the list of
algorithms is not exhaustive. As well, some algorithms that are
REQUIRED by [RFC4253] are missing, notably "ssh-dss" and "diffie-
hellman-group1-sha1" due to their weak security and there being
alternatives that are widely supported.
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' grouping appears when populated with some data.
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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@2020-05-20.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
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"This module defines a common features, identities, and
groupings for Secure Shell (SSH).
Copyright (c) 2020 IETF Trust and the persons identified
as authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with
or without modification, is permitted pursuant to, and
subject to the license terms contained in, the Simplified
BSD License set forth in Section 4.c of the IETF Trust's
Legal Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info).
This version of this YANG module is part of RFC EEEE
(https://www.rfc-editor.org/info/rfcEEEE); see the RFC
itself for full legal notices.;
The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED',
'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document
are to be interpreted as described in BCP 14 (RFC 2119)
(RFC 8174) when, and only when, they appear in all
capitals, as shown here.";
revision 2020-05-20 {
description
"Initial version";
reference
"RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
}
// Features
feature ssh-ecc {
description
"Elliptic Curve Cryptography is supported for SSH.";
reference
"RFC 5656: Elliptic Curve Algorithm Integration in the
Secure Shell Transport Layer";
}
feature ssh-x509-certs {
description
"X.509v3 certificates are supported for SSH per RFC 6187.";
reference
"RFC 6187: X.509v3 Certificates for Secure Shell
Authentication";
}
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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:
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The Secure Shell (SSH) Transport Layer Protocol";
}
identity ecdsa-sha2-nistp256 {
if-feature "ssh-ecc and ssh-sha2";
base public-key-alg-base;
description
"Elliptic Curve Digital Signature Algorithm (ECDSA) using the
nistp256 curve and the SHA2 family of hashing algorithms.";
reference
"RFC 5656: Elliptic Curve Algorithm Integration in the
Secure Shell Transport Layer";
}
identity ecdsa-sha2-nistp384 {
if-feature "ssh-ecc and ssh-sha2";
base public-key-alg-base;
description
"Elliptic Curve Digital Signature Algorithm (ECDSA) using the
nistp384 curve and the SHA2 family of hashing algorithms.";
reference
"RFC 5656: Elliptic Curve Algorithm Integration in the
Secure Shell Transport Layer";
}
identity ecdsa-sha2-nistp521 {
if-feature "ssh-ecc and ssh-sha2";
base public-key-alg-base;
description
"Elliptic Curve Digital Signature Algorithm (ECDSA) using the
nistp521 curve and the SHA2 family of hashing algorithms.";
reference
"RFC 5656: Elliptic Curve Algorithm Integration in the
Secure Shell Transport Layer";
}
identity x509v3-ssh-rsa {
if-feature "ssh-x509-certs";
base public-key-alg-base;
description
"RSASSA-PKCS1-v1_5 signature scheme using a public key stored
in an X.509v3 certificate and using SHA-1 as the hashing
algorithm.";
reference
"RFC 6187: X.509v3 Certificates for Secure Shell
Authentication";
}
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identity x509v3-rsa2048-sha256 {
if-feature "ssh-x509-certs and ssh-sha2";
base public-key-alg-base;
description
"RSASSA-PKCS1-v1_5 signature scheme using a public key stored
in an X.509v3 certificate and using SHA-256 as the hashing
algorithm. RSA keys conveyed using this format MUST have a
modulus of at least 2048 bits.";
reference
"RFC 6187: X.509v3 Certificates for Secure Shell
Authentication";
}
identity x509v3-ecdsa-sha2-nistp256 {
if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
base public-key-alg-base;
description
"Elliptic Curve Digital Signature Algorithm (ECDSA)
using the nistp256 curve with a public key stored in
an X.509v3 certificate and using the SHA2 family of
hashing algorithms.";
reference
"RFC 6187: X.509v3 Certificates for Secure Shell
Authentication";
}
identity x509v3-ecdsa-sha2-nistp384 {
if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
base public-key-alg-base;
description
"Elliptic Curve Digital Signature Algorithm (ECDSA)
using the nistp384 curve with a public key stored in
an X.509v3 certificate and using the SHA2 family of
hashing algorithms.";
reference
"RFC 6187: X.509v3 Certificates for Secure Shell
Authentication";
}
identity x509v3-ecdsa-sha2-nistp521 {
if-feature "ssh-ecc and ssh-x509-certs and ssh-sha2";
base public-key-alg-base;
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
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"RFC 6187: X.509v3 Certificates for Secure Shell
Authentication";
}
identity key-exchange-alg-base {
description
"Base identity used to identify key exchange algorithms.";
}
identity diffie-hellman-group14-sha1 {
base key-exchange-alg-base;
description
"Diffie-Hellman key exchange with SHA-1 as HASH and
Oakley Group 14 (2048-bit MODP Group).";
reference
"RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
}
identity diffie-hellman-group-exchange-sha1 {
if-feature "ssh-dh-group-exchange";
base key-exchange-alg-base;
description
"Diffie-Hellman Group and Key Exchange with SHA-1 as HASH.";
reference
"RFC 4419: Diffie-Hellman Group Exchange for the
Secure Shell (SSH) Transport Layer Protocol";
}
identity diffie-hellman-group-exchange-sha256 {
if-feature "ssh-dh-group-exchange and ssh-sha2";
base key-exchange-alg-base;
description
"Diffie-Hellman Group and Key Exchange with SHA-256 as HASH.";
reference
"RFC 4419: Diffie-Hellman Group Exchange for the
Secure Shell (SSH) Transport Layer Protocol";
}
identity ecdh-sha2-nistp256 {
if-feature "ssh-ecc and ssh-sha2";
base key-exchange-alg-base;
description
"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";
}
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identity ecdh-sha2-nistp384 {
if-feature "ssh-ecc and ssh-sha2";
base key-exchange-alg-base;
description
"Elliptic Curve Diffie-Hellman (ECDH) key exchange using the
nistp384 curve and the SHA2 family of hashing algorithms.";
reference
"RFC 5656: Elliptic Curve Algorithm Integration in the
Secure Shell Transport Layer";
}
identity ecdh-sha2-nistp521 {
if-feature "ssh-ecc and ssh-sha2";
base key-exchange-alg-base;
description
"Elliptic Curve Diffie-Hellman (ECDH) key exchange using the
nistp521 curve and the SHA2 family of hashing algorithms.";
reference
"RFC 5656: Elliptic Curve Algorithm Integration in the
Secure Shell Transport Layer";
}
identity encryption-alg-base {
description
"Base identity used to identify encryption algorithms.";
}
identity triple-des-cbc {
base encryption-alg-base;
description
"Three-key 3DES in CBC mode.";
reference
"RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
}
identity aes128-cbc {
base encryption-alg-base;
description
"AES in CBC mode, with a 128-bit key.";
reference
"RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
}
identity aes192-cbc {
base encryption-alg-base;
description
"AES in CBC mode, with a 192-bit key.";
reference
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"RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
}
identity aes256-cbc {
base encryption-alg-base;
description
"AES in CBC mode, with a 256-bit key.";
reference
"RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
}
identity aes128-ctr {
if-feature "ssh-ctr";
base encryption-alg-base;
description
"AES in SDCTR mode, with 128-bit key.";
reference
"RFC 4344: The Secure Shell (SSH) Transport Layer Encryption
Modes";
}
identity aes192-ctr {
if-feature "ssh-ctr";
base encryption-alg-base;
description
"AES in SDCTR mode, with 192-bit key.";
reference
"RFC 4344: The Secure Shell (SSH) Transport Layer Encryption
Modes";
}
identity aes256-ctr {
if-feature "ssh-ctr";
base encryption-alg-base;
description
"AES in SDCTR mode, with 256-bit key.";
reference
"RFC 4344: The Secure Shell (SSH) Transport Layer Encryption
Modes";
}
identity mac-alg-base {
description
"Base identity used to identify message authentication
code (MAC) algorithms.";
}
identity hmac-sha1 {
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base mac-alg-base;
description
"HMAC-SHA1";
reference
"RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
}
identity hmac-sha2-256 {
if-feature "ssh-sha2";
base mac-alg-base;
description
"HMAC-SHA2-256";
reference
"RFC 6668: SHA-2 Data Integrity Verification for the
Secure Shell (SSH) Transport Layer Protocol";
}
identity hmac-sha2-512 {
if-feature "ssh-sha2";
base mac-alg-base;
description
"HMAC-SHA2-512";
reference
"RFC 6668: SHA-2 Data Integrity Verification for the
Secure Shell (SSH) Transport Layer Protocol";
}
// Groupings
grouping transport-params-grouping {
description
"A reusable grouping for SSH transport parameters.";
reference
"RFC 4253: The Secure Shell (SSH) Transport Layer Protocol";
container host-key {
description
"Parameters regarding host key.";
leaf-list host-key-alg {
type identityref {
base public-key-alg-base;
}
ordered-by user;
description
"Acceptable host key algorithms in order of descending
preference. The configured host key algorithms should
be compatible with the algorithm used by the configured
private key. Please see Section 5 of RFC EEEE for
valid combinations.
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If this leaf-list is not configured (has zero elements)
the acceptable host key algorithms are implementation-
defined.";
reference
"RFC EEEE: YANG Groupings for SSH Clients and SSH Servers";
}
}
container key-exchange {
description
"Parameters regarding key exchange.";
leaf-list key-exchange-alg {
type identityref {
base key-exchange-alg-base;
}
ordered-by user;
description
"Acceptable key exchange algorithms in order of descending
preference.
If this leaf-list is not configured (has zero elements)
the acceptable key exchange algorithms are implementation
defined.";
}
}
container encryption {
description
"Parameters regarding encryption.";
leaf-list encryption-alg {
type identityref {
base encryption-alg-base;
}
ordered-by user;
description
"Acceptable encryption algorithms in order of descending
preference.
If this leaf-list is not configured (has zero elements)
the acceptable encryption algorithms are implementation
defined.";
}
}
container mac {
description
"Parameters regarding message authentication code (MAC).";
leaf-list mac-alg {
type identityref {
base mac-alg-base;
}
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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".
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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:
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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 EEEE
name: ietf-ssh-server
namespace: urn:ietf:params:xml:ns:yang:ietf-ssh-server
prefix: sshs
reference: RFC EEEE
name: ietf-ssh-common
namespace: urn:ietf:params:xml:ns:yang:ietf-ssh-common
prefix: sshcmn
reference: RFC EEEE
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-14 (work in
progress), March 2020.
[I-D.ietf-netconf-keystore]
Watsen, K., "A YANG Data Model for a Keystore", draft-
ietf-netconf-keystore-16 (work in progress), March 2020.
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[I-D.ietf-netconf-trust-anchors]
Watsen, K., "A YANG Data Model for a Truststore", draft-
ietf-netconf-trust-anchors-09 (work in progress), March
2020.
[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, .
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[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,
.
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[RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
.
8.3. URIs
[1] https://tools.ietf.org/html/draft-ietf-netconf-crypto-types
[2] https://tools.ietf.org/html/draft-ietf-netconf-trust-anchors
[3] https://tools.ietf.org/html/draft-ietf-netconf-keystore
[4] https://tools.ietf.org/html/draft-ietf-netconf-tcp-client-server
[5] https://tools.ietf.org/html/draft-ietf-netconf-ssh-client-server
[6] https://tools.ietf.org/html/draft-ietf-netconf-tls-client-server
[7] https://tools.ietf.org/html/draft-ietf-netconf-http-client-server
[8] https://tools.ietf.org/html/draft-ietf-netconf-netconf-client-
server
[9] https://tools.ietf.org/html/draft-ietf-netconf-restconf-client-
server
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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.
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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.
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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.//)
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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".
A.16. 15 to 16
o Removed unnecessary if-feature statements in the -client and
-server modules.
o Cleaned up some description statements in the -client and -server
modules.
o Fixed a canonical ordering issue in ietf-ssh-common detected by
new pyang.
A.17. 16 to 17
o Removed choice local-or-external by removing the 'external' case
and flattening the 'local' case and adding a "client-auth-config-
supported" feature.
o Updated examples to include the "*-key-format" nodes.
o Augmented-in "must" expressions ensuring that locally-defined
public-key-format are "ct:ssh-public-key-format" (must expr for
ref'ed keys are TBD).
A.18. 17 to 18
o Removed leaf-list 'other' from ietf-ssh-server.
o Removed unused 'external-client-auth-supported' feature.
o Added features client-auth-password, client-auth-hostbased, and
client-auth-none.
o Renamed 'host-key' to 'public-key' for when refering to
'publickey' based auth.
o Added new feature-protected 'hostbased' and 'none' to the 'user'
node's config.
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o Added new feature-protected 'hostbased' and 'none' to the 'client-
identity' node's config.
o Updated examples to reflect new "bag" addition to truststore.
o Refined truststore/keystore groupings to ensure the key formats
"must" be particular values.
o Switched to using truststore's new "public-key" bag (instead of
separate "ssh-public-key" and "raw-public-key" bags.
o Updated client/server examples to cover ALL cases (local/ref x
cert/raw-key/psk).
A.19. 18 to 19
o Updated the "keepalives" containers to address Michal Vasko's
request to align with RFC 8071.
o Removed algorithm-mapping tables from the "SSH Common Model"
section
o Removed 'algorithm' node from examples.
o Added feature "client-identity-publickey"
o Removed "choice auth-type", as auth-types aren't exclusive.
o Renamed both "client-certs" and "server-certs" to "ee-certs"
o Switch "must" to assert the public-key-format is "subject-public-
key-info-format" when certificates are used.
o Added a "Note to Reviewers" note to first page.
Acknowledgements
The authors would like to thank for following for lively discussions
on list and in the halls (ordered by last name): Andy Bierman, Martin
Bjorklund, Benoit Claise, Mehmet Ersue, Balazs Kovacs, Radek Krejci,
David Lamparter, Ladislav Lhotka, Alan Luchuk, Tom Petch, Juergen
Schoenwaelder, Phil Shafer, Sean Turner, Michal Vasko, Bert Wijnen,
and Liang Xia.
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Authors' Addresses
Kent Watsen
Watsen Networks
EMail: kent+ietf@watsen.net
Gary Wu
Cisco Systems
EMail: garywu@cisco.com
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