Network Working Group M. Wasserman

Network Working Group M. Wasserman
Request for Comments: 4742 ThingMagic
Category: Standards Track T. Goddard
ICEsoft Technologies, Inc.
December 2006

Using the NETCONF Configuration Protocol over Secure SHell (SSH)

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The IETF Trust (2006).

Abstract

This document describes a method for invoking and running the Network
Configuration Protocol (NETCONF) within a Secure Shell (SSH) session
as an SSH subsystem.

Table of Contents

1. Introduction ....................................................2
2. Requirements Terminology ........................................2
3. Starting NETCONF over SSH .......................................2
3.1. Capabilities Exchange ......................................3
4. Using NETCONF over SSH ..........................................5
5. Exiting the NETCONF Subsystem ...................................6
6. Security Considerations .........................................6
7. IANA Considerations .............................................7
8. Acknowledgements ................................................7
9. References ......................................................8
9.1. Normative References .......................................8
9.2. Informative References .....................................8

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RFC 4742 NETCONF over SSH December 2006

1. Introduction

The NETCONF protocol [RFC4721] is an XML-based protocol used to
manage the configuration of networking equipment. NETCONF is defined
to be session-layer and transport independent, allowing mappings to
be defined for multiple session-layer or transport protocols. This
document defines how NETCONF can be used within a Secure Shell (SSH)
session, using the SSH connection protocol [RFC4254] over the SSH
transport protocol [RFC4253]. This mapping will allow NETCONF to be
executed from a secure shell session by a user or application.

Throughout this document, the terms "client" and "server" are used to
refer to the two ends of the SSH transport connection. The client
actively opens the SSH connection, and the server passively listens
for the incoming SSH connection. The terms "manager" and "agent" are
used to refer to the two ends of the NETCONF protocol session. The
manager issues NETCONF remote procedure call (RPC) commands, and the
agent replies to those commands. When NETCONF is run over SSH using
the mapping defined in this document, the client is always the
manager, and the server is always the agent.

2. Requirements Terminology

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

3. Starting NETCONF over SSH

To run NETCONF over SSH, the client will first establish an SSH
transport connection using the SSH transport protocol, and the client
and server will exchange keys for message integrity and encryption.
The client will then invoke the "ssh-userauth" service to
authenticate the user, as described in the SSH authentication
protocol [RFC4252]. Once the user has been successfully
authenticated, the client will invoke the "ssh-connection" service,
also known as the SSH connection protocol.

After the ssh-connection service is established, the client will open
a channel of type "session", which will result in an SSH session.

Once the SSH session has been established, the user (or application)
will invoke NETCONF as an SSH subsystem called "netconf". Subsystem
support is a feature of SSH version 2 (SSHv2) and is not included in
SSHv1. Running NETCONF as an SSH subsystem avoids the need for the
script to recognize shell prompts or skip over extraneous
information, such as a system message that is sent at shell start-up.
However, even when a subsystem is used, some extraneous messages may

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RFC 4742 NETCONF over SSH December 2006

be printed by the user's start-up scripts. Implementations MUST skip
over these messages by searching for an 'xml' start directive, which
MUST be followed by a <hello> element in the 'NETCONF' namespace.

In order to allow NETCONF traffic to be easily identified and
filtered by firewalls and other network devices, NETCONF servers MUST
default to providing access to the "netconf" SSH subsystem only when
the SSH session is established using the IANA-assigned TCP port
<830>. Servers SHOULD be configurable to allow access to the netconf
SSH subsystem over other ports.

A user (or application) could use the following command line to
invoke NETCONF as an SSH subsystem on the IANA-assigned port:

[user@client]$ ssh -s server.example.org -p <830> netconf

Note that the -s option causes the command ("netconf") to be invoked
as an SSH subsystem.

3.1. Capabilities Exchange

The server MUST indicate its capabilities by sending an XML document
containing a <hello> element as soon as the NETCONF session is
established. The user (or application) can parse this message to
determine which NETCONF capabilities are supported by the server.

The client must also send an XML document containing a <hello>
element to indicate the client's capabilities to the server. The
document containing the <hello> element MUST be the first XML
document that the client sends after the NETCONF session is
established.

The following example shows a capability exchange. Messages sent by
the client are marked with "C:", and messages sent by the server are
marked with "S:".

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RFC 4742 NETCONF over SSH December 2006

S: <?xml version="1.0" encoding="UTF-8"?>
S: <hello>
S: <capabilities>
S: <capability>
S: urn:ietf:params:xml:ns:netconf:base:1.0
S: </capability>
S: <capability>
S: urn:ietf:params:ns:netconf:capability:startup:1.0
S: </capability>
S: </capabilities>
S: <session-id>4<session-id>
S: </hello>
S: ]]>]]>

C: <?xml version="1.0" encoding="UTF-8"?>
C: <hello>
C: <capabilities>
C: <capability>
C: urn:ietf:params:xml:ns:netconf:base:1.0
C: </capability>
C: </capabilities>
C: </hello>
C: ]]>]]>

Although the example shows the server sending a <hello> message
followed by the client's message, both sides will send the message as
soon as the NETCONF subsystem is initialized, perhaps simultaneously.

As the previous example illustrates, a special character sequence,
]]>]]>, MUST be sent by both the client and the server after each XML
document in the NETCONF exchange. This character sequence cannot
legally appear in an XML document, so it can be unambiguously used to
identify the end of the current document, allowing resynchronization
of the NETCONF exchange in the event of an XML syntax or parsing
error.

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RFC 4742 NETCONF over SSH December 2006

4. Using NETCONF over SSH

A NETCONF over SSH session consists of the manager and agent
exchanging complete XML documents. Once the session has been
established and capabilities have been exchanged, the manager will
send complete XML documents containing <rpc> elements to the server,
and the agent will respond with complete XML documents containing
<rpc-reply> elements.

To continue the example given above, an NETCONF over SSH session to
retrieve a set of configuration information might look like this:

C: <?xml version="1.0" encoding="UTF-8"?>
C: <rpc message-id="105"
C: xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
C: <get-config>
C: <source><running/></source>
C: <config xmlns="http://example.com/schema/1.2/config">
C: <users/>
C: </config>
C: </get-config>
C: </rpc>
C: ]]>]]>

S: <?xml version="1.0" encoding="UTF-8"?>
S: <rpc-reply message-id="105"
S: xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
S: <config xmlns="http://example.com/schema/1.2/config">
S: <users>
S: <user><name>root</name><type>superuser</type></user>
S: <user><name>fred</name><type>admin</type></user>
S: <user><name>barney</name><type>admin</type></user>
S: </users>
S: </config>
S: </rpc-reply>
S: ]]>]]>

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RFC 4742 NETCONF over SSH December 2006

5. Exiting the NETCONF Subsystem

Exiting NETCONF is accomplished using the <close-session> operation.
An agent will process RPC messages from the manager in the order in
which they are received. When the agent processes a <close-session>
command, the agent shall respond and close the SSH session channel.
The agent MUST NOT process any RPC commands received on the current
session after the <close-session> command.

To continue the example used in previous sections, an existing
NETCONF subsystem session could be closed as follows:

C: <?xml version="1.0" encoding="UTF-8"?>
C: <rpc message-id="106"
C: xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
C: <close-session/>
C: </rpc>
C: ]]>]]>

S: <?xml version="1.0" encoding="UTF-8"?>
S: <rpc-reply id="106"
S: xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
S: <ok/>
S: </rpc-reply>
S: ]]>]]>

6. Security Considerations

NETCONF is used to access configuration and state information and to
modify configuration information, so the ability to access this
protocol should be limited to users and systems that are authorized
to view the agent's configuration and state or to modify the agent's
configuration.

The identity of the server MUST be verified and authenticated by the
client according to local policy before password-based authentication
data or any configuration or state data is sent to or received from
the server. The identity of the client MUST also be verified and
authenticated by the server according to local policy to ensure that
the incoming client request is legitimate before any configuration or
state data is sent to or received from the client. Neither side
should establish a NETCONF over SSH connection with an unknown,
unexpected, or incorrect identity on the opposite side.

Configuration or state data may include sensitive information, such
as usernames or security keys. So, NETCONF should only be used over
communications channels that provide strong encryption for data

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RFC 4742 NETCONF over SSH December 2006

privacy. This document defines a NETCONF over SSH mapping that
provides for support of strong encryption and authentication.

This document requires that servers default to allowing access to the
"netconf" SSH subsystem only when using a specific TCP port assigned
by IANA for this purpose. This will allow NETCONF over SSH traffic
to be easily identified and filtered by firewalls and other network
nodes. However, it will also allow NETCONF over SSH traffic to be
more easily identified by attackers.

This document also recommends that servers be configurable to allow
access to the "netconf" SSH subsystem over other ports. Use of that
configuration option without corresponding changes to firewall or
network device configuration may unintentionally result in the
ability for nodes outside the firewall or other administrative
boundary to gain access to "netconf" SSH subsystem.

7. IANA Considerations

IANA assigned a TCP port number that is the default port for NETCONF
over SSH sessions as defined in this document.

IANA assigned port <830> for this purpose.

IANA is also requested to assign "netconf" as an SSH Service Name as
defined in [RFC4250], as follows:

Service Name Reference
------------- ---------
netconf RFC 4742

8. Acknowledgements

This document was written using the xml2rfc tool described in RFC
2629 [RFC2629].

Extensive input was received from the other members of the NETCONF
design team, including: Andy Bierman, Weijing Chen, Rob Enns, Wes
Hardaker, David Harrington, Eliot Lear, Simon Leinen, Phil Shafer,
Juergen Schoenwaelder, and Steve Waldbusser. The following people
have also reviewed this document and provided valuable input: Olafur
Gudmundsson, Sam Hartman, Scott Hollenbeck, Bill Sommerfeld, and Bert
Wijnen.

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RFC 4742 NETCONF over SSH December 2006

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC4250] Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH)
Protocol Assigned Numbers", RFC 4250, January 2006.

[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Authentication Protocol", RFC 4252, January 2006.

[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, January 2006.

[RFC4254] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Connection Protocol", RFC 4254, January 2006.

[RFC4721] Enns, R., Ed., "NETCONF Configuration Protocol", RFC 4721,
December 2006.

9.2. Informative References

[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
June 1999.

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RFC 4742 NETCONF over SSH December 2006

Authors' Addresses

Margaret Wasserman
ThingMagic
One Broadway, 5th Floor
Cambridge, MA 02142
USA

Phone: +1 781 405-7464
EMail: [email protected]
URI: http://www.thingmagic.com

Ted Goddard
ICEsoft Technologies, Inc.
Suite 300, 1717 10th St. NW
Calgary, AB T2M 4S2
Canada

Phone: +1 403 663-3322
EMail: [email protected]
URI: http://www.icesoft.com

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RFC 4742 NETCONF over SSH December 2006

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