Network Working Group
Request for Comments: 4742
Category: Standards Track
M. Wasserman
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 © The IETF Trust (2006).


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

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 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 -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:".

   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.

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="">
   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="">
   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: ]]>]]>

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 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.

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.

Authors' Addresses

   Margaret Wasserman
   One Broadway, 5th Floor
   Cambridge, MA  02142
   Phone: +1 781 405-7464
   Ted Goddard
   ICEsoft Technologies, Inc.
   Suite 300, 1717 10th St. NW
   Calgary, AB  T2M 4S2
   Phone: +1 403 663-3322

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Copyright © The IETF Trust (2006).

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