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rfc:rfc6242

Internet Engineering Task Force (IETF) M. Wasserman Request for Comments: 6242 Painless Security, LLC Obsoletes: 4742 June 2011 Category: Standards Track ISSN: 2070-1721

         Using the NETCONF Protocol over Secure Shell (SSH)

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.  This document obsoletes RFC 4742.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6242.

Copyright Notice

 Copyright (c) 2011 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
 (http://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.

Wasserman Standards Track [Page 1] RFC 6242 NETCONF over SSH June 2011

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 . . . . . . . . . . . . . . . . . . . .  4
   4.1.  Framing Protocol . . . . . . . . . . . . . . . . . . . . .  5
   4.2.  Chunked Framing Mechanism  . . . . . . . . . . . . . . . .  5
   4.3.  End-of-Message Framing Mechanism . . . . . . . . . . . . .  7
 5.  Exiting the NETCONF Subsystem  . . . . . . . . . . . . . . . .  8
 6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
 7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
 8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
 9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   9.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
   9.2.  Informative References . . . . . . . . . . . . . . . . . . 10
 Appendix A.  Changes from RFC 4742 . . . . . . . . . . . . . . . . 11

1. Introduction

 The NETCONF protocol [RFC6241] 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.
 Although this document gives specific examples of how NETCONF
 messages are sent over an SSH connection, use of this transport is
 not restricted to the messages shown in the examples below.  This
 transport can be used for any NETCONF message.

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 SSH client will first establish an SSH
 transport connection using the SSH transport protocol, and the SSH
 client and SSH server will exchange keys for message integrity and
 encryption.  The SSH client will then invoke the "ssh-userauth"
 service to authenticate the user, as described in the SSH

Wasserman Standards Track [Page 2] RFC 6242 NETCONF over SSH June 2011

 authentication protocol [RFC4252].  Once the user has been
 successfully authenticated, the SSH client will invoke the
 "ssh-connection" service, also known as the SSH connection protocol.
 The username provided by the SSH implementation will be made
 available to the NETCONF message layer as the NETCONF username
 without modification.  If the username does not comply to the NETCONF
 requirements on usernames [RFC6241], i.e., the username is not
 representable in XML, the SSH session MUST be dropped.  Any
 transformations applied to the authenticated identity of the SSH
 client made by the SSH server (e.g., via authentication services or
 mappings to system accounts) are outside the scope of this document.
 After the ssh-connection service is established, the SSH client will
 open a channel of type "session", which will result in an SSH
 session.
 Once the SSH session has been established, the NETCONF client 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.
 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

 As specified in [RFC6241], the NETCONF server indicates its
 capabilities by sending an XML document containing a <hello> element
 as soon as the NETCONF session is established.  The NETCONF client
 can parse this message to determine which NETCONF capabilities are
 supported by the NETCONF server.

Wasserman Standards Track [Page 3] RFC 6242 NETCONF over SSH June 2011

 As [RFC6241] states, the NETCONF client also sends an XML document
 containing a <hello> element to indicate the NETCONF client's
 capabilities to the NETCONF server.  The document containing the
 <hello> element is the first XML document that the NETCONF client
 sends after the NETCONF session is established.
 The following example shows a capability exchange.  Data sent by the
 NETCONF client are marked with "C:", and data sent by the NETCONF
 server are marked with "S:".
 S: <?xml version="1.0" encoding="UTF-8"?>
 S: <hello xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
 S:   <capabilities>
 S:     <capability>
 S:       urn:ietf:params:netconf:base:1.1
 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 xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
 C:   <capabilities>
 C:     <capability>
 C:       urn:ietf:params:netconf:base:1.1
 C:     </capability>
 C:   </capabilities>
 C: </hello>
 C: ]]>]]>
 Although the example shows the NETCONF server sending a <hello>
 message followed by the NETCONF client's <hello> message, both sides
 will send the message as soon as the NETCONF subsystem is
 initialized, perhaps simultaneously.

4. Using NETCONF over SSH

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

Wasserman Standards Track [Page 4] RFC 6242 NETCONF over SSH June 2011

4.1. Framing Protocol

 The previous version of this document defined the character sequence
 "]]>]]>" as a message separator, under the assumption that it could
 not be found in well-formed XML documents.  However, this assumption
 is not correct.  It can legally appear in XML attributes, comments,
 and processing instructions.  In order to solve this problem, and at
 the same time be compatible with existing implementations, this
 document defines the following framing protocol.
 The <hello> message MUST be followed by the character sequence
 ]]>]]>.  Upon reception of the <hello> message, the receiving peer's
 SSH Transport layer conceptually passes the <hello> message to the
 Messages layer.  If the :base:1.1 capability is advertised by both
 peers, the chunked framing mechanism (see Section 4.2) is used for
 the remainder of the NETCONF session.  Otherwise, the old end-of-
 message-based mechanism (see Section 4.3) is used.

4.2. Chunked Framing Mechanism

 This mechanism encodes all NETCONF messages with a chunked framing.
 Specifically, the message follows the ABNF [RFC5234] rule Chunked-
 Message:
      Chunked-Message = 1*chunk
                        end-of-chunks
      chunk           = LF HASH chunk-size LF
                        chunk-data
      chunk-size      = 1*DIGIT1 0*DIGIT
      chunk-data      = 1*OCTET
      end-of-chunks   = LF HASH HASH LF
      DIGIT1          = %x31-39
      DIGIT           = %x30-39
      HASH            = %x23
      LF              = %x0A
      OCTET           = %x00-FF
 The chunk-size field is a string of decimal digits indicating the
 number of octets in chunk-data.  Leading zeros are prohibited, and
 the maximum allowed chunk-size value is 4294967295.

Wasserman Standards Track [Page 5] RFC 6242 NETCONF over SSH June 2011

 As an example, the message:
     <rpc message-id="102"
          xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
       <close-session/>
     </rpc>
 could be encoded as (using '\n' as a visible representation of the
 LineFeed character):
 C:  \n#4\n
 C:  <rpc
 C:  \n#18\n
 C:   message-id="102"\n
 C:  \n#79\n
 C:       xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">\n
 C:    <close-session/>\n
 C:  </rpc>
 C:  \n##\n
 Conceptually, the SSH Transport layer encodes messages sent by the
 Messages layer, and decodes messages received on the SSH channel
 before passing them to the Messages layer.
 The examples for the chunked framing mechanism show all LineFeeds,
 even those that are not used as part of the framing mechanism.  Note
 that the SSH transport does not interpret the XML content; thus, it
 does not care about any optional XML-specific LineFeeds.
 In the second and third chunks quoted above, each line is terminated
 by a LineFeed.  For all the XML lines (except the last one), this
 example treats the LineFeed as part of the chunk-data and so
 contributing to the chunk-size.
 Note that there is no LineFeed character after the <rpc> end tag in
 this message.  The LineFeed required by the start of the end-of-
 chunks block immediately follows the last '>' character in the
 message.
 If the chunk-size and the chunk-size value respectively are invalid
 or if an error occurs during the decoding process, the peer MUST
 terminate the NETCONF session by closing the corresponding SSH
 channel.  Implementations MUST ensure they are not vulnerable for a
 buffer overrun.

Wasserman Standards Track [Page 6] RFC 6242 NETCONF over SSH June 2011

4.3. End-of-Message Framing Mechanism

 This mechanism exists for backwards compatibility with
 implementations of previous versions of this document.  It is only
 used when the remote peer does not advertise a base protocol version
 supporting chunked encoding, i.e., a NETCONF implementation only
 supporting :base:1.0.
 When this mechanism is used, the special character sequence ]]>]]>,
 MUST be sent by both the NETCONF client and the NETCONF server after
 each message (XML document) in the NETCONF exchange.  Conceptually,
 the SSH Transport layer passes any data found in between the ]]>]]>
 characters to the Messages layer.
 A NETCONF over SSH session, using the backwards-compatible end-of-
 message framing 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: ]]>]]>

Wasserman Standards Track [Page 7] RFC 6242 NETCONF over SSH June 2011

5. Exiting the NETCONF Subsystem

 Exiting NETCONF is accomplished using the <close-session> operation.
 A NETCONF server will process NETCONF messages from the NETCONF
 client in the order in which they are received.  When the NETCONF
 server processes a <close-session> operation, the NETCONF server
 SHALL respond and close the SSH session channel.  The NETCONF server
 MUST NOT process any NETCONF messages received after the
 <close-session> operation.
 To continue the example used in Section 4.2, an existing NETCONF
 subsystem session could be closed as follows:
 C: \n#140\n
 C: <?xml version="1.0" encoding="UTF-8"?>\n
 C: <rpc message-id="106"\n
 C:      xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">\n
 C:   <close-session/>\n
 C: </rpc>
 C: \n##\n
 S: \n#139\n
 S: <?xml version="1.0" encoding="UTF-8"?>\n
 S: <rpc-reply id="106"\n
 S:            xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">\n
 S:   <ok/>\n
 S: </rpc-reply>
 S: \n##\n

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

Wasserman Standards Track [Page 8] RFC 6242 NETCONF over SSH June 2011

 Configuration or state data may include sensitive information, such
 as usernames or security keys.  So, NETCONF requires 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 SSH 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 SSH 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 of the firewall or other administrative
 boundaries to gain access to the "netconf" SSH subsystem.
 RFC 4742 assumes that the end-of-message (EOM) sequence, ]]>]]>,
 cannot appear in any well-formed XML document, which turned out to be
 mistaken.  The EOM sequence can cause operational problems and open
 space for attacks if sent deliberately in RPC messages.  It is
 however believed that the associated threat is not very high.  This
 document still uses the EOM sequence for the initial <hello> message
 to avoid incompatibility with existing implementations.  When both
 peers implement base:1.1 capability, a proper framing protocol
 (chunked framing mechanism; see Section 4.2) is used for the rest of
 the NETCONF session, to avoid injection attacks.

7. IANA Considerations

 Based on the previous version of this document, RFC 4742, IANA
 assigned the TCP port 830 as the default port for NETCONF over SSH
 sessions.
 IANA had also assigned "netconf" as an SSH Subsystem Name, as defined
 in [RFC4250], as follows:
            Subsystem Name                  Reference
            --------------                  ---------
            netconf                         RFC 4742
 IANA updated these allocations to refer to this document.

Wasserman Standards Track [Page 9] RFC 6242 NETCONF over SSH June 2011

8. Acknowledgements

 Ted Goddard was a co-author on earlier versions of this document.
 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, Balazs
 Lengyel, Bert Wijnen, Mehmet Ersue, Martin Bjorklund, Lada Lothka,
 Kent Watsen, and Tom Petch.

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.
 [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
            Specifications: ABNF", STD 68, RFC 5234, January 2008.
 [RFC6241]  Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
            and A.  Bierman, Ed., "Network Configuration Protocol
            (NETCONF)", RFC 6241, June 2011.

9.2. Informative References

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

Wasserman Standards Track [Page 10] RFC 6242 NETCONF over SSH June 2011

Appendix A. Changes from RFC 4742

 This section lists major changes between this document and RFC 4742.
 o  Introduced the new chunked framing mechanism to solve known
    security issues with the EOM framing.
 o  Extended text in Security Considerations; added text on EOM
    issues.
 o  Added examples to show new chunked encoding properly; highlighted
    the location of new lines.
 o  Added text for NETCONF username handling following the
    requirements on usernames in [RFC6241].
 o  Changed use of the terms "client/server" and "manager/agent" to
    "SSH client/server" and "NETCONF client/server".
 o  Consistently used the term "operation", instead of "command" or
    "message".
 o  Integrated errata verified for RFC 4742 as of the date of
    publication of this document.  See errata for RFC 4742 at
    http://www.rfc-editor.org.

Author's Address

 Margaret Wasserman
 Painless Security, LLC
 356 Abbott Street
 North Andover, MA  01845
 USA
 Phone: +1 781 405-7464
 EMail: mrw@painless-security.com
 URI:   http://www.painless-security.com

Wasserman Standards Track [Page 11]

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