NETCONF over SSH: Using the NETCONF Protocol over Secure Shell (SSH)

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.

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
   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.

   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.

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.

   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.

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: ]]>]]>

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.

   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.

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.

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