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Versions: (RFC 4742) 00 01 02 03 04 05 06 07
08 RFC 6242
Network Working Group M. Wasserman
Internet-Draft Painless Security, LLC
Obsoletes: 4742 (if approved) T. Goddard
Intended status: Standards Track ICEsoft Technologies, Inc.
Expires: August 27, 2011 February 23, 2011
Using the NETCONF Configuration Protocol over Secure Shell (SSH)
draft-ietf-netconf-rfc4742bis-07.txt
Abstract
This document describes a method for invoking and running the NETCONF
protocol within a Secure Shell (SSH) session as an SSH subsystem.
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
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This Internet-Draft will expire on August 27, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 3
3. Starting NETCONF over SSH . . . . . . . . . . . . . . . . . . 3
3.1. Capabilities Exchange . . . . . . . . . . . . . . . . . . 4
4. Using NETCONF over SSH . . . . . . . . . . . . . . . . . . . . 5
4.1. Framing protocol . . . . . . . . . . . . . . . . . . . . . 5
4.2. Chunked Framing Mechanism . . . . . . . . . . . . . . . . 6
4.3. End-of-message Framing Mechanism . . . . . . . . . . . . . 7
5. Exiting the NETCONF Subsystem . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Changes from RFC4742bis-05 to RFC4742-bis-06 . . . . . . . 11
9.2. Changes from RFC4742bis-04 to RFC4742-bis-05 . . . . . . . 11
9.3. Changes from RFC4742bis-03 to RFC4742-bis-04 . . . . . . . 11
9.4. Changes from RFC4742bis-02 to RFC4742-bis-03 . . . . . . . 11
9.5. Changes from RFC4742bis-01 to RFC4742-bis-02 . . . . . . . 11
9.6. Changes from RFC4742bis-00 to RFC4742-bis-01 . . . . . . . 12
9.7. Changes from RFC4742 to RFC4742bis-00 . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Changes from RFC 4742 . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The NETCONF protocol [I-D.ietf-netconf-4741bis] 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.
How the NETCONF Server extracts the SSH user name from the SSH layer
is implementation-dependent.
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
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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
The NETCONF server MUST indicate 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.
The NETCONF client must also send an XML document containing a
<hello> element to indicate the NETCONF client's capabilities to the
NETCONF server. The document containing the <hello> element MUST be
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:".
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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:xml:ns: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:xml:ns: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.
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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 transfer 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):
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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.
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 an error occurs during the decoding process, the peer MUST
terminate the NETCONF session by closing the corresponding SSH
channel.
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:
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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:
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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 should only be used over
communications channels that provide strong encryption for data
privacy. This document defines a NETCONF over SSH mapping which
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
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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
boundary to gain access to "netconf" SSH subsystem.
RFC 4742 assumes that the 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 port 830 as the default port for NETCONF over SSH sessions.
IANA has also assigned "netconf" as an SSH Subsystem Name, as defined
in [RFC4250], as follows:
Subsystem Name Reference
-------------- ---------
netconf RFC 4742
IANA is requested to update these allocations to reference this
document when it is published as an RFC.
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
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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,
Ken Watson, and Tom Petch.
9. Change Log
[RFC Editor: Please remove this section before publication as an
RFC.]
9.1. Changes from RFC4742bis-05 to RFC4742-bis-06
o Updated chunked encoding to include location of new lines.
o Updated examples to show new chunked encoding properly.
o Minor editorial fixes.
9.2. Changes from RFC4742bis-04 to RFC4742-bis-05
o Added option for chunked encoding.
o Added 1.1 capability to enable chunked encoding.
9.3. Changes from RFC4742bis-03 to RFC4742-bis-04
o Shortened text about EOM sequence.
o Added text to Security Considerations about EOM issues.
9.4. Changes from RFC4742bis-02 to RFC4742-bis-03
o Added intended status and "obsoletes" to headers.
o Very minor editorial changes.
9.5. Changes from RFC4742bis-01 to RFC4742-bis-02
o Removed unneeded wording about client/server, made unnecessary by
previous changes.
o Stated that how a server extracts the SSH user name is
implementation-dependent.
o Further fixes to operation/message/data wording.
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9.6. Changes from RFC4742bis-00 to RFC4742-bis-01
o Changed use of client/server, manager/agent to SSH client/server
and NETCONF client/server.
o Consistently used term operation, instead of command or message.
o Clarified some sections based on review feedback.
o Fixed several typos.
9.7. Changes from RFC4742 to RFC4742bis-00
o Integrated previously-approved errata from
http://rfc-editor.org/errata_search.php?rfc=4742
o Removed text requiring implementations to skip to an XML start
directive at the beginning of a session.
o Made it clear the ]]>]]> is illegal in XML documents only outside
of comments.
10. References
10.1. Normative References
[I-D.ietf-netconf-4741bis]
Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "Network Configuration Protocol (NETCONF)",
draft-ietf-netconf-4741bis-09 (work in progress),
February 2011.
[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.
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[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
10.2. 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 Stated that the extraction of the SSH user name by a NETCONF
server is implementation-dependent.
o Changed use of the terms client/server, manager/agent to SSH
client/server and NETCONF client/server.
o Consistently used the term operation, instead of command or
message.
o Integrated previously-approved errata from
http://rfc-editor.org/errata_search.php?rfc=4742
Authors' Addresses
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
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Ted Goddard
ICEsoft Technologies, Inc.
Suite 300, 1717 10th St. NW
Calgary, AB T2M 4S2
Canada
Phone: +1 403 663-3322
Email: ted.goddard@icesoft.com
URI: http://www.icesoft.com
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