Internet DRAFT - draft-ietf-netconf-reverse-ssh
draft-ietf-netconf-reverse-ssh
NETCONF Working Group K. Watsen
Internet-Draft Juniper Networks
Updates: 4253 (if approved) July 2014
Intended status: Standards Track
Expires: January 02, 2015
NETCONF Call Home using SSH
draft-ietf-netconf-reverse-ssh-06
Abstract
This document presents a technique for a NETCONF server to request
that a NETCONF client initiates a SSH connection to the NETCONF
server, a technique referred to as 'call home'. Call home is needed
to support deployments where the NETCONF client is otherwise unable
to initiate a SSH connection to the NETCONF server directly.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 02, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
Watsen Expires January 02, 2015 [Page 1]
Internet-Draft NETCONF Call Home using SSH July 2014
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. Requirements Terminology . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2.1. Applicability Statement . . . . . . . . . . . . . . . . . 3
2.2. Update to RFC 4253 . . . . . . . . . . . . . . . . . . . 3
2.3. Draft Naming . . . . . . . . . . . . . . . . . . . . . . 3
3. Benefits to Device Management . . . . . . . . . . . . . . . . 3
4. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. SSH Server Identification and Verification . . . . . . . . . 5
6. Device Configuration . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 9
A.1. 05 to 06 . . . . . . . . . . . . . . . . . . . . . . . . 9
A.2. 04 to 05 . . . . . . . . . . . . . . . . . . . . . . . . 10
A.3. 03 to 04 . . . . . . . . . . . . . . . . . . . . . . . . 10
A.4. 02 to 03 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.5. 01 to 02 . . . . . . . . . . . . . . . . . . . . . . . . 11
A.6. 00 to 01 . . . . . . . . . . . . . . . . . . . . . . . . 11
1. 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].
2. Introduction
Watsen Expires January 02, 2015 [Page 2]
Internet-Draft NETCONF Call Home using SSH July 2014
This document presents a technique for a NETCONF server to request
that a NETCONF [RFC6241] client initiates a SSH [RFC4251] connection
to the NETCONF server, a technique referred to as 'call home'. Call
home is needed to support deployments where the NETCONF client is
otherwise unable to initiate a SSH connection to the NETCONF server
directly.
2.1. Applicability Statement
The techniques described in this document are suitable for network
management scenarios such as the ones described in section 3.
However, these techniques SHOULD only be used for a NETCONF server to
initiate a connection to a NETCONF client, as described in this
document.
The reason for this restriction is that different protocols have
different security assumptions. The NETCONF over SSH specification
requires NETCONF clients and servers to verify the identity of the
other party before starting the NETCONF protocol (section 6 of
[RFC6242]). This contrasts with the base SSH protocol, which does
not require programmatic verification of the other party (section
9.3.4 of [RFC4251] and section 4 of [RFC4252]). In such
circumstances, allowing the SSH server to contact the SSH client
would open new vulnerabilities. Therefore, any use of call home with
SSH for purposes other than NETCONF will need a thorough, contextual
security analysis.
2.2. Update to RFC 4253
This document updates the SSH Transport Layer Protocol [RFC4253] only
by removing the restriction in Section 4 (Connection Setup) of
[RFC4252] that the SSH Client must initiate the transport connection.
Security implications related to this change are discussed in
Security Considerations (Section 7).
2.3. Draft Naming
(this section should be removed if this draft becomes an RFC)
This draft's name includes the string "reverse-ssh", and yet
currently nowhere in this draft is there any reference to reversing
SSH. This appearant ommision comes from the -05 edit of this draft,
where "Reverse SSH" was changed to "Call Home" throughout. If this
draft becomes an RFC, its name would no longer contain the obsolete
"reverse-ssh" reference, thus self-correcting this inconsistency.
3. Benefits to Device Management
Watsen Expires January 02, 2015 [Page 3]
Internet-Draft NETCONF Call Home using SSH July 2014
The SSH protocol is nearly ubiquitous for device management, as it is
the transport for the command-line applications `ssh`, `scp`, and
`sftp` and is the required transport for the NETCONF protocol
[RFC6241]. However, all these SSH-based protocols expect the network
element to be the SSH server.
NETCONF over SSH Call Home enables the network element to
consistently be the SSH server regardless of which peer initiates the
underlying TCP connection. Maintaining the role of SSH server is
both necessary and desirable. It is necessary because SSH channels
and subsystems can only be opened on the SSH server. It is desirable
because it conveniently leverages infrastructure that may be deployed
for host-key verification and user authentication.
Call home is useful for both initial deployment and on-going device
management and may be used to enable any of the following scenarios:
o The network element may proactively call home after being powered
on for the first time to register itself with its management
system.
o The network element may access the network in a way that
dynamically assigns it an IP address and it doesn't register its
assigned IP addressed to a mapping service.
o The network element may be configured in "stealth mode" and thus
doesn't have any open ports for the management system to connect
to.
o The network element may be deployed behind a firewall that doesn't
allow SSH access to the internal network.
o The network element may be deployed behind a firewall that
implements network address translation (NAT) for all internal
network IP addresses, thus complicating the ability for a
management system to connect to it.
o The operator may prefer to have network elements initiate
management connections believing it is easier to secure one open-
port in the data center than to have an open port on each network
element in the network.
One key benefit of using SSH as the transport protocol is its ability
to multiplex an unspecified number of independently flow-controlled
TCP sessions [RFC4254]. This is valuable as the network element only
needs to be configured to initiate a single call home connection to a
management system, regardless the number of NETCONF channels the
management system wants to open.
Watsen Expires January 02, 2015 [Page 4]
Internet-Draft NETCONF Call Home using SSH July 2014
4. Protocol
The NETCONF server's perspective (e.g., the network element)
o The NETCONF server initiates a TCP connection to the NETCONF
client on the IANA-assigned SSH for NETCONF Call Home port YYYY.
o The TCP connection is accepted and a TCP session is established.
o Using this TCP connection, the NETCONF server immediately starts
the SSH server protocol. That is, the next message sent on the
TCP stream is SSH's Protocol Version Exchange message (section
4.2, [RFC4253]).
o The SSH connection is established.
The NETCONF client's perspective (e.g., the management system)
o The NETCONF client listens for TCP connections on the IANA-
assigned NETCONF over SSH Call Home port YYYY.
o The NETCONF client accepts an incoming TCP connection and a TCP
session is established.
o Using this TCP connection, the NETCONF client immediately starts
the SSH Client protocol, starting with sending the SSH's Protocol
Version Exchange message (section 4.2, [RFC4253]).
o The SSH connection is established.
5. SSH Server Identification and Verification
When the management system accepts a new incoming TCP connection on
the NETCONF over SSH Call Home port, it starts the SSH client
protocol. As the SSH client, it MUST authenticate the SSH server, by
both identifying the network element and verifying its SSH host key.
Due to call home having the network element initiate the TCP
connection, the management system MAY identify the remote peer using
the source IP address of the TCP connection. However, identifying
the remote peer using the source IP address of the TCP connection is
NOT RECOMMENDED as it can only work in networks that use known static
addresses.
To support network elements having dynamically-assigned IP addresses,
or deployed behind gateways that translate their IP addresses (e.g.,
NAT), the management system MAY identify the device using its SSH
host key. For instance, a fingerprint of the network element's host
Watsen Expires January 02, 2015 [Page 5]
Internet-Draft NETCONF Call Home using SSH July 2014
key could itself be used as an identifier since each device has a
statistically unique host key. However, identifying the remote peer
using its host key directly is NOT RECOMMENDED as it requires the
host key to be manually verified the first time the network element
connects and anytime its host key changes thereafter.
Yet another option for identifying the network element is for its
host key to encode the network element's identity, such as if the
host key were a certificate. This option enables the host key to
change over time, so long as it continues to encode the same
identity, but brings the next issue of how the management system can
verify the network element's host key is authentic.
The security of SSH is anchored in the ability for the SSH client to
verify the SSH server's host key. Typically this is done by
comparing the host key presented by the SSH server with one that was
previously configured on the SSH client, looking it up in a local
database using the identity of the SSH client as the lookup key.
Nothing changes regarding this requirement due to the direction
reversal of the underlying TCP connection. To ensure security, the
management system MUST verify the network element's SSH host key each
time a SSH session is established.
However, configuring distinct host keys on the management system
doesn't scale well, which is an important consideration to a network
management system. A more scalable strategy for the management
system is for the network element's manufacturer to sign the network-
element's host key with a common trusted key, such as a certificate
authority. Then, when the network-element is deployed, the
management system only needs to trust a single certificate, which
vouches for the authenticity of the various network element host
keys.
Since both the identification and verification issues are addressed
using certificates, this draft RECOMMENDS network elements use a host
key that can encode a unique identifier (e.g., its serial number) and
be signed by a common trust anchor (e.g., a certificate authority).
Examples of suitable public host keys are the X.509v3 keys defined in
defined in [RFC6187] and the PGP keys defined in [RFC4253].
6. Device Configuration
How to configure a device to initiate a NETCONF over SSH Call Home
connection is outside the scope of this document, as implementations
can support this protocol using a proprietary configuration data
model. That said, a YANG [RFC6020] model to configure NETCONF over
SSH Call Home is specified in [draft-ietf-netconf-server-model].
Watsen Expires January 02, 2015 [Page 6]
Internet-Draft NETCONF Call Home using SSH July 2014
7. Security Considerations
This RFC deviates from standard SSH protocol usage by allowing the
SSH server to initiate the TCP connection. This conflicts with
section 4 of the SSH Transport Layer Protocol RFC [RFC4253], which
states "The client initiates the connection". However this statement
is made without rationalization and it's not clear how it impacts the
security of the protocol, so this section analyzes the security
offered by having the client initiate the connection.
First, assuming the SSH server is not using a public host key
algorithm that certifies its identity, the security of the protocol
doesn't seem to be sensitive to which peer initiates the connection.
That is, it is still the case that reliable distribution of host keys
(or their fingerprints) should occur prior to first connection and
that verification for subsequent connections happens by comparing the
host keys in a locally cached database. It does not seem to matter
if the SSH server's host name is derived from user-input or extracted
from the TCP layer, potentially via a reverse-DNS lookup. Once the
host name-to-key association is stored in a local database, no man-
in-the-middle attack is possible due to the attacker being unable to
guess the real SSH server's private key (Section 9.3.4 (Man-in-the-
middle) of [RFC4251]).
That said, this RFC recommends implementations use a public host key
algorithm that certifies the SSH server's identity. The identity can
be any unique identifier, such as a device's serial number or a
deployment-specific value. If this recommendation is followed, then
no information from the TCP layer would be needed to lookup the
device in a local database and therefore the directionality of the
TCP layer is clearly inconsequential.
The SSH protocol negotiates which algorithms it will use during key
exchange (Section 7.1 (Algorithm Negotiation) in [RFC4253]). The
algorithm selected is essentially the first compatible algorithm
listed by the SSH client that is also listed by the SSH server. For
a network management application, there may be a need to advertise a
large number of algorithms to be compatible with the various devices
it manages. The SSH client SHOULD order its list of public host key
algorithms such that all the certifiable public host key algorithms
are listed first. Additionally, when possible, SSH servers SHOULD
only list certifiable public host key algorithms. Note that since
the SSH server would have to be configured to know which IP address
it is to connect to, it is expected that it will also be configured
to know which host key algorithm to use for the particular
application, and hence only needs to list just that one public host
key algorithm.
Watsen Expires January 02, 2015 [Page 7]
Internet-Draft NETCONF Call Home using SSH July 2014
This RFC suggests implementations can use a device's serial number as
a form of identity. A potential concern with using a serial number
is that the SSH protocol passes the SSH server's host-key in the
clear and many times serial numbers encode revealing information
about the device, such as what kind of device it is and when it was
manufactured. While there is little security in trying to hide this
information from an attacker, it is understood that some deployments
may want to keep this information private. If this is a concern,
deployments SHOULD use an alternate unique identifier, if even just
the hash of the device's serial number.
An attacker could DoS the application by having it perform
computationally expensive operations, before deducing that the
attacker doesn't posses a valid key. This is no different than any
secured service and all common precautions apply (e.g., blacklisting
the source address after a set number of unsuccessful login
attempts).
8. IANA Considerations
This document requests that IANA assigns a TCP port number in the
"Registered Port Numbers" range with the service name "netconf-ssh-
ch". This port will be the default port for NETCONF over SSH Call
Home protocol and will be used when the NETCONF server is to initiate
a connection to a NETCONF client using SSH. Below is the
registration template following the rules in [RFC6335].
Service Name: netconf-ssh-ch
Transport Protocol(s): TCP
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: NETCONF over SSH Call Home
Reference: RFC XXXX
Port Number: YYYY
9. Acknowledgements
The author would like to thank for following for lively discussions
on list and in the halls (ordered by last name): Andy Bierman, Martin
Bjorklund, Mehmet Ersue, Wes Hardaker, Stephen Hanna, David
Harrington, Jeffrey Hutzelman, Radek Krejci, Alan Luchuk, Mouse, Russ
Mundy, Tom Petch, Peter Saint-Andre, Joe Touch, Sean Turner, Bert
Wijnen.
10. References
10.1. Normative References
Watsen Expires January 02, 2015 [Page 8]
Internet-Draft NETCONF Call Home using SSH July 2014
[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, December 2005.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
Protocol Architecture ", RFC 4251, 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.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the
Network Configuration Protocol (NETCONF) ", RFC 6020,
October 2010.
[RFC6187] Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure
Shell Authentication ", RFC 6187, March 2011.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
Bierman, "NETCONF Configuration Protocol", RFC 6241, June
2011.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, June 2011.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", RFC 6335, August
2011.
10.2. Informative References
[draft-ietf-netconf-server-model]
Watsen, K. and J. Schoenwaelder, "A YANG Data Model for
NETCONF Server Configuration", RFC 6242, June 2011.
Appendix A. Change Log
A.1. 05 to 06
Watsen Expires January 02, 2015 [Page 9]
Internet-Draft NETCONF Call Home using SSH July 2014
Changed title to "NETCONF Call Home using SSH"
Revised the Abstract and Introduction to better explain what the
document regards.
Changed "MUST" to "SHOULD" in the Applicability Statement.
Added a "Draft Naming" section explaining why, despite its name,
reversing SSH is nowhere in the text
Added PGP keys as another kind of SSH host key encoding identity
and signed by a trust anchor.
Revised the Device Considerations section to more clearly explain
why a device configuration data model is out of scope, and hence
an Informative reference.
Clarified Security Considerations section on use of serial
numbers.
A.2. 04 to 05
Changed "Reverse SSH" to "Call Home"
Added references to Applicability Statement
A.3. 03 to 04
Changed title to "Reverse SSH for NETCONF Call Home" (changed
again in -05)
Removed statement on how other SSH channels might be used for
other protocols
Improved language on how the management system, as the SSH client,
MUST authenticate the SSH server
Clarified that identifying the network element using source IP
address is NOT RECOMMENDED
Clarified that identifying the NE using simple certificate
comparison is NOT RECOMMENDED
Device Configuration section now more clearly states that the YANG
model is out of scope
Change requested port name to "netconf-ssh-ch"
Watsen Expires January 02, 2015 [Page 10]
Internet-Draft NETCONF Call Home using SSH July 2014
General edits for grammer, capitalization, and spellings
A.4. 02 to 03
Updated Device Configuration section to reference
[draft-ietf-netconf-server-model]
A.5. 01 to 02
Added Applicability Statement
Removed references to ZeroConf / ZeroTouch
Clarified the protocol section
Added a section for identification and verification
A.6. 00 to 01
Removed the hmac-* family of algorithms
Author's Address
Kent Watsen
Juniper Networks
EMail: kwatsen@juniper.net
Watsen Expires January 02, 2015 [Page 11]
