Reverse Secure Shell (Reverse SSH)
draft-ietf-netconf-reverse-ssh-02
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
|
|
|---|---|---|---|
| Author | Kent Watsen | ||
| Last updated | 2013-10-29 | ||
| Replaced by | draft-ietf-netconf-call-home, draft-ietf-netconf-call-home, RFC 8071 | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-netconf-reverse-ssh-02
NETCONF Working Group K. Watsen
Internet-Draft Juniper Networks
Updates: 4253 (if approved) November 2013
Intended status: Standards Track
Expires: May 05, 2014
Reverse Secure Shell (Reverse SSH)
draft-ietf-netconf-reverse-ssh-02
Abstract
This memo presents a technique for a NETCONF server to initiate a SSH
connection to a NETCONF client. This is accomplished by the NETCONF
client listening on IANA-assigned TCP port YYYY and starting the SSH
client protocol immediately after accepting a TCP connection on it.
This role-reversal is necessary as the NETCONF server must also be
the SSH Server, in order for the NETCONF client to open the IANA-
assigned SSH subsystem "netconf".
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 May 05, 2014.
Copyright Notice
Copyright (c) 2013 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
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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 . . . . . . . . . . . . . . . . . 2
2.2. Update to RFC 4253 . . . . . . . . . . . . . . . . . . . 3
3. Benefits to Device Management . . . . . . . . . . . . . . . . 3
4. The Reverse SSH Protocol . . . . . . . . . . . . . . . . . . 4
5. SSH Server Identification and Verification . . . . . . . . . 5
6. Device Configuration . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
10. Normative References . . . . . . . . . . . . . . . . . . . . 14
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 15
A.1. 01 to 02 . . . . . . . . . . . . . . . . . . . . . . . . 15
A.2. 00 to 01 . . . . . . . . . . . . . . . . . . . . . . . . 15
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
This memo presents a technique for a NETCONF [RFC6241] server to
initiate a Secure Shell (SSH) [RFC4251] connection to a NETCONF
client. This is accomplished by the NETCONF client listening on
IANA-assigned TCP port YYYY and starting the SSH client protocol
immediately after accepting a TCP connection on it. This role-
reversal is necessary as the NETCONF server must also be the SSH
Server, in order for the NETCONF client to open the IANA-assigned SSH
subsystem "netconf" [RFC6242].
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 MUST only be used for a NETCONF server to
initiate a connection to a NETCONF client, as described in this
document.
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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. This contrasts
with the base SSH protocol, which does not require programmatic
verification of the other party. In such circumstances, allowing the
SSH server to contact the SSH client would open new vulnerabilities.
Therefore, any use of Reverse 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 the
Security Considerations (Section 7) section.
3. Benefits to Device Management
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.
Reverse SSH 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
conviently leverages infrastructure that may be deployed for host-key
verification and user authentication.
Reverse SSH 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.
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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 Reverse SSH connection to
the management system, regardless the number of TCP-based protocols
the management system wishes to support. For instance, in addition
to having a SSH channel for NETCONF, management system may "pin up"
channels for Syslog, SNMP, or file-transfers.
4. The Reverse SSH 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 Reverse SSH port YYYY.
o The TCP connection is accecpted 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]).
The NETCONF client's perspective (e.g. the management system)
o The NETCONF client listens for TCP connections on the IANA-
assigned SSH 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]).
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5. SSH Server Identification and Verification
When the management system accepts a new incoming connection, it
needs to authenticate the remote peer. Ultimately, this comes down
to identifying the peer and verifying its SSH host key.
Due to Reverse SSH having the network element initiate the TCP
connection, the first data the management system has to identify it
with is the source IP address of the TCP connection. But this
approach only works in networks that only use static addresses.
To support network-elements having dynamically-assigned IP addresses
or deployed behind gateways that translate their IP address (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
key could be used as an identifier since the probability of collision
is acceptibly low. But this solution requires the management system
to be configured with each device's host key each time it changes.
Yet another option for identifying the network element if for it's
host key to encode its identity, such as if it were a certificate.
This option enables the host key to change over time, but brings the
next issue of how the mangement element can verify the network
element's host key is authentice.
The security of SSH is anchored in the ability for the SSH client to
verify the SSH server's hostkey. 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 is to have the network
element's host key signed by a common trusted key, such as a
certificate authority. Thus, the mangement system only needs to
trust a single public key, which vouches for the authenticity of the
network element's specific public key.
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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 (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].
6. Device Configuration
This section defines a YANG [RFC6020] module to configure Reverse SSH
on the device. This YANG module enables a NETCONF client to
generically manage a NETCONF server's Reverse SSH configuration. Key
aspects of this YANG module include support for more than one
application, more than one server per application, and a reconnection
strategy.
Configuration Example
<config xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
<reverse-ssh xmlns="urn:ietf:params:xml:ns:yang:ietf-reverse-ssh">
<applications>
<application>
<name>config-mgr</name>
<description>
This entry requests the device to periodically
connect to the Configuration Manager application
</description>
<servers>
<server>
<host>config-mgr1.acme.com</host>
<port>7022</port>
</server>
<server>
<host>config-mgr2.acme.com</host>
<port>7022</port>
</server>
</servers>
<host-keys>
<host-key>
<name>ssh_host_key_cert</name>
</host-key>
<host-key>
<name>ssh_host_key_cert2</name>
</host-key>
</host-keys>
<connection-type>
<periodic>
<timeout-mins>8</timeout-mins>
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<linger-secs>10</linger-secs>
</periodic>
</connection-type>
<reconnect-strategy>
<start-with>last-connected</start-with>
<interval-secs>10</interval-secs>
<count-max>4</count-max>
</reconnect-strategy>
<keep-alive-strategy>
<interval-secs>5</interval-secs>
<count-max>3</count-max>
</keep-alive-strategy>
</application>
<application>
<name>log-monitor</name>
<description>
This entry requests the device to mantain a
persistent connection to the Log Monitoring
application
</description>
<servers>
<server>
<host>log-mon1.acme.com</host>
<port>7514</port>
</server>
<server>
<host>log-monitor2.acme.com</host>
<port>7514</port>
</server>
</servers>
<host-keys>
<host-key>
<name>ssh_host_key_hmac</name>
</host-key>
</host-keys>
<connection-type>
<persistent/>
</connection-type>
<reconnect-strategy>
<start-with>last-connected</start-with>
<interval-secs>10</interval-secs>
<count-max>4</count-max>
</reconnect-strategy>
<keep-alive-strategy>
<interval-secs>5</interval-secs>
<count-max>3</count-max>
</keep-alive-strategy>
</application>
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</applications>
</reverse-ssh>
</config>
The YANG Module
module ietf-reverse-ssh {
namespace "urn:ietf:params:xml:ns:yang:ietf-reverse-ssh";
prefix "rssh";
import ietf-inet-types { prefix inet; }
organization
"IETF NETCONF (Network Configuration Protocol) Working Group";
contact
"WG Web: <http://tools.ietf.org/wg/netconf/>
WG List: <mailto:netconf@ietf.org>
WG Chair: Bert Wijnen
<mailto:bertietf@bwijnen.net>
WG Chair: Mehmet Ersue
<mailto:mehmet.ersue@nsn.com>
Editor: Kent Watsen
<mailto:kwatsen@juniper.net>";
revision 2013-06-18 {
description "Initial conception";
reference "RFC XXXX: Reverse SSH";
}
// RFC Ed.: replace XXXX with actual
// RFC number and remove this note
container reverse-ssh {
container applications {
description
"All the application that the device
initiates Reverse SSH connections to";
list application {
key name;
min-elements 1;
leaf name {
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mandatory true;
type string {
length 1..64;
}
description
"The name of the application the device is
connecting to";
}
leaf description {
type string;
description
"An optional description for the application";
}
container servers {
description
"An ordered listing of the application's
servers that the device should attempt
connecting to.";
list server {
key host;
min-elements 1;
ordered-by user;
leaf host {
mandatory true;
type inet:host;
description
"IP address or domain-name for
the server";
}
leaf port {
type inet:port-number;
description
"The IP port for this server.
The device will use the
IANA-assigned port if not
specified.";
}
}
}
container host-keys {
description
"An ordered listing of the SSH host keys the
device should advertise to the application.";
list host-key {
key name;
min-elements 1;
ordered-by user;
leaf name {
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mandatory true;
type string {
length 1..64;
}
description
"The name of a host key the device
should advertise during the SSH
key exchange.";
}
}
}
container connection-type {
description "Indicates the application's
preference for how the device's
connection is maintained.";
choice connection-type {
default persistent-connection;
case persistent-connection {
leaf persistent {
type empty;
}
}
case periodic-connection {
container periodic {
leaf timeout-mins {
type uint8;
default 5;
units minutes;
description
"The maximum amount of unconnected
time the device will wait until
establishing a connection to the
applications again to send it.
The device may establish a
connection before this time if
it has data it needs to send to
the device.";
}
leaf linger-secs {
type uint8;
default 30;
units seconds;
description
"The amount of time the device should
wait after last receiving data from
or sending data to the device before
closing its connection to the app.";
}
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}
}
}
}
container reconnect-strategy {
leaf start-with {
default first-listed;
type enumeration {
enum first-listed;
enum last-connected;
}
}
leaf interval-secs {
type uint8;
units seconds;
default 5;
description
"time delay between connection attempts";
}
leaf count-max {
type uint8;
default 3;
description
"num times try to connect to a server";
}
}
container keep-alive-strategy {
leaf interval-secs {
type uint8;
units seconds;
default 15;
description
"Sets a timeout interval in seconds after
which if no data has been received from
the client, a message will be sent to
request a response from the SSH client.
A value of '0' indicates that no messages
should be sent.";
}
leaf count-max {
type uint8;
default 3;
description
"Sets the number of keep alive messages
that may be sent without receiving any
response from the SSH client before
assuming the SSH client is no longer
alive. If this threshold is reached
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the device will disconnect the SSH
session. The keep alive interval timer
is reset after each transmission. Thus,
an unresponsive SSH client will be
disconnected after approximately
'count-max * interval-secs' seconds.";
}
}
}
}
}
}
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 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-th-
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 (Algortihm Negotition) in [RFC4253]). The
algorithm selected is essentially the first compatible algorithm
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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 needs 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.
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 MAY consider using instead a hash of the device's serial
number or an application-specified unique identifier.
An attacker could DoS the application by having it to 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 "reverse-ssh".
This port will be the default port for the Reverse SSH protocol and
will be used when the NETCONF server needs to initiate a connection
to a NETCONF client using SSH. Below is the registration template
following the rules in [RFC6335].
Service Name: reverse-ssh
Transport Protocol(s): TCP
Assignee: IESG <iesg@ietf.org>
Contact: IETF Chair <chair@ietf.org>
Description: Reverse SSH (call home)
Reference: RFC XXXX
Port Number: YYYY
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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, Mouse, Russ Mundy, Tom Petch, Peter
Saint-Andre, Joe Touch, Sean Turner, Bert Wijnen.
10. 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, Ed., "The Secure Shell (SSH)
Protocol Assigned Numbers ", RFC 4250, December 2005.
[RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Protocol Architecture ", RFC 4251, January 2006.
[RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Authentication Protocol ", RFC 4252, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol ", RFC 4253, January 2006.
[RFC4254] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Connection Protocol ", RFC 4254, January 2006.
[RFC6020] Bjorklund, M., Ed., "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., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "NETCONF Configuration Protocol", RFC
6241, June 2011.
[RFC6242] Wasserman, M., Ed., "Using the NETCONF Protocol over
Secure Shell (SSH)", RFC 6242, June 2011.
[RFC6335] Cotton, M., Ed., Eggert, L., Ed., Touch, J., Ed.,
Westerlund, M., Ed., and S. Cheshire, Ed., "Internet
Assigned Numbers Authority (IANA) Procedures for the
Management of the Service Name and Transport Protocol Port
Number Registry", RFC 6335, August 2011.
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Appendix A. Change Log
A.1. 01 to 02
Added Applicability Statement
Removed references to ZeroConf / ZeroTouch
Clarified the protocol section
Added a section for identification and verification
A.2. 00 to 01
removed the hmac-* family of algorithms
Author's Address
Kent Watsen
Juniper Networks
EMail: kwatsen@juniper.net
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