NETCONF Working Group K. Watsen
Internet-Draft Juniper Networks
Intended status: Standards Track November 24, 2015
Expires: May 27, 2016

NETCONF Call Home and RESTCONF Call Home
draft-ietf-netconf-call-home-13

Abstract

This RFC presents NETCONF Call Home and RESTCONF Call Home, which enable a NETCONF or RESTCONF server to initiate a secure connection to a NETCONF or RESTCONF client respectively.

Editorial Note (To be removed by RFC Editor)

This draft contains many placeholder values that need to be replaced with finalized values at the time of publication. This note summarizes all of the substitutions that are needed. Please note that no other RFC Editor instructions are specified anywhere else in this document.

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Artwork in this document contains placeholder values for ports pending IANA assignment from "draft-ietf-netconf-call-home". Please apply the following replacements:

The following two Appendix sections are to be removed prior to publication:

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/.

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This Internet-Draft will expire on May 27, 2016.

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Table of Contents

1. Introduction

This RFC presents NETCONF Call Home and RESTCONF Call Home, which enable a NETCONF or RESTCONF server to initiate a secure connection to a NETCONF or RESTCONF client respectively.

NETCONF Call Home supports both of the secure transports used by the NETCONF protocol [RFC6241], SSH and TLS. The NETCONF protocol's binding to SSH is defined in [RFC6242]. The NETCONF protocol's binding to TLS is defined in [RFC7589].

RESTCONF Call Home only supports TLS, the same as the RESTCONF protocol [draft-ietf-netconf-restconf]. The RESTCONF protocol's binding to TLS is defined in [draft-ietf-netconf-restconf].

The SSH protocol is defined in [RFC4253]. The TLS protocol is defined in [RFC5246]. Both the SSH and TLS protocols are layered on top of the TCP protocol, which is defined in [RFC793].

Both NETCONF Call Home and RESTCONF Call Home preserve all but one of the client/server roles in their respective protocol stacks, as compared to client-initiated NETCONF and RESTCONF connections. The one and only role reversal that occurs is at the TCP layer; that is, which peer is the TCP-client and which is the TCP-server.

For example, a network element is traditionally the TCP-server. However, when calling home, the network element becomes the TCP-client. The network element's secure transport layer roles (SSH-server, TLS-server) and its application layer roles (NETCONF-server, RESTCONF-server) both remain the same.

Having consistency in both the secure transport layer (SSH, TLS) and application layer (NETCONF, RESTCONF) roles conveniently enables deployed network management infrastructure to support call home also. For instance, existing certificate chains and user authentication mechanisms are unaffected by call home.

1.1. Motivation

Call home is generally useful for both the initial deployment and on-going management of networking elements. Here are some scenarios enabled by call home:

1.2. 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].

1.3. Applicability Statement

The techniques described in this document are suitable for network management scenarios such as the ones described in Section 1.1. However, these techniques are only defined for NETCONF Call Home and RESTCONF Call Home, as described in this document.

The reason for this restriction is that different protocols have different security assumptions. The NETCONF and RESTCONF protocols require clients and servers to verify the identity of the other party. This requirement is specified for the NETCONF protocol in Section 2.2 of [RFC6241], and is specified for the RESTCONF protocol in Sections 2.4 and 2.5 of [draft-ietf-netconf-restconf]).

This contrasts with the base SSH and TLS protocols, which do not require programmatic verification of the other party (section 9.3.4 of [RFC4251], section 4 of [RFC4252], and section 7.3 of [RFC5246]). In such circumstances, allowing the SSH/TLS server to contact the SSH/TLS client would open new vulnerabilities. Any use of call home with SSH/TLS for purposes other than NETCONF or RESTCONF will need a thorough, contextual security analysis, similar to that performed in the process of publishing this document.

1.4. Relation to RFC 4253

This document uses the SSH Transport Layer Protocol [RFC4253] with the exception that the statement "The client initiates the connection" made in Section 4 (Connection Setup) does not apply. Assuming the reference to client means "SSH client" and the reference to connection means "TCP connection", this statement doesn't hold true in call home, where the network element is the SSH server and yet still initiates the TCP connection. Security implications related to this change are discussed in Security Considerations (Section 5).

1.5. The NETCONF/RESTCONF Convention

Throughout the remainder of this document, the term "NETCONF/RESTCONF" is used as an abbreviation in place of the text "the NETCONF or the RESTCONF". The NETCONF/RESTCONF abbreviation is not intended to require or to imply that a client or server must implement both the NETCONF standard and the RESTCONF standard.





2. Solution Overview

The diagram below illustrates call home from a protocol layering perspective:

       NETCONF/RESTCONF                    NETCONF/RESTCONF
            Server                              Client
              |                                    |
              |         1. TCP                     |
              |----------------------------------->|
              |                                    |
              |                                    |
              |         2. SSH/TLS                 |
              |<-----------------------------------|
              |                                    |
              |                                    |
              |         3. NETCONF/RESTCONF        |
              |<-----------------------------------|
              |                                    |

             Note: arrows point from the "client" to 
               the "server" at each protocol layer 

This diagram makes the following points:

  1. The NETCONF/RESTCONF server begins by initiating a TCP connection to the NETCONF/RESTCONF client.
  2. Using this TCP connection, the NETCONF/RESTCONF client initiates a SSH/TLS session to the NETCONF/RESTCONF server.
  3. Using this SSH/TLS session, the NETCONF/RESTCONF client intiates a NETCONF/RESTCONF session to the NETCONF/RESTCONF server.




3. The NETCONF or RESTCONF Client

The term "client" is defined in [RFC6241], Section 1.1 "client". In the context of network management, the NETCONF/RESTCONF client might be a network management system.

3.1. Protocol Operation

C1
The NETCONF/RESTCONF client listens for TCP connection requests from NETCONF/RESTCONF servers. The client MUST support accepting TCP connections on the IANA-assigned ports defined in Section 6, but MAY be configured to listen a defferent port.
C2
The NETCONF/RESTCONF client accepts an incoming TCP connection request and a TCP connection is established.
C3
Using this TCP connection, the NETCONF/RESTCONF client starts either the SSH-client [RFC4253] or the TLS-client [RFC5246] protocol. For example, assuming the use of the IANA-assigned ports, the SSH-client protocol is started when the connection is accepted on port PORT-X and the TLS-client protocol is started when the connection is accepted on either port PORT-Y or PORT-Z.
C4
If using TLS, the NETCONF/RESTCONF client MUST advertise "peer_allowed_to_send", as defined by [RFC6520]. This is required so NETCONF/RESTCONF servers can depend on it being there for call home connections, when keep-alives are needed the most.
C5
As part of establishing an SSH or TLS connection, the NETCONF/RESTCONF client MUST validate the server's presented host key or certificate. This validation MAY be accomplished by certificate path validation or by comparing the host key or certificate to a previously trusted or "pinned" value. If a certificate is presented and it contains revocation checking information, the NETCONF/RESTCONF client SHOULD check the revocation status of the certificate. If it is determined that a certificate has been revoked, the client MUST immediately close the connection.
C6
If certificate path validation is used, the NETCONF/RESTCONF client MUST ensure that the presented certificate has a valid chain of trust to a preconfigured Issuer and that the presented certificate encodes an "identifier" [RFC6125] that the client had awareness of prior to the connection attempt. Clients SHOULD ensure that the Issuer used to authenticate the presented certificate defines the namespace for the identifiers of interest. How identifiers are encoded in certificates MAY be determined by a policy associated with the certificate's Issuer. For instance, a given Issuer may be known to only sign IDevID certificates [Std-802.1AR-2009] having a unique identifier (e.g., serial number) in the X.509 certificate's "CommonName" field.
C7
After the server's host key or certificate is validated, the SSH or TLS protocol proceeds as normal to establish a SSH or TLS connection. When performing client-authentcation with the NETCONF/RESTCONF server, the NETCONF/RESTCONF client MUST ensure to only use credentials that it had previously associated for the NETCONF/RESTCONF server’s presented host key or server certificate.
C8
Once the SSH or TLS connection is established, the NETCONF/RESTCONF client starts either the NETCONF-client [RFC6241] or RESTCONF-client [draft-ietf-netconf-restconf] protocol. Assuming the use of the IANA-assigned ports, the NETCONF-client protocol is started when the connection is accepted on either port PORT-X or PORT-Y and the RESTCONF-client protocol is started when the connection is accepted on port PORT-Z.

3.2. Configuration Data Model

How a NETCONF or RESTCONF client is configured is outside the scope of this document. For instance, such configuration might be used to enable listening for call home connections, configuring trust anchors, or configuring identifiers for expected connections.




4. The NETCONF or RESTCONF Server

The term "server" is defined in [RFC6241], Section 1.1 "server". In the context of network management, the NETCONF/RESTCONF server might be a network element or a device.

4.1. Protocol Operation

S1
The NETCONF/RESTCONF server initiates a TCP connection request to the NETCONF/RESTCONF client. The server MUST support connecting to one of the IANA-assigned ports defined in Section 6, but MAY be configured to connect to a different port. Using the IANA-assigned ports, the server connects to port PORT-X for NETCONF over SSH, port PORT-Y for NETCONF over TLS, and port PORT-Z for RESTCONF over TLS.
S2
The TCP connection request is accepted and a TCP connection is established.
S3
Using this TCP connection, the NETCONF/RESTCONF server starts either the SSH-server [RFC4253] or the TLS-server [RFC5246] protocol, depending on how it is configured. For example, assuming the use of the IANA-assigned ports, the SSH-server protocol is used after connecting to the remote port PORT-X and the TLS-server protocol is used after connecting to one of the remote ports PORT-Y or PORT-Z.
S4
As part of establishing the SSH or TLS connection, the NETCONF/RESTCONF server will send its host key or certificate to the client. If a certificate is sent, the server MUST also send all intermediate certificates leading up to the certificate's trust anchor. How to send a list of certificates is defined for SSH in [RFC6187] Section 2.1, and for TLS in [RFC5246] Section 7.4.2.
S5
Establishing an SSH or TLS session requires server authentication of client credentials in all cases except with RESTCONF, where some client authentication schemes occur after the secure transport connection (TLS) has been established. If transport (SSH or TLS) level client authentication is required, and the client is unable to successfully authenticate itself to the server in an amount of time defined by local policy, the server MUST close the connection.
S6
Once the SSH or TLS connection is established, the NETCONF/RESTCONF server starts either the NETCONF-server [RFC6241] or RESTCONF-server [draft-ietf-netconf-restconf] protocol, depending on how it is configured. Assuming the use of the IANA-assigned ports, the NETCONF-server protocol is used after connecting to remote port PORT-X or PORT-Y, and the RESTCONF-server protocol is used after connecting to remote port PORT-Z.
S7
If a persistent connection is desired, the NETCONF/RESTCONF server, as the connection initiator, SHOULD actively test the aliveness of the connection using a keep-alive mechanism. For TLS based connections, the NETCONF/RESTCONF server SHOULD send HeartbeatRequest messages, as defined by [RFC6520]. For SSH based connections, per section 4 of [RFC4254], the NETCONF/RESTCONF server SHOULD send a SSH_MSG_GLOBAL_REQUEST message with the purposely nonexistent "request name" value "keepalive@ietf.org" and the "want reply" value set to '1'.

4.2. Configuration Data Model

How a NETCONF or RESTCONF server is configured is outside the scope of this document. This includes configuration that might be used to specify hostnames, IP addresses, ports, algorithms, or other relevant parameters. That said, a YANG [RFC6020] model for configuring NETCONF and RESTCONF servers, including call home, is provided in [draft-ietf-netconf-server-model].

5. Security Considerations

The security considerations described in [RFC6242] and [RFC7589], and by extension [RFC4253], [RFC5246], and [draft-ietf-netconf-restconf] apply here as well.

This RFC deviates from standard SSH and TLS usage by having the SSH/TLS server initiate the underlying TCP connection. This reversal is incongruous with [RFC4253], which says "the client initiates the connection" and also [RFC6125], which says "the client MUST construct a list of acceptable reference identifiers, and MUST do so independently of the identifiers presented by the service." To account for these variances, this RFC requires that the NETCONF/RESTCONF client validate the SSH host key or certificate via certificate path validation to a preconfigured Issuer certificate or by comparing the host key or certificate to a previously trusted or "pinned" value. Furthermore, if certificate path validation is used, this RFC requires that the client be able to match a presented identifier encoded in the certificate with an identifier the client was preconfigured to expect.

Internet facing hosts running NETCONF or RESTCONF call home will be fingerprinted via scanning tools such as `zmap` [zmap]. Both SSH and TLS provide many ways in which a device can be fingerprinted. SSH and TLS servers are fairly mature and able to withstand an attack, but SSH and TLS clients may not be as robust. Implementers and deployments need to ensure that software update mechanisms are provided so that vulnerabilities can be fixed in a timely fashion.

An attacker could launch a denial of service (DoS) attack on the NETCONF/RESTCONF client by having it perform computationally expensive operations, before deducing that the attacker doesn't posses a valid key. For instance, in TLS 1.3 [draft-ietf-tls-tls13], the ClientHello message contains a Key Share value based on an asymmetric key operation. This is similar to any other secured service and all common precautions apply (e.g., temporarily blacklisting the source address after a set number of unsuccessful login attempts).

For cases when the NETCONF/RESTCONF server presents an X.509 certificate, the NETCONF/RESTCONF client needs to ensure that the Issuer cartificate used for certificate path validation is unique to the manufacturer of the server. This is especially important when the client-authentication mechanism passes a shared secret (e.g., a password) to the server. Not doing so could otherwise lead to a case where the client sends the shared secret to another device that happens to have the same identity value (e.g., serial number) as the server the client was configured to expect.

When using call home with the RESTCONF protocol, special care is required when using some HTTP authentication mechanisms, especially the Basic [RFC7617] and Digest [RFC7616] authentication schemes, which convey a shared key. Implementations and deployments should be sure to review the Security Considerations section for any client authentication scheme used.

6. IANA Considerations

This RFC requests that IANA assigns three TCP port numbers in the "Registered Port Numbers" range with the service names "netconf-ch-ssh", "netconf-ch-tls", and "restconf-ch-tls". These ports will be the default ports for NETCONF Call Home and RESTCONF Call Home protocols. Below is the registration template following the rules in [RFC6335].

Service Name:           netconf-ch-ssh
Transport Protocol(s):  TCP
Assignee:               IESG <iesg@ietf.org>
Contact:                IETF Chair <chair@ietf.org>
Description:            NETCONF Call Home (SSH)
Reference:              RFC XXXX
Port Number:            PORT-X

Service Name:           netconf-ch-tls
Transport Protocol(s):  TCP
Assignee:               IESG <iesg@ietf.org>
Contact:                IETF Chair <chair@ietf.org>
Description:            NETCONF Call Home (TLS)
Reference:              RFC XXXX
Port Number:            PORT-Y

Service Name:           restconf-ch-tls
Transport Protocol(s):  TCP
Assignee:               IESG <iesg@ietf.org>
Contact:                IETF Chair <chair@ietf.org>
Description:            RESTCONF Call Home (TLS)
Reference:              RFC XXXX
Port Number:            PORT-Z

7. Acknowledgements

The author would like to thank the 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, Juergen Schoenwaelder, Peter Saint-Andre, Joe Touch, Hannes Tschofenig, Sean Turner, and Bert Wijnen.

8. References

8.1. Normative References

[draft-ietf-netconf-restconf] Bierman, A., Bjorklund, M. and K. Watsen, "RESTCONF Protocol", Internet-Draft draft-ieft-netconf-restconf-04, 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, January 2006.
[RFC4252] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252, January 2006.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006.
[RFC4254] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Connection Protocol", RFC 4254, DOI 10.17487/RFC4254, January 2006.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and Verification of Domain-Based Application Service Identity within Internet Public Key Infrastructure Using X.509 (PKIX) Certificates in the Context of Transport Layer Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March 2011.
[RFC6187] Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure Shell Authentication", RFC 6187, DOI 10.17487/RFC6187, March 2011.
[RFC6241] Enns, R., Bjorklund, M., Schoenwaelder, J. and A. Bierman, "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, 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", BCP 165, RFC 6335, DOI 10.17487/RFC6335, August 2011.
[RFC6520] Seggelmann, R., Tuexen, M. and M. Williams, "Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) Heartbeat Extension", RFC 6520, DOI 10.17487/RFC6520, February 2012.
[RFC7589] Badra, M., Luchuk, A. and J. Schoenwaelder, "Using the NETCONF Protocol over Transport Layer Security (TLS) with Mutual X.509 Authentication", RFC 7589, DOI 10.17487/RFC7589, June 2015.
[RFC793] Postel, J., "TRANSMISSION CONTROL PROTOCOL", STD 7, September 1981.

8.2. Informative References

[draft-ietf-netconf-server-model] Watsen, K. and J. Schoenwaelder, "NETCONF Server Configuration Model", 2014.
[draft-ietf-tls-tls13] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", 2015.
[RFC6020] Bjorklund, M., "YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF)", RFC 6020, DOI 10.17487/RFC6020, October 2010.
[RFC7616] Shekh-Yusef, R., Ahrens, D. and S. Bremer, "HTTP Digest Access Authentication", RFC 7616, DOI 10.17487/RFC7616, September 2015.
[RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme", RFC 7617, DOI 10.17487/RFC7617, September 2015.
[Std-802.1AR-2009] IEEE SA-Standards Board, "IEEE Standard for Local and metropolitan area networks - Secure Device Identity", December 2009.
[zmap] Durumeric, Z., Wustrow, E. and J. Halderman, "ZMap: Fast Internet-Wide Scanning and its Security Applications", 2013.

In proceedings of the 22nd USENIX Security Symposium

Appendix A. Change Log

A.1. 00 to 01

A.2. 01 to 02

A.3. 02 to 03

A.4. 03 to 04

A.5. 04 to 05

A.6. 05 to 06

A.7. 06 to 07

A.8. 07 to 08

A.9. 08 to 09

A.10. 09 to 10

A.11. 10 to 11

A.12. 11 to 12

A.13. 12 to 13

Appendix B. Open Issues

All issues with this draft are tracked using GitHub issues. Please see: https://github.com/netconf-wg/call-home/issues to see currently opened issues.

Author's Address

Kent Watsen Juniper Networks EMail: kwatsen@juniper.net