Internet Engineering Task Force (IETF) K. Watsen Request for Comments: 8071 Juniper Networks Category: Standards Track February 2017 ISSN: 2070-1721 NETCONF Call Home and RESTCONF Call Home 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. 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 7841. 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/rfc8071. Copyright Notice Copyright (c) 2017 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 1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Requirements Terminology . . . . . . . . . . . . . . . . 3 1.3. Applicability Statement . . . . . . . . . . . . . . . . . 4 1.4. Relation to RFC 4253 . . . . . . . . . . . . . . . . . . 4 1.5. The NETCONF/RESTCONF Convention . . . . . . . . . . . . . 4 2. Solution Overview . . . . . . . . . . . . . . . . . . . . . . 5 3. The NETCONF or RESTCONF Client . . . . . . . . . . . . . . . 5 3.1. Client Protocol Operation . . . . . . . . . . . . . . . . 5 3.2. Client Configuration Data Model . . . . . . . . . . . . . 7 4. The NETCONF or RESTCONF Server . . . . . . . . . . . . . . . 7 4.1. Server Protocol Operation . . . . . . . . . . . . . . . . 7 4.2. Server Configuration Data Model . . . . . . . . . . . . . 8 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11 7.1. Normative References . . . . . . . . . . . . . . . . . . 11 7.2. Informative References . . . . . . . . . . . . . . . . . 12 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13 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 Network Configuration Protocol (NETCONF) [RFC6241], Secure Shell (SSH), and Transport Layer Security (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 [RFC8040]. The RESTCONF protocol's binding to TLS is defined in [RFC8040]. 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 initially assumes the role of the TCP client. The network element's secure transport-layer roles (SSH server, TLS server) and its application-layer roles (NETCONF server, RESTCONF server) all 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 ongoing management of networking elements. Here are some scenarios enabled by call home: o The network element may proactively "call home" after being powered on for the first time in order 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, but does not register its assigned IP address to a mapping service (e.g., dynamic DNS). o The network element may be deployed behind a firewall that implements Network Address Translation (NAT) for all internal network IP addresses. o The network element may be deployed behind a firewall that does not allow any management access to the internal network. o The network element may be configured in "stealth mode", and thus does not have any open ports for the management system to connect to. 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. 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 [RFC8040]. 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 risk assessment. A risk assessment for this RFC is in the Security Considerations section (Section 5). 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 of RFC 4253 does not apply. Assuming the reference to the client means "SSH client" and the reference to the 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 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. Figure 1: Call Home Sequence Diagram 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 an SSH/TLS session to the NETCONF/RESTCONF server. 3. Using this SSH/TLS session, the NETCONF/RESTCONF client initiates 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. In the context of network management, the NETCONF/RESTCONF client might be a network management system. 3.1. Client 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 to a different 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 4334 and the TLS client protocol is started when the connection is accepted on either port 4335 or port 4336. C4 When using TLS, the NETCONF/RESTCONF client MUST advertise "peer_allowed_to_send", as defined by [RFC6520]. This is required so that 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 certificate, and that the presented certificate encodes an "identifier" [RFC6125] that the client was aware of before the connection attempt. 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., a 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 an SSH or TLS connection. When performing client authentication with the NETCONF/RESTCONF server, the NETCONF/RESTCONF client MUST 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 [RFC8040] protocol. Assuming the use of the IANA-assigned ports, the NETCONF client protocol is started when the connection is accepted on either port 4334 or port 4335 and the RESTCONF client protocol is started when the connection is accepted on port 4336. 3.2. Client Configuration Data Model How a NETCONF or RESTCONF client is configured is outside the scope of this document. For instance, such a configuration might be used to enable listening for call home connections, configuring trusted certificate issuers, or configuring identifiers for expected connections. That said, YANG [RFC7950] data modules for configuring NETCONF and RESTCONF clients, including call home, are provided in [NETCONF-MODELS] and [RESTCONF-MODELS]. 4. The NETCONF or RESTCONF Server The term "server" is defined in [RFC6241], Section 1.1. In the context of network management, the NETCONF/RESTCONF server might be a network element or a device. 4.1. Server Protocol Operation S1 The NETCONF/RESTCONF server initiates a TCP connection request to the NETCONF/RESTCONF client. The source port may be per local policy or randomly assigned by the operating system. 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 4334 for NETCONF over SSH, port 4335 for NETCONF over TLS, and port 4336 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 4334 and the TLS server protocol is used after connecting to either remote port 4335 or remote port 4336.
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 a well-known and trusted issuer. 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- level (SSH or TLS) 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 [RFC8040] 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 4334 or remote port 4335, and the RESTCONF server protocol is used after connecting to remote port 4336. 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 server SHOULD send an SSH_MSG_GLOBAL_REQUEST message with a purposely nonexistent "request name" value (e.g., firstname.lastname@example.org) and the "want reply" value set to '1'. 4.2. Server 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, YANG [RFC7950] data modules for configuring NETCONF and RESTCONF servers, including call home, are provided in [NETCONF-MODELS] and [RESTCONF-MODELS].
5. Security Considerations The security considerations described in [RFC6242] and [RFC7589], and by extension [RFC4253], [RFC5246], and [RFC8040] 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." Risks associated with these variances are centered around server authentication and the inability for clients to compare an independently constructed reference identifier to one presented by the server. To mitigate against these risks, this RFC requires that the NETCONF/RESTCONF client validate the server's SSH host key or certificate, by certificate path validation to a preconfigured issuer certificate, or by comparing the host key or certificate to a previously trusted or "pinned" value. Furthermore, when a certificate is used, this RFC requires that the client be able to match an identifier encoded in the presented certificate with an identifier the client was preconfigured to expect (e.g., a serial number). For cases when the NETCONF/RESTCONF server presents an X.509 certificate, NETCONF/RESTCONF clients should ensure that the preconfigured issuer certificate used for certificate path validation is unique to the manufacturer of the server. That is, the certificate should not belong to a third-party certificate authority that might issue certificates for more than one manufacturer. This is especially important when a client authentication mechanism passing a shared secret (e.g., a password) to the server is used. Not doing so could otherwise lead to a case where the client sends the shared secret to another server that happens to have the same identity (e.g., a serial number) as the server the client was configured to expect. Considerations not associated with server authentication follow next. Internet-facing hosts running NETCONF Call Home or RESTCONF Call Home will be fingerprinted via scanning tools such as "zmap" [zmap]. Both SSH and TLS provide many ways in which a host can be fingerprinted. SSH and TLS servers are fairly mature and able to withstand attacks, 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 possess a valid key. For instance, in TLS 1.3 [TLS1.3], the ClientHello message contains a Key Share value based on an expensive asymmetric key operation. Common precautions mitigating DoS attacks are recommended, such as temporarily blacklisting the source address after a set number of unsuccessful login attempts. When using call home with the RESTCONF protocol, special care is required when using some HTTP authentication schemes, especially the Basic [RFC7617] and Digest [RFC7616] schemes, which convey a shared secret (e.g., a password). Implementers and deployments should be sure to review the Security Considerations section in the RFC for any HTTP client authentication scheme used. 6. IANA Considerations IANA has assigned three TCP port numbers in the "User Ports" 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 Port Number: 4334 Transport Protocol(s): TCP Description: NETCONF Call Home (SSH) Assignee: IESG <email@example.com> Contact: IETF Chair <firstname.lastname@example.org> Reference: RFC 8071 Service Name: netconf-ch-tls Port Number: 4335 Transport Protocol(s): TCP Description: NETCONF Call Home (TLS) Assignee: IESG <email@example.com> Contact: IETF Chair <firstname.lastname@example.org> Reference: RFC 8071 Service Name: restconf-ch-tls Port Number: 4336 Transport Protocol(s): TCP Description: RESTCONF Call Home (TLS) Assignee: IESG <email@example.com> Contact: IETF Chair <firstname.lastname@example.org> Reference: RFC 8071
7. References 7.1. Normative References [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, <http://www.rfc-editor.org/info/rfc793>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, January 2006, <http://www.rfc-editor.org/info/rfc4251>. [RFC4252] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Authentication Protocol", RFC 4252, DOI 10.17487/RFC4252, January 2006, <http://www.rfc-editor.org/info/rfc4252>. [RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006, <http://www.rfc-editor.org/info/rfc4253>. [RFC4254] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Connection Protocol", RFC 4254, DOI 10.17487/RFC4254, January 2006, <http://www.rfc-editor.org/info/rfc4254>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>. [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, <http://www.rfc-editor.org/info/rfc6125>. [RFC6187] Igoe, K. and D. Stebila, "X.509v3 Certificates for Secure Shell Authentication", RFC 6187, DOI 10.17487/RFC6187, March 2011, <http://www.rfc-editor.org/info/rfc6187>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed., and A. Bierman, Ed., "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011, <http://www.rfc-editor.org/info/rfc6241>. [RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011, <http://www.rfc-editor.org/info/rfc6242>. [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, <http://www.rfc-editor.org/info/rfc6335>. [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, <http://www.rfc-editor.org/info/rfc6520>. [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, <http://www.rfc-editor.org/info/rfc7589>. [RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017, <http://www.rfc-editor.org/info/rfc8040>. 7.2. Informative References [NETCONF-MODELS] Watsen, K., Wu, G., and J. Schoenwaelder, "NETCONF Client and Server Models", Work in Progress, draft-ietf-netconf- netconf-client-server-01, November 2016. [RESTCONF-MODELS] Watsen, K. and J. Schoenwaelder, "RESTCONF Client and Server Models", Work in Progress draft-ietf-netconf- restconf-client-server-01, November 2016. [RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP Digest Access Authentication", RFC 7616, DOI 10.17487/RFC7616, September 2015, <http://www.rfc-editor.org/info/rfc7616>.
[RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme", RFC 7617, DOI 10.17487/RFC7617, September 2015, <http://www.rfc-editor.org/info/rfc7617>. [RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, DOI 10.17487/RFC7950, August 2016, <http://www.rfc-editor.org/info/rfc7950>. [Std-802.1AR-2009] IEEE, "IEEE Standard for Local and metropolitan area networks - Secure Device Identity", IEEE Std 802.1AR-2009, DOI 10.1109/IEEESTD.2009.5367679, December 2009, <http://standards.ieee.org/findstds/ standard/802.1AR-2009.html>. [TLS1.3] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", Work in Progress, draft-ietf-tls-tls13-18, October 2016. [zmap] Durumeric, Z., Wustrow, E., and J. Halderman, "ZMap: Fast Internet-Wide Scanning and its Security Applications", 22nd Usenix Security Symposium, August 2013, <https://zmap.io/paper.html>. Acknowledgements The author would like to thank the following (ordered by last name) for lively discussions on the mailing list and in the halls: Jari Arkko, Andy Bierman, Martin Bjorklund, Ben Campbell, Spencer Dawkins, Mehmet Ersue, Stephen Farrell, Wes Hardaker, Stephen Hanna, David Harrington, Jeffrey Hutzelman, Simon Josefsson, Radek Krejci, Suresh Krishnan, Barry Leiba, Alan Luchuk, Kathleen Moriarty, Mouse, Russ Mundy, Tom Petch, Peter Saint-Andre, Joseph Salowey, Juergen Schoenwaelder, Martin Stiemerling, Joe Touch, Hannes Tschofenig, Sean Turner, and Bert Wijnen. Author's Address Kent Watsen Juniper Networks Email: email@example.com