NETCONF Working Group                                     Mohamad Badra
Internet Draft                                         LIMOS Laboratory
Intended status: Standards Track                           May 27,                           June 5, 2008
Expires: November December 2008

                NETCONF over Transport Layer Security (TLS)
                       draft-ietf-netconf-tls-02.txt
                       draft-ietf-netconf-tls-03.txt

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Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   The Network Configuration Protocol (NETCONF) provides mechanisms to
   install, manipulate, and delete the configuration of network devices.
   This document describes how to use the Transport Layer Protocol (TLS)
   to secure NETCONF exchanges.

Table of Contents

   1. Introduction...................................................3
      1.1. Conventions used in this document.........................3
   2. NETCONF over TLS...............................................3
      2.1. Connection Initiation.....................................3
      2.2. Connection Closure........................................4
   3. Endpoint Authentication and Identification.....................4
      3.1. Server Identity...........................................5
      3.2. Client Identity...........................................6
      3.3. Password-Based Authentication.............................6
   4. Cipher Suite Requirements......................................7
   5. Security Considerations........................................7
   6. IANA Considerations............................................7 Considerations............................................8
   7. Acknowledgments................................................8
   A. Appendix - Test Vectors for the PSK Derivation Function........9
   B. Appendix - Enabling Third Party Authentication using Passwords10
      B.1. Working Group discussion at the 71st IETF meeting........12
   Normative References.............................................13 References..............................................9
   Authors' Addresses...............................................14 Addresses...............................................10
   Intellectual Property and Copyright Statements...................14 Statement..................................10
   Disclaimer of Validity...........................................11

1. Introduction

   The NETCONF protocol [RFC4741] defines a simple mechanism through
   which a network device can be managed.  NETCONF is connection-
   oriented, requiring a persistent connection between peers.  This
   connection must provide reliable, sequenced data delivery, integrity
   and confidentiality and peers authentication.  This document
   describes how to use TLS [RFC4346] to secure NETCONF connections.

   Throughout this document, the terms "client" and "server" are used to
   refer to the two ends of the TLS connection.  The client actively
   opens the TLS connection, and the server passively listens for the
   incoming TLS connection.  The terms "manager" and "agent" are used to
   refer to the two ends of the NETCONF protocol session.  The manager
   issues NETCONF remote procedure call (RPC) commands, and the agent
   replies to those commands.  When NETCONF is run over TLS using the
   mapping defined in this document, the client is always the manager,
   and the server is always the agent.

1.1. Conventions used in this document

   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. NETCONF over TLS

   Since TLS is application protocol-independent, NETCONF can operate on
   top of the TLS protocol transparently.  This document defines how
   NETCONF can be used within a Transport Layer Security (TLS) session.

2.1. Connection Initiation

   The peer acting as the NETCONF manager MUST also act as the TLS
   client.  It MUST connect to the server that passively listens for the
   incoming TLS connection on the IANA-to-be-assigned TCP port <TBA>.
   It MUST therefore send the TLS ClientHello to begin the TLS
   handshake.  Once the TLS handshake has been finished, the client and
   the server MAY then send their NETCONF exchanges.  In particular, the
   client will send complete XML documents to the server containing
   <rpc> elements, and the server will respond with complete XML
   documents containing <rpc-reply> elements.  The client MAY indicate
   interest in receiving event notifications from a NETCONF server by
   creating a subscription to receive event notifications [I-D.ietf-- [I-D.ietf-
   netconf-notification], in which the NETCONF server replies to
   indicate whether the subscription request was successful and, if it
   was successful, begins sending the event notifications to the NETCONF
   client as the events occur within the system.  All these elements are
   encapsulated into TLS records of type "application data".  These
   records are protected using the TLS material keys.

   Current NETCONF messages don't include a message's length.  This
   document uses consequently the same delimiter sequence defined in
   [RFC4742] and therefore the special character sequence, ]]>]]>, to
   delimit XML documents.

2.2. Connection Closure

   Either NETCONF peer MAY stop the NETCONF connection at any time and
   therefore notify the other NETCONF peer that no more data on this
   channel will be sent and that any data received after a closure
   request will be ignored.  This MAY happen when no data is received
   from a connection for a long time, where the application decides what
   "long" means.

   TLS has the ability for secure connection closure using the Alert
   protocol.  When the NETCONF peer closes the NETCONF connection, it
   MUST send a TLS close_notify alert before closing the TCP connection.
   Any data received after a closure alert is ignored.

   Unless a fatal error has occurred, each party is required to send a
   close_notify alert before closing the write side of the connection
   [RFC4346].  The other party MUST respond with a close_notify alert of
   its own and close down the connection immediately, discarding any
   pending writes.  It is not required for the initiator of the close to
   wait for the responding close_notify alert before closing the read
   side of the connection.

3. Endpoint Authentication and Identification

   NETCONF requires that its transport provide mutual authentication of
   client and server, so cipher suites that are anonymous or which only
   authenticate the server to the client MUST NOT be used with NETCONF.
   This document specifies how to use TLS with endpoint authentication,
   which can be based on either preshared keys [RFC4279] or public key
   certificates [RFC4346].  Some cipher suites (e.g.
   TLS_RSA_PSK_WITH_AES_128_CBC_SHA) use both.  Section 3.1 describes
   how the client authenticates the server if public key certificates
   are provided by the server, section 3.2 describes how the server
   authenticates the client if public key certificates are provided by
   the client, and section 3.3 describes how the client and server
   mutually authenticate one another using a password.

3.1. Server Identity

   During the TLS negotiation, the client MUST carefully examine the
   certificate presented by the server to determine if it meets their
   expectations.  Particularly, the client MUST check its understanding
   of the server hostname against the server's identity as presented in
   the server Certificate message, in order to prevent man-in-the-middle
   attacks.

   Matching is performed according to these rules [RFC4642]:

      - The client MUST use the server hostname it used to open the
        connection (or the hostname specified in TLS "server_name"
        extension [RFC4366]) as the value to compare against the server
        name as expressed in the server certificate.  The client MUST
        NOT use any form of the server hostname derived from an
        insecure remote source (e.g., insecure DNS lookup).  CNAME
        canonicalization is not done.

      - If a subjectAltName extension of type dNSName is present in the
        certificate, it MUST be used as the source of the server's
        identity.

      - Matching is case-insensitive.

      - A "*" wildcard character MAY be used as the left-most name
        component in the certificate.  For example, *.example.com would
        match a.example.com, foo.example.com, etc., but would not match
        example.com.

      - If the certificate contains multiple names (e.g., more than one
        dNSName field), then a match with any one of the fields is
        considered acceptable.

   If the match fails, the client MUST either ask for explicit user
   confirmation or terminate the connection and indicate the server's
   identity is suspect.

   Additionally, clients MUST verify the binding between the identity of
   the servers to which they connect and the public keys presented by
   those servers.  Clients SHOULD implement the algorithm in Section 6
   of [RFC5280] for general certificate validation, but MAY supplement
   that algorithm with other validation methods that achieve equivalent
   levels of verification (such as comparing the server certificate
   against a local store of already-verified certificates and identity
   bindings).

   If the client has external information as to the expected identity of
   the server, the hostname check MAY be omitted.

3.2. Client Identity

   Typically, the server has no external knowledge of what the client's
   identity ought to be and so checks (other than that the client has a
   certificate chain rooted in an appropriate CA) are not possible.  If
   a server has such knowledge (typically from some source external to
   NETCONF or TLS) it MUST check the identity as described above.

3.3. Password-Based Authentication

   [RFC4279] supports authentication based on pre-shared keys (PSKs).
   These pre-shared keys are symmetric keys, shared in advance among the
   communicating parties.

   The PSK can be generated in many ways and its length is variable.
   Implementation of this document MAY rely on [RFC4279] to enable
   password based user authentication.  In this case, the password is
   used to generate the PSK.  It is RECOMMENDED that implementations
   that allow the administrator to manually configure the password also
   provide functionality for generating a new random password, taking
   [RFC4086] into account.

   This document generates the PSK from the password as follow:

    PSK = SHA-1(SHA-1(psk_identity + "Key Pad for Netconf" + password) +
                psk_identity_hint)

   Where + means concatenation.

   The label "Key Pad for Netconf" is an ASCII string.

   The psk_identity_hint is initially defined in section 5.1 of
   [RFC4279].  The psk_identity_hint can do double duty and also provide
   a form of server authentication in the case where the user has the
   same password on a number of NETCONF servers.  If a hint is provided,
   the psk_identity_hint is encoded in the same way as in [RFC4279] and
   should be a string representation of the name of the server
   recognizable to the administrator or his software.  In the case where
   the user types a server name to connect to, it should be that string.
   If the string the user enters differs from the one returned as
   psk_identity_hint, the software could display the server's name and
   ask the user to confirm.  For automated scripts, the names could be
   expected to match.  It is highly recommended that implementations set
   the psk_identity_hint to the DNS name of the NETCONF server (i.e.,
   the TLS server).

   It is RECOMMENDED that users choose different passwords for the
   different servers they manage.

      Note 1: The NETCONF over TLS implementation need not store the
      password in clear text, but rather can store the value of the
      inner SHA-1 (SHA-1(SHA-1(password + psk_identity SHA-1, (SHA-1(psk_identity + "Key Pad for
      Netconf") Netconf" + psk_identity_hint)),
      password)), which could not be used as a password equivalent for
      applications other than NETCONF.  Deriving the PSK from a password
      is not secure.  This construction is used because it is
      anticipated that people will do it anyway.

      Note 2: [RFC4279] defines some conformance requirements for the
      PSK, for the PSK identity encoding and for the identity hint. The
      same requirements apply here as well; in particular on the
      password.  Moreover, the management interface by which the
      password is provided MUST accept ASCII strings of at least 64
      octets and MUST NOT add a null terminator before using them as
      shared secrets.  It MUST also accept a HEX encoding of the
      password.  The management interface MAY accept other encodings if
      the algorithm for translating the encoding to a binary string is
      specified.

4. Cipher Suite Requirements

   A compliant implementation of the protocol specified in this document
   MUST implement the cipher suite TLS_DHE_PSK_WITH_AES_128_CBC_SHA and
   MAY implement any TLS cipher suite that provides mutual
   authentication.

5. Security Considerations

   The security considerations described throughout [RFC4346] and
   [RFC4279] apply here as well.

   This document in its current version doesn't support third party
   authentication due to the fact that TLS doesn't specify this way of
   authentication and that NETCONF depends on the transport protocol for
   the authentication service.  If third party authentication is needed,
   BEEP or SSH transport can be used.

   As with all schemes involving shared keys and passwords, special care
   should be taken to protect the shared values and passwords as well as
   to limit their exposure over time.  Alternatively, using certificates
   would provide better protection.

6. IANA Considerations

   IANA is requested to assign a TCP port number that will be the
   default port for NETCONF over TLS sessions as defined in this
   document.

   IANA has assigned port <TBA> for this purpose.

7. Acknowledgments

   A significant amount of the text in Section 3.1 was lifted from
   [RFC4642].

   The author would like to acknowledge David Harrington, Miao Fuyou,
   Eric Rescorla, Juergen Schoenwaelder, Simon Josefsson, Olivier
   Coupelon and the NETCONF mailing list members for their comments on
   the document.  The author appreciates also Bert Wijnen, Mehmet Ersue
   and Dan Romascanu for their efforts on issues resolving discussion,
   and Charlie Kaufman for the thorough review of this document and for
   the helpful comments on the password-based authentication.

A. Appendix - Test Vectors for the PSK Derivation Function

   The test vectors for the PSK derivation function in this document
   have been cross-verified by two independent implementations.  An
   implementation that concurs with the results provided in this
   document should be interoperable with other similar implementations.

         password = password
         psk_identity = psk_identity
         psk_identity_hint = psk_identity_hint

         The inner SHA-1 value (in hex):

         inner := SHA-1(password + psk_identity SHA-1(psk_identity + "Key Pad for Netconf") Netconf" + password)
               == SHA-1("psk_identityKey Pad for Netconfpassword")
               => 6d6eeb6a b8d0466b 45245d07 47d86726 b41b868c

         The outer SHA-1 value (in hex):

         outer := SHA-1(inner + psk_identity_hint)
               => 88f3824b 3e5659f5 2d00e959 bacab954 b6540344

B. Appendix - Enabling Third Party Authentication using Passwords

   During the 71st IETF meeting, several proposals have been proposed to
   enable third party authentication that could be used in combination
   with existing user authentication databases such as RADIUS. They are
   listed below. More details on those proposals may be found at
   https://www3.ietf.org/proceedings/08mar/slides/netconf-1/netconf-
   1.htm and
   http://www.psg.com/lists/netconf/netconf.2008/msg00125.html.

   We summarize them as following:

   1. Defining <user-login> RPC:
      --------------------------

     This option relies on JUNOS mechanism to enable an authentication
     function via third parties. It consists of establishing a TLS with
     no manager authentication, leaving the <request-login> RPC as the
     only valid RPC.  Anything else is an error.

     Once the TLS session is established, the agent MUST authenticate
     the manager by emitting the following <rpc> tag element:

         <rpc-reply message-id="101"
            xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
              <challenge>Password:</challenge>
         </rpc-reply>

       In which the manager MUST reply with the following:

         <rpc>
            <request-login>
               <challenge-response>password</challenge-response>
            </request-login>
         </rpc>

     The rules to handle this were pretty simple:

        - The <request-login> RPC could only be performed if the session
          wasn't authenticated.

        - No other RPCs could be performed if the session wasn't
          authenticated.

        - The transport protocol can authenticate the session
          (internally).

     Pros and cons:

     o   is simple to do. But

     o   might raise questions from the security ADs; NETCONF assumes
         the authentication is part of the transport not NETCONF.

     o   only works for plaintext passwords (SASL PLAIN).

   2. Enhancing TLS:
      --------------

     The second option consists of extending TLS so the manager
     authentication becomes part of TLS. This extension, detailed in
     http://tools.ietf.org/id/draft-badra-tls-password-ext-01.txt,
     defines a new extension and a new TLS message to the TLS protocol
     to enable TLS client authentication using passwords. The extension
     is used to convey the manager login, whereas the new message is
     defined and sent by the manager to prove its knowledge of the
     password.

     Steps during the TLS negotiation:

         - The manager adds such an extension to its TLS ClientHello.

         - If the agent agrees on using this extension, it will notify
           the manager and replies with its certificate and/or its
           authenticated public key.

         - The manager generates a premaster secret and encrypts it
           using the agent public key.

         - The manager then computes the session key using the premaster
           secret and encrypts, among others, its password with the
           computed key.

         - The agent decrypts the premaster secret and computes the same
           key to decrypt the password.

         - The agent checks with a database (or AAA infrastructures) to
           verify the password and then to authenticate the manager.

     Pros and cons

     o   is simple to do. But
     o   It is indeed not easy to convince TLS WG to add password
         authentication extension to TLS.

   3. Running BEEP over TLS:
      ----------------------

     It looks complex for a solution, requires that all implementations
     do actually support BEEP.

   4. Extending NETCONF with a message to start TLS:
      ----------------------------------------------

     This option consists of extending NETCONF with a new message to
     start the TLS negotiation and to perform an authentication
     mechanism based on RFC4422 (SASL) or on any similar protocol.

       Pros and cons

       o   simple to do. But

       o   might raise questions from the security ADs; NETCONF assumes
           the authentication is part of the transport not NETCONF.
           Moreover, it adds complexity related to the use of SASL
           PLAIN.

   5. Enable SSH (RFC4742 and TLS (as defined through this document:
      --------------------------------------------------------------

     Since SSH already defines a password-based authentication and
     because this protocol MUST be implemented as a security protocol
     for NETCONF, users can rely on SSH for password authentication, and
     on TLS for authentication using PSK or certificates. This means the
     agent SHOULD passively listen for the incoming SSH (respectively
     TLS) connection on port 830 (respectively port <TBA-by-IANA>).

       Pros and cons

       o   simple to do.

       o   already specified by RFC4742 and by the current document.

B.1. Working Group discussion at the 71st IETF meeting

   Some of the options have been found as not practical in the WG
   session during 71st meeting.

   Options #2 and #3 have not been supported in the WG session.

   Option #1 and # 4 seems to be against the security design for
   NETCONF. Whether #5 or other options can be accepted by the WG
   members needs to be discussed on the mailing list.

Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
             Housley, R., and W. Polk, "Internet X.509 Public Key
             Infrastructure Certificate and Certificate Revocation List
             (CRL) Profile", RFC 5280, May 2008.

   [RFC4086] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
             "Randomness Requirements for Security", BCP 106, RFC 4086,
             June 2005.

   [RFC4279] Eronen, P. and H. Tschofenig., "Pre-Shared Key Ciphersuites
             for Transport Layer Security (TLS)", RFC 4279, December
             2005.

   [RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol 1.1", RFC 4346, April 2006.

   [RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
             and T. Wright, "Transport Layer Security (TLS) Extensions",
             RFC 4366, April 2006.

   [RFC4642] Murchison, K., Vinocur, J., Newman, C., "Using Transport
             Layer Security (TLS) with Network News Transfer Protocol
             (NNTP)", RFC 4642, October 2006

   [RFC4741] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
             December 2006.

   [RFC4742] Wasserman, M. and T. Goddard, "Using the NETCONF
             Configuration Protocol over Secure Shell (SSH)", RFC 4742,
             December 2006.

   [I-D.ietf-netconf-notification]
             Chisholm, S. and H. Trevino, "NETCONF Event Notifications",
             draft-ietf-netconf-notification-12.txt,
             draft-ietf-netconf-notification-13.txt, (work in progress),
             February
             May 2008.

Authors' Addresses

   Mohamad Badra
   LIMOS Laboratory - UMR6158, CNRS
   France

   Email: badra@isima.fr

Contributors

   Ibrahim Hajjeh
   INEOVATION
   France

   Email: hajjeh@ineovation.com

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