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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 4217

                                                    Paul Ford-Hutchinson
<draft-murray-auth-ftp-ssl-06.txt>                            IBM UK Ltd
                                                        Martin Carpenter
                                                            Verisign Ltd
                                                              Tim Hudson
INTERNET-DRAFT (draft)                                 RSA Australia Ltd
                                                             Eric Murray
                                                        Wave Systems Inc
                                                          Volker Wiegand
                                                              SuSE Linux

                                                    18th September, 2000
This document expires on 17th March, 2001


                         Securing FTP with TLS


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html













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Index
      1. .......... Abstract
      2. .......... Introduction
      3. .......... Audience
      4. .......... Session negotiation on the control port
      5. .......... Response to FEAT command
      6. .......... Data Connection Behaviour
      7. .......... Mechanisms for the AUTH Command
      8. .......... SASL Considerations
      9. .......... Data Connection Security
      10. ......... A discussion of negotiation behaviour
      11. ......... Who negotiates what, where and how
      12. ......... Timing Diagrams
      13. ......... Implications of [FTP-EXT]
      14. ......... Discussion of the 'REIN' command
      15. ......... Security Considerations
      16. ......... IANA Considerations
      17. ......... Network Management
      18. ......... Internationalization
      19. ......... Scalability & Limits
      20. ......... Applicability
      21. ......... Acknowledgements
      22. ......... References
      23. ......... Authors' Contact Addresses
                                  Appendices
      A. .......... Summary of [RFC-2246]
      B. .......... Summary of [RFC-2228]
























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1. Abstract

   This document describes a mechanism that can be used by FTP clients
   and servers to implement security and authentication using the TLS
   protocol defined by [RFC-2246] and the extensions to the FTP protocol
   defined by [RFC-2228].  It describes the subset of the extensions
   that are required and the parameters to be used; discusses some of
   the policy issues that clients and servers will need to take;
   considers some of the implications of those policies and discusses
   some expected behaviours of implementations to allow interoperation.
   This document is intended to provide TLS support for FTP in a similar
   way to that provided for SMTP in [RFC-2487].

   TLS is not the only mechanism for securing file transfer, however it
   does offer some of the following positive attributes:-

      - Flexible security levels.  TLS can support privacy, integrity,
      authentication or some combination of all of these.  This allows
      clients and servers to dynamically, during a session, decide on
      the level of security required for a particular data transfer,

      - It is possible to use X.509 certificates to authenticate client
      users and not just client hosts.

      - Formalised public key management.  By use of X.509 public
      certificates during the authentication phase, certificate
      management can be built into a central function.  Whilst this may
      not be desirable for all uses of secured file transfer, it offers
      advantages in certain structured environments such as access to
      corporate data sources.

      - Co-existence and interoperation with authentication mechanisms
      that are already in place for the HTTPS protocol.  This allows web
      browsers to incorporate secure file transfer using the same
      infrastructure that has been set up to allow secure web browsing.

   The TLS protocol is a development of the Netscape Communication
   Corporation's SSL protocol and this document can be used to allow the
   FTP protocol to be used with either SSL or TLS.  The actual protocol
   used will be decided by the negotiation of the protected session by
   the TLS/SSL layer.

   Note that this specification is in accordance with the FTP RFC
   [RFC-959] and relies on the TLS protocol [RFC-2246] and the FTP
   security extensions [RFC-2228].






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2.  Introduction

   This document is an attempt to describe how three other documents
   should combined to provide a useful, interoperable, secure file
   transfer protocol.  Those documents are:-


      RFC 959 [RFC-959]

         The description of the Internet File Transfer Protocol

      RFC 2246 [RFC-2246]

         The description of the Transport Layer Security protocol
         (developed from the Netscape Secure Sockets Layer (SSL)
         protocol version 3.0).

      RFC 2228 [RFC-2228]

         Extensions to the FTP protocol to allow negotiation of security
         mechanisms to allow authentication, privacy and message
         integrity.

   The File Transfer Protocol (FTP) currently defined in [RFC-959] and
   in place on the Internet is an excellent mechanism for exchanging
   files.  The security extensions to FTP in [RFC-2228] offer a
   comprehensive set of commands and responses that can be used to add
   authentication, integrity and privacy to the FTP protocol.  The TLS
   protocol is a popular (due to its wholesale adoption in the HTTP
   environment) mechanism for generally securing a socket connection.
   There are many ways in which these three protocols can be combined
   which would ensure that interoperation is impossible.  This document
   describes one method by which FTP can operate securely in such a way
   as to provide both flexibility and interoperation.  This necessitates
   a brief description of the actual negotiation mechanism (if used); a
   much more detailed description of the policies and practices that
   would be required and a discussion of the expected behaviours of
   clients and servers to allow either party to impose their security
   requirements on the FTP session.


3.  Audience

   This document is aimed at developers who wish to use TLS as a
   security mechanism to secure FTP clients and/or servers.


4.  Session negotiation on the control port



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   4.1  Negotiated Session Security

      In this scenario, the server listens on the normal FTP control
      port {FTP-PORT} and the session initiation is not secured at all.
      Once the client wishes to secure the session, the AUTH command is
      sent and the server may then allow TLS negotiation to take place.

    4.1.1  Client wants a secured session

        If a client wishes to attempt to secure a session then it
        should, in accordance with [RFC-2228] send the AUTH command with
        the parameter requesting TLS or SSL {TLS-PARM}.


        The client then needs to behave according to its policies
        depending on the response received from the server and also the
        result of the TLS negotiation.  i.e. A client which receives an
        'AUTH' rejection may choose to continue with the session
        unprotected if it so desires.

    4.1.2  Server wants a secured session

        The FTP protocol does not allow a server to directly dictate
        client behaviour, however the same effect can be achieved by
        refusing to accept certain FTP commands until the session is
        secured to an acceptable level to the server.

    4.2  Implicit Session Security

      In this scenario, the server listens on a distinct port {FTP-
      TLSPORT} to the normal unsecured FTP server.  Upon connection, the
      client is expected to start the TLS negotiation.  If the
      negotiation fails or succeeds at an unacceptable level of security
      then it will be a client and/or server policy decision to
      disconnect the session.

5.  Response to the FEAT command

   The FEAT command (introduced in [RFC-2389]) allows servers with
   additional features to advertise these to a client by responding to
   the FEAT command.  If a server supports the 'FEAT' command then it
   MUST advertise supported 'AUTH', 'PBSZ' and 'PROT' commands in the
   reply as described in section 3.2 of [RFC-2389].  Additionally, the
   'AUTH' command should have a reply that identifies 'TLS' as one of
   the possible parameters to 'AUTH'.  It is not necessary to identify
   the 'SSL', 'TLS-P' or 'TLS-C' parameters separately.

   Example reply (in same style is [RFC-2389])



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      C> FEAT
      S> 211-Extensions supported
      S>  AUTH TLS
      S>  PBSZ
      S>  PROT
      S> 211 END


6. Data Connection Behaviour

   The Data Connection in the FTP model can be used in one of three
   ways.  (Note: these descriptions are not necessarily placed in exact
   chronological order, but do describe the steps required. - See
   diagrams later for clarification)

         i) Classic FTP client/server data exchange

         - The client obtains a port, sends the port number to the
         server, the server connects to the client.  The client issues a
         send or receive request to the server on the control connection
         and the data transfer commences on the data connection.

         ii) Firewall-Friendly client/server data exchange (as discussed
         in [RFC-1579]) using the PASV command to reverse the direction
         of the data connection.

         - The client requests that the server open a port, the server
         obtains a port and returns the address and port number to the
         client.  The client connects to the server on this port.  The
         client issues a send or receive request  on the control
         connection and the data transfer commences on the data
         connection.

         iii) Client initiated server/server data exchange (proxy or
         PASV connections)

         - The client requests that server A opens a port, server A
         obtains a port and returns it to the client.  The client sends
         this port number to server B.  Server B connects to server A.
         The client sends a send or receive request to server A and the
         complement to server B and the data transfer commences.  In
         this model server A is the proxy or PASV host and is a client
         for the Data Connection to server B.

   For i) and ii) the FTP client will be the TLS client and the FTP
   server will be the TLS server.

   That is to say, it does not matter which side initiates the



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   connection with a connect() call or which side reacts to the
   connection via the accept() call, the FTP client as defined in
   [RFC-959] is always the TLS client as defined in [RFC-2246].

   In scenario iii) there is a problem in that neither server A nor
   server B is the TLS client given the fact that an FTP server must act
   as a TLS server for Firewall-Friendly FTP [RFC-1579].  Thus this is
   explicitly excluded in the security extensions document [RFC-2228],
   and in this document.



7. Mechanisms for the AUTH Command

   The AUTH command takes a single parameter to define the security
   mechanism to be negotiated.  As the SSL/TLS protocols self-negotiate
   their levels there is no need to distinguish SSL vs TLS in the
   application layer.  The proposed mechanism name for negotiating
   SSL/TLS will be the character string 'TLS'.  This will allow the
   client and server to negotiate SSL or TLS on the control connection
   without altering the protection of the data channel.  To protect the
   data channel as well, the PBSZ, PROT command sequence should be used.
   We call this "Explicit Data Channel Protection".

   However, there are clients and servers that exist today which use the
   string 'SSL' to indicate that negotiation should take place on the
   control connection and that the data connection should be implicitly
   protected (i.e. the PBSZ 0, PROT P command sequence is not required
   but the client and server will protect the data channel as if it
   had). This is "Implicit Data Channel Protection" and is included
   primarily for backward compatibility.

   To allow for streamlining of the negotiation, whilst allowing the
   'SSL' string to sink peacefully into disuse, the strings 'TLS-P' and
   'TLS-C' will also be defined.  'TLS-C' will be a synonym for 'TLS'
   and 'TLS-P' a synonym for 'SSL'. Thus we allow for strict compliance
   with [RFC-2228] by use of 'TLS' or 'TLS-C' and a quicker (2 less
   commands) and perhaps more sensible option 'TLS-P' which will
   implicitly secure the data connection at the same time as securing
   the control connection.

   Note: Regardless of the manner in which the data connection is
   secured (either implicitly by use of 'TLS-P', 'SSL' or connection to
   a well-known port for FTP protocol over TLS, or explicitly by use of
   the PBSZ/PROT sequence) the data connection state may be modified by
   the client issuing the PROT command with the new desired level of
   data channel protection and the server replying in the affirmative.
   This data channel protection negotiation can happen at any point in



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   the session (even straight after a PORT or PASV command) and as often
   as is required.

      See also Section 16, "IANA Considerations".


8. SASL Considerations

   SASL is the Simple Authentication Security Layer. Currently, its
   definition can be found in the internet draft [RFC-2222]. This
   document attempts to define the means by which a connection-based
   protocol may identify and authenticate a client user to a server,
   with additional optional negotiation of protection for the remainder
   of that session.

   Unfortunately, the SASL paradigm does not fit in neatly with the FTP-
   TLS protocol, mainly due to the fact that FTP uses two (independent)
   connections, and under FTP-TLS these may be at different (and
   possibly renegotiable) protection levels. Consequently, it is
   envisaged that SASL will sit underneath TLS on the control
   connection, and TLS (on both, either or neither connection) will be
   used for privacy and integrity (with optional authentication from TLS
   on either connection).


9. Data Connection Security

   The Data Connection security level is determined by two factors.

      1) The mechanism used to negotiate security on the control
      connection will dictate the default (i.e. un-negotiated) security
      level of the data port.

      2) The PROT command, as specified in [RFC-2228] allows
      client/server negotiation of the security level of the data
      connection.  Once a PROT command has been issued by the client and
      accepted by the server by returning the '200' reply, the security
      of subsequent data connections should be at that level until
      another PROT command is issued and accepted; the session ends; or
      the security of the session (via an AUTH command) is re-
      negotiated).

   Data Connection Security Negotiation (the PROT command)

      Note: In line with [RFC-2228], there is no facility for securing
      the Data connection with an insecure Control connection.

      The command defined in [RFC-2228] to negotiate data connection



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      security is the PROT command.  As defined there are four values
      that the PROT command parameter can take.

          'C' - Clear - neither Integrity nor Privacy

          'S' - Safe - Integrity without Privacy

          'E' - Confidential - Privacy without Integrity

          'P' - Private - Integrity and Privacy

      As TLS negotiation encompasses (and exceeds) the
      Safe/Confidential/Private distinction, only Private (use TLS) and
      Clear (don't use TLS) are used.

      For TLS, the data connection can have one of two security levels.

         1) Clear

         2)Private

      With 'Clear' protection level, the data connection is made without
      TLS at all.  Thus the connection is unauthenticated and has no
      privacy or integrity.  This might be the desired behaviour for
      servers sending file lists, pre-encrypted data or non-sensitive
      data (e.g. for anonymous FTP servers).

      If the data connection security level is 'Private' then a TLS
      negotiation must take place, to the satisfaction of the Client and
      Server prior to any data being transmitted over the connection.
      The TLS layers of the Client and Server will be responsible for
      negotiating the exact TLS Cipher Suites that will be used (and
      thus the eventual security of the connection).


      In addition, the PBSZ (protection buffer size) command, as
      detailed in [RFC-2228], is compulsory prior to any PROT command.
      This document also defines a data channel encapsulation mechanism
      for protected data buffers.  For FTP-TLS, which appears to the FTP
      application as a streaming protection mechanism, this is not
      required.  Thus the PBSZ command must still be issued, but must
      have a parameter of '0' to indicate that no buffering is taking
      place and the data connection should not be encapsulated. Note
      that PBSZ 0 is not in the grammar of [RFC-2228], section 8.1,
      where it is stated:
         PBSZ <sp> <decimal-integer> <CRLF> <decimal-integer> ::= any
         decimal integer from 1 to (2^32)-1
      However it should be noted that using a value of '0' to mean a



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      streaming protocol is a reasonable use of '0' for that parameter
      and is not ambiguous.

   Initial Data Connection Security

      For backward compatibility and ease of implementation the
      following rules govern the initial expected protection setting of
      the data connection.

         Connections accepted on the 'secure FTP' port (see
         {FTP-TLSPORT}).
            The initial state of the data connection will be 'Private'
            (Although this does not follow [RFC-2228], this is how such
            clients tend to work today).

         Connections accepted on the normal FTP port {FTP-PORT} with
         TLS/SSL negotiated via an 'AUTH SSL' command.
            The initial state of the data connection will be 'Private'
            (Although this does not follow [RFC-2228], this is how such
            clients tend to work today).

         Connections accepted on the normal FTP port {FTP-PORT} with
         TLS/SSL negotiated via an 'AUTH TLS' command.
            The initial state of the data connection will be 'Clear'
            (this is the correct behaviour as indicated by [RFC-2228].)

      Note: Connections made on other ports may be still behave in one
      of these ways, but that will be a local configuration issue.


10. A Discussion of Negotiation Behaviour

   All these discussions assume that the negotiation has taken place by
   issuing the AUTH command with a mechanism that does not implicitly
   protect the data channel.  Using a mechanism which does implicitly
   secure the data channel or connecting to a port which is implicitly
   protected will have similar issues.

   10.1. The server's view of the control connection

      A server may have a policy statement somewhere that might:

         - Deny any command before TLS is negotiated (this might cause
         problems if a SITE or some such command is required prior to
         login)
         - Deny certain commands before TLS is negotiated (such as USER,
         PASS or ACCT)
         - Deny insecure USER commands for certain users (e.g. not



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         ftp/anonymous)
         - Deny secure USER commands for certain users (e.g.
         ftp/anonymous)
         - Define the level(s) of TLS/SSL to be allowed
         - Define the CipherSuites allowed to be used (perhaps on a per
         host/domain/...  basis)
         - Allow TLS authentication as a substitute for local
         authentication.
         - Define data connection policies (see next section)

         Note: The TLS negotiation may not be completed satisfactorily
         for the server, in which case it can be one of these states.

            The TLS negotiation failed completely

         In this case, the control connection should still be up in
         unprotected mode and the server should issue an unprotected
         '421' reply to end the session.

            The TLS negotiation completed successfully, but the server
            decides that the session parameters are not acceptable (e.g.
            Distinguished Name in the client certificate is not
            permitted to use the server)

         In this case, the control connection should still be up in a
         protected state, so the server can either continue to refuse to
         service commands or issue a '421' reply and close the
         connection.

            The TLS negotiation failed during the TLS handshake

         In this case, the control connection is in an unknown state and
         the server should simply drop the control connection.

      Server code will be responsible for implementing the required
      policies and ensuring that the client is prevented from
      circumventing the chosen security by refusing to service those
      commands that are against policy.

   10.2. The server's view of the data connection

      The server can take one of four basic views of the data connection

         1 - Don't allow encryption at all (in which case the PROT
         command should not allow any value other than 'C' - if it is
         allowed at all)
         2 - Allow the client to choose protection or not
         3 - Insist on data protection (in which case the PROT command



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         must be issued prior to the first attempted data transfer)
         4 - Decide on one of the above three for each and every data
         connection

      The server should only check the status of the data protection
      level (for options 3 and 4 above) on the actual command that will
      initiate the data transfer (and not on the PORT or PASV).  The
      following commands cause data connections to be opened and thus
      may be rejected (before any 1xx) message due to an incorrect PROT
      setting.


         STOR
         RETR
         NLST
         LIST
         STOU
         APPE
         MLST (if [FTP-EXT] is implemented)
         MLSD (if [FTP-EXT] is implemented)


      The reply to indicate that the PROT setting is incorrect is
       '521 data connection cannot be opened with this PROT setting'
      If the protection level indicates that TLS is required, then it
      should be negotiated once the data connection is made.  Thus, the
      '150' reply only states that the command can be used given the
      current PROT level.  Should the server not like the TLS
      negotiation then it will close the data port immediately and
      follow the '150' command with a '522' reply indicating that the
      TLS negotiation failed or was unacceptable.  (Note: this means
      that the application can pass a standard list of CipherSuites to
      the TLS layer for negotiation and review the one negotiated for
      applicability in each instance).

      It is quite reasonable for the server to insist that the data
      connection uses a TLS cached session.  This might be a cache of a
      previous data connection or of the control connection.  If this is
      the reason for the the refusal to allow the data transfer then the
      '522' reply should indicate this.
      Note: this has an important impact on client design, but allows
      servers to minimise the cycles used during TLS negotiation by
      refusing to perform a full negotiation with a previously
      authenticated client.

      It should be noted that the TLS authentication of the server will
      be authentication of the server host itself and not a user on the
      server host.



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   10.3. The client's view of the control connection

      In most cases it is likely that the client will be using TLS
      because the server would refuse to interact insecurely.  To allow
      for this, clients must be able to be flexible enough to manage the
      securing of a session at the appropriate time and still allow the
      user/server policies to dictate exactly when in the session the
      security is negotiated.

      In the case where it is the client that is insisting on the
      securing of the session, it will need to ensure that the
      negotiations are all completed satisfactorily and will need to be
      able to inform the user sensibly should the server not support, or
      be prepared to use, the required security levels.

      Clients must be coded in such a manner as to allow the timing of
      the AUTH, PBSZ and PROT commands to be flexible and dictated by
      the server.  It is quite reasonable for a server to refuse certain
      commands prior to these commands, similarly it is quite possible
      that a SITE or quoted command might be needed by a server prior to
      the AUTH.  A client must allow a user to override the timing of
      these commands to suit a specific server.
      For example, a client should not insist on sending the AUTH as the
      first command in a session, nor should it insist on issuing a
      PBSZ, PROT pair directly after the AUTH.  This may well be the
      default behaviour, but must be overridable by a user.

      Note: The TLS negotiation may not be completed satisfactorily for
      the client, in which case it will be in one of these states:

            The TLS negotiation failed completely

            In this case, the control connection should still be up in
            unprotected mode and the client should issue an unprotected
            QUIT command to end the session.

            The TLS negotiation completed successfully, but the client
            decides that the session parameters are not acceptable (e.g.
            Distinguished Name in certificate is not the actual server
            expected)

            In this case, the control connection should still be up in a
            protected state, so the client should issue a protected QUIT
            command to end the session.

            The TLS negotiation failed during the TLS handshake

            In this case, the control connection is in an unknown state



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            and the client should simply drop the control connection.

   10.4. The client's view of the data connection

   Client security policies

      Clients do not typically have 'policies' as such, instead they
      rely on the user defining their actions and, to a certain extent,
      are reactive to the server policy.  Thus a client will need to
      have commands that will allow the user to switch the protection
      level of the data connection dynamically, however, there may be a
      general 'policy' that attempts all LIST and NLST commands on a
      Clear connection first (and automatically switches to Private if
      it fails).  In this case there would need to be a user command
      available to ensure that a given data transfer was not attempted
      on an insecure data connection.

      Clients also need to understand that the level of the PROT setting
      is only checked for a particular data transfer after that transfer
      has been requested.  Thus a refusal by the server to accept a
      particular data transfer should not be read by the client as a
      refusal to accept that data protection level in toto, as not only
      may other data transfers be acceptable at that protection level,
      but it is entirely possible that the same transfer may be accepted
      at the same protection level at a later point in the session.

      It should be noted that the TLS authentication of the client
      should be authentication of a user on the client host and not the
      client host itself.






















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11. Who negotiates what, where and how

   11.1. Do we protect at all ?

      Client issues AUTH <Mechanism>, server accepts or rejects.
      If server needs AUTH, then it refuses to accept certain commands
      until it gets a successfully protected session.

   11.2. What level of protection do we use on the Control connection ?

      Decided entirely by the TLS CipherSuite negotiation.

   11.3. Do we protect data connections in general ?

      Client issues PROT command, server accepts or rejects.


   11.4. Is protection required for a particular data transfer ?

      A client would already have issued a PROT command if it required
      the connection to be protected.
      If a server needs to have the connection protected then it will
      reply to the STOR/RETR/NLST/... command with a '522' indicating
      that the current state of the data connection protection level is
      not sufficient for that data transfer at that time.

   11.5. What level of protection is required for a particular data
   transfer ?

      Decided entirely by the TLS CipherSuite negotiation.

   Thus it can be seen that, for flexibility, it is desirable for the
   FTP application to be able to interact with the TLS layer upon which
   it sits to define and discover the exact TLS CipherSuites that are to
   be/have been negotiated and make decisions accordingly.  However it
   should be entirely possible, using the mechanisms described in this
   document, to have a TLS client or server sitting on top of a generic
   'TLS socket layer'.  In this case, interoperability for a client with
   a smart TLS-aware server may not be possible due to server policies.












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12. Timing Diagrams

   12.1. Establishing a protected session

           Client                                 Server
  control          data                   data               control
====================================================================

                                                             socket()
                                                             bind()
  socket()
  connect()  ----------------------------------------------> accept()
  AUTH TLS   ---------------------------------------------->
            <----------------------------------------------  234
  TLSneg()  <----------------------------------------------> TLSneg()
  PBSZ 0     ---------------------------------------------->
            <----------------------------------------------  200
  PROT P     ---------------------------------------------->
            <----------------------------------------------  200
  USER fred  ---------------------------------------------->
            <----------------------------------------------  331
  PASS pass  ---------------------------------------------->
            <----------------------------------------------  230

Note: the order of the PBSZ/PROT pair and the USER/PASS pair (with
respect to each other) is not important (i.e. the USER/PASS can happen
prior to the PBSZ/PROT - or indeed the server can refuse to allow a
PBSZ/PROT pair until the USER/PASS pair has happened).























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   12.2. A standard data transfer without protection.

           Client                                 Server
  control          data                   data               control
====================================================================

                   socket()
                   bind()
  PORT w,x,y,z,a,b ----------------------------------------->
      <----------------------------------------------------- 200
  STOR file ------------------------------------------------>
                                          socket()
                                          bind()
      <----------------------------------------------------- 150
                   accept() <-----------  connect()
                   write()   -----------> read()
                   close()   -----------> close()
      <----------------------------------------------------- 226

































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   12.3. A firewall-friendly data transfer without protection

           Client                                 Server
  control          data                   data               control
====================================================================

  PASV -------------------------------------------------------->
                                          socket()
                                          bind()
      <------------------------------------------ 227 (w,x,y,z,a,b)
                   socket()
  STOR file --------------------------------------------------->
                   connect()  ----------> accept()
      <-------------------------------------------------------- 150
                   write()    ----------> read()
                   close()    ----------> close()
      <-------------------------------------------------------- 226


    Note: Implementors should be aware that then connect()/accept()
    function is performed prior to the receipt of the reply from the
    STOR command. This contrasts with situation when (non-firewall-
    friendly) PORT is used prior to the STOR, and the accept()/connect()
    is performed after the reply from the aforementioned STOR has been
    dealt with.


























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   12.4. A standard data transfer with protection

           Client                                 Server
  control          data                   data               control
====================================================================

                   socket()
                   bind()
  PORT w,x,y,z,a,b -------------------------------------------->
      <-------------------------------------------------------- 200
  STOR file --------------------------------------------------->
                                          socket()
                                          bind()
      <-------------------------------------------------------- 150
                   accept()  <----------  connect()
                   TLSneg()  <----------> TLSneg()
                   TLSwrite() ----------> TLSread()
                   close()    ----------> close()
      <-------------------------------------------------------- 226
































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   12.5. A firewall-friendly data transfer with protection

           Client                                 Server
  control          data                   data               control
====================================================================

  PASV -------------------------------------------------------->
                                          socket()
                                          bind()
      <------------------------------------------ 227 (w,x,y,z,a,b)
                   socket()
  STOR file --------------------------------------------------->
                   connect()  ----------> accept()
      <-------------------------------------------------------- 150
                   TLSneg()   <---------> TLSneg()
                   TLSwrite()  ---------> TLSread()
                   close()     ---------> close()
      <-------------------------------------------------------- 226

































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13. Implications of [FTP-EXT]

   13.1. MLST and MLSD

      MLST and MLSD are directory listing commands and should be treated
      in the same manner as NLST and LIST for the purposes of this
      document.












































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14. Discussion of the 'REIN' command

   The 'REIN' command, defined in [RFC-959], allows the user to reset
   the state of the FTP session.
      REINITIALIZE (REIN)
         This command terminates a USER, flushing all I/O and account
         information, except to allow any transfer in progress to be
         completed.  All parameters are reset to the default settings
         and the control connection is left open.  This is identical to
         the state in which a user finds himself immediately after the
         control connection is opened.  A USER command may be expected
         to follow.
   The defined behaviour for TLS protected FTP sessons will depend on
   the manner of session initialisation.
   If the session has been explicity protected (see section 4.1) then
   the TLS session(s) will be cleared and the control and data
   connections revert to unprotected, clear communications.  It will be
   acceptable to use cached TLS sessions for subsequent connections,
   however a server should not mandate this.
   If the session is implicitly protected  (see section 4.2) then the
   control connection will continue to be protected using the exisiting
   negotiated TLS session and the data connection will revert to being
   implicitly protected, irrespective of any 'PROT' commands preceding
   the 'REIN'.



























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15. Security Considerations

   This entire document deals with security considerations related to
   the File Transfer Protocol.

   15.1. Verification of Authentication tokens

      15.1.1. Server Certificates

         Although it is entirely an implementation decision, it is
         recommended that certificates used for server authentication of
         the TLS session contain the server identification information
         in a similar manner to those used for http servers.  (i.e.
         SubjectCommonName or SubjectAltName of type dNSName).

         Note that, if there is any future extensions to the FTP
         protocol to allow multi-homed servers, then the interaction of
         such a mechanism, the REIN commands and the certificate
         presented by the server in the TLS handshake will need to be
         considered carefully.

      15.1.2. Client Certificates

         - Deciding which client certificates to allow and defining
         which fields define what authentication information is entirely
         a server implementation issue.

         - It is also server implementation issue to decide if the
         authentication token presented for the data connection must
         match the one used for the corresponding control connection.

   15.2. Addressing FTP Security Considerations [RFC-2577]

      15.2.1. Bounce Attack

         A bounce attack should be harder in a secured FTP environment
         because:

            - The FTP server that is being used to initiate a false
            connection will always be a 'server' in the TLS context.
            Therefore, only services that act as 'clients' in the TLS
            context could be vulnerable.  This would be a counter-
            intuitive way to implement TLS on a service.

            - The FTP server would detect that the authentication
            credentials for the data connection are not the same as
            those for the control connection, thus the server policies
            could be set to drop the data connection.



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            - Genuine users are less likely to initiate such attacks
            when the authentication is strong and malicious users are
            less likely to gain access to the FTP server if the
            authentication is not easily subverted (password guessing,
            network tracing, etc...)

      15.2.2. Restricting Access

         This document presents a strong mechanism for solving the issue
         raised in this section.

      15.2.3. Protecting Passwords

         The twin solutions of strong authentication and data
         confidentiality ensure that this is not an issue when TLS is
         used to protect the control session.

      15.2.4. Privacy

         The TLS protocol ensures data confidentiality by encryption.
         Privacy (e.g. access to download logs, user profile
         information, etc...) is outside the scope of this document (and
         [RFC-2577] presumably)

      15.2.5. Protecting Usernames

         This is not an issue when TLS is used as the primary
         authentication mechanism.

      15.2.6. Port Stealing

         This proposal will do little for the Denial of Service element
         of this section, however, strong authentication on the data
         connection will prevent unauthorised connections retrieving or
         submitting files.

      15.2.7. Software-Base Security Problems

         Nothing in this proposal will affect the discussion in this
         section.




16. IANA Considerations

   {FTP-PORT} - The port assigned to the FTP control connection is 21.




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   {FTP-TLSPORT} - A port to be assigned by the IANA for native TLS FTP
   connections on the control socket.  This has been provisionally
   reserved as port 990.

   {TLS-PARM} - A parameter for the AUTH command to indicate that TLS is
   required.  It is recommended that 'TLS', 'TLS-C', 'SSL' and 'TLS-P'
   are acceptable, and mean the following :-

       'TLS' or 'TLS-C' - the TLS protocol or the SSL protocol will be
      negotiated on the control connection.  The default protection
      setting for the Data connection is 'Clear'.

       'SSL' or 'TLS-P' - the TLS protocol or the SSL protocol will be
      negotiated on the control connection.  The default protection
      setting for the Data connection is 'Private'.  This is primarily
      for backward compatibility.

         Note - [RFC-2228] states that these parameters are case-
         insensitive.


17. Network Management

   NONE


18. Internationalization

   NONE


19. Scalability & Limits

   There are no issues other than those concerned with the ability of
   the server to refuse to have a complete TLS negotiation for each and
   every data connection, which will allow servers to retain throughput
   whilst using cycles only when necessary.














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20. Applicability

   This mechanism is generally applicable as a mechanism for securing
   the FTP protocol.  It is unlikely that anonymous FTP clients or
   servers will require such security (although some might like the
   authentication features without the privacy).


21. Acknowledgements

   o Netscape Communications Corporation for the original SSL protocol.

   o Eric Young for the SSLeay libraries.

   o University of California, Berkley for the original implementations
   of FTP and ftpd on which the initial implementation of these
   extensions were layered.

   o IETF CAT working group.

   o IETF TLS working group.

   o IETF FTPEXT working group.




























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22. References

   [RFC-959] J. Postel, "File Transfer Protocol"
      RFC 959, October 1985.

   [RFC-1579] Bellovin, S., "Firewall-Friendly FTP"
      RFC 1579, February 1994.

   [RFC-2222] J. Myers, "Simple Authentication and Security Layer"
      RFC 2222, October 1997.

   [RFC-2228] M. Horowitz, S. Lunt, "FTP Security Extensions"
      RFC 2228, October 1997.

   [RFC-2246] T. Dierks, C. Allen, "The TLS Protocol Version 1.0"
      RFC 2246, January 1999.

   [RFC-2389] P Hethmon, R.Elz, "Feature Negotiation Mechanism for the
   File Transfer Protocol"
      RFC 2389, August 1998.

   [RFC-2487] P Hoffman, "SMTP Service Extension for Secure SMTP over
   TLS"
      RFC 2487, January 1999.

   [RFC-2577] M Allman, S Ostermann "FTP Security Considerations"
      RFC 2577, May 1999.

   [FTP-EXT] R Elz, P Hethmon "Extensions to FTP"
      draft-ietf-ftpext-mlst-07.txt, June 1999.

   [SRA-FTP] "SRA - Secure RPC Authentication for TELNET and FTP Version
   1.1"
      file://ftp.funet.fi/security/login/telnet/doc/sra/sra.README

















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23. Authors' Contact Addresses

Please send comments to Paul Ford-Hutchinson at the address below

        Tim Hudson                  Paul Ford-Hutchinson
           RSA Data Security           IBM UK Ltd
             Australia Pty Ltd         PO Box 31
                                       Birmingham Road
                                       Warwick
                                       United Kingdom
  tel -   +61 7 3227 4444             +44 1926 462005
  fax -   +61 7 3227 4400             +44 1926 496482
email - tjh@rsasecurity.com.au    paulfordh@uk.ibm.com

        Martin Carpenter            Eric Murray
           Verisign Ltd                Wave Systems Inc.
email -  mcarpenter@verisign.com    ericm@lne.com

        Volker Wiegand
           SuSE Linux
email -  wiegand@suse.de






























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                               Appendices

A.  Summary of [RFC-2246]

   The TLS protocol was developed by the IETF TLS working group.  It is
   based on the SSL protocol proposed by Netscape Communications
   Corporation.  The structure of the start of a TLS session allows
   negotiation of the level of the protocol to be used - in this way, a
   client or server can simultaneously support TLS and SSL and negotiate
   the most appropriate for the connection.

   The TLS protocol defines three security mechanisms that may be used
   (almost) independently.  They are Authentication, Integrity and
   Privacy.  It is possible to have an Authenticated session with no
   Privacy and with or without Integrity (useful for anonymous FTP
   sites, or sites with pre-encrypted data). For example, sessions with
   Authentication, Privacy and Integrity would be useful for control
   connections over an insecure network and data connections
   transferring confidential material.

   The TLS protocol allows unauthenticated sessions; server
   authentication or client and server authentication.  There is no
   mechanism for authenticating a client without first authenticating
   the server.

   The basic mechanism of the TLS protocol is that (for an
   Authenticated, Private session) asymmetric encryption is used to
   authenticate clients and servers and exchange a session key for
   symmetric encryption which is to be used for the rest of the session.

   The structure of the TLS session initialisation is that the client
   initiates the session with a 'ClientHello' message.  The server will
   respond with a 'ServerHello' and the session negotiation will
   continue.

   The TLS protocol allows session caching which is achieved by the
   client requesting that the server re-use a session context (Cipher
   Suite and symmetric key) in the ClientHello message.  There is no
   reason why a second connection could not request a 'cached' session
   with the same context as an existing session.











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B.  Summary of [RFC-2228]



   Extensions to FTP

   The FTP Security Extensions document has 8 new commands to enhance
   the FTP protocol to allow negotiation of security and exchange of
   security data.  Three of these commands (the AUTH, PBSZ and PROT
   commands) are used by this document to allow an FTP client to
   negotiate TLS with the server.  The other commands are not required.

   i) AUTH

      This command is a request by the client to use an authentication
      and/or security mechanism.

      The client will issue an 'AUTH <Mechanism>' command
       which will be a request to the server to secure the control
      connection using the TLS (or SSL) protocol. It also governs the
      initial protection setting of the data channel (which may be
      changed by a subsequent PROT command).

   ii) ADAT

      This command is used to transmit security data required by the
      security mechanism agreed in a preceding AUTH command.
      This document does not use the ADAT command.

   iii) PROT

      This command is used by the client to instruct the type of
      security that is required on the Data connection.

      The 'PROT C' command will mean that TLS should not be used to
      secure the data connection; 'PROT P' means that TLS should be
       used.  'PROT E' and 'PROT S' are not defined and generate
       a '536' reply from the server.

   iv) PBSZ

      This command is used to negotiate the size of the buffer to be
      used during secured data transfer.

      The PBSZ command must be issued prior to the PROT command.  The
      PBSZ command cannot be sent on an insecure control connection.
      For FTP and TLS the only valid value for the parameter is '0', all
      other values should receive a '200' reply with the text 'PBSZ=0'



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      included.

   v) CCC

      This command is used to specify that the control channel no longer
      requires protection.
      This document does not use the CCC command.

   vi) MIC

      This command is used to send a normal FTP command with integrity
      protection.
      This document does not use the MIC command.

   vii) CONF

      This command is used to send a normal FTP command with
      confidentiality protection (encrypted).
      This document does not use the CONF command.

   viii) ENC

      This command is used to send a normal FTP command with
      confidentiality and integrity protection.
      This document does not use the ENC command.


























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Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

This document expires on 17th March, 2001






















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