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Versions: (draft-ietf-ftpext2-hosts) 00 01 02 03 04 05 RFC 7151

Network Working Group                                         P. Hethmon
Internet-Draft                                          Hethmon Brothers
Updates: 959 (if approved)                                   R. McMurray
Intended status: Standards Track                   Microsoft Corporation
Expires: November 15, 2013                                  May 14, 2013


         File Transfer Protocol HOST Command for Virtual Hosts
               draft-hethmon-mcmurray-ftpext-ftp-hosts-02

Abstract

   The File Transfer Protocol, as defined in RFC 959, does not provide a
   way for FTP clients and servers to differentiate between multiple DNS
   names that are registered for a single IP address.  This document
   defines a new FTP command that provides a mechanism for FTP clients
   and servers to identify individual virtual hosts on an FTP server.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 15, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as



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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Document Conventions . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Basic Tokens . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Server Replies . . . . . . . . . . . . . . . . . . . . . .  4
   3.  The HOST command . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Syntax of the HOST command . . . . . . . . . . . . . . . .  5
     3.2.  HOST command semantics . . . . . . . . . . . . . . . . . .  8
       3.2.1.  REIN command semantics . . . . . . . . . . . . . . . .  9
       3.2.2.  User-PI usage of HOST  . . . . . . . . . . . . . . . .  9
       3.2.3.  State Diagrams . . . . . . . . . . . . . . . . . . . . 11
     3.3.  HOST command errors  . . . . . . . . . . . . . . . . . . . 17
     3.4.  FEAT response for HOST command . . . . . . . . . . . . . . 18
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Unworkable Alternatives . . . . . . . . . . . . . . . 21
     A.1.  Overloading the CWD command  . . . . . . . . . . . . . . . 21
     A.2.  Overloading the ACCT command . . . . . . . . . . . . . . . 22
     A.3.  Overloading the USER command . . . . . . . . . . . . . . . 22
     A.4.  Conclusion . . . . . . . . . . . . . . . . . . . . . . . . 23
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24






















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1.  Introduction

   It is common on the Internet for many DNS names to resolve to a
   single IP address.  This practice has introduced the concept of a
   "virtual host", where a host appears to exist as an independent
   entity, but in reality shares its physical resources with one or more
   similar hosts.

   Such an arrangement presents some problems for FTP servers, as an FTP
   server distinguishes incoming FTP connections by their IP addresses
   rather than their DNS names.  Therefore, all DNS names that share a
   common IP address are handled by the same FTP server and share the
   same Network Virtual File System (NVFS).

   This means that different virtual hosts cannot offer different
   virtual file systems to clients, nor can they offer different
   authentication systems.  Any scheme to overcome this issue needs to
   indicate not only the destination IP address, but also the virtual
   host name that is associated with the desired virtual FTP server.
   Typical user-FTP processes currently use hostnames to perform
   hostname to IP address resolution and then ignore hostnames for the
   rest of the FTP session, therefore any mechanism to overcome this
   issue would require modifications to the user-PI and server-PI.

   It should be noted that this same problem existed for HTTP/1.0 as
   defined in [RFC1945], and was resolved in HTTP/1.1 as defined in
   [RFC2616] through the addition of the Host request header field.  The
   goal of this document is to bring a similar level of feature parity
   to FTP by introducing a new HOST command that allows user-FTP
   processes to specify which virtual host to connect to for a server-
   FTP process that is handling requests for multiple virtual hosts on a
   single IP address.


2.  Document Conventions

   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 [RFC2119].

   In examples, "C>" and "S>" indicate lines sent by the client and
   server, respectively.

   This document also uses notation defined in [RFC0959] and [RFC1123].
   In particular, the terms "reply", "user", "NVFS", "NVT", "file",
   "pathname", "FTP commands", "DTP", "user-FTP process", "user-PI",
   "user-DTP", "server-FTP process", "server-PI", "server-DTP", "mode",
   "type", "control connection", "data connection", and "ASCII", are all



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   used here as defined there.

   Syntax required is defined using the Augmented BNF defined in
   [RFC5234].  Some general ABNF definitions are required throughout the
   document; those will be defined later in this section.  At first
   reading, it may be wise to simply recall that these definitions exist
   here, and skip to the next section.

2.1.  Basic Tokens

   This document imports the core definitions given in Appendix B of
   [RFC5234].  There definitions will be found for basic ABNF elements
   like ALPHA, DIGIT, SP, etc.  To that, the following term is added for
   use in this document.

        TCHAR = VCHAR / SP / HTAB    ; visible plus white space

   The VCHAR (from [RFC5234]) and TCHAR rules give basic character types
   from varying sub-sets of the ASCII character set for use in various
   commands and responses.

   Note that in ABNF, string literals are case insensitive.  That
   convention is preserved in this document, and implies that FTP
   commands and parameters that are added by this specification have
   values that can be represented in any case.  That is, "HOST" is the
   same as "host", "Host", "HoSt", etc., and "ftp.example.com" is the
   same as "Ftp.Example.Com", "fTp.eXample.cOm", etc.

2.2.  Server Replies

   Section 4.2 of [RFC0959] defines the format and meaning of replies by
   the server-PI to FTP commands from the user-PI.  Those reply
   conventions are used here without change.

        error-response = error-code SP *TCHAR CRLF
        error-code     = ("4" / "5") 2DIGIT

   Implementers should note that the ABNF syntax (which was not used in
   [RFC0959]) used in this document, and other FTP related documents,
   sometimes shows replies using the one line format.  Unless otherwise
   explicitly stated, that is not intended to imply that multi-line
   responses are not permitted.  Implementers should assume that, unless
   stated to the contrary, any reply to any FTP command (including QUIT)
   can be of the multi-line format described in [RFC0959].

   Throughout this document, replies will be identified by the three-
   digit code that is their first element.  Thus the term "500 reply"
   means a reply from the server-PI using the three digit code "500".



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3.  The HOST command

   A new command "HOST" is added to the FTP command set in order to
   allow a server-FTP process to determine which of possibly many
   virtual hosts to which the client wishes to connect.  This command
   MUST be issued before the user is authenticated, as this will allow
   the authentication scheme and set of legal users to be dependent upon
   the virtual host that is chosen.

   Server-FTP processes that conform to this specification MUST treat a
   situation where the HOST command is issued more than once before the
   user has been authenticated as though a previous HOST command was not
   sent, and return the appropriate reply for the new HOST command.
   Server-FTP processes MUST treat a situation where the HOST command is
   issued after the user has been authenticated as an erroneous sequence
   of commands and return a 503 reply.

   Servers should note that the response to the HOST command is a
   sensible time to send their "welcome" message.  This allows the
   message to be personalized for any virtual hosts that are supported,
   and also allows the client to determine the supported languages, or
   representations, for the message, and other messages, via the FEAT
   response, and select an appropriate one via the LANG command.  See
   [RFC2640] for more information.

   It should be noted that user-PI implementations that were created
   before the introduction of the HOST command will not support this new
   command.  A similar problem existed with the introduction of the Host
   header for HTTP in [RFC2616], and HTTP server implementations had to
   determine how best accomodate HTTP requests from down-level clients
   that did not support the Host header.  With this in mind, Server-FTP
   processes will need to determine how best to accomodate FTP requests
   from down-level FTP clients that do not support the HOST command, but
   those considerations are outside the scope of this document.

3.1.  Syntax of the HOST command

   The HOST command is defined as follows.













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     host-command  = "HOST" SP hostname CRLF
     hostname      = domain / IP-literal

     domain        = sub-domain *("." sub-domain)
     sub-domain    = let-dig [ldh-str]
     let-dig       = ALPHA / DIGIT
     ldh-str       = *( ALPHA / DIGIT / "-" ) let-dig

     IP-literal    = ( "[" IPv6address "]" ) / IPv4address

     IPv6address   =                              6( h16 ":" ) ls32
                     /                       "::" 5( h16 ":" ) ls32
                     / [               h16 ] "::" 4( h16 ":" ) ls32
                     / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
                     / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
                     / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
                     / [ *4( h16 ":" ) h16 ] "::"              ls32
                     / [ *5( h16 ":" ) h16 ] "::"              h16
                     / [ *6( h16 ":" ) h16 ] "::"

     ls32          = ( h16 ":" h16 ) / IPv4address
                     ; least-significant 32 bits of address

     h16           = 1*4HEXDIG
                     ; 16 bits of address represented in hexadecimal

     IPv4address   = dec-octet "." dec-octet "." dec-octet "." dec-octet

     dec-octet     = DIGIT                 ; 0-9
                     / %x31-39 DIGIT       ; 10-99
                     / "1" 2DIGIT          ; 100-199
                     / "2" %x30-34 DIGIT   ; 200-249
                     / "25" %x30-35        ; 250-255

     host-response = host-ok / error-response
     host-ok       = "220" [ SP *TCHAR ] CRLF

   The "hostname" rule is a restricted form of the "host" rule specified
   in [RFC3986].  Details of the additional restrictions imposed by this
   document are given further down in this section in the discussion of
   the syntax; they aim at simplifying implementations by only allowing
   what currently is specified precisely and in use on the Internet.
   The "IPv4address" and "IPv6address" ABNF rules and all subordinate
   ABNF rules given above are taken literally from [RFC3986] and
   reproduced here for the convenience of readers; however, [RFC3986]
   remains the normative specification for the syntactic form of IPv4
   and IPv6 address literals, in order to ensure identical presentation
   in 'ftp' URI hostname parts and in the protocol element specified



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   here.

   As with all FTP commands, the "HOST" command word is case
   independent, and MAY be specified in any character case desired.

   The "hostname" (given as a parameter) specifies the virtual host to
   which access is desired.  This SHOULD be the same host name that was
   used to obtain the IP address to which the FTP control connection was
   made, after any client conversions have been completed that convert
   an abbreviated or local alias to a complete (fully qualified) domain
   name, but before resolving a DNS alias (owner of a CNAME resource
   record) to its canonical name.

   Internationalization of domain names is only supported through the
   use of IDNA "A-Labels" for <sub-domain> as described in [RFC5890].
   For example, the following HOST command specifies an
   internationalized domain name:

        HOST xn--e1afmkfd.com

   If the user was given an IPv4 or IPv6 literal address, and
   consequently was not required to derive the literal address from a
   hostname, the client MAY send the HOST command with the IPv4 or IPv6
   literal address as specified to it.  While it may seem counter-
   intuitive to specify a literal address by using the HOST command
   after the client has already connected to the server using a literal
   address, this should be expected behavior because a user-FTP process
   should not be required to differentiate between a fully qualified
   domain name and an IPv4 or IPv6 network literal address.  That being
   said, if the IPv4 or IPv6 literal address specified by the client
   does not match the literal address for the server, the server MUST
   respond with a 504 reply to indicate that the IPv4 or IPv6 literal
   address is not valid.

   When the hostname parameter contains a literal address, square
   brackets are expected to disambiguate IPv6 address syntax from port
   numbers syntax.  Therefore, if the literal address is an IPv6
   address, the IPv6 address is required to be enclosed in square
   brackets (after eliminating any syntax that might also - but is not
   required to - be enclosed in brackets, and from which the server
   deduced that a literal address had been specified.)  For example, the
   following examples MAY be sent if the client had been instructed to
   respectively connect to "192.0.2.1", "FE80::c000:0201", or
   "192.0.2.1" and IPv6 syntax is preferred:

        HOST 192.0.2.1
        HOST [FE80::c000:0201]
        HOST [::192.0.2.1]



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   The client MUST NOT send the port number as part of the HOST command,
   even when the client has been instructed to connect to a non-standard
   port.  The reason for this requirement is that the user-PI will have
   established a connection to the server-PI before the HOST command is
   sent, therefore specifying a different port with the HOST command has
   no meaning.  For example, the server-PI MUST respond with a 501 reply
   if the client sends a HOST command with syntax like either of the
   following examples:

        HOST 192.0.2.1:2112
        HOST [FE80::c000:0201]:2112

   The hostname parameter is otherwise to be treated as a fully
   qualified domain name or relative name as those terms are defined in
   section 3.1 of [RFC1034].  This implies that the name is to be
   treated as a case-independent string, meaning that uppercase ASCII
   characters are to be treated as equivalent to their corresponding
   lowercase ASCII characters, but otherwise preserved as given.  It
   also implies some limits on the length of the parameter and of the
   components that create its internal structure.  Those limits are not
   altered in any way here.

   Neither [RFC1034] nor [RFC1035] impose any other restrictions upon
   what kinds of names can be stored in the DNS.  This specification,
   however, only allows the use of names that can be inferred from the
   ABNF grammar given for the "hostname".  Similarly, this specification
   restricts address literals to the IPv4 and IPv6 address families well
   established in the Internet.

3.2.  HOST command semantics

   Upon receiving the HOST command, before authenticating the user-PI, a
   server-FTP process SHOULD validate that the hostname given represents
   a valid virtual host for that server, and, if it is valid, establish
   the appropriate environment for that virtual host.  The resultant
   actions needed to create that environment are not specified here, and
   may range from doing nothing at all, to performing a simple change of
   working directory, changing authentication schemes and/or username
   and password lists, or making much more elaborate state changes -
   such as creating isolated environments for each FTP session.

   The "220" reply code for the HOST command is the same as the code
   that is used in the initial "welcome" message that is sent after the
   connection is established.

   If the hostname specified would normally be acceptable, but for any
   reason is temporarily unavailable, the server-FTP process SHOULD
   reply to the HOST command with a 421 reply and close the connection.



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   In this particular situation, the server-FTP process MAY choose to
   keep the connection open in order to allow the user-PI an opportunity
   to choose another virtual host with a subsequent HOST command.

   If the hostname specified is unknown at the server, or if the server
   is otherwise unwilling to treat the particular connection as a
   connection to the hostname specified, the server SHOULD respond with
   a 504 reply.

3.2.1.  REIN command semantics

   As specified in [RFC0959], the REIN command returns the state of the
   connection to what it was immediately after the transport connection
   was opened.  This specification makes no changes to that behavior.
   The effect of a HOST command MUST be reset if a REIN command is
   performed, and a new HOST command MUST be issued afterwards in order
   to connect to a virtual host.

3.2.2.  User-PI usage of HOST

   A user-PI that conforms to this specification MUST send the HOST
   command after opening the transport connection, or after any REIN
   command, before attempting to authenticate the user with the USER
   command.  The following example illustrates what a typical login
   sequence might look like when the HOST command is used:

        C> HOST ftp.example.com
        S> 220 Host accepted
        C> USER foo
        S> 331 Password required
        C> PASS bar
        S> 230 User logged in

   If a user-PI sends an additional HOST command before attempting to
   authenticate the user, a server-FTP process that conforms to this
   specification MUST treat the additional HOST command as though a
   previous HOST command was not sent, and return the appropriate reply
   for the new HOST command.  For example, if a user specifies the wrong
   virtual host by mistake, sending a subsequent HOST command will
   rectify the error.  The following example illustrates what the login
   sequence might look like when the HOST command is sent twice before a
   user has been authenticated:









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        C> HOST foo.example.com
        S> 220 Host accepted
        C> HOST bar.example.com
        S> 220 Host accepted
        C> USER foo
        S> 331 Password required
        C> PASS bar
        S> 230 User logged in

   The HOST command can be used in combination with the ACCT command to
   differentiate between a user's various accounts on a specific virtual
   host.  In this scenario, the user-PI sends a HOST command which the
   server-PI uses to route activity to the correct virtual host; the
   user-PI sends credentials using the USER and PASS commands which the
   server-PI validates; then, the user-PI sends an ACCT command to
   specify any additional account information for the server-PI
   implementation.  The following example illustrates a sequential
   series of client commands that specify both a HOST and ACCT, with the
   server responses omitted for brevity:

        C> HOST ftp.example.com
        C> USER foo
        C> PASS bar
        C> ACCT project1

   This is also true when the HOST command is used with the AUTH and
   ADAT commands that are discussed in [RFC2228] and [RFC4217].  In this
   scenario, the user-PI sends a HOST command which the server-PI uses
   to route activity to the correct virtual host, then the user-PI uses
   the AUTH and ADAT commands to negotiate the security mechanism and
   relevant authentication token(s) with the server-PI, then the user-PI
   sends user credentials using the USER and PASS commands which the
   server-PI validates.  After which the user-PI MAY send an ACCT
   command to specify any additional account information for the
   server-PI implementation.  The following example illustrates a
   sequential series of client commands that specify both a HOST and
   ACCT when used in conjunction with the security commands that are
   discussed in [RFC2228] and [RFC4217], with the server responses
   omitted for brevity:

        C> HOST ftp.example.com
        C> AUTH <mechanism-name>
        C> ADAT <base64data>
        C> USER foo
        C> PASS bar
        C> ACCT project1





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3.2.3.  State Diagrams

   The state diagrams in this section illustrate typical sequences for
   command and reply interchange between the user-PI and server-PI.
   These diagrams are modeled on the similar diagrams in section 6 of
   [RFC0959].

   In each diagram, the (B) "begin" state is assumed to occur after the
   transport connection has opened, or after a REIN command has
   succeeded.  Other commands (such as FEAT [RFC2389]) that require no
   authentication may have intervened.

   Additionally, a three-digit reply indicates a precise server reply
   code.  A single digit on a reply path indicates any server reply that
   begins with that digit, except where a precise server reply code is
   defined on another path.  For example, a single digit "5" will apply
   to "500", "501", "502", etc., when those reply codes are not
   expressly defined in the diagram.  For each command there are three
   possible outcomes: success (S), failure (F), and error (E).  In the
   state diagrams below we use the symbol B for "begin", and the symbol
   W for "wait for reply".

   For each of these diagrams, without any state transitions being
   shown, a REIN command will return the diagram from any wait state to
   the (B) "begin" state.


























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   The state diagram in Figure 1 shows a typical sequence of flow of
   control when HOST is used with USER and PASS to log in to a
   particular FTP virtual host.

              +---+   HOST    +---+ 1,3,5
              | B |---------->| W |-----------------
              +---+           +---+                 |
                               | |                  |
                     2,500,502 | | 4,501,503,504    |
                 --------------   -----------       |
                |                            |      V
                V                   1        |    +---+
              +---+   USER    +---+-------------->| E |
              |   |---------->| W | 2        |    +---+
              +---+           +---+-------   |      ^
                               | |        |  |      |
                             3 | | 4,5    |  |      |
                 --------------   -----   |  |      |
                |                      |  |  |      |
                |                -------------------
                |              1|      |  |  |
                V               |      |   ------>+---+
              +---+   PASS    +---+ 2  |     |    | S |
              |   |---------->| W |-------------->+---+
              +---+           +---+    |     |
                                |      |     |
                                |4,5   |     |
                                |      |      --->+---+
                                |       --------->| F |
                                 ---------------->+---+

            Figure 1: Typical login sequence with HOST command



















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   After a user has logged in, an additional account may be required by
   the server and specified by the client by using ACCT command.  With
   this in mind, the state diagram in Figure 2 shows a typical sequence
   of flow of control when HOST is used with USER and PASS to log in to
   an FTP virtual host and ACCT is used to specify an account.

              +---+   HOST    +---+ 1,3,5
              | B |---------->| W |-----------------
              +---+           +---+                 |
                               | |                  |
                     2,500,502 | | 4,501,503,504    |
                 --------------   -------------     |
                |                              |    |
                V                   1          |    V
              +---+   USER    +---+-------------->+---+
              |   |---------->| W | 2       ----->| E |
              +---+           +---+------  |  --->+---+
                               | |       | | | |
                             3 | | 4,5   | | | |
                 --------------   -----  | | | |
                |                      | | | | |
                |                      | | | | |
                |                ----------  | |
                |              1|      | |   | |
                V               |      | |   | |
              +---+   PASS    +---+ 2  |  ------->+---+
              |   |---------->| W |-------------->| S |
              +---+           +---+   ----------->+---+
                               | |   | |     | |
                             3 | |4,5| |     | |
                 --------------   --------   |  ----
                |                    | |  |  |      |
                |                    | |  |  |      |
                |                ------------       |
                |            1,3|    | |  |         |
                V               |   2| |  |         V
              +---+   ACCT    +---+--  |   ------>+---+
              |   |---------->| W | 4,5 --------->| F |
              +---+           +---+-------------->+---+

           Figure 2: Login sequence with HOST and ACCT commands










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   The state diagram in Figure 3 shows a typical sequence of flow of
   control when HOST is used with the AUTH and ADAT commands that are
   discussed in [RFC2228].  (NOTE: Section 4 provides additional
   information about using the HOST command with TLS.)















































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              +---+   HOST    +---+ 1,3,5
              | B |---------->| W |------------------
              +---+           +---+                  |
                               | |                   |
                     2,500,502 | | 4,501,503,504     |
                 --------------   -------------      |
                |                              |     |
                V                              |     |
              +---+   AUTH    +---+ 4,5        |     |
              |   |---------->| W |----------->|     |
              +---+           +---+            |     |
                           334 | |             |     |
                 --------------  |             |     |
                |            234 |             |     |
                |    ------------              |     |
                V   |               4,5        |     |
              +---+ | ADAT    +---+----------->|     |
              |   |---------->| W | 335        |     |
              +---+ |         +---+-----       |     |
                ^   |           |       |      |     |
                |   |           |       |      |     |
                 -----------------------       |     |
                    |           |              |     |
                ----        235 |              |     |
               |  --------------               |     |
               | |                             |     V
               V V                  1          |   +---+
              +---+   USER    +---+--------------->| E |
              |   |---------->| W | 2          |   +---+
              +---+           +---+-------     |     ^
                               | |        |    |     |
                             3 | | 4,5    |    |     |
                 --------------   ------  |    |     |
                |                       | |    |     |
                |                --------------------
                |              1|       | |    |
                V               |       |  ------->+---+
              +---+   PASS    +---+ 2   |      |   | S |
              |   |---------->| W |--------------->+---+
              +---+           +---+     |      |
                                |       |      |
                                |4,5    |      |
                                |       |       -->+---+
                                |        --------->| F |
                                 ----------------->+---+

         Figure 3: Login sequence with HOST and AUTH/ADAT commands




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   After a user has logged in with the security commands that are
   discussed in [RFC2228], an additional account may be required by the
   server and specified by the client by using ACCT command.  The state
   diagram in Figure 4 shows a typical sequence of flow of control when
   HOST is used with the AUTH and ADAT commands to log in to an FTP
   virtual host and ACCT is used to specify an account.

              +---+   HOST    +---+ 1,3,5
              | B |---------->| W |------------------
              +---+           +---+                  |
                               | |                   |
                     2,500,502 | | 4,501,503,504     |
                +--------------   --------------     |
                |                               |    |
                V                               |    |
              +---+   AUTH    +---+ 4,5         |    |
              |   |---------->| W |------------>|    |
              +---+           +---+             |    |
                           334 | |              |    |
                 --------------  |              |    |
                |            234 |              |    |
                |    ------------               |    |
                V   |               4,5         |    |
              +---+ | ADAT    +---+------------>|    |
              |   |---------->| W | 335         |    |
              +---+ |         +---+-----        |    |
                ^   |           |       |       |    |
                |   |           |       |       |    |
                 -----------------------        |    |
                    |           |               |    |
                ----         235|               |    |
               |  --------------                |    |
               | |                              |    |
               V V                  1           |    V
              +---+   USER    +---+--------------->+---+
              |   |---------->| W | 2        ----->| E |
              +---+           +---+-------  |  --->+---+
                               | |        | | | |
                             3 | | 4,5    | | | |
                 --------------   ------  | | | |
                |                       | | | | |
                |                -----------  | |
                |              1|       | |   | |
                V               |       | |   | |
              +---+   PASS    +---+ 2   |  ------->+---+
              |   |---------->| W |--------------->| S |
              +---+           +---+   ------------>+---+
                               | |   |  |     | |



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                             3 | |4,5|  |     | |
                 --------------   ---------   |  ----
                |                    |  |  |  |      |
                |                -------------       |
                |            1,3|    |  |  |         |
                V               |   2|  |  |         V
              +---+   ACCT    +---+--   |   ------>+---+
              |   |---------->| W | 4,5  --------->| F |
              +---+           +---+--------------->+---+

      Figure 4: Login sequence with HOST and AUTH/ADAT/ACCT commands

3.3.  HOST command errors

   The server-PI SHOULD return a 500 or 502 reply if the HOST command is
   unrecognized or unimplemented, as specified in [RFC0959].  For
   example, a server-PI that predates or otherwise chooses not conform
   to this specification would be expected to return a 500 or 502 reply.

   As discussed in section 3 of this document, if a HOST command is sent
   after a user has been authenticated, the server MUST treat the
   situation as an invalid sequence of commands and return a 503 reply.

   A 501 reply SHOULD be sent if the hostname given is syntactically
   invalid, and a 504 reply SHOULD be sent if a syntactically valid
   hostname is not a valid virtual host name for the server.  In all
   such cases, the server-FTP process MUST do one of the following:

   a.  Ignore the HOST command and act as if a HOST command had not been
       sent.  A user-FTP process MAY then send a subsequent HOST command
       with a different hostname.

   b.  Close the connection.

   A user-PI receiving a 500 or 502 reply to a HOST command SHOULD
   assume that the server-PI does not implement virtual servers by using
   the HOST command.  The user-PI MAY then proceed to login as if the
   HOST command had not been sent.

   A user-PI receiving an error reply that is different from the errors
   that have been described here SHOULD assume that the virtual HOST is
   unavailable, and terminate communications.

   A server-PI that receives a USER command to begin the authentication
   sequence without having received a HOST command SHOULD NOT reject the
   USER command.  Clients conforming to earlier FTP specifications do
   not send HOST commands.  In this case the server MAY act as if some
   default virtual host had been explicitly selected, or MAY enter an



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   environment that is different from that of any supported virtual
   hosts, perhaps one in which a union of all available accounts exists
   and which presents an NVFS that appears to contain subdirectories
   that contain the NVFS for all supported virtual hosts.

3.4.  FEAT response for HOST command

   When replying to the FEAT command [RFC2389], a server-FTP process
   that supports the HOST command MUST include a line containing the
   single word "HOST".  This word is case insensitive, and MAY be sent
   in any mixture of upper or lower case, however it SHOULD be sent in
   upper case.  That is, the response SHOULD be:

        C> FEAT
        S> 211- <any descriptive text>
        S>  ...
        S>  HOST
        S>  ...
        S> 211 End

   The ellipses indicate place holders where other features may be
   included, and are not required.  The one-space indentation of the
   feature lines is mandatory [RFC2389].


4.  Security Considerations

   As discussed in section 3 of this document, a server implementation
   MUST treat an additional HOST command that was sent before a user has
   been authenticated as though a previous HOST command was not sent.
   In this situation, the server implementation MUST reset the
   authentication environment, as that would allow for segregation
   between the security environments for each virtual host on an FTP
   server.  The implementation details for security environments may
   vary greatly based on the requirements of each server implementation
   and operating system, and those details are outside the scope of the
   protocol itself.  For example, a virtual host "foo.example.com" on an
   FTP server might use a specific username and password list, while the
   virtual host "bar.example.com" on the same FTP server might use a
   different username and password list.  In such a scenario, resetting
   the security environment is necessary for the virtual servers to
   appear to behave independently from a client perspective, while the
   actual server implementation details are irrelevant at the protocol
   level.

   Section 15.1.1 of [RFC4217] discusses the use of X.509 certificates
   for server authentication.  Taking the information from that document
   into account, when securing FTP sessions with the security mechanisms



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   that are defined in [RFC4217], client implementations SHOULD verify
   that the hostname which they specify in the parameter for the HOST
   command matches the identity that is specified in the server's X.509
   certificate in order to prevent man-in-the-middle attacks.

   When the HOST command is used in combination with the FTP security
   extensions that were introduced in [RFC2228] and [RFC4217], the HOST
   command SHOULD precede the security handshake when the user-PI is not
   providing the "server_name" in the extended client hello as defined
   in [RFC6066].  This allows both user-PI and server-FTP processes to
   map an FTP HOST with the correct server name in the server's
   certificate.  If the HOST command is sent after the security
   handshake, then mapping an FTP HOST to a security certificate will
   not take place before the secure session is established and the
   user-PI will be unable to verify the server's identity as presented
   in the server certificate message.

   If the user-PI is providing the "server_name" in the extended client
   hello as defined in Section 3 of [RFC6066], the user-PI MAY provide
   the HOST command after the security handshake because the client will
   be able to verify the server's identity as presented in the server
   Certificate message.  However, the server-PI MUST verify that the
   name in the HOST command matches the "server_name" that is provided
   in the extended client hello.

   A general discussion of issues related to the security of FTP can be
   found in [RFC2577].


5.  IANA Considerations

   IANA is requested to register the following FTP extension according
   to the procedure established by [RFC5797]:

   +------+---------+-------------+------+------+----------------------+
   | cmd  | FEAT    | description | type | conf | RFC#s/References and |
   |      | Code    |             |      |      | Notes                |
   +------+---------+-------------+------+------+----------------------+
   | HOST | HOST    | Hostname    | a    | o    | TBD                  |
   +------+---------+-------------+------+------+----------------------+

     NOTE TO RFC EDITOR: Please update TBD in the above table with the
                         number of this document.


6.  References





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6.1.  Normative References

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol
              (FTP)", STD 9, RFC 959, October 1985.

   [RFC1034]  Mockapetris, P., "Domain Names - Concepts and Facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain Names - Implementation and
              Specification", STD 13, RFC 1035, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts --
              Application and Support", STD 3, RFC 1123, October 1989.

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

   [RFC2228]  Horowitz, M. and S. Lunt, "FTP Security Extensions",
              RFC 2228, October 1997.

   [RFC2389]  Hethmon, P. and R. Elz, "Feature negotiation mechanism for
              the File Transfer Protocol", RFC 2389, August 1998.

   [RFC2640]  Curtin, W., "Internationalization of the File Transfer
              Protocol", RFC 2640, July 1999.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", RFC 3986,
              January 2005.

   [RFC4217]  Ford-Hutchinson, P., "Securing FTP with TLS", RFC 4217,
              October 2005.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", RFC 5234, January 2008.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, August 2010.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

6.2.  Informative References

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.




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   [RFC2577]  Allman, M. and S. Ostermann, "FTP Security
              Considerations", RFC 2577, May 1999.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC5797]  Klensin, J. and A. Hoenes, "FTP Command and Extension
              Registry", RFC 5797, March 2010.


Appendix A.  Unworkable Alternatives

   Due to the level of scope for adding a new command to FTP, a brief
   discussion of suggested alternatives to a HOST command and their
   respective limitations is warranted.  The suggested alternatives that
   are discussed in this appendix have been proposed in the past, but
   each of these ideas was deemed insufficient for the reasons that are
   listed within each section of the appendix.

A.1.  Overloading the CWD command

   One suggested method to emulate a form of virtual hosts would be for
   the client to simply send a "CWD" command after connecting, using the
   virtual host name as the argument to the CWD command.  This would
   allow the server-FTP process to implement the file stores of the
   virtual hosts as sub-directories in its NVFS.  This suggestion is
   simple in concept, and most server-FTP implementations support this
   without requiring any code changes.  While this method is simple to
   describe, and to implement, it suffers from several drawbacks:

   a.  The "CWD" command is available only after the user-PI has
       authenticated itself to the server-FTP process.  Thus, all
       virtual hosts would be required to share a common authentication
       scheme if they used this method.

   b.  To make the virtual host truly transparent, either the server-FTP
       process needs to be modified to include information that shows
       the special nature of this first CWD command (negating most of
       the advantage of this scheme), or all users must see the same
       identical NVFS view upon connecting (they must connect in the
       same initial directory), or the NVFS must implement the full set
       of virtual host directories at each possible initial directory
       for any possible user.

   c.  Unless the server is specially modified, a user connecting this
       way to a virtual host would be able to easily move to any other
       virtual host supported at the same server-FTP process, exposing



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       the nature of the virtual host.

A.2.  Overloading the ACCT command

   Another suggested method would be to simply overload the "ACCT" for
   FTP virtual hosts, but this proposal is unacceptable for several
   reasons with regard to when the ACCT command is sent during the
   request flow.  Sections 5.4 and 6 of [RFC0959] document the request
   flow for a login sequence as USER -> PASS -> ACCT.  This flow of
   commands may be acceptable when you are considering a single user
   having multiple accounts on an FTP server, but fails to differentiate
   between virtual hosts when you consider the following two issues:

   a.  The first problem with overloading the ACCT command is
       certificate negotiation when using the FTP security extensions
       that are documented in [RFC2228] and [RFC4217].  In order to
       safeguard user credentials, security mechanism and certificate
       negotiation must occur before login credentials are sent by the
       client.  The problem with using the ACCT command in this scenario
       is that there is no way of ensuring that the certificate matches
       the correct virtual host before the user credentials are sent.

   b.  The second problem with overloading the ACCT command is how user
       credentials are implemented for FTP virtual hosts.  FTP server
       implementations may allow the use of custom user credentials on a
       per-virtual-host basis.  For example, in one particular
       implementation the virtual host negotiation occurs, and then the
       user credentials are looked up using the account mechanism that
       is specific to that virtual host.  So once again the virtual host
       negotiation must take place before the user credentials are sent.

A.3.  Overloading the USER command

   An additional suggestion would be to overload well-known syntax
   through the existing USER command, as illustrated in the following
   example:

        C> USER foo@example.com
        S> 331 Password required
        C> PASS bar
        S> 230 User logged in

   In this example, the user "foo" might be attempting to log on to the
   virtual host "example.com" on an FTP server.  This suggestion may
   seem plausible at first, but introduces several implementation
   problems.  For example:





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   a.  Some network environments already use the "username@hostname"
       syntax for network credentials, where the "hostname" portion
       refers to the location of the user's credentials within the
       network hierarchy.  Using the "foo@example.com" syntax it becomes
       difficult to differentiate between the user "foo" logging into a
       virtual host named "example.com" on an FTP server versus the user
       "foo@example.com" logging into an FTP server with no specified
       virtual host.

   b.  When using the FTP security extensions that are documented in
       [RFC2228] and [RFC4217], security mechanism and certificate
       negotiation must occur before login credentials are sent by the
       client.  More specifically, the AUTH/ADAT commands must be sent
       before the USER command in order to safeguard user credentials.
       If you overload the USER command, there is no way of ensuring
       that the certificate matches the correct virtual host before the
       user credentials are sent by the client.

A.4.  Conclusion

   The conclusion from the examination of the existing possibilities
   seems to be that in order to obtain an adequate emulation of "real"
   FTP servers, client and server modifications to support virtual hosts
   are necessary.  Therefore a new FTP command seems the most likely
   solution to provide the required level of support.


Appendix B.  Acknowledgements

   Robert Elz and Paul Hethmon provided a detailed discussion of the
   HOST command in their Internet draft titled "Extensions to FTP" as
   part of their work with the FTPEXT Working Group at the IETF.  Their
   work formed the basis for much of this document, and their help has
   been greatly appreciated.  They would also like to credit Bernhard
   Rosenkraenzer for having first suggested and described the HOST
   command.

   Several people have provided a wealth of constructive feedback about
   earlier versions of this document that have helped to shape its
   development; many of their suggestions have been incorporated, and
   their contributions are gratefully acknowledged.  There are far too
   many to mention here, but authors of this document would like to
   specifically thank Alexey Melnikov, Alfred Hoenes, John Klensin, Joe
   Touch, Paul Ford-Hutchinson, Daniel Stenberg, Mykyta Yevstifeyev, and
   Anthony Bryan for their assistance.  In addition, Alec Rowell's help
   in making sections of this document more readable was invaluable.





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Authors' Addresses

   Paul Hethmon
   Hethmon Brothers
   2305 Chukar Road
   Knoxville, TN  37923
   USA

   Email: phethmon@hethmon.com


   Robert McMurray
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   Email: robmcm@microsoft.com

































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