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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 draft-ietf-ftpext2-hosts

Network Working Group                                         P. Hethmon
Internet-Draft                                          Hethmon Brothers
Updates: 959                                                 R. McMurray
Intended status: Standards Track                               Microsoft
Expires: December 20, 2010                                     June 2010


         File Transfer Protocol HOST Command for Virtual Hosts
                  draft-hethmon-mcmurray-ftp-hosts-15

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   (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.

Abstract

   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.

Table of Contents

   1. Introduction.....................................................2
   2. Document Conventions  ...........................................3
     2.1. Basic Tokens  ...............................................3
     2.2. Server Replies  .............................................4
   3. The HOST command ................................................4
     3.1. Syntax of the HOST command ..................................5
     3.2. HOST command semantics ......................................7
       3.2.1. REIN command semantics ..................................8
       3.2.2. User-PI usage of HOST ...................................8
       3.2.3. State Diagrams ..........................................9
     3.3. HOST command errors ........................................12
     3.4. FEAT response for HOST command .............................13
   4. Security Considerations ........................................14
   5. IANA Considerations ............................................14
   6. References .....................................................14
     6.1 Normative References ........................................14
     6.2 Informative References ......................................15
   Appendix A: Unworkable Alternatives ...............................16
     A.1. Overloading the CWD command ................................16
     A.2. Overloading the ACCT command ...............................16
     A.3. Overloading the USER command ...............................17
     A.4. Conclusion .................................................18
   Acknowledgments ...................................................18
   Authors' Addresses ................................................18

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,
   not their DNS names, because hosts are uniquely identified by their
   address rather than name.  That is, all DNS names that share an IP
   address are handled by the same FTP server and share the same Network

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   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.  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 mutiple 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 [RFC959] 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 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

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   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 [RFC959] 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
   [RFC959]) 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 [RFC959].

   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".

3. The HOST command

   A new command "HOST" is added to the FTP command set to allow the
   server-FTP process to determine to which of possibly many virtual
   hosts the client wishes to connect.  This command SHOULD be issued
   before the user is authenticated, allowing the authentication scheme,
   and set of legal users, to be dependent upon the virtual host chosen.
   Server-FTP processes SHOULD treat a situation where the HOST command
   is issued after the user has been authenticated using one of the
   following two behaviors:

      a. Treat the late HOST command as an erroneous sequence of
      commands and return a 503 reply.

      b. Treat the late HOST command as though a REIN command was sent
      before the HOST command and reset the user-PI to the state that
      existed after the TCP connection was first established and before
      the initial user authentication and then return the appropriate

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      reply for the HOST command.

   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.

3.1. Syntax of the HOST command

   The HOST command is defined as follows.

     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


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   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 Punycode as described in [RFC3492].

   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]

   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:

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        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, allows only the of names that can be inferred from the ABNF
   grammar given for the "hostname".

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, to changing authentication schemes and/or username
   and password lists, to making much more elaborate state changes, as
   necessary.

   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.  This reply code is used deliberately
   in order to allow the implementation of a front-end FTP server as a
   wrapper, which simply waits for the HOST command, and then invokes a
   server that is compliant with [RFC959] in the appropriate environment
   for the particular hostname received.

   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.
   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.


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3.2.1. REIN command semantics

   As specified in [RFC959], 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 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

   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 certificate 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

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   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

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
   [RFC959].

   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".

   In each of these diagrams, a REIN command will return the diagram to
   the (B) "begin" state.

   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                   1         |     V
              +---+   USER    +---+-------------->+---+
              |   |---------->| W | 2         |   | E |
              +---+           +---+------   ----->+---+

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

         Figure 1: Typical login sequence with HOST command

   The state diagram in Figure 2 shows the flow of control when a HOST
   command is sent after a user has already successfully logged in to a
   virtual host with USER and PASS.

                                 ------------------------------
                                |                              |
                                |                              |
                                V                              |
              +---+   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               |      |  ------->+---+  HOST  |
              +---+   PASS    +---+ 2  |      |   | S |--------
              |   |---------->| W |-------------->+---+
              +---+           +---+    |      |
                                |      |      |
                                |4,5   |      |
                                |      |      |

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                                |      |       -->+---+
                                |       --------->| F |
                                 ---------------->+---+

         Figure 2: Login sequence with repeated HOST command

   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 3 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 3: Login sequence with HOST and ACCT commands

   When the HOST command is used in combination with the FTP security
   extensions that were introduced in [RFC2228], it SHOULD precede
   the security handshake.  This allows both user-PI and server-FTP

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   processes to map an FTP HOST to security data appropriately.  The
   state diagram in Figure 4 shows a typical sequence of flow of control
   when HOST is used with the AUTH and ADAT commands that are discussed
   in [RFC2228].

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

         Figure 4: 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 5 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 |------------>|    |
              +---+           +---+             |    |
                          234 | |               |    |
                     ---------  | 334           |    |
                    |           |               |    |
                 ---------------|-------        |    |
                |   |           |       |       |    |
                V   |           V  335  |       |    |
              +---+ | ADAT    +---+-----        |    |
              |   |---------->| W | 4,5         |    |
              +---+ |         +---+------------>|    |
                    |           |               |    |
                ----         235|               |    |
               |  --------------                |    |
               | |                              |    |
               V V                  1           |    V
              +---+   USER    +---+--------------->+---+
              |   |---------->| W | 2        ----->| E |
              +---+           +---+-------  |  --->+---+
                               | |        | | | |
                             3 | | 4,5    | | | |
                 --------------   ------  | | | |
                |                       | | | | |
                |                -----------  | |
                |              1|       | |   | |
                V               |       | |   | |
              +---+   PASS    +---+ 2   |  ------->+---+
              |   |---------->| W |--------------->| S |
              +---+           +---+   ------------>+---+
                               | |   |  |     | |
                             3 | |4,5|  |     | |
                 --------------   ---------   |  ----
                |                    |  |  |  |      |
                |                -------------       |
                |            1,3|    |  |  |         |

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

         Figure 5: Login sequence with HOST and AUTH/ADAT/ACCT commands

3.3. HOST command errors

   The server-PI SHOULD reply with a 500 or 502 reply if the HOST
   command is unrecognized or unimplemented.

   As discussed in section 3 of this document, if a HOST command is sent
   after a user has been authenticated the server SHOULD do one of the
   following:

      a. Send a 503 reply for an invalid sequence of commands.

      b. Treat the HOST command as though a REIN command was sent and
      reset the user-PI to the state that existed after the previous
      HOST command was sent and before the user had been authenticated,
      and then return the appropriate reply for the HOST command.

   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 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
   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

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   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

   With the introduction of virtual hosts to FTP, server implementers
   will need to take some care to ensure that the integrity of
   potentially sensitive information is maintained.  For example, while
   hostnames may generally be assumed to be publicly available DNS
   names, this may not always be the situation.  Some organizations may
   use private hostnames, and that information SHOULD be protected when
   transmitted between the client and server by using a strong method of
   encryption.

   Server implementations SHOULD reset the security environment when a
   HOST command is sent after a user has logged in.  This allows for
   individual authentication environments for each virtual host on an
   FTP server.  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 virtual servers to appear
   to behave independently.

   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]:

   +-------+------+-------------------+------+------+------------------+

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   | cmd   | FEAT | description       | type | conf | RFC#s/References |
   |       | Code |                   |      |      | and Notes        |
   +-------+------+-------------------+------+------+------------------+
   | HOST  | HOST | Hostname          | a    | o    | [#1]             |
   +-------+------+-------------------+------+------+------------------+

   [[ RFC Editor: Please update note #1 in the above table with the
   number of this document. ]]

6. References

6.1.  Normative References

   [RFC959]  Postel, J., Reynolds, J., "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., Lunt, S., "FTP Security Extensions",
             RFC 2228, October 1997

   [RFC2389] Hethmon, P., Elz, R., "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

   [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
             for Internationalized Domain Names in Applications (IDNA)",
             RFC 3492, March 2003

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

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

6.2.  Informative References

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

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

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

   [RFC5797] Klensin, J., Hoenes, A., "FTP Command and Extension
             Registry", 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
      the nature of the virtual host.

A.2. Overloading the ACCT command

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   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 [RFC959] 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:

      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

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      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.

Acknowledgments

   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.

   Alexey Melnikov, Alfred Hoenes, John Klensin, and Joe Touch have made
   several suggestions about earlier versions of this document; many of
   their suggestions have been incorporated, and their contributions are
   gratefully acknowledged.  In addition, Alec Rowell's assitance in
   making sections of this document more readable was invaluable.

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 98052-6399


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   Email: robmcm@microsoft.com



















































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