path: root/doc/rfc/rfc7151.txt

Internet Engineering Task Force (IETF)                        P. Hethmon
Request for Comments: 7151                              Hethmon Brothers
Updates: 959                                                 R. McMurray
Category: Standards Track                          Microsoft Corporation
ISSN: 2070-1721                                               March 2014


         File Transfer Protocol HOST Command for Virtual Hosts

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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7151.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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






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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 ...............................9
           3.2.3. State Diagrams .....................................11
      3.3. HOST Command Errors .......................................16
      3.4. FEAT Response for HOST Command ............................17
   4. Security Considerations ........................................17
   5. IANA Considerations ............................................19
   6. References .....................................................19
      6.1. Normative References ......................................19
      6.2. Informative References ....................................20
   Appendix A. Unworkable Alternatives ...............................21
     A.1. Overloading the CWD Command ................................21
     A.2. Overloading the ACCT Command ...............................21
     A.3. Overloading the USER Command ...............................22
     A.4. Conclusion .................................................23
   Appendix B. Acknowledgements ......................................23

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, because
   an FTP server distinguishes incoming FTP connections by IP addresses
   rather than 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
   hostname 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




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   rest of the FTP session; therefore, any mechanism to overcome this
   issue would require modifications to the user protocol interpreter
   (user-PI) and server protocol interpreter (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 [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.

   The required syntax is defined using the Augmented BNF defined in
   [RFC5234].  Some general ABNF definitions are required throughout the
   document; they will be defined in subsequent sections.

   With the increased use of virtualization technologies, there may be
   several possible definitions for the term "virtual host".  This
   document follows the definition from Section 4.1.14 of [RFC3875],
   where several virtual hosts share the same IP address, and hostnames
   are used by the server-FTP process to route user-PI sessions to the
   appropriate virtual host.

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 subsets 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.  Similarly, because domain names
   are defined to be case insensitive, "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 used in this document
   and other FTP-related documents (but that was not used in [RFC959])
   sometimes shows replies using the one-line format.  Unless otherwise
   explicitly stated, multi-line responses are also 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 in order to
   allow a server-FTP process to determine to which of possibly many
   virtual hosts the client wishes to connect.  If a HOST command is
   sent, it MUST be issued before the user is authenticated, as this
   will allow the authentication scheme and set of authorized users to
   be dependent upon the virtual host that is chosen.

   Server-FTP processes MUST treat a situation in which the HOST command
   is issued more than once before the user has been authenticated as
   though only the last HOST command had been sent, and return the
   appropriate reply for the last HOST command.  Server-FTP processes





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   MUST treat a situation in which 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.
   It also allows the client to determine, via the FEAT response, the
   languages or representations supported by the server 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 to accommodate 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 accommodate
   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.  Note that [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 here.

      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   = <see [RFC3986] Section 3.2.2>
      IPv4address   = <see [RFC3986] Section 3.2.2>

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






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   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 in the discussion of the syntax that occurs later
   in this section; they aim at simplifying implementations by only
   allowing what currently is specified precisely and in use on the
   Internet.

   As with all FTP commands, the "HOST" command word is case independent
   and can 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 hostname 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 Internationalized Domain Names for Applications (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
   counterintuitive 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




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   following examples MAY be sent if the client had been instructed to
   connect to "192.0.2.1", "2001:db8::c000:201", or "::192.0.2.1",
   respectively, and IPv6 syntax is preferred:

      HOST 192.0.2.1
      HOST [2001:db8::c000:201]
      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:

      HOST 192.0.2.1:2112
      HOST [2001:db8::c000:201]: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] imposes 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 on 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.



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   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 is
   temporarily unavailable, the server-FTP process SHOULD respond to the
   HOST command with a 421 reply and close the connection.

      Example:

      The server-FTP process is shutting down, so the server-FTP process
      responds to the HOST command with a 421 reply and closes the
      connection.  In this scenario, the 421 reply informs the client it
      can retry at another time.

   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.

      Examples:

      The particular virtual host that was specified by the HOST command
      is disabled at the server.  The server responds with a 504 reply
      and keeps the connection open in order to allow the user-PI an
      opportunity to specify another virtual host with a subsequent HOST
      command.

      Alternatively, the server-FTP process might choose to route all
      connections with unknown hostnames to a different virtual host so
      that no connection attempts will result in failed connections.
      This design would be implementation specific and outside the scope
      of this specification.

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 afterwards in order
   to connect to a virtual host.









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3.2.2.  User-PI Usage of HOST

   A user-PI 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 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 hostname 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:

      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



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   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 HOST and ACCT
   commands 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

   An exception to the above scenario would be when a user-PI is
   providing the hostname in the "server_name" extension of a Transport
   Layer Security (TLS) extended client hello as discussed in [RFC6066].
   When the user-PI specifies the hostname in the "server_name"
   extension of a TLS extended client hello, the server-PI MUST verify
   that the hostname in the HOST command matches the value of the
   "server_name" extension.  The following example illustrates a
   sequential series of client commands that specify the HOST command
   when used in conjunction with the TLS extensions that are discussed
   in [RFC6066], with the server responses omitted for brevity:

      C> AUTH TLS
      C> HOST ftp.example.com
      C> USER foo
      C> PASS bar

   Additional security information about using the HOST command with the
   security extensions that are discussed in [RFC2228], [RFC4217], and
   [RFC6066] is provided in Section 4 of this document.










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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
   [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), or 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 the 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

   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 the 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 [RFC959].  For
   example, a server-PI that predates or otherwise does 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 hostname 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 log in 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 that conform 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 the server MAY



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   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 that 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, but it SHOULD be
   sent in upper case so as to maximize interoperability with disparate
   implementations.  That is, the response SHOULD be:

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

   The ellipses indicate placeholders where other features may be
   included but 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
   that are defined in [RFC4217], client implementations SHOULD verify



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   that the hostname that 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-FTP 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 the correct security
   certificate will not take place before the secure session is
   established.

   For example, if a server-FTP process has multiple virtual hosts
   defined and no hostname has been sent from a user-FTP process, the
   server-FTP process will be unable to route the connection to the
   correct virtual host when the connection is established.  In this
   situation, the server-FTP process will be forced to choose a virtual
   host that will respond.  When the user-PI attempts to negotiate a
   secure connection, the virtual host to which the connection was
   routed will respond with its server certificate during the security
   handshake.  If the virtual host that was chosen by the server-FTP
   process does not match the virtual host to which the user-FTP process
   had intended to connect, the user-PI will be unable to verify the
   server's identity as presented in the server certificate message.

   However, 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 server will be able to route the connection to the
   correct virtual host based on the contents of the "server_name"
   extension and the client will be able to verify the server's identity
   as presented in the corresponding 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.

   In general, client implementations SHOULD protect user credentials by
   using the FTP security extensions that were introduced in [RFC2228]
   and [RFC4217]; a detailed discussion for securing FTP sessions can be
   found in those documents, and a general discussion of security issues
   related to FTP can be found in [RFC2577].







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5.  IANA Considerations

   IANA has registered 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    | RFC 7151             |
   +------+---------+-------------+------+------+----------------------+

6.  References

6.1.  Normative References

   [RFC959]   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, B., "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", STD 66, RFC
              3986, January 2005.

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





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   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, 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. Nielsen, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [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.

   [RFC3875]  Robinson, D. and K. Coar, "The Common Gateway Interface
              (CGI) Version 1.1", RFC 3875, October 2004.

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
























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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 listed
   within each section of this 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 hostname as the argument to the CWD command.  This would
   allow the server-FTP process to implement the file stores of the
   virtual hosts as subdirectories 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 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

   Another suggested method would be to simply overload the ACCT command
   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




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   having multiple accounts on an FTP server, but it 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, negotiation of the security mechanism
       and certificate 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 it 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 virtual host that is named "example.com" on an FTP server
       versus the user "foo@example.com" logging into an FTP server with
       no specified virtual host.




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   b.  When using the FTP security extensions that are documented in
       [RFC2228] and [RFC4217], negotiation of the security mechanism
       and certificate 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

   After examining the above alternatives, and in order to obtain an
   adequate emulation of "real" FTP servers, it was concluded that
   supporting virtual hosts will require both client and server
   modifications.  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 of 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 has 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 the authors of this document would like to
   specifically thank Alexey Melnikov, Alfred Hoenes, John Klensin, Joe
   Touch, Paul Ford-Hutchinson, Daniel Stenberg, Mykyta Yevstifeyev,
   Alec Rowell, Jaroslav Dunajsky, Wade Hilmo, Anthony Bryan, and Barry
   Leiba for their assistance.















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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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