File Transfer Protocol HOST Command for Virtual Hosts
RFC 7151

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.

Hethmon & McMurray           Standards Track                    [Page 1]
RFC 7151           FTP HOST Command for Virtual Hosts         March 2014

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