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An FTP Application Layer Gateway (ALG) for IPv6-to-IPv4 Translation
RFC 6384

Internet Engineering Task Force (IETF)                    I. van Beijnum
Request for Comments: 6384                      Institute IMDEA Networks
Category: Standards Track                                   October 2011
ISSN: 2070-1721

  An FTP Application Layer Gateway (ALG) for IPv6-to-IPv4 Translation

Abstract

   The File Transfer Protocol (FTP) has a very long history, and despite
   the fact that today other options exist to perform file transfers,
   FTP is still in common use.  As such, in situations where some client
   computers only have IPv6 connectivity while many servers are still
   IPv4-only and IPv6-to-IPv4 translators are used to bridge that gap,
   it is important that FTP is made to work through these translators to
   the best possible extent.

   FTP has an active and a passive mode, both as original commands that
   are IPv4-specific and as extended, IP version agnostic commands.  The
   only FTP mode that works without changes through an IPv6-to-IPv4
   translator is extended passive.  However, many existing FTP servers
   do not support this mode, and some clients do not ask for it.  This
   document specifies a middlebox that may solve this mismatch.

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/rfc6384.

Copyright Notice

   Copyright (c) 2011 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

Van Beijnum                  Standards Track                    [Page 1]
RFC 6384                 An IPv6-to-IPv4 FTP ALG            October 2011

   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  ALG Overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Control Channel Translation  . . . . . . . . . . . . . . . . .  5
     5.1.  Language Negotiation . . . . . . . . . . . . . . . . . . .  7
   6.  EPSV to PASV Translation . . . . . . . . . . . . . . . . . . .  8
   7.  EPRT to PORT Translation . . . . . . . . . . . . . . . . . . .  9
     7.1.  Stateless EPRT Translation . . . . . . . . . . . . . . . .  9
     7.2.  Stateful EPRT Translation  . . . . . . . . . . . . . . . . 10
   8.  Default Port 20 Translation  . . . . . . . . . . . . . . . . . 10
   9.  Both PORT and PASV . . . . . . . . . . . . . . . . . . . . . . 11
   10. Default Behavior . . . . . . . . . . . . . . . . . . . . . . . 11
   11. The ALGS Command . . . . . . . . . . . . . . . . . . . . . . . 12
   12. Timeouts and Translating to NOOP . . . . . . . . . . . . . . . 13
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14
   16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 15
     17.2. Informative References . . . . . . . . . . . . . . . . . . 15

1.  Introduction

   [RFC0959] specifies two modes of operation for FTP: active mode, in
   which the server connects back to the client, and passive mode, in
   which the server opens a port for the client to connect to.  Without
   additional measures, active mode with a client-supplied port does not
   work through NATs or firewalls.  With active mode, the PORT command
   has an IPv4 address as its argument, and with passive mode, the
   server responds to the PASV command with an IPv4 address.  This makes
   both the passive and active modes, as originally specified in

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