Behavior Engineering for Hindrance                        I. van Beijnum
Avoidance                                                 IMDEA Networks
Internet-Draft                                         September 1, 2010
Intended status: Standards Track
Expires: March 5, 2011

                An FTP ALG for IPv6-to-IPv4 translation


   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, it is important that in the
   situation where some client computers are IPv6-only while many
   servers are still IPv4-only and IPv6-to-IPv4 translators are used to
   bridge that gap, FTP is made to work through these translators as
   best it can.

   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 describes specifies a middlebox that may solve this

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 5, 2011.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Notational Conventions . . . . . . . . . . . . . . . . . . . .  4
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  ALG overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Control channel translation  . . . . . . . . . . . . . . . . .  5
   6.  EPSV to PASV translation . . . . . . . . . . . . . . . . . . .  7
   7.  EPRT to PORT translation . . . . . . . . . . . . . . . . . . .  8
     7.1.  Stateless EPRT translation . . . . . . . . . . . . . . . .  8
     7.2.  Stateful EPRT translation  . . . . . . . . . . . . . . . .  9
   8.  Default port 20 translation  . . . . . . . . . . . . . . . . .  9
   9.  Both PORT and PASV . . . . . . . . . . . . . . . . . . . . . . 10
   10. Default behavior . . . . . . . . . . . . . . . . . . . . . . . 10
   11. The ALGS command . . . . . . . . . . . . . . . . . . . . . . . 10
   12. Timeouts and translating to NOOP . . . . . . . . . . . . . . . 12
   13. IANA considerations  . . . . . . . . . . . . . . . . . . . . . 13
   14. Security considerations  . . . . . . . . . . . . . . . . . . . 13
   15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 13
   16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     17.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15

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1.  Introduction

   [RFC0959] specifies two modes of operation for FTP: active mode, in
   which the server connects back to the client and passive mode, where
   the server opens a port for the client to connect to.  Without
   additional action, 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 in 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 [RFC0959]
   incompatible with IPv6.  These issues were solved in [RFC2428], which
   introduces the EPSV (extended passive) command, where the server only
   responds with a port number, and the EPRT (extended port) command,
   which allows the client to supply either an IPv4 or an IPv6 address
   (and a port) to the server.

   A survey done in April of 2009 of 25 randomly picked and/or well-
   known FTP sites reachable over IPv4 showed that only 12 of them
   supported EPSV over IPv4.  Additionally, only 2 of those 12 indicated
   that they supported EPSV in response to the FEAT command introduced
   in [RFC2389] that asks the server to list its supported features.
   One supported EPSV but not FEAT.  In 5 cases, issuing the EPSV
   command to the server led to a significant delay, in 3 cases followed
   by a control channel reset.  All 25 servers were able to successfully
   complete a transfer in traditional passive PASV mode as required by
   [RFC1123].  More tests showed that the use of an address family
   argument with the EPSV command is widely mis- or unimplemented in
   servers.  The additional tests with more servers showed that
   approximately 65% of FTP servers support EPSV successfully and around
   96% support PASV successfully.  Clients were not extensively tested,
   but previous experience from the author suggests that most clients
   support PASV, with the notable exception of the command line client
   included with Windows, which only supports active mode.  This FTP
   client uses the original PORT command when running over IPv4 and EPRT
   when running over IPv6.

   Although these issues can and should be addressed by modifying, where
   necessary, clients and servers support EPSV successfully, such
   modifications may not appear widely in a timely fashion.  Also,
   network operators who may want to deploy IPv6-to-IPv4 translation
   generally don't have control over client or server implementations.
   As such, this document standardizes an FTP Application Layer Gateway
   that will allow unmodified IPv6 FTP clients to interact with
   unmodified IPv4 FTP servers successfully when using FTP for simple
   file transfers between a single client and a single server.

   Clients that want to engage in more complex behavior, such as server-
   to-server transfers, may make an FTP application layer gateway (ALG)

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   go into transparent mode by issuing an AUTH or ALGS commands as
   explained in Section 5.

   The recommendations and specifications in this document apply to all
   forms of IPv6-to-IPv4 translation, including stateless translation
   such as [RFC2765] or [I-D.ietf-behave-v6v4-xlate] as well as stateful
   translation such as [I-D.ietf-behave-v6v4-xlate-stateful].

2.  Notational Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Where these words appear in lower case, they SHOULD NOT be
   interpreted within the context of [RFC2119], but rather, in
   accordance with regular English usage.

3.  Terminology

   Within the context of this document, the words "client" and "server"
   refer to FTP client and server implementations, respectively.  An FTP
   server is understood to be an implementation of the FTP protocol
   running on a server system with a stable address, waiting for clients
   to connect and issue commands that eventually start data transfers.
   Clients interact with servers using the FTP protocol, and store
   (upload files) and retrieve (download files) to and from one or more
   servers.  This either happens interactively under control of a user,
   or is done as an unattended background process.  Most operating
   systems provide a web browser that implements a basic FTP client, as
   well as a command line client.  Third-party FTP clients are also
   widely available.

   Other terminology is derived from the documents listed in the
   reference section.  Note that this document cannot be fully
   understood on its own; it depends on background and terminology
   outlined in the references.

4.  ALG overview

   The most robust way to solve an IP version mismatch between FTP
   clients and FTP servers would be by changing clients and servers
   rather than using an IPv6-to-IPv4 translator for the data channel and
   using an application layer gateway on the control channel.  As such,
   it is recommended to update FTP clients and servers as required for

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   IPv6-to-IPv4 translation support where possible, to allow proper
   operation of the FTP protocol without the need for ALGs.

   On the other hand, network operators often have little influence over
   the FTP clients their customers run, let alone the FTP servers used
   throughout the Internet.  For those operators, deploying an ALG may
   be the only way to provide a satisfactory customer experience.  So,
   even though not the preferred solution, this document standardizes
   the functionality of such an ALG in order to promote consistent
   behavior between ALGs in an effort to minimize their harmful effects.

   Operators are encouraged to only deploy an FTP ALG for IPv6-to-IPv4
   translation when the FTP ALG is clearly needed.  In the presence of
   the ALG, EPSV commands that could be handled directly by conforming
   servers are translated into PASV commands, introducing unnecessary
   complexity and reducing robustness.  As such a "set and forget"
   policy on ALGs is not recommended.

   Note that the translation of EPSV through all translators and EPRT
   through a stateless translator is relatively simple but supporting
   translation of EPRT through a stateful translator is relatively
   difficult, because in the latter case a translation mapping must be
   set up for each data transfer using parameters that must be learned
   from the client/server interaction on the control channel.  This
   needs to happen before the EPRT command can be translated into a PORT
   command and passed on to the server.  As such, an ALG used with a
   stateful translator MUST support EPSV and MAY support EPRT.  However,
   an ALG used with a stateless translator SHOULD also support EPRT.

   The ALG functionality is described as a function separate from the
   IPv6-to-IPv4 translation function.  However, in the case of EPRT
   translation, the ALG and translator functions need to be tightly
   coupled, so if EPRT translation is supported, it is assumed that the
   ALG and IPv6-to-IPv4 translation functions are integrated within a
   single device.

5.  Control channel translation

   The IPv6-to-IPv4 FTP ALG intercepts all TCP sessions towards port 21
   for IPv6 destination addresses that map to IPv4 destinations
   reachable through an IPv6-to-IPv4 translator.  The FTP ALG implements
   the Telnet protocol ([RFC0854]) used for control channel interactions
   to the degree necessary to interpret commands and responses and re-
   issue those commands and responses, modifying them as outlined below.
   Telnet option negotiation attempts by either the client or the
   server, except for those allowed by [RFC1123], MUST be rejected by
   the FTP ALG without relaying those attempts.  This avoids the

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   situation where the client and the server negotiate Telnet options
   that are unimplemented by the FTP ALG.

   There are two ways to implement the control channel ALG:

   1.  The ALG terminates the IPv6 TCP session, sets up a new IPv4 TCP
       session towards the IPv4 FTP server, and relays commands and
       responses back and forth between the two sessions.

   2.  Packets that are part of the control channel are translated

   As they ultimately provide the same result either implementation
   strategy (or any other that is functionally equivalent) MAY be used.

   In the second case, an implementation MUST have the ability to track
   and update TCP sequence numbers when translating packets and break up
   packets into smaller packets after translation, as the control
   channel translation could modify the length of the payload portion of
   the packets in question.  Also, FTP commands/responses or Telnet
   negotiations could straddle packet boundaries, so in order to be able
   to perform the ALG function, it can prove necessary to reconstitute
   Telnet negotiations and FTP commands and responses from multiple

   If the client issues the AUTH command, then the client is attempting
   to negotiate [RFC2228] security mechanisms which are likely to be
   incompatible with the FTP ALG function.  In this situation, the FTP
   ALG MUST switch to transparently forwarding all data on the control
   channel in both directions until the end of the control channel
   session.  This requirement applies regardless of the response from
   the server.  In other words, it is the fact that the client attempts
   the AUTH negotiation that requires the ALG to become transparent,
   whether or not the attempt is successful.  The transparency
   requirement applies to the commands and responses flowing between the
   client and the server.  It is possible that commands or responses
   that were sent through the ALG before the AUTH command was issued
   were changed in length so TCP sequence numbers in packets entering
   the ALG and packets exiting the ALG no longer match.  In transparent
   mode, the ALG MUST continue to adjust sequence numbers if it was
   doing so before entering transparent mode as the result of the AUTH
   command.  The ALGS command (Section 11) MAY have a similar effect as
   the AUTH command, depending on the argument used.

   There have been FTP ALGs for the purpose of making active FTP work
   through IPv4 NATs for a long time.  Another type of ALG would be one
   that imposes restrictions required by security policies.  Multiple
   ALGs of different types can be implemented as a single entity.  If

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   such a multi-purpose ALG forbids the use of the AUTH command for
   policy reasons, the side effect of making the ALG stop performing the
   translations described here, as well as other possible interventions
   related to IPv6-to-IPv4 translation, MUST be retained even if the ALG
   responds to the AUTH command with an error and does not propagate the
   command to the server.  This way, any time a client issues the AUTH
   command, it knows that an ALG will be in transparent mode afterwards.
   Implementers are further advised that unlike hosts behind an IPv4
   NAT, IPv6 hosts using an IPv6-to-IPv4 translator will normally have
   the ability to execute FTP over IPv6 without interference from the
   IPv6-to-IPv4 translator or the ALG, so an IPv6-to-IPv4 translation
   FTP ALG is not the best place to implement security policies.

6.  EPSV to PASV translation

   Although many IPv4 FTP servers support the EPSV command, some servers
   react adversely to this command, and there is no reliable way to
   detect in advance that this will happen.  As such, an FTP ALG MUST
   translate all occurrences of the EPSV command issued by the client to
   the PASV command, and reformat a 227 response as a corresponding 229
   response.  However, an ALG MAY forego EPSV to PASV translation if it
   has positive knowledge, either through administrative configuration
   or learned dynamically, that EPSV will be successful without
   translation to PASV.

   For instance, if the client issues EPSV (or EPSV 2 to indicate IPv6
   as the network protocol), this is translated to the PASV command.  If
   the server with address then responds with:

      227 Entering Passive Mode (192,0,2,31,237,19)

   The FTP ALG reformats this as:

      229 Entering Extended Passive Mode (|||60691|)

   The ALG SHOULD ignore the IPv4 address in the server's 227 response.
   This is the behavior that is exhibited by most clients and is needed
   to work with servers that include [RFC1918] addresses in their 227
   responses.  However, if the 227 response contains an IPv4 address
   that does not match the destination of the control channel, the FTP
   ALG MAY send the following response to the client instead of the 229

      425 Can't open data connection.

   It is important that the response is in the 4xx range to indicate a
   temporary condition.

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   If the client issues an EPSV command with a numeric argument other
   than 2, the ALG MUST NOT pass the command on to the server, but
   rather respond with a 522 error:

      522 Network protocol not supported

   If the client issues EPSV ALL, the FTP ALG MUST NOT pass this command
   to the server, but respond with:

      504 Command not implemented for that parameter

   This avoids the situation where an FTP server reacts adversely to
   receiving a PASV command after the client indicated that it will only
   use EPSV during this session.

7.  EPRT to PORT translation

   Should the IPv6 client issue an EPRT command, the FTP ALG MAY
   translate this EPRT command to a PORT command.  The translation is
   different depending on whether the translator is a stateless one-to-
   one translator or a stateful one-to-many translator.

7.1.  Stateless EPRT translation

   If the address specified in the EPRT command is the IPv6 address used
   by the client for the control channel session, then the FTP ALG
   reformats the EPRT command into a PORT command with the IPv4 address
   that maps to the client's IPv6 address.  The port number must be
   preserved for compatibility with stateless translators.  For
   instance, if the client with IPv6 address 2001:db8:2::31 issues the
   following EPRT command:

      EPRT |2|2001:db8:2::31|5282|

   Assuming the IPv4 address that goes with 2001:db8:2::31 is, the FTP ALG reformats this as:

      PORT 192,0,2,31,20,162

   If the address specified in the EPRT command is an IPv4 address or an
   IPv6 address that is not the IPv6 address used by the client for the
   control session, the ALG SHOULD NOT attempt any translation, but pass
   along the command unchanged.

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7.2.  Stateful EPRT translation

   If the address in the EPRT command is the IPv6 address used by the
   client for the control channel, the stateful translator selects an
   unused port number in combination with the IPv4 address used for the
   control channel towards the FTP server, and sets up a mapping from
   that transport address to the one specified by the client in the EPRT
   command.  The PORT command with the IPv4 address and port used on the
   IPv4 side of the mapping is only issued towards the server once the
   mapping is created.  Initially, the mapping is such that either any
   transport address or the FTP server's IPv4 address with any port
   number is accepted as a source, but once the three-way handshake is
   complete, the mapping SHOULD be narrowed to only match the negotiated
   TCP session.

   If the address specified in the EPRT command is an IPv4 address or an
   IPv6 address that is not the IPv6 address used by the client for the
   control session, the ALG SHOULD NOT attempt any translation, but pass
   along the command unchanged.

   If the client with IPv6 address 2001:db8:2::31 issues the EPRT

      EPRT |2|2001:db8:2::31|5282|

   And the stateful translator uses the address on its IPv4
   interface, a mapping with destination address and
   destination port 60192 towards 2001:db8:2::31 port 5282 may be
   created, after which the FTP ALG reformats the EPRT command as:

      PORT 192,0,2,31,235,32

8.  Default port 20 translation

   If the client does not issue an EPSV/PASV or EPRT/PORT command prior
   to initiating a file transfer, it is invoking the default active FTP
   behavior where the server sets up a TCP session towards the client.
   In this situation, the source port number is the default FTP data
   port (port 20) and the destination port is the port the client uses
   as the source port for the control channel session.

   In the case of a stateless translator, this does not pose any
   problems.  In the case of a stateful translator, the translator MAY
   accept incoming connection requests from the server on the IPv4 side
   if the transport addresses match that of an existing FTP control
   channel session, with the exception that the control channel session
   uses port 21 and the new session port 20.  In this case, a mapping is

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   set up towards the same transport address on the IPv6 side that is
   used for the matching FTP control channel session.

   Note that if multiple clients are using the same IPv6-to-IPv4
   translator to communicate with the same FTP server, and for each
   client the IPv6-to-IPv4 translator uses the same source address on
   its IPv4 side, it may not be possible to correlate incoming FTP data
   channel sessions with the intended IPv6 host unambiguously.  In such
   cases, the IPv6-to-IPv4 translator SHOULD refuse the connection
   establishment attempt by returning a TCP reset packet.  An ALG/
   translator MAY monitor the progress of FTP control channels and only
   attempt to perform a mapping when an FTP client has started a file
   transfer without issuing the EPSV, PASV, EPRT or PORT commands.

9.  Both PORT and PASV

   [RFC0959] allows a client to issue both PORT and PASV to use non-
   default ports on both sides of the connection.  However, this is
   incompatible with the notion that with PASV, the data connection is
   made from the client to the server, while PORT reaffirms the default
   behavior where the server connects to the client.  As such, the
   behavior of an ALG is undefined when a client issues both PASV and
   PORT.  Implementations SHOULD NOT try to detect the situation where
   both PASV and PORT commands are issued prior to a command that
   initiates a transfer, but rather, apply the same translation they
   would have if there had not been a PASV command prior to a PORT
   command or a PORT command prior to a PASV command.

10.  Default behavior

   Whenever the client issues a command which the ALG is not set up to
   translate, either because the command is not specified in this
   document, the command is not part of any FTP specification, the ALG
   functionality is disabled administratively for the command in
   question, or translation does not apply for any other reason, the
   command MUST be passed on to the server without modification, and the
   server response MUST be passed on to the client without modification.
   For example, if the client issues the PASV command, this command is
   passed on to the server transparently, and the server's response to
   the client.

11.  The ALGS command

   ALGs SHOULD support the new ALGS (ALG status) command that allows
   clients to query and set the ALG's status.  Note that this command

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   MUST NOT be implemented in FTP servers.  If those recognize the
   command, the best course of action would be to return a 202 response:

      202 Command not implemented, superfluous at this site

   However, as FTP servers don't implement the command, there is no
   reason for them to specifically recognize this command, and returning
   any 50x response that is normally returned when commands are not
   recognized is appropriate.  A client can use the ALGS command to
   request the ALG's status and to enable and disable EPSV to PASV and,
   if implemented, EPRT to PORT translation.  There are three possible
   arguments to the ALGS command:

                   The ALG is requested to return the EPSV and EPRT
                   translation status.

                   The ALG is requested to enable translation.

                   The ALG is requested to disable translation.

   The ALG SHOULD enable or disable translation as requested.  If EPRT
   to PORT translation is supported, ALGS ENABLE64 enables it and ALGS
   DISABLE64 disables it along with enabling or disabling EPSV to PASV
   translation.  If EPRT to PORT translation is not supported, ALGS
   ENABLE64 only enables EPSV to PASV translation.  After an ALGS
   command with any of the three supported arguments, the ALG returns a
   216 response indicating the type of translation that will be
   performed.  There are four possible keywords that follow the 216
   response code:

   216 NONE
                   Neither EPSV nor EPRT translation is performed.

   216 EPSV
                   EPSV is translated to PASV, no EPRT translation is

   216 EPRT
                   EPRT is translated to PORT, no EPSV translation is

                   EPSV is translated to PASV, EPRT is translated to

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   The translation type MAY be followed by a space and additional
   descriptive text until end-of-line.  Failure to set the requested
   translation mode is not an error condition, and is thus indicated by
   the applicable keyword following the 216 response, and not with an
   error code response.

   If the ALGS command is not implemented, the command SHOULD be passed
   on to the server without modification.  If there is no argument to
   the ALGS command, or the argument is not one of STATUS64, ENABLE64 or
   DISABLE64 (or an argument specified by a supported newer document), a
   504 error SHOULD be returned.

   The Augmented Backus-Naur Form (ABNF) notation (see [RFC5234]) of the
   ALGS command and its response are as follows:

   algs-command      = "ALGS" SP algs-token CRLF
   algs-token        = "STATUS64" / "ENABLE64" / "DISABLE64"
                       / algs-ext-token
   algs-ext-token    = *VCHAR

   algs-response     = (ok-response / error-response) CRLF
   ok-response       = "216" SP response-token [ freetext ]
   response-token    = "NONE" / "EPSV" / "EPRT" / "EPSVEPRT"
   error-response    = not-implemented / invalid-parameter
   not-implemented   = "502" [ freetext ]
   invalid-parameter = "504" [ freetext ]
   freetext          = (SP *VCHAR)

12.  Timeouts and translating to NOOP

   Wherever possible, control channels should not time out while there
   is an active data channel.  A timeout of at least 30 seconds is
   recommended for data channel mappings created by the FTP ALG that are
   waiting for initial packets.

   Whenever a command from the client is not propagated to the server,
   the FTP ALG instead issues a NOOP command in order to keep the
   keepalive state between the client and the server synchronized.  The
   response to the NOOP command MUST NOT be relayed back to the client.
   An implementation MAY wait for the server to return the 200 response
   to the NOOP and translate that 200 response into the response the ALG
   is required to return to the client.  This way, the ALG never has to
   create new packets to send to the client, but it can limit itself to
   modifying packets transmitted by the server.  If the server responds
   with something other than 200 to the NOOP command, the ALG MUST tear
   down the control channel session and log an error.

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13.  IANA considerations

   IANA is requested to add to the FTP Commands and Extensions registry
   the following entry:

   Command Name

   FEAT Code

                              FTP64 ALG status

   Command Type

   Conformance Requirements

                              RFC TBD Section 11

   [TO BE REMOVED: This registration should take place at the following

14.  Security considerations

   In the majority of cases, FTP is used without further security
   mechanisms.  This allows an attacker with passive interception
   capabilities to obtain the login credentials, and an attacker that
   can modify packets to change the data transferred.  However, FTP can
   be used with TLS in order to solve these issues.  IPv6-to-IPv4
   translation and the FTP ALG do not impact the security issues in the
   former case nor the use of TLS in the latter case.  However, if FTP
   is used with TLS or another authentication mechanism, the ALG
   function is not performed so only passive transfers from a server
   that implements EPSV or a client that supports PASV will succeed.

   For general FTP security considerdations, see [RFC2577].

15.  Contributors

   Dan Wing, Kentaro Ebisawa, Remi Denis-Courmont, Mayuresh Bakshi,
   Sarat Kamisetty, Reinaldo Penno, Alun Jones, Dave Thaler, Mohammed

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   Boucadair, Mikael Abrahamsson, Dapeng Liu, Michael Liu, Andrew
   Sullivan, Anthony Bryan and Ed Jankiewicz contributed ideas and
   comments.  Dan Wing ran experiments with a large number of FTP
   servers that were very illuminating; many of the choices underlying
   this document are based on his results.  This document adopts several
   design decisions from [I-D.liu-behave-ftp64].

16.  Acknowledgements

   Iljitsch van Beijnum is partly funded by Trilogy, a research project
   supported by the European Commission under its Seventh Framework

17.  References

17.1.  Normative References

   [RFC0854]  Postel, J. and J. Reynolds, "Telnet Protocol
              Specification", STD 8, RFC 854, May 1983.

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

   [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., "FTP Security Extensions", RFC 2228,
              October 1997.

   [RFC2428]  Allman, M., Ostermann, S., and C. Metz, "FTP Extensions
              for IPv6 and NATs", RFC 2428, September 1998.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

17.2.  Informative References

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

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

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

   [RFC2765]  Nordmark, E., "Stateless IP/ICMP Translation Algorithm
              (SIIT)", RFC 2765, February 2000.

              Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers",
              draft-ietf-behave-v6v4-xlate-stateful-12 (work in
              progress), July 2010.

              Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", draft-ietf-behave-v6v4-xlate-05 (work in
              progress), December 2009.

              Liu, D. and Z. Cao, "IPv6 IPv4 translation FTP
              considerations", draft-liu-behave-ftp64-03 (work in
              progress), August 2009.

              Bernstein, D., "PASV security and PORT security", 2000,

Author's Address

   Iljitsch van Beijnum
   IMDEA Networks
   Avda. del Mar Mediterraneo, 22
   Leganes, Madrid  28918


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