Behavior Engineering for Hindrance                        I. van Beijnum
Avoidance                                                 IMDEA Networks
Internet-Draft                                         December 17, 2009
Intended status: Informational
Expires: June 20, 2010

              IPv6-to-IPv4 translation FTP considerations


   The File Transfer Protocol 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
   don't support this mode, and some clients don't ask for it.  This
   document describes server, client and middlebox (if any) behavior
   that minimizes this problem.

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   This Internet-Draft will expire on June 20, 2010.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the BSD License.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Client recommendations . . . . . . . . . . . . . . . . . . . .  4
   4.  ALG functionality  . . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  Control channel translation  . . . . . . . . . . . . . . .  6
     4.2.  EPSV to PASV translation . . . . . . . . . . . . . . . . .  7
     4.3.  EPRT to PORT translation . . . . . . . . . . . . . . . . .  8
       4.3.1.  Stateless EPRT translation . . . . . . . . . . . . . .  8
       4.3.2.  Stateful EPRT translation  . . . . . . . . . . . . . .  8
     4.4.  Default port 20 translation  . . . . . . . . . . . . . . .  9
     4.5.  Both PORT and PASV . . . . . . . . . . . . . . . . . . . .  9
     4.6.  Timeouts . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Server implementation recommendations . . . . . . . . 11
   Appendix B.  Acknowledgements  . . . . . . . . . . . . . . . . . . 12
   Appendix C.  Document and discussion information . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12

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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 doesn't
   work through NATs or firewalls.  And in both cases, an IPv4 address
   is specified, making both the original passive and active modes
   incompatible with IPv6.  These issues were solved in [RFC2428], which
   introduces the EPSV (extended passive) mode, 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 ([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 weren't 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.  It uses the original PORT
   command when running over IPv4 and EPRT when running over IPv6.

   Considering the above, this document describes the following


      *  Allow EPSV (even for IPv4-only servers)

      *  Use a predictable address in the response to the PASV command


      *  Use EPSV over IPv6 rather than EPRT

      *  Fall back to PASV if EPSV fails (even over IPv6)

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      *  Don't use certain modes and options that trigger server bugs

   Additionally, there are guidelines for operators choosing to
   implement application layer gateway functionality to provide
   connectivity between unupdated servers and/or clients.  Clients that
   want to engage in more complex behavior, such as server-to-server
   transfers, may make an FTP ALG go into transparent mode by issuing an
   AUTH command.

   The recommendations 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].

   The FTP protocol allows for complex interactions, such as the
   situation where a client connects to two servers and directs the
   servers to exchange data between them.  No attempt is made to address
   these other than through making ALGs transparent after an AUTH

2.  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 and start data transfers.  Clients
   interact with servers using the FTP protocol, and store (upload) or
   retrieve (download) files to/from one or more servers, either
   interactively under control of a user, or 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.

3.  Client recommendations

   All FTP clients should support EPSV when communicating over IPv6 and
   always attempt to use EPSV mode unless explicitly configured to use

   It is highly recommended that FTP clients react by retrying with PASV
   when the EPSV command fails, either because of an error response by
   the server (40x, 42x, 50x and 52x responses), because the data

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   connection couldn't be created or because the control channel session
   was terminated.  In the latter two cases, a client may cache the name
   or address of the FTP server and issue PASV rather than EPSV in
   future sessions.  In that case, the cache entry should be cleared if
   older than 7 days and the server indicates EPSV support in its FEAT
   response where it previously did not indicate EPSV support in its
   FEAT response.  There is always a risk that an error was the result
   of a condition unrelated to IPv6-to-IPv4 translation.  However,
   retrying with a PASV request has little potential for harm, so unless
   the error is clearly unrelated, retrying with PASV is the appropriate

   When after attempting to initiate EPSV and/or EPRT modes
   unsuccessfully and a client retries with PASV, the server will
   respond to the PASV command with an IPv4 address that the client must
   use to connect to for the data connection.  Even if the client has
   IPv4 reachability, it should ignore the server-supplied address and
   set up a data connection towards the IPv6 address of the server that
   is used for the control channel session.  However, the port number
   used for the data connection is taken from the 227 response to the
   PASV command.

   The main rationale for ignoring the IPv4 address in the 227 response,
   even if the client has IPv4 connectivity, is the fact that most
   servers will only allow a data connection from the same client
   address as seen in the control channel connection, see
   <>.  Using IPv6 for the control
   channel and IPv4 for the data channel means that the source address
   will almost certainly be different in both cases, making it unlikely
   that the data connection can be established successfully.

   Clients should refrain from using any arguments with the EPSV
   command.  "EPSV 2" to request IPv6 will fail across an IPv6-to-IPv4
   translator.  Also, this command is often not handled properly by IPv6
   servers.  "EPSV ALL" indicates that the client will use EPSV for all
   transfers, but an ALG may translate EPSV commands to PASV commands,
   conflicting with the earlier "EPSV ALL".

4.  ALG functionality

   The use of FTP application layer gateways for compatibility with
   IPv6-to-IPv4 translators is rejected by many within the IETF
   community.  As such, it is recommended to update FTP clients and
   servers as required for IPv6-to-IPv4 translation support where
   possible, to allow proper operation of the FTP protocol without the
   need for ALGs.

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   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 describes the
   functionality of such an ALG in order to promote consistent behavior
   between ALGs in an effort to minimize their harmful effects.
   However, the situation with regard to FTP servers and -clients,
   especially in IPv6-heavy deployments, may change fast, so within
   relatively little time it may become feasible to stop running an ALG.
   Operators are encouraged to keep revisiting the issue.

   Note that the translation of EPSV through all translators and EPRT
   through a stateless translator is relatively simple and translation
   of EPRT through a stateful translator relatively difficult.  As such,
   an ALG used with a stateful translator may choose to support only
   EPSV.  However, an ALG used with a stateless translator should also
   support EPRT.

4.1.  Control channel translation

   The IPv6-to-IPv4 FTP ALG intercepts all TCP sessions towards IPv4
   port 21 destinations.  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.  Option
   negotiation attempts by either the client or the server, except for
   those allowed by [RFC1123], should be rejected by the FTP ALG without
   relaying those attempts.  This avoids the situation where the client
   and the server negotiate options unknown to 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

   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 may modify the length of the payload portion of
   the packets in question.  Also, FTP commands/responses or Telnet
   negotiations may straddle packet boundaries, so in order to be able
   to perform the ALG function, it may be necessary to reconstitute
   Telnet negotiations and FTP commands and responses from multiple

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   If the client issues the AUTH command 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, not
   whether or not the attempt is successful.

   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 can be implemented as a single entity.  Should such a multi-
   purpose ALG forbid 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 doesn't propagate the command
   to the server.  (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 ALG.)

4.2.  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 may
   translate all occurrences of the EPSV command issued by the client to
   the PASV command, and reformat a 227 response as a corresponding 229

   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|)

   If the server's 227 response contains an IPv4 address that doesn't
   match the destination of the control channel, the FTP ALG should send
   the following response to the client:

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      425 Can't open data connection.

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

   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.

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

      202 Command not implemented.

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

4.3.  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.

4.3.1.  Stateless EPRT translation

   If the address specified in the EPRT command is the client's IPv6
   address, 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

   If the address specified in the EPRT command is not the client's IPv4
   address, the ALG's response is undefined.  It may pass along the
   command unchanged, respond with an error, or attempt to perform an
   appropriate translation.

4.3.2.  Stateful EPRT translation

   If the address in the EPRT command is the IPv6 address of the control
   channel client's address, 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

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   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 is narrowed to only match the negotiated TCP

   If the address in the EPRT command is not the client's IPv6 address,
   the ALG's response is undefined.

4.4.  Default port 20 translation

   If the client doesn't issue an EPSV/PASV or EPRT/PORT command, 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 in 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
   should 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 set up towards the same transport address on the IPv6 side
   that is used for the matching FTP control channel session.

   So for instance, the client is 2001:db8:31::6 and the server is  The translator has prefix 2001:db8:ffff:fffff::/96 as its
   translator prefix and as its IPv4 address.  On the IPv6
   side, the transport addresses for an FTP control channel session
   could then be 2001:db8:31::6,49152 to 2001:db8:ffff:ffff::c000:204,21
   on the IPv6 side and,60000 to,21 on the IPv4 side.
   If then the FTP server initiates a session from,20 to,60000, the translator sets up a mapping from those addresses
   to source 2001:db8:ffff:ffff::c000:204,20 destination 2001:db8:31::

   If there is no (unambiguous) match for an existing data channel
   session when an incoming session request on port 20 arrives, the
   connection is refused with a TCP RST.

4.5.  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

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   behavior of an ALG is undefined when a client issues both PASV and

4.6.  Timeouts

   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 mappings created by the FTP ALG that are waiting for
   initial packets.

   Whenever a command from the client isn't 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 is not relayed back to the client.

5.  IANA considerations


6.  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 don't 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.

7.  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.

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

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   [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.

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

              Bagnulo, M., Matthews, P., and I. Beijnum, "NAT64: Network
              Address and Protocol Translation from IPv6 Clients to IPv4
              Servers", draft-ietf-behave-v6v4-xlate-stateful-07 (work
              in progress), December 2009.

              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.

Appendix A.  Server implementation recommendations

   As EPSV works through IPv6-to-IPv4 translation transparently without
   additional effort on the part of the client, the server or an
   application layer gateway, it is highly recommended that all servers
   implement EPSV.

   [RFC2428] suggests that the EPSV mode is useful both for clients with
   IPv6 connectivity and for clients operating behind a NAT device.  As
   such, it is common for IPv6-capable clients to use EPSV even when
   communicating over IPv4.  If a server doesn't implement EPSV and
   responds with a 501 or 502 error, the client simply retries with
   PASV.  This works well with both servers that have working EPSV and
   servers that don't implement EPSV.  However, there is a class of
   servers that does implement EPSV, but is unable to use this mode
   because the data connection can't be established successfully.  This
   is very likely the result of a middlebox monitoring the control
   channel interactions, and creating firewall or translation state
   according to the information 227 response after a PASV command.  If
   the server supports EPSV but the middlebox doesn't, the result is
   that the data connection cannot be established and the data transfer

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   To avoid this, it is highly recommended that server implementers
   include a configuration setting that makes it possible to disable
   EPSV and EPRT support and respond with a 502 (command not
   implemented) error instead.  Server operators can thus disable EPSV
   support in servers located behind PASV-only middleboxes so clients
   that issue EPSV can fall back to PASV gracefully rather than a

   The test performed by Dan Wing showed that existing implementations
   present the address used for the server side of the control channel
   connection in the 227 response to a PASV command.  Clients conforming
   to this specification depend on this behavior and it allows ALGs to
   translate a 227 PASV response to a 229 EPSV response without loss of
   information; as such it is highly recommended that servers continue
   to implement this limitation.

   Many servers that support the FEAT command do not list EPSV and EPRT
   as a supported feature in the response to the FEAT command.  It is
   recommended that EPSV and EPRT capability is included in the FEAT
   response, unless EPSV and/or EPRT are administratively disabled as
   outlined above.

Appendix B.  Acknowledgements

   Kentaro Ebisawa, Remi Denis-Courmont, Mayuresh Bakshi, Sarat
   Kamisetty, Reinaldo Penno, Alun Jones, Dave Thaler, Mohammed
   Boucadair, Mikael Abrahamsson and Dapeng Liu 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
   important design decisions from [I-D.liu-behave-ftp64].

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

Appendix C.  Document and discussion information

   Please direct questions and comments to the BEHAVE mailinglist.  The
   latest version of this document will always be available at

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Author's Address

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


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