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Network Working Group M. Allman Request for Comments: 2428 NASA Lewis/Sterling Software Category: Standards Track S. Ostermann

                                                       Ohio University
                                                               C. Metz
                                                         The Inner Net
                                                        September 1998
                  FTP Extensions for IPv6 and NATs

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.


 The specification for the File Transfer Protocol assumes that the
 underlying network protocol uses a 32-bit network address
 (specifically IP version 4).  With the deployment of version 6 of the
 Internet Protocol, network addresses will no longer be 32-bits.  This
 paper specifies extensions to FTP that will allow the protocol to
 work over IPv4 and IPv6.  In addition, the framework defined can
 support additional network protocols in the future.

1. Introduction

 The keywords, such as MUST and SHOULD, found in this document are
 used as defined in RFC 2119 [Bra97].
 The File Transfer Protocol [PR85] only provides the ability to
 communicate information about IPv4 data connections.  FTP assumes
 network addresses will be 32 bits in length.  However, with the
 deployment of version 6 of the Internet Protocol [DH96] addresses
 will no longer be 32 bits long.  RFC 1639 [Pis94] specifies
 extensions to FTP to enable its use over various network protocols.
 Unfortunately, the mechanism can fail in a multi-protocol
 environment.  During the transition between IPv4 and IPv6, FTP needs
 the ability to negotiate the network protocol that will be used for
 data transfer.

Allman, et. al. Standards Track [Page 1] RFC 2428 FTP Extensions for IPv6 and NATs September 1998

 This document provides a specification for a way that FTP can
 communicate data connection endpoint information for network
 protocols other than IPv4.  In this specification, the FTP commands
 PORT and PASV are replaced with EPRT and EPSV, respectively.  This
 document is organized as follows.  Section 2 outlines the EPRT
 command and Section 3 outlines the EPSV command.  Section 4 defines
 the utilization of these two new FTP commands.  Section 5 briefly
 presents security considerations.  Finally, Section 6 provides

2. The EPRT Command

 The EPRT command allows for the specification of an extended address
 for the data connection.  The extended address MUST consist of the
 network protocol as well as the network and transport addresses.  The
 format of EPRT is:
 The EPRT command keyword MUST be followed by a single space (ASCII
 32).  Following the space, a delimiter character (<d>) MUST be
 specified.  The delimiter character MUST be one of the ASCII
 characters in range 33-126 inclusive.  The character "|" (ASCII 124)
 is recommended unless it coincides with a character needed to encode
 the network address.
 The <net-prt> argument MUST be an address family number defined by
 IANA in the latest Assigned Numbers RFC (RFC 1700 [RP94] as of the
 writing of this document).  This number indicates the protocol to be
 used (and, implicitly, the address length).  This document will use
 two of address family numbers from [RP94] as examples, according to
 the following table:
      AF Number   Protocol
      ---------   --------
      1           Internet Protocol, Version 4 [Pos81a]
      2           Internet Protocol, Version 6 [DH96]
 The <net-addr> is a protocol specific string representation of the
 network address.  For the two address families specified above (AF
 Number 1 and 2), addresses MUST be in the following format:
      AF Number   Address Format      Example
      ---------   --------------      -------
      1           dotted decimal
      2           IPv6 string         1080::8:800:200C:417A
                  defined in [HD96]

Allman, et. al. Standards Track [Page 2] RFC 2428 FTP Extensions for IPv6 and NATs September 1998

 The <tcp-port> argument must be the string representation of the
 number of the TCP port on which the host is listening for the data
 The following are sample EPRT commands:
      EPRT |1||6275|
      EPRT |2|1080::8:800:200C:417A|5282|
 The first command specifies that the server should use IPv4 to open a
 data connection to the host "" on TCP port 6275.  The
 second command specifies that the server should use the IPv6 network
 protocol and the network address "1080::8:800:200C:417A" to open a
 TCP data connection on port 5282.
 Upon receipt of a valid EPRT command, the server MUST return a code
 of 200 (Command OK).  The standard negative error code 500 and 501
 [PR85] are sufficient to handle most errors (e.g., syntax errors)
 involving the EPRT command.  However, an additional error code is
 needed.  The response code 522 indicates that the server does not
 support the requested network protocol.  The interpretation of this
 new error code is:
      5yz Negative Completion
      x2z Connections
      xy2 Extended Port Failure - unknown network protocol
 The text portion of the response MUST indicate which network
 protocols the server does support.  If the network protocol is
 unsupported, the format of the response string MUST be:
      <text stating that the network protocol is unsupported> \
 Both the numeric code specified above and the protocol information
 between the characters '(' and ')' are intended for the software
 automata receiving the response; the textual message between the
 numeric code and the '(' is intended for the human user and can be
 any arbitrary text, but MUST NOT include the characters '(' and ')'.
 In the above case, the text SHOULD indicate that the network protocol
 in the EPRT command is not supported by the server.  The list of
 protocols inside the parenthesis MUST be a comma separated list of
 address family numbers.  Two example response strings follow:
      Network protocol not supported, use (1)
      Network protocol not supported, use (1,2)

Allman, et. al. Standards Track [Page 3] RFC 2428 FTP Extensions for IPv6 and NATs September 1998

3. The EPSV Command

 The EPSV command requests that a server listen on a data port and
 wait for a connection.  The EPSV command takes an optional argument.
 The response to this command includes only the TCP port number of the
 listening connection.  The format of the response, however, is
 similar to the argument of the EPRT command.  This allows the same
 parsing routines to be used for both commands.  In addition, the
 format leaves a place holder for the network protocol and/or network
 address, which may be needed in the EPSV response in the future.  The
 response code for entering passive mode using an extended address
 MUST be 229.  The interpretation of this code, according to [PR85]
      2yz Positive Completion
      x2z Connections
      xy9 Extended Passive Mode Entered
 The text returned in response to the EPSV command MUST be:
      <text indicating server is entering extended passive mode> \
 The portion of the string enclosed in parentheses MUST be the exact
 string needed by the EPRT command to open the data connection, as
 specified above.
 The first two fields contained in the parenthesis MUST be blank.  The
 third field MUST be the string representation of the TCP port number
 on which the server is listening for a data connection.  The network
 protocol used by the data connection will be the same network
 protocol used by the control connection.  In addition, the network
 address used to establish the data connection will be the same
 network address used for the control connection.  An example response
 string follows:
      Entering Extended Passive Mode (|||6446|)
 The standard negative error codes 500 and 501 are sufficient to
 handle all errors involving the EPSV command (e.g., syntax errors).
 When the EPSV command is issued with no argument, the server will
 choose the network protocol for the data connection based on the
 protocol used for the control connection.  However, in the case of
 proxy FTP, this protocol might not be appropriate for communication
 between the two servers.  Therefore, the client needs to be able to
 request a specific protocol.  If the server returns a protocol that
 is not supported by the host that will be connecting to the port, the

Allman, et. al. Standards Track [Page 4] RFC 2428 FTP Extensions for IPv6 and NATs September 1998

 client MUST issue an ABOR (abort) command to allow the server to
 close down the listening connection.  The client can then send an
 EPSV command requesting the use of a specific network protocol, as
 If the requested protocol is supported by the server, it SHOULD use
 the protocol.  If not, the server MUST return the 522 error messages
 as outlined in section 2.
 Finally, the EPSV command can be used with the argument "ALL" to
 inform Network Address Translators that the EPRT command (as well as
 other data commands) will no longer be used.  An example of this
 command follows:
 Upon receipt of an EPSV ALL command, the server MUST reject all data
 connection setup commands other than EPSV (i.e., EPRT, PORT, PASV, et
 al.).  This use of the EPSV command is further explained in section

4. Command Usage

 For all FTP transfers where the control and data connection(s) are
 being established between the same two machines, the EPSV command
 MUST be used.  Using the EPSV command benefits performance of
 transfers that traverse firewalls or Network Address Translators
 (NATs).  RFC 1579 [Bel94] recommends using the passive command when
 behind firewalls since firewalls do not generally allow incoming
 connections (which are required when using the PORT (EPRT) command).
 In addition, using EPSV as defined in this document does not require
 NATs to change the network address in the traffic as it is forwarded.
 The NAT would have to change the address if the EPRT command was
 used.  Finally, if the client issues an "EPSV ALL" command, NATs may
 be able to put the connection on a "fast path" through the
 translator, as the EPRT command will never be used and therefore,
 translation of the data portion of the segments will never be needed.
 When a client only expects to do two-way FTP transfers, it SHOULD
 issue this command as soon as possible.  If a client later finds that
 it must do a three-way FTP transfer after issuing an EPSV ALL
 command, a new FTP session MUST be started.

Allman, et. al. Standards Track [Page 5] RFC 2428 FTP Extensions for IPv6 and NATs September 1998

5. Security Issues

 The authors do not believe that these changes to FTP introduce new
 security problems.  A companion Work in Progress [AO98] is a more
 general discussion of FTP security issues and techniques to reduce
 these security problems.

6. Conclusions

 The extensions specified in this paper will enable FTP to operate
 over a variety of network protocols.


 [AO98]   Allman, M., and S. Ostermann, "FTP Security
          Considerations", Work in Progress.
 [Bel94]  Bellovin, S., "Firewall-Friendly FTP", RFC 1579, February
 [Bra97]  Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels", BCP 14, RFC 2119, March 1997.
 [DH96]   Deering, S., and R. Hinden, "Internet Protocol, Version 6
          (IPv6) Specification", RFC 1883, December 1995.
 [HD96]   Hinden, R., and S. Deering, "IP Version 6 Addressing
          Architecture", RFC 2373, July 1998.
 [Pis94]  Piscitello, D., "FTP Operation Over Big Address Records
          (FOOBAR)", RFC 1639, June 1994.
 [Pos81a] Postel, J., "Internet Protocol", STD 5, RFC 791, September
 [Pos81b] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
          September 1981.
 [PR85]   Postel, J., and J. Reynolds, "File Transfer Protocol (FTP)",
          STD 9, RFC 959, October 1985.
 [RP94]   Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
          1700, October 1994.  See also:

Allman, et. al. Standards Track [Page 6] RFC 2428 FTP Extensions for IPv6 and NATs September 1998

Authors' Addresses

 Mark Allman
 NASA Lewis Research Center/Sterling Software
 21000 Brookpark Rd.  MS 54-2
 Cleveland, OH  44135
 Phone: (216) 433-6586
 Shawn Ostermann
 School of Electrical Engineering and Computer Science
 Ohio University
 416 Morton Hall
 Athens, OH  45701
 Phone: (740) 593-1234
 Craig Metz
 The Inner Net
 Box 10314-1954
 Blacksburg, VA  24062-0314
 Phone:  (DSN) 754-8590

Allman, et. al. Standards Track [Page 7] RFC 2428 FTP Extensions for IPv6 and NATs September 1998

Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an

Allman, et. al. Standards Track [Page 8]

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