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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.
+
+Abstract
+
+   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
+   conclusions.
+
+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:
+
+           EPRT<space><d><net-prt><d><net-addr><d><tcp-port><d>
+
+   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      132.235.1.2
+        2           IPv6 string         1080::8:800:200C:417A
+                    representations
+                    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
+   connection.
+
+   The following are sample EPRT commands:
+
+        EPRT |1|132.235.1.2|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 "132.235.1.2" 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> \
+            (prot1,prot2,...,protn)
+
+   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]
+   is:
+
+        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> \
+            (<d><d><d><tcp-port><d>)
+
+   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
+   follows:
+
+        EPSV<space><net-prt>
+
+   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:
+
+        EPSV<space>ALL
+
+   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.
+
+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.
+
+References
+
+   [AO98]   Allman, M., and S. Ostermann, "FTP Security
+            Considerations", Work in Progress.
+
+   [Bel94]  Bellovin, S., "Firewall-Friendly FTP", RFC 1579, February
+            1994.
+
+   [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
+            1981.
+
+   [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:
+            http://www.iana.org/numbers.html
+
+
+
+
+
+
+
+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
+   EMail: mallman@lerc.nasa.gov
+   http://gigahertz.lerc.nasa.gov/~mallman/
+
+
+   Shawn Ostermann
+   School of Electrical Engineering and Computer Science
+   Ohio University
+   416 Morton Hall
+   Athens, OH  45701
+
+   Phone: (740) 593-1234
+   EMail: ostermann@cs.ohiou.edu
+
+
+   Craig Metz
+   The Inner Net
+   Box 10314-1954
+   Blacksburg, VA  24062-0314
+
+   Phone:  (DSN) 754-8590
+   EMail: cmetz@inner.net
+
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+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
+   English.
+
+   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
+   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
+   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
+   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
+   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
+   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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+Allman, et. al.             Standards Track                     [Page 8]
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