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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]
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-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]
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-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]
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-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]
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-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]
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-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]
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-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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-RFC 2428 FTP Extensions for IPv6 and NATs September 1998
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-
-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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