Network Working Group M. Allman

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.

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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]

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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)

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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

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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.

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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

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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: [email protected]
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: [email protected]

Craig Metz
The Inner Net
Box 10314-1954
Blacksburg, VA 24062-0314

Phone: (DSN) 754-8590
EMail: [email protected]

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