FTPEXT Working Group Mark Allman
Internet Draft NASA Lewis/Sterling Software
Expires: November 1, 1998 Shawn Ostermann
Ohio University
Craig Metz
The Inner Net
May 1, 1998
FTP Extensions for IPv6
<draft-ietf-ftpext-ftp-over-ipv6-02.txt>
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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].
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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.
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:
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AF Number Address Format Example
--------- -------------- -------
1 dotted decimal 132.235.1.2
2 IPv6 string 1080::8:800:200C:417A
representations
defined in [HD96]
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)
In this document, any text enclosed within "<>" is informational
text that can be written in any language but MUST NOT include
parentheses. 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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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 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>
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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.
5. Security Issues
The authors do not believe that these changes to FTP introduce new
security problems. A companion Internet Draft [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.
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References
[AO98] Mark Allman and Shawn Ostermann. FTP Security
Considerations, March 1998. I-D
draft-ietf-ftpext-sec-consider-01.txt (work in progress).
[Bel94] S. Bellovin. Firewall-Friendly FTP, February 1994. RFC
1579.
[Bra97] Scott Bradner. Key words for use in RFCs to Indicate
Requirement Levels, March 1997. RFC 2119.
[DH96] S. Deering and R. Hinden. Internet Protocol, Version 6
(IPv6) Specification, January 1996. RFC 1883.
[HD96] R. Hinden and S. Deering. IP Version 6 Addressing
Architecture, January 1996. RFC 1884.
[Pis94] D. Piscitello. FTP Operation Over Big Address Records
(FOOBAR), June 1994. RFC 1639.
[Pos81a] J. Postel. Internet Protocol, September 1981. RFC 791.
[Pos81b] J. Postel. Transmission Control Protocol, September 1981.
RFC 793.
[PR85] J. Postel and J. Reynolds. File Transfer Protocol (FTP),
October 1985. RFC 959.
[RP94] J. Reynolds and J. Postel. Assigned Numbers, October 1994.
RFC 1700.
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Author's Addresses:
Mark Allman
NASA Lewis Research Center/Sterling Software
21000 Brookpark Rd. MS 54-2
Cleveland, OH 44135
Phone: (216) 433-6586
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
ostermann@cs.ohiou.edu
Craig Metz
The Inner Net
Box 10314-1954
Blacksburg, VA 24062-0314
Phone: (DSN) 754-8590
cmetz@inner.net
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