Internet DRAFT - draft-ietf-ftpext2-ftp64
draft-ietf-ftpext2-ftp64
FTPEXT2 D. Liu
Internet-Draft China Mobile
Intended status: BCP Iljitsch van. Beijnum
Expires: July 11, 2012 IMDEA Networks
Z. Cao
China Mobile
January 11, 2012
FTP consideration for IPv4/IPv6 transition
draft-ietf-ftpext2-ftp64-02
Abstract
The File transfer protocol(FTP) has a long histroy,, but still being
widely used. The first concept of FTP was described RFC 114, and
then was specified in RFC 354. FTP can work in IPv4 environment and
then was extended to IPv6. RFC 2428 defines IPv6 extensions of FTP.
In the IPv6-IPv4 translation scenario, considerations should be
applied to FTP client, server and translation box to ensure FTP
protocol work properly. This document discusses the details for FTP
to work in IPv4-IPv6 transitiion scenario. This document gives
recommendation regarding how IPv6 FTP client should behave in 6to4
scenario.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 26, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions used in this document . . . . . . . . . . . . . . . 4
3. Client considerations . . . . . . . . . . . . . . . . . . . . . 4
4. Server considerations . . . . . . . . . . . . . . . . . . . . . 5
5. FTP ALG considerations . . . . . . . . . . . . . . . . . . . . 5
5.1. FTP ALG limitations . . . . . . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
Figure 1 illustrated the IPv6-IPv4 translation FTP scenario.
+----------------------------------------------- -----+
| |
| |
| +----------------+ +--------------+ |
| | IPv6 Network | | IPv4 Network | |
| | +-----------+ | +-----------+ | +----------+ | |
| | |IPv6 |--|--|Translation|--|-|IPv4 | | |
| | |FTP Client | | | Box | | |FTP Server| | |
| | +-----------+ | +-----------+ | +----------+ | |
| | | | | |
| +----------------+ +--------------+ |
| |
| |
+------------------------------------------------ ----+
Figure 1 IPv6-IPv4 translation FTP scenario.
Figure 1
The IPv6 FTP client situated in an IPv6 network and tries to
communicate with an IPv4 server that situated in an IPv4 network
through a translation box in the middle.
It should be noted that in some scenario, the FTP client that running
on the IPv6 host maybe legacy IPv4 FTP client. Here "legacy IPv4 FTP
client" refers to the FTP client software that only support IPv4.In
this case, the communication will not succeed by only introducing
translation box in the network. That case is out the scope of this
document.
FTP has two operation modes: passive mode and active mode. In
passive mode, the server provides port used for the client to connect
to. In active mode, the server connects back to the client, using
the IP address and port number which provide by the client.
RFC 2428 specifies IPv6 extension of FTP. Two new commands, EPRT/
EPSV are specified. The EPRT command is an extension of PORT, it
could provide IPv6 address and port number to the server. The EPSV
command is an extension of PASV, when client sends this command, the
server should responses its port number used for the client to
connect.
Many serves do not support EPSV command today, but most of them could
support PASV mode. This document provides guidelines and extensions
for implementing IPv6 FTP client to avoid the problems when an IPv6
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FTP client communicating with an IPv4 server through a translation
box.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
ALG: Application layer gateway
3. Client considerations
According to RFC 2428, the IPv6 client SHOULD support EPSV and EPRT
command. From the backward compatibility's perspective, this
document suggests that the IPv6 FTP client SHOULD support both EPSV
and PASV. The reason is that during the early stage of IPv6
transition, many FTP servers will be located in the IPv4 Internet
which may not support EPSV command. This requirement implies that
the IPv6 FTP client supports both IPv4 and IPv6 protocol version.
This requirement is reasonable since backward compatibility to IPv4
is one of the basic requirements for any IPv6 applications especially
in the early stage of IPv6 transition.
Most of today's dedicated IPv4 FTP client software uses passive mode
as the default mode. According to RFC 2428, for IPv6 FTP client,
EPSV command MUST be used when the control and data connection
established between the same two machines. The reasons that both
IPv4 and IPv6 FTP client prefer passive mode includes: 1. Active
mode of FTP may introduce security issues. For example, the attacker
may use PORT/EPRT command to specify a victim host's IP and port,
then the FTP serve will continually to send TCP SYN to the victim
host to try to establish data connection. This kind of attack is
recognized as FTP reflects attack. 2. Passive mode of FTP may
benefit when traverse firewalls or NAT. using passive mode also
eliminate FTP ALG in the middle box.
From the above analysis, it is suggested that FTP client SHOULD use
passive mode instead of active mode whenever it is possible. For
IPv6 FTP client, according to RFC 2428, it should use EPSV command.
In IPv4-IPv6 transition scenario, an IPv6 client may try to
communicate with an IPv4 server. In this case, the IPv4 server may
not support EPSV command and the EPSV command may fail. This
document suggests that the IPv6 FTP client SHOULD retry with PASV
command when EPSV command fails. The IPv4 FTP server will respond to
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PASV command with a message that contains an IPv4 address and port
number of the FTP server that used for the client to connect to. The
client MUST ignores the IPv4 address provided in the response; it
should use the control connection's IP address to connect to the
server to establish the data connection. The approach that simply
ignoring PASV respond message's IP address and use control channel's
IP address could not only simply the FTP client software's
implementation but also can avoid the problems caused by using the
IPv4 address that included in the response message. For example, if
the FTP client has a private IPv4 connection and a public IPv6
connection, if it tries to use the IPv4 connection to establish data
connection with the server, it will never succeed. Another example
is: in an IPv4 world if for example a PASV reply contains network
address prefix, e.g. 192.168.0.0/16, which can happen if using
encrypted communications and going though NAT and the server hasn't
configured the "external" facing IP (And since the session is
encrypted, NAT can't see the address to change it). The client
should identify this (and similar) wrong IPs and reuse the control
connection's IP.
4. Server considerations
This document does not enforce any requirement for FTP server since
this document considers the IPv6 FTP client communicating with IPv4
FTP server scenario. The IPv4 FTP server maybe just an ordinary IPv4
FTP server.
5. FTP ALG considerations
This document argues that since FTP is a protocol that could avoid
ALG by slightly adjusting the operation of the IPv6 FTP client it is
not recommended the translation box to implement FTP ALG.
Adjusting the operation of IPv6 client is feasible because IPv6 is
not widely deployed and there are not much IPv6 FTP client deployed
right now. It is a good chance to give this guideline before the
widely deployment of IPv6 and IPv6 FTP client.
5.1. FTP ALG limitations
Implementing FTP ALG in the translation box may have some
limitations, such as:
1) FTP ALG may case to increase the complexity of translation box,
since FTP ALG needs to understand FTP protocol and translate the
application layer payload and update the header of FTP control
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packets. ALG could also cause the decline of the translation box's
performance.
2) From the evolution perspective, if the network continues to
provide support of FTP ALG all the time, the ALG function of the
translation box will become more and more complex.
6. Security Considerations
FTP security is discussed in RFC 2577 [RFC 2577]. The recommendation
that is defined in this document will not impact FTP security.
7. IANA Considerations
None
8. Acknowledgments
The authors want to thanks the following people for their useful
suggestions: Robert Oslin,Anthony Bryan,John C Klensin,Mykyta
Yevstifeyev.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9.2. Informative References
[FTP Security Considerations]
M. Allman, "FTP Security Considerations", 1999,
<http://tools.ietf.org/html/rfc2577>.
[I-D.draft-ietf-behave-ftp64-11]
I. van Beijnum, "An FTP ALG for IPv6-to-IPv4 translation",
2011,
<http://tools.ietf.org/html/draft-ietf-behave-ftp64-11>.
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Authors' Addresses
Dapeng Liu
China Mobile
Unit2, 28 Xuanwumenxi Ave,Xuanwu District
Beijing 100053
China
Email: liudapeng@chinamobile.com
Iljitsch van Beijnum
IMDEA Networks
Avda. del Mar Mediterraneo, 22, Leganes
Madrid 28918
Spain
Email: iljitsch@muada.com
Zhen Cao
China Mobile
Unit2, 28 Xuanwumenxi Ave,Xuanwu District
Beijing 100053
China
Email: caozhen@chinamobile.com
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