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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." 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 Liu, et al. Expires April 26, 2012 [Page 1] Internet-Draft FTP consider for IPv4/IPv6 transition October 2011 Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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 Liu, et al. Expires April 26, 2012 [Page 2] Internet-Draft FTP consider for IPv4/IPv6 transition October 2011 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 Liu, et al. Expires April 26, 2012 [Page 3] Internet-Draft FTP consider for IPv4/IPv6 transition October 2011 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 Liu, et al. Expires April 26, 2012 [Page 4] Internet-Draft FTP consider for IPv4/IPv6 transition October 2011 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 Liu, et al. Expires April 26, 2012 [Page 5] Internet-Draft FTP consider for IPv4/IPv6 transition October 2011 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, . [I-D.draft-ietf-behave-ftp64-11] I. van Beijnum, "An FTP ALG for IPv6-to-IPv4 translation", 2011, . Liu, et al. Expires April 26, 2012 [Page 6] Internet-Draft FTP consider for IPv4/IPv6 transition October 2011 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 Liu, et al. Expires April 26, 2012 [Page 7]