An FTP Application Layer Gateway (ALG) for IPv4-to-IPv6 Translation
draft-tsou-behave-ftp46-01
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Tina Tsou
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Simon Perreault
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Jing Huang
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2013-09-16
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Internet Engineering Task Force T. Tsou, Ed.
Internet-Draft Huawei Technologies (USA)
Intended status: Informational S. Perreault
Expires: March 20, 2014 Viagenie
J. Huang
Huawei Technologies
September 16, 2013
An FTP Application Layer Gateway (ALG) for IPv4-to-IPv6 Translation
draft-tsou-behave-ftp46-01
Abstract
An FTP ALG for NAT64 was defined in RFC 6384. Its scope was limited
to an IPv6 client connecting to an IPv4 server. This memo supports
the case of an IPv4 client connecting to an IPv6 server.
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 March 20, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents
1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. PASV to EPSV . . . . . . . . . . . . . . . . . . . . . . . . 3
5. EPSV (IPv4) to EPSV (IPv6) . . . . . . . . . . . . . . . . . 4
6. Command to disable FTP ALG . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
8. Security Considerations . . . . . . . . . . . . . . . . . . . 6
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
10. Normative References . . . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. Overview
During the transition from IPv4 to IPv6, some operators need to
deploy NAT in their network. Some subscribers have the need to run
IPv4 based FTP servers at home, and some of the FTP [RFC0959] control
messages carry IP address and port number in the payload, which will
cause a NAT traversal problem.
[RFC6384] defines FTP ALG for NAT64, but only for the case where the
FTP client is on the inside of the NAT64. The case where an FTP
server is on the inside of the NAT64 is not covered.
When the FTP server is behind NAT, it can publish its service address
via a HTTP redirect server and a DDNS system which needs to support
both IP address and port rather than IP address only, or other
possible methods. The FTP server can listen on any possible ports,
not just port 21; FTP server can get it external IP address and port
via some technology like UPnP, and then publish the acquired IP
address and port as its URI, ftp://203.0.113.1:1200, port 1200 is
allocated by NAT.
1.1. Requirements Language
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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].
2. Terminology
3. Scenarios
There can be several scenarios if NAT is involved in the network.
a) In this scenario, the FTP client is behind NAT, FTP ALG needs to
handle the EPRT / PORT command in FTP active mode, translating the IP
address and port. This scenario has been covered by [RFC6384], but
only for NAT64. This scenario for other kinds of NAT has not been
covered.
+--------+ +---------+ +-------+ +---------+ +--------+
| FTP | | | | | | | | FTP |
| Client +----+ IPv6 +----+ NAT64 +----+ IPv4 +----+ Server +
| | | Network | | | | Network | | |
| IPv6 | | | | | | | | IPv4 |
+--------+ +---------+ +-------+ +---------+ +--------+
FTP Client Behind NAT
b) If the FTP server is behind a NAT, FTP passive mode will be the
only working mode, the EPSV / PASV command and the response will be
processed by FTP ALG. This memo covers this scenario.
+--------+ +---------+ +-------+ +---------+ +--------+
| FTP | | | | | | | | FTP |
| Server +----+ IPv6 +----+ NAT64 +----+ IPv4 +----+ Client +
| | | Network | | | | Network | | |
| IPv6 | | | | | | | | IPv4 |
+--------+ +---------+ +-------+ +---------+ +--------+
FTP Server Behind NAT
4. PASV to EPSV
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If FTP client issues PASV command to FTP server, FTP ALG translates
PASV command into EPSV command [RFC2428], setting the "net-prt" field
to 2 (IPv6). The response of EPSV command is translated into PASV
response. FTP ALG allocates an IPv4 address and port for the EPSV
response message, and builds a NAT mapping entry if the NAT is
stateful. The source address of the EPSV response message and the
"tcp-port" in the payload are used for the NAT mapping. The
allocated IPv4 address and port are put into the PASV response
message.
For instance, in the IPv4 side of NAT64, FTP server's address is
203.0.113.1. FTP client issues a PASV command to FTP server, and it
is translated into EPSV command by FTP AGL, as shown below:
PASV command:
PASV
EPSV command:
EPSV 2
When FTP server returns a success response of EPSV containing tcp-
port 3000, FTP AGL allocates an IPv4 address 203.0.113.1 and tcp-port
2000 corresponding to the tcp-port 3000 in the EPSV response message,
and puts the allocated IP address and port into PASV response
message, as shown below:
EPSV success response:
229 Entering Passive Mode (|||3000|)
PASV success response:
227 Entering Passive Mode (203,0,113,1,7,208)
5. EPSV (IPv4) to EPSV (IPv6)
If FTP client issues EPSV command to FTP server, FTP ALG modifies the
"net-prt", change the value from 1 (IPv4) to 2 (IPv6). The response
of IPv6 EPSV command is also translated. FTP ALG allocates an IPv4
address and port for the EPSV response message.
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[RFC2428] requires that "the network address used to establish the
data connection will be the same network address used for the control
connection", so NAT MUST to make sure that IPv4 address for control
connection and IPv4 address for data connection for a FTP server must
be the same, which means all the mappings for an IPv6 address MUST
have the same external IPv4 address.
For instance, in the IPv4 side of NAT64, FTP server's address is
203.0.113.1. The FTP client issues an IPv4 EPSV command to FTP
server, and it is translated into IPv6 EPSV command by FTP AGL, as
shown below:
EPSV (IPv4) command:
EPSV 1
EPSV (IPv6) command:
EPSV 2
When FTP server returns a success response of EPSV containing port
3000, FTP AGL will allocate an IPv4 address 203.0.113.1 and port 2000
corresponding to the port 3000 in the EPSV response message, and put
the allocated port into PASV response message, as shown below:
EPSV (IPv6) success response:
229 Entering Passive Mode (|||3000|)
EPSV (IPv4) success response:
229 Entering Passive Mode (|||2000|)
6. Command to disable FTP ALG
Command ALGS defined in [RFC6384] is extended, three more possible
arguments are added:
ALGS STATUS46 to return the EPSV and EPRT translation status.
ALGS ENABLE46 to enable translation.
ALGS DISABLE46 to disable translation.
7. IANA Considerations
This memo includes no request to IANA.
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8. Security Considerations
RFC6384's security considerations applies to this document.
9. Acknowledgements
10. Normative References
[RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol", STD
9, RFC 959, October 1985.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2428] Allman, M., Ostermann, S., and C. Metz, "FTP Extensions
for IPv6 and NATs", RFC 2428, September 1998.
[RFC6384] van Beijnum, I., "An FTP Application Layer Gateway (ALG)
for IPv6-to-IPv4 Translation", RFC 6384, October 2011.
Authors' Addresses
Tina Tsou (editor)
Huawei Technologies (USA)
2330 Central Expressway
Santa Clara CA 95050
USA
Phone: +1 408 330 4424
Email: tina.tsou.zouting@huawei.com
Simon Perreault
Viagenie
246 Aberdeen
Quebec, QC G1R 2E1
Canada
Phone: +1 418 656 9254
Email: simon.perreault@viagenie.ca
URI: http://viagenie.ca
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Jing Huang
Huawei Technologies
Huawei Area F, Bantian, Longgang District
Shenzhen 518129
China
Email: James.huang@huawei.com
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