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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Behavior Engineering for Hindrance I. van Beijnum 3 Avoidance Institute IMDEA Networks 4 Internet-Draft May 2, 2011 5 Intended status: Standards Track 6 Expires: November 3, 2011 8 An FTP ALG for IPv6-to-IPv4 translation 9 draft-ietf-behave-ftp64-09 11 Abstract 13 The File Transfer Protocol (FTP) has a very long history, and despite 14 the fact that today, other options exist to perform file transfers, 15 FTP is still in common use. As such, it is important that in the 16 situation where some client computers only have IPv6 connectivity 17 while many servers are still IPv4-only and IPv6-to-IPv4 translators 18 are used to bridge that gap, FTP is made to work through these 19 translators as best it can. 21 FTP has an active and a passive mode, both as original commands that 22 are IPv4-specific, and as extended, IP version agnostic commands. 23 The only FTP mode that works without changes through an IPv6-to-IPv4 24 translator is extended passive. However, many existing FTP servers 25 do not support this mode, and some clients do not ask for it. This 26 document specifies a middlebox that may solve this mismatch. 28 Status of this Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on November 3, 2011. 45 Copyright Notice 47 Copyright (c) 2011 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4 64 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 65 4. ALG overview . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 5. Control channel translation . . . . . . . . . . . . . . . . . 5 67 5.1. Language negotiation . . . . . . . . . . . . . . . . . . . 7 68 6. EPSV to PASV translation . . . . . . . . . . . . . . . . . . . 8 69 7. EPRT to PORT translation . . . . . . . . . . . . . . . . . . . 9 70 7.1. Stateless EPRT translation . . . . . . . . . . . . . . . . 9 71 7.2. Stateful EPRT translation . . . . . . . . . . . . . . . . 10 72 8. Default port 20 translation . . . . . . . . . . . . . . . . . 10 73 9. Both PORT and PASV . . . . . . . . . . . . . . . . . . . . . . 11 74 10. Default behavior . . . . . . . . . . . . . . . . . . . . . . . 11 75 11. The ALGS command . . . . . . . . . . . . . . . . . . . . . . . 11 76 12. Timeouts and translating to NOOP . . . . . . . . . . . . . . . 13 77 13. IANA considerations . . . . . . . . . . . . . . . . . . . . . 13 78 14. Security considerations . . . . . . . . . . . . . . . . . . . 14 79 15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14 80 16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15 81 17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 82 17.1. Normative References . . . . . . . . . . . . . . . . . . . 15 83 17.2. Informative References . . . . . . . . . . . . . . . . . . 15 84 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16 86 1. Introduction 88 [RFC0959] specifies two modes of operation for FTP: active mode, in 89 which the server connects back to the client and passive mode, where 90 the server opens a port for the client to connect to. Without 91 additional measures, active mode with a client-supplied port does not 92 work through NATs or firewalls. With active mode, the PORT command 93 has an IPv4 address as its argument, and in passive mode, the server 94 responds to the PASV command with an IPv4 address. This makes both 95 the passive and active modes as originally specified in [RFC0959] 96 incompatible with IPv6. These issues were solved in [RFC2428], which 97 introduces the EPSV (extended passive) command, where the server only 98 responds with a port number, and the EPRT (extended port) command, 99 which allows the client to supply either an IPv4 or an IPv6 address 100 (and a port) to the server. 102 A survey done in April of 2009 of 25 randomly picked and/or well- 103 known FTP sites reachable over IPv4 showed that only 12 of them 104 supported EPSV over IPv4. Additionally, only 2 of those 12 indicated 105 that they supported EPSV in response to the FEAT command introduced 106 in [RFC2389] that asks the server to list its supported features. 107 One supported EPSV but not FEAT. In 5 cases, issuing the EPSV 108 command to the server led to a significant delay, in 3 cases followed 109 by a control channel reset. All 25 servers were able to successfully 110 complete a transfer in traditional PASV passive mode as required by 111 [RFC1123]. More testing showed that the use of an address family 112 argument with the EPSV command is widely mis- or unimplemented in 113 servers. The additional tests with more servers showed that 114 approximately 65% of FTP servers support EPSV successfully and around 115 96% support PASV successfully. Clients were not extensively tested, 116 but previous experience from the author suggests that most clients 117 support PASV, with the notable exception of the command line client 118 included with Windows, which only supports active mode. This FTP 119 client uses the original PORT command when running over IPv4 and EPRT 120 when running over IPv6. 122 Although these issues can and should be addressed by modifying 123 clients and servers to support EPSV successfully (see 124 [I-D.liu-ftp64-extension]), such modifications may not appear widely 125 in a timely fashion. Also, network operators who may want to deploy 126 IPv6-to-IPv4 translation generally don't have control over client or 127 server implementations. As such, this document standardizes an FTP 128 Application Layer Gateway (ALG) that will allow unmodified IPv6 FTP 129 clients to interact with unmodified IPv4 FTP servers successfully 130 when using FTP for simple file transfers between a single client and 131 a single server. 133 Clients that want to engage in more complex behavior, such as server- 134 to-server transfers, may make an FTP application layer gateway (ALG) 135 go into transparent mode by issuing AUTH or ALGS commands as 136 explained in Section 5. 138 The recommendations and specifications in this document apply to all 139 forms of IPv6-to-IPv4 translation, including stateless translation 140 such as [RFC6145] as well as stateful translation such as [RFC6146]. 142 2. Notational Conventions 144 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 145 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 146 document are to be interpreted as described in [RFC2119]. 148 3. Terminology 150 Within the context of this document, the words "client" and "server" 151 refer to FTP client and server implementations, respectively. An FTP 152 server is understood to be an implementation of the FTP protocol 153 running on a server system with a stable address, waiting for clients 154 to connect and issue commands that eventually start data transfers. 155 Clients interact with servers using the FTP protocol, and store 156 (upload) files to, and retrieve (download) files from one or more 157 servers. This either happens interactively under control of a user, 158 or is done as an unattended background process. Most operating 159 systems provide a web browser that implements a basic FTP client, as 160 well as a command line client. Third-party FTP clients are also 161 widely available. 163 Other terminology is derived from the documents listed in the 164 references section. Note that this document cannot be fully 165 understood on its own; it depends on background and terminology 166 outlined in the references. 168 4. ALG overview 170 The most robust way to solve an IP version mismatch between FTP 171 clients and FTP servers would be by changing clients and servers 172 rather than using an IPv6-to-IPv4 translator for the data channel and 173 using an application layer gateway on the control channel. As such, 174 it is recommended to update FTP clients and servers as required for 175 IPv6-to-IPv4 translation support where possible, to allow proper 176 operation of the FTP protocol without the need for ALGs. 178 On the other hand, network operators or even network administrators 179 within an organization often have little influence over the FTP 180 client and server implementations used over the network. For those 181 operators and administrators, deploying an ALG may be the only way to 182 provide a satisfactory customer experience. So, even though not the 183 preferred solution, this document standardizes the functionality of 184 such an ALG in order to promote consistent behavior between ALGs in 185 an effort to minimize their harmful effects. 187 Operators and administrators are encouraged to only deploy an FTP ALG 188 for IPv6-to-IPv4 translation when the FTP ALG is clearly needed. In 189 the presence of the ALG, EPSV commands that could be handled directly 190 by conforming servers are translated into PASV commands, introducing 191 additional complexity and reducing robustness. As such a "set and 192 forget" policy on ALGs is not recommended. 194 Note that the translation of EPSV through all translators and EPRT 195 through a stateless translator is relatively simple but supporting 196 translation of EPRT through a stateful translator is relatively 197 difficult, because in the latter case a translation mapping must be 198 set up for each data transfer using parameters that must be learned 199 from the client/server interaction over the control channel. This 200 needs to happen before the EPRT command can be translated into a PORT 201 command and passed on to the server. As such, an ALG used with a 202 stateful translator MUST support EPSV and MAY support EPRT. However, 203 an ALG used with a stateless translator SHOULD also support EPRT. 205 The ALG functionality is described as a function separate from the 206 IPv6-to-IPv4 translation function. However, in the case of EPRT 207 translation, the ALG and translator functions need to be tightly 208 coupled, so if EPRT translation is supported, it is assumed that the 209 ALG and IPv6-to-IPv4 translation functions are integrated within a 210 single device. 212 5. Control channel translation 214 The IPv6-to-IPv4 FTP ALG intercepts all TCP sessions towards port 21 215 for IPv6 destination addresses that map to IPv4 destinations 216 reachable through an IPv6-to-IPv4 translator. The FTP ALG implements 217 the Telnet protocol ([RFC0854]) used for control channel interactions 218 to the degree necessary to interpret commands and responses and re- 219 issue those commands and responses, modifying them as outlined below. 220 Telnet option negotiation attempts by either the client or the 221 server, except for those allowed by [RFC1123], MUST be rejected by 222 the FTP ALG without relaying those attempts. This avoids the 223 situation where the client and the server negotiate Telnet options 224 that are unimplemented by the FTP ALG. 226 There are two ways to implement the control channel ALG: 228 1. The ALG terminates the IPv6 TCP session, sets up a new IPv4 TCP 229 session towards the IPv4 FTP server, and relays commands and 230 responses back and forth between the two sessions. 232 2. Packets that are part of the control channel are translated 233 individually. 235 As they ultimately provide the same result, either implementation 236 strategy, or any other that is functionally equivalent, can be used. 238 In the second case, an implementation MUST have the ability to track 239 and update TCP sequence numbers when translating packets as well as 240 the ability to break up packets into smaller packets after 241 translation, as the control channel translation could modify the 242 length of the payload portion of the packets in question. Also, FTP 243 commands/responses or Telnet negotiations could straddle packet 244 boundaries, so in order to be able to perform the ALG function, it 245 can prove necessary to reconstitute Telnet negotiations and FTP 246 commands and responses from multiple packets. 248 If the client issues the AUTH command, then the client is attempting 249 to negotiate [RFC2228] security mechanisms which are likely to be 250 incompatible with the FTP ALG function. In this situation, the FTP 251 ALG MUST switch to transparently forwarding all data on the control 252 channel in both directions until the end of the control channel 253 session. This requirement applies regardless of the response from 254 the server. In other words, it is the fact that the client attempts 255 the AUTH negotiation that requires the ALG to become transparent, 256 whether or not the attempt is successful. The transparency 257 requirement applies to the commands and responses flowing between the 258 client and the server. It is possible that commands or responses 259 that were sent through the ALG before the AUTH command was issued 260 were changed in length so TCP sequence numbers in packets entering 261 the ALG and packets exiting the ALG no longer match. In transparent 262 mode, the ALG MUST continue to adjust sequence numbers if it was 263 doing so before entering transparent mode as the result of the AUTH 264 command. The ALGS command (Section 11) can also be used to disable 265 the ALG functionality, but the control channel MUST then still be 266 monitored for subsequent ALGS commands that re-enable the ALG 267 functionality. 269 There have been FTP ALGs for the purpose of making active FTP work 270 through IPv4 NATs for a long time. Another type of ALG would be one 271 that imposes restrictions required by security policies. Multiple 272 ALGs of different types can be implemented as a single entity. If 273 such a multi-purpose ALG forbids the use of the AUTH command for 274 policy reasons, the side effect of making the ALG stop performing the 275 translations described here, as well as other possible interventions 276 related to IPv6-to-IPv4 translation, MUST be retained even if the ALG 277 responds to the AUTH command with an error and does not propagate the 278 command to the server. This way, any time a client issues the AUTH 279 command, it knows that an ALG will be in transparent mode afterwards. 280 Implementers are further advised that unlike hosts behind an IPv4 281 NAT, IPv6 hosts using an IPv6-to-IPv4 translator will normally have 282 the ability to execute FTP over IPv6 without interference from the 283 IPv6-to-IPv4 translator or the ALG, so an IPv6-to-IPv4 translation 284 FTP ALG is not the best place to implement security policies. 286 5.1. Language negotiation 288 [RFC2640] specifies the ability for clients and servers to negotiate 289 the language used between the two of them in the descriptive text 290 that accompanies server response codes. Ideally, IPv6-to-IPv4 FTP 291 ALGs would support this feature, so that if a non-default language is 292 negotiated by a client and a server, the ALG also transmits its text 293 messages for translated responses in the negotiated language. 294 However, even if the ALG supports negotiation of the feature, there 295 is no way to make sure that the ALG has text strings for all possible 296 languages. So the situation where the client and server try to 297 negotiate a language that the ALG doesn't support can't be avoided. 298 The proper behavior for an FTP ALG in this situation may be addressed 299 in a future specification, as the same issue is present in IPv4-to- 300 IPv4 FTP ALGs. For the time being, ALG implementations may employ 301 one of the following strategies regarding LANG negotiation: 303 1. Monitor LANG negotiation, and send text in the negotiated 304 language if text in that language is available. If not, text is 305 sent in the default language. 307 2. Not monitor LANG negotiation. Text is sent in the default 308 language. 310 3. Block LANG negotiation by translating the LANG command to a NOOP 311 command, and translating the resulting 200 response into a 312 response appropriate for unsupported commands, such as 500. Text 313 is sent in the default language. 315 In the first two cases, if a language is negotiated, text transmitted 316 by the client or the server MUST be assumed to be encoded in UTF-8 317 rather than be limited to 7-bit ASCII. An ALG that implements the 318 first or second option MUST translate and/or forward commands and 319 responses containing UTF-8 encoded text when those occur. The ALG 320 itself MUST NOT generate characters outside the 7-bit ASCII range 321 unless it implements the first option and a language was negotiated. 323 Note that [RFC2640] section 3.1 specifies new handling for spaces and 324 the CR character in path names. ALGs that don't block LANG 325 negotiation SHOULD comply with the specified rules for path handling. 326 Implementers should especially note that the NUL (%x00) character is 327 used as an escape whenever a CR character occurs in a pathname. 329 In the sections that follow, a number of well-known response numbers 330 are shown, along with the descriptive text that is associated with 331 that response number. However, this text is not part of the 332 specification of the response. As such, implementations MAY use the 333 response text shown or they MAY show a different response text for a 334 given response number. Requirements language only applies to the 335 response number. 337 6. EPSV to PASV translation 339 Although many IPv4 FTP servers support the EPSV command, some servers 340 react adversely to this command (see Section 1 for examples), and 341 there is no reliable way to detect in advance that this will happen. 342 As such, an FTP ALG SHOULD translate all occurrences of the EPSV 343 command issued by the client to the PASV command, and reformat a 227 344 response as a corresponding 229 response. However, an ALG MAY forego 345 EPSV to PASV translation if it has positive knowledge, either through 346 administrative configuration or learned dynamically, that EPSV will 347 be successful without translation to PASV. 349 For instance, if the client issues EPSV (or EPSV 2 to indicate IPv6 350 as the network protocol), this is translated to the PASV command. If 351 the server with address 192.0.2.31 then responds with: 353 227 Entering Passive Mode (192,0,2,31,237,19) 355 The FTP ALG reformats this as: 357 229 Entering Extended Passive Mode (|||60691|) 359 The ALG SHOULD ignore the IPv4 address in the server's 227 response. 360 This is the behavior that is exhibited by most clients and is needed 361 to work with servers that include [RFC1918] addresses in their 227 362 responses. However, if the 227 response contains an IPv4 address 363 that does not match the destination of the control channel, the FTP 364 ALG MAY send a 425 response to the client instead of the 229 365 response, e.g.: 367 425 Can't open data connection 369 It is important that the response is in the 4xx range to indicate a 370 temporary condition. 372 If the client issues an EPSV command with a numeric argument other 373 than 2, the ALG MUST NOT pass the command on to the server, but 374 rather respond with a 522 error, e.g.: 376 522 Network protocol not supported 378 If the client issues EPSV ALL, the FTP ALG MUST NOT pass this command 379 to the server, but respond with a 504 error, e.g.: 381 504 Command not implemented for that parameter 383 This avoids the situation where an FTP server reacts adversely to 384 receiving a PASV command after the client used the EPSV ALL command 385 to indicate that it will only use EPSV during this session. 387 7. EPRT to PORT translation 389 Should the IPv6 client issue an EPRT command, the FTP ALG MAY 390 translate this EPRT command to a PORT command. The translation is 391 different depending on whether the translator is a stateless one-to- 392 one translator or a stateful one-to-many translator. 394 7.1. Stateless EPRT translation 396 If the address specified in the EPRT command is the IPv6 address used 397 by the client for the control channel session, then the FTP ALG 398 reformats the EPRT command into a PORT command with the IPv4 address 399 that maps to the client's IPv6 address. The port number MUST be 400 preserved for compatibility with stateless translators. For 401 instance, if the client with IPv6 address 2001:db8:2::31 issues the 402 following EPRT command: 404 EPRT |2|2001:db8:2::31|5282| 406 Assuming the IPv4 address that goes with 2001:db8:2::31 is 407 192.0.2.31, the FTP ALG reformats this as: 409 PORT 192,0,2,31,20,162 411 If the address specified in the EPRT command is an IPv4 address or an 412 IPv6 address that is not the IPv6 address used by the client for the 413 control session, the ALG SHOULD NOT attempt any translation, but pass 414 along the command unchanged. 416 7.2. Stateful EPRT translation 418 If the address in the EPRT command is the IPv6 address used by the 419 client for the control channel, the stateful translator selects an 420 unused port number in combination with the IPv4 address used for the 421 control channel towards the FTP server, and sets up a mapping from 422 that transport address to the one specified by the client in the EPRT 423 command. The PORT command with the IPv4 address and port used on the 424 IPv4 side of the mapping is only issued towards the server once the 425 mapping is created. Initially, the mapping is such that either any 426 transport address or the FTP server's IPv4 address with any port 427 number is accepted as a source, but once the three-way handshake is 428 complete, the mapping SHOULD be narrowed to only match the negotiated 429 TCP session. 431 If the address specified in the EPRT command is an IPv4 address or an 432 IPv6 address that is not the IPv6 address used by the client for the 433 control session, the ALG SHOULD NOT attempt any translation, but pass 434 along the command unchanged. 436 If the client with IPv6 address 2001:db8:2::31 issues the EPRT 437 command: 439 EPRT |2|2001:db8:2::31|5282| 441 And the stateful translator uses the address 192.0.2.31 on its IPv4 442 interface, a mapping with destination address 192.0.2.31 and 443 destination port 60192 towards 2001:db8:2::31 port 5282 may be 444 created, after which the FTP ALG reformats the EPRT command as: 446 PORT 192,0,2,31,235,32 448 8. Default port 20 translation 450 If the client does not issue an EPSV/PASV or EPRT/PORT command prior 451 to initiating a file transfer, it is invoking the default active FTP 452 behavior where the server sets up a TCP session towards the client. 453 In this situation, the source port number is the default FTP data 454 port (port 20) and the destination port is the port the client uses 455 as the source port for the control channel session. 457 In the case of a stateless translator, this does not pose any 458 problems. In the case of a stateful translator, the translator MAY 459 accept incoming connection requests from the server on the IPv4 side 460 if the transport addresses match that of an existing FTP control 461 channel session, with the exception that the control channel session 462 uses port 21 and the new session port 20. In this case, a mapping is 463 set up towards the same transport address on the IPv6 side that is 464 used for the matching FTP control channel session. 466 An ALG/translator MAY monitor the progress of FTP control channels 467 and only attempt to perform a mapping when an FTP client has started 468 a file transfer without issuing the EPSV, PASV, EPRT or PORT 469 commands. 471 9. Both PORT and PASV 473 [RFC0959] allows a client to issue both PORT and PASV to use non- 474 default ports on both sides of the connection. However, this is 475 incompatible with the notion that with PASV, the data connection is 476 made from the client to the server, while PORT reaffirms the default 477 behavior where the server connects to the client. As such, the 478 behavior of an ALG is undefined when a client issues both PASV and 479 PORT. Implementations SHOULD NOT try to detect the situation where 480 both PASV and PORT commands are issued prior to a command that 481 initiates a transfer, but rather, apply the same translation they 482 would have if there had not been a PASV command prior to a PORT 483 command or a PORT command prior to a PASV command. 485 10. Default behavior 487 Whenever the client issues a command which the ALG is not set up to 488 translate, either because the command is not specified in this 489 document, the command is not part of any FTP specification, the ALG 490 functionality is disabled administratively for the command in 491 question, or translation does not apply for any other reason, the 492 command MUST be passed on to the server without modification, and the 493 server response MUST be passed on to the client without modification. 494 For example, if the client issues the PASV command, this command is 495 passed on to the server transparently, and the server's response to 496 the client. 498 11. The ALGS command 500 ALGs MUST support the new ALGS (ALG status) command that allows 501 clients to query and set the ALG's status. FTP servers (as opposed 502 to ALGs) MUST NOT perform any actions upon receiving the ALGS 503 command. FTP servers MUST still send a response, however. If FTP 504 servers recognize the ALGS command, the best course of action would 505 be to return a 202 response: 507 202 Command not implemented, superfluous at this site 509 However, there is no reason for FTP servers to specifically recognize 510 this command; returning any 50x response that is normally returned 511 when commands are not recognized is appropriate. 513 A client can use the ALGS command to request the ALG's status and to 514 enable and disable EPSV to PASV and, if implemented, EPRT to PORT 515 translation. There are three possible arguments to the ALGS command: 517 ALGS STATUS64 518 The ALG is requested to return the EPSV and EPRT 519 translation status. 521 ALGS ENABLE64 522 The ALG is requested to enable translation. 524 ALGS DISABLE64 525 The ALG is requested to disable translation. 527 The ALG MUST enable or disable EPSV to PASV translation as requested. 528 If EPRT to PORT translation is supported, ALGS ENABLE64 SHOULD enable 529 it and ALGS DISABLE64 SHOULD disable it along with enabling or 530 disabling EPSV to PASV translation, respectively. If EPRT to PORT 531 translation is not supported, ALGS ENABLE64 only enables EPSV to PASV 532 translation. After an ALGS command with any of the three supported 533 arguments, the ALG MUST return a 216 response indicating the type of 534 translation that will be performed. 536 216 NONE 537 Neither EPSV nor EPRT translation is performed. 539 216 EPSV 540 EPSV is translated to PASV, no EPRT translation is 541 performed. 543 216 EPRT 544 EPRT is translated to PORT, no EPSV translation is 545 performed. 547 216 EPSVEPRT 548 EPSV is translated to PASV, EPRT is translated to 549 PORT. 551 The translation type MAY be followed by a space and additional 552 descriptive text until end-of-line. If the ALG is unable to set the 553 requested translation mode, for instance, because of lack of certain 554 resources, this is not considered an error condition. In those 555 cases, the ALG responds with a 216 response followed by the keyword 556 that indicates the current translation status of the ALG. 558 If there is no argument to the ALGS command, or the argument is not 559 one of STATUS64, ENABLE64 or DISABLE64 (or an argument specified by a 560 supported newer document), a 504 or 502 error SHOULD be returned. 562 The Augmented Backus-Naur Form (ABNF) notation (see [RFC5234]) of the 563 ALGS command and its response are as follows: 565 algs-command = "ALGS" SP algs-token CRLF 566 algs-token = "STATUS64" / "ENABLE64" / "DISABLE64" 568 algs-response = (ok-response / error-response) CRLF 569 ok-response = "216" SP response-token [ freetext ] 570 response-token = "NONE" / "EPSV" / "EPRT" / "EPSVEPRT" 571 error-response = not-implemented / invalid-parameter 572 not-implemented = "502" [ freetext ] 573 invalid-parameter = "504" [ freetext ] 574 freetext = (SP *VCHAR) 576 12. Timeouts and translating to NOOP 578 Wherever possible, control channels SHOULD NOT time out while there 579 is an active data channel. A timeout of at least 30 seconds is 580 RECOMMENDED for data channel mappings created by the FTP ALG that are 581 waiting for initial packets. 583 Whenever a command from the client is not propagated to the server, 584 the FTP ALG instead issues a NOOP command in order to keep the 585 keepalive state between the client and the server synchronized. The 586 response to the NOOP command MUST NOT be relayed back to the client. 587 An implementation MAY wait for the server to return the 200 response 588 to the NOOP command and translate that 200 response into the response 589 the ALG is required to return to the client. This way, the ALG never 590 has to create new packets to send to the client, but it can limit 591 itself to modifying packets transmitted by the server. If the server 592 responds with something other than 200 to the NOOP command, the ALG 593 SHOULD tear down the control channel session and log an error. 595 13. IANA considerations 597 IANA is requested to add to the FTP Commands and Extensions registry 598 the following entry: 600 Command Name 601 ALGS 603 FEAT Code 604 -N/A- 606 Description 607 FTP64 ALG status 609 Command Type 610 -N/A- 612 Conformance Requirements 613 o 615 Reference 616 RFC TBD Section 11 618 [TO BE REMOVED: This registration should take place at the following 619 location: http://www.iana.org/assignments/ftp-commands-extensions/ 620 ftp-commands-extensions.xhtml] 622 14. Security considerations 624 In the majority of cases, FTP is used without further security 625 mechanisms. This allows an attacker with passive interception 626 capabilities to obtain the login credentials, and an attacker that 627 can modify packets to change the data transferred. However, FTP can 628 be used with TLS in order to solve these issues. IPv6-to-IPv4 629 translation and the FTP ALG do not impact the security issues in the 630 former case nor the use of TLS in the latter case. However, if FTP 631 is used with TLS as per [RFC4217], or another authentication 632 mechanism that the ALG is aware of, the ALG function is not performed 633 so only passive transfers from a server that implements EPSV or a 634 client that supports PASV will succeed. 636 For general FTP security considerations, see [RFC2577]. 638 15. Contributors 640 Dan Wing, Kentaro Ebisawa, Remi Denis-Courmont, Mayuresh Bakshi, 641 Sarat Kamisetty, Reinaldo Penno, Alun Jones, Dave Thaler, Mohammed 642 Boucadair, Mikael Abrahamsson, Dapeng Liu, Michael Liu, Andrew 643 Sullivan, Anthony Bryan and Ed Jankiewicz contributed ideas and 644 comments. Dan Wing ran experiments with a large number of FTP 645 servers that were very illuminating; many of the choices underlying 646 this document are based on his results. 648 16. Acknowledgements 650 Iljitsch van Beijnum is partly funded by Trilogy, a research project 651 supported by the European Commission under its Seventh Framework 652 Program. 654 17. References 656 17.1. Normative References 658 [RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol 659 Specification", STD 8, RFC 854, May 1983. 661 [RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol", 662 STD 9, RFC 959, October 1985. 664 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application 665 and Support", STD 3, RFC 1123, October 1989. 667 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 668 Requirement Levels", BCP 14, RFC 2119, March 1997. 670 [RFC2228] Horowitz, M., "FTP Security Extensions", RFC 2228, 671 October 1997. 673 [RFC2428] Allman, M., Ostermann, S., and C. Metz, "FTP Extensions 674 for IPv6 and NATs", RFC 2428, September 1998. 676 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 677 Specifications: ABNF", STD 68, RFC 5234, January 2008. 679 17.2. Informative References 681 [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and 682 E. Lear, "Address Allocation for Private Internets", 683 BCP 5, RFC 1918, February 1996. 685 [RFC2389] Hethmon, P. and R. Elz, "Feature negotiation mechanism for 686 the File Transfer Protocol", RFC 2389, August 1998. 688 [RFC2577] Allman, M. and S. Ostermann, "FTP Security 689 Considerations", RFC 2577, May 1999. 691 [RFC2640] Curtin, B., "Internationalization of the File Transfer 692 Protocol", RFC 2640, July 1999. 694 [RFC4217] Ford-Hutchinson, P., "Securing FTP with TLS", RFC 4217, 695 October 2005. 697 [RFC6145] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation 698 Algorithm", RFC 6145, April 2011. 700 [RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful 701 NAT64: Network Address and Protocol Translation from IPv6 702 Clients to IPv4 Servers", RFC 6146, April 2011. 704 [I-D.liu-ftp64-extension] 705 Liu, D., Beijnum, I., and Z. Cao, "FTP extension for IPv4/ 706 IPv6 transition", draft-liu-ftp64-extension-00 (work in 707 progress), October 2010. 709 [Bernstein] 710 Bernstein, D., "PASV security and PORT security", 2000, 711 . 713 Author's Address 715 Iljitsch van Beijnum 716 Institute IMDEA Networks 717 Avda. del Mar Mediterraneo, 22 718 Leganes, Madrid 28918 719 Spain 721 Email: iljitsch@muada.com