Network Working Group P. Hethmon
Internet-Draft Hethmon Brothers
Updates: 959 R. McMurray
Intended status: Standards Track Microsoft
Expires: May 19, 2010 November 2009
File Transfer Protocol HOST Command
draft-hethmon-mcmurray-ftp-hosts-09
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Abstract
The File Transfer Protocol, as defined in RFC 959 and Section 4
of RFC 1123, is one of the oldest and most widely used protocols on
the Internet.
This document addresses the subject of creating multi-homed hostname-
based FTP servers on a single IP address. This is achieved by
extending the FTP specification to add a HOST command that is used
to specify individual FTP hosts.
Table of Contents
1. Introduction.....................................................2
2. Document Conventions ...........................................4
2.1. Basic Tokens ...............................................4
2.2. Server Replies .............................................5
3. The HOST command ................................................5
3.1. Syntax of the HOST command ..................................6
3.2. HOST command semantics ......................................7
3.2.1. The REIN command .......................................7
3.2.2. User-PI usage of HOST ...................................8
3.2.3. State Diagrams ..........................................9
3.3. HOST command errors ........................................11
3.4. FEAT response for HOST command .............................12
4. Security Considerations ........................................12
5. IANA Considerations ............................................12
6. References .....................................................12
6.1 Normative References ........................................12
6.2 Informative References ......................................13
7. Acknowledgments ................................................13
8. Authors' Addresses .............................................13
1. Introduction
It is common on the Internet for many domain names to be allocated
to a single IP address. This practice has introduced the concept of
a "virtual host", where a host appears to exist as an independent
entity, but in reality shares all of its resources with one or more
similar hosts.
Such an arrangement presents some problems for FTP servers, as all
an FTP server can detect is an incoming FTP connection to a
particular IP address. That is, all domain names that share an IP
address also share the same FTP server, and more importantly, its
Network Virtual File System (NVFS).
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This means that different virtual hosts cannot offer different
virtual file systems to clients, nor can they offer different
authentication systems.
No scheme can overcome this without modifications of some kind to the
user-PI and to the user-FTP process. That process is the only entity
that knows which virtual host is required. It has performed the
domain name-to-IP address translation, and thus has the original
domain name available.
One method that could be used to allow a style of virtual host would
be for the client to simply send a "CWD" command after connecting,
using the virtual host name as the argument to the CWD command.
This would allow the server-FTP process to implement the file stores
of the virtual hosts as sub-directories in its NVFS. This is simple,
and most server-FTP implementations support this without requiring
any code changes.
While that method is simple to describe, and to implement, it suffers
from several drawbacks. First, the "CWD" command is available only
after the user-PI has authenticated itself to the server-FTP process.
Thus, all virtual hosts would be required to share a common
authentication scheme if they used this method. Secondly, to make
the virtual host truly transparent, either the server-FTP process
needs to be modified to include information that shows the special
nature of this first CWD command (negating most of the advantage of
this scheme), or all users must see the same identical NVFS view upon
connecting (they must connect in the same initial directory), or the
NVFS must implement the full set of virtual host directories at each
possible initial directory for any possible user. Thirdly, and
again, unless the server is specially modified, a user connecting
this way to a virtual host would be able to easily move to any other
virtual host supported at the same server-FTP process, exposing the
nature of the virtual host.
Another suggested method is to simply overload the "ACCT" for FTP
virtual hosts, but this proposal is unacceptable for several
reasons with regard to when the ACCT command is sent during the
request flow. Sections 5.4 and 6 of [RFC959] document the request
flow for a login sequence as USER -> PASS -> ACCT. This flow of
commands may be acceptable when you are considering a single user
having multiple accounts on an FTP server, but fails to
differentiate between virtual hosts when you consider the following
two issues:
a. The first problem with overloading the ACCT command is
certificate negotiation when using the FTP security extensions
that are documented in [RFC2228] and [RFC4217]. In order to
safeguard user credentials, security mechanism and certificate
negotiation must occur before login credentials are sent by the
client. The problem with using the ACCT command in this scenario
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is that there is no way of ensuring that the certificate matches
the correct virtual host before the user credentials are sent.
b. The second problem with overloading the ACCT command is how
user credentials are implemented for FTP virtual hosts. FTP
server implementations may allow the use of custom user
credentials on a per-virtual-host basis. For example, in one
particular implementation the virtual host negotiation occurs,
then the user credentials are looked up using the account
mechanism that is specific to that virtual host. So once again
the virtual host negotiation must take place before the user
credentials are sent.
The conclusion from the examination of the existing possibilities
seems to be that to obtain an adequate emulation of "real" FTP
servers, server modifications to support virtual hosts are
necessary. A new FTP command seems the most likely solution to
provide the required level of support.
2. Document Conventions
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 [RFC2119].
In examples, "C>" and "S>" indicate lines sent by the client and
server, respectively.
This document also uses notation defined in [RFC959] and [RFC1123].
In particular, the terms "reply", "user", "NVFS", "NVT", "file",
"pathname", "FTP commands", "DTP", "user-FTP process", "user-PI",
"user-DTP", "server-FTP process", "server-PI", "server-DTP", "mode",
"type", "control connection", "data connection", and "ASCII", are
all used here as defined there.
Syntax required is defined using the Augmented BNF defined in
[RFC5234]. Some general ABNF definitions are required throughout the
document, those will be defined later in this section. At first
reading, it may be wise to simply recall that these definitions
exist here, and skip to the next section.
2.1. Basic Tokens
This document imports the core definitions given in Appendix A of
[RFC5234]. There definitions will be found for basic ABNF elements
like ALPHA, DIGIT, SP, etc. To that, the following term is added
for use in this document.
TCHAR = VCHAR / SP / HTAB ; visible plus white space
The VCHAR (from [RFC5234]) and TCHAR rules give basic character
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types from varying sub-sets of the ASCII character set for use in
various commands and responses.
Note that in ABNF, string literals are case insensitive. That
convention is preserved in this document, and implies that FTP
commands added by this specification have names that can be
represented in any case. That is, "HOST" is the same as "host",
"Host" and "HoSt" etc. However note that ALPHA, in particular, is
case sensitive.
2.2. Server Replies
Section 4.2 of [RFC959] defines the format and meaning of replies
by the server-PI to FTP commands from the user-PI. Those reply
conventions are used here without change.
error-response = error-code SP *TCHAR CRLF
error-code = ("4" / "5") 2DIGIT
Implementers should note that the ABNF syntax (which was not used in
[RFC959]) used in this document, and other FTP related documents,
sometimes shows replies using the one line format. Unless otherwise
explicitly stated, that is not intended to imply that multi-line
responses are not permitted. Implementers should assume that, unless
stated to the contrary, any reply to any FTP command (including QUIT)
may be of the multi-line format described in [RFC959].
Throughout this document, replies will be identified by the three
digit code that is their first element. Thus the term "500 reply"
means a reply from the server-PI using the three digit code "500".
3. The HOST command
A new command "HOST" is added to the FTP command set to allow the
server-FTP process to determine to which of possibly many virtual
hosts the client wishes to connect. This command SHOULD be issued
before the user is authenticated, allowing the authentication scheme,
and set of legal users, to be dependent upon the virtual host chosen.
Server-FTP processes may permit the HOST command to be issued after
the user has been authenticated, or may treat that as an erroneous
sequence of commands. The behavior of a server-FTP process that
allows late HOST commands is undefined. One reasonable
interpretation would be for the user-PI to be returned to the state
that existed after the TCP connection was first established, before
user authentication.
Servers should note that the response to the HOST command is a
sensible time to send their "welcome" message. This allows the
message to be personalized for any virtual hosts that are supported,
and also allows the client to determine the supported languages, or
representations, for the message, and other messages, via the FEAT
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response, and select an appropriate one via the LANG command. See
[RFC2640] for more information.
3.1. Syntax of the HOST command
The HOST command is defined as follows.
host-command = "Host" SP hostname CRLF
hostname = 1*DNCHAR 1*( "." 1*DNCHAR ) [ "." ]
DNCHAR = ALPHA / DIGIT / "-" / "_" / "$" /
"!" / "%" / "[" / "]" / ":"
host-response = host-ok / error-response
host-ok = "220" [ SP *TCHAR ] CRLF
As with all FTP commands, the "HOST" command word is case
independent, and may be specified in any character case desired.
The "hostname" (given as a parameter) specifies the virtual host to
which access is desired. It should normally be the same name that
was used to obtain the IP address to which the FTP control connection
was made, after any client conversions have been completed that
convert an abbreviated or local alias to a complete (fully qualified)
domain name, but before resolving a DNS alias (owner of a CNAME
resource record) to its canonical name.
If the client was given a network literal address, and consequently
was not required to derive it from a hostname, the client should send
the HOST command with the network address, as specified to it.
Square brackets are expected to disambiguate port numbers syntax from
IPv6 address syntax. Therefore, if the network address is an IPv6
network address, the address should be enclosed in square brackets
(after eliminating any syntax that might also - but is not required
to - be enclosed in brackets, and from which the server deduced that
a literal address had been specified.) For example:
HOST 192.0.2.1
HOST [FE80::c000:0201]
HOST [::192.0.2.1]
should be sent if the client had been instructed to respectively
connect to "192.0.2.1", "FE80::c000:0201", or "192.0.2.1" and IPv6
syntax is preferred.
The client MUST NOT send the port number as part of the HOST
command, even when the client has been instructed to connect to a
non-standard port. For example, the server-PI should reply with
a 501 reply if the client sends a HOST command with syntax like
"HOST 192.0.2.1:2112" or "HOST [FE80::c000:0201]:2112".
The parameter is otherwise to be treated as a fully qualified domain
name or relative name as those terms are defined in section 3.1 of
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[RFC1034]. This implies that the name is to be treated as a
case-independent string, in that uppercase ASCII characters are to
be treated as equivalent to their corresponding lowercase ASCII
characters, but otherwise preserved as given. It also implies some
limits on the length of the parameter and of the components that
create its internal structure. Those limits are not altered in any
way here.
Neither [RFC1034] nor [RFC1035] impose any other restrictions upon
what kinds of names can be stored in the DNS. This specification,
however, allows only a restricted set of names for the purposes of
the HOST command. Those restrictions can be inferred from the ABNF
grammar given for the "hostname".
3.2. HOST command semantics
Upon receiving the HOST command, before authenticating the user-PI, a
server-FTP process should validate that the hostname given represents
a valid virtual host for that server, and, if it is valid, establish
the appropriate environment for that virtual host. The resultant
actions needed to create that environment are not specified here, and
may range from doing nothing at all, to performing a simple change of
working directory, to making much more elaborate state changes, as
required.
If the hostname specified is unknown at the server, or if the server
is otherwise unwilling to treat the particular connection as a
connection to the hostname specified, the server will respond with a
504 reply.
Note: servers may require that the name specified is in some sense
equivalent to the particular network address that was used to reach
the server.
If the hostname specified would normally be acceptable, but for any
reason is temporarily unavailable, the server SHOULD reply to the
HOST command with a 421 reply.
The "220" reply code for the HOST command is the same as the code
that is used in the initial "welcome" message that is sent after
the connection is established. This reply code is used deliberately
in order to allow the implementation of the front-end FTP server as
a wrapper, which simply waits for the HOST command, and then invokes
a server that is compliant with [RFC959] in the appropriate
environment for the particular hostname received.
3.2.1. REIN command semantics
As specified in [RFC959], the REIN command returns the state of
the connection to that it was immediately after the transport
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connection was opened. This specification makes no changes to that
behavior. The effect of a HOST command will be lost if a REIN
command is performed, and a new HOST command must be issued.
Implementers of user-FTP should be aware that server-FTP
implementations that implement the HOST command as a wrapper around
older implementations will be unable to correctly implement the REIN
command. If they do, REIN will typically return the server-FTP to
the state that it was in immediately after the HOST command was
issued, instead of to the state that it was in immediately after the
connection was opened.
3.2.2. User-PI usage of HOST
A user-PI that conforms to this specification, MUST send the HOST
command after opening the transport connection, or after any REIN
command, before attempting to authenticate the user with the USER
command.
The HOST command can be used in combination with the ACCT command
to differentiate between a user's various accounts on a specific
virtual host. In this scenario, the user-PI sends a HOST command
which the server-PI uses to route activity to the correct virtual
host, the user-PI sends credentials using the USER and PASS commands
which the server-PI validates, then the user-PI sends an ACCT
command to specify any additional account information for the
server-PI implementation. The following example illustrates a
series of client commands that specify both a HOST and ACCT, with
the server responses omitted for brevity:
C> HOST foobar.com
C> USER foo
C> PASS bar
C> ACCT project1
This is also true when the HOST command is used with the AUTH and
ADAT commands that are discussed in [RFC2228] and [RFC4217]. In
this scenario, the user-PI sends a HOST command which the server-PI
uses to route activity to the correct virtual host, then the user-PI
uses the AUTH and ADAT commands to negotiate the security mechanism
and certificate with the server-PI, then the user-PI sends user
credentials using the USER and PASS commands which the server-PI
validates. After which the user-PI may send an ACCT command to
specify any additional account information for the server-PI
implementation. The following example illustrates a series of client
commands that specify both a HOST and ACCT when used in conjunction
with the security commands that are discuseed in [RFC2228] and
[RFC4217], with the server responses omitted for brevity:
C> HOST foobar.com
C> AUTH <mechanism-name>
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C> ADAT <base64data>
C> USER foo
C> PASS bar
C> ACCT project1
3.2.3. State Diagrams
The state diagrams in this section illustrate typical sequences for
command and reply interchange between the user-PI and server-PI.
These diagrams are modeled on the similar diagrams in section 6 of
[RFC959].
In both diagrams, the (B) "begin" state is assumed to occur after
the transport connection has opened, or after a REIN command has
succeeded. Other commands (such as FEAT [RFC2389]) that require no
authentication may have intervened.
In each diagram, a three-digit reply indicates a precise server reply
code. A single digit on a reply path indicates any server reply that
begins with that digit, except where a precise server reply code is
defined on another path. For example, a single digit "5" will apply
to "500", "501", "502", etc., when those reply codes are not
expressly defined in the diagram. For each command there are three
possible outcomes: success (S), failure (F), and error (E). In the
state diagrams below we use the symbol B for "begin", and the
symbol W for "wait for reply".
+---+ HOST +---+ 1,3,5
| B |---------->| W |-----------------
+---+ +---+ |
| | |
2,500,502 | | 4,501,503,504 |
-------------- ------------- |
| | |
V 1 | V
+---+ USER +---+-------------->+---+
| |---------->| W | 2 ----->| E |
+---+ +---+------ | --->+---+
| | | | | |
3 | | 4,5 | | | |
-------------- ----- | | | |
| | | | | |
| | | | | |
| ---------- | |
| 1| | | | |
V | | | | |
+---+ PASS +---+ 2 | ------->+---+
| |---------->| W |-------------->| S |
+---+ +---+ ----------->+---+
| | | | | |
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3 | |4,5| | | |
-------------- -------- | ----
| | | | | |
| | | | | |
| ------------ |
| 1,3| | | | |
V | 2| | | V
+---+ ACCT +---+-- | ------>+---+
| |---------->| W | 4,5 --------->| F |
+---+ +---+-------------->+---+
When the HOST command is used in combination with the FTP security
extensions that were introduced in [RFC2228], it SHOULD precede
the security handshake. This allows both user-PI and server-FTP
processes to map an FTP HOST to security data appropriately.
The following state diagram shows a typical sequence of flow of
control when HOST is used with the AUTH and ADAT commands that are
discussed in [RFC2228]. The explanations that were given for the
previous diagram also apply to this diagram.
+---+ HOST +---+ 1,3,5
| B |---------->| W |-----------------
+---+ +---+ |
| | |
2,500,502 | | 4,501,503,504 |
+-------------- ------------- |
| | |
V | |
+---+ AUTH +---+ 4,5 | |
| |---------->| W |----------->| |
+---+ +---+ | |
234 | | | |
--------- | 334 | |
| | | |
---------------|------ | |
| | | | | |
V | V 335 | | |
+---+ | ADAT +---+---- | |
| |---------->| W | 4,5 | |
+---+ | +---+----------->| |
| | | |
---- 235| | |
| -------------- | |
| | | |
V V 1 | V
+---+ USER +---+-------------->+---+
| |---------->| W | 2 ----->| E |
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+---+ +---+------ | --->+---+
| | | | | |
3 | | 4,5 | | | |
-------------- ----- | | | |
| | | | | |
| | | | | |
| ---------- | |
| 1| | | | |
V | | | | |
+---+ PASS +---+ 2 | ------->+---+
| |---------->| W |-------------->| S |
+---+ +---+ ----------->+---+
| | | | | |
3 | |4,5| | | ----
-------------- -------- | |
| | | | | |
| | | | | |
| ------------ |
| 1,3| | | | |
V | 2| | | V
+---+ ACCT +---+-- | ------>+---+
| |---------->| W | 4,5 --------->| F |
+---+ +---+-------------->+---+
3.3. HOST command errors
The server-PI should reply with a 500 or 502 reply if the HOST
command is unrecognized or unimplemented. A 503 reply may be sent
if the HOST command is given after a previous HOST command, or after
a user has been authenticated. Alternately, the server may define
behavior that allows the HOST command after a user has been
authenticated. A 501 reply should be sent if the hostname given is
syntactically invalid, and a 504 reply should be sent if a
syntactically valid hostname is not a valid virtual host name for the
server. In all such cases, the server-FTP process should act as if
no HOST command had been given.
A user-PI receiving a 500 or 502 reply should assume that the
server-PI does not implement virtual servers by using the HOST
command. The user-PI may then proceed to login as if the HOST
command had succeeded, and may attempt a CWD command to the hostname
after authenticating the user.
A user-PI receiving an error reply that is different from the errors
that have been described here should assume that the virtual HOST is
unavailable, and terminate communications.
A server-PI that receives a USER command, beginning the
authentication sequence, without having received a HOST command,
SHOULD NOT reject the USER command. Clients conforming to earlier
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FTP specifications do not send HOST commands. In this case the
server may act as if some default virtual host had been explicitly
selected, or may enter an environment different from that of any
supported virtual hosts, perhaps one in which a union of all
available accounts exists, and which presents an NVFS that appears
to contain subdirectories that contain the NVFS for all supported
virtual hosts.
3.4. FEAT response for HOST command
A server-FTP process that supports the HOST command and virtual FTP
servers MUST include, in the response to the FEAT command [RFC2389],
a feature line indicating that the HOST command is supported. This
line should contain the single word "HOST". This command word is
case insensitive, but it SHOULD be transmitted in upper case only.
That is, the response SHOULD be:
C> FEAT
S> 211- <any descriptive text>
S> ...
S> HOST
S> ...
S> 211 End
The ellipses indicate place holders where other features may be
included, and are not required. The single space indentation of each
feature line is mandatory [RFC2389].
4. Security Considerations
With the introduction of virtual hosts to FTP, and the possible
accompanying multiple authentication environments, server
implementers will need to take some care to ensure that integrity is
maintained.
A general discussion of issues related to the security of FTP can be
found in [RFC2577].
5. IANA Considerations
This document has no IANA actions.
6. References
6.1. Normative References
[RFC959] Postel, J., Reynolds, J., "File Transfer Protocol (FTP)",
STD 9, RFC 959, October 1985
[RFC1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987
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[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987
[RFC1123] Braden, R,. "Requirements for Internet Hosts --
Application and Support", STD 3, RFC 1123, October 1989
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[RFC2228] Horowitz, M., Lunt, S., "FTP Security Extensions",
RFC 2228, October 1997
[RFC2389] Hethmon, P., Elz, R., "Feature negotiation mechanism for
the File Transfer Protocol", RFC 2389, August 1998
[RFC2640] Curtin, W., "Internationalization of the File Transfer
Protocol", RFC 2640, July 1999
[RFC4217] Ford-Hutchinson, P., "Securing FTP with TLS", RFC 4217,
October 2005
[RFC5234] Crocker, D., Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 5234, January 2008
6.2. Informative References
[RFC2577] Allman, M., Ostermann, S., "FTP Security Considerations",
RFC 2577, May 1999
7. Acknowledgments
Robert Elz and Paul Hethmon provided a detailed discussion of the
HOST command in their Internet draft titled "Extensions to FTP"
as part of their work with the FTPEXT Working Group at the IETF.
Their work formed the basis for much of this document, and their
help has been greatly appreciated. They would also like to credit
Bernhard Rosenkraenzer for having first suggested and described the
HOST command.
8. Authors' Addresses
Paul Hethmon
Hethmon Brothers
2305 Chukar Road
Knoxville, TN 37923 USA
Email: phethmon@hethmon.com
Robert McMurray
Microsoft Corporation
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One Microsoft Way
Redmond, WA 98052-6399
Email: robmcm@microsoft.com
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