Network Working Group P. Hethmon
Internet-Draft Hethmon Brothers
Updates: 959 R. McMurray
Intended status: Standards Track Microsoft
Expires: December 20, 2010 June 2010
File Transfer Protocol HOST Command for Virtual Hosts
draft-hethmon-mcmurray-ftp-hosts-15
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. This document may contain material
from IETF Documents or IETF Contributions published or made publicly
available before November 10, 2008. The person(s) controlling the
copyright in some of this material may not have granted the IETF
Trust the right to allow modifications of such material outside the
IETF Standards Process. Without obtaining an adequate license from
the person(s) controlling the copyright in such materials, this
document may not be modified outside the IETF Standards Process, and
derivative works of it may not be created outside the IETF Standards
Process, except to format it for publication as an RFC or to
translate it into languages other than English.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
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".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on December 20, 2010.
Copyright
Copyright (c) 2010 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
Provisions Relating to IETF Documents
Hethmon & McMurray Expires December 20, 2010 [Page 1]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
(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.
Abstract
This document defines a new FTP command that provides a mechanism for
FTP clients and servers to identify individual virtual hosts on an
FTP server.
Table of Contents
1. Introduction.....................................................2
2. Document Conventions ...........................................3
2.1. Basic Tokens ...............................................3
2.2. Server Replies .............................................4
3. The HOST command ................................................4
3.1. Syntax of the HOST command ..................................5
3.2. HOST command semantics ......................................7
3.2.1. REIN command semantics ..................................8
3.2.2. User-PI usage of HOST ...................................8
3.2.3. State Diagrams ..........................................9
3.3. HOST command errors ........................................12
3.4. FEAT response for HOST command .............................13
4. Security Considerations ........................................14
5. IANA Considerations ............................................14
6. References .....................................................14
6.1 Normative References ........................................14
6.2 Informative References ......................................15
Appendix A: Unworkable Alternatives ...............................16
A.1. Overloading the CWD command ................................16
A.2. Overloading the ACCT command ...............................16
A.3. Overloading the USER command ...............................17
A.4. Conclusion .................................................18
Acknowledgments ...................................................18
Authors' Addresses ................................................18
1. Introduction
It is common on the Internet for many DNS names to resolve 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 its physical resources with one or
more similar hosts.
Such an arrangement presents some problems for FTP servers, as an FTP
server distinguishes incoming FTP connections by their IP addresses,
not their DNS names, because hosts are uniquely identified by their
address rather than name. That is, all DNS names that share an IP
address are handled by the same FTP server and share the same Network
Hethmon & McMurray Expires December 20, 2010 [Page 2]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
Virtual File System (NVFS).
This means that different virtual hosts cannot offer different
virtual file systems to clients, nor can they offer different
authentication systems. Any scheme to overcome this issue needs to
indicate not only the destination IP address, but also the virtual
host name that is associated with the desired virtual FTP server.
Typical user-FTP processes currently use hostnames to perform
hostname to IP address resolution and then ignore hostnames for the
rest of the FTP session, therefore any mechanism to overcome this
issue would require modifications to the user-PI and server-PI.
It should be noted that this same problem existed for HTTP/1.0 as
defined in [RFC1945], and was resolved in HTTP/1.1 as defined in
[RFC2616] through the addition of the Host request header. The goal
of this document is to bring a similar level of feature parity to FTP
by introducing a new HOST command that allows user-FTP processes to
specify which virtual host to connect to for a server-FTP process
that is handling requests for mutiple virtual hosts on a single IP
address.
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 B 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
Hethmon & McMurray Expires December 20, 2010 [Page 3]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
The VCHAR (from [RFC5234]) and TCHAR rules give basic character
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 and parameters that are added by this specification have
values that can be represented in any case. That is, "HOST" is the
same as "host", "Host", "HoSt", etc., and "ftp.example.com" is the
same as "Ftp.Example.Com", "fTp.eXample.cOm", etc.
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)
can 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 SHOULD treat a situation where the HOST command
is issued after the user has been authenticated using one of the
following two behaviors:
a. Treat the late HOST command as an erroneous sequence of
commands and return a 503 reply.
b. Treat the late HOST command as though a REIN command was sent
before the HOST command and reset the user-PI to the state that
existed after the TCP connection was first established and before
the initial user authentication and then return the appropriate
Hethmon & McMurray Expires December 20, 2010 [Page 4]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
reply for the HOST command.
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
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 = domain / IP-literal
domain = sub-domain *("." sub-domain)
sub-domain = let-dig [ldh-str]
let-dig = ALPHA / DIGIT
ldh-str = *( ALPHA / DIGIT / "-" ) let-dig
IP-literal = ( "[" IPv6address "]" ) / IPv4address
IPv6address = 6( h16 ":" ) ls32
/ "::" 5( h16 ":" ) ls32
/ [ h16 ] "::" 4( h16 ":" ) ls32
/ [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
/ [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
/ [ *3( h16 ":" ) h16 ] "::" h16 ":" ls32
/ [ *4( h16 ":" ) h16 ] "::" ls32
/ [ *5( h16 ":" ) h16 ] "::" h16
/ [ *6( h16 ":" ) h16 ] "::"
ls32 = ( h16 ":" h16 ) / IPv4address
; least-significant 32 bits of address
h16 = 1*4HEXDIG
; 16 bits of address represented in hexadecimal
IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet
dec-octet = DIGIT ; 0-9
/ %x31-39 DIGIT ; 10-99
/ "1" 2DIGIT ; 100-199
/ "2" %x30-34 DIGIT ; 200-249
/ "25" %x30-35 ; 250-255
host-response = host-ok / error-response
host-ok = "220" [ SP *TCHAR ] CRLF
Hethmon & McMurray Expires December 20, 2010 [Page 5]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
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. This SHOULD be the same host 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.
Internationalization of domain names is only supported through the
use of Punycode as described in [RFC3492].
If the user was given an IPv4 or IPv6 literal address, and
consequently was not required to derive the literal address from a
hostname, the client MAY send the HOST command with the IPv4 or
IPv6 literal address as specified to it. While it may seem
counter-intuitive to specify a literal address by using the HOST
command after the client has already connected to the server using
a literal address, this should be expected behavior because a
user-FTP process should not be required to differentiate between a
fully qualified domain name and an IPv4 or IPv6 network literal
address. That being said, if the IPv4 or IPv6 literal address
specified by the client does not match the literal address for the
server, the server MUST respond with a 504 reply to indicate that
the IPv4 or IPv6 literal address is not valid.
When the hostname parameter contains a literal address, square
brackets are expected to disambiguate IPv6 address syntax from port
numbers syntax. Therefore, if the literal address is an IPv6
address, the IPv6 address is required to 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, the
following examples MAY 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:
HOST 192.0.2.1
HOST [FE80::c000:0201]
HOST [::192.0.2.1]
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. The reason for this requirement is that the user-PI will have
established a connection to the server-PI before the HOST command is
sent, therefore specifying a different port with the HOST command has
no meaning. For example, the server-PI MUST respond with a 501 reply
if the client sends a HOST command with syntax like either of the
following examples:
Hethmon & McMurray Expires December 20, 2010 [Page 6]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
HOST 192.0.2.1:2112
HOST [FE80::c000:0201]:2112
The hostname 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 [RFC1034]. This implies that the name is to be
treated as a case-independent string, meaning 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 the of names that 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 changing authentication schemes and/or username
and password lists, to making much more elaborate state changes, as
necessary.
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 a 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.
If the hostname specified would normally be acceptable, but for any
reason is temporarily unavailable, the server-FTP process SHOULD
reply to the HOST command with a 421 reply and close the connection.
In this particular situation, the server-FTP process MAY choose to
keep the connection open in order to allow the user-PI an opportunity
to choose another virtual host with a subsequent HOST command.
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 SHOULD respond with
a 504 reply.
Hethmon & McMurray Expires December 20, 2010 [Page 7]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
3.2.1. REIN command semantics
As specified in [RFC959], the REIN command returns the state of
the connection to what it was immediately after the transport
connection was opened. This specification makes no changes to that
behavior. The effect of a HOST command MUST be reset if a REIN
command is performed, and a new HOST command MUST be issued in order
to connect to a virtual host.
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 following example illustrates what a typical login
sequence might look like when the HOST command is used:
C> HOST ftp.example.com
S> 220 Host accepted
C> USER foo
S> 331 Password required
C> PASS bar
S> 230 User logged in
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
sequential series of client commands that specify both a HOST and
ACCT, with the server responses omitted for brevity:
C> HOST ftp.example.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 sequential
series of client commands that specify both a HOST and ACCT when
Hethmon & McMurray Expires December 20, 2010 [Page 8]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
used in conjunction with the security commands that are discussed
in [RFC2228] and [RFC4217], with the server responses omitted for
brevity:
C> HOST ftp.example.com
C> AUTH <mechanism-name>
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 each diagram, 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.
Additionally, 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".
In each of these diagrams, a REIN command will return the diagram to
the (B) "begin" state.
The state diagram in Figure 1 shows a typical sequence of flow of
control when HOST is used with USER and PASS to log in to a
particular FTP virtual host.
+---+ HOST +---+ 1,3,5
| B |---------->| W |-----------------
+---+ +---+ |
| | |
2,500,502 | | 4,501,503,504 |
-------------- ------------ |
| | |
V 1 | V
+---+ USER +---+-------------->+---+
| |---------->| W | 2 | | E |
+---+ +---+------ ----->+---+
Hethmon & McMurray Expires December 20, 2010 [Page 9]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
| | | | |
3 | | 4,5 | | |
-------------- ----- | | |
| | | | |
| ---------- |
| 1| | | |
V | | ------->+---+
+---+ PASS +---+ 2 | | | S |
| |---------->| W |-------------->+---+
+---+ +---+ | |
| | |
|4,5 | |
| | -->+---+
| --------->| F |
---------------->+---+
Figure 1: Typical login sequence with HOST command
The state diagram in Figure 2 shows the flow of control when a HOST
command is sent after a user has already successfully logged in to a
virtual host with USER and PASS.
------------------------------
| |
| |
V |
+---+ 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 | | ------->+---+ HOST |
+---+ PASS +---+ 2 | | | S |--------
| |---------->| W |-------------->+---+
+---+ +---+ | |
| | |
|4,5 | |
| | |
Hethmon & McMurray Expires December 20, 2010 [Page 10]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
| | -->+---+
| --------->| F |
---------------->+---+
Figure 2: Login sequence with repeated HOST command
After a user has logged in, an additional account may be required by
the server and specified by the client by using ACCT command. With
this in mind, the state diagram in Figure 3 shows a typical sequence
of flow of control when HOST is used with USER and PASS to log in to
an FTP virtual host and ACCT is used to specify an account.
+---+ 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 |
+---+ +---+ ----------->+---+
| | | | | |
3 | |4,5| | | |
-------------- -------- | ----
| | | | | |
| | | | | |
| ------------ |
| 1,3| | | | |
V | 2| | | V
+---+ ACCT +---+-- | ------>+---+
| |---------->| W | 4,5 --------->| F |
+---+ +---+-------------->+---+
Figure 3: Login sequence with HOST and ACCT commands
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
Hethmon & McMurray Expires December 20, 2010 [Page 11]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
processes to map an FTP HOST to security data appropriately. The
state diagram in Figure 4 shows a typical sequence of flow of control
when HOST is used with the AUTH and ADAT commands that are discussed
in [RFC2228].
+---+ 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 |
+---+ +---+------- ----->+---+
| | | | |
3 | | 4,5 | | |
-------------- ------ | | |
| | | | |
| ----------- |
| 1| | | |
V | | ------->+---+
+---+ PASS +---+ 2 | | | S |
| |---------->| W |--------------->+---+
+---+ +---+ | |
| | |
|4,5 | |
| | -->+---+
| --------->| F |
---------------->+---+
Figure 4: Login sequence with HOST and AUTH/ADAT commands
Hethmon & McMurray Expires December 20, 2010 [Page 12]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
After a user has logged in with the security commands that are
discussed in [RFC2228], an additional account may be required by the
server and specified by the client by using ACCT command. The state
diagram in Figure 5 shows a typical sequence of flow of control
when HOST is used with the AUTH and ADAT commands to log in to
an FTP virtual host and ACCT is used to specify an account.
+---+ 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 |
+---+ +---+------- | --->+---+
| | | | | |
3 | | 4,5 | | | |
-------------- ------ | | | |
| | | | | |
| ----------- | |
| 1| | | | |
V | | | | |
+---+ PASS +---+ 2 | ------->+---+
| |---------->| W |--------------->| S |
+---+ +---+ ------------>+---+
| | | | | |
3 | |4,5| | | |
-------------- --------- | ----
| | | | | |
| ------------- |
| 1,3| | | | |
Hethmon & McMurray Expires December 20, 2010 [Page 13]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
V | 2| | | V
+---+ ACCT +---+-- | ------>+---+
| |---------->| W | 4,5 --------->| F |
+---+ +---+--------------->+---+
Figure 5: Login sequence with HOST and AUTH/ADAT/ACCT commands
3.3. HOST command errors
The server-PI SHOULD reply with a 500 or 502 reply if the HOST
command is unrecognized or unimplemented.
As discussed in section 3 of this document, if a HOST command is sent
after a user has been authenticated the server SHOULD do one of the
following:
a. Send a 503 reply for an invalid sequence of commands.
b. Treat the HOST command as though a REIN command was sent and
reset the user-PI to the state that existed after the previous
HOST command was sent and before the user had been authenticated,
and then return the appropriate reply for the HOST command.
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 MUST do one of the following:
a. Ignore the HOST command and act as if as if a HOST command
had not been sent. A user-FTP process MAY then send a subsequent
HOST command with a different hostname.
b. Close the connection.
A user-PI receiving a 500 or 502 reply to a HOST command 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 not been sent.
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 to begin the authentication
sequence without having received a HOST command SHOULD NOT reject the
USER command. Clients conforming to earlier 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 that is 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
Hethmon & McMurray Expires December 20, 2010 [Page 14]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
that contain the NVFS for all supported virtual hosts.
3.4. FEAT response for HOST command
When replying to the FEAT command [RFC2389], a server-FTP process
that supports the HOST command MUST include a line containing the
single word "HOST". This word is case insensitive, and MAY be sent
in any mixture of upper or lower case, however it SHOULD be sent in
upper case. 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 one-space indentation of the
feature lines is mandatory [RFC2389].
4. Security Considerations
With the introduction of virtual hosts to FTP, server implementers
will need to take some care to ensure that the integrity of
potentially sensitive information is maintained. For example, while
hostnames may generally be assumed to be publicly available DNS
names, this may not always be the situation. Some organizations may
use private hostnames, and that information SHOULD be protected when
transmitted between the client and server by using a strong method of
encryption.
Server implementations SHOULD reset the security environment when a
HOST command is sent after a user has logged in. This allows for
individual authentication environments for each virtual host on an
FTP server. For example, a virtual host "foo.example.com" on an FTP
server might use a specific username and password list, while the
virtual host "bar.example.com" on the same FTP server might use a
different username and password list. In such a scenario, resetting
the security environment is necessary for virtual servers to appear
to behave independently.
A general discussion of issues related to the security of FTP can be
found in [RFC2577].
5. IANA Considerations
IANA is requested to register the following FTP extension according
to the procedure established by [RFC5797]:
+-------+------+-------------------+------+------+------------------+
Hethmon & McMurray Expires December 20, 2010 [Page 15]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
| cmd | FEAT | description | type | conf | RFC#s/References |
| | Code | | | | and Notes |
+-------+------+-------------------+------+------+------------------+
| HOST | HOST | Hostname | a | o | [#1] |
+-------+------+-------------------+------+------+------------------+
[[ RFC Editor: Please update note #1 in the above table with the
number of this document. ]]
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
[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
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications (IDNA)",
RFC 3492, March 2003
[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
[RFC1945] Berners-Lee, T., Fielding, R., Frystyk, H., "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996
Hethmon & McMurray Expires December 20, 2010 [Page 16]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
[RFC2577] Allman, M., Ostermann, S., "FTP Security Considerations",
RFC 2577, May 1999
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., Berners-Lee, T., "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999
[RFC5797] Klensin, J., Hoenes, A., "FTP Command and Extension
Registry", March 2010
Appendix A: Unworkable Alternatives
Due to the level of scope for adding a new command to FTP, a brief
discussion of suggested alternatives to a HOST command and their
respective limitations is warranted. The suggested alternatives that
are discussed in this appendix have been proposed in the past, but
each of these ideas was deemed insufficient for the reasons that are
listed within each section of the appendix.
A.1. Overloading the CWD command
One suggested method to emulate a form of virtual hosts 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 suggestion is
simple in concept, and most server-FTP implementations support this
without requiring any code changes. While this method is simple to
describe, and to implement, it suffers from several drawbacks:
a. 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.
b. 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.
c. 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.
A.2. Overloading the ACCT command
Hethmon & McMurray Expires December 20, 2010 [Page 17]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
Another suggested method would be 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
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,
and 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.
A.3. Overloading the USER command
An additional suggestion would be to overload well-known syntax
through the existing USER command, as illustrated in the following
example:
C> USER foo@example.com
S> 331 Password required
C> PASS bar
S> 230 User logged in
In this example, the user "foo" might be attempting to log on to the
virtual host "example.com" on an FTP server. This suggestion may
seem plausible at first, but introduces several implementation
problems. For example:
a. Some network environments already use the "username@hostname"
syntax for network credentials, where the "hostname" portion
refers to the location of the user's credentials within the
network hierarchy. Using the "foo@example.com" syntax it becomes
difficult to differentiate between the user "foo" logging into a
Hethmon & McMurray Expires December 20, 2010 [Page 18]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
virtual host named "example.com" on an FTP server versus the user
"foo@example.com" logging into an FTP server with no specified
virtual host.
b. When using the FTP security extensions that are documented in
[RFC2228] and [RFC4217], security mechanism and certificate
negotiation must occur before login credentials are sent by the
client. More specifically, the AUTH/ADAT commands must be sent
before the USER command in order to safeguard user credentials.
If you overload the USER command, there is no way of ensuring that
the certificate matches the correct virtual host before the user
credentials are sent by the client.
A.4. Conclusion
The conclusion from the examination of the existing possibilities
seems to be that in order to obtain an adequate emulation of "real"
FTP servers, client and server modifications to support virtual hosts
are necessary. Therefore a new FTP command seems the most likely
solution to provide the required level of support.
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.
Alexey Melnikov, Alfred Hoenes, John Klensin, and Joe Touch have made
several suggestions about earlier versions of this document; many of
their suggestions have been incorporated, and their contributions are
gratefully acknowledged. In addition, Alec Rowell's assitance in
making sections of this document more readable was invaluable.
Authors' Addresses
Paul Hethmon
Hethmon Brothers
2305 Chukar Road
Knoxville, TN 37923 USA
Email: phethmon@hethmon.com
Robert McMurray
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399
Hethmon & McMurray Expires December 20, 2010 [Page 19]
INTERNET-DRAFT FTP HOST Command for Virtual Hosts June 2010
Email: robmcm@microsoft.com
Hethmon & McMurray Expires December 20, 2010 [Page 20]
