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IETF RFC 7151
Last modified on Wednesday, March 12th, 2014
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Internet Engineering Task Force (IETF) P. Hethmon
Request for Comments: 7151 Hethmon Brothers
Updates: 959 R. McMurray
Category: Standards Track Microsoft Corporation
ISSN: 2070-1721 March 2014
File Transfer Protocol HOST Command for Virtual Hosts
 Abstract
The File Transfer Protocol, as defined in RFC 959, does not provide a
way for FTP clients and servers to differentiate between multiple DNS
names that are registered for a single IP address. 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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 7151.
Copyright Notice
Copyright (c) 2014 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Hethmon & McMurray Standards Track PAGE 1 
RFC 7151 FTP HOST Command for Virtual Hosts March 2014
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 ...............................9
3.2.3. State Diagrams .....................................11
3.3. HOST Command Errors .......................................16
3.4. FEAT Response for HOST Command ............................17
4. Security Considerations ........................................17
5. IANA Considerations ............................................19
6. References .....................................................19
6.1. Normative References ......................................19
6.2. Informative References ....................................20
Appendix A. Unworkable Alternatives ...............................21
A.1. Overloading the CWD Command ................................21
A.2. Overloading the ACCT Command ...............................21
A.3. Overloading the USER Command ...............................22
A.4. Conclusion .................................................23
Appendix B. Acknowledgements ......................................23
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, because
an FTP server distinguishes incoming FTP connections by IP addresses
rather than DNS names. Therefore, all DNS names that share a common
IP address are handled by the same FTP server and share the same
Network 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
hostname 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
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
rest of the FTP session; therefore, any mechanism to overcome this
issue would require modifications to the user protocol interpreter
(user-PI) and server protocol interpreter (server-PI).
It should be noted that this same problem existed for HTTP/1.0 as
defined in [RFC 1945] and was resolved in HTTP/1.1 as defined in
[RFC 2616] through the addition of the Host request header field. 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 multiple 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 [RFC 2119].
In examples, "C>" and "S>" indicate lines sent by the client and
server, respectively.
This document also uses notation defined in [RFC 959] and [RFC 1123].
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.
The required syntax is defined using the Augmented BNF defined in
[RFC 5234]. Some general ABNF definitions are required throughout the
document; they will be defined in subsequent sections.
With the increased use of virtualization technologies, there may be
several possible definitions for the term "virtual host". This
document follows the definition from Section 4.1.14 of [RFC 3875],
where several virtual hosts share the same IP address, and hostnames
are used by the server-FTP process to route user-PI sessions to the
appropriate virtual host.
2.1. Basic Tokens
This document imports the core definitions given in Appendix B of
[RFC 5234]. 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
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The VCHAR (from [RFC 5234]) and TCHAR rules give basic character types
from varying subsets 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. Similarly, because domain names
are defined to be case insensitive, "ftp.example.com" is the same as
"Ftp.Example.Com", "fTp.eXample.cOm", etc.
2.2. Server Replies
Section 4.2 of [RFC 959] 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 used in this document
and other FTP-related documents (but that was not used in [RFC 959])
sometimes shows replies using the one-line format. Unless otherwise
explicitly stated, multi-line responses are also 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 [RFC 959].
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 in order to
allow a server-FTP process to determine to which of possibly many
virtual hosts the client wishes to connect. If a HOST command is
sent, it MUST be issued before the user is authenticated, as this
will allow the authentication scheme and set of authorized users to
be dependent upon the virtual host that is chosen.
Server-FTP processes MUST treat a situation in which the HOST command
is issued more than once before the user has been authenticated as
though only the last HOST command had been sent, and return the
appropriate reply for the last HOST command. Server-FTP processes
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MUST treat a situation in which the HOST command is issued after the
user has been authenticated as an erroneous sequence of commands and
return a 503 reply.
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.
It also allows the client to determine, via the FEAT response, the
languages or representations supported by the server and select an
appropriate one via the LANG command. See [RFC 2640] for more
information.
It should be noted that user-PI implementations that were created
before the introduction of the HOST command will not support this new
command. A similar problem existed with the introduction of the Host
header for HTTP in [RFC 2616], and HTTP server implementations had to
determine how best to accommodate HTTP requests from down-level
clients that did not support the Host header. With this in mind,
server-FTP processes will need to determine how best to accommodate
FTP requests from down-level FTP clients that do not support the HOST
command, but those considerations are outside the scope of this
document.
3.1. Syntax of the HOST Command
The HOST command is defined as follows. Note that [RFC 3986] remains
the normative specification for the syntactic form of IPv4 and IPv6
address literals, in order to ensure identical presentation in 'ftp'
URI hostname parts and in the protocol element specified here.
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 = <see [RFC 3986] Section 3.2.2>
IPv4address = <see [RFC 3986] Section 3.2.2>
host-response = host-ok / error-response
host-ok = "220" [ SP *TCHAR ] CRLF
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The "hostname" rule is a restricted form of the "host" rule specified
in [RFC 3986]. Details of the additional restrictions imposed by this
document are given in the discussion of the syntax that occurs later
in this section; they aim at simplifying implementations by only
allowing what currently is specified precisely and in use on the
Internet.
As with all FTP commands, the "HOST" command word is case independent
and can 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 hostname 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 Internationalized Domain Names for Applications (IDNA)
"A-labels" for <sub-domain> as described in [RFC 5890]. For example,
the following HOST command specifies an internationalized
domain name:
HOST xn--e1afmkfd.com
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
counterintuitive 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
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
following examples MAY be sent if the client had been instructed to
connect to "192.0.2.1", "2001:db8::c000:201", or "::192.0.2.1",
respectively, and IPv6 syntax is preferred:
HOST 192.0.2.1
HOST [2001:db8::c000:201]
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:
HOST 192.0.2.1:2112
HOST [2001:db8::c000:201]: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 [RFC 1034]. 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 [RFC 1034] nor [RFC 1035] imposes any other restrictions upon
what kinds of names can be stored in the DNS. This specification,
however, only allows the use of names that can be inferred from the
ABNF grammar given for the "hostname". Similarly, this specification
restricts address literals to the IPv4 and IPv6 address families well
established on the Internet.
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, changing authentication schemes and/or username
and password lists, or making much more elaborate state changes --
such as creating isolated environments for each FTP session.
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
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.
If the hostname specified would normally be acceptable, but is
temporarily unavailable, the server-FTP process SHOULD respond to the
HOST command with a 421 reply and close the connection.
Example:
The server-FTP process is shutting down, so the server-FTP process
responds to the HOST command with a 421 reply and closes the
connection. In this scenario, the 421 reply informs the client it
can retry at another time.
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.
Examples:
The particular virtual host that was specified by the HOST command
is disabled at the server. The server responds with a 504 reply
and keeps the connection open in order to allow the user-PI an
opportunity to specify another virtual host with a subsequent HOST
command.
Alternatively, the server-FTP process might choose to route all
connections with unknown hostnames to a different virtual host so
that no connection attempts will result in failed connections.
This design would be implementation specific and outside the scope
of this specification.
3.2.1. REIN Command Semantics
As specified in [RFC 959], 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 afterwards in order
to connect to a virtual host.
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
3.2.2. User-PI Usage of HOST
A user-PI 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
If a user-PI sends an additional HOST command before attempting to
authenticate the user, a server-FTP process MUST treat the additional
HOST command as though a previous HOST command was not sent and
return the appropriate reply for the new HOST command. For example,
if a user specifies the wrong virtual hostname by mistake, sending a
subsequent HOST command will rectify the error. The following
example illustrates what the login sequence might look like when the
HOST command is sent twice before a user has been authenticated:
C> HOST foo.example.com
S> 220 Host accepted
C> HOST bar.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
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
This is also true when the HOST command is used with the AUTH and
ADAT commands that are discussed in [RFC 2228] and [RFC 4217]. 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
relevant authentication token(s) 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 HOST and ACCT
commands when used in conjunction with the security commands that are
discussed in [RFC 2228] and [RFC 4217], 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
An exception to the above scenario would be when a user-PI is
providing the hostname in the "server_name" extension of a Transport
Layer Security (TLS) extended client hello as discussed in [RFC 6066].
When the user-PI specifies the hostname in the "server_name"
extension of a TLS extended client hello, the server-PI MUST verify
that the hostname in the HOST command matches the value of the
"server_name" extension. The following example illustrates a
sequential series of client commands that specify the HOST command
when used in conjunction with the TLS extensions that are discussed
in [RFC 6066], with the server responses omitted for brevity:
C> AUTH TLS
C> HOST ftp.example.com
C> USER foo
C> PASS bar
Additional security information about using the HOST command with the
security extensions that are discussed in [RFC 2228], [RFC 4217], and
[RFC 6066] is provided in Section 4 of this document.
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
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
[RFC 959].
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 [RFC 2389]) 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), or error (E). In the
state diagrams below, we use the symbol "B" for "begin" and the
symbol "W" for "wait for reply".
For each of these diagrams, without any state transitions being
shown, a REIN command will return the diagram from any wait state to
the (B) "begin" state.
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
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
V 1 | +---+
+---+ USER +---+-------------->| E |
| |---------->| W | 2 | +---+
+---+ +---+------- | ^
| | | | |
3 | | 4,5 | | |
-------------- ----- | | |
| | | | |
| -------------------
| 1| | | |
V | | ------>+---+
+---+ PASS +---+ 2 | | | S |
| |---------->| W |-------------->+---+
+---+ +---+ | |
| | |
|4,5 | |
| | --->+---+
| --------->| F |
---------------->+---+
Figure 1: Typical Login Sequence with HOST Command
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
After a user has logged in, an additional account may be required by
the server and specified by the client by using the ACCT command.
With this in mind, the state diagram in Figure 2 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 2: Login Sequence with HOST and ACCT Commands
The state diagram in Figure 3 shows a typical sequence of flow of
control when HOST is used with the AUTH and ADAT commands that are
discussed in [RFC 2228]. (NOTE: Section 4 provides additional
information about using the HOST command with TLS.)
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
+---+ HOST +---+ 1,3,5
| B |---------->| W |------------------
+---+ +---+ |
| | |
2,500,502 | | 4,501,503,504 |
-------------- ------------- |
| | |
V | |
+---+ AUTH +---+ 4,5 | |
| |---------->| W |----------->| |
+---+ +---+ | |
334 | | | |
-------------- | | |
| 234 | | |
| ------------ | |
V | 4,5 | |
+---+ | ADAT +---+----------->| |
| |---------->| W | 335 | |
+---+ | +---+----- | |
^ | | | | |
| | | | | |
----------------------- | |
| | | |
---- 235 | | |
| -------------- | |
| | | V
V V 1 | +---+
+---+ USER +---+--------------->| E |
| |---------->| W | 2 | +---+
+---+ +---+------- | ^
| | | | |
3 | | 4,5 | | |
-------------- ------ | | |
| | | | |
| --------------------
| 1| | | |
V | | ------->+---+
+---+ PASS +---+ 2 | | | S |
| |---------->| W |--------------->+---+
+---+ +---+ | |
| | |
|4,5 | |
| | -->+---+
| --------->| F |
----------------->+---+
Figure 3: Login Sequence with HOST and AUTH/ADAT Commands
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RFC 7151 FTP HOST Command for Virtual Hosts March 2014
After a user has logged in with the security commands that are
discussed in [RFC 2228], an additional account may be required by the
server and specified by the client by using the ACCT command. The
state diagram in Figure 4 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 |------------>| |
+---+ +---+ | |
334 | | | |
-------------- | | |
| 234 | | |
| ------------ | |
V | 4,5 | |
+---+ | ADAT +---+------------>| |
| |---------->| W | 335 | |
+---+ | +---+----- | |
^ | | | | |
| | | | | |
----------------------- | |
| | | |
---- 235| | |
| -------------- | |
| | | |
V V 1 | V
+---+ USER +---+--------------->+---+
| |---------->| W | 2 ----->| E |
+---+ +---+------- | --->+---+
| | | | | |
3 | | 4,5 | | | |
-------------- ------ | | | |
| | | | | |
| ----------- | |
| 1| | | | |
V | | | | |
+---+ PASS +---+ 2 | ------->+---+
| |---------->| W |--------------->| S |
+---+ +---+ ------------>+---+
| | | | | |
Hethmon & McMurray Standards Track PAGE 15
RFC 7151 FTP HOST Command for Virtual Hosts March 2014
3 | |4,5| | | |
-------------- --------- | ----
| | | | | |
| ------------- |
| 1,3| | | | |
V | 2| | | V
+---+ ACCT +---+-- | ------>+---+
| |---------->| W | 4,5 --------->| F |
+---+ +---+--------------->+---+
Figure 4: Login Sequence with HOST and AUTH/ADAT/ACCT Commands
3.3. HOST Command Errors
The server-PI SHOULD return a 500 or 502 reply if the HOST command is
unrecognized or unimplemented, as specified in [RFC 959]. For
example, a server-PI that predates or otherwise does not conform to
this specification would be expected to return a 500 or 502 reply.
As discussed in Section 3 of this document, if a HOST command is sent
after a user has been authenticated, the server MUST treat the
situation as an invalid sequence of commands and return a 503 reply.
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 hostname 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 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 log in 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 that conform 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 the server MAY
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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 that presents an NVFS that appears to contain
subdirectories that contain the NVFS for all supported virtual hosts.
3.4. FEAT Response for HOST Command
When replying to the FEAT command [RFC 2389], a server-FTP process
that supports the HOST command MUST include a line containing the
single word "HOST". This word is case insensitive, but it SHOULD be
sent in upper case so as to maximize interoperability with disparate
implementations. That is, the response SHOULD be:
C> FEAT
S> 211- <any descriptive text>
S> ...
S> HOST
S> ...
S> 211 End
The ellipses indicate placeholders where other features may be
included but are not required. The one-space indentation of the
feature lines is mandatory [RFC 2389].
4. Security Considerations
As discussed in Section 3 of this document, a server implementation
MUST treat an additional HOST command that was sent before a user has
been authenticated as though a previous HOST command was not sent.
In this situation, the server implementation MUST reset the
authentication environment, as that would allow for segregation
between the security environments for each virtual host on an FTP
server. The implementation details for security environments may
vary greatly based on the requirements of each server implementation
and operating system, and those details are outside the scope of the
protocol itself. 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 the virtual servers to
appear to behave independently from a client perspective, while the
actual server implementation details are irrelevant at the protocol
level.
Section 15.1.1 of [RFC 4217] discusses the use of X.509 certificates
for server authentication. Taking the information from that document
into account, when securing FTP sessions with the security mechanisms
that are defined in [RFC 4217], client implementations SHOULD verify
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that the hostname that they specify in the parameter for the HOST
command matches the identity that is specified in the server's X.509
certificate in order to prevent man-in-the-middle attacks.
When the HOST command is used in combination with the FTP security
extensions that were introduced in [RFC 2228] and [RFC 4217], the HOST
command SHOULD precede the security handshake when the user-PI is not
providing the "server_name" in the extended client hello as defined
in [RFC 6066]. This allows both user-FTP and server-FTP processes to
map an FTP HOST with the correct server name in the server's
certificate. If the HOST command is sent after the security
handshake, then mapping an FTP HOST to the correct security
certificate will not take place before the secure session is
established.
For example, if a server-FTP process has multiple virtual hosts
defined and no hostname has been sent from a user-FTP process, the
server-FTP process will be unable to route the connection to the
correct virtual host when the connection is established. In this
situation, the server-FTP process will be forced to choose a virtual
host that will respond. When the user-PI attempts to negotiate a
secure connection, the virtual host to which the connection was
routed will respond with its server certificate during the security
handshake. If the virtual host that was chosen by the server-FTP
process does not match the virtual host to which the user-FTP process
had intended to connect, the user-PI will be unable to verify the
server's identity as presented in the server certificate message.
However, if the user-PI is providing the "server_name" in the
extended client hello as defined in Section 3 of [RFC 6066], the
user-PI MAY provide the HOST command after the security handshake
because the server will be able to route the connection to the
correct virtual host based on the contents of the "server_name"
extension and the client will be able to verify the server's identity
as presented in the corresponding server certificate message.
However, the server-PI MUST verify that the name in the HOST command
matches the "server_name" that is provided in the extended client
hello.
In general, client implementations SHOULD protect user credentials by
using the FTP security extensions that were introduced in [RFC 2228]
and [RFC 4217]; a detailed discussion for securing FTP sessions can be
found in those documents, and a general discussion of security issues
related to FTP can be found in [RFC 2577].
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5. IANA Considerations
IANA has registered the following FTP extension according to the
procedure established by [RFC 5797]:
+------+---------+-------------+------+------+----------------------+
| cmd | FEAT | description | type | conf | RFC#s/References and |
| | Code | | | | Notes |
+------+---------+-------------+------+------+----------------------+
| HOST | HOST | Hostname | a | o | RFC 7151 |
+------+---------+-------------+------+------+----------------------+
6. References
6.1. Normative References
[RFC 959] Postel, J. and J. Reynolds, "File Transfer Protocol
(FTP)", STD 9, RFC 959, October 1985.
[RFC 1034] Mockapetris, P., "Domain Names - Concepts and Facilities",
STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
[RFC 1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2228] Horowitz, M. and S. Lunt, "FTP Security Extensions", RFC
2228, October 1997.
[RFC 2389] Hethmon, P. and R. Elz, "Feature negotiation mechanism for
the File Transfer Protocol", RFC 2389, August 1998.
[RFC 2640] Curtin, B., "Internationalization of the File Transfer
Protocol", RFC 2640, July 1999.
[RFC 3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC 4217] Ford-Hutchinson, P., "Securing FTP with TLS", RFC 4217,
October 2005.
Hethmon & McMurray Standards Track PAGE 19
RFC 7151 FTP HOST Command for Virtual Hosts March 2014
[RFC 5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC 5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC 6066] Eastlake, D., "Transport Layer Security (TLS) Extensions:
Extension Definitions", RFC 6066, January 2011.
6.2. Informative References
[RFC 1945] Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.
[RFC 2577] Allman, M. and S. Ostermann, "FTP Security
Considerations", RFC 2577, May 1999.
[RFC 2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC 3875] Robinson, D. and K. Coar, "The Common Gateway Interface
(CGI) Version 1.1", RFC 3875, October 2004.
[RFC 5797] Klensin, J. and A. Hoenes, "FTP Command and Extension
Registry", RFC 5797, March 2010.
Hethmon & McMurray Standards Track PAGE 20
RFC 7151 FTP HOST Command for Virtual Hosts March 2014
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 listed
within each section of this 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 hostname as the argument to the CWD command. This would
allow the server-FTP process to implement the file stores of the
virtual hosts as subdirectories 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 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
Another suggested method would be to simply overload the ACCT command
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 [RFC 959] 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
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having multiple accounts on an FTP server, but it 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 [RFC 2228] and [RFC 4217]. In order to
safeguard user credentials, negotiation of the security mechanism
and certificate 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 it 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 virtual host that is named "example.com" on an FTP server
versus the user "foo@example.com" logging into an FTP server with
no specified virtual host.
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b. When using the FTP security extensions that are documented in
[RFC 2228] and [RFC 4217], negotiation of the security mechanism
and certificate 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
After examining the above alternatives, and in order to obtain an
adequate emulation of "real" FTP servers, it was concluded that
supporting virtual hosts will require both client and server
modifications. Therefore, a new FTP command seems the most likely
solution to provide the required level of support.
Appendix B. Acknowledgements
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 of 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.
Several people have provided a wealth of constructive feedback about
earlier versions of this document that has helped to shape its
development; many of their suggestions have been incorporated, and
their contributions are gratefully acknowledged. There are far too
many to mention here, but the authors of this document would like to
specifically thank Alexey Melnikov, Alfred Hoenes, John Klensin, Joe
Touch, Paul Ford-Hutchinson, Daniel Stenberg, Mykyta Yevstifeyev,
Alec Rowell, Jaroslav Dunajsky, Wade Hilmo, Anthony Bryan, and Barry
Leiba for their assistance.
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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
USA
EMail: robmcm@microsoft.com
Hethmon & McMurray Standards Track PAGE 24
RFC TOTAL SIZE: 54330 bytes
PUBLICATION DATE: Wednesday, March 12th, 2014
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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