Re: TSV-DIR review of draft-hethmon-mcmurray-ftp-hosts-11.txt

[Joe Touch <[e-mail]>]

Hi, Robert,

Some clarification below, which I hope will be useful. Overall, most of
my points were minor and I'd be glad to discuss them either offline or
on the list.

Joe

Robert McMurray wrote:
> Hi, Joe.
> 
> Thanks very much for the feedback. You make some good points, and I 
> am working on a new draft where I have incorporated a lot of the
> semantic changes that you recommended as they will make some areas of
> the draft a little clearer. As far as changing the title of the draft
> is concern, I am not opposed to changing the title, but I tried to
> follow the examples from other RFCs. Looking at a couple of recent
> RFCs like 5789 and 5689 perhaps you might prefer "HOST Command for
> File Transfer Protocol."

My concern is whether it would be useful to be more clear about the
reason for the command, rather than just naming the command. I
appreciate that this may not have been addressed in other RFCs for FTP
commands, but would prefer if this were decided on its own merit rather
than merely past examples. Regardless, this is a very minor concern, though.

> I have a few key points that I would like to discuss with regard to 
> your analysis of the draft, although I would like to mention in
> advance that I believe that some areas of the draft and your
> subsequent analysis might be saying the same thing and we only differ
> semantically. If so, I might need to clarify something in order to
> remove any ambiguity between what I wrote and what you understood.
> <grin>
> 
> --------------------
> 1. Identifying the use of the HOST command with regard to which type
> of host names to which the draft is referring.
> --------------------
> 
> Just to make sure that we're on the same page, the use of the HOST 
> command will be the most advantageous to servers that want to support
>  multiple FQDN-based FTP sites on a single server. For example, the
> FTP sites ftp.foo.com and ftp.bar.com are both physically located on
> a server at 192.168.1.1 and those two names are mapped via DNS to
> that IP address, or the FTP sites foo.example.com and bar.example.com
> are both physically located on a server at 192.168.2.1 and once again
> those two names are mapped via DNS to that IP address. As you alluded
> to in your email, HTTP accommodates this through the use of the Host
> header.

Yes, we're on the same page here.

> It is the intention for the HOST command to work in much the same 
> way, in that a typical scenario will involve a user-FTP process
> making a connection to an FTP server using an IP address, and then
> send a HOST command with the name of the host to connect to that the
> server-FTP process will process accordingly. In actual practice this
> will most often resemble a person entering an FQDN into an FTP client
> software application and that application will look up the IP address
> to physically connect to the FTP server. Once the connection is
> established using the IP address, the FTP client application will
> then send the HOST command with the FQDN for the desired FTP site,
> which the FTP server will use to route the connection according to
> its design.
> 
> So with that in mind, the parameter for the HOST command is supposed 
> to be indicating a hostname; this will typically be an FQDN, but
> that's not a requirement. For example, Windows networks can use short
> host names for network names, but everything in the above scenario
> would still hold true for a Windows network where the virtual hosts
> "foobar1" and "foobar2" are both physically located on the same FTP
> server.
>
> But in any case, my expectation was that the parameter for the HOST 
> command should be mapped to the IP address. As your email indicated, 
> these are multiple virtual hosts on a single physical host. If I have
> not made that clear then please let me know.

Nope - we're on the same page there. I now see why you want to ensure it
is a FQDN, though I think there's utility to considering this more
generally. More below...

> -------------------- 
> 2. Overloading the USER command to differentiate hosts.
> --------------------
> 
> This was an early consideration of ours, and I have seen that there 
> are some limited FTP server implementations that resemble something 
> similar to that already. As just one example, some versions of the 
> Microsoft FTP server allow for USER syntax like "USER
> hostname|username" or "USER hostname\username" where the hostname is
> used to internally route the FTP request to the correct FTP site on
> the server. There are several reasons why this suggestion is
> impractical outside of a limited-scope server implementation, but one
> of my arguments against overloading the USER command stems from a
> series of discussions that were held several years ago where it was
> agreed that syntax like "username@hostname" is unfortunately already
> overloaded in a variety of scenarios and therefore simply unworkable
> in a practical sense.
> 
> For example, in network environments where "username@hostname is 
> already in use as a network login where the hostname portion refers
> to the location of the user's credentials within the network
> hierarchy, that would necessitate having to determine how to
> implement differentiation between FTP hosts and network credential
> stores, and you could easily wind up with syntax like "USER 
> username@credentialstore@ftphostname" or "USER 
> username@ftphostname@credentialstore", or you could wind up with a 
> scenario where an FTP service won't know the difference between "USER
> username@credentialstore" and "USER username@ftphostname".

I believe your version of the command already overloads these two as
equivalent, i.e., "HOST vhostname" already overloads the command with
both indicating the credentialstore of the subsequent USER command and
the ftphostname. As a result, I don't see why putting this at the end of
the username needs to be confusing - it means the same thing (i.e., both
credentialstore and ftphostname).

My point here is that if you're discussing alternatives, you need to
address why this alternative was not used. There may be useful reasons
(in specific, using a separate command allows you to reuse some error
codes more usefully), but you're also incurring an extra round trip,
which people tend to count these days.

> -------------------- 
> 3. Passing the parameter for the HOST command as an opaque string.

> --------------------
> 
> I disagree with the suggestion that the parameter for the HOST 
> command should be an opaque string rather than constraining the
> parameter through the protocol. The intention of this draft is to
> allow an FTP client to specify the correct hostname (typically an
> FQDN) for an FTP server to use after an FTP client has connected, so
> that limits the parameter for the HOST command to hostnames that are
> already constrained by protocol, although I tried to make allowances
> for an FTP client to send the IP address via the HOST command rather
> than force the client to know the difference between an FQDN and an
> IP address. (An actual human using an FTP client should be able to
> tell the difference, of course, but their FTP software application
> might not. <grin>)

My primary concern with assuming this is a FQDN is giving the
misimpression that FQDNs name Internet hosts, rather than the idea that
this command really uses an arbitrary string to identify a vhost --
which has no requirement of how it must be named (AFAICT).

I don't disagree that the app might check and only allow FQDNs, but I
don't see why the protocol needs to check this. Glad to discuss this
further if useful, but again this is a minor point.

Joe

> In any event, I am still working on an updated draft. Thanks again 
> for the feedback.
> 
> Robert
> 
> 
> -----Original Message-----
> From: Joe Touch [mailto:[e-mail]]
> Sent: Thursday, May 06, 2010 10:48 AM
> To: IETF discussion list
> Cc: TSV Dir; [e-mail]; [e-mail]
> Subject: TSV-DIR review of draft-hethmon-mcmurray-ftp-hosts-11.txt
> 
> Hi, all,
> 
> I've reviewed this document as part of the transport area 
> directorate's ongoing effort to review key IETF documents. These 
> comments were written primarily for the transport area directors, but
>  are copied to the document's authors for their information and to
> allow them to address any issues raised. The authors should consider
> this review together with any other last-call comments they receive.
> Please always CC [e-mail] if you reply to or forward this
> review.
> 
> The document defines a new FTP command called "HOST" that allows an 
> FTP server to support virtual hosts, i.e., multiple hosts on the same
> IP address.
> 
> The document does raise an important concern from a transport 
> viewpoint. The document extends FTP to support the same kind of 
> virtual hosting common in the web. Although the Internet defines
> hosts by IP address, virtual hosts are specified in various ways,
> including DNS names, that may or may not map to the IP address of the
> connection to the server.
> 
> The extension is discussed in ways that imply that the user is 
> connecting to a HOST as indicated in the command, rather than the IP 
> address of the FTP connection. IMO, the HOST command ought to be 
> presented more as a way to provide context within a host, not to 
> indicate a host. That might be acheived by a name change (VHOST?)
> and/or by revising some of the discussion accordingly.
> 
> Thus, I would strongly suggest the document be revised as follows. 
> Notes are included below that indicate places where thes critical
> issues should be addressed, flagged by "####". -- This document
> should emphasize that the argument to the host command is, at the
> protocol level, an opaque string passed to the application that
> indicates only a session context (including authentication, file 
> system, etc.). There should be no attempt for the protocol to
> validate, translate, or interpret that string - e.g., as matching the
> IP address of the connection, or providing a valid IP address or
> fqdn. That validation can be performed by the server implementation,
> but should not be a constraint of the protocol.
> 
> There are places where SHOULDs are used where I expected MUSTs. Any 
> place a SHOULD is used, the document ought to include a description
> of what happens if not, i.e., if the SHOULD is not followed, and it'd
> be useful to also include reasons why and conditions where this might
> be useful or valid.
> 
> Also, in terminology, the document would benefit from a bit of 
> clarification and revision, notably its title and description of the 
> purpose of this command. The HOST command performes much the same 
> function as HTTP including the full URL in the GET command, which 
> similarly allows virtual hosting. The document should avoid confusing
>  this with multi-homing, which is not necessarily related. --
> 
> I have provided some other feedback interspersed in the text below 
> (flagged with "++++"). The most notable is the issue of how to handle
>  multiple HOST commands, the potential for HOST commands after the
> USER command (without a REIN command), and updating the state machine
> to include the effects of REIN interactions with the HOST command.
> 
> I also found numerous places where "should" or "must" are used but 
> not capitalized; the document should be checked for these uses to
> confirm the intent. If case is important, then Section 2 should
> include text to indicate such (as noted there).
> 
> I'd like to also suggest that the alternatives discussed might 
> consider an opportunity to integrate this command with the USER
> command to accept "name[@hoststring]" arguments, e.g.,
> [e-mail] This syntax would suffice, and might need only
> an additional error code (to indicate that the hoststring is not
> valid, rather than the entire command argument), rather than defining
> a new name and introducing an additional round trip into the login
> process. It seems equally compatible with the wrapper approach
> discussed (which, IMO, should also be noted as a goal of the design,
> FWIW).
> 
> I'd be glad to discuss this further on whatever list would be
> useful.
> 
> Joe
> 
> ---------------------
> 
> Network Working Group                                         P. Hethmon
> Internet-Draft                                          Hethmon Brothers
> Updates: 959                                                 R. McMurray
> Intended status: Standards Track                               Microsoft
> Expires: October 6, 2010                                      April 2010
> 
> 
>                   File Transfer Protocol HOST Command
>                   draft-hethmon-mcmurray-ftp-hosts-11
> 
> ####
> The title should more accurately indicate the nature of the command,
> e.g., "The File Transfer Protocol HOST Command for Virtual Hosts"
> ####
> 
> 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.
> 
> ####
> Multihoming is when a single host has more than one IP address This 
> extension supports 'virtual hosts', where more than one
> fully-qualified DNS name (fqdn) resolves to the same IP address.
> 
> The terminology needs to be updated throughout to address this issue.
> ####
> 
> 1. Introduction
> 
>    It is common on the Internet for many domain names to be allocated
> 
> ++++
> It is common in the Internet to allow multiple DNS host names to
> resolve to the same IP address.
> 
> (they're not allocated, and they're not domain names)
> ++++
> 
>    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
> 
> ++++
> physical resources
> ++++
> 
>    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
> 
> ####
> an FTP server distinguishes incoming FTP connections by their IP 
> address, not their DNS names, because hosts are uniquely identified
> by address rather than name. That is, all DNS names that share an
> IP...
> ####
> 
>    address also share the same FTP server, and more importantly, its
> 
> ++++
> are handled by the same FTP server
> ++++
> 
> 
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> 
> 
> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
>    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.
> 
> ++++
> [it's worth pointing out here that a similar situation existed for 
> the web many years ago, and how it was resolved, and that you want to
> do the same thing here basically]
> ++++
> 
>    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
> 
> ####
> I think you mean to say that "Any scheme to overcome this needs to 
> indicate not only the destination IP address, but also a name
> associated with the desired virtual FTP server. This information is
> currently discarded in the FTP application at the user interface of
> the client."
> 
> (note: I used the term "name" - not DNS name here; see preface text 
> as to why)
> ####
> 
>    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.
> 
> ++++
> I don't know why the following section is in the intro. You do need
> to discuss alternatives not pursued, but maybe later.
> ++++
> 
>    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.
> 
> ++++
> I think the above is too glib. The first point is worth noting - that
> it is available only after the connection is established, and so
> cannot be used to determine per-host parameters (including, but not
> limited to, authentication). The second point can easily be overcome
> with an optional standard prefix to the CWD command, e.g.:
> 
>         CWD fqdn:{path}
> 
> The ability of the user to move between virtual FTP servers can be 
> seen as a benefit, not a liability. The question is whether the
> benefit is needed, and whether it presents new problems (e.g.,
> re-authenticating on different servers may not be feasible).
> ++++
> 
>    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:
> 
> ++++
> I think this is easier to state. The problem is that you need to do 
> ACCT before you do USER or PASS, since you don't know on what virtual
> server to validate the first two.
> 
> I.e., same issue as before. Don't beat a dead horse, just note it and
> move on.
> ++++
> 
>       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
> 
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> 
> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
>       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.
> 
>    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.
> 
> ++++
> I think you may need to say:
> 
>   a new command that *precedes* USER is required.
> 
> However, I think it isn't. You can easily do the colon trick:
> 
>          vhostname:username
> 
> or even the at-sign trick:
> 
>         username@vhostname
> 
> That seems like it's also potentially more easily understood to users
> (it's already supported by many FTP command lines using that
> syntax), and reduces the login by one round trip exchange as well.
> ++++
> 
> 
> 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].
> 
> ++++
> The document uses these terms in lower case, presumably not as key
> words. If that is desired, it would be useful to be explicit about the
> difference, e.g., by including the following text:
> 
>    When used in lower case, these words have their conventional meaning
>    and do not convey the interpretations 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
> 
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> 
> 
> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 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)
>    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,
> 
> ####
> When can this command ever come AFTER a user is authenticated? Is 
> that ever intended to be valid? If not, then change the SHOULD to
> MUST. However, as noted above, embedding this info in the USER
> command would have achieved the same effect without requiring
> protocol machinery to determine the order of commands.
> ####
> 
>    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 a 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
> 
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> 
> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
>       appropriate reply for the HOST command.
> 
> ++++
> This is further evidence that this info might be easier to embed in 
> the USER command. The REIN effectively does a logout, and then the
> next USER command would be expected to have the needed vhostname
> information anyway.
> ++++
> 
>    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
> 
> ####
> IMO,
>      hostname = bytetring
> 
> I see no reason to specify it further. To the protocol, it can/should
> be opaque. The server can decide whether it likes it or not. If I
> want to call a vhost "mickey_mouse!!@@##" I should be able to, IMO.
> 
> The rest of the syntax can be omitted.
> ####
> 
>      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
> 
> 
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> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 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.  It should normally 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.
> 
> ####
> I disagree with the above paragraph. The contents of the hostname 
> argument MUST be opaque to the protocol.
> 
> The remainder of this section (and/or document) needs to be updated 
> accordingly, IMO.
> 
> That also means that the client can send what it wants - fully 
> qualified or not. That's up to the client implentation to decide. The
>  semantics of that name should be meaningless to the protocol.> ####
> 
>    Internationalization of domain names is only supported through the
>    use of Punycode as described in [RFC3492].
> 
> ####
> This might say that internationalization of the opaque string is 
> handled via Punycode. It should not refer to domain names per se;
> that is an interpretation that can be made by the server, but must
> not be assumed in the protocol.
> ####
> 
>    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 should 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 SHOULD respond with a 504 reply to indicate that
>    the IPv4 or IPv6 literal address is not valid.
> 
> ####
> First, these strings ought to be opaque to the protocol. The server 
> can decide to check them, but the protocol MUST NOT, IMO.
> 
> Note, though, that even if the server interprets them, let's discuss 
> whether the server should do the above or not (i.e., this would be an
> implementation/configuration choice, not a protocol requirement, if
> so).
> 
> If you are intending to support only virtual hosting, then there is 
> never a need to send the IP address in a HOST command. However, if
> you do want to allow it for completeness, then it MUST respond with a
> 504 if the address does not match (this can't be a SHOULD unless
> there is a valid reason for either another response, or a valid
> action, and this doc doesn't argue for either).
> 
> However, there could be a case where the FTP server listens on 
> multiple addresses on a multihomed host. In that case, this command 
> would appear to provide a valid way to connect to one address and
> access the data on the other. Since this would succeed (even if not 
> intended!) if the user had provided a fqdn, I see no reason why it 
> should fail when they provide an IP address. Consider:
>        a.example.net = 10.0.0.1
>        b.example.com = 192.0.0.2
> 
> A user might issue:
>   ftp a.example.net [e-mail]
>       connecting to:    10.0.0.1
>       issuing:          HOST b.example.net
> 
> if that works (and I can't see why it wouldn't, so far), then the
> following should also work:
> 
>   ftp 10.0.0.1 joe@192.0.0.2
>       connecting to:    10.0.0.1
>       issuing:          HOST 192.0.0.2
> 
> If you don't want either example to work, then you missed a step on
> the HOST command:
> 
>     HOST fqdn
>          resolve fqdn to a set of IP addresses "S"
>          the current connection IP destination address MUST
>          match an address in S, otherwise the server MUST
>          return an error
> 
> HOWEVER, I strongly discourage this sort of rule. Overall, IMO, the 
> HOST command takes an *arbitrary* argument that is interpreted *only*
> as the context of access control and the NVFS. As a result, *all*
> such checks that attempt to match any contents of this identifier to
> the current connection MUST NOT be prohibited by the protocol (the
> server configuration MAY check these or not, as supported and/or
> configured).
> ####
> 
>    When the hostname parameter contains a literal address, square
>    brackets are expected to disambiguate port numbers syntax from IPv6
>    address syntax.
> 
> ++++
> This doesn't make sense; below you prohibit the use of port numbers
> in the command, so why do you need to disambiguate them?
> ++++
> 
>    Therefore, if the literal address is an IPv6
>    address, the IPv6 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 respond 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".
> 
> ####
> If raw addresses are deemed useful, then ports should be as well.
> 
> Further ports MUST be allowed to be specified either by their number
> or their service name (e.g., "ftp"), IMO.
> 
> I would assume that service names MUST NOT be translated or validated
>  by the protocol, just as the fqdn MUST NOT be. The server
> implementation MAY check/translate these as needed to coordinate with
> their configuration, but this is outside the scope of the protocol,
> IMO.
> ####
> 
>    The hostname parameter is otherwise to be treated as a fully
> 
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> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
>    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.
> 
> ####
> None of this should be validated, translated, or checked in the
> protocol. The server implementation can decide whether it likes the
> name, or it matches its configuration tables.
> ####
> 
>    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 making much more elaborate state changes, as
>    required.
> 
> ++++
> It might be useful to mention other changes, such as using different
> username/password lists.
> ++++
> 
>    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.
> 
> ####
>Agreed - that is a server configuration issue. It's important to note
> here that this MUST NOT be checked in the protocol itself, though.
> ####
> 
>    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.
> 
> ++++
> If you say SHOULD, you ought to indicate conditions under which you
> might not, i.e., when would you not reply with 421? would you reply
> with a different code, or just wait? It's not useful to include
> SHOULDs without giving a hint as to why they are not MUSTs, IMO.
> ++++
> 
>    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.
> 
> ++++
> A wrapper could as easily wait for a custom response command. I am 
> not sure why the response command is the same as for the initial
> connection. Many other commands have their own response codes; IMO,
> this should as well:
> 
>             260 HOST command accepted
> ++++
> 
> 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
>    connection was opened.  This specification makes no changes to that
> 
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> 
>    behavior.  The effect of a HOST command will be lost if a REIN
>    command is performed, and a new HOST command must be issued.
> 
> ++++
> The effect of a HOST command MUST be reset if a REIN command is performed.
> (i.e., the FTP authentication and NVFS parameters are set to the
> initial connection default after a REIN command).
> ++++
> 
>    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.
> 
> ++++
> This seems like a problem; it seems like you need a few things here:
> 
>      1) a way to find out what the current HOST environment is
> 
>      CHOSTIS
>      SHOST a.example.com
> 
>      2) a way to set the host to the default of a server, e.g.,
>      using the HOST command with no argument??
> 
>      CHOST
>      SHOST set to server default
> 
> (note: you now need to allow successive HOST commands to be permitted
> after either login on REIN)
> ++++
> 
> 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:
> 
>         CHOST foobar.com
>         S220 Host accepted
> 
> ++++
> I think you need to say:
>           S220 Service ready for new user
> 
> However, I prefer:
>           S260 HOST accepted
> ++++
> 
>         CUSER foo
>         S331 Password required
>         CPASS bar
>         S230 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:
> 
>         CHOST foobar.com
>         CUSER foo
>         CPASS bar
>         CACCT 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
> 
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> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
>    implementation. The following example illustrates a sequential
>    series of client commands that specify both a HOST and ACCT when
>    used in conjunction with the security commands that are discussed
>    in [RFC2228] and [RFC4217], with the server responses omitted for
>    brevity:
> 
>         CHOST foobar.com
>         CAUTH <mechanism-name>         CADAT <base64data>         CUSER foo
>         CPASS bar
>         CACCT 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".
> 
> ####
> I think this should allow for multiple HOST commands.
> 
> It should also show how the REIN command sets you back to a state where
> you can accept new HOST commands.
> ####
> 
>               +---+   HOST    +---+ 1,3,5
>               | B |---------->| W |-----------------
>               +---+           +---+                 |
>                                | |                  |
>                      2,500,502 | | 4,501,503,504    |
>                  --------------   -------------     |
>                 |                              |    |
>                 V                   1          |    V
>               +---+   USER    +---+-------------->+---+
>               |   |---------->| W | 2       ----->| E |
>               +---+           +---+------  |  --->+---+
>                                | |       | | | |
>                              3 | | 4,5   | | | |
>                  --------------   -----  | | | |
>                 |                      | | | | |
> 
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> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
>                 |                      | | | | |
>                 |                ----------  | |
>                 |              1|      | |   | |
>                 V               |      | |   | |
>               +---+   PASS    +---+ 2  |  ------->+---+
>               |   |---------->| W |-------------->| S |
>               +---+           +---+   ----------->+---+
>                                | |   | |     | |
>                              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        |    |
> 
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> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
>               +---+ |         +---+----------->|    |
>                     |           |              |    |
>                 ----         235|              |    |
>                |  --------------               |    |
>                | |                             |    |
>                V 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 |
>               +---+           +---+-------------->+---+
> 
> 
> 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
> 
> ++++
> Please explain why successive HOST commands should be invalid. I 
> can't see a reason, and there's potential utility (e.g., to allow
> REIN wrappers, as described earlier).
> ++++
> 
>    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.
> 
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> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
> 
>    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
>    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:
> 
>         CFEAT
>         S211- <any descriptive text>         S ...
>         S HOST
>         S ...
>         S211 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 the integrity
>    of user credentials is maintained. A general discussion of issues
>    related to the security of FTP can be found in [RFC2577].
> 
> ++++
> I think you need to have more here, esp. on how the server MUST reset
>  security parameters when a HOST command is issued, and new commands
> MUST be interpreted in the security context of the most recent HOST
> command.
> ++++
> 
> ####
> There's also the issue of whether a server should validate the HOST 
> command parameters as matching the incoming connection. IMO, that is 
> something that the server should be checking, not the protocol, but
> it might be important to note whether there should be a default (MUST
>  validate?) or whether there should be no such assumptions because
> the HOST command argument is treated by the protocol as an opaque
> string (IMO, that's the better answer here).
> ####
> 
> 5. IANA Considerations
> 
>    IANA is requested to register the following FTP extension according
>    to the procedure established by [RFC5797]:
> 
>    +-------+------+-------------------+------+------+------------------+
>    | cmd   | FEAT | description       | type | conf | RFC#s/References |
> 
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> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
>    |       | 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
> 
>    [RFC2577] Allman, M., Ostermann, S., "FTP Security Considerations",
>              RFC 2577, May 1999
> 
> 
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> INTERNET-DRAFT      File Transfer Protocol HOST Command       April 2010
> 
>    [RFC5797] Klensin, J., Hoenes, A., "FTP Command and Extension
>              Registry", March 2010
> 
> 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.
> 
>    Alexey Melnikov, Alfred Hoenes, and John Klensin have made several
>    suggestions about earlier versions of this document, most of which
>    have been incorporated and their contributions are gratefully
>    acknowledged.
> 
> 8. Authors' Addresses
> 
>    Paul Hethmon
>    Hethmon Brothers
>    2305 Chukar Road
>    Knoxville, TN 37923 USA
> 
>    Email: [e-mail]
> 
>    Robert McMurray
>    Microsoft Corporation
>    One Microsoft Way
>    Redmond, WA 98052-6399
> 
>    Email: [e-mail]
> 
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> Hethmon & McMurray           Expires October 6, 2010           [Page 15]
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