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FTPEXT Working Group R. Elz
Internet Draft University of Melbourne
Expiration Date: April 2000
P. Hethmon
Hethmon Brothers
October 1999
Extensions to FTP
draft-ietf-ftpext-mlst-08.txt
Status of this Memo
This document is an Internet-Draft and is NOT offered in accordance
with Section 10 of RFC2026, and the author does not provide the IETF
with any rights other than to publish as an Internet-Draft.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
This entire section has been prepended to this document automatically
during formatting without any direct involvement by the author(s) of
this draft.
Elz & Hethmon [Expires April 2000] [Page 1]
Internet Draft draft-ietf-ftpext-mlst-08.txt October 1999
Abstract
In order to overcome the problems caused by the undefined format of
the current FTP LIST command output, a new command is needed to
transfer standardized listing information from Server-FTP to User-
FTP. Commands to enable this are defined in this document.
In order to allow consenting clients and servers to interact more
freely, a quite basic, and optional, virtual file store structure is
defined.
This proposal also extends the FTP protocol to allow character sets
other than US-ASCII[1] by allowing the transmission of 8-bit
characters and the recommended use of UTF-8[2] encoding.
Much implemented, but long undocumented, mechanisms to permit
restarts of interrupted data transfers in STREAM mode, are also
included here.
Lastly, the HOST command has been added to allow a style of "virtual
site" to be constructed.
Changed in this version of this document: Minor corrections as
discussed on the mailing list, including fixing many typographical
errors; Additional examples. This paragraph will be deleted from the
final version of this document.
Elz & Hethmon [Expires April 2000] [Page 2]
Internet Draft draft-ietf-ftpext-mlst-08.txt October 1999
Table of Contents
Abstract ................................................ 2
1 Introduction ............................................ 4
2 Document Conventions .................................... 4
2.1 Basic Tokens ............................................ 5
2.2 Pathnames ............................................... 5
2.3 Times ................................................... 7
2.4 Server Replies .......................................... 8
3 File Modification Time (MDTM) ........................... 8
3.1 Syntax .................................................. 9
3.2 Error responses ......................................... 9
3.3 FEAT response for MDTM .................................. 9
3.4 MDTM Examples ........................................... 10
4 File SIZE ............................................... 11
4.1 Syntax .................................................. 11
4.2 Error responses ......................................... 11
4.3 FEAT response for SIZE .................................. 12
4.4 Size Examples ........................................... 12
5 Restart of Interrupted Transfer (REST) .................. 13
5.1 Restarting in STREAM Mode ............................... 13
5.2 Error Recovery and Restart .............................. 14
5.3 Syntax .................................................. 14
5.4 FEAT response for REST .................................. 16
5.5 REST Example ............................................ 16
6 Virtual FTP servers ..................................... 16
6.1 The HOST command ........................................ 18
6.2 Syntax of the HOST command .............................. 18
6.3 HOST command semantics .................................. 19
6.4 HOST command errors ..................................... 21
6.5 FEAT response for HOST command .......................... 22
7 A Trivial Virtual File Store (TVFS) ..................... 23
7.1 TVFS File Names ......................................... 23
7.2 TVFS Path Names ......................................... 24
7.3 FEAT Response for TVFS .................................. 25
7.4 OPTS for TVFS ........................................... 26
7.5 TVFS Examples ........................................... 26
8 Listings for Machine Processing (MLST and MLSD) ......... 28
8.1 Format of MLSx Requests ................................. 29
8.2 Format of MLSx Response ................................. 29
8.3 Filename encoding ....................................... 32
8.4 Format of Facts ......................................... 33
8.5 Standard Facts .......................................... 33
8.6 System Dependent and Local Facts ........................ 41
8.7 MLSx Examples ........................................... 42
8.8 FEAT response for MLSx .................................. 50
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8.9 OPTS parameters for MLST ................................ 51
9 Impact On Other FTP Commands ............................ 55
10 Character sets and Internationalization ................. 56
11 IANA Considerations ..................................... 56
11.1 The OS specific fact registry ........................... 56
11.2 The OS specific filetype registry ....................... 57
12 Security Considerations ................................. 57
13 References .............................................. 58
Acknowledgments ......................................... 59
Copyright ............................................... 60
Editors' Addresses ...................................... 60
1. Introduction
This document amends the File Transfer Protocol (FTP) [3]. Five new
commands are added: "SIZE", "HOST", "MDTM", "MLST", and "MLSD". The
existing command "REST" is modified. Of those, the "SIZE" and "MDTM"
commands, and the modifications to "REST" have been in wide use for
many years. The others are new.
These commands allow a client to restart an interrupted transfer in
transfer modes not previously supported in any documented way, to
support the notion of virtual hosts, and to obtain a directory
listing in a machine friendly, predictable, format.
An optional structure for the server's file store (NVFS) is also
defined, allowing servers that support such a structure to convey
that information to clients in a standard way, thus allowing clients
more certainty in constructing and interpreting path names.
2. Document Conventions
This document makes use of the document conventions defined in BCP14
[4]. That provides the interpretation of capitalized imperative
words like MUST, SHOULD, etc.
This document also uses notation defined in STD 9 [3]. In
particular, the terms "reply", "user", "NVFS", "file", "pathname",
"FTP commands", "DTP", "user-FTP process", "user-PI", "user-DTP",
"server-FTP process", "server-PI", "server-DTP", "mode", "type",
"NVT", "control connection", "data connection", and "ASCII", are all
used here as defined there.
Syntax required is defined using the Augmented BNF defined in [5].
Some general ABNF definitions are required throughout the document,
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those will be defined later in this section. At first reading, it
may be wise to simply recall that these definitions exist here, and
skip to the next section.
2.1. Basic Tokens
This document imports the core definitions given in Appendix A of
[5]. There definitions will be found for basic ABNF elements like
ALPHA, DIGIT, SP, etc. To that, the following terms are added for
use in this document.
TCHAR = VCHAR / SP / HTAB ; visible plus white space
RCHAR = ALPHA / DIGIT / "," / "." / ":" / "!" /
"@" / "#" / "$" / "%" / "^" /
"&" / "(" / ")" / "-" / "_" /
"+" / "?" / "/" / "\" / "'" /
DQUOTE ; <"> -- double quote character (%x22)
The VCHAR (from [5]), TCHAR, and RCHAR types give basic character
types from varying sub-sets of the ASCII character set for use in
various commands and responses.
token = 1*RCHAR
A "token" is a string whose precise meaning depends upon the context
in which it is used. In some cases it will be a value from a set of
possible values maintained elsewhere. In others it might be a string
invented by one party to an FTP conversation from whatever sources it
finds relevant.
Note that in ABNF, string literals are case insensitive. That
convention is preserved in this document, and implies that FTP
commands added by this specification have names that can be
represented in any case. That is, "MDTM" is the same as "mdtm",
"Mdtm" and "MdTm" etc. However note that ALPHA, in particular, is
case sensitive. That implies that a "token" is a case sensitive
value. That implication is correct.
2.2. Pathnames
Various FTP commands take pathnames as arguments, or return pathnames
in responses. When the MLST command is supported, as indicated in
the response to the FEAT command [6], pathnames are to be transferred
in one of the following two formats.
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pathname = utf-8-name / raw
utf-8-name = <a UTF-8 encoded Unicode string>
raw = <any string not being a valid UTF-8 encoding>
Which format is used is at the option of the user-PI or server-PI
sending the pathname. UTF-8 encodings [2] contain enough internal
structure that it is always, in practice, possible to determine
whether a UTF-8 or raw encoding has been used, in those cases where
it matters. While it is useful for the user-PI to be able to
correctly display a pathname received from the server-PI to the user,
it is far more important for the user-PI to be able to retain and
retransmit the identical pathname when required. Implementations are
advised against converting a UTF-8 pathname to a local encoding, and
then attempting to invert the encoding later. Note that ASCII is a
subset of UTF-8.
Unless otherwise specified, the pathname is terminated by the CRLF
that terminates the FTP command, or by the CRLF that ends a reply.
Any trailing spaces preceding that CRLF form part of the name.
Exactly one space will precede the pathname and serve as a separator
from the preceding syntax element. Any additional spaces form part
of the pathname. See [7] for a fuller explanation of the character
encoding issues. All implementations supporting MLST MUST support
[7].
Implementations should also beware that the control connection uses
Telnet NVT conventions [8], and that the Telnet IAC character, if
part of a pathname sent over the control connection, MUST be
correctly escaped as defined by the Telnet protocol.
Implementors should also be aware that although Telnet NVT
conventions are used over the control connections, Telnet option
negotiation MUST NOT be attempted. See section 4.1.2.12 of [9].
2.2.1. Pathname Syntax
Except where TVFS is supported (see section 7) this specification
imposes no syntax upon pathnames. Nor does it restrict the character
set from which pathnames are created. This does not imply that the
NVFS is required to make sense of all possible pathnames. Server-PIs
may restrict the syntax of valid pathnames in their NVFS in any
manner appropriate to their implementation or underlying file system.
Similarly, a server-PI may parse the pathname, and assign meaning to
the components detected.
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2.2.2. Wildcarding
For the commands defined in this specification, all pathnames are to
be treated literally. That is, for a pathname given as a parameter
to a command, the file whose name is identical to the pathname given
is implied. No characters from the pathname may be treated as
special or "magic", thus no pattern matching (other than for exact
equality) between the pathname given and the files present in the
NVFS of the Server-FTP is permitted.
Clients that desire some form of pattern matching functionality must
obtain a listing of the relevant directory, or directories, and
implement their own filename selection procedures.
2.3. Times
The syntax of a time value is:
time-val = 14DIGIT [ "." 1*DIGIT ]
The leading, mandatory, fourteen digits are to be interpreted as, in
order from the leftmost, four digits giving the year, with a range of
1000-9999, two digits giving the month of the year, with a range of
01-12, two digits giving the day of the month, with a range of 01-31,
two digits giving the hour of the day, with a range of 00-23, two
digits giving minutes past the hour, with a range of 00-59, and
finally, two digits giving seconds past the minute, with a range of
00-60 (with 60 being used only at a leap second). Years in the tenth
century, and earlier, cannot be expressed. This is not considered a
serious defect of the protocol.
The optional digits, which are preceded by a period, give decimal
fractions of a second. These may be given to whatever precision is
appropriate to the circumstance, however implementations MUST NOT add
precision to time-vals where that precision does not exist in the
underlying value being transmitted.
Symbolically, a time-val may be viewed as
YYYYMMDDHHMMSS.sss
The "." and subsequent digits ("sss") are optional. However the "."
MUST NOT appear unless at least one following digit also appears.
Time values are always represented in UTC (GMT), and in the Gregorian
calendar regardless of what calendar may have been in use at the date
and time indicated at the location of the server-PI.
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The technical differences between GMT, TAI, UTC, UT1, UT2, etc, are
not considered here. A server-FTP process should always use the same
time reference, so the times it returns will be consistent. Clients
are not expected to be time synchronized with the server, so the
possible difference in times that might be reported by the different
time standards is not considered important.
2.4. Server Replies
Section 4.2 of [3] 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
Implementors should note that the ABNF syntax (which was not used in
[3]) 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. Implementors 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 [3].
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. File Modification Time (MDTM)
The FTP command, MODIFICATION TIME (MDTM), can be used to determine
when a file in the server NVFS was last modified. This command has
existed in many FTP servers for many years, as an adjunct to the REST
command for STREAM mode, thus is widely available. However, where
supported, the "modify" fact which can be provided in the result from
the new MLST command is recommended as a superior alternative.
When attempting to restart a RETRieve, if the User-FTP makes use of
the MDTM command, or "modify" fact, it can check and see if the
modification time of the source file is more recent than the
modification time of the partially transferred file. If it is, then
most likely the source file has changed and it would be unsafe to
restart the previously incomplete file transfer.
When attempting to restart a STORe, the User FTP can use the MDTM
command to discover the modification time of the partially
transferred file. If it is older than the modification time of the
file that is about to be STORed, then most likely the source file has
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changed and it would be unsafe to restart the file transfer.
Note that using MLST (described below) where available, can provide
this information, and much more, thus giving an even better
indication that a file has changed, and that restarting a transfer
would not give valid results.
Note that this is applicable to any RESTart attempt, regardless of
the mode of the file transfer.
3.1. Syntax
The syntax for the MDTM command is:
mdtm = "MdTm" SP pathname CRLF
As with all FTP commands, the "MDTM" command label is interpreted in
a case insensitive manner.
The "pathname" specifies an object in the NVFS which may be the
object of a RETR command. Attempts to query the modification time of
files that are unable to be retrieved generate undefined responses.
The server-PI will respond to the MDTM command with a 213 reply
giving the last modification time of the file whose pathname was
supplied, or a 550 reply if the file does not exist, the modification
time is unavailable, or some other error has occurred.
mdtm-response = "213" SP time-val CRLF /
error-response
3.2. Error responses
Where the command is correctly parsed, but the modification time is
not available, either because the pathname identifies no existing
entity, or because the information is not available for the entity
named, then a 550 reply should be sent. Where the command cannot be
correctly parsed, a 500 or 501 reply should be sent, as specified in
[3].
3.3. FEAT response for MDTM
When replying to the FEAT command [6], an FTP server process that
supports the MDTM command MUST include a line containing the single
word "MDTM". This MAY be sent in upper or lower case, or a mixture
of both (it is case insensitive) but SHOULD be transmitted in upper
case only. That is, the response SHOULD be
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C> Feat
S> 211- <any descriptive text>
S> ...
S> MDTM
S> ...
S> 211 End
The ellipses indicate place holders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory [6].
3.4. MDTM Examples
If we assume the existence of three files, A B and C, and a directory
D, and no other files at all, then the MTDM command may behave as
indicated. The "C>" lines are commands from user-PI to server-PI,
the "S>" lines are server-PI replies.
C> MDTM A
S> 213 19980615100045.014
C> MDTM B
S> 213 19980615100045.014
C> MDTM C
S> 213 19980705132316
C> MDTM D
S> 550 D is not retrievable
C> MDTM E
S> 550 No file named "E"
C> mdtm file6
S> 213 19990929003355
C> MdTm 19990929043300 File6
S> 213 19991005213102
C> MdTm 19990929043300 file6
S> 550 19990929043300 file6: No such file or directory.
From that we can conclude that both A and B were last modified at the
same time (to the nearest millisecond), and that C was modified 21
days and several hours later.
The times are in GMT, so file A was modified on the 15th of June,
1998, at approximately 11am in London (summer time was then in
effect), or perhaps at 8pm in Melbourne, Australia, or at 6am in New
York. All of those represent the same absolute time of course. The
location where the file was modified, and consequently the local wall
clock time at that location, is not available.
There is no file named "E" in the current directory, but there are
files named both "file6" and "19990929043300 File6". The
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modification times of those files were obtained. There is no file
named "19990929043300 file6".
4. File SIZE
The FTP command, SIZE OF FILE (SIZE), is used to obtain the transfer
size of a file from the server-FTP process. That is, the exact
number of octets (8 bit bytes) which would be transmitted over the
data connection should that file be transmitted. This value will
change depending on the current STRUcture, MODE and TYPE of the data
connection, or a data connection which would be created were one
created now. Thus, the result of the SIZE command is dependent on
the currently established STRU, MODE and TYPE parameters.
The SIZE command returns how many octets would be transferred if the
file were to be transferred using the current transfer structure,
mode and type. This command is normally used in conjunction with the
RESTART (REST) command. The server-PI might need to read the
partially transferred file, do any appropriate conversion, and count
the number of octets that would be generated when sending the file in
order to correctly respond to this command. Estimates of the file
transfer size MUST NOT be returned, only precise information is
acceptable.
4.1. Syntax
The syntax of the SIZE command is:
size = "Size" SP pathname CRLF
The server-PI will respond to the SIZE command with a 213 reply
giving the transfer size of the file whose pathname was supplied, or
an error response if the file does not exist, the size is
unavailable, or some other error has occurred. The value returned is
in a format suitable for use with the RESTART (REST) command for mode
STREAM, provided the transfer mode and type are not altered.
size-response = "213" SP 1*DIGIT CRLF /
error-response
4.2. Error responses
Where the command is correctly parsed, but the size is not available,
either because the pathname identifies no existing entity, or because
the entity named cannot be transferred in the current MODE and TYPE
(or at all), then a 550 reply should be sent. Where the command
cannot be correctly parsed, a 500 or 501 reply should be sent, as
specified in [3].
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4.3. FEAT response for SIZE
When replying to the FEAT command [6], an FTP server process that
supports the SIZE command MUST include a line containing the single
word "SIZE". This word is case insensitive, and MAY be sent in any
mixture of upper or lower case, however it SHOULD be sent in upper
case. That is, the response SHOULD be
C> FEAT
S> 211- <any descriptive text>
S> ...
S> SIZE
S> ...
S> 211 END
The ellipses indicate place holders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory [6].
4.4. Size Examples
Consider a text file "Example" stored on a Unix(TM) server where each
end of line is represented by a single octet. Assume the file
contains 112 lines, and 1830 octets total. Then the SIZE command
would produce:
C> TYPE I
S> 200 Type set to I.
C> size Example
S> 213 1830
C> TYPE A
S> 200 Type set to A.
C> Size Example
S> 213 1942
Notice that with TYPE=A the SIZE command reports an extra 112 octets.
Those are the extra octets that need to be inserted, one at the end
of each line, to provide correct end of line semantics for a transfer
using TYPE=A. Other systems might need to make other changes to the
transfer format of files when converting between TYPEs and MODEs.
The SIZE command takes all of that into account.
Since calculating the size of a file with this degree of precision
may take considerable effort on the part of the server-PI, user-PIs
should not used this command unless this precision is essential (such
as when about to restart an interrupted transfer). For other uses,
the "Size" fact of the MLST command (see section 8.5.7) ought be
requested.
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5. Restart of Interrupted Transfer (REST)
To avoid having to resend the entire file if the file is only
partially transferred, both sides need some way to be able to agree
on where in the data stream to restart the data transfer.
The FTP specification [3] includes three modes of data transfer,
Stream, Block and Compressed. In Block and Compressed modes, the
data stream that is transferred over the data connection is
formatted, allowing the embedding of restart markers into the stream.
The sending DTP can include a restart marker with whatever
information it needs to be able to restart a file transfer at that
point. The receiving DTP can keep a list of these restart markers,
and correlate them with how the file is being saved. To restart the
file transfer, the receiver just sends back that last restart marker,
and both sides know how to resume the data transfer. Note that there
are some flaws in the description of the restart mechanism in RFC 959
[3]. See section 4.1.3.4 of RFC 1123 [9] for the corrections.
5.1. Restarting in STREAM Mode
In Stream mode, the data connection contains just a stream of
unformatted octets of data. Explicit restart markers thus cannot be
inserted into the data stream, they would be indistinguishable from
data. For this reason, the FTP specification [3] did not provide the
ability to do restarts in stream mode. However, there is not really
a need to have explicit restart markers in this case, as restart
markers can be implied by the octet offset into the data stream.
Because the data stream defines the file in STREAM mode, a different
data stream would represent a different file. Thus, an offset will
always represent the same position within a file. On the other hand,
in other modes than STREAM, the same file can be transferred using
quite different octet sequences, and yet be reconstructed into the
one identical file. Thus an offset into the data stream in transfer
modes other than STREAM would not give an unambiguous restart point.
If the data representation TYPE is IMAGE, and the STRUcture is File,
for many systems the file will be stored exactly in the same format
as it is sent across the data connection. It is then usually very
easy for the receiver to determine how much data was previously
received, and notify the sender of the offset where the transfer
should be restarted. In other representation types and structures
more effort will be required, but it remains always possible to
determine the offset with finite, but perhaps non-negligible, effort.
In the worst case an FTP process may need to open a data connection
to itself, set the appropriate transfer type and structure, and
actually transmit the file, counting the transmitted octets.
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If the user-FTP process is intending to restart a retrieve, it will
directly calculate the restart marker, and send that information in
the RESTart command. However, if the user-FTP process is intending
to restart sending the file, it needs to be able to determine how
much data was previously sent, and correctly received and saved. A
new FTP command is needed to get this information. This is the
purpose of the SIZE command, as documented in section 4.
5.2. Error Recovery and Restart
STREAM MODE transfers with FILE STRUcture may be restarted even
though no restart marker has been transferred in addition to the data
itself. This is done by using the SIZE command, if needed, in
combination with the RESTART (REST) command, and one of the standard
file transfer commands.
When using TYPE ASCII or IMAGE, the SIZE command will return the
number of octets that would actually be transferred if the file were
to be sent between the two systems. I.e. with type IMAGE, the SIZE
normally would be the number of octets in the file. With type ASCII,
the SIZE would be the number of octets in the file including any
modifications required to satisfy the TYPE ASCII CR-LF end of line
convention.
5.3. Syntax
The syntax for the REST command when the current transfer mode is
STREAM is:
rest = "Rest" SP 1*DIGIT CRLF
The numeric value gives the number of octets of the immediately
following transfer to not actually send, effectively causing the
transmission to be restarted at a later point. A value of zero
effectively disables restart, causing the entire file to be
transmitted. The server-PI will respond to the REST command with a
350 reply, indicating that the REST parameter has been saved, and
that another command, which should be either RETR or STOR, should
then follow to complete the restart.
rest-response = "350" SP *TCHAR CRLF /
error-response
Server-FTP processes may permit transfer commands other than RETR and
STOR, such as APPE and STOU, to complete a restart, however, this is
not recommended. STOU (store unique) is undefined in this usage, as
storing the remainder of a file into a unique filename is rarely
going to be useful. If APPE (append) is permitted, it MUST act
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identically to STOR when a restart marker has been set. That is, in
both cases, octets from the data connection are placed into the file
at the location indicated by the restart marker value.
The REST command is intended to complete a failed transfer. Use with
RETR is comparatively well defined in all cases, as the client bears
the responsibility of merging the retrieved data with the partially
retrieved file. If it chooses to use the data obtained other than to
complete an earlier transfer, or if it chooses to re-retrieve data
that had been retrieved before, that is its choice. With STOR,
however, the server must insert the data into the file named. The
results are undefined if a client uses REST to do other than restart
to complete a transfer of a file which had previously failed to
completely transfer. In particular, if the restart marker set with a
REST command is not at the end of the data currently stored at the
server, as reported by the server, or if insufficient data are
provided in a STOR that follows a REST to extend the destination file
to at least its previous size, then the effects are undefined.
The REST command must be the last command issued before the data
transfer command which is to cause a restarted rather than complete
file transfer. The effect of issuing a REST command at any other
time is undefined. The server-PI may react to a badly positioned
REST command by issuing an error response to the following command,
not being a restartable data transfer command, or it may save the
restart value and apply it to the next data transfer command, or it
may silently ignore the inappropriate restart attempt. Because of
this, a user-PI that has issued a REST command, but which has not
successfully transmitted the following data transfer command for any
reason, should send another REST command before the next data
transfer command. If that transfer is not to be restarted, then
"REST 0" should be issued.
An error-response will follow a REST command only when the server
does not implement the command, or the restart marker value is
syntactically invalid for the current transfer mode. That is, in
STREAM mode, if something other than one or more digits appears in
the parameter to the REST command. Any other errors, including such
problems as restart marker out of range, should be reported when the
following transfer command is issued. Such errors will cause that
transfer request to be rejected with an error indicating the invalid
restart attempt.
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5.4. FEAT response for REST
Where a server-FTP process supports RESTart in STREAM mode, as
specified here, it MUST include in the response to the FEAT command
[6], a line containing exactly the string "REST STREAM". This string
is not case sensitive, but SHOULD be transmitted in upper case.
Where REST is not supported at all, or supported only in block or
compressed modes, the REST line MUST NOT be included in the FEAT
response. Where required, the response SHOULD be
C> feat
S> 211- <any descriptive text>
S> ...
S> REST STREAM
S> ...
S> 211 end
The ellipses indicate place holders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory [6].
5.5. REST Example
Assume that the transfer of a largish file has previously been
interrupted after 802816 octets had been received, that the previous
transfer was with TYPE=I, and that it has been verified that the file
on the server has not since changed.
C> TYPE I
S> 200 Type set to I.
C> PORT 127,0,0,1,15,107
S> 200 PORT command successful.
C> REST 802816
S> 350 Restarting at 802816. Send STORE or RETRIEVE
C> RETR cap60.pl198.tar
S> 150 Opening BINARY mode data connection
[...]
S> 226 Transfer complete.
6. Virtual FTP servers
It has become common in the Internet for many domain names to be
allocated to a single IP address. This 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,
other such hosts.
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Such an arrangement presents some problems for FTP Servers, as all
the FTP Server can detect is an incoming FTP connection to a
particular IP address. That is, all domain names which share the IP
address also share the FTP server, and more importantly, its NVFS.
This means that the various virtual hosts cannot offer different
virtual file systems to clients, nor can they offer different
authentication systems.
No scheme can overcome this without modifications of some kind to the
user-PI and the user-FTP process. That process is the only entity
that knows which virtual host is required. It has performed the
domain name to IP address translation, and thus has the original
domain name available.
One method which 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 supported by essentially all server-FTP implementations 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. Second, either the server-FTP process needs
to be modified to understand 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, or the virtual host will not be
truly transparent. Third, and again unless the server is specially
modified, a user connecting this way to a virtual host would be able
to trivially move to any other virtual host supported at the same
server-FTP process, exposing the nature of the virtual host.
Other schemes overloading other existing FTP commands have also been
proposed. None of those have sufficient merit to be worth
discussion.
The conclusion from the examination of the possibilities seems to be
that to obtain an adequate emulation of "real" FTP servers, server
modifications to support virtual hosts are required. A new command
seems most likely to provide the support required.
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6.1. The HOST command
A new command "HOST" is added to the FTP command set to allow
server-FTP process to determine to which of possibly many virtual
hosts the client wishes to connect. This command is intended to be
issued before the user is authenticated, allowing the authentication
scheme, and set of legal users, to be dependent upon the virtual host
chosen. Server-FTP processes may, if they desire, permit the HOST
command to be issued after the user has been authenticated, or may
treat that as an erroneous sequence of commands. The behavior of the
server-FTP process which does allow late HOST commands is undefined.
One reasonable interpretation would be for the user-PI to be returned
to the state that existed after the TCP connection was first
established, before user authentication.
Servers should note that the response to the HOST command is a
sensible time to send their "welcome" message. This allows the
message to be personalized for any virtual hosts that are supported,
and also allows the client to have determined supported languages, or
representations, for the message, and other messages, via the FEAT
response, and selected an appropriate one via the LANG command. See
[7] for more information.
6.2. Syntax of the HOST command
The HOST command is defined as follows.
host-command = "Host" SP hostname CRLF
hostname = 1*DNCHAR 1*( "." 1*DNCHAR ) [ "." ]
DNCHAR = ALPHA / DIGIT / "-" / "_" / "$" /
"!" / "%" / "[" / "]" / ":"
host-response = host-ok / error-response
host-ok = "220" [ SP *TCHAR ] CRLF
As with all FTP commands, the "host" command word is case
independent, and may be specified in any character case desired.
The "hostname" given as a parameter specifies the virtual host to
which access is desired. It should normally be the same name that
was used to obtain the IP address to which the FTP control connection
was made, after any client conversions to convert an abbreviated or
local alias to a complete (fully qualified) domain name, but before
resolving a DNS alias (owner of a CNAME resource record) to its
canonical name.
If the client was given a network literal address, and consequently
was not required to derive it from a hostname, it should send the
HOST command with the network address, as specified to it, enclosed
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in brackets (after eliminating any syntax, which might also be
brackets, but is not required to be, from which the server deduced
that a literal address had been specified.) That is, for example
HOST [10.1.2.3]
should be sent if the client had been instructed to connect to
"10.1.2.3", or "[10.1.2.3]", or perhaps even IPv4:10.1.2.3. The
method of indicating to a client that a literal address is to be used
is beyond the scope of this specification.
The parameter is otherwise to be treated as a "complete domain name",
as that term is defined in section 3.1 of RFC 1034 [10]. That
implies that the name is to be treated as a case independent string,
in that upper case ASCII characters are to be treated as equivalent
to the corresponding lower case 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.
RFC 1034 imposes no other restrictions upon what kinds of names can
be stored in the DNS. Nor does RFC 1035. This specification,
however, allows only a restricted set of names for the purposes of
the HOST command. Those restrictions can be inferred from the ABNF
grammar given for the "hostname".
6.3. 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 so, establish the
appropriate environment for that virtual host. The meaning of that
is not specified here, and may range from doing nothing at all, or
performing a simple change of working directory, to much more
elaborate state changes, as required.
If the hostname specified is unknown at the server, or if the server
is otherwise unwilling to treat the particular connection as a
connection to the hostname specified, the server will respond with a
504 reply.
Note: servers may require that the name specified is in some sense
equivalent to the particular network address that was used to reach
the server.
If the hostname specified would normally be acceptable, but for any
reason is temporarily unavailable, the server SHOULD reply to the
HOST command with a 434 reply.
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The "220" reply code for the HOST command is the same as the code
used on the initial connection established "welcome" message. This
is done deliberately so as to allow the implementation to implement
the front end FTP server as a wrapper which simply waits for the HOST
command, and then invokes an older, RFC959 compliant, server in the
appropriate environment for the particular hostname received.
6.3.1. The REIN command
As specified in [3], the REIN command returns the state of the
connection to that it was immediately after the transport connection
was opened. That is not changed here. The effect of a HOST command
will be lost if a REIN command is performed, a new HOST command must
be issued.
Implementors of user-FTP should be aware that server-FTP
implementations which implement the HOST command as a wrapper around
older implementations will be unable to correctly implement the REIN
command. In such an implementation, REIN will typically return the
server-FTP to the state that existed immediately after the HOST
command was issued, instead of to the state immediately after the
connection was opened.
6.3.2. 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 state diagram shows a typical sequence of flow of
control, where the "B" (begin) state is assumed to occur after the
transport connection has opened, or a REIN command has succeeded.
Other commands (such as FEAT [6]) which require no authentication may
have intervened. This diagram is modeled upon (and largely borrowed
from) the similar diagram in section 6 of [3].
In this diagram, a three digit reply indicates that precise server
reply code, a single digit on a reply path indicates any server reply
beginning with that digit, other than any three digit replies that
might take another path.
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+---+ 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 |
+---+ +---+-------------->+---+
6.4. HOST command errors
The server-PI shall 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 accept the
command at such a time, with server defined behavior. A 501 reply
should be sent if the hostname given is syntactically invalid, and a
504 reply 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.
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A user-PI receiving a 500 or 502 reply should assume that the
server-PI does not implement the HOST command style virtual server.
It may then proceed to login as if the HOST command had succeeded,
and perhaps, attempt a CWD command to the hostname after
authenticating the user.
A user-PI receiving some other error reply 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 all
supported virtual hosts, perhaps one in which a union of all
available accounts exists, and which presents a NVFS which appears to
contain sub-directories containing the NVFS for all virtual hosts
supported.
6.5. 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 [6], a
feature line indicating that the HOST command is supported. This
line should contain the single word "HOST". This MAY be sent in
upper or lower case, or a mixture of both (it is case insensitive)
but SHOULD be transmitted in upper case only. That is, the response
SHOULD be
C> Feat
S> 211- <any descriptive text>
S> ...
S> HOST
S> ...
S> 211 End
The ellipses indicate place holders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory [6].
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7. A Trivial Virtual File Store (TVFS)
Traditionally, FTP has placed almost no constraints upon the file
store (NVFS) provided by a server. This specification does not alter
that. However, it has become common for servers to attempt to
provide at least file system naming conventions modeled loosely upon
those of the UNIX(TM) file system. That is, a tree structured file
system, built of directories, each of which can contain other
directories, or other kinds of files, or both. Each file and
directory has a file name relative to the directory that contains it,
except for the directory at the root of the tree, which is contained
in no other directory, and hence has no name of its own.
That which has so far been described is perfectly consistent with the
standard FTP NVFS and access mechanisms. The "CWD" command is used
to move from one directory to an embedded directory. "CDUP" may be
provided to return to the parent directory, and the various file
manipulation commands ("RETR", "STOR", the rename commands, etc) are
used to manipulate files within the current directory.
However, it is often useful to be able to reference files other than
by changing directories, especially as FTP provides no guaranteed
mechanism to return to a previous directory. The Trivial Virtual
File Store (TVFS), if implemented, provides that mechanism.
7.1. TVFS File Names
Where a server implements the TVFS, no elementary filename shall
contain the character "/". Where the underlying natural file store
permits files, or directories, to contain the "/" character in their
names, a server-PI implementing TVFS must encode that character in
some manner whenever file or directory names are being returned to
the user-PI, and reverse that encoding whenever such names are being
accepted from the user-PI.
The encoding method to be used is not specified here. Where some
other character is illegal in file and directory names in the
underlying file store, a simple transliteration may be sufficient.
Where there is no suitable substitute character a more complex
encoding scheme, possibly using an escape character, is likely to be
required.
With the one exception of the unnamed root directory, a TVFS file
name may not be empty. That is, all other file names contain at
least one character.
With the sole exception of the "/" character, any valid IS10646
character [11] may be used in a TVFS filename. When transmitted,
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file name characters are encoded using the UTF-8 encoding [2].
7.2. TVFS Path Names
A TVFS "Path Name" combines the file or directory name of a target
file or directory, with the directory names of zero or more enclosing
directories, so as to allow the target file or directory to be
referenced other than when the server's "current working directory"
is the directory directly containing the target file or directory.
By definition, every TVFS file or directory name is also a TVFS path
name. Such a path name is valid to reference the file from the
directory containing the name, that is, when that directory is the
server-FTP's current working directory.
Other TVFS path names are constructed by prefixing a path name by a
name of a directory from which the path is valid, and separating the
two with the "/" character. Such a path name is valid to reference
the file or directory from the directory containing the newly added
directory name.
Where a path name has been extended to the point where the directory
added is the unnamed root directory, the path name will begin with
the "/" character. Such a path is known as a fully qualified path
name. Fully qualified paths may, obviously, not be further extended,
as, by definition, no directory contains the root directory. Being
unnamed, it cannot be represented in any other directory. A fully
qualified path name is valid to reference the named file or directory
from any location (that is, regardless of what the current working
directory may be) in the virtual file store.
Any path name which is not a fully qualified path name may be
referred to as a "relative path name" and will only correctly
reference the intended file when the current working directory of the
server-FTP is a directory from which the relative path name is valid.
As a special case, the path name "/" is defined to be a fully
qualified path name referring to the root directory. That is, the
root directory does not have a directory (or file) name, but does
have a path name. This special path name may be used only as is as a
reference to the root directory. It may not be combined with other
path names using the rules above, as doing so would lead to a path
name containing two consecutive "/" characters, which is an undefined
sequence.
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7.2.1. Notes
+ It is not required, or expected, that there be only one fully
qualified path name that will reference any particular file or
directory.
+ As a caveat, though the TVFS file store is basically tree
structured, there is no requirement that any file or directory
have only one parent directory.
+ As defined, no TVFS path name will ever contain two consecutive
"/" characters. Such a name is not illegal however, and may be
defined by the server for any purpose that suits it. Clients
implementing this specification should not assume any semantics
at all for such names.
+ Similarly, other than the special case path that refers to the
root directory, no TVFS path name constructed as defined here
will ever end with the "/" character. Such names are also not
illegal, but are undefined.
+ While any legal IS10646 character is permitted to occur in a TVFS
file or directory name, other than "/", server FTP
implementations are not required to support all possible IS10646
characters. The subset supported is entirely at the discretion
of the server. The case (where it exists) of the characters that
make up file, directory, and path names may be significant.
Unless determined otherwise by means unspecified here, clients
should assume that all such names are comprised of characters
whose case is significant. Servers are free to treat case (or
any other attribute) of a name as irrelevant, and hence map two
names which appear to be distinct onto the same underlying file.
+ There are no defined "magic" names, like ".", ".." or "C:".
Servers may implement such names, with any semantics they choose,
but are not required to do so.
+ TVFS imposes no particular semantics or properties upon files,
guarantees no access control schemes, or any of the other common
properties of a file store. Only the naming scheme is defined.
7.3. FEAT Response for TVFS
In response to the FEAT command [6] a server that wishes to indicate
support for the TVFS as defined here will include a line that begins
with the four characters "TVFS" (in any case, or mixture of cases,
upper case is not required). Servers SHOULD send upper case.
Such a response to the FEAT command MUST NOT be returned unless the
server implements TVFS as defined here.
Later specifications may add to the TVFS definition. Such additions
should be notified by means of additional text appended to the TVFS
feature line. Such specifications, if any, will define the extra
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text.
Until such a specification is defined, servers should not include
anything after "TVFS" in the TVFS feature line. Clients, however,
should be prepared to deal with arbitrary text following the four
defined characters, and simply ignore it if unrecognized.
A typical response to the FEAT command issued by a server
implementing only this specification would be:
C> feat
S> 211- <any descriptive text>
S> ...
S> TVFS
S> ...
S> 211 end
The ellipses indicate place holders where other features may be
included, and are not required. The one space indentation of the
feature lines is mandatory [6], and is not counted as one of the
first four characters for the purposes of this feature listing.
The TVFS feature adds no new commands to the FTP command repertoire.
7.4. OPTS for TVFS
There are no options in this TVFS specification, and hence there is
no OPTS command defined.
7.5. TVFS Examples
Assume a TVFS file store is comprised of a root directory, which
contains two directories (A and B) and two non-directory files (X and
Y). The A directory contains two directories (C and D) and one other
file (Z). The B directory contains just two non-directory files (P
and Q) and the C directory also two non-directory files (also named P
and Q, by chance). The D directory is empty, that is, contains no
files or directories.
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This structure may depicted graphically as...
(unnamed root)
/ | \ \
/ | \ \
A X B Y
/|\ / \
/ | \ / \
C D Z P Q
/ \
/ \
P Q
Given this structure, the following fully qualified path names exist.
/
/A
/B
/X
/Y
/A/C
/A/D
/A/Z
/A/C/P
/A/C/Q
/B/P
/B/Q
It is clear that none of the paths / /A /B or /A/D refer to the same
directory, as the contents of each is different. Nor do any of / /A
/A/C or /A/D. However /A/C and /B might be the same directory, there
is insufficient information given to tell. Any of the other path
names (/X /Y /A/Z /A/C/P /A/C/Q /B/P and /B/Q) may refer to the same
underlying files, in almost any combination.
If the current working directory of the server-FTP is /A then the
following path names, in addition to all the fully qualified path
names, are valid
C
D
Z
C/P
C/Q
These all refer to the same files or directories as the corresponding
fully qualified path with "/A/" prepended.
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That those path names all exist does not imply that the TVFS sever
will necessarily grant any kind of access rights to the named paths,
or that access to the same file via different path names will
necessarily be granted equal rights.
None of the following relative paths are valid when the current
directory is /A
A
B
X
Y
B/P
B/Q
P
Q
Any of those could be made valid by changing the server-FTP's current
working directory to the appropriate directory. Note that the paths
"P" and "Q" might refer to different files depending upon which
directory is selected to cause those to become valid TVFS relative
paths.
8. Listings for Machine Processing (MLST and MLSD)
The MLST and MLSD commands are intended to standardize the file and
directory information returned by the Server-FTP process. These
commands differ from the LIST command in that the format of the
replies is strictly defined although extensible.
Two commands are defined, MLST which provides data about exactly the
object named on its command line, and no others. MLSD on the other
hand will list the contents of a directory if a directory is named,
otherwise a 501 reply will be returned. In either case, if no object
is named, the current directory is assumed. That will cause MLST to
send a one line response, describing the current directory itself,
and MLSD to list the contents of the current directory.
In the following, the term MLSx will be used wherever either MLST or
MLSD may be inserted.
The MLST and MLSD commands also extend the FTP protocol as presented
in RFC 959 [3] and RFC 1123 [9] to allow that transmission of 8-bit
data over the control connection. Note this is not specifying
character sets which are 8-bit, but specifying that FTP
implementations are to specifically allow the transmission and
reception of 8-bit bytes, with all bits significant, over the control
connection. That is, all 256 possible octet values are permitted.
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The MLSx command allows both UTF-8/Unicode and "raw" forms as
arguments, and in responses both to the MLST and MLSD commands, and
all other FTP commands which take pathnames as arguments.
8.1. Format of MLSx Requests
The MLST and MLSD commands each allow a single optional argument.
This argument may be either a directory name or, for MLST only, a
filename. For these purposes, a "filename" is the name of any entity
in the server NVFS which is not a directory. Where TVFS is
supported, any TVFS relative path name valid in the current working
directory, or any TVFS fully qualified path name, may be given. If a
directory name is given then MLSD must return a listing of the
contents of the named directory, otherwise it issues a 501 reply, and
does not open a data connection. In all cases for MLST, a single set
of fact lines (usually a single fact line) containing the information
about the named file or directory shall be returned over the control
connection, without opening a data connection.
If no argument is given then MLSD must return a listing of the
contents of the current working directory, and MLST must return a
listing giving information about the current working directory
itself. For these purposes, the contents of a directory are whatever
filenames (not pathnames) the server-PI will allow to be referenced
when the current working directory is the directory named, and which
the server-PI desires to reveal to the user-PI.
No title, header, or summary, lines, or any other formatting, other
than as is specified below, is ever returned in the output of an MLST
or MLSD command.
If the Client-FTP sends an invalid argument, the Server-FTP MUST
reply with an error code of 501.
The syntax for the MLSx command is:
mlst = "MLst" [ SP pathname ] CRLF
mlsd = "MLsD" [ SP pathname ] CRLF
8.2. Format of MLSx Response
The format of a response to an MLSx command is as follows:
mlst-response = control-response / error-response
mlsd-response = ( initial-response final-response ) /
error-response
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control-response = "250-" [ response-message ] CRLF
1*( SP entry CRLF )
"250" [ SP response-message ] CRLF
initial-response = "150" [ SP response-message ] CRLF
final-response = "226" SP response-message CRLF
response-message = *TCHAR
data-response = *( entry CRLF )
entry = [ facts ] SP pathname
facts = 1*( fact ";" )
fact = factname "=" value
factname = "Size" / "Modify" / "Create" /
"Type" / "Unique" / "Perm" /
"Lang" / "Media-Type" / "CharSet" /
os-depend-fact / local-fact
os-depend-fact = <IANA assigned OS name> "." token
local-fact = "X." token
value = *RCHAR
Upon receipt of a MLSx command, the server will verify the parameter,
and if invalid return an error-response. For this purpose, the
parameter should be considered to be invalid if the client issuing
the command does not have permission to perform the request
operation.
If valid, then for an MLST command, the server-PI will send the first
(leading) line of the control response, the entry for the pathname
given, or the current directory if no pathname was provided, and the
terminating line. Normally exactly one entry would be returned, more
entries are permitted only when required to represent a file that is
to have multiple "Type" facts returned.
Note that for MLST the fact set is preceded by a space. That is
provided to guarantee that the fact set cannot be accidentally
interpreted as the terminating line of the control response, but is
required even when that would not be possible. Exactly one space
exists between the set of facts and the pathname. Where no facts are
present, there will be exactly two leading spaces before the
pathname. No spaces are permitted in the facts, any other spaces in
the response are to be treated as being a part of the pathname.
If the command was an MLSD command, the server will open a data
connection as indicated in section 3.2 of RFC959 [3]. If that fails,
the server will return an error-response. If all is OK, the server
will return the initial-response, send the appropriate data-response
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over the new data connection, close that connection, and then send
the final-response over the control connection. The grammar above
defines the format for the data-response, which defines the format of
the data returned over the data connection established.
The data connection opened for a MLSD response shall be a connection
as if the "TYPE L 8", "MODE S", and "STRU F" commands had been given,
whatever FTP transfer type, mode and structure had actually been set,
and without causing those settings to be altered for future commands.
That is, this transfer type shall be set for the duration of the data
connection established for this command only. While the content of
the data sent can be viewed as a series of lines, implementations
should note that there is no maximum line length defined.
Implementations should be prepared to deal with arbitrarily long
lines.
The facts part of the specification would contain a series of "file
facts" about the file or directory named on the same line. Typical
information to be presented would include file size, last
modification time, creation time, a unique identifier, and a
file/directory flag.
The complete format for a successful reply to the MLSD command would
be:
facts SP pathname CRLF
facts SP pathname CRLF
facts SP pathname CRLF
...
Note that the format is intended for machine processing, not human
viewing, and as such the format is very rigid. Implementations MUST
NOT vary the format by, for example, inserting extra spaces for
readability, replacing spaces by tabs, including header or title
lines, or inserting blank lines, or in any other way alter this
format. Exactly one space is always required after the set of facts
(which may be empty). More spaces may be present on a line if, and
only if, the file name presented contains significant spaces. The
set of facts must not contain any spaces anywhere inside it. Facts
should be provided in each output line only if they both provide
relevant information about the file named on the same line, and they
are in the set requested by the user-PI. There is no requirement
that the same set of facts be provided for each file, or that the
facts presented occur in the same order for each file.
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8.3. Filename encoding
An FTP implementation supporting the MLSx commands must be 8-bit
clean. This is necessary in order to transmit UTF-8 encoded
filenames. This specification recommends the use of UTF-8 encoded
filenames. FTP implementations SHOULD use UTF-8 whenever possible to
encourage the maximum interoperability.
Filenames are not restricted to UTF-8, however treatment of arbitrary
character encodings is not specified by this standard. Applications
are encouraged to treat non-UTF-8 encodings of filenames as octet
sequences.
Note that this encoding is unrelated to that of the contents of the
file, even if the file contains character data.
Further information about filename encoding for FTP may be found in
"Internationalization of the File Transfer Protocol" [7].
8.3.1. Notes about the Filename
The filename returned in the MLST response should be the same name as
was specified in the MLST command, or, where TVFS is supported, a
fully qualified TVFS path naming the same file. Where no argument
was given to the MLST command, the server-PI may either include an
empty filename in the response, or it may supply a name that refers
to the current directory, if such a name is available. Where TVFS is
supported, a fully qualified path name of the current directory
SHOULD be returned.
Filenames returned in the output from an MLSD command SHOULD be
unqualified names within the directory named, or the current
directory if no argument was given. That is, the directory named in
the MLSD command SHOULD NOT appear as a component of the filenames
returned.
If the server-FTP process is able, and the "type" fact is being
returned, it MAY return in the MLSD response, an entry whose type is
"cdir", which names the directory from which the contents of the
listing were obtained. Where TVFS is supported, the name MAY be the
fully qualified path name of the directory, or MAY be any other path
name which is valid to refer to that directory from the current
working directory of the server-FTP. Where more than one name
exists, multiple of these entries may be returned. In a sense, the
"cdir" entry can be viewed as a heading for the MLSD output.
However, it is not required to be the first entry returned, and may
occur anywhere within the listing.
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When TVFS is supported, a user-PI can refer to any file or directory
in the listing by combining a type "cdir" name, with the appropriate
name from the directory listing using the procedure defined in
section 7.2.
Alternatively, whether TVFS is supported or not, the user-PI can
issue a CWD command ([3]) giving a name of type "cdir" from the
listing returned, and from that point reference the files returned in
the MLSD response from which the cdir was obtained by using the
filename components of the listing.
8.4. Format of Facts
The "facts" for a file in a reply to a MLSx command consist of
information about that file. The facts are a series of keyword=value
pairs each followed by semi-colon (";") characters. An individual
fact may not contain a semi-colon in its name or value. The complete
series of facts may not contain the space character. See the
definition or "RCHAR" in section 2.1 for a list of the characters
that can occur in a fact value. Not all are applicable to all facts.
A sample of a typical series of facts would be: (spread over two
lines for presentation here only)
size=4161;lang=en-US;modify=19970214165800;create=19961001124534;
type=file;x.myfact=foo,bar;
8.5. Standard Facts
This document defines a standard set of facts as follows:
size -- Size in octets
modify -- Last modification time
create -- Creation time
type -- Entry type
unique -- Unique id of file/directory
perm -- File permissions, whether read, write, execute is
allowed for the login id.
lang -- Language of the filename per IANA[12] registry.
media-type -- MIME media-type of file contents per IANA registry.
charset -- Character set per IANA registry (if not UTF-8)
Fact names are case-insensitive. Size, size, SIZE, and SiZe are the
same fact.
Further operating system specific keywords could be specified by
using the IANA operating system name as a prefix (examples only):
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OS/2.ea -- OS/2 extended attributes
MACOS.rf -- MacIntosh resource forks
UNIX.mode -- Unix file modes (permissions)
Implementations may define keywords for experimental, or private use.
All such keywords MUST begin with the two character sequence "x.".
As type names are case independent, "x." and "X." are equivalent.
For example:
x.ver -- Version information
x.desc -- File description
x.type -- File type
8.5.1. The type Fact
The type fact needs a special description. Part of the problem with
current practices is deciding when a file is a directory. If it is a
directory, is it the current directory, a regular directory, or a
parent directory? The MLST specification makes this unambiguous
using the type fact. The type fact given specifies information about
the object listed on the same line of the MLST response.
Five values are possible for the type fact:
file -- a file entry
cdir -- the listed directory
pdir -- a parent directory
dir -- a directory or sub-directory
OS.name=type -- an OS or file system dependent file type
The syntax is defined to be:
type-fact = type-label "=" type-val
type-label = "Type"
type-val = "File" / "cdir" / "pdir" / "dir" /
os-type
8.5.1.1. type=file
The presence of the type=file fact indicates the listed entry is a
file containing non-system data. That is, it may be transferred from
one system to another of quite different characteristics, and perhaps
still be meaningful.
8.5.1.2. type=cdir
The type=cdir fact indicates the listed entry contains a pathname of
the directory whose contents are listed. An entry of this type will
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only be returned as a part of the result of an MLSD command when the
type fact is included, and provides a name for the listed directory,
and facts about that directory. In a sense, it can be viewed as
representing the title of the listing, in a machine friendly format.
It may appear at any point of the listing, it is not restricted to
appearing at the start, though frequently may do so, and may occur
multiple times. It MUST NOT be included if the type fact is not
included, or there would be no way for the user-PI to distinguish the
name of the directory from an entry in the directory.
Where TVFS is supported by the server-FTP, this name may be used to
construct path names with which to refer to the files and directories
returned in the same MLSD output (see section 7.2). These path names
are only expected to work when the server-PI's position in the NVFS
file tree is the same as its position when the MLSD command was
issued, unless a fully qualified path name results.
Where TVFS is not supported, the only defined semantics associated
with a "type=cdir" entry are that, provided the current working
directory of the server-PI has not been changed, a pathname of type
"cdir" may be used as an argument to a CWD command, which will cause
the current directory of the server-PI to change so that the
directory which was listed in its current working directory.
8.5.1.3. type=dir
If present, the type=dir entry gives the name of a directory. Such
an entry typically cannot be transferred from one system to another
using RETR, etc, but should (permissions permitting) be able to be
the object of an MLSD command.
8.5.1.4. type=pdir
If present, which will occur only in the response to a MLSD command
when the type fact is included, the type=pdir entry represents a
pathname of the parent directory of the listed directory. As well as
having the properties of a type=dir, a CWD command that uses the
pathname from this entry should change the user to a parent directory
of the listed directory. If the listed directory is the current
directory, a CDUP command may also have the effect of changing to the
named directory. User-FTP processes should note not all responses
will include this information, and that some systems may provide
multiple type=pdir responses.
Where TVFS is supported, a "type=pdir" name may be a relative path
name, or a fully qualified path name. A relative path name will be
relative to the directory being listed, not to the current directory
of the server-PI at the time.
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For the purposes of this type value, a "parent directory" is any
directory in which there is an entry of type=dir which refers to the
directory in which the type=pdir entity was found. Thus it is not
required that all entities with type=pdir refer to the same
directory. The "unique" fact (if supported) can be used to determine
whether there is a relationship between the type=pdir entries or not.
8.5.1.5. System defined types
Files types that are specific to a specific operating system, or file
system, can be encoded using the "OS." type names. The format is:
os-type = "OS." os-name "=" os-type
os-name = <an IANA registered operating system name>
os-type = token
The "os-name" indicates the specific system type which supports the
particular localtype. OS specific types are registered by the IANA
using the procedures specified in section 11. The "os-type" provides
the system dependent information as to the type of the file listed.
The os-name and os-type strings in an os-type are case independent.
"OS.unix=block" and "OS.Unix=BLOCK" represent the same type (or
would, if such a type were registered.)
Note: Where the underlying system supports a file type which is
essentially an indirect pointer to another file, the NVFS
representation of that type should normally be to represent the file
which the reference indicates. That is, the underlying basic file
will appear more than once in the NVFS, each time with the "unique"
fact (see immediately following section) containing the same value,
indicating that the same file is represented by all such names.
User-PIs transferring the file need then transfer it only once, and
then insert their own form of indirect reference to construct
alternate names where desired, or perhaps even copy the local file if
that is the only way to provide two names with the same content. A
file which would be a reference to another file, if only the other
file actually existed, may be represented in any OS dependent manner
appropriate, or not represented at all.
8.5.1.6. Multiple types
Where a file is such that it may validly, and sensibly, treated by
the server-PI as being of more than one of the above types, then
multiple entries should be returned, each with its own "Type" fact of
the appropriate type, and each containing the same pathname. This
may occur, for example, with a structured file, which may contain
sub-files, and where the server-PI permits the structured file to be
treated as a unit, or treated as a directory allowing the sub-files
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within it to be referenced.
8.5.2. The unique Fact
The unique fact is used to present a unique identifier for a file or
directory in the NVFS accessed via a server-FTP process. The value
of this fact should be the same for any number of pathnames that
refer to the same underlying file. The fact should have different
values for names which reference distinct files. The mapping between
files, and unique fact tokens should be maintained, and remain
consistent, for at least the lifetime of the control connection from
user-PI to server-PI.
unique-fact = "Unique" "=" token
This fact would be expected to be used by Server-FTPs whose host
system allows things such as symbolic links so that the same file may
be represented in more than one directory on the server. The only
conclusion that should be drawn is that if two different names each
have the same value for the unique fact, they refer to the same
underlying object. The value of the unique fact (the token) should
be considered an opaque string for comparison purposes, and is a case
dependent value. The tokens "A" and "a" do not represent the same
underlying object.
8.5.3. The modify Fact
The modify fact is used to determine the last time the content of the
file (or directory) indicated was modified. Any change of substance
to the file should cause this value to alter. That is, if a change
is made to a file such that the results of a RETR command would
differ, then the value of the modify fact should alter. User-PIs
should not assume that a different modify fact value indicates that
the file contents are necessarily different than when last retrieved.
Some systems may alter the value of the modify fact for other
reasons, though this is discouraged wherever possible. Also a file
may alter, and then be returned to its previous content, which would
often be indicated as two incremental alterations to the value of the
modify fact.
For directories, this value should alter whenever a change occurs to
the directory such that different filenames would (or might) be
included in MLSD output of that directory.
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modify-fact = "Modify" "=" time-val
8.5.4. The create Fact
The create fact indicates when a file, or directory, was first
created. Exactly what "creation" is for this purpose is not
specified here, and may vary from server to server. About all that
can be said about the value returned is that it can never indicate a
later time than the modify fact.
create-fact = "Create" "=" time-val
Implementation Note: Implementors of this fact on UNIX(TM) systems
should note that the unix "stat" "st_ctime" field does not give
creation time, and that unix file systems do not record creation
time at all. Unix (and POSIX) implementations will normally not
include this fact.
8.5.5. The perm Fact
The perm fact is used to indicate access rights the current FTP user
has over the object listed. Its value is always an unordered
sequence of alphabetic characters.
perm-fact = "Perm" "=" *pvals
pvals = "a" / "c" / "d" / "e" / "f" /
"l" / "m" / "p" / "r" / "w"
There are ten permission indicators currently defined. Many are
meaningful only when used with a particular type of object. The
indicators are case independent, "d" and "D" are the same indicator.
The "a" permission applies to objects of type=file, and indicates
that the APPE (append) command may be applied to the file named.
The "c" permission applies to objects of type=dir (and type=pdir,
type=cdir). It indicates that files may be created in the directory
named. That is, that a STOU command is likely to succeed, and that
STOR and APPE commands might succeed if the file named did not
previously exist, but is to be created in the directory object that
has the "c" permission. It also indicates that the RNTO command is
likely to succeed for names in the directory.
The "d" permission applies to all types. It indicates that the
object named may be deleted, that is, that the RMD command may be
applied to it if it is a directory, and otherwise that the DELE
command may be applied to it.
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The "e" permission applies to the directory types. When set on an
object of type=dir, type=cdir, or type=pdir it indicates that a CWD
command naming the object should succeed, and the user should be able
to enter the directory named. For type=pdir it also indicates that
the CDUP command may succeed (if this particular pathname is the one
to which a CDUP would apply.)
The "f" permission for objects indicates that the object named may be
renamed - that is, may be the object of an RNFR command.
The "l" permission applies to the directory file types, and indicates
that the listing commands, LIST, NLST, and MLSD may be applied to the
directory in question.
The "m" permission applies to directory types, and indicates that the
MKD command may be used to create a new directory within the
directory under consideration.
The "p" permission applies to directory types, and indicates that
objects in the directory may be deleted, or (stretching naming a
little) that the directory may be purged. Note: it does not indicate
that the RMD command may be used to remove the directory named
itself, the "d" permission indicator indicates that.
The "r" permission applies to type=file objects, and for some
systems, perhaps to other types of objects, and indicates that the
RETR command may be applied to that object.
The "w" permission applies to type=file objects, and for some
systems, perhaps to other types of objects, and indicates that the
STOR command may be applied to the object named.
Note: That a permission indicator is set can never imply that the
appropriate command is guaranteed to work - just that it might.
Other system specific limitations, such as limitations on
available space for storing files, may cause an operation to
fail, where the permission flags may have indicated that it was
likely to succeed. The permissions are a guide only.
Implementation note: The permissions are described here as they apply
to FTP commands. They may not map easily into particular
permissions available on the server's operating system. Servers
are expected to synthesize these permission bits from the
permission information available from operating system. For
example, to correctly determine whether the "D" permission bit
should be set on a directory for a server running on the
UNIX(TM) operating system, the server should check that the
directory named is empty, and that the user has write permission
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on both the directory under consideration, and its parent
directory.
Some systems may have more specific permissions than those
listed here, such systems should map those to the flags defined
as best they are able. Other systems may have only more broad
access controls. They will generally have just a few possible
permutations of permission flags, however they should attempt to
correctly represent what is permitted.
8.5.6. The lang Fact
The lang fact describes the natural language of the filename for use
in display purposes. Values used here should be taken from the
language registry of the IANA. See [13] for the syntax, and
procedures, related to language tags.
lang-fact = "Lang" "=" token
Server-FTP implementations MUST NOT guess language values. Language
values must be determined in an unambiguous way such as file system
tagging of language or by user configuration. Note that the lang
fact provides no information at all about the content of a file, only
about the encoding of its name.
8.5.7. The size Fact
The size fact applies to non-directory file types and should always
reflect the approximate size of the file. This should be as accurate
as the server can make it, without going to extraordinary lengths,
such as reading the entire file. The size is expressed in units of
octets of data in the file.
Given limitations in some systems, Client-FTP implementations must
understand this size may not be precise and may change between the
time of a MLST and RETR operation.
Clients that need highly accurate size information for some
particular reason should use the SIZE command as defined in section
4. The most common need for this accuracy is likely to be in
conjunction with the REST command described in section 5. The size
fact, on the other hand, should be used for purposes such as
indicating to a human user the approximate size of the file to be
transferred, and perhaps to give an idea of expected transfer
completion time.
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size-fact = "Size" "=" 1*DIGIT
8.5.8. The media-type Fact
The media-type fact represents the IANA media type of the file named,
and applies only to non-directory types. The list of values used
must follow the guidelines set by the IANA registry.
media-type = "Media-Type" "=" <per IANA guidelines>
Server-FTP implementations MUST NOT guess media type values. Media
type values must be determined in an unambiguous way such as file
system tagging of media-type or by user configuration. This fact
gives information about the content of the file named. Both the
primary media type, and any appropriate subtype should be given,
separated by a slash "/" as is traditional.
8.5.9. The charset Fact
The charset fact provides the IANA character set name, or alias, for
the encoded pathnames in a MLSx response. The default character set
is UTF-8 unless specified otherwise. FTP implementations SHOULD use
UTF-8 if possible to encourage maximum interoperability. The value
of this fact applies to the pathname only, and provides no
information about the contents of the file.
charset-type = "Charset" "=" token
8.5.10. Required facts
Servers are not required to support any particular set of the
available facts. However, servers SHOULD, if conceivably possible,
support at least the type, perm, size, unique, and modify facts.
8.6. System Dependent and Local Facts
By using an system dependent fact, or a local fact, a server-PI may
communicate to the user-PI information about the file named which is
peculiar to the underlying file system.
8.6.1. System Dependent Facts
System dependent fact names are labeled by prefixing a label
identifying the specific information returned by the name of the
appropriate operating system from the IANA maintained list of
operating system names.
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The value of an OS dependent fact may be whatever is appropriate to
convey the information available. It must be encoded as a "token" as
defined in section 2.1 however.
In order to allow reliable interoperation between users of system
dependent facts, the IANA will maintain a registry of system
dependent fact names, their syntax, and the interpretation to be
given to their values. Registrations of system dependent facts are
to be accomplished according to the procedures of section 11.
8.6.2. Local Facts
Implementations may also make available other facts of their own
choosing. As the method of interpretation of such information will
generally not be widely understood, server-PIs should be aware that
clients will typically ignore any local facts provided. As there is
no registration of locally defined facts, it is entirely possible
that different servers will use the same local fact name to provide
vastly different information. Hence user-PIs should be hesitant
about making any use of any information in a locally defined fact
without some other specific assurance that the particular fact is one
that they do comprehend.
Local fact names all begin with the sequence "X.". The rest of the
name is a "token" (see section 2.1). The value of a local fact can
be anything at all, provided it can be encoded as a "token".
8.7. MLSx Examples
The following examples are all taken from dialogues between existing
FTP clients and servers. Because of this, not all possible
variations of possible response formats are shown in the examples.
This should not be taken as limiting the options of other server
implementors. Where the examples show OS dependent information, that
is to be treated as being purely for the purposes of demonstration of
some possible OS specific information that could be defined. As at
the time of the writing of this document, no OS specific facts or
file types have been defined, the examples shown here should not be
treated as in any way to be preferred over other possible similar
definitions. Consult the IANA registries to determine what types and
facts have been defined.
In the examples shown, only relevant commands and responses have been
included. This is not to imply that other commands (including
authentication, directory modification, PORT or PASV commands, or
similar) would not be present in an actual connection, or were not,
in fact, actually used in the examples before editing. Note also
that the formats shown are those that are transmitted between client
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and server, not formats which would normally ever be reported to the
user of the client.
In the examples, lines that begin "C> " were sent over the control
connection from the client to the server, lines that begin "S> " were
sent over the control connection from the server to the client, and
lines that begin "D> " were sent from the server to the client over a
data connection created just to send those lines and closed
immediately after. No examples here show data transferred over a
data connection from the client to the server. In all cases, the
prefixes shown above, including the one space, have been added for
the purposes of this document, and are not a part of the data
exchanged between client and server.
8.7.1. Simple MLST
C> PWD
S> 257 "/tmp" is current directory.
C> MLst cap60.pl198.tar.gz
S> 250- Listing cap60.pl198.tar.gz
S> Type=file;Size=1024990;Perm=r; /tmp/cap60.pl198.tar.gz
S> 250 End
The client first asked to be told the current directory of the
server. This was purely for the purposes of clarity of this example.
The client then requested facts about a specific file. The server
returned the "250-" first control-response line, followed by a single
line of facts about the file, followed by the terminating "250 "
line. The text on the control-response line and the terminating line
can be anything the server decides to send. Notice that the fact
line is indented by a single space. Notice also that there are no
spaces in the set of facts returned, until the single space before
the filename. The filename returned on the fact line is a fully
qualified pathname of the file listed. The facts returned show that
the line refers to a file, that file contains approximately 1024990
bytes, though more or less than that may be transferred if the file
is retrieved, and a different number may be required to store the
file at the client's file store, and the connected user has
permission to retrieve the file but not to do anything else
particularly interesting.
8.7.2. MLST of a directory
C> PWD
S> 257 "/" is current directory.
C> MLst tmp
S> 250- Listing tmp
S> Type=dir;Modify=19981107085215;Perm=el; /tmp
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S> 250 End
Again the PWD is just for the purposes of demonstration for the
example. The MLST fact line this time shows that the file listed is
a directory, that it was last modified at 08:52:15 on the 7th of
November, 1998 UTC, and that the user has permission to enter the
directory, and to list its contents, but not to modify it in any way.
Again, the fully qualified path name of the directory listed is
given.
8.7.3. MLSD of a directory
C> MLSD tmp
S> 150 BINARY connection open for MLSD tmp
D> Type=cdir;Modify=19981107085215;Perm=el; tmp
D> Type=cdir;Modify=19981107085215;Perm=el; /tmp
D> Type=pdir;Modify=19990112030508;Perm=el; ..
D> Type=file;Size=25730;Modify=19940728095854;Perm=; capmux.tar.z
D> Type=file;Size=1830;Modify=19940916055648;Perm=r; hatch.c
D> Type=file;Size=25624;Modify=19951003165342;Perm=r; MacIP-02.txt
D> Type=file;Size=2154;Modify=19950501105033;Perm=r; uar.netbsd.patch
D> Type=file;Size=54757;Modify=19951105101754;Perm=r; iptnnladev.1.0.sit.hqx
D> Type=file;Size=226546;Modify=19970515023901;Perm=r; melbcs.tif
D> Type=file;Size=12927;Modify=19961025135602;Perm=r; tardis.1.6.sit.hqx
D> Type=file;Size=17867;Modify=19961025135602;Perm=r; timelord.1.4.sit.hqx
D> Type=file;Size=224907;Modify=19980615100045;Perm=r; uar.1.2.3.sit.hqx
D> Type=file;Size=1024990;Modify=19980130010322;Perm=r; cap60.pl198.tar.gz
S> 226 MLSD completed
In this example notice that there is no leading space on the fact
lines returned over the data connection. Also notice that two lines
of "type=cdir" have been given. These show two alternate names for
the directory listed, one a fully qualified pathname, and the other a
local name relative to the servers current directory when the MLSD
was performed. Note that all other filenames in the output are
relative to the directory listed, though the server could, if it
chose, give a fully qualified path name for the "type=pdir" line.
This server has chosen not to. The other files listed present a
fairly boring set of files that are present in the listed directory.
Note that there is no particular order in which they are listed.
They are not sorted by filename, by size, or by modify time. Note
also that the "perm" fact has an empty value for the file
"capmux.tar.z" indicating that the connected user has no permissions
at all for that file. This server has chosen to present the "cdir"
and "pdir" lines before the lines showing the content of the
directory, it is not required to do so. The "size" fact does not
provide any meaningful information for a directory, so is not
included in the fact lines for the directory types shown.
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8.7.4. A more complex example
C> MLst test
S> 250- Listing test
S> Type=dir;Perm=el;Unique=keVO1+ZF4 test
S> 250 End
C> MLSD test
S> 150 BINARY connection open for MLSD test
D> Type=cdir;Perm=el;Unique=keVO1+ZF4; test
D> Type=pdir;Perm=e;Unique=keVO1+d?3; ..
D> Type=OS.unix=slink:/foobar;Perm=;Unique=keVO1+4G4; foobar
D> Type=OS.unix=chr-13/29;Perm=;Unique=keVO1+5G4; device
D> Type=OS.unix=blk-11/108;Perm=;Unique=keVO1+6G4; block
D> Type=file;Perm=awr;Unique=keVO1+8G4; writable
D> Type=dir;Perm=cpmel;Unique=keVO1+7G4; promiscuous
D> Type=dir;Perm=;Unique=keVO1+1t2; no-exec
D> Type=file;Perm=r;Unique=keVO1+EG4; two words
D> Type=file;Perm=r;Unique=keVO1+IH4; leading space
D> Type=file;Perm=r;Unique=keVO1+1G4; file1
D> Type=dir;Perm=cpmel;Unique=keVO1+7G4; incoming
D> Type=file;Perm=r;Unique=keVO1+1G4; file2
D> Type=file;Perm=r;Unique=keVO1+1G4; file3
D> Type=file;Perm=r;Unique=keVO1+1G4; file4
S> 226 MLSD completed
C> MLSD test/incoming
S> 150 BINARY connection open for MLSD test/incoming
D> Type=cdir;Perm=cpmel;Unique=keVO1+7G4; test/incoming
D> Type=pdir;Perm=el;Unique=keVO1+ZF4; ..
D> Type=file;Perm=awdrf;Unique=keVO1+EH4; bar
D> Type=file;Perm=awdrf;Unique=keVO1+LH4;
D> Type=file;Perm=rf;Unique=keVO1+1G4; file5
D> Type=file;Perm=rf;Unique=keVO1+1G4; file6
D> Type=dir;Perm=cpmdelf;Unique=keVO1+!s2; empty
S> 226 MLSD completed
For the purposes of this example the fact set requested has been
modified to delete the "size" and "modify" facts, and add the
"unique" fact. First, facts about a filename have been obtained via
MLST. Note that no fully qualified path name was given this time.
That was because the server was unable to determine that information.
Then having determined that the filename represents a directory, that
directory has been listed. That listing also shows no fully
qualified path name, for the same reason, thus has but a single
"type=cdir" line. This directory (which was created especially for
the purpose) contains several interesting files. There are some with
OS dependent file types, several sub-directories, and several
ordinary files.
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Not much can be said here about the OS dependent file types, as none
of the information shown there should be treated as any more than
possibilities. It can be seen that the OS type of the server is
"unix" though, which is one of the OS types in the IANA registry of
Operating System names.
Of the three directories listed, "no-exec" has no permission granted
to this user to access at all. From the "Unique" fact values, it can
be determined that "promiscuous" and "incoming" in fact represent the
same directory. Its permissions show that the connected user has
permission to do essentially anything other than to delete the
directory. That directory was later listed. It happens that the
directory can not be deleted because it is not empty.
Of the normal files listed, two contain spaces in their names. The
file called " leading space" actually contains two spaces in its
name, one before the "l" and one between the "g" and the "s". The
two spaces that separate the facts from the visible part of the path
name make that clear. The file "writable" has the "a" and "w"
permission bits set, and consequently the connected user should be
able to STOR or APPE to that file.
The other four file names, "file1", "file2", "file3", and "file4" all
represent the same underlying file, as can be seen from the values of
the "unique" facts of each. It happens that "file1" and "file2" are
Unix "hard" links, and that "file3" and "file4" are "soft" or
"symbolic" links to the first two. None of that information is
available via standard MLST facts, it is sufficient for the purposes
of FTP to note that all represent the same file, and that the same
data would be fetched no matter which of them was retrieved, and that
all would be simultaneously modified were data stored in any.
Finally, the sub-directory "incoming" is listed. Since "promiscuous"
is the same directory there would be no point listing it as well. In
that directory, the files "file5" and "file6" represent still more
names for the "file1" file we have seen before. Notice the entry
between that for "bar" and "file5". Though it is not possible to
easily represent it in this document, that shows a file with a name
comprising exactly three spaces (" "). A client will have no
difficulty determining that name from the output presented to it
however. The directory "empty" is, as its name implies, empty,
though that is not shown here. It can, however, be deleted, as can
file "bar" and the file whose name is three spaces. All the files
that reside in this directory can be renamed. This is a consequence
of the UNIX semantics of the directory that contains them being
modifiable.
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8.7.5. More accurate time information
C> MLst file1
S> 250- Listing file1
S> Type=file;Modify=19990929003355.237; file1
S> 250 End
In this example, the server-FTP is indicating that "file1" was last
modified 237 milliseconds after 00:33:55 UTC on the 29th of
September, 1999.
8.7.6. A different server
C> MLST
S> 250-Begin
S> type=dir;unique=AQkAAAAAAAABCAAA; /
S> 250 End.
C> MLSD .
S> 150 Opening ASCII mode data connection for MLS.
D> type=cdir;unique=AQkAAAAAAAABCAAA; /
D> type=dir;unique=AQkAAAAAAAABEAAA; bin
D> type=dir;unique=AQkAAAAAAAABGAAA; etc
D> type=dir;unique=AQkAAAAAAAAB8AwA; halflife
D> type=dir;unique=AQkAAAAAAAABoAAA; incoming
D> type=dir;unique=AQkAAAAAAAABIAAA; lib
D> type=dir;unique=AQkAAAAAAAABWAEA; linux
D> type=dir;unique=AQkAAAAAAAABKAEA; ncftpd
D> type=dir;unique=AQkAAAAAAAABGAEA; outbox
D> type=dir;unique=AQkAAAAAAAABuAAA; quake2
D> type=dir;unique=AQkAAAAAAAABQAEA; winstuff
S> 226 Listing completed.
C> MLSD linux
S> 150 Opening ASCII mode data connection for MLS.
D> type=cdir;unique=AQkAAAAAAAABWAEA; /linux
D> type=pdir;unique=AQkAAAAAAAABCAAA; /
D> type=dir;unique=AQkAAAAAAAABeAEA; firewall
D> type=file;size=12;unique=AQkAAAAAAAACWAEA; helo_world
D> type=dir;unique=AQkAAAAAAAABYAEA; kernel
D> type=dir;unique=AQkAAAAAAAABmAEA; scripts
D> type=dir;unique=AQkAAAAAAAABkAEA; security
S> 226 Listing completed.
C> MLSD linux/kernel
S> 150 Opening ASCII mode data connection for MLS.
D> type=cdir;unique=AQkAAAAAAAABYAEA; /linux/kernel
D> type=pdir;unique=AQkAAAAAAAABWAEA; /linux
D> type=file;size=6704;unique=AQkAAAAAAAADYAEA; k.config
D> type=file;size=7269221;unique=AQkAAAAAAAACYAEA; linux-2.0.36.tar.gz
D> type=file;size=12514594;unique=AQkAAAAAAAAEYAEA; linux-2.1.130.tar.gz
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S> 226 Listing completed.
Note that this server returns its "unique" fact value in quite a
different format. It also returns fully qualified path names for the
"pdir" entry.
8.7.7. Some IANA files
C> MLSD .
S> 150 BINARY connection open for MLSD .
D> Type=cdir;Modify=19990219183438; /iana/assignments
D> Type=pdir;Modify=19990112030453; ..
D> Type=dir;Modify=19990219073522; media-types
D> Type=dir;Modify=19990112033515; character-set-info
D> Type=dir;Modify=19990112033529; languages
D> Type=file;Size=44242;Modify=19990217230400; character-sets
D> Type=file;Size=1947;Modify=19990209215600; operating-system-names
S> 226 MLSD completed
C> MLSD media-types
S> 150 BINARY connection open for MLSD media-types
D> Type=cdir;Modify=19990219073522; media-types
D> Type=cdir;Modify=19990219073522; /iana/assignments/media-types
D> Type=pdir;Modify=19990219183438; ..
D> Type=dir;Modify=19990112033045; text
D> Type=dir;Modify=19990219183442; image
D> Type=dir;Modify=19990112033216; multipart
D> Type=dir;Modify=19990112033254; video
D> Type=file;Size=30249;Modify=19990218032700; media-types
S> 226 MLSD completed
C> MLSD character-set-info
S> 150 BINARY connection open for MLSD character-set-info
D> Type=cdir;Modify=19990112033515; character-set-info
D> Type=cdir;Modify=19990112033515; /iana/assignments/character-set-info
D> Type=pdir;Modify=19990219183438; ..
D> Type=file;Size=1234;Modify=19980903020400; windows-1251
D> Type=file;Size=4557;Modify=19980922001400; tis-620
D> Type=file;Size=801;Modify=19970324130000; ibm775
D> Type=file;Size=552;Modify=19970320130000; ibm866
D> Type=file;Size=922;Modify=19960505140000; windows-1258
S> 226 MLSD completed
C> MLSD languages
S> 150 BINARY connection open for MLSD languages
D> Type=cdir;Modify=19990112033529; languages
D> Type=cdir;Modify=19990112033529; /iana/assignments/languages
D> Type=pdir;Modify=19990219183438; ..
D> Type=file;Size=2391;Modify=19980309130000; default
D> Type=file;Size=943;Modify=19980309130000; tags
D> Type=file;Size=870;Modify=19971026130000; navajo
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D> Type=file;Size=699;Modify=19950911140000; no-bok
S> 226 MLSD completed
C> PWD
S> 257 "/iana/assignments" is current directory.
This example shows some of the IANA maintained files that are
relevant for this specification in MLSD format. Note that these
listings have been edited by deleting many entries, the actual
listings are much longer.
8.7.8. A stress test of case (in)dependence
The following example is intended to make clear some cases where case
dependent strings are permitted in the MLSx commands, and where case
independent strings are required.
C> MlsD .
S> 150 BINARY connection open for MLSD .
D> Type=pdir;Modify=19990929011228;Perm=el;Unique=keVO1+ZF4; ..
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+Bd8; FILE2
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+aG8; file3
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+ag8; FILE3
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+bD8; file1
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+bD8; file2
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+Ag8; File3
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+bD8; File1
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+Bd8; File2
D> Type=file;Size=4096;Modify=19990929011440;Perm=r;Unique=keVO1+bd8; FILE1
S> 226 MLSD completed
Note first that the "MLSD" command, shown here as "MlsD" is case
independent. Clients may issue this command in any case, or
combination of cases, they desire. This is the case for all FTP
commands.
Next, notice the labels of the facts. These are also case
independent strings, Server-FTP is permitted to return them in any
case they desire. User-FTP must be prepared to deal with any case,
though it may do this by mapping the labels to a common case if
desired.
Then, notice that there are nine objects of "type" file returned. In
a case independent NVFS these would represent three different file
names, "file1", "file2", and "file3". With a case dependent NVFS all
nine represent different file names. Either is possible, server-FTPs
may implement a case dependent or a case independent NVFS. User-FTPs
must allow for case dependent selection of files to manipulate on the
server.
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Lastly, notice that the value of the "unique" fact is case dependent.
In the example shown, "file1", "File1", and "file2" all have the same
"unique" fact value "keVO1+bD8", and thus all represent the same
underlying file. On the other hand, "FILE1" has a different "unique"
fact value ("keVO1+bd8") and hence represents a different file.
Similarly, "FILE2" and "File2" are two names for the same underlying
file, whereas "file3", "File3" and "FILE3" all represent different
underlying files.
That the approximate sizes ("size" fact) and last modification times
("modify" fact) are the same in all cases might be no more than a
coincidence.
It is not suggested that the operators of server-FTPs create NVFS
which stress the protocols to this extent, however both user and
server implementations must be prepared to deal with such extreme
examples.
8.8. FEAT response for MLSx
When responding to the FEAT command, a server-FTP process that
supports MLST, and MLSD, plus internationalization of pathnames, MUST
indicate that this support exists. It does this by including a MLST
feature line. As well as indicating the basic support, the MLST
feature line indicates which MLST facts are available from the
server, and which of those will be returned if no subsequent "OPTS
MLST" command is sent.
mlst-feat = SP "MLST" [SP factlist] CRLF
factlist = 1*( factname ["*"] ";" )
The initial space shown in the mlst-feat response is that required by
the FEAT command, two spaces are not permitted. If no factlist is
given, then the server-FTP process is indicating that it supports
MLST, but implements no facts. Only pathnames can be returned. This
would be a minimal MLST implementation, and useless for most
practical purposes. Where the factlist is present, the factnames
included indicate the facts supported by the server. Where the
optional asterisk appears after a factname, that fact will be
included in MLST format responses, until an "OPTS MLST" is given to
alter the list of facts returned. After that, subsequent FEAT
commands will return the asterisk to show the facts selected by the
most recent "OPTS MLST".
Note that there is no distinct FEAT output for MLSD. The presence of
the MLST feature indicates that both MLST and MLSD are supported.
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8.8.1. Examples
C> Feat
S> 211- Features supported
S> REST STREAM
S> MDTM
S> SIZE
S> TVFS
S> UTF8
S> MLST Type*;Size*;Modify*;Perm*;Unique*;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
Aside from some features irrelevant here, this server indicates that
it supports MLST including several, but not all, standard facts, all
of which it will send by default. It also supports two OS dependent
facts, and one locally defined fact. The latter three must be
requested expressly by the client for this server to supply them.
C> Feat
S> 211-Extensions supported:
S> CLNT
S> MDTM
S> MLST type*;size*;modify*;UNIX.mode*;UNIX.owner;UNIX.group;unique;
S> PASV
S> REST STREAM
S> SIZE
S> TVFS
S> Compliance Level: 19981201 (IETF mlst-05)
S> 211 End.
Again, in addition to some irrelevant features here, this server
indicates that it supports MLST, four of the standard facts, one of
which ("unique") is not enabled by default, and several OS dependent
facts, one of which is provided by the server by default. This
server actually supported more OS dependent facts. Others were
deleted for the purposes of this document to comply with document
formatting restrictions.
8.9. OPTS parameters for MLST
For the MLSx commands, the Client-FTP may specify a list of facts it
wishes to be returned in all subsequent MLSx commands until another
OPTS MLST command is sent. The format is specified by:
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mlst-opts = "OPTS" SP "MLST"
[ SP 1*( factname ";" ) ]
By sending the "OPTS MLST" command, the client requests the server to
include only the facts listed as arguments to the command in
subsequent output from MLSx commands. Facts not included in the
"OPTS MLST" command MUST NOT be returned by the server. Facts that
are included should be returned for each entry returned from the MLSx
command where they meaningfully apply. Facts requested that are not
supported, or which are inappropriate to the file or directory being
listed should simply be omitted from the MLSx output. This is not an
error. Note that where no factname arguments are present, the client
is requesting that only the file names be returned. In this case,
and in any other case where no facts are included in the result, the
space that separates the fact names and their values from the file
name is still required. That is, the first character of the output
line will be a space, (or two characters will be spaces when the line
is returned over the control connection,) and the file name will
start immediately thereafter.
Clients should note that generating values for some facts can be
possible, but very expensive, for some servers. It is generally
acceptable to retrieve any of the facts that the server offers as its
default set before any "OPTS MLST" command has been given, however
clients should use particular caution before requesting any facts not
in that set. That is, while other facts may be available from the
server, clients should refrain from requesting such facts unless
there is a particular operational requirement for that particular
information, which ought be more significant than perhaps simply
improving the information displayed to an end user.
Note, there is no "OPTS MLSD" command, the fact names set with the
"OPTS MLST" command apply to both MLST and MLSD commands.
Servers are not required to accept "OPTS MLST" commands before
authentication of the user-PI, but may choose to permit them.
8.9.1. OPTS MLST Response
The "response-message" from [6] to a successful OPTS MLST command has
the following syntax.
mlst-opt-resp = "MLST OPTS" [ SP 1*( factname ";" ) ]
This defines the "response-message" as used in the "opts-good"
message in RFC2389 [6].
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The facts named in the response are those which the server will now
include in MLST (and MLSD) response, after the processing of the
"OPTS MLST" command. Any facts from the request not supported by the
server will be omitted from this response message. If no facts will
be included, the list of facts will be empty. Note that the list of
facts returned will be the same as those marked by a trailing
asterisk ("*") in a subsequent FEAT command response. There is no
requirement that the order of the facts returned be the same as that
in which they were requested, or that in which they will be listed in
a FEAT command response, or that in which facts are returned in MLST
responses. The fixed string "MLST OPTS" in the response may be
returned in any case, or mixture of cases.
8.9.2. Examples
C> Feat
S> 211- Features supported
S> MLST Type*;Size;Modify*;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> OptS Mlst Type;UNIX.mode;Perm;
S> 201 MLST OPTS Type;Perm;UNIX.mode;
C> Feat
S> 211- Features supported
S> MLST Type*;Size;Modify;Perm*;Unique;UNIX.mode*;UNIX.chgd;X.hidden;
S> 211 End
C> opts MLst lang;type;charset;create;
S> 201 MLST OPTS Type;
C> Feat
S> 211- Features supported
S> MLST Type*;Size;Modify;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> OPTS mlst size;frogs;
S> 201 MLST OPTS Size;
C> Feat
S> 211- Features supported
S> MLST Type;Size*;Modify;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> opts MLst unique type;
S> 501 Invalid MLST options
C> Feat
S> 211- Features supported
S> MLST Type;Size*;Modify;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
For the purposes of this example, features other than MLST have been
deleted from the output to avoid clutter. The example shows the
initial default feature output for MLST. The facts requested are
then changed by the client. The first change shows facts that are
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available from the server being selected. Subsequent FEAT output
shows the altered features as being returned. The client then
attempts to select some standard features which the server does not
support. This is not an error, however the server simply ignores the
requests for unsupported features, as the FEAT output that follows
shows. Then, the client attempts to request a non-standard, and
unsupported, feature. The server ignores that, and selects only the
supported features requested. Lastly, the client sends a request
containing a syntax error (spaces cannot appear in the factlist.) The
server-FTP sends an error response and completely ignores the
request, leaving the fact set selected as it had been previously.
Note that in all cases, except the error response, the response lists
the facts that have been selected.
C> Feat
S> 211- Features supported
S> MLST Type*;Size*;Modify*;Perm*;Unique*;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> Opts MLST
S> 201 MLST OPTS
C> Feat
S> 211- Features supported
S> MLST Type;Size;Modify;Perm;Unique;UNIX.mode;UNIX.chgd;X.hidden;
S> 211 End
C> MLst tmp
S> 250- Listing tmp
S> /tmp
S> 250 End
C> OPTS mlst unique;size;
S> 201 MLST OPTS Size;Unique;
C> MLst tmp
S> 250- Listing tmp
S> Unique=keVO1+YZ5; /tmp
S> 250 End
C> OPTS mlst unique;type;modify;
S> 201 MLST OPTS Type;Modify;Unique;
C> MLst tmp
S> 250- Listing tmp
S> Type=dir;Modify=19990930152225;Unique=keVO1+YZ5; /tmp
S> 250 End
C> OPTS mlst fish;cakes;
S> 201 MLST OPTS
C> MLst tmp
S> 250- Listing tmp
S> /tmp
S> 250 End
C> OptS Mlst Modify;Unique;
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S> 201 MLST OPTS Modify;Unique;
C> MLst tmp
S> 250- Listing tmp
S> Modify=19990930152225;Unique=keVO1+YZ5; /tmp
S> 250 End
C> opts MLst fish cakes;
S> 501 Invalid MLST options
C> MLst tmp
S> 250- Listing tmp
S> Modify=19990930152225;Unique=keVO1+YZ5; /tmp
S> 250 End
This example shows the effect of changing the facts requested upon
subsequent MLST commands. Notice that a syntax error leaves the set
of selected facts unchanged. Also notice exactly two spaces
preceding the pathname when no facts were selected, either
deliberately, or because none of the facts requested were available.
9. Impact On Other FTP Commands
Along with the introduction of MLST, traditional FTP commands must be
extended to allow for the use of more than US-ASCII or EBCDIC
character sets. In general, the support of MLST requires support for
arbitrary character sets wherever filenames and directory names are
allowed. This applies equally to both arguments given to the
following commands and to the replies from them, as appropriate.
CWD
RETR
STOR
STOU
APPE
RNFR
RNTO
DELE
RMD
MKD
PWD
STAT
The arguments to all of these commands should be processed the same
way that MLST commands and responses are processed with respect to
handling embedded spaces, CRs and NULs. See section 2.2.
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10. Character sets and Internationalization
FTP commands are protocol elements, and are always expressed in
ASCII. FTP responses are composed of the numeric code, which is a
protocol element, and a message, which is often expected to convey
information to the user. It is not expected that users normally
interact directly with the protocol elements, rather the user FTP-
process constructs the commands, and interprets the results, in the
manner best suited for the particular user. Explanatory text in
responses generally has no particular meaning to the protocol. The
numeric codes provide all necessary information. Server-PIs are free
to provide the text in any language that can be adequately
represented in ASCII, or where an alternative language and
representation has been negotiated (see [7]) in that language and
representation.
Pathnames are expected to be encoded in UTF-8 allowing essentially
any character to be represented in a pathname. Meaningful pathnames
are defined by the server NVFS.
No restrictions at all are placed upon the contents of files
transferred using the FTP protocols. Unless the "media-type" fact is
provided in a MLSx response nor is any advice given here which would
allow determining the content type. That information is assumed to
be obtained via other means.
11. IANA Considerations
This specification makes use of some lists of values currently
maintained by the IANA, and creates two new lists for the IANA to
maintain. It does not add any values to any existing registries.
The existing IANA registries used by this specification are modified
using mechanisms specified elsewhere.
11.1. The OS specific fact registry
A registry of OS specific fact names shall be maintained by the IANA.
The OS names for the OS portion of the fact name must be taken from
the IANA's list of registered OS names. To add a fact name to this
OS specific registry of OS specific facts, an applicant must send to
the IANA a request, in which is specified the OS name, the OS
specific fact name, a definition of the syntax of the fact value,
which must conform to the syntax of a token as given in this
document, and a specification of the semantics to be associated with
the particular fact and its values. Upon receipt of such an
application, and if the combination of OS name and OS specific fact
name has not been previously defined, the IANA will add the
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specification to the registry.
Any examples of OS specific facts found in this document are to be
treated as examples of possible OS specific facts, and do not form a
part of the IANA's registry merely because of being included in this
document.
11.2. The OS specific filetype registry
A registry of OS specific file types shall be maintained by the IANA.
The OS names for the OS portion of the fact name must be taken from
the IANA's list of registered OS names. To add a file type to this
OS specific registry of OS specific file types, an applicant must
send to the IANA a request, in which is specified the OS name, the OS
specific file type, a definition of the syntax of the fact value,
which must conform to the syntax of a token as given in this
document, and a specification of the semantics to be associated with
the particular fact and its values. Upon receipt of such an
application, and if the combination of OS name and OS specific file
type has not been previously defined, the IANA will add the
specification to the registry.
Any examples of OS specific file types found in this document are to
be treated as potential OS specific file types only, and do not form
a part of the IANA's registry merely because of being included in
this document.
12. Security Considerations
This memo does not directly concern security. It is not believed
that any of the mechanisms documented here impact in any particular
way upon the security of FTP.
Implementing the SIZE command, and perhaps some of the facts of the
MDLx commands, may impose a considerable load on the server, which
could lead to denial of service attacks. Servers have, however,
implemented this for many years, without significant reported
difficulties.
With the introduction of virtual hosts to FTP, and the possible
accompanying multiple authentication environments, server
implementors will need to take some care to ensure that integrity is
maintained.
The FEAT and OPTS commands may be issued before the FTP
authentication has occurred [6]. This allows unauthenticated clients
to determine which of the features defined here are supported, and to
negotiate the fact list for MLSx output. No actual MLSx commands may
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be issued however, and no problems with permitting the selection of
the format prior to authentication are foreseen.
A general discussion of issues related to the security of FTP can be
found in [14].
13. References
[1] Coded Character Set--7-bit American Standard Code for Information
Interchange, ANSI X3.4-1986.
[2] Yergeau, F., "UTF-8, a transformation format of Unicode and ISO
10646", RFC 2044, October 1996.
[3] Postel, J., Reynolds, J., "File Transfer Protocol (FTP)",
STD 9, RFC 959, October 1985
[4] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997
[5] Crocker, D., Overell, P., "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997
[6] Hethmon, P., Elz, R., "Feature negotiation mechanism for the
File Transfer Protocol", RFC 2389, August 1998
[7] Curtin, W., "Internationalization of the File Transfer Protocol",
RFC 2640, July 1999
[8] Postel, J., Reynolds, J., "Telnet protocol Specification"
STD 8, RFC 854, May 1983
[9] Braden, R,. "Requirements for Internet Hosts -- Application
and Support", STD 3, RFC 1123, October 1989
[10] Mockapetris, P., "Domain Names - Concepts and Facilities"
STD 13, RFC 1034, November 1987
[11] ISO/IEC 10646-1:1993 "Universal multiple-octet coded character set
(UCS) -- Part 1: Architecture and basic multilingual plane",
International Standard -- Information Technology, 1993
[12] Internet Assigned Numbers Authority. http://www.iana.org
Email: iana@iana.org.
[13] Alvestrand, H., "Tags for the Identification of Languages"
RFC 1766, March 1995
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[14] Allman, M., Ostermann, S., "FTP Security Considerations"
RFC 2577, May 1999
Acknowledgments
This document is a product of the FTPEXT working group of the IETF.
The following people are among those who have contributed to this
document:
Alex Belits
D. J. Bernstein
Dave Cridland
Martin J. Duerst
Mike Gleason
Mark Harris
Alun Jones
James Matthews
Luke Mewburn
Jan Mikkelsen
Keith Moore
Buz Owen
Mark Symons
Stephen Tihor
and the entire FTPEXT working group of the IETF.
Apologies are offered to any inadvertently omitted.
Bernhard Rosenkraenzer suggested the HOST command, and initially
described it.
The description of the modifications to the REST command and the MDTM
and SIZE commands comes from a set of modifications suggested for
RFC959 by Rick Adams in 1989. A draft containing just those
commands, edited by David Borman, has been merged with this document.
Mike Gleason provided access to the FTP server used in some of the
examples.
All of the examples in this document are taken from actual
client/server exchanges, though some have been edited for brevity, or
to meet document formatting requirements.
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Copyright
This document is in the public domain. Any and all copyright
protection that might apply in any jurisdiction is expressly
disclaimed.
Editors' Addresses
Robert Elz
University of Melbourne
Department of Computer Science
Parkville, Vic 3052
Australia
Email: kre@munnari.OZ.AU
Paul Hethmon
Hethmon Brothers
2305 Chukar Road
Knoxville, TN 37923 USA
Phone: +1 423 690 8990
Email: phethmon@hethmon.com
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