6. 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. This 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 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.
6.1. TVFS File Names
Where a server implements the TVFS, no elementary file name 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
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  may be used in a TVFS file name. When transmitted,
file name characters are encoded using the UTF-8 encoding . Note
that the two-character sequence CR LF occurring in a file name will
make that name impossible to transmit over a data connection.
Consequently, it should be avoided, or if that is impossible to
achieve, it MUST be encoded in some reversible way.
6.2. TVFS Pathnames
A TVFS "Pathname" 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
pathname. Such a pathname 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 pathnames are constructed by prefixing a pathname by a
name of a directory from which the path is valid, and separating the
two with the "/" character. Such a pathname is valid to reference
the file or directory from the directory containing the newly added
Where a pathname has been extended to the point where the directory
added is the unnamed root directory, the pathname will begin with the
"/" character. Such a path is known as a fully qualified pathname.
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
pathname 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 pathname that is not a fully qualified pathname may be referred
to as a "relative pathname" and will only correctly reference the
intended file when the current working directory of the server-FTP is
a directory from which the relative pathname is valid.
As a special case, the pathname "/" is defined to be a fully
qualified pathname referring to the root directory. That is, the
root directory does not have a directory (or file) name, but does
have a pathname. This special pathname may be used only as is as a
reference to the root directory. It may not be combined with other
pathnames using the rules above, as doing so would lead to a pathname
containing two consecutive "/" characters, which is an undefined
+ It is not required, or expected, that there be only one fully
qualified pathname that will reference any particular file or
+ 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 pathname 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
for such names.
+ Similarly, other than the special case path that refers to the
root directory, no TVFS pathname 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 pathnames 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 that
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.
6.3. FEAT Response for TVFS
In response to the FEAT command  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
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:
S> 211- <any descriptive text>
S> 211 end
The ellipses indicate place holders where other features may be
included, but are not required. The one-space indentation of the
feature lines is mandatory  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.
6.4. OPTS for TVFS
There are no options in this TVFS specification, and hence there is
no OPTS command defined.
6.5. 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. This structure may depicted graphically as...
/ | \ \
/ | \ \
A X B Y
/|\ / \
/ | \ / \
C D Z P Q
Given this structure, the following fully qualified pathnames exist.
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
pathnames (/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 pathnames, in addition to all the fully qualified
pathnames, are valid
These all refer to the same files or directories as the corresponding
fully qualified path with "/A/" prepended.
That those pathnames 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 pathnames will
necessarily be granted equal rights.
None of the following relative paths are valid when the current
directory is /A
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
7. 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 and MLSD. MLST provides data about
exactly the object named on its command line, and no others. MLSD,
on the other, lists the contents of a directory if a directory is
named, otherwise a 501 reply is 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 STD 9, RFC 959  and STD 3, RFC 1123  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.
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.
7.1. 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
file name. For these purposes, a "file name" is the name of any
entity in the server NVFS which is not a directory. Where TVFS is
supported, any TVFS relative pathname valid in the current working
directory, or any TVFS fully qualified pathname, 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
file or directory names (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.
Note that omitting the argument is the only defined way to obtain a
listing of the current directory, unless a pathname that represents
the directory happens to be known. In particular, there is no
defined shorthand name for the current directory. This does not
prohibit any particular server-PI implementing such a shorthand.
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
7.2. 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 ) /
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 = *SCHAR
Upon receipt of an 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
If the parameter is 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. In
this case, the pathname component of every response MUST be
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 STD 9, RFC 959 . 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 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
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
The complete format for a successful reply to the MLSD command would
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 pathname 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. See section 7.9 (page 51).
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
7.2.1. Error Responses to MLSx commands
Many of the 4xy and 5xy responses defined in section 4.2 of STD 9,
RFC 959  are possible in response to the MLST and MLSD commands.
In particular, syntax errors can generate 500 or 501 replies. Giving
a pathname that exists but is not a directory as the argument to a
MLSD command generates a 501 reply. Giving a name that does not
exist, or for which access permission (to obtain directory
information as requested) is not granted will elicit a 550 reply.
Other replies (530, 553, 503, 504, and any of the 4xy replies) are
also possible in appropriate circumstances.
7.3. File Name Encoding
An FTP implementation supporting the MLSx commands must be 8-bit
clean. This is necessary in order to transmit UTF-8 encoded file
names. This specification recommends the use of UTF-8 encoded file
names. FTP implementations SHOULD use UTF-8 whenever possible to
encourage the maximum inter-operability.
File names 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 file
names 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 file name encoding for FTP may be found in
"Internationalization of the File Transfer Protocol" .
7.3.1. Notes about the File Name
The file name 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 file name 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 pathname of the current directory SHOULD
File names 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 file names
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 pathname of the directory, or MAY be any other
pathname that 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.
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
Alternatively, whether TVFS is supported or not, the user-PI can
issue a CWD command () 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 file
name components of the listing.
7.4. 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)
7.5. 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 file name per IANA  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
Further operating system specific keywords could be specified by
using the IANA operating system name as a prefix (examples only):
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.
x.ver -- Version information
x.desc -- File description
x.type -- File type
7.5.1. 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" /
The value of the type fact (the "type-val") is a case independent
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.
The type=cdir fact indicates the listed entry contains a pathname of
the directory whose contents are listed. An entry of this type will
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 pathnames with which to refer to the files and directories
returned in the same MLSD output (see section 6.2). These pathnames
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 pathname 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 that was listed in its current working directory.
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.
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
pathname, or a fully qualified pathname. A relative pathname will be
relative to the directory being listed, not to the current directory
of the server-PI at the time.
For the purposes of this type value, a "parent directory" is any
directory in which there is an entry of type=dir that 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 and supplied) can be used
to determine whether there is a relationship between the type=pdir
entries or not.
188.8.131.52. 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-kind
os-name = <an IANA registered operating system name>
os-kind = token
The "os-name" indicates the specific system type that supports the
particular localtype. OS specific types are registered by the IANA
using the procedures specified in section 10. The "os-kind" provides
the system dependent information as to the type of the file listed.
The os-name and os-kind 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 that is
essentially an indirect pointer to another file, the NVFS
representation of that type should normally be to represent the file
that 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.
184.108.40.206. 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
within it to be referenced. When this is done, the pathname returned
with each entry MUST be identical to the others representing the same
7.5.2. 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 that 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
7.5.3. 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
For directories, this value should alter whenever a change occurs to
the directory such that different file names would (or might) be
included in MLSD output of that directory.
modify-fact = "Modify" "=" time-val
7.5.4. 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.
7.5.5. 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.
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 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.
7.5.6. The lang Fact
The lang fact describes the natural language of the file name for use
in display purposes. Values used here should be taken from the
language registry of the IANA. See  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.
7.5.7. 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
size-fact = "Size" "=" 1*DIGIT
7.5.8. 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.
7.5.9. 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 inter-operability. The value
of this fact applies to the pathname only, and provides no
information about the contents of the file.
charset-type = "Charset" "=" token
7.5.10. 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.