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Easily Parsed LIST Format (EPLF)
INTERNET-DRAFT draft-bernstein-eplf-02.txt (expires 1 August 1997)
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Status of this memo
This memo provides information for the Internet community. This memo
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Abstract
The File Transfer Protocol (FTP) supports two commands that list
files: NLST and LIST. The NLST response is easy to parse but provides
very little information. The LIST response provides more information,
but in a format that varies from system to system. The most common
LIST formats are undocumented and impossible to parse reliably.
This document defines Easily Parsed LIST Format (EPLF), a format
for the LIST response that is usable by humans yet easy for programs
to handle. This format is supported by anonftpd, a secure FTP server.
One visible advantage of EPLF is that a browser can easily display
dates in the viewer's time zone and native language. EPLF also makes
it straightforward for an indexing program to automatically traverse
an FTP area and for a mirroring program to avoid downloading the same
file twice.
Easily Parsed LIST Format (EPLF)
D. J. Bernstein, djb@pobox.com
19970201
1. Introduction
The File Transfer Protocol (FTP) supports two commands that list
files: NLST and LIST. The NLST response is easy to parse but provides
very little information. The LIST response provides more information,
but in a format that varies from system to system. The most common
LIST formats are undocumented and impossible to parse reliably.
This document defines Easily Parsed LIST Format (EPLF), a format
for the LIST response that is usable by humans yet easy for programs
to handle. This format is supported by anonftpd, a secure FTP server.
One visible advantage of EPLF is that a browser can easily display
dates in the viewer's time zone and native language. EPLF also makes
it straightforward for an indexing program to automatically traverse
an FTP area and for a mirroring program to avoid downloading the same
file twice.
EPLF also corrects a design flaw in FTP's handling of LIST arguments.
An EPLF server must respond to ``LIST filename'' with information
about that file and no others, even if that file is a directory. A
client that wants an EPLF list of the contents of a directory must
first CWD to that directory. A client that merely wants a list of
file names in a different directory may use NLST.
In this document, a string of 8-bit bytes may be written in two
different forms: as a series of hexadecimal numbers between angle
brackets, or as a sequence of ASCII characters between double quotes.
For example, <68 65 6c 6c 6f 20 77 6f 72 6c 64 21> is a string of
length 12; it is the same as the string "hello world!".
2. Format
An EPLF response to LIST is a series of lines, each line specifying a
different file. Each line begins with "+", continues with a series of
facts about the file, and ends with <09> followed by the file name.
Each fact is zero or more bytes of information, terminated by "," and
not containing <09>.
There are several possible facts, each of which appears at most once,
in any order:
"r"
If this file name is supplied in a RETR command, the RETR
should succeed. The server must supply this fact unless it is
aware of file type problems, permission problems, or other
reasons that RETR will fail. The presence of "r" does not
guarantee success: for example, the file may be removed or
renamed, or the RETR may suffer a temporary failure.
"/"
If this file name is supplied in a CWD command, the CWD should
succeed. As with "r", the server must supply this fact unless
it is aware of reasons that CWD will fail. The presence of "/"
does not guarantee success.
"i"[ident]
This file has identifier [ident]. [ident] is a sequence of
bytes not including "," or <09>. If two files on the same FTP
server (not necessarily in the same LIST response) have the
same [ident], those files have the same contents; a successful
RETR of each file should produce the same results, and a
successful CWD to each file should lead to the same working
directory. (Under UNIX, for example, [dev].[ino] could be used
as [ident], where [dev] and [ino] are the device number and
inode number of the file.)
"s"[size]
The size of this file is [size]. [size] is a sequence of ASCII
digits specifying a number. If the file is retrieved in TYPE I
and is not modified, it will contain exactly [size] bytes. This
fact should not be supplied if "r" is not supplied.
"m"[time]
This file was last modified at [time]. [time] is a sequence of
ASCII digits specifying a number of seconds, real time, since
the beginning of 1970 GMT. This fact cannot be used for files
modified before 1970 GMT.
Further facts may be defined in the future. Pieces of the fact-space
beginning with "x" will be parcelled out to organizations that would
like to define their own facts. Facts beginning with "X" are reserved
for experimental use.
All facts other than "/" and "r" are optional. Any statement of
adherence to EPLF by a server FTP implementation must include a list
of facts supported by that implementation other than "/" and "r".
The server is under no obligation to ensure that LISTs in different
directories produce disjoint lists of targets. For example, some
servers may list a special ".." name that refers to the parent
directory, or a "/" name that refers to the top directory. To avoid
loops, a client attempting to traverse the FTP area must notice that
the identifiers of these directories are the same as identifiers of
directories already traversed.
The server is also under no obligation to list all possible targets
of RETR or CWD in a LIST command. Some servers may avoid listing
special names such as ".." or "/". A client that wishes to return to
a directory must use PWD and record the reply rather than relying on
any useful meaning of CDUP, CWD .., or CWD /.
Operating systems support a wide variety of means for obtaining the
contents of a file from its name. For example, many systems support
symbolic links: if ONE is a link to TWO, any reference to ONE is
first replaced by a reference to TWO. Such information is irrelevant
to FTP and is not displayed by any of the above facts. (Under UNIX
this means that the server should use stat(), not lstat().)
Servers are permitted to use arbitrary characters in file names,
except for <0a> and <0d>. Beware that the characters <00>, <09>,
<20>, and <ff> cause all sorts of trouble, ranging from inadequacies
in the syntax of FTP commands to misinterpretation by some clients.
3. Examples
Here is a typical EPLF response:
"+i8388621.48594,m825718503,r,s280," <09> "djb.html" <0d 0a>
"+i8388621.50690,m824255907,/," <09> "514" <0d 0a>
"+i8388621.48598,m824253270,r,s612," <09> "514.html" <0d 0a>
A typical EPLF-ignorant client will show the response to the user:
ftp> dir
200 Okay.
150 I'm looking through the directory. Trying to connect...
+i8388621.48594,m825718503,r,s280, djb.html
+i8388621.50690,m824255907,/, 514
+i8388621.48598,m824253270,r,s612, 514.html
226 Finished transferring 127 bytes.
ftp>
A more sophisticated client (in the Pacific timezone) might instead
display the following human-readable listing:
Tue Feb 13 15:58:27 1996 514/
612 bytes Tue Feb 13 15:14:30 1996 514.html
280 bytes Fri Mar 1 14:15:03 1996 djb.html
4. Sample code
The following C function takes a pointer to a string containing one
line of an EPLF response. It assumes that the original response did
not contain <00>, and that the trailing <0d 0a> has been replaced by
<00>. It returns a pointer to the filename, or 0 if the line does not
appear to be an EPLF response.
char *eplf_name(line) char *line;
{
if (*line != 43) return 0;
while (*line) if (*line++ == 9) return line;
return 0;
}
The following C function takes a pointer as above, and prints a
human-readable listing as shown in section 3. It assumes that the
local character set is ASCII, that file modification times fit into a
local time_t, and that file sizes fit into a local unsigned long. It
also assumes that time_t is interpreted as a number of seconds since
the beginning of 1970 GMT. (A more portable function could use
mktime() to discover the time_t representation of 1970 GMT.) Note
that its output is not machine-readable, since the file name might
contain the local newline sequence.
#include <time.h>
int eplf_readable(line) char *line;
{
int flagcwd = 0; time_t when = 0;
int flagsize = 0; unsigned long size;
if (*line++ != '+') return 0;
while (*line)
switch (*line)
{
case '\t':
if (flagsize) printf("%10lu bytes ",size);
else printf(" ");
if (when) printf("%24.24s",ctime(&when));
else printf(" ");
printf(" %s%s\n",line + 1,flagcwd ? "/" : "");
return 1;
case 's':
flagsize = 1; size = 0;
while (*++line && (*line != ','))
size = size * 10 + (*line - '0');
break;
case 'm':
while (*++line && (*line != ','))
when = when * 10 + (*line - '0');
break;
case '/':
flagcwd = 1;
default:
while (*line) if (*line++ == ',') break;
}
return 0;
}
5. Acknowledgments
Thanks to Scott Schwartz for pointing out that "i"[ident] was
originally overspecified. Thanks to Benjamin Riefenstahl for
several helpful suggestions.
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