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rfc:rfc691

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

                                                      Brian Harvey
                                                             SU-AI

Re: File Transfer Protocol May 28, 1975 Ref: RFC 354, 385, 414, 448, 454, 630, 542, 640 1

                     One More Try on the FTP                         2
 This is a slight revision of RFC 686, mainly differing in the
 discussion of print files.  Reading several RFCs that I (sigh)
 never heard of before writing 686 has convinced me that although
 I was right all along it was for the wrong reasons.  The list of
 reply codes is also slightly different to reflect the four lists
 in RFCs 354, 454, 542, and 640 more completely.  Let me also
 suggest that if there are no objections before June 1, everyone
 take it as official that HELP should return 200, that SRVR should
 be used as discussed below, and that "permanent" 4xx errors be
 changed to 5xx.  And thanks to Jon Postel who just spent all
 evening helping me straighten this all out.                        2a
 Aside from a cry of anguish by the site responsible for the
 security hassle described below, I've only had one comment on
 this, which was unfavorable but, alas, unspecific.  Let me just
 say, in the hopes of avoiding more such, that I am not just
 trying to step on toes for the fun of it, and that I don't think
 the positive changes to FTP-1 proposed here are necessarily the
 best possible thing.  What they are, I think, is easily doable.
 The great-FTP-in-the-sky isn't showing any signs of universal
 acceptability, and it shouldn't stand in the way of solving
 immediate problems.                                                2b
                    Leaving Well Enough Alone                        3

I recently decided it was time for an overhaul of our FTP user and server programs. This was my first venture into the world of network protocols, and I soon discovered that there was a lot we were doing wrong–and a few things that everyone seemed to be doing differently from each other. When I enquired about this, the response from some quarters was "Oh, you're running Version 1!" 4

Since, as far as I can tell, all but one network host are running version 1, and basically transferring files OK, it seems to me that the existence on paper of an unused protocol should not stand in the way of maintaining the current one unless there is a good reason to

                                 1

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

believe that the new one is either imminent or strongly superior or both. (I understand, by the way, that FTP-2 represents a lot of thought and effort by several people who are greater network experts than I, and that it isn't nice of me to propose junking all that work, and I hereby apologize for it.) Let me list what strike me as the main differences in FTP-2 and examine their potential impact on the world. 5

 1.  FTP-2 uses TELNET-2.  The main advantage of the new Telnet
 protocol is that it allows flexible negotiation about things like
 echoing.  But the communicators in the case of FTP are computer
 programs, not people, and don't want any echoing anyway.  The
 argument that new hosts might not know about old Telnet seems an
 unlikely one for quite some time to come; if TELNET-2 ever does
 really take over the world, FTP-1 could be implemented in it.      5a
 2.  FTP-2 straightens out the "print file" mess.  First of all,
 there are two separate questions here: what command one ought to
 give to establish a print file transfer, and which end does what
 sort of conversion.  For the second question, although all of the
 FTP-1 documents are confusing on the subject, I think it is
 perfectly obvious what to do: if the user specifies, and the
 server accepts, an ASCII or EBCDIC print file transfer parameter
 sequence, then the data sent over the network should contain
 Fortran control characters.  That is, the source file should
 contain Fortran controls, and should be sent over the net as is,
 and reformatted if necessary not by the SERVER as the protocol
 says but by the RECIPIENT (server for STOR, user for RETR). (The
 "Telnet print file" non-issue will be debunked below.)
 As a non-Fortran-user I may be missing something here but I don't
 think so; it is just like the well-understood TYPE E in which the
 data is sent in EBCDIC and the recipient can format it for local
 use as desired.  One never reformats a file from ASCII to EBCDIC
 at the sending end.  Perhaps the confusion happened because the
 protocol authors had in mind using these types to send files
 directly to a line printer at the server end, and indeed maybe
 that's all it's good for and nobody's user program will implement
 TYPE P RETR.                                                       5b
 As for the specific commands used to negotiate such a transfer,
 there may currently be some confusion because the most recent
 FTP-1 document on the subject (RFC 454) invents a new command,
 FORM, which is not in general use as far as I know.  (Most of my
                                 2

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

 experiments have been on PDP-10s; perhaps other systems have
 adopted this command.)  FTP-2 puts the format argument in the
 TYPE command as a second argument. Either way, using a
 two-dimensional scheme to specify the combinations of
 ASCII/EBCDIC and ASA/normal conveys no more information than the
 present A-P-E-F scheme.  FTP-2 also introduces the notion of
 Telnet formatted vs. non-print files.  These types are used when
 a Telnet format oriented system is sending a file to an ASA
 oriented one, and the recipient needs to know, not what is coming
 over the net, but how to solve a local file storage problem.  It
 is unnecessary and unfair for hosts to have to negotiate
 something which does not acttually affect what gets sent over the
 net.  It is unnecessary because the sending user process (there
 is no problem if the user process is receiving) need not
 understand what the issue is, it need only make the server
 understand by transmitting a message from the human user to the
 server process.  Any TYPE parameter must be understood by both
 processes even if the user treats it just like some other type.    5c
 To take a specific example, if I want to send an ASCII file to a
 360, my FTP user program needs to have built into it the
 knowledge that there are two TYPEs which are really the same, AN
 and AT in the FTP-2 notation. If tomorrow someone needs to know
 the ultimate use of a binary file (for instance, the old PDP-6
 DECtape format stores dump files differently from ordinary data
 files), I will have to add another piece of information to my FTP
 user and server (maybe they try to read such a file from me).
 Instead, information which affects only the RECIPIENT of a file,
 and not the format AS SENT OVER THE NET, should be specified in
 some form which the sending process can ignore.  This is what the
 SRVR command should be used for.                                   5d
 If a user at a 360 wants to retrieve a "Telnet print file" from
 another system, he might tell his FTP user process something like  5e
    TYPE A
    DISP PRINT
    RETR FOO etc.                                                  5e1
 (or whatever syntax they use in their FTP).  If a user at a 10
 wants to send such a file to a 360, he would say                   5f
    TYPE A
                                 3

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

    SRVR PRINT
    STOR FOO etc.                                                  5f1
 His FTP user program would send on the SRVR command without
 comment. Suppose that the transformation is one which might be
 used in either direction between the same two hosts.  (This is
 not the case for the Telnet print file thing because two 360s
 would be using ASA format.)  Then the user process could accept
 the equivalent of DISP PRINT from the user, and if the transfer
 turned out to be a STOR it would decide to send SRVR PRINT first.
 In this way the FTP user program can be written so that the human
 user types the same command regardless of the direction of
 transfer.                                                          5g
 Thus, FTP servers which care about the distinction between Telnet
 print and non-print could implement SRVR N and SRVR T.  Ideally
 the SRVR parameters should be registered with Jon Postel to avoid
 conflicts, although it is not a disaster if two sites use the
 same parameter for different things.  I suggest that parameters
 be allowed to be more than one letter, and that an initial letter
 X be used for really local idiosyncracies.  The following should
 be considered as registered:                                       5h
    T - Telnet print file                                          5h1
    N - Normal.                                                    5h2
       Means to turn off any previous SRVR in effect. (This makes
       "non-print" the default case, rather than
       making "Telnet print" and "non-print" equal.  It is
       probably a good idea if a user program can count on
       being able to turn off an earlier SRVR without having
       to know a specific inverse for it.  Servers which do not
       implement any other SRVR parameters need not implement
       SRVR N either; user processes shouldn't send SRVR N
       just for the hell of it.)
 3.  FTP-2 reshuffles reply codes somewhat.  There have been four
 attempts altogether, that I know of, at specifying a list of
 reply codes: RFCs 354 and 454 for FTP-1, and RFCs 542 and 640 for
 FTP-2. There is not much to choose from among the first three of
 these, which are basically the same, except for a slight increase
 in specificity each time through, e.g., the introduction of reply
                                 4

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

 code 456 for a rename which fails because a file of the same
 (new) name already exists.  This increased specificity of reply
 codes doesn't seem to be much of a virtue; if a rename operation
 fails, it is the human user, not the FTP user program, who needs
 to know that it was because of a name conflict rather than some
 other file system error.  I am all for putting such information
 in the text part of FTP replies.  Some real problems are actually
 addressed in the reply code revision of RFC 640, in which the
 basic scheme for assigning reply code numbers is more rational
 than either the FTP-1 scheme or the original FTP-2 scheme.
 However, I think that most of the benefits of RFC 640 can be
 obtained in a way which does not require cataclysmic
 reprogramming.  More on this below.                                5i
 4.  FTP-2 was established by a duly constituted ARPAnet committee
 and we are duty-bound to implement it.  I don't suppose anyone
 would actually put it that baldly, but I've heard things which
 amounted to that.  It's silly.                                     5j
 5.  FTP-2 specifies default sockets for the data connection.
 Most places use the default sockets already anyway, and it is
 easy enough to ignore the 255 message if you want to.  This is a
 security issue, of course, and I'm afraid that I can't work up
 much excitement about helping the CIA keep track of what anti-war
 demonstrations I attended in 1968 and which Vietnamese hamlets to
 bomb for the greatest strategic effect even if they do pay my
 salary indirectly.  I could rave about this subject for pages,
 and probably will if I ever get around to writing an argument
 against MAIL-2, but for now let me just get one anecdote off my
 chest:  I have access to an account at an ARPAnet host because I
 am responsible at my own site for local maintenance of a program
 which was written by, and is maintained by, someone at the other
 site.  However, the other site doesn't really trust us outsiders
 (the account is shared by people in my position at several other
 hosts) to protect their vital system security, so every week they
 run a computer program to generate a new random password for the
 account (last week's was HRHPUK) and notify us all by network
 mail.  Well, on my system and at least one of the others, that
 mail isn't read protected.  I delete my mail when I read it, but
 since it is hard enough remembering HRHPUK without them changing
 it every week, I naturally write it in a file on our system.
 That file could in principle be read protected but it isn't,
 since sometimes I'm in someone else's office when I want to use
                                 5

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

 it, and the other passwords in it are for open guest accounts
 which are widely known.  Moral #1: Security freaks are pretty
 weird.  Moral #2: If you have a secret don't keep it on the
 ARPAnet.  (In the past week I have heard about two newly
 discovered holes in TENEX security.)                               5k
 6.  FTP-2 is available online and FTP-1 isn't, so new hosts can't
 find out how to do it.  Aargh!!!  What a reason for doing
 anything! Surely it would be less costly for someone to type it
 in again than for everyone to reprogram.  Meanwhile these new
 hosts can ask Jon or Geoff or Bobby or even me for help in
 getting FTP up.                                                    5l
 7.  FTP-2 has some changes to the strange MODEs and STRUs.  This
 is another thing I can't get too excited about.  We support only
 MODE S and STRU F and that will probably still be true even if we
 are forced into FTP-2.  If the relatively few people who do very
 large file transfers need to improve the restart capability, they
 can do so within FTP-1 without impacting the rest of us.  The
 recent implementation of paged file transfers by TENEX shows that
 problems of individual systems can be solved within the FTP-1
 framework. If the IBM people have some problem about record
 structure in FTP-1, for example, let them solve it in FTP-1, and
 whatever the solution is, nobody who isn't affected has to
 reprogram.                                                         5m

Well, to sum up, I am pretty happy with the success I've had transferring files around the network the way things are. When I do run into trouble it's generally because some particular host hasn't implemented some particular feature of FTP-1, and there's no reason to suppose they'll do it any faster if they also have to convert to FTP-2 at the same time. The main thing about FTP-2, as I said at the beginning, is that its existence is an excuse for not solving problems in FTP-1. Some such problems are quite trivial except for the fact that people are reluctant to go against anything in the protocol document, as if the latter were the Holy Writ. A few actually require some coordinated effort. Here is my problem list: 6

 1.  It is almost true that an FTP user program can understand
 reply codes by the following simple algorithm:                     6a
    a. Replies starting with 0 or 1 should be typed out and
    otherwise ignored.                                             6a1
                                 6

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

    b. Replies starting with 2 indicate success (of this step or
    of the whole operation, depending on the command).             6a2
    c. Replies starting with 4 or 5 indicate failure of the
    command.                                                       6a3
    d. Replies starting with 3 are only recognized in three cases:
    the initial 300 message, the 330 password request, and the
    350 MAIL response.  (Note that the user program need not
    distinguish which 300 message it got, merely whether or not it
    is expecting one right now.)                                   6a4
 The only real problem with this, aside from bugs in a few servers
 whose maintainers tell me they're working on it, is the HELP
 command, which is not in the original protocol and which returns
 0xx, 1xx, or 2xx depending on the server.  (Sometimes more than
 one message is returned.) The word from one network protocol
 expert at BBN is that (a) 050 or 030 is the correct response to
 HELP, and (b) there is a perfectly good mechanism in the protocol
 for multi-line responses.  Unfortunately this does not do much
 good in dealing with reality.  There seems to be a uniform
 procedure for handling the STAT command:                           6b
    151 information
    151 information
    151 ...
    151 information
    200 END OF STATUS                                              6b1
 which fits right in with the above algorithm.  This is despite
 the fact that 1xx is supposed to constitute a positive response
 to a command like STAT, so that according to RFC 354 it ought to
 be                                                                 6c
    151-information
    information
    ...
    151 information                                                6c1
 instead.  RFC 414, which approves of the 200 reply for STAT, also
 gives 200 for HELP.  (It seems to me, by the way, that 050 and
 030 aren't good enough as responses to HELP since they
 "constitute neither a positive nor a negative acknowledgement" of
                                 7

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

 the HELP command and thus don't tell the user program when it
 ought to ask the human user what to do next.)  I suggest that,
 despite RFC 354, a 200 response be given by all servers at the
 end of whatever other HELP it gives as of, let's say, June 1.
 The alternatives are either to let the current rather chaotic
 situation continue forever while waiting for FTP-2, or to try to
 standardize everyone on a multi-line 1xx for both HELP and STAT.
 I'm against changing STAT, which works perfectly for everyone as
 far as I can tell, and it should be clear that I'm against
 waiting for FTP-2.  Unfortunately there is no real mechanism for
 "officially" adopting my plan, but I bet if TENEX does it on June
 1 the rest of the world will come along.                           6d
 2.  Another reply code problem is the use of 9xx for
 "experimental" replies not in the protocol.  This includes the
 BBN mail-forwarding message and one other that I know of.  This
 procedure is sanctioned by RFC 385, but it seems like a bad idea
 to me.  For one thing, the user program has no way of knowing
 whether the reply is positive, negative, or irrelevant.  The
 examples I've been burned by all should have been 0xx messages.
 I propose that all such messages be given codes in the 000-599
 range, chosen to fit the scheme given above for interpreting
 reply codes.  x9x or xx9 could be used to indicate experiments.    6e
 3.  One more on reply codes: RFC 630 (the one about the TENEX mod
 to the reply codes for MAIL and MLFL) raises the issue of
 "temporary" versus "permanent" failures within the 4xx category.
 RFC 640 deals with this question in the FTP-2 context by changing
 the meaning of 4xx and 5xx so that the former are for temporary
 errors and the latter are for permanent errors.  I like this
 idea, and I think it could easily be adapted for FTP-1 use in a
 way which would allow people to ignore the change and still win.
 At present, I believe that the only program which attempts to
 distinguish between temporary and permanent errors is the TENEX
 mailer.  For other programs, no distinction is currently made
 between 4xx and 5xx responses; both indicate failure, and any
 retrials are done by the human user based on the text part of the
 message.  A specific set of changes to the reply codes is
 proposed below.                                                    6f
 Perhaps I should make a few more points about RFC 640, since it's
 the best thing about FTP-2 and the only argument for it I find at
                                 8

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

 all convincing.  Let me try to pick out the virtues of 640 and
 indicate how they might be achieved in FTP-1.                      6g
    a.  The 3xx category is used uniformly for "positive
    intermediate replies" where further negotiation in the Telnet
    connection is required, as for RNFR.  I'm afraid this one
    can't be changed without affecting existing user programs.
    (One of my goals here is to enable existing user programs to
    work while some servers continue as now and others adopt the
    suggestions I make below.) However, although this 3xx idea is
    logically pleasing, it is not really necessary for a
    simple-minded user program to be able to interpret replies.
    The only really new 3xx in RFC 640 is the 350 code for RNFR.
    But this would only be a real
    improvement for the user program if there were also a 2xx code
    which might be returned after RNFR, which is not the case.
    640 also abolishes the 300 initial connection message with
    220, but again there is clearly no conflict here.              6g1
    b.  The use of 1xx is expanded to include what is now the 250
    code for the beginning of a file transfer.  The idea is that a
    1xx message doesn't affect the state of the user process, but
    this is not really true.  Consider the file transfer commands.
    The state diagram on page 13 of RFC 640 is slightly
    misleading. It appears as if 1xx replies are simply ignored by
    the user program.  In reality, that little loop hides a lot of
    work: the file transfer itself!  If the server replied to the
    file transfer command immediately with a 2xx message, it would
    be a bug in the server, not a successful transfer.  The real
    state diagram is more like                                     6g2
       B --> cmd --> W --> 1 --> W --> 2 --> S
    (with branches out from the "W"s for bad replies).  It should
    be clear from this diagram that the user program, if it trusts
    the server to know what it's doing, can expect a 2xx instead
    of the 1xx without getting confused, since it knows which of
    the W states it's in.  In fact, the use of 1xx in file
    transfer is very different from its other uses, which are
    indeed more like the 0xx and 1xx replies in FTP-1.  I'd call
    this particular point a bug in RFC 640.                        6g3
    c.  Automatic programs which use FTP (like mailers) can decide
                                 9

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

    whether to queue or abandon an unsuccessful transfer based on
    the distinction between 4xx and 5xx codes.  I like this
    idea, although those temporary errors virtually never happen
    in real life.  This could be accomplished in FTP-1 by moving
    many of the 4xx replies to 5xx.  Mailers would be modified to
    use the first digit to decide whether or not to retry.  This
    scheme does not cause any catastrophes if some server is slow
    in converting; it merely leads to unnecessary retries.  A few
    CPU cycles would be wasted in the month following the official
    switch.  Thus, this feature is very different from (a) and
    (b), which could lead to catastrophic failures if not
    implemented all at once.  (Yes, I know that FTP-2 is supposed
    to be done on a different ICP socket.  I am not discussing
    FTP-2 but whether its virtues can be transferred to FTP-1.)
    The specific codes involved are listed below.                  6g4
    d.  The use of the second digit to indicate the type of
    message.  (The proposed division is not totally clean;
    for example, why is 150 ("file status okay; about to open
    data connection") considered to be more about the file
    system than about the data connection?)  This can easily
    be done, since the second digit is not currently important
    to any user process--the TENEX mailer is, in this plan,
    already due for modification because of (c).  Since this
    is mostly an aesthetic point, I'm hesitant to do it if it
    would be difficult for anyone.  In particular, I would want to
    leave the 25x messages alone, in case some user programs
    distinguish these.  This is especially likely for the ones
    which are entirely meant for the program: 251 and 255.
    Therefore I propose that if this idea is adopted in FTP-1
    the meanings of x2x and x5x be interchanged.  This proposal is
    reflected in the specific list below.                          6g5

Let me summarize the specific changes to FTP-1 I'd like to see made, most of which are merely documentation changes to reflect reality: 7

 1.  HELP should return 200.  All commands should return 2xx if
 successful, and I believe all do except HELP.                      7a
 2.  The definition of 1xx messages should be changed to read:
 "Informative replies to status inquiries.  These constitute
 neither a positive nor a negative acknowledgment."                 7b
                                 10

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

 3.  Experimental reply codes should be of the form x9x or xx9,
 where the first digit is chosen to reflect the significance of
 the reply to automated user programs.  Reply codes greater than
 599 are not permitted.  The xx9 form should be used if the reply
 falls into one of the existing categories for the second digit.
 User programs are encouraged to determine the significance of the
 reply from the first digit, rather than requiring a specific
 reply code, when possible.                                         7c
 4.  The STAT command with no argument is considered a request for
 a directory listing for the current working directory, except
 that it may be given along with TELNET SYNCH while a transfer is
 in progress, in which case it is a request for the status of that
 transfer.  (Everyone seems to do the first part of this.  I'm not
 sure if anyone actually implements the second.  This is just
 getting the protocol to agree with reality.)  The reply to a STAT
 command should be zero or more 1xx messages followed by a 200.     7d
 5.  TYPEs P and F mean that the source file contains ASA control
 characters and that the recipient program should reformat it if
 necessary.  Servers which care about Telnet-print vs. non-print
 should implement SRVR T and SRVR N.  All user processes should
 provide a way for the human user to specify an arbitrary SRVR
 command.                                                           7e
 6.  (This is just a resolution of a loose end in documentation.)
 Nested reply codes are not allowed.  I don't think this really
 needs more discussion; they never happen and can't possibly work,
 and FTP user programs shouldn't have to worry about them.          7f
 Here is a list of the current FTP-1 replies, and how they should
 be renumbered for the new scheme.  The changes from 4xx to 5xx
 should be REQUIRED as of June 1; changes in the second or third
 digit are not so important.  (As explained above, it will not be
 catastrophic even if some hosts do not meet the requirement.) The
 list also contains one new possible reply adapted from RFC 640.
 Replies invented in RFC 454 are so noted; since some of them are
 for commands largely not implemented like REIN, they may be
 irrelevant.                                                        7g
    OLD   NEW   TEXT
                                                                   7g1
    0x0   0x0   (These messages are not very well defined nor very
                                 11

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

                important.  Servers should use their judgment.)
    100   110   System status reply.  (Since nobody does STAT as
    in
                the protocol, this may be a moot point.)
    110   111   System busy doing...  (This RFC 454 message could
                easily be considered an example of the one above,
                but since the 454 authors want to distinguish it,
                here it is in another number.)
    150   150   "File status reply."  (If this were really that,
    it
                would be switched to 120, but I believe what is
    meant
                is the response to a bare STAT in mid-transfer,
    which
                is more a connection status reply than a file
    status
                reply.)
    151   121   Directory listing reply.
    200   200   Last command ok.
    201   251   ABOR ok.                                           7g2
    202   252   ABOR ignored, no transfer in progress.
    new   206   Command ignored, superfluous here.
    230   230   Login complete.
    231   231   Logout complete.  (RFC 454: Closing connection.)
    232   232   Logout command will be processed when transfer is
                complete.                                          7g3
    233   233   Logout complete, parameters reinitialized.  (RFC
    454               for REIN)                                    7g4
    250   250   Transfer started correctly.
    251   251   MARK yyyy = mmmm
    252   252   Transfer completed ok.
    253   223   Rename ok.
    254   224   Delete ok.
    255   255   SOCK nnnn
    256   256   Mail completed ok.
    300   300   Connection greeting
    301   301   Command incomplete (no crlf)
    330   330   Enter password                                     7g5
    331   331   Enter account (RFC 454)
    350   350   Enter mail.                                        7g6
    400   huh?  "This service not implemented."  I don't
    understand
                this; how does it differ from 506?  If it means no
                                 12

NWG/RFC# 691 BH 6-JUN-75 23:15 32700 One More Try on the FTP

    FTP
                at all, who gave the message?  Flush.              7g7
    401   451   Service not accepting users now, goodbye.
    430   430   Foo, you are a password hacker!
    431   531   Invalid user or password.
    432   532   User invalid for this service.
    433   533   Need account to write files.
    434   454   Logout by operator.
    435   455   Logout by system.
    436   456   Service shutting down.
    450   520   File not found.
    451   521   Access denied.
    452   452   Transfer incomplete, connection closed.            7g8
    453   423   Transfer incomplete, insufficient storage space.
    454   454   Can't connect to your socket.
    455   425   Random file system error (RFC 454)                 7g9
    456   526   Name duplication, rename failed (RFC 454)
    457   557   Bad transfer parameters (TYPE, BYTE, etc) (RFC
    454)
    500   500   Command gibberish.
    501   501   Argument gibberish.
    502   502   Argument missing.
    503   503   Arguments conflict.
    504   504   You can't get there from here.
    505   505   Command conflicts with previous command.
    506   506   Action not implemented.
    507   507   Some other problem.  (RFC 454)
    550   520   Bad syntax in pathname.  (RFC454)                 7g10
                                 13
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