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RFC 691 - One more try on the FTP


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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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