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rfc:rfc505

Network Working Group M. A. Padlipsky Request for Comments: 505 MIT-Multics NIC: 16156 25 June 1973

          Two Solutions to a File Transfer Access Problem
 In RFC #87, Bob Bressler raises the issue of how one can use the File
 Transfer Protocol to send a file to a user on another system without
 knowing that user's password.  In RFC 501, Kan Pogran points out
 certain objections to Bressler's solution of having a "daemon"
 process do the job -- in particular, the fact that it would require
 an interpretive access control mechanism in the daemon different from
 most system's normal access control mechanisms.  Because Ken felt
 that it would be too much of a digression in RFC 501 for him to cover
 the following points fully, I decided it might be of interest to deal
 with them separately: There are at least two solutions to the problem
 Bob raised in RFC 487 -- in regard to "my" sending "him" a file
 without knowing his password -- which don't give rise to the problems
 noted in RFC 501.  One hinges on adding a convention to the FTP, the
 other on adding a command.
 The first solution is very straightforward.  Instead of having me
 push the file, he could pull it.  That is, he uses his own "principal
 identifies" (thus solving access permission problems at his end) and
 his own User FTP to extract the file with the aid of my Server FTP.
 All this requires is that 1) I give appropriate access permission on
 my end, and 2) he have the ability to use my Server FTP.  The second
 condition is met by either a) his having an account on my system, or
 b) my system's having a known account for "free" Server FTP use. (*)
 So standing the model on its head solves the functional problem,
 although he has to pay for the User FTP.  But, then, it's he who
 wants the file, so why shouldn't he?  On the other hand, "he" might
 not be logged in right now and I might be -- and by the time he can
 get logged in my system might be scheduled to be down.  Fortunately,
 there's also a moderately straightforward solution to the problem as
 originally posed.  This goes back to the mechanism used to prevent
 capricious and/or malicious card input on Multics: Instead of placing
 input (card deck or transferred file) directly into the alleged
 recipient's directory, place it in a "pool" directory and merely
 inform the recipient of its arrival.  If he really wanted it, he then
 copies it into his own directory.  That way, unauthorized people
 can't freeload on somebody else's directory (and the pool is, of
 course, periodically purged), nor can they clobber others' already-
 existing files.

Padlipsky [Page 1] RFC 505 Two Solutions to a File Transfer 25 June 1973

[1]

 This second solution has the virtue of requiring fewer steps than the
 first, and would work even when the first wouldn't; so even though it
 would require another FTP command, I propose the addition of a new
 FTP "POOL" command, which does what the last paragraph described.
 Depending on the various Servers' protection mechanisms, the pooled
 files could be made readable only by the declared recipients.  This
 would, for example, offer an easy way to get some privacy for "mail"
 (which otherwise is likely to be readable by anybody who can write
 it), although other solutions to that particular problem of course
 exist.  At any rate, the POOL command's syntax would be POOL id name
 where id is a valid user identifier on the Server, and name is the
 desired name to be placed on the about-to-be-transferred file in the
 Server's pool directory.  (*) (Servers must, of course, do whatever
 pre- or post-fixing to name is necessary to make it unique within the
 pool.)  The transfer then takes place in the same manner as with
 STOR, and on successful completion the Server sends a message to id
 that he should pick up name (suitably) modified to look like a local
 pathname) if he wants it. The message should also identify the
 putative sender (even though it might have come in from a free
 account).  The id should, naturally, be validated before starting the
 transfer.
 The question has been raised locally as to why we don't simply take a
 pooled view of STOR on Multics and forget about pushing for a new
 command.  To do so would have two drawbacks, I feel: first, I think
 we'd be remiss in our duty as NWG participants if we failed to
 attempt to offer solutions to protocol problems to the Network
 community as a whole.  Second, on a less pious but more practical
 note, if we don't know the id we have to infer it from the pathname,
 which rules out abbreviations and forces senders to have to know too
 much  about our internal structure. (The alternative of requiring an
 additional argument to the STOR is subject to the same objection.  It
 is also subject to the objection that protocols really shouldn't be
 unilaterally extended.  Of course, we could go to "site-specific
 parameters", but that's complicating life so much that the
 alternative of no unsolicited files seems preferable.)  Therefore, I
 think that POOL would be worthwhile unless no other Servers have
 enough access control for it to be necessary anywhere but on Multics.
 At the very least, having the protocol specify an "access id"
 optional argument to STOR seems desirable.

[2]

 Input as to whether any of the other Servers has file access control
 abilities similar to those of Multics would be useful in clarifying
 whether this whole area is one which needs specific treatment at the

Padlipsky [Page 2] RFC 505 Two Solutions to a File Transfer 25 June 1973

 Protocol level, or merely needs internally acceptable handling at our
 end.  In the meantime, if you're trying to send an unsolicited file
 to us for free, you can use the NETML mechanism with no directory
 qualification on the target pathname in the STOR, then MAIL the file
 name to the intended recipient, who will copy the file into his own
 directory (from, in our syntax, >udd>Cnet>anonymous).  That's all
 pretty complicated, but it sure does go to show that higher-level
 protocols need to know an awful lot about the various operating
 systems.  At any rate, comment on either Bressler's Problem, POOL,
 STOR, or other people's access control mechanisms would all be
 appreciated.

Endnotes

 [1] (*) For b),I suggest that the USER NETML / PASS NETML discipline
 of RFC 491 be extended.  That is, Hosts which allow free use of their
 FTP Servers should accept that pair of FTP commands as an indication
 to commence free service.  Whether this leads to a login of a dummy
 user or a passoff to a daemon process is a matter of local
 implementation preference, of course.
 [2] (*) Note that this definition relieves the user of having to know
 the Server's pathname for the pool directory.
       [ This RFC was put into machine readable form for entry ]
        [ into the online RFC archives by Thomas Farmer 11/98 ]

Padlipsky [Page 3]

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