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]
RFC, FYI, BCP