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Keywords: FTP
Network Working Group K. Pogran
Request for Comments: 501 MIT-Multics
NIC: 15718 11 May 1973
Un-Muddling "Free File Transfer"
As the ARPA Network begin to mature, we find ourselves addressing
issues and concepts deliberately put off and left untouched at
earlier stages of Network development. Among the issues now coming
to the fore are access control, user authentication, and accounting.
These issues arise immediately out of efforts to develop uniform
methods for providing limited "free" access to the File Transfer
Servers of the host systems, to meet user needs for mail transmission
and similar services.
Several proposals have been made, described by such phrases as
"login-less mail", "'free' accounts", "free file transfer", etc.
These proposals inevitably have imbedded in them some particular
notion of how such things as access control and user authentication
are accomplished and these proposals, which knowingly or unknowingly
make presumptions about the implementation of such mechanisms,
inevitably meet with strong criticism from implementors whose
systems' mechanisms are quite different.
In RFC 467, Bob Bressler proposes ways of helping out users who wish
to transfer files to or from "systems which have some flavor of
security, but on which the user has no access privileges".
Unfortunately, beginning with the first paragraph of the RFC, the
notions of access controls on files (examples of protection
mechanisms), and control of access to the system (user
authentication) are thoroughly muddled. In addition, he makes
sweeping assumptions about the nature and use of accounting
mechanisms and accounts at server sites. RFC 487 also has buried
deep within it assumptions about the nature of the access control and
user authentication aspects of File Transfer Server implementations.
What's needed at this juncture, of course, is a lucid discussion of
the general concepts involved in protection mechanisms, and file
system access controls in particular. Well, you won't find that in
the remainder of this RFC. What you will find is perhaps enough of a
discussion to un-muddle that which RFC 487 has muddled; the rest will
have to come down the pike at a later time.
In many systems, mechanisms which control access to the system,
mechanism which control access to files, and accounting mechanisms
all mesh at the moment at which a prospective user of the system is
authenticated: the system has checked his user-name, password,
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RFC 501 Un-Muddling "Free File Transfer" 11 May 1973
account, or whatever, and decided that he is, indeed, a valid user of
the system. This user, who would like to have some information
processing performed on his behalf, is termed a principal in "privacy
and protection" parlance. Some number of processes are initially set
up for this principal, and some (presumably unforgeable) principal
identifier is associated with the process(es), so that their requests
for access to files and other system resources may be properly
validated. In addition, the identify of the account to be charged
for the resources consumed by these processes is associated with the
processes at this time [1], although on some systems, a process may
change its account identifier at any time.
The first question is: Whose principal identifier does a File
Transfer Server process use? There are at least two possibilities: 1)
the File Transfer Server can run as a "system daemon" process, with
(usually) a highly privileged principal identifier. When acting on
behalf of a user, it must, itself, interpretively evaluate that
user's access to a desired file. Also, it must be able to charge
that user's account for the resources it uses. 2) A File Transfer
Server process can be given the user's own principal identifier.
With this implementation, validation of the user's access to files is
performed automatically by the usual file system mechanisms.
Paragraph four of RFC 487 clearly presumes implementation 1): "If a
user connects to an FTP server and makes a file request without
supplying a user name-password, the server should then examine the
file access parameters ..." Systems truly concerned about protection
may prefer implementation 2), and for good reason -- it follows the
"principle of least privilege", which states that a process should
execute with as little access privilege as it requires to perform its
tasks properly. Running a File Transfer Server process with a user's
principal identifier rather than with that of a system daemon leaves
the system far less susceptible to damage caused by incorrect actions
of the File Transfer Server. [2]
The next question is: Whom do you charge for file transfers? Bressler
tries to set down some guidelines for determining who to charge for
"non-logged-in" (read: "free") file transfer usage: "Clearly, storing
a file in a user's directory can be charged to that user." How is the
word "storing" used here? Surely, "that user" can be billed for the
disk or other storage medium charges incurred by that file which is
now taking up space, but is it legitimate to charge "that user" for
the I/O and/or CPU resources used by someone else to transfer a file
over the Network, and place it into that user's directory? For
example, should the recipient of Network mail be charged for the
resources consumed by someone else in sending it? (Would you care to
pay the postage for all the junk mail that arrives in your home (U.S.
Mail) mailbox?).
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RFC 501 Un-Muddling "Free File Transfer" 11 May 1973
Over the telephone, Bob explained to me that he desired a mechanism
which would, for example, enable me, at his request, to transfer a
file from my system to his directory on his system, without requiring
that I know his password. All well and good. In this situation, it
would make sense to charge Bressler's account for this file transfer.
But how does an un-authenticated user tell a server "Charge this to
Bressler's account because he says it's OK"? Pitfalls abound. The
file Transfer Server process needs to be able to charge an arbitrary
user's account; this again presupposes implementation 1) of the File
Transfer Server described above (or else any user process in the
system would have the capability of charging any user's account; this
seems undesirable). A more reasonable approach would be to charge
that instance of the File Transfer Server process to a general
"Network services" account. Mechanisms for accomplishing this are
presented in RFC 491. [3]
RFC 487 matter-of-factly suggests that retrieval of files in "system"
directories should be charged to "overhead". Here too, some broad
assumptions are made about the nature of accounting mechanisms and
accounts at server sites. In addition, an undesirable loss of
generality is imposed upon the File Transfer Server: It is now
required to have the capability of distinguishing the pathnames of
"system" files from those of "user" files. In a number of systems,
there is no syntactic distinction between the two, and the same
general mechanisms can be used to manipulate both kinds of files (if
a distinction between them can be made at all). The addition of code
to the File Transfer Server which examines the pathname given for
each request, to determine which sort it is, seems to be antithetical
to the goals of uniformity and generality that many of today's
systems have achieved.
The statement that a Network user's file transfer activity can be
charged to a system-wide "overhead" account contains two assumptions:
Such an account cannot be presumed to exist on all systems;
furthermore, if it does exist, in some cases it just isn't the right
account to charge. Certainly, a good case can be made that the cost
of fostering inter-user communication within the ARPA Network
community (which is what "free" file transfer amounts to) should be
borne by ARPA, meaning that such activity should be charged to ARPA-
funded accounts. If a host system's operation is entirely funded by
ARPA (or if its management doesn't care who pays for this activity),
then it makes sense to charge "free" file transfer activity to a
"system overhead" account. On the other hand, that isn't the correct
course of action for a host system whose operation is not funded by
ARPA, for charging "free" file transfers to "system overhead" would
result in passing the cost along to local customers who may have no
interest at all in the ARPA Network.
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RFC 501 Un-Muddling "Free File Transfer" 11 May 1973
Lastly, Bressler suggests that for file retrieval, CPU charges "may
be sufficiently small to not cause a major problem". To believe this
is naivete. It may appear to be true for a system which doesn't
charge the user for time spent executing supervisor code, or I/O
routines, where Network software overhead doesn't show up in the
user's bill. In this case, Network software overhead must contribute
to "general system overhead", the cost of which must be borne by all
users. I don't think many people in the Network community would
consider the actual (as opposed to charged) CPU time spent
transferring a file to be negligible. Certainly, if a system is a
very popular or busy one from a Network standpoint, the cumulative
CPU time spent on "free" file transfers, viewed at the end of an
accounting period (a week? a month? a year?) will not be negligible!
In this RFC, I've picked apart Bob Bressler's RFC 487, mostly because
of its confusion of several distinct (although related) issues, and
the implementation assumptions it contains which conflict with (or
badly bend out of shape) mechanisms and design philosophies existing
on other systems (in particular, the system I am most familiar with,
Multics) [4]. The applicability of the discussions in this RFC, I
think goes beyond that: We've got to acknowledge that it's difficult
to propose Network-wide mechanisms for providing desirable services
without building in assumptions about how they are to be implemented.
We're at a point where we're asking for fairly sophisticated
services, and proposing correspondingly sophisticated mechanisms.
It's time to begin talking about how various systems accomplish such
things as user authentication, access control, and so on, so that we
can all gain a clearer understanding of such issues, and be able to
propose mechanisms with fewer implementation assumptions built into
them.
END NOTES:
[1] On some systems, there is a one-to-one correspondence between
principals and accounts.
[2] It should be noted that systems which choose implementation 2)
may require a user authentication sequence (USER, PASS, and possibly
ACCT commands) before permitting any file transfers, as explicitly
stated on page 17 of RFC 354 (NIC 10596), and page 20 of RFC 4554
(NIC 14333). This authentication sequence would be required to
ascertain the principal identifier to be associated with the newly-
spawned File Transfer Server process; the process is not allowed to
proceed until its principal identifier has been established.
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RFC 501 Un-Muddling "Free File Transfer" 11 May 1973
[3] Note that there are at least two scenarios for accomplishing the
transfer Bressler desires: Either I "push" the file, using my "User
FTP" and his system's "FTP Server", or he "pulls" the file, using his
"User FTP" and my system's "FTP Server". Bob chose the first
scenario; it can be argued that, since it is Bob who wanted the file
transferred, the second scenario is the more appropriate one. A
forthcoming RFC by Mike Padlipsky expands on these points, as well as
an entirely different alternative.
[4] Padlipsky keeps insisting that I've also shown the superiority of
implementation 2) of the File Transfer Server (described above), but
I resist that conclusion. Those interested may want to look at his
Unified User-Level Protocol specification, which is based on a
similar premise.
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Pogran [Page 5]