[OpenAFS] Is member of a machine group honored as system:authuser?

[Jeffrey Altman]

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On 1/24/2018 2:31 PM, Ximeng Guan wrote:
> Hello,
>=20
> I am trying to make some effective use of machine groups in AFS to acco=
mmodate certain requirement of licensed software. I read about the featur=
e, and noticed that in the 1998 edition of the book "Managing AFS, The An=
drew File System" by Richard Campbell, the following text appeared in Cha=
pter 7 p.230:
>=20
> "...
> There is one final quirk to the implementation: it's common for several=
 top-level directories of the AFS namespace to be permitted only to syste=
m:authuser, that is, any user can access the rest of the namespace, but o=
nly if the user has been authenticated as a user, any user, of the curren=
t cell. Machine groups are intended to be useful for any person logged in=
 to a workstation so that software licenses can be honestly followed. The=
refore, when an unauthenticated user is using a machine that is a member =
of a group entry on an ACL, the user's implicit credential is elevated to=
 system:authuser, but only if the machine entry in the group is an exact =
match, not a wildcard.
>=20
> This rule permits any user of a given desktop to effectively have syste=
m:authuser credentials for a directory. As long as that directory has an =
ACL that includes the specific machine's IP address as a member of a grou=
p entry, any user of the desktop, and only that desktop, would have acces=
s to the directory.=20
> ...
> "

This text was true prior to IBM AFS 3.2 but has not been true for any
release since IBM AFS 3.3.  As of AFS 3.3 the Current Protection Set for
a host includes neither system:anyuser nor system:authuser.  In other
words, hosts are not users.

> That is exactly what we have in the top-level directories in our cell: =
We have "system:authuser rl" on the ACL of root.cell.=20
>=20
> Access list for . is
> Normal rights:
>   system:administrators rlidwka
>   system:authuser rl
>=20
> Then when I create a machine-based pts entry 10.12.8.31, add it to a ne=
w group named machinegrp, and wait for >2 hours to let it be effective (a=
ccording to dafileserver's man page)
>=20
> $ pts member machinegrp
> Members of machinegrp (id: -250) are:
>   10.12.8.31
>=20
> I would expect that a local user on 10.12.8.31, even without an AFS tok=
en, would be able to "cd" into the top directory of the cell. But in real=
ity that does not happen. An unauthenticated user is denied of access.=20

This is working as designed because the ACL does not include the host
identity.
>=20
> When I explicitly put "machinegrp rl" on the ACL of the cell's top dire=
ctory (root.cell), an unauthenticated user is indeed able to access the A=
FS space.=20
>=20
> This is not quite convenient, because to allow the user of that specifi=
c machine to launch a license software installed in a certain (deep) dire=
ctory under AFS, for example /afs/cellname/tools/vendors/abc/softwarexx/b=
in, we would have to explicitly place "machinegrp l" on the ACL of the pa=
rent directories of ./bin from /softwarexx all the way up to /cellname.=20
>=20
> Then if we have another software and another machine group, we will hav=
e to do the same again, and the ACL of our root.cell directory will soon =
be populated with machine group entries. That does not seem to be an eleg=
ant solution.=20
>=20
> Did I miss anything here?=20

Perhaps.

The problem you are attempting to solve is that there exist directories
and files that must be accessible only from a particular subset of
trusted machines.  This data is only supposed to be visible to users of
those machines and no one else.

What you are possibly missing is that IP ACLs are not a form of
authentication and they cannot be used to provide any integrity
protection or wire privacy.  Any data that is accessed by the host
without a user's AFS token is going to be transmitted in the clear.
In addition, IP addresses can be spoofed.


This use case is one that AuriStorFS was explicitly designed to address.
 AuriStorFS client hosts can be keyed using Kerberos v5 principals.
AuriStor's RX security class supports combined identity authentication
providing the file server both the identity of the user and the identity
of the host.  Finally, the AuriStorFS Access Control language permits
different access permissions to be granted to each of the following
combinations:

  authenticated user on unauthenticated host
  authenticated user on authenticated host
  anonymous user on authenticated host
  anonymous user on anonymous host

The anonymous user on authenticated host communications with the file
server are authenticated using the host principal and all data is both
integrity protected and encrypted for wire privacy.

Jeffrey Altman
AuriStor, Inc.



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fn:Jeffrey Altman
n:Altman;Jeffrey
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adr:Suite 6B;;255 West 94Th Street;New York;New York;10025-6985;United St=
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email;internet:jaltman@auristor.com
title:Founder and CEO
tel;work:+1-212-769-9018
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