Re: NFS exporting

[smb () research att com (smb () research att com)]

Here's what I posted to Firewalls on ``secure'' NFS.  You may want to
consult its archives for more on NFS; they're on ftp.greatcircle.com.

                --Steve Bellovin

------- Forwarded Message

To: jak () mentat com
cc: Marcus J Ranum <mjr () tis com>, firewalls () GreatCircle COM
Subject: Re: General questions from a firewall neophyte  
Date: Wed, 30 Mar 94 09:48:59 EST


> 2.  Little seems to be said about secure NFS or RPC in firewall
> discussions.  Am I perhaps missing something?  We have employees
> with machines at home who would like to NFS mount file systems across
> the Internet.  I gather that some sites actually permit this on
> isolated machines, but product literature and papers I've read don't
> seem to talk about this much.  I know that "secure NFS" has its
> own discussion group, but it seems like firewalls shouldn't completely
> ignore the topic.

Marcus covered everything quite well; let me add a few more details
about secure NFS.

First of all, ``secure NFS'' is an incorrect term.  Rather, it's NFS
using DES-authenticated RPC.  Any other RPC-based service could use
DES authentication; however, no other standard ones do.

Second, the DES authentication key is exchanged using Diffie-Hellman
exponential key exchange.  Unfortunately, the modulus size used by Sun
is too small -- it's been cryptanalyzed by LaMacchia and Odlyzko.
Worse yet, the user's private key is stored in /etc/publickey
protected by DES encryption, along with a cleartext public key.  And
the DES key?  The user's password, of course.  Can you say ``password
cracking''?  In other words, using this feature negates the beneficial
effect of using a shadow password file.

Additionally, the key distribution mechanism seems to be very closely
tied to NIS.  At least, I couldn't make it work without enabling NIS,
though admittedly I didn't try particularly hard.  And I'm *not* going
to run NIS over the Internet, thank you!  It might be possible to set
up all the keys via NIS on the central site and hand-carry them to
home machines.  But then they might have to run NIS locally, which is
a pain.

There are more issues as well, but the margin of this note is too
small for them to fit.

                --Steve Bellovin