Mailinglist Archive: opensuse-security (409 mails)

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RE: [suse-security] Fileserver access from public networks?
  • From: "Reckhard, Tobias" <tobias.reckhard@xxxxxxxxxxx>
  • Date: Mon, 19 Aug 2002 08:28:11 +0200
  • Message-id: <96C102324EF9D411A49500306E06C8D1019894D6@xxxxxxxxxxxxxxxxx>
> The usual scenario is a lan, with the internal servers (e.g. the file
> server), secured by firewall solutions, a dmz, with external
> servers like www
> or ftp, also secured with a firewall.

Yep, a tried and trusted architecture.

> Everybody tells you: isolate the fileserver from public
> networks, don't use
> smb or appleshare across the internet. Ok.
> But we have the following scenario: we are at university
> here, students want
> to acces data from the internet. So there must be some kind
> of internet
> access to the file server. Here are my questions:
> - Why do people run ftp servers to share files, but tell me
> that cifs(smb)
> and appleshare are "insecure" on public networks. Both
> encrypt passwords...
> and data is not encrypted in ftp, too (?). I is much simpler
> for users to use
> the same protocol (smb/applehare) in university networks and
> at home (and ftp
> doesn't keep type and creator information important for the
> mac-clients).

You're misunderstanding what 'everybody' tells you. When they say "isolate
the fileserver from public networks, don't use smb or appleshare across the
internet", they mean:

* Isolate the file server from public networks,
* make sure the data on the file server, which is assumed to be sensitive,
doesn't leave the private network,
* don't allow access to the file server across the Internet without adequate
protection. Don't use cleartext protocols such as SMB or Appleshare.

FTP and HTTP aren't alternatives to CIFS or Appleshare, they're worse when
you're talking about file server access. FTP servers are good to provide
free access to public data, same with HTTP servers.

You *can* use HTTPS for secure, i.e. strongly encrypted and mutually
authenticated access to file resources. Or you can employ VPN techniques.

> - I don't want to have one external server and one internal.
> I'm almost sure
> that just the file I need when connecting from the outside
> will always be on
> the internal server than ;-), and how to explain our users
> that they have one
> account, but are to store data ont wo file servers... Is it
> the only solution
> to have one internal and one external file server, not connected?

It is usually sensible, security-wise, to keep a file server with sensitive
data well away from the Internet, away from any direct contact. The same
applies to databases, BTW. If you were to ask me, you'd need to have very
good reasons to place such a beast in a network that's accessible from the
Internet without at least a sound application layer gateway inbetween. The
alternative is to place a server in the DMZ, which is supplied with copies
of (*only*) the necessary data stored on the internal box. It makes things
less transparent to users, yes, but I'm sure it can be made to be entirely

> - If I really install a second external file server, what
> about linking it
> into the internal one? So I could create a subdir
> "internet_box" in users'
> home dir's, pointing to their nfs-mounted directories on the
> external server.
> So they could decide to make their files internet-accessable
> or not (some
> will have all their data on the external server, while seeing
> only one file
> server, while others who only work from university network
> won't use this
> directory at all). What about this scenario?

We security folks typically don't like to use NFS or other remote filesystem
schemes, including SMB, across security boundaries. It's not as bad to use
them across the DMZ-internal net boundary as it is between the Internet and
a private net, but it still gives us very creepy feelings.. If you need to
use NFS or SMB, SMB is preferred security-wise.

> How do you implement such installations? We are going to
> expand our students'
> computer lab soon, and I want to have a clear structure of
> servers and
> networks before.

* Set up a second file server in the DMZ and a replication scheme to
(automatically) transfer, i.e. push the relevant data from the internal
machine to the DMZ.
* Arrange for the students to be able to access the DMZ server via VPN,
HTTPS or SMB-over-SSL. Full authentication would be mandatory.
* If files need to be modified from the outside, I'd try to use something
with the check-out/check-in functionality of CVS. Since this would probably
involve inbound communication between the DMZ server and the file server,
I'd put a strong focus on security, making sure the DMZ server can't make
the file server do anything it shouldn't and also making sure that it can't
perform a DoS against anything important.
* If files need to be inserted from the outside, I'd try to go with an
'incoming' area on the DMZ server, from which the internal server
periodically pulls and subsequently removes new data, which it then sorts to
destination directories. These may then be published on the DMZ server in
part or whole.

Ideally, I'd use a document management system. It'd probably be most
appealing to users and most practical if it used WWW techniques.


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