security_level.txt   [plain text]

!==
!== security_level.txt for Samba release 2.2.0-alpha3 24 Mar 2001
!==
Contributor:	Andrew Tridgell
Updated:	June 27, 1997
Status:		Current

Subject:	Description of SMB security levels.
===========================================================================

Samba supports the following options to the global smb.conf parameter
"security =":
	share, user, server

Note: Samba-2.0.0 now adds the "domain" security mode. Please refer to
the smb.conf man page for usage information and to the document
docs/textdocs/DOMAIN_MEMBER.txt for further background details.

Of the above, "security = server" means that Samba reports to clients that
it is running in "user mode" but actually passes off all authentication
requests to another "user mode" server. This requires an additional
parameter "password server =" that points to the real authentication server.
That real authentication server can be another Samba server or can be a
Windows NT server, the later natively capable of encrypted password support.

Below is a more complete description of security levels.
===========================================================================

A SMB server tells the client at startup what "security level" it is
running. There are two options "share level" and "user level". Which
of these two the client receives affects the way the client then tries
to authenticate itself. It does not directly affect (to any great
extent) the way the Samba server does security. I know this is
strange, but it fits in with the client/server approach of SMB. In SMB
everything is initiated and controlled by the client, and the server
can only tell the client what is available and whether an action is
allowed. 

I'll describe user level security first, as its simpler. In user level
security the client will send a "session setup" command directly after
the protocol negotiation. This contains a username and password. The
server can either accept or reject that username/password
combination. Note that at this stage the server has no idea what
share the client will eventually try to connect to, so it can't base
the "accept/reject" on anything other than:

- the username/password
- the machine that the client is coming from

If the server accepts the username/password then the client expects to
be able to mount any share (using a "tree connection") without
specifying a password. It expects that all access rights will be as
the username/password specified in the "session setup". 

It is also possible for a client to send multiple "session setup"
requests. When the server responds it gives the client a "uid" to use
as an authentication tag for that username/password. The client can
maintain multiple authentication contexts in this way (WinDD is an
example of an application that does this)


Ok, now for share level security. In share level security the client
authenticates itself separately for each share. It will send a
password along with each "tree connection" (share mount). It does not
explicitly send a username with this operation. The client is
expecting a password to be associated with each share, independent of
the user. This means that samba has to work out what username the
client probably wants to use. It is never explicitly sent the
username. Some commercial SMB servers such as NT actually associate
passwords directly with shares in share level security, but samba
always uses the unix authentication scheme where it is a
username/password that is authenticated, not a "share/password".

Many clients send a "session setup" even if the server is in share
level security. They normally send a valid username but no
password. Samba records this username in a list of "possible
usernames". When the client then does a "tree connection" it also adds
to this list the name of the share they try to connect to (useful for
home directories) and any users listed in the "user =" smb.conf
line. The password is then checked in turn against these "possible
usernames". If a match is found then the client is authenticated as
that user.

Finally "server level" security. In server level security the samba
server reports to the client that it is in user level security. The
client then does a "session setup" as described earlier. The samba
server takes the username/password that the client sends and attempts
to login to the "password server" by sending exactly the same
username/password that it got from the client. If that server is in
user level security and accepts the password then samba accepts the
clients connection. This allows the samba server to use another SMB
server as the "password server". 

You should also note that at the very start of all this, where the
server tells the client what security level it is in, it also tells
the client if it supports encryption. If it does then it supplies the
client with a random "cryptkey". The client will then send all
passwords in encrypted form. You have to compile samba with encryption
enabled to support this feature, and you have to maintain a separate
smbpasswd file with SMB style encrypted passwords. It is
cryptographically impossible to translate from unix style encryption
to SMB style encryption, although there are some fairly simple management
schemes by which the two could be kept in sync.