SAMBA Project Documentation
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Chapter 9. Security levels

9.1. Introduction

Samba supports the following options to the global smb.conf parameter

[global]
+security = [share|user(default)|domain|ads]

Please refer to the smb.conf man page for usage information and to the document +DOMAIN_MEMBER.html for further background details +on domain mode security. The Windows 2000 Kerberos domain security model +(security = ads) is described in the ADS-HOWTO.html.

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.

9.2. 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.

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