Samba-3 Server Types and Security Modes

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Official Samba-3 HOWTO and Reference Guide, The

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This chapter is from the book

This chapter is from the book 

Official Samba-3 HOWTO and Reference Guide, The

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3.3 Samba Security Modes

In this section the function and purpose of Samba’s security modes are described. An accurate understanding of how Samba implements each security mode as well as how to configure MS Windows clients for each mode will significantly reduce user complaints and administrator heartache.

In the SMB/CIFS networking world, there are only two types of security: User Level and Share Level. We refer to these collectively as security levels. In implementing these two security levels, Samba provides flexibilities that are not available with Microsoft Windows NT4/200x servers. In actual fact, Samba implements Share Level security only one way, but has four ways of implementing User Level security. Collectively, we call the Samba implementations Security Modes. They are known as: SHARE, USER, DOMAIN, ADS, and SERVER modes. They are documented in this chapter.

An 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. This may sound 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.

3.3.1 User Level Security

We will describe User Level Security first, as its simpler. In User Level Security, the client will send a session setup request directly following protocol negotiation. This request provides a username and password. The server can either accept or reject that username/password combination. 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:

1. the username/password.

2. the name of the client machine.

If the server accepts the username/password then the client expects to be able to mount shares (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).

3.3.1.1 Example Configuration

The smb.conf parameter that sets user level security is:

security = user

This is the default setting since Samba-2.2.x.

3.3.2 Share Level Security

In Share Level security, the client authenticates itself separately for each share. It sends a password along with each tree connection (share mount). It does not explicitly send a username with this operation. The client expects 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 pair that is authenticated, not a share/password pair.

To understand the MS Windows networking parallels, one should think in terms of MS Windows 9x/Me where one can create a shared folder that provides read-only or full access, with or without a 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 parameter in the smb.conf file. The password is then checked in turn against these possible usernames. If a match is found then the client is authenticated as that user.

3.3.2.1 Example Configuration

The smb.conf parameter that sets Share Level security is:

security = share
   

There are reports that recent MS Windows clients do not like to work with share mode security servers. You are strongly discouraged from using Share Level security.

3.3.3 Domain Security Mode (User Level Security)

When Samba is operating in security = domain mode, the Samba server has a domain security trust account (a machine account) and causes all authentication requests to be passed through to the Domain Controllers. In other words, this configuration makes the Samba server a Domain Member server.

3.3.3.1 Example Configuration

Samba as a Domain Member Server

This method involves addition of the following parameters in the smb.conf file:

security = domain
   workgroup = MIDEARTH
   

In order for this method to work, the Samba server needs to join the MS Windows NT security domain. This is done as follows:

1. On the MS Windows NT Domain Controller, using the Server Manager, add a machine account for the Samba server.

2. On the UNIX/Linux system execute:

root# net rpc join -U administrator%password

root# smbpasswd -j DOMAIN_NAME -r PDC_NAME     -U Administrator%password

root# net rpc join -U Administrator%password

Use of this mode of authentication does require there to be a standard UNIX account for each user in order to assign a UID once the account has been authenticated by the remote Windows DC. This account can be blocked to prevent logons by clients other than MS Windows through means such as setting an invalid shell in the /etc/passwd entry.

An alternative to assigning UIDs to Windows users on a Samba member server is presented in Chapter 20, Winbind: Use of Domain Accounts.

For more information regarding Domain Membership, see Chapter 6, Domain Membership.

3.3.4 ADS Security Mode (User Level Security)

Both Samba-2.2, and Samba-3 can join an Active Directory domain. This is possible if the domain is run in native mode. Active Directory in native mode perfectly allows NT4-style Domain Members. This is contrary to popular belief. Active Directory in native mode prohibits only the use of Backup Domain Controllers running MS Windows NT4.

If you are using Active Directory, starting with Samba-3 you can join as a native AD member. Why would you want to do that? Your security policy might prohibit the use of NT-compatible authentication protocols. All your machines are running Windows 2000 and above and all use Kerberos. In this case Samba as an NT4-style domain would still require NT-compatible authentication data. Samba in AD-member mode can accept Kerberos tickets.

3.3.4.1 Example Configuration

realm = your.kerberos.REALM
   security = ADS
   

The following parameter may be required:

password server = your.kerberos.server
   

Please refer to Chapter 6, Domain Membership and Section 6.4 for more information regarding this configuration option.

3.3.5 Server Security (User Level Security)

Server Security Mode is left over from the time when Samba was not capable of acting as a Domain Member server. It is highly recommended not to use this feature. Server security mode has many drawbacks that include:

• Potential Account Lockout on MS Windows NT4/200x password servers.

• Lack of assurance that the password server is the one specified.

• Does not work with Winbind, which is particularly needed when storing profiles remotely.

• This mode may open connections to the password server, and keep them open for extended periods.

• Security on the Samba server breaks badly when the remote password server suddenly shuts down.

• With this mode there is NO security account in the domain that the password server belongs to for the Samba server.

In Server Security Mode 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 client’s connection. This allows the Samba server to use another SMB server as the password server.

You should also note that at the 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, it supplies the client with a random cryptkey. The client will then send all passwords in encrypted form. Samba supports this type of encryption by default.

The parameter 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. The real authentication server can be another Samba server, or it can be a Windows NT server, the latter being natively capable of encrypted password support.

3.3.5.1 Example Configuration

Using MS Windows NT as an Authentication Server

This method involves the additions of the following parameters in the smb.conf file:

encrypt passwords = Yes
   security = server
   password server = "NetBIOS_name_of_a_DC"
   

There are two ways of identifying whether or not a username and password pair is valid. One uses the reply information provided as part of the authentication messaging process, the other uses just an error code.

The downside of this mode of configuration is the fact that for security reasons Samba will send the password server a bogus username and a bogus password and if the remote server fails to reject the username and password pair then an alternative mode of identification of validation is used. Where a site uses password lock out after a certain number of failed authentication attempts this will result in user lockouts.

Use of this mode of authentication requires a standard UNIX account for the user. This account can be blocked to prevent logons by non-SMB/CIFS clients.