The following procedures outline the implementation of the security measures discussed so far.
11.3.1. Share Access Controls
Access control entries placed on the share itself act as a filter at the time a when CIFS/SMB client (such as Windows XP Pro) attempts to make a connection to the Samba server.
CREATE/EDIT/DELETE SHARE ACLS
11.3.2. Share Definition Controls
Share-definition-based access controls can be used like a checkpoint or like a pile-driver. Just as a checkpoint can be used to require someone who wants to get through to meet certain requirements, so it is possible to require the user (or group the user belongs to) to meet specified credential-related objectives. It can be likened to a pile-driver by overriding default controls in that having met the credential-related objectives, the user can be granted powers and privileges that would not normally be available under default settings.
It must be emphasized that the controls discussed here can act as a filter or give rights of passage that act as a superstructure over normal directory and file access controls. However, share-level ACLs act at a higher level than do share definition controls because the user must filter through the share-level controls to get to the share-definition controls. The proper hierarchy of controls implemented by Samba and Windows networking consists of:
184.108.40.206 Checkpoint Controls
Consider the following extract from a smb.conf file defining the share called Apps:
[Apps] comment = Application Share path = /data/apps read only = Yes valid users = @Employees
This definition permits only those who are members of the group called Employees to access the share.
If there is an ACL on the share itself to permit read/write access for all Employees as well as read/write for the group Doctors, both groups are permitted through to the share. However, at the moment an attempt is made to set up a connection to the share, a member of the group Doctors, who is not also a member of the group Employees, would immediately fail to validate.
Consider another example. In this case, you want to permit all members of the group Employees except the user patrickj to access the Apps share. This can be easily achieved by setting a share-level ACL permitting only Employees to access the share, and then in the share definition controls excluding just patrickj. Here is how that might be done:
[Apps] comment = Application Share path = /data/apps read only = Yes invalid users = patrickj
Let us assume that you want to permit the user gbshaw to manage any file in the UNIX/Linux file system directory /data/apps, but you do not want to grant any write permissions beyond that directory tree. Here is one way this can be done:
[Apps] comment = Application Share path = /data/apps read only = Yes invalid users = patrickj admin users = gbshaw
Now we have a set of controls that permits only Employees who are also members of the group Doctors, excluding the user patrickj, to have read-only privilege, but the user gbshaw is granted administrative rights. The administrative rights conferred upon the user gbshaw permit operation as if that user has logged in as the user root on the UNIX/Linux system and thus, for access to the directory tree that has been shared (exported), permit the user to override controls that apply to all other users on that resource.
There are additional checkpoint controls that may be used. For example, if for the same share we now want to provide the user peters with the ability to write to one directory to which he has write privilege in the UNIX file system, you can specifically permit that with the following settings:
[Apps] comment = Application Share path = /data/apps read only = Yes invalid users = patrickj admin users = gbshaw write list = peters
This is a particularly complex example at this point, but it begins to demonstrate the possibilities. You should refer to the online manual page for the smb.conf file for more information regarding the checkpoint controls that Samba implements.
220.127.116.11 Override Controls
Override controls implemented by Samba permit actions like the adoption of a different identity during file system operations, the forced overwriting of normal file and directory permissions, and so on. You should refer to the online manual page for the smb.conf file for more information regarding the override controls that Samba implements.
In the following example, you want to create a Windows networking share that any user can access. However, you want all read and write operations to be performed as if the user billc and member of the group Mentors read/write the files. Here is one way this can be done:
[someshare] comment = Some Files Everyone May Overwrite path = /data/somestuff read only = No force user = billc force group = Mentors
That is all there is to it. Well, it is almost that simple. The downside of this method is that users are logged onto the Windows client as themselves, and then immediately before accessing the file, Samba makes system calls to change the effective user and group to the forced settings specified, completes the file transaction, and then reverts to the actually logged-on identity. This imposes significant overhead on Samba. The alternative way to effectively achieve the same result (but with lower system CPU overheads) is described next.
The use of the force user or the force group may also have a severe impact on system (particularly on Windows client) performance. If opportunistic locking is enabled on the share (the default), it causes an oplock break to be sent to the client even if the client has not opened the file. On networks that have high traffic density, or on links that are routed to a remote network segment, oplock breaks can be lost. This results in possible retransmission of the request, or the client may time-out while waiting for the file system transaction (read or write) to complete. The result can be a profound apparent performance degradation as the client continually attempts to reconnect to overcome the effect of the lost oplock break, or time-out.
11.3.3. Share Point Directory and File Permissions
Samba has been designed and implemented so that it respects as far as is feasible the security and user privilege controls that are built into the UNIX/Linux operating system. Samba does nothing with respect to file system access that violates file system permission settings, unless it is explicitly instructed to do otherwise through share definition controls. Given that Samba obeys UNIX file system controls, this chapter does not document simple information that can be obtained from a basic UNIX training guide. Instead, one common example of a typical problem is used to demonstrate the most effective solution referred to in the immediately preceding paragraph.
One of the common issues that repeatedly pops up on the Samba mailing lists involves the saving of Microsoft Office files (Word and Excel) to a network drive. Here is the typical sequence:
There have been many postings over the years that report the same basic problem. Frequently Samba users want to know when this "bug" will be fixed. The fact is, this is not a bug in Samba at all. Here is the real sequence of what happens in this case.
When the user saves a file, MS Word creates a new (temporary) file. This file is naturally owned by the user who creates the file (billc) and has the permissions that follow that user's default settings within the operating system (UNIX/Linux). When MS Word has finished writing the file to disk, it then renames the new (temporary) file to the name of the old one. MS Word does not change the ownership or permissions to what they were on the original file. The file is thus a totally new file, and the old one has been deleted in the process.
Samba received a request to create a new file, and then to rename the file to a new name. The old file that has the same name is now automatically deleted. Samba has no way of knowing that the new file should perhaps have the same ownership and permissions as the old file. To Samba, these are entirely independent operations.
The question is, "How can we solve the problem?"
The solution is simple. Use UNIX file system permissions and controls to your advantage. Follow these simple steps to create a share in which all files will consistently be owned by the same user and the same group:
USING DIRECTORY PERMISSIONS TO FORCE FILE USER AND GROUP OWNERSHIP
11.3.4. Managing Windows 200x ACLs
Samba must translate Windows 2000 ACLs to UNIX POSIX ACLs. This has some interesting side effects because there is not a one-to-one equivalence between them. The as-close-as-possible ACLs match means that some transactions are not possible from MS Windows clients. One of these is to reset the ownership of directories and files. If you want to reset ownership, this must be done from a UNIX/Linux login.
There are two possible ways to set ACLs on UNIX/Linux file systems from a Windows network workstation, either via File Manager or via the Microsoft Management Console (MMC) Computer Management interface.
18.104.22.168 Using the MMC Computer Management Interface
22.214.171.124 Using MS Windows Explorer (File Manager)
The following alternative method may be used from a Windows workstation. In this example we work with a domain called MEGANET, a server called MASSIVE, and a share called Apps. The underlying UNIX/Linux share point for this share is /data/apps.
126.96.36.199 Setting Posix ACLs in UNIX/Linux
Yet another alternative method for setting desired security settings on the shared resource files and directories can be achieved by logging into UNIX/Linux and setting POSIX ACLs directly using command-line tools. Here is an example session on the same resource as in the immediately preceding example on a SUSE 9 Linux system:
11.3.5. Key Points Learned
The mish-mash of issues were thrown together into one chapter because it seemed like a good idea. Looking back, this chapter could be broken into two, but it's too late now. It has been done. The highlights covered are as follows: