Using Samba-5. Browsing and Advanced Disk Shares-P2

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Using Samba-5. Browsing and Advanced Disk Shares-P2

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  1. If Samba is the primary domain controller for your workgroup or NT domain, it should also be the domain master browser. The domain master browser is a special machine that has the NetBIOS resource type and is used to propagate browse lists to and from each of the local master browsers in individual subnets across the domain. To force Samba to become the domain master browser, set the following in the [global] section of the smb.conf: [global] domain master = yes If you have a Windows NT server on the network acting as a primary domain controller (PDC), we recommend that you do not use Samba to become the domain master browser. The reverse is true as well: if Samba is taking on the responsibilities of a PDC, we recommend making it the domain master browser. Splitting the PDC and the domain master browser will cause unpredictable errors to occur on the network. 5.1.5.13 remote browse sync The global remote browse sync option specifies that Samba should synchronize its browse lists with local master browsers in other subnets. However, the synchronization can occur only with other Samba servers, and not with Windows computers. For example, if your Samba server was a master browser on the subnet 192.168.235, and Samba local master browsers
  2. existed on other subnets at 192.168.234.92 and 192.168.236.2, you could specify the following: remote browse sync = 192.168.234.92 192.168.236.2 The Samba server would then directly contact the other machines on the address list and synchronize browse lists. You can also say: remote browse sync = 192.168.234.255 192.168.236.255 This forces Samba to broadcast queries to determine the IP addresses of the local master browser on each subnet, with which it will then synchronize browse lists. This only works, however, if your router doesn't block directed broadcast requests ending in 255. 5.1.5.14 remote announce Samba servers are capable of providing browse lists to foreign subnets with the remote announce option. This is typically sent to the local master browser of the foreign subnet in question. However, if you do not know the address of the local master browser, you can do the following: [global]
  3. remote announce = 192.168.234.255/ACCOUNTING \ 192.168.236.255/ACCOUNTING With this, Samba will broadcast host announcements to all machines on subnets 192.168.234 and 192.168.236, which will hopefully reach the local master browser of the subnet. You can also specify exact IP addresses, if they are known. 5.2 Filesystem Differences One of the biggest issues for which Samba has to correct is the difference between Unix and non-Unix filesystems. This includes items such as handling symbolic links, hidden files, and dot files. In addition, file permissions can also be a headache if not accounted for properly. This section describes how to use Samba to make up for some of those annoying differences, and even how to add some new functionality of its own. 5.2.1 Hiding and Vetoing Files There are some cases when we need to ensure that a user cannot see or access a file at all. Other times, we don't want to keep a user from accessing a file - we just want to hide it when they view the contents of the directory. On Windows systems, an attribute of files allows them to be hidden from a folder listing. With Unix, the traditional way of hiding files in a directory is to precede them with a dot (.). This prevents items such as configuration files or defaults from being seen when performing an ordinary ls command.
  4. Keeping a user from accessing a file at all, however, involves working with permissions on files and or directories. The first option we should discuss is the boolean hide dot files. This option does exactly what it says. When set to yes, the option treats files beginning with a period (.) as hidden. If set to no, those files are always shown. The important thing to remember is that the files are only hidden. If the user has chosen to show all hidden files while browsing (e.g., using the Folder Options menu item under the View menu in Windows 98), they will still be able to see the files, as shown in Figure 5.2. Figure 5.2: Hidden files in the [data] share Instead of simply hiding files beginning with a dot, you can also specify a string pattern to Samba for files to hide, using the hide files option. For example, let's assume that we specified the following in our example [data] share: [data]
  5. path = /home/samba/data browseable = yes guest ok = yes writeable = yes case sensitive = no hide files = /*.java/*README*/ Each entry for this option must begin, end, or be separated from another with a slash ( / ) character, even if there is only one pattern listed. This convention allows spaces to appear in filenames. In this example, the share directory would appear as shown in Figure 5.3. Again, note that we have set the Windows 98 option to view hidden files for the window. Figure 5.3: Hiding files based on filename patterns If we want to prevent users from seeing files at all, we can instead use the veto files option. This option, which takes the same syntax as the hide
  6. files option, specifies a list of files that should never be seen by the user. For example, let's change the [data] share to the following: [data] path = /home/samba/data browseable = yes guest ok = yes writeable = yes case sensitive = no veto files = /*.java/*README*/ The syntax of this option is identical to the hide files configuration option: each entry must begin, end, or be separated from another with a slash ( /) character, even if there is only one pattern listed. By doing so, the files hello.java and README will simply disappear from the directory, and the user will not be able to access them through SMB. There is one other question that we need to address. What happens if the user tries to delete a directory that contains vetoed files? This is where the delete veto files option comes in. If this boolean option is set to yes, the user is allowed to delete both the regular files and the vetoed files in the directory, and the directory itself will be removed. If the option is set to no,
  7. the user will not be able to delete the vetoed files, and consequently the directory will not be deleted either. From the user's perspective, the directory will appear to be empty, but cannot be removed. The dont descend directive specifies a list of directories whose contents Samba should not allow to be visible. Note that we say contents, not the directory itself. Users will be able to enter a directory marked as such, but they are prohibited from descending the directory tree any farther - they will always see an empty folder. For example, let's use this option with a more basic form of the share that we defined earlier in the chapter: [data] path = /home/samba/data browseable = yes guest ok = yes writeable = yes case sensitive = no dont descend = config defaults In addition, let's assume that the /home/samba/data directory has the following contents:
  8. drwxr-xr-x 6 tom users 1024 Jun 13 09:24 . drwxr-xr-x 8 root root 1024 Jun 10 17:53 .. -rw-r--r-- 2 tom users 1024 Jun 9 11:43 README drwxr-xr-x 3 tom users 1024 Jun 13 09:28 config drwxr-xr-x 3 tom users 1024 Jun 13 09:28 defaults drwxr-xr-x 3 tom users 1024 Jun 13 09:28 market If the user then connects to the share, he or she would see the directories shown in Figure 5.4. However, the contents of the /config and /defaults directories would appear empty to the user, even if other folders or files existed in them. In addition, users cannot write any data to the folder (which prevents them from creating a file or folder with the same name as one that is already there but invisible). If a user attempts to do so, he or she will receive an "Access Denied" message. dont descend is an administrative option, not a security option, and is not a substitute for good file permissions. Figure 5.4: Contents of the [data] share with dont descend
  9. 5.2.2 Links DOS and NT filesystems don't have symbolic links; Windows 95/98/NT systems approximate this with "shortcuts" instead. Therefore, when a client tries to open a symbolic link on a Samba server share, Samba attempts to follow the link to find the real file and let the client open it, as if he or she were on a Unix machine. If you don't want to allow this, set the follow symlinks option: [data] path = /home/samba/data browseable = yes guest ok = yes writeable = yes case sensitive = no
  10. follow symlinks = no You can test this by creating a directory on the Unix server inside the share as the user that you are logging in with. Enter the following commands: % mkdir hello; cd hello % cat "This is a test" >hello.txt % ln -s hello.txt "Link to hello" This results in the two files shown in the window in Figure 5.5. Normally, if you click on either one, you will receive a file which has the text "This is a test" inside of it. However, with the follow symlinks option set to no, you should receive an error similar to the dialog in Figure 5.5 if you click on "Link to hello."
  11. Figure 5.5: An error dialog trying to follow symbolic links when forbidden by Samba Finally, let's discuss the wide links option. This option, if set to yes, allows the client user to follow symbolic links that point outside the shared directory tree, including files or directories at the other end of the link. For example, let's assume that we modified the [data] share as follows: [data] path = /home/samba/data browseable = yes guest ok = yes writeable = yes case sensitive = no follow symlinks = yes wide links = yes
  12. As long as the follow symlinks option is enabled, this will cause Samba to follow all symbolic links outside the current share tree. If we create a file outside the share (for example, in someone's home directory) and then create a link to it in the share as follows: ln -s ~tom/datafile ./datafile then you will be able to open the file in Tom's directory as per the target file's permissions. 5.2.3 Filesystem Options Table 5.4 shows a breakdown of the options we discussed earlier. We recommend the defaults for most, except those listed in the following descriptions. Table 5.4: Filesystem Configuration Options Option Parameters Function Default Scope unix boolean Provides Unix user's full no Global realname name to client.
  13. Table 5.4: Filesystem Configuration Options Option Parameters Function Default Scope dont string (list of Indicates a list of None Share descend directories) directories whose contents Samba should make invisible to clients. follow boolean If set to no, Samba will yes Share symlinks not honor symbolic links. getwd boolean If set to yes, Samba will yes Global cache use a cache for getwd( ) calls. wide boolean If set to yes, Samba will yes Share links follow symbolic links outside the share. hide dot boolean If set to yes, treats Unix yes Share files hidden files as hidden files
  14. Table 5.4: Filesystem Configuration Options Option Parameters Function Default Scope in Windows. hide string (list of List of file patterns to treat None Share files files) as hidden. veto string (list of List of file patterns to None Share files files) never show. delete boolean If set to yes, will delete no Share veto files matched by veto files files when the directory they reside in is deleted. 5.2.3.1 unix realname Some programs require a full username in order to operate. For example, a Windows email program often needs to associate a username with a given real name. If your system password file contains the real names of users in the GCOS field, the unix realname option instructs Samba to provide this information to clients. Without it, the name of the user will simply be
  15. his or her login ID. For example, if your Unix password file contains the following line: rcollins:/KaBfco47Rer5:500:500:Robert Collins: /home/rcollins:/bin/ksh And the option in the configuration file is: [global] unix realname = yes then the name Robert Collins will be provided to any client that requests the real name of user rcollins. You typically don't need to bother with this option. 5.2.3.2 dont descend The dont descend option can be used to specify various directories that should appear empty to the client. Note that the directory itself will still appear. However, Samba will not show any of the contents of the directory to the client user. This is not a good option to use as a security feature (a user could probably find a way around it); it really is meant only as a convenience to keep client users from browsing into directories that might have sensitive files. See our example earlier in this section.
  16. 5.2.3.3 follow symlinks This option, which is discussed in greater detail earlier, controls whether Samba will follow a symbolic link in the Unix operating system to the target, or if it should return an error to the client user. If the option is set to yes, the target of the link will be interpreted as the file. 5.2.3.4 getwd cache This global option specifies whether Samba should use a local cache for the Unix getwd() ( get current working directory) system call. You can override the default value of yes as follows: [global] getwd cache = no Setting this option to yes can significantly increase the time it takes to resolve the working directory, especially if the wide links option is set to no. You should normally not need to alter this option. 5.2.3.5 wide links This option specifies whether the client user can follow symbolic links that point outside the shared directory tree. This includes any files or directories at the other end of the link, as long as the permissions are correct for the user. The default value for this option is yes. Note that this option will not
  17. be honored if the follow symlinks options is set to no. Setting this option to no slows smbd considerably. 5.2.3.6 hide files The hide files option provides one or more directory or filename patterns to Samba. Any file matching this pattern will be treated as a hidden file from the perspective of the client. Note that this simply means that the DOS hidden attribute is set, which may or may not mean that the user can actually see it while browsing. Each entry in the list must begin, end, or be separated from another entry with a slash ( /) character, even if there is only one pattern listed. This allows spaces to appear in the list. Asterisks can be used as a wildcard to represent zero or more characters. Questions marks can be used to represent exactly one character. For example: hide files = /.jav*/README.???/ 5.2.3.7 hide dot files The hide dot files option hides any files on the server that begin with a dot (.) character, in order to mimic the functionality behind several shell commands that are present on Unix systems. Like hide files, those files that begin with a dot have the DOS hidden attribute set, which doesn't necessarily guarantee that a client cannot view them. The default value for this option is yes.
  18. 5.2.3.8 veto files More stringent than the hidden files state is the state provided by the veto files configuration option. Samba won't even admit these files exist. You cannot list or open them from the client. In reality, this isn't a trustworthy security option. It is actually a mechanism to keep PC programs from deleting special files, such as ones used to store the resource fork of a Macintosh file on a Unix filesystem. If both Windows and Macs are sharing the same files, this can prevent ill-advised power users from removing files the Mac users need. The syntax of this option is identical to that of the hide files configuration option: each entry must begin, end, or be separated from another with a slash ( / ) character, even if only one pattern is listed. Asterisks can be used as a wildcard to represent zero or more characters. Questions marks can be used to represent exactly one character. For example: veto files = /*config/*default?/ This option is primarily administrative - not a substitute for good file permissions. 5.2.3.9 delete veto files This option tells Samba to delete vetoed files when a user attempts to delete the directory in which they reside. The default value is no. This means if a
  19. user tries to delete a directory that contains a vetoed file, the file (and the directory) will not be deleted. Instead, the directory will remain and appear to be empty from the perspective of the user. If set to yes, the directory and the vetoed files will be deleted. 5.3 File Permissions and Attributes on MS-DOS and Unix DOS was never intended to be a multiuser, networked operating system. Unix, on the other hand, was designed that way from the start. Consequently, there are inconsistencies and gaps in coverage between the two filesystems that Samba must not only be aware of, but also provide solutions for. One of the biggest gaps is how Unix and DOS handle permissions with files. Let's take a look at how Unix assigns permissions. All Unix files have read, write, and execute bits for three classifications of users: owner, group, and world. These permissions can be seen at the extreme left-hand side when a ls -al command is issued in a Unix directory. For example: -rwxr--r-- 1 tom users 2014 Apr 13 14:11 access.conf Windows, on the other hand, has four principal bits that it uses with any file: read-only, system, hidden, and archive. You can view these bits by right- clicking on the file and choosing the Properties menu item. You should see a dialog similar to Figure 5.6.[ 1]
  20. [1] The system checkbox will probably be greyed for your file. Don't worry about that - you should still be able to see when the box is checked and when it isn't. Figure 5.6: DOS and Windows file properties The definition of each of those bits follows: Read-only The file's contents can be read by a user but cannot be written to. System This file has a specific purpose required by the operating system.
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