[Chapter 6] 6.2 Controlling Access to Shares

6.2 Controlling Access to Shares

Often you will need to restrict the users who can access a specific share for security reasons. This is very easy to do with Samba since it contains a wealth of options for creating practically any security configuration. Let's introduce a few configurations that you might want to use in your own Samba setup.

WARNING: Again, if you are connecting with Windows 98 or NT 4.0 with Service Pack 3 (or above), those clients will send encrypted passwords to the Samba server. If Samba is not configured for this, it will continually refuse the connection. This chapter describes how to set up Samba for encrypted passwords. See the Section 6.4, Passwords" section.

We've seen what happens when you specify valid users. However, you are also allowed to specify a list of invalid users - users who should never be allowed access to Samba or its shares. This is done with the invalid users option. We hinted at one frequent use of this option earlier: a global default with the [homes] section to ensure that various system users and superusers cannot be forged for access. For example:

[global]
	invalid users = root bin daemon adm sync shutdown \
						halt mail news uucp operator gopher
	auto services = dave peter bob

[homes]
	browsable = no
	writeable = yes

The invalid users option, like valid users, can take group names as well as usernames. In the event that a user or group appears in both lists, the invalid users option takes precedence and the user or group will be denied access to the share.

At the other end of the spectrum, you can explicitly specify users who will be allowed superuser (root) access to a share with the admin users option. An example follows:

[sales]
		path = /home/sales
		comment = Fiction Corp Sales Data
		writeable = yes
		valid users = tom dick harry
		admin users = mike

This option takes both group names and usernames. In addition, you can specify NIS netgroups by preceding them with an @ as well; if the netgroup is not found, Samba will assume that you are referring to a standard Unix group.

Be careful if you assign an entire group administrative privileges to a share. The Samba team highly recommends you avoid using this option, as it essentially gives root access to the specified users or groups for that share.

If you wish to force read-only or read-write access to users who access a share, you can do so with the read list and write list options, respectively. These options can be used on a per-share basis to restrict a writable share or grant write access to specific users in a read-only share, respectively. For example:

[sales]
		path = /home/sales
		comment = Fiction Corp Sales Data
		read only = yes
		write list = tom dick

The write list option cannot override Unix permissions. If you've created the share without giving the write-list user write permission on the Unix system, he or she will be denied write access regardless of the setting of write list.

6.2.1 Guest Access

As mentioned earlier, you can specify users who have guest access to a share. The options that control guest access are easy to work with. The first option, guest account, specifies the Unix account that guest users should be assigned when connecting to the Samba server. The default value for this is set during compilation, and is typically nobody. However, you may want to reset the guest user to ftp if you have trouble accessing various system services.

If you wish to restrict access in a share only to guests - in other words, all clients connect as the guest account when accessing the share - you can use the guest only option in conjunction with the guest ok option, as shown in the following example:

[sales]
		path = /home/sales
		comment = Fiction Corp Sales Data
		writeable = yes
		guest ok = yes
		guest account = ftp
		guest only = yes

Make sure you specify yes for both guest only and guest ok in this scenario; otherwise, Samba will not use the guest acount that you specify.

6.2.2 Access Control Options

Table 6.1 summarizes the options that you can use to control access to shares.

Table 6.1: Share-level Access Options
Option	Parameters	Function	Default	Scope
`admin users`	string (list of usernames)	Specifies a list of users who can perform operations as root.	None	Share
`valid users`	string (list of usernames)	Specifies a list of users that can connect to a share.	None	Share
`invalid users`	string (list of usernames)	Specifies a list of users that will be denied access to a share.	None	Share
`read list`	string (list of usernames)	Specifies a list of users that have read-only access to a writable share.	None	Share
`write list`	string (list of usernames)	Specifies a list of users that have read-write access to a read-only share.	None	Share
`max connections`	numerical	Indicates the maximum number of connections for a share at a given time.	`0`	Share
`guest only (only guest)`	boolean	Specifies that this share allows only guest access.	`no`	Share
`guest account`	string (name of account)	Names the Unix account that will be used for guest access.	`nobody`	Share

6.2.2.1 admin users

This option specifies a list of users that perform file operations as if they were root. This means that they can modify or destroy any other user's work, no matter what the permissions. Any files that they create will have root ownership and will use the default group of the admin user. The admin users option is used to allow PC users to act as administrators for particular shares. We urge you to avoid this option.

6.2.2.2 valid users and invalid users

These two options let you enumerate the users and groups who are granted or denied access to a particular share. You can enter a list of comma-delimited users, or indicate an NIS or Unix group name by prefixing the name with an at-sign (@).

The important rule to remember with these options is that any name or group in the invalid users list will always be denied access, even if it is included (in any form) in the valid users list. By default, neither option has a value associated with it. If both options have no value, any user is allowed to access the share.

6.2.2.3 read list and write list

Like the valid users and invalid users options, this pair of options specifies which users have read-only access to a writeable share and read-write access to a read-only share, respectively. The value of either options is a list of users. read list overrides any other Samba permissions granted - as well as Unix file permissions on the server system - to deny users write access. write list overrides other Samba permissions to grant write access, but cannot grant write access if the user lacks write permissions for the file on the Unix system. You can specify NIS or Unix group names by prefixing the name with an at sign (such as @users). Neither configuration option has a default value associated with it.

6.2.2.4 max connections

This option specifies the maximum number of client connections that a share can have at any given time. Any connections that are attempted after the maximum is reached will be rejected. The default value is 0, which means that an unlimited number of connections are allowed. You can override it per share as follows:

[accounting]
	max connections = 30

This option is useful in the event that you need to limit the number of users who are accessing a licensed program or piece of data concurrently.

6.2.2.5 guest only

This share-level option (sometimes called only guest) forces a connection to a share to be performed with the user specified by the guest account option. The share to which this is applied must explicitly specify guest ok = yes in order for this option to be recognized by Samba. The default value for this option is no.

6.2.2.6 guest account

This option specifies the name of account to be used for guest access to shares in Samba. The default for this option varies from system to system, but it is often set to nobody. Some default user accounts have trouble connecting as guest users. If that occurs on your system, the Samba team recommends using the ftp account as the guest user.

6.2.3 Username Options

Table 6.2 shows two additional options that Samba can use to correct for incompatibilities in usernames between Windows and Unix.

Table 6.2: Username Options
Option	Parameters	Function	Default	Scope
`username map`	string (fully-qualified pathname)	Sets the name of the username mapping file.	None	Global
`username level`	numerical	Indicates the number of capital letters to use when trying to match a username.	`0`	Global

6.2.3.1 username map

Client usernames on an SMB network can be relatively large (up to 255 characters), while usernames on a Unix network often cannot be larger than eight characters. This means that an individual user may have one username on a client and another (shorter) one on the Samba server. You can get past this issue by mapping a free-form client username to a Unix username of eight or fewer characters. It is placed in a standard text file, using a format that we'll describe shortly. You can then specify the pathname to Samba with the global username map option. Be sure to restrict access to this file; make the root user the file's owner and deny write access to others. Otherwise, an untrusted user who can access the file can easily map their client username to the root user of the Samba server.

You can specify this option as follows:

[global]
	username map = /etc/samba/usermap.txt

Each of the entries in the username map file should be listed as follows: the Unix username, followed by an equal sign (=), followed by one or more whitespace-separated SMB client usernames. Note that unless instructed otherwise, (i.e., a guest connection), Samba will expect both the client and the server user to have the same password. You can also map NT groups to one or more specific Unix groups using the @ sign. Here are some examples:

jarwin = JosephArwin
manderso = MarkAnderson
users = @account

Also, you can use the asterisk to specify a wildcard that matches any free-form client username as an entry in the username map file:

nobody = *

Comments in the file can be specified as lines beginning with (#) and (;).

Note that you can also use this file to redirect one Unix user to another user. Be careful if you do so because Samba and your client may not notify the user that the mapping has been made and Samba may be expecting a different password.

6.2.3.2 username level

SMB clients (such as Windows) will often send usernames in SMB connection requests entirely in capital letters; in other words, client usernames are not necessarily case sensitive. On a Unix server, however, usernames are case sensitive: the user ANDY is different from the user andy. By default, Samba attacks this problem by doing the following:

Checking for a user account with the exact name sent by the client
Testing the username in all lowercase letters
Testing the username in lowercase letters with only the first letter capitalized

If you wish to have Samba attempt more combinations of uppercase and lowercase letters, you can use the username level global configuration option. This option takes an integer value that specifies how many letters in the username should be capitalized when attempting to connect to a share. You can specify this options as follows:

[global]
	username level = 3

In this case, Samba will then attempt all permutations of usernames it can compute having three capital letters. The larger the number, the more computations Samba will have to perform to match the username and the longer the authentication will take.