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In 6, we looked at some of the access controls that can be applied to Samba shares. We'll review them here, focusing on how they affect access to file shares specifically. Table 11.1 lists the set of access control parameters we'll cover in the next few sections of the text. Table 11.1 Useful Access Control Parameters
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hosts allow hosts deny
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Allow/restrict hosts
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username valid users invalid users only user force user guest ok guest only guest account group force group
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Restrict/allow users
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Enforce named group rights
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By File Operation
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read only read list writeable write list force force force force security mode create mode directory mode directory security mode
Read-only access
Write access
Specify permissions
security mask create mask directory mask directory security mask
Hide
veto files hide files hide dot files delete veto files dont descend
Hide/restrict specified files
Browse Rights
The least-effective method of access control is security by obscurity. I mention this because many sites will use the browseable parameter to hide a share and assume that is sufficient to protect it from unauthorized access. They subscribe to a cyber philosophical twist on old sayings like, "What your users don't know can't hurt you" and "User ignorance is system administrator bliss". There are certainly good reasons for making a share nonbrowseable. Just remember that although a share doesn't show up in a browse list, users can still access it if they know the name a priori: browseable = <yes, no>
Host Access
The most basic form of Samba access control is by host name or IP address. Use the hosts allow and hosts deny parameters in the global and share sections of smb.conf to restrict access to those hosts that are trusted members of your workgroup or domain. You can specify computers by name, IP number, or IP group by using an IP class value or subnet mask. The except keyword can be used in conjunction with group values like a subnet mask to exclude individual or groups of machines (Example 11.4).
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Example 11.4 Restricting Access by Host hosts allow = 192.168. except 192.168.0.10 hosts deny = frodo, bilbo hosts allow = 149.125.10.0/255.255.255.0
User and Group Access
A finer level of access restraint can be applied at the user level. Samba allows you to limit right of entry to specific individuals through the username, only user, valid users, and invalid users parameters (Example 11.5). Per the particular parameter employed, the client-supplied user name is checked against the associated user list before access is granted or denied. If the user name is prefixed with an "@" sign, or an "&" sign it is assumed to correspond to an NIS netgroup. If it is prefixed with a "+" sign, the name is checked against the UNIX group file. Example 11.5 Restricting Access by User Name invalid users = root system +wheel only user = fred The following rules are applied when a client requests access to a share or service. Access is granted if: An unrestricted client passes a user name and password that maps to a permitted UNIX account name. A client previously registered a user name with the server and follows up by supplying a valid password. The client's NetBIOS name and previously used user name are checked against the supplied password. A client with a previously validated user name/password pair supplies a validation token to the server. The share is configured with the user list parameter and the client supplies a valid password for one of the usernames in the list. A share is configured with a guest account parameter and the client supplies a valid guest account name. Sometimes you will want to make a share available to anyone. Adding the guest ok or guest only parameters to a share section definition will accomplish this. Note that even though you are granting public access to the resource, you will still want to restrict operation privileges on the share by mapping guest access to a valid UNIX account. Indicate the desired user account name as a value to the guest account parameter. A
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good candidate account with limited privileges is the nobody account which is commonly used for UNIX anonymous access services (Example 11.6). Example 11.6 Restricting Guest Access guest ok = true guest account = nobody
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