4.6 Networking Options with Samba

If you're running Samba on a multi-homed machine (that is, one on multiple subnets), or even if you want to implement a security policy on your own subnet, you should take a close look at the networking configuration options:

For the purposes of this exercise, let's assume that our Samba server is connected to a network with more than one subnet. Specifically, the machine can access both the 192.168.220.* and 134.213.233.* subnets. Here are our additions to the ongoing configuration file for the networking configuration options:

[global]
	netbios name = HYDRA
	server string = Samba %v on (%L)
	workgroup = SIMPLE

	#  Networking configuration options
	hosts allow = 192.168.220. 134.213.233. localhost
	hosts deny = 192.168.220.102
	interfaces = 192.168.220.100/255.255.255.0 \
					134.213.233.110/255.255.255.0
	bind interfaces only = yes

[data]
	path = /home/samba/data
	guest ok = yes
	comment = Data Drive
	volume = Sample-Data-Drive
	writeable = yes
	

Let's first talk about the hosts allow and hosts deny options. If these options sound familiar, you're probably thinking of the hosts.allow and hosts.deny files that are found in the /etc directories of many Unix systems. The purpose of these options is identical to those files; they provide a means of security by allowing or denying the connections of other hosts based on their IP addresses. Why not just use the hosts.allow and hosts.deny files themselves? Because there may be services on the server that you want others to access without giving them access Samba's disk or printer shares

With the hosts allow option above, we've specified a cropped IP address: 192.168.220. (Note that there is still a third period; it's just missing the fourth number.) This is equivalent to saying: "All hosts on the 192.168.220 subnet." However, we've explicitly specified in a hosts deny line that 192.168.220.102 is not to be allowed access.

You might be wondering: why will 192.168.220.102 be denied even though it is still in the subnet matched by the hosts allow option? Here is how Samba sorts out the rules specified by hosts allow and hosts deny :

1. If there are no allow or deny options defined anywhere in smb.conf, Samba will allow connections from any machine allowed by the system itself.

2. If there are hosts allow or hosts deny options defined in the [global] section of smb.conf, they will apply to all shares, even if the shares have an overriding option defined.

3. If there is only a hosts allow option defined for a share, only the hosts listed will be allowed to use the share. All others will be denied.

4. If there is only a hosts deny option defined for a share, any machine which is not on the list will be able to use the share.

5. If both a hosts allow and hosts deny option are defined, a host must appear in the allow list and not appear in the deny list (in any form) in order to access the share. Otherwise, the host will not be allowed.

   WARNING: Take care that you don't explicitly allow a host to access a share, but then deny access to the entire subnet of which the host is part.

Let's look at another example of that final item. Consider the following options:

hosts allow = 111.222.
hosts deny = 111.222.333.

In this case, only the hosts that belong to the subnet 111.222.*.* will be allowed access to the Samba shares. However, if a client belongs to the 111.222.333.* subnet, it will be denied access, even though it still matches the qualifications outlined by hosts allow. The client must appear on the hosts allow list and must not appear on the hosts deny list in order to gain access to a Samba share. If a computer attempts to access a share to which it is not allowed access, it will receive an error message.

The other two options that we've specified are the interfaces and the bind interface only address. Let's look at the interfaces option first. Samba, by default, sends data only from the primary network interface, which in our example is the 192.168.220.100 subnet. If we would like it to send data to more than that one interface, we need to specify the complete list with the interfaces option. In the previous example, we've bound Samba to interface with both subnets (192.168.220 and 134.213.233) on which the machine is operating by specifying the other network interface address: 134.213.233.100. If you have more than one interface on your computer, you should always set this option as there is no guarantee that the primary interface that Samba chooses will be the right one.

Finally, the bind interfaces only option instructs the nmbd process not to accept any broadcast messages other than those subnets specified with the interfaces option. Note that this is different from the hosts allow and hosts deny options, which prevent machines from making connections to services, but not from receiving broadcast messages. Using the bind interfaces only option is a way to shut out even datagrams from foreign subnets from being received by the Samba server. In addition, it instructs the smbd process to bind to only the interface list given by the interfaces option. This restricts the networks that Samba will serve.

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