Using Samba

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 explicity 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.

Networking Options

The networking options we introduced above are summarized in Table 4.5.

Table 4-5. Networking Configuration Options

Option	Parameters	Function	Default	Scope
hosts allow (allow hosts)	string (list of hostnames)	Specifies the machines that can connect to Samba.	none	Share
hosts deny (deny hosts)	string (list of hostnames)	Specifies the machines that cannot connect to Samba.	none	Share
interfaces	string (list of IP/netmask combinations)	Sets the network interfaces Samba will respond to. Allows correcting defaults.	system-dependent	Global
bind interfaces only	boolean	If set to yes, Samba will bind only to those interfaces specified by the interfaces option.	no	Global
socket address	string (IP address)	Sets IP address to listen on, for use with multiple virtual interfaces on a server.	none	Global

hosts allow

The hosts allow option (sometimes written as allow hosts) specifies the machines that have permission to access shares on the Samba server, written as a comma- or space-separated list of names of machines or their IP addresses. You can gain quite a bit of security by simply placing your LAN’s subnet address in this option. For example, we specified the following in our example:

hosts allow = 192.168.220. localhost

Note that we placed localhost after the subnet address. One of the most common mistakes when attempting to use the hosts allow option is to accidentally disallow the Samba server from communicating with itself. The smbpasswd program will occasionally need to connect to the Samba server as a client in order to change a user’s encrypted password. In addition, local browsing propagation requires local host access. If this option is enabled and the localhost address is not specified, the locally-generated packets requesting the change of the encrypted password will be discarded by Samba, and browsing propagation will not work properly. To avoid this, explicitly allow the loopback address (either localhost or 127.0.0.1) to be used.^[19]

You can specify any of the following formats for this option:

• Hostnames, such as ftp.example.com .

• IP addresses, like 130.63.9.252.

• Domain names, which can be differentiated from individual hostnames because they start with a dot. For example, .ora.com represents all machines within the ora.com domain.

• Netgroups, which start with an at-sign, such as @printerhosts. Netgroups are available on systems running yellow pages/NIS or NIS+, but rarely otherwise. If netgroups are supported on your system, there should be a netgroups manual page that describes them in more detail.

• Subnets, which end with a dot. For example, 130.63.9. means all the machines whose IP addresses begin with 130.63.9.

• The keyword ALL, which allows any client access.

• The keyword EXCEPT followed by more one or more names, IP addresses, domain names, netgroups, or subnets. For example, you could specify that Samba allow all hosts except those on the 192.168.110 subnet with hosts allow = ALL EXCEPT 192.168.110. (remember the trailing dot).

Using the ALL keyword is almost always a bad idea, since it means that anyone on any network can browse your files if they guess the name of your server.

Note that there is no default value for the hosts allow configuration option, although the default course of action in the event that neither option is specified is to allow access from all sources. In addition, if you specify this option in the [global] section of the configuration file, it will override any hosts allow options defined shares.

hosts deny

The hosts deny option (also deny hosts) specifies machines that do not have permission to access a share, written as a comma- or space-separated list of machine names or their IP addresses. Use the same format as specifying clients as the hosts allow option above. For example, to restrict access to the server from everywhere but example.com, you could write:

hosts deny = ALL EXCEPT .example.com

Like hosts allow, there is no default value for the hosts deny configuration option, although the default course of action in the event that neither option is specified is to allow access from all sources. Also, if you specify this option in the [global] section of the configuration file, it will override any hosts deny options defined in shares. If you wish to deny hosts access to specific shares, omit both the hosts allow and hosts deny options in the [global] section of the configuration file.

interfaces

The interfaces option outlines the network addresses to which you want the Samba server to recognize and respond. This option is handy if you have a computer that resides on more than one network subnet. If this option is not set, Samba searches for the primary network interface of the server (typically the first Ethernet card) upon startup and configures itself to operate on only that subnet. If the server is configured for more than one subnet and you do not specify this option, Samba will only work on the first subnet it encounters. You must use this option to force Samba to serve the other subnets on your network.

The value of this option is one or more sets of IP address/netmask pairs, such as the following:

interfaces = 192.168.220.100/255.255.255.0 192.168.210.30/255.255.255.0

You can optionally specify a CIDR format bitmask, as follows:

interfaces = 192.168.220.100/24 192.168.210.30/24

The bitmask number specifies the first number of bits that will be turned on in the netmask. For example, the number 24 means that the first 24 (of 32) bits will be activated in the bit mask, which is the same as saying 255.255.255.0. Likewise, 16 would be equal to 255.255.0.0, and 8 would be equal to 255.0.0.0.

Tip

This option may not work correctly if you are using DHCP.

bind interfaces only

The bind interfaces only option can be used to force the smbd and nmbd processes to serve SMB requests to only those addresses specified by the interfaces option. The nmbd process normally binds to the all addresses interface (0.0.0.0.) on ports 137 and 138, allowing it to receive broadcasts from anywhere. However, you can override this behavior with the following:

bind interfaces only = yes

This will cause both Samba processes to ignore any packets whose origination address does not match the broadcast address(es) specified by the interfaces option, including broadcast packets. With smbd, this option will cause Samba to not serve file requests to subnets other than those listed in the interfaces option. You should avoid using this option if you want to allow temporary network connections, such as those created through SLIP or PPP. It’s very rare that this option is needed, and it should only be used by experts.

Tip

If you set bind interfaces only to yes , you should add the localhost address (127.0.01) to the “interfaces” list. Otherwise, smbpasswd will be unable to connect to the server using its default mode in order to change a password.

socket address

The socket address option dictates which of the addresses specified with the interfaces parameter Samba should listen on for connections. Samba accepts connections on all addresses specified by default. When used in an smb.conf file, this option will force Samba to listen on only one IP address. For example:

interfaces = 192.168.220.100/24 192.168.210.30/24
socket address = 192.168.210.30

This option is a programmer’s tool and we recommend that you do not use it.

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^[19] Starting with Samba 2.0.5, localhost will automatically be allowed unless it is explicitly denied.