samba-mirror/docs/Samba-HOWTO-Collection/Securing.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE book PUBLIC "-//Samba-Team//DTD DocBook V4.2-Based Variant V1.0//EN" "http://www.samba.org/samba/DTD/samba-doc">
<chapter id="securing-samba">

<chapterinfo>
	&author.tridge;
	&author.jht;
	<pubdate>May 26, 2003</pubdate>
</chapterinfo>

<title>Securing Samba</title>

<sect1>
<title>Introduction</title>
<para>
This note was attached to the Samba 2.2.8 release notes as it contained an
important security fix. The information contained here applies to Samba
installations in general.
</para>

<blockquote>
<para>
A new apprentice reported for duty to the chief engineer of a boiler house. He said, <quote>Here I am,
if you will show me the boiler I'll start working on it.</quote> Then engineer replied, <quote>You're leaning
on it!</quote>
</para>
</blockquote>

<para>
Security concerns are just like that. You need to know a little about the subject to appreciate
how obvious most of it really is. The challenge for most of us is to discover that first morsel
of knowledge with which we may unlock the secrets of the masters.
</para>

</sect1>

<sect1>
<title>Features and Benefits</title>

<para>
There are three levels at which security principals must be observed in order to render a site
at least moderately secure. They are the perimeter firewall, the configuration of the host
server that is running Samba and Samba itself.
</para>

<para>
Samba permits a most flexible approach to network security. As far as possible Samba implements
the latest protocols to permit more secure MS Windows file and print operations.
</para>

<para>
Samba may be secured from connections that originate from outside the local network. This may be
done using <emphasis>host-based protection</emphasis> (using Samba's implementation of a technology
known as <quote>tcpwrappers,</quote> or it may be done be using <emphasis>interface-based exclusion</emphasis>
so &smbd; will bind only to specifically permitted interfaces. It is also
possible to set specific share or resource-based exclusions, for example on the <smbconfsection>[IPC$]</smbconfsection>
auto-share. The <smbconfsection>[IPC$]</smbconfsection> share is used for browsing purposes as well as to establish
TCP/IP connections.
</para>

<para>
Another method by which Samba may be secured is by setting Access Control Entries (ACEs) in an Access 
Control List (ACL) on the shares themselves. This is discussed in <link linkend="AccessControls">File, Directory and Share Access Controls</link>.
</para>

</sect1>

<sect1>
<title>Technical Discussion of Protective Measures and Issues</title>

<para>
The key challenge of security is the fact that protective measures suffice at best
only to close the door on known exploits and breach techniques. Never assume that
because you have followed these few measures that the Samba server is now an impenetrable
fortress! Given the history of information systems so far, it is only a matter of time
before someone will find yet another vulnerability.
</para>

	<sect2>
	<title>Using Host-Based Protection</title>

	<para>
	In many installations of Samba, the greatest threat comes from outside
	your immediate network. By default, Samba will accept connections from
	any host, which means that if you run an insecure version of Samba on
	a host that is directly connected to the Internet you can be
	especially vulnerable.
	</para>

	<para>
	One of the simplest fixes in this case is to use the <smbconfoption><name>hosts allow</name></smbconfoption> and
	<smbconfoption><name>hosts deny</name></smbconfoption> options in the Samba &smb.conf; configuration file to only
	allow access to your server from a specific range of hosts. An example might be:
	</para>

	<para><smbconfblock>
<smbconfoption><name>hosts allow</name><value>127.0.0.1 192.168.2.0/24 192.168.3.0/24</value></smbconfoption>
<smbconfoption><name>hosts deny</name><value>0.0.0.0/0</value></smbconfoption>
	</smbconfblock></para>

	<para>
	The above will only allow SMB connections from <constant>localhost</constant> (your own
	computer) and from the two private networks 192.168.2 and 192.168.3. All other
	connections will be refused as soon as the client sends its first packet. The refusal
	will be marked as <errorname>not listening on called name</errorname> error.
	</para>

	</sect2>

	<sect2>
	<title>User-Based Protection</title>

	<para>
	If you want to restrict access to your server to valid users only, then the following
	method may be of use. In the &smb.conf; <smbconfsection>[global]</smbconfsection> section put:
	</para>

	<para><smbconfblock>
<smbconfoption><name>valid users</name><value>@smbusers, jacko</value></smbconfoption>
	</smbconfblock></para>

	<para>
	This restricts all server access to either the user <emphasis>jacko</emphasis>
	or to members of the system group <emphasis>smbusers</emphasis>.
	</para>

	</sect2>

	<sect2>

	<title>Using Interface Protection</title>

	<para>
	By default, Samba will accept connections on any network interface that
	it finds on your system. That means if you have a ISDN line or a PPP
	connection to the Internet then Samba will accept connections on those
	links. This may not be what you want.
	</para>

	<para>
	You can change this behavior using options like this:
	</para>

	<para><smbconfblock>
<smbconfoption><name>interfaces</name><value>eth* lo</value></smbconfoption>
<smbconfoption><name>bind interfaces only</name><value>yes</value></smbconfoption>
	</smbconfblock></para>

	<para>
	This tells Samba to only listen for connections on interfaces with a
	name starting with <constant>eth</constant> such as <constant>eth0, eth1</constant> plus on the loopback
	interface called <constant>lo</constant>. The name you will need to use depends on what
	OS you are using. In the above, I used the common name for Ethernet
	adapters on Linux.
	</para>

	<para>
	If you use the above and someone tries to make an SMB connection to
	your host over a PPP interface called <constant>ppp0,</constant> then they will get a TCP
	connection refused reply. In that case, no Samba code is run at all as
	the operating system has been told not to pass connections from that
	interface to any Samba process.
	</para>

	</sect2>

	<sect2>
	<title>Using a Firewall</title>

	<para>
	Many people use a firewall to deny access to services they do not
	want exposed outside their network. This can be a good idea,
	although I recommend using it in conjunction with the above
	methods so you are protected even if your firewall is not active
	for some reason.
	</para>

	<para>
	If you are setting up a firewall, you need to know what TCP and
	UDP ports to allow and block. Samba uses the following:
	</para>

	<simplelist>
		<member>UDP/137 - used by nmbd</member>
		<member>UDP/138 - used by nmbd</member>
		<member>TCP/139 - used by smbd</member>
		<member>TCP/445 - used by smbd</member>
	</simplelist>

	<para>
	The last one is important as many older firewall setups may not be
	aware of it, given that this port was only added to the protocol in
	recent years. 
	</para>

	</sect2>

	<sect2>
	<title>Using IPC$ Share-Based Denials </title>

	<para>
	If the above methods are not suitable, then you could also place a
	more specific deny on the IPC$ share that is used in the recently
	discovered security hole. This allows you to offer access to other
	shares while denying access to IPC$ from potentially un-trustworthy
	hosts.
	</para>

	<para>
	To do this you could use:
	</para>

	<para><smbconfblock>
<smbconfsection>[IPC$]</smbconfsection>
<smbconfoption><name>hosts allow</name><value>192.168.115.0/24 127.0.0.1</value></smbconfoption>
<smbconfoption><name>hosts deny</name><value>0.0.0.0/0</value></smbconfoption>
	</smbconfblock></para>

	<para>
	This instructs Samba that IPC$ connections are not allowed from
	anywhere except from the two listed network addresses (localhost and the 192.168.115
	subnet). Connections to other shares are still allowed. As the
	IPC$ share is the only share that is always accessible anonymously,
	this provides some level of protection against attackers that do not
	know a valid username/password for your host.
	</para>

	<para>
	If you use this method, then clients will be given an <errorname>`access denied'</errorname>
	reply when they try to access the IPC$ share. Those clients will not be able to
	browse shares, and may also be unable to access some other resources.  This is not
	recommended unless you cannot use one of the other methods listed above for some reason.
	</para>

	</sect2>

	<sect2>
	<title>NTLMv2 Security</title>

	<para>
	To configure NTLMv2 authentication, the following registry keys are worth knowing about:
	</para>

	<para>
		<screen>
		[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
		"lmcompatibilitylevel"=dword:00000003
		</screen>
	</para>

	<para>
	The value 0x00000003 means send NTLMv2 response only. Clients will use NTLMv2 authentication,
	use NTLMv2 session security if the server supports it. Domain Controllers accept LM,
	NTLM and NTLMv2 authentication.
	</para>

	<para>
		<screen>
		[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0]
		"NtlmMinClientSec"=dword:00080000
		</screen>
	</para>

	<para>
	The value 0x00080000 means permit only NTLMv2 session security. If either NtlmMinClientSec or
	NtlmMinServerSec is set to 0x00080000, the connection will fail if NTLMv2
	session security is not negotiated.
	</para>
	</sect2>
</sect1>

<sect1>
<title>Upgrading Samba</title>

<para>
Please check regularly on <ulink noescape="1" url="http://www.samba.org/">http://www.samba.org/</ulink> for updates and
important announcements. Occasionally security releases are made and 
it is highly recommended to upgrade Samba when a security vulnerability
is discovered. Check with your OS vendor for OS specific upgrades.
</para>

</sect1>

<sect1>
<title>Common Errors</title>

<para>
If all of Samba and host platform configuration were really as intuitive as one might like them to be, this
section would not be necessary. Security issues are often vexing for a support person to resolve, not
because of the complexity of the problem, but for the reason that most administrators who post what turns
out to be a security problem request are totally convinced that the problem is with Samba.
</para>

	<sect2>
	<title>Smbclient Works on Localhost, but the Network Is Dead</title>

	<para>
	This is a common problem. Red Hat Linux (and others) installs a default firewall.
	With the default firewall in place, only traffic on the loopback adapter (IP address 127.0.0.1)
	is allowed through the firewall.
	</para>

	<para>
	The solution is either to remove the firewall (stop it) or modify the firewall script to
	allow SMB networking traffic through. See section above in this chapter.
	</para>

	</sect2>

	<sect2>
	<title>Why Can Users Access Home Directories of Other Users?</title>

	<para>
	<quote>
	We are unable to keep individual users from mapping to any other user's
	home directory once they have supplied a valid password! They only need
	to enter their own password. I have not found any method to configure
	Samba so that  users may map only their own home directory.
	</quote>
	</para>

	<para><quote>
	User xyzzy can map his home directory. Once mapped user xyzzy can also map
	anyone else's home directory.
	</quote></para>

	<para>
	This is not a security flaw, it is by design. Samba allows users to have
	exactly the same access to the UNIX file system as when they were logged
	onto the UNIX box, except that it only allows such views onto the file 
	system as are allowed by the defined shares.
	</para>

	<para>
	If your UNIX home directories are set up so that one user can happily <command>cd</command>
	into another users directory and execute <command>ls</command>, the UNIX security solution is to change file
	permissions on the user's home directories such that the <command>cd</command> and <command>ls</command> are denied.
	</para>

	<para>
	Samba tries very hard not to second guess the UNIX administrators security policies, and
	trusts the UNIX admin to set the policies and permissions he or she desires.
	</para>

	<para>
	Samba allows the behavior you require. Simply put the <smbconfoption><name>only user</name><value>%S</value></smbconfoption>
	option in the <smbconfsection>[homes]</smbconfsection> share definition.
	</para>

	<para>
	The <smbconfoption><name>only user</name><value></value></smbconfoption> works in conjunction with the <smbconfoption><name>users</name><value>list</value></smbconfoption>,
	so to get the behavior you require, add the line :
	<smbconfblock>
<smbconfoption><name>users</name><value>%S</value></smbconfoption>
</smbconfblock>
	this is equivalent to adding
	<smbconfblock>
<smbconfoption><name>valid users</name><value>%S</value></smbconfoption>
	</smbconfblock>
	to the definition of the <smbconfsection>[homes]</smbconfsection> share, as recommended in
	the &smb.conf; man page.
	</para>
	</sect2>

</sect1>
</chapter>