samba-mirror/docs-xml/Samba3-HOWTO/TOSHARG-Securing.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<!DOCTYPE chapter 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>
<indexterm><primary>security</primary></indexterm>
<indexterm><primary>direct internet access</primary></indexterm>
<indexterm><primary>firewall</primary></indexterm>
<indexterm><primary>private network</primary></indexterm>
<indexterm><primary>barriers</primary></indexterm>
<indexterm><primary>deterents</primary></indexterm>
<indexterm><primary>secured networks</primary></indexterm>
The information contained in this chapter applies in general to all Samba installations. Security is
everyone's concern in the information technology world. A surprising number of Samba servers are being
installed on machines that have direct internet access, thus security is made more critical than it would have been had the
server been located behind a firewall and on a private network. Paranoia regarding server security is causing
some  network administrators to insist on the installation of robust firewalls even on servers that are located
inside secured networks. This chapter provides information to assist the administrator who understands
how to create the needed barriers and deterents against <quote>the enemy</quote>, no matter where [s]he may
come from.
</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>
<indexterm><primary>moderately secure</primary></indexterm>
<indexterm><primary>perimeter firewall</primary></indexterm>
<indexterm><primary>host security</primary></indexterm>
<indexterm><primary>Samba security</primary></indexterm>
There are three levels at which security principles 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>
<indexterm><primary>host-based protection</primary></indexterm>
<indexterm><primary>interface-based exclusion</primary></indexterm>
<indexterm><primary>resource-based exclusion</primary></indexterm>
Samba can be secured from connections that originate from outside the local network. This can 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 name="[IPC$]"/> autoshare. The <smbconfsection
name="[IPC$]"/> share is used for browsing purposes as well as to establish TCP/IP connections.
</para>

<para>
<indexterm><primary>Access Control Entries</primary><see>ACE</see></indexterm>
<indexterm><primary>ACL</primary></indexterm>
<indexterm><primary>controls</primary></indexterm>
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 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, 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>
<indexterm><primary>outside threat</primary></indexterm>
<indexterm><primary>insecure</primary></indexterm>
<indexterm><primary>Internet</primary></indexterm>
	In many installations of Samba, the greatest threat comes from outside
	your immediate network. By default, Samba accepts 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>
<indexterm><primary>allow access</primary></indexterm>
<indexterm><primary>range of hosts</primary></indexterm>
	One of the simplest fixes in this case is to use the <smbconfoption name="hosts allow"/> and
	<smbconfoption name="hosts deny"/> options in the Samba &smb.conf; configuration file to
	allow access to your server only from a specific range of hosts. An example might be:
	<smbconfblock>
	<smbconfoption name="hosts allow">127.0.0.1 192.168.2.0/24 192.168.3.0/24</smbconfoption>
	<smbconfoption name="hosts deny">0.0.0.0/0</smbconfoption>
	</smbconfblock>
	</para>

	<para>
<indexterm><primary>localhost</primary></indexterm>
<indexterm><primary>private networks</primary></indexterm>
<indexterm><primary>called name</primary></indexterm>
	The above will allow SMB connections only 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 <literal>not listening on called name</literal> 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 name="[global]"/> section put:
	<smbconfblock>
	<smbconfoption name="valid users">@smbusers, jacko</smbconfoption>
	</smbconfblock>
	</para>

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

	</sect2>

	<sect2>

	<title>Using Interface Protection</title>

	<para>
<indexterm><primary>network interface</primary></indexterm>
<indexterm><primary>accept connections</primary></indexterm>
<indexterm><primary>Internet</primary></indexterm>
	By default, Samba accepts connections on any network interface that
	it finds on your system. That means if you have an 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:
	<smbconfblock>
	<smbconfoption name="interfaces">eth* lo</smbconfoption>
	<smbconfoption name="bind interfaces only">yes</smbconfoption>
	</smbconfblock>
	</para>

	<para>
<indexterm><primary>interfaces</primary></indexterm>
<indexterm><primary>loopback interface</primary></indexterm>
<indexterm><primary>Ethernet adapters</primary></indexterm>
<indexterm><primary>listen for connections</primary></indexterm>
	This tells Samba to listen for connections only on interfaces with a name starting with
	<constant>eth</constant> such as <constant>eth0</constant> or <constant>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>
<indexterm><primary>PPP</primary></indexterm>
<indexterm><primary>SMB</primary></indexterm>
<indexterm><primary>cracker</primary></indexterm>
<indexterm><primary>confirm address</primary></indexterm>
	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 [s]he will get a TCP connection refused reply. In that case, no Samba code
	is run at all, because the operating system has been told not to pass connections from that interface to any
	Samba process. However, the refusal helps a would-be cracker by confirming that the IP address provides
	valid active services.
	</para>

	<para>
<indexterm><primary>ignore connection</primary></indexterm>
<indexterm><primary>refusing connection</primary></indexterm>
<indexterm><primary>exploitation</primary></indexterm>
<indexterm><primary>denial of service</primary></indexterm>
<indexterm><primary>firewall</primary></indexterm>
	A better response would be to ignore the connection (from, for example, ppp0) altogether. The
	advantage of ignoring the connection attempt, as compared with refusing it, is that it foils those who
	probe an interface with the sole intention of finding valid IP addresses for later use in exploitation
	or denial of service attacks. This method of dealing with potential malicious activity demands the
	use of appropriate firewall mechanisms.
	</para>

	</sect2>

	<sect2 id="firewallports">
	<title>Using a Firewall</title>

	<para>
<indexterm><primary>deny access</primary></indexterm>
<indexterm><primary>exposed</primary></indexterm>
<indexterm><primary>firewall active</primary></indexterm>
	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:
<indexterm><primary>Port 135/TCP</primary></indexterm>
<indexterm><primary>Port 137/UDP</primary></indexterm>
<indexterm><primary>Port 138/UDP</primary></indexterm>
<indexterm><primary>Port 139/TCP</primary></indexterm>
<indexterm><primary>Port 445/TCP</primary></indexterm>
	</para>

	<simplelist>
		<member>Port 135/TCP - used by smbd</member>
		<member>Port 137/UDP - used by nmbd</member>
		<member>Port 138/UDP - used by nmbd</member>
		<member>Port 139/TCP - used by smbd</member>
		<member>Port 445/TCP - used by smbd</member>
	</simplelist>

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

	<para>
<indexterm><primary>configuring a firewall</primary></indexterm>
<indexterm><primary>high order ports</primary></indexterm>
<indexterm><primary>block incoming packets</primary></indexterm>
	When configuring a firewall, the high order ports (1024-65535) are often used for outgoing connections and
	therefore should be permitted through the firewall. It is prudent to block incoming packets on the high order
	ports except for established connections.
	</para>

	</sect2>

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

	<para>
<indexterm><primary>IPC$</primary></indexterm>
<indexterm><primary>deny</primary></indexterm>
<indexterm><primary>security hole</primary></indexterm>
	If the above methods are not suitable, then you could also place a more specific deny on the IPC$ 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 untrustworthy hosts.
	</para>

	<para>
	To do this you could use:
	<smbconfblock>
	<smbconfsection name="[IPC$]"/>
	<smbconfoption name="hosts allow">192.168.115.0/24 127.0.0.1</smbconfoption>
	<smbconfoption name="hosts deny">0.0.0.0/0</smbconfoption>
	</smbconfblock>
	</para>

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

	<para>
<indexterm><primary>access denied</primary></indexterm>
<indexterm><primary>IPC$</primary></indexterm>
<indexterm><primary>browse shares</primary></indexterm>
	If you use this method, then clients will be given an <literal>`access denied'</literal> reply when they try
	to access the IPC$ 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 for some reason you cannot use one of the other methods
	just discussed.
	</para>

	</sect2>

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

	<para>
<indexterm><primary>NTLMv2</primary></indexterm>
	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 to 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 negotiated.
	</para>
	</sect2>
</sect1>

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

<para>
<indexterm><primary>updates</primary></indexterm>
<indexterm><primary>important announcements</primary></indexterm>
<indexterm><primary>security vulnerability</primary></indexterm>
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 promptly 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 Samba and host platform configurations were really as intuitive as one might like them to be, this
chapter would not be necessary. Security issues are often vexing for a support person to resolve, not because
of the complexity of the problem, but because 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. Linux vendors tend to install 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 <link linkend="firewallports">the Using a 
	Firewall</link> section.
	</para>

	</sect2>

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

	<para>
	<quote>
<indexterm><primary>mapping home directory</primary></indexterm>
<indexterm><primary>own home directory</primary></indexterm>
	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>
<indexterm><primary>security flaw</primary></indexterm>
<indexterm><primary>defined shares</primary></indexterm>
	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 on to the UNIX box, except that it only allows such views onto the file
	system as are allowed by the defined shares.
	</para>

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

	<para>
<indexterm><primary>security policies</primary></indexterm>
<indexterm><primary>permissions</primary></indexterm>
	Samba tries very hard not to second guess the UNIX administrator's 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">%S</smbconfoption>
	option in the <smbconfsection name="[homes]"/> share definition.
	</para>

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

</sect1>
</chapter>