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<?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>
This note was attached to the Samba 2.2.8 release notes because it contains 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 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>
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 name="[IPC$]"/>
autoshare. The <smbconfsection name="[IPC$]"/> 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 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>
        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>
        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:
        </para>

        <para><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>
        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 <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 name="[global]"/> section put:
        </para>

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

        <para>
        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>
        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:
        </para>

        <para><smbconfblock>
<smbconfoption name="interfaces">eth* lo</smbconfoption>
<smbconfoption name="bind interfaces only">yes</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 or 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 he or she 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.
        </para>

        </sect2>

        <sect2 id="firewallports">
        <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 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>
        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>
        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:
        </para>

        <para><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>
        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>
        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 for some reason you cannot use one of the other methods just discussed.
        </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 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>
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 configurations 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 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. 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 <link linkend="firewallports">the Using a 
        firewall</link> section.
        </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 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>
        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>