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<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter�18.�Securing Samba</title><link rel="stylesheet" href="../samba.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.74.0"><link rel="home" href="index.html" title="The Official Samba 3.4.x HOWTO and Reference Guide"><link rel="up" href="optional.html" title="Part�III.�Advanced Configuration"><link rel="prev" href="locking.html" title="Chapter�17.�File and Record Locking"><link rel="next" href="InterdomainTrusts.html" title="Chapter�19.�Interdomain Trust Relationships"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Chapter�18.�Securing Samba</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="locking.html">Prev</a>�</td><th width="60%" align="center">Part�III.�Advanced Configuration</th><td width="20%" align="right">�<a accesskey="n" href="InterdomainTrusts.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="securing-samba"></a>Chapter�18.�Securing Samba</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Andrew</span> <span class="orgname">Samba Team</span> <span class="surname">Tridgell</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><code class="email"><<a class="email" href="mailto:tridge@samba.org">tridge@samba.org</a>></code></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">John</span> <span class="othername">H.</span> <span class="orgname">Samba Team</span> <span class="surname">Terpstra</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><code class="email"><<a class="email" href="mailto:jht@samba.org">jht@samba.org</a>></code></p></div></div></div></div><div><p class="pubdate">May 26, 2003</p></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="securing-samba.html#id2618082">Introduction</a></span></dt><dt><span class="sect1"><a href="securing-samba.html#id2618181">Features and Benefits</a></span></dt><dt><span class="sect1"><a href="securing-samba.html#id2618327">Technical Discussion of Protective Measures and Issues</a></span></dt><dd><dl><dt><span class="sect2"><a href="securing-samba.html#id2618343">Using Host-Based Protection</a></span></dt><dt><span class="sect2"><a href="securing-samba.html#id2618497">User-Based Protection</a></span></dt><dt><span class="sect2"><a href="securing-samba.html#id2618557">Using Interface Protection</a></span></dt><dt><span class="sect2"><a href="securing-samba.html#firewallports">Using a Firewall</a></span></dt><dt><span class="sect2"><a href="securing-samba.html#id2618907">Using IPC$ Share-Based Denials </a></span></dt><dt><span class="sect2"><a href="securing-samba.html#id2619056">NTLMv2 Security</a></span></dt></dl></dd><dt><span class="sect1"><a href="securing-samba.html#id2619110">Upgrading Samba</a></span></dt><dt><span class="sect1"><a href="securing-samba.html#id2619154">Common Errors</a></span></dt><dd><dl><dt><span class="sect2"><a href="securing-samba.html#id2619170">Smbclient Works on Localhost, but the Network Is Dead</a></span></dt><dt><span class="sect2"><a href="securing-samba.html#id2619198">Why Can Users Access Other Users' Home Directories?</a></span></dt></dl></dd></dl></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2618082"></a>Introduction</h2></div></div></div><p>

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 “<span class="quote">the enemy</span>”, no matter where [s]he may

come from.

</p><div class="blockquote"><blockquote class="blockquote"><p>

A new apprentice reported for duty to the chief engineer of a boiler house. He said, “<span class="quote">Here I am,

if you will show me the boiler I'll start working on it.</span>” Then engineer replied, “<span class="quote">You're leaning

on it!</span>”

</p></blockquote></div><p>

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.

</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2618181"></a>Features and Benefits</h2></div></div></div><p>

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.

</p><p>

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.

</p><p>

Samba can be secured from connections that originate from outside the local network. This can be done using

<span class="emphasis"><em>host-based protection</em></span>, using Samba's implementation of a technology known as

“<span class="quote">tcpwrappers,</span>” or it may be done be using <span class="emphasis"><em>interface-based exclusion</em></span> so

<span class="application">smbd</span> will bind only to specifically permitted interfaces. It is also possible to set specific share- or

resource-based exclusions, for example, on the <em class="parameter"><code>[IPC$]</code></em> autoshare. The <em class="parameter"><code>[IPC$]</code></em> share is used for browsing purposes as well as to establish TCP/IP connections.

</p><p>

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

<a class="link" href="AccessControls.html" title="Chapter�16.�File, Directory, and Share Access Controls">File, Directory, and Share Access Controls</a>.

</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2618327"></a>Technical Discussion of Protective Measures and Issues</h2></div></div></div><p>

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.

</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2618343"></a>Using Host-Based Protection</h3></div></div></div><p>

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.

</p><p>

One of the simplest fixes in this case is to use the <a class="link" href="smb.conf.5.html#HOSTSALLOW" target="_top">hosts allow</a> and

<a class="link" href="smb.conf.5.html#HOSTSDENY" target="_top">hosts deny</a> options in the Samba <code class="filename">smb.conf</code> configuration file to

allow access to your server only from a specific range of hosts. An example might be:

</p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2618433"></a><em class="parameter"><code>hosts allow = 127.0.0.1 192.168.2.0/24 192.168.3.0/24</code></em></td></tr><tr><td><a class="indexterm" name="id2618445"></a><em class="parameter"><code>hosts deny = 0.0.0.0/0</code></em></td></tr></table><p>

</p><p>

The above will allow SMB connections only from <code class="constant">localhost</code> (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 <code class="literal">not listening on called name</code> error.

</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2618497"></a>User-Based Protection</h3></div></div></div><p>

If you want to restrict access to your server to valid users only, then the following

method may be of use. In the <code class="filename">smb.conf</code> <em class="parameter"><code>[global]</code></em> section put:

</p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2618525"></a><em class="parameter"><code>valid users = @smbusers, jacko</code></em></td></tr></table><p>

</p><p>

This restricts all server access either to the user <span class="emphasis"><em>jacko</em></span>

or to members of the system group <span class="emphasis"><em>smbusers</em></span>.

</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2618557"></a>Using Interface Protection</h3></div></div></div><p>

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.

</p><p>

You can change this behavior using options like this:

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</p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2618600"></a><em class="parameter"><code>interfaces = eth* lo</code></em></td></tr><tr><td><a class="indexterm" name="id2618611"></a><em class="parameter"><code>bind interfaces only = yes</code></em></td></tr></table><p>

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</p><p>

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This tells Samba to listen for connections only on interfaces with a name starting with

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<code class="constant">eth</code> such as <code class="constant">eth0</code> or <code class="constant">eth1</code>, plus on the loopback interface called

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<code class="constant">lo</code>. The name you will need to use depends on what OS you are using. In the above, I used

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the common name for Ethernet adapters on Linux.

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</p><p>

111

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If you use the above and someone tries to make an SMB connection to your host over a PPP interface called

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<code class="constant">ppp0</code>, then [s]he will get a TCP connection refused reply. In that case, no Samba code

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is run at all, because the operating system has been told not to pass connections from that interface to any

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Samba process. However, the refusal helps a would-be cracker by confirming that the IP address provides

119

valid active services.

120

</p><p>

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125

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A better response would be to ignore the connection (from, for example, ppp0) altogether. The

127

advantage of ignoring the connection attempt, as compared with refusing it, is that it foils those who

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probe an interface with the sole intention of finding valid IP addresses for later use in exploitation

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or denial of service attacks. This method of dealing with potential malicious activity demands the

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use of appropriate firewall mechanisms.

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</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="firewallports"></a>Using a Firewall</h3></div></div></div><p>

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133

134

135

Many people use a firewall to deny access to services they do not want exposed outside their network. This can

136

be a good idea, although I recommend using it in conjunction with the above methods so you are protected even

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if your firewall is not active for some reason.

138

</p><p>

139

If you are setting up a firewall, you need to know what TCP and UDP ports to allow and block. Samba uses

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the following:

141

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144

145

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</p><table class="simplelist" border="0" summary="Simple list"><tr><td>Port 135/TCP - used by smbd</td></tr><tr><td>Port 137/UDP - used by nmbd</td></tr><tr><td>Port 138/UDP - used by nmbd</td></tr><tr><td>Port 139/TCP - used by smbd</td></tr><tr><td>Port 445/TCP - used by smbd</td></tr></table><p>

147

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The last one is important because many older firewall setups may not be aware of it, given that this port

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was only added to the protocol in recent years.

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</p><p>

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152

153

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When configuring a firewall, the high order ports (1024-65535) are often used for outgoing connections and

155

therefore should be permitted through the firewall. It is prudent to block incoming packets on the high order

156

ports except for established connections.

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</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2618907"></a>Using IPC$ Share-Based Denials </h3></div></div></div><p>

158

159

160

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If the above methods are not suitable, then you could also place a more specific deny on the IPC$ share that

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is used in the recently discovered security hole. This allows you to offer access to other shares while

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denying access to IPC$ from potentially untrustworthy hosts.

164

</p><p>

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To do this you could use:

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</p><table class="simplelist" border="0" summary="Simple list"><tr><td> </td></tr><tr><td><em class="parameter"><code>[IPC$]</code></em></td></tr><tr><td><a class="indexterm" name="id2618963"></a><em class="parameter"><code>hosts allow = 192.168.115.0/24 127.0.0.1</code></em></td></tr><tr><td><a class="indexterm" name="id2618975"></a><em class="parameter"><code>hosts deny = 0.0.0.0/0</code></em></td></tr></table><p>

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</p><p>

168

169

170

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This instructs Samba that IPC$ connections are not allowed from anywhere except the two listed network

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addresses (localhost and the 192.168.115 subnet). Connections to other shares are still allowed. Because the

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IPC$ share is the only share that is always accessible anonymously, this provides some level of protection

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against attackers who do not know a valid username/password for your host.

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</p><p>

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177

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If you use this method, then clients will be given an <code class="literal">`access denied'</code> reply when they try

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to access the IPC$ share. Those clients will not be able to browse shares and may also be unable to access

181

some other resources. This is not recommended unless for some reason you cannot use one of the other methods

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just discussed.

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</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2619056"></a>NTLMv2 Security</h3></div></div></div><p>

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To configure NTLMv2 authentication, the following registry keys are worth knowing about:

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</p><p>

187

</p><pre class="screen">

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[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]

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"lmcompatibilitylevel"=dword:00000003

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</pre><p>

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</p><p>

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The value 0x00000003 means to send NTLMv2 response only. Clients will use NTLMv2 authentication;

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use NTLMv2 session security if the server supports it. Domain controllers accept LM,

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NTLM, and NTLMv2 authentication.

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</p><p>

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</p><pre class="screen">

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[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0]

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"NtlmMinClientSec"=dword:00080000

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</pre><p>

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</p><p>

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The value 0x00080000 means permit only NTLMv2 session security. If either NtlmMinClientSec or

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NtlmMinServerSec is set to 0x00080000, the connection will fail if NTLMv2

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session security is negotiated.

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</p></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2619110"></a>Upgrading Samba</h2></div></div></div><p>

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Please check regularly on <a class="ulink" href="http://www.samba.org/" target="_top">http://www.samba.org/</a> for

209

updates and important announcements. Occasionally security releases are made, and it is highly recommended to

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upgrade Samba promptly when a security vulnerability is discovered. Check with your OS vendor for OS-specific

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upgrades.

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</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2619154"></a>Common Errors</h2></div></div></div><p>

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If all Samba and host platform configurations were really as intuitive as one might like them to be, this

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chapter would not be necessary. Security issues are often vexing for a support person to resolve, not because

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of the complexity of the problem, but because most administrators who post what turns out to be a security

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problem request are totally convinced that the problem is with Samba.

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</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2619170"></a>Smbclient Works on Localhost, but the Network Is Dead</h3></div></div></div><p>

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This is a common problem. Linux vendors tend to install a default firewall.

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With the default firewall in place, only traffic on the loopback adapter (IP address 127.0.0.1)

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is allowed through the firewall.

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</p><p>

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The solution is either to remove the firewall (stop it) or modify the firewall script to

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allow SMB networking traffic through. See <a class="link" href="securing-samba.html#firewallports" title="Using a Firewall">the Using a

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Firewall</a> section.

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</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2619198"></a>Why Can Users Access Other Users' Home Directories?</h3></div></div></div><p>

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“<span class="quote">

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We are unable to keep individual users from mapping to any other user's home directory once they have

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supplied a valid password! They only need to enter their own password. I have not found any method to

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configure Samba so that users may map only their own home directory.

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</span>”

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</p><p>“<span class="quote">

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User xyzzy can map his home directory. Once mapped, user xyzzy can also map anyone else's home directory.

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</span>”</p><p>

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This is not a security flaw, it is by design. Samba allows users to have exactly the same access to the UNIX

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file system as when they were logged on to the UNIX box, except that it only allows such views onto the file

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system as are allowed by the defined shares.

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</p><p>

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If your UNIX home directories are set up so that one user can happily <code class="literal">cd</code>

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into another user's directory and execute <code class="literal">ls</code>, the UNIX security solution is to change file

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permissions on the user's home directories so that the <code class="literal">cd</code> and <code class="literal">ls</code> are denied.

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</p><p>

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Samba tries very hard not to second guess the UNIX administrator's security policies and

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trusts the UNIX admin to set the policies and permissions he or she desires.

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</p><p>

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Samba allows the behavior you require. Simply put the <a class="link" href="smb.conf.5.html#ONLYUSER" target="_top">only user = %S</a>

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option in the <em class="parameter"><code>[homes]</code></em> share definition.

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</p><p>

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The <a class="link" href="smb.conf.5.html#ONLYUSER" target="_top">only user</a> works in conjunction with the <a class="link" href="smb.conf.5.html#USERS" target="_top">users = list</a>,

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so to get the behavior you require, add the line:

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</p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2619377"></a><em class="parameter"><code>users = %S</code></em></td></tr></table><p>

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This is equivalent to adding

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</p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2619395"></a><em class="parameter"><code>valid users = %S</code></em></td></tr></table><p>

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to the definition of the <em class="parameter"><code>[homes]</code></em> share, as recommended in

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the <code class="filename">smb.conf</code> man page.

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</p></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="locking.html">Prev</a>�</td><td width="20%" align="center"><a accesskey="u" href="optional.html">Up</a></td><td width="40%" align="right">�<a accesskey="n" href="InterdomainTrusts.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter�17.�File and Record Locking�</td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top">�Chapter�19.�Interdomain Trust Relationships</td></tr></table></div></body></html>