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<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter 15. Securing Samba</title><link rel="stylesheet" href="samba.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.60.1"><link rel="home" href="index.html" title="SAMBA Project Documentation"><link rel="up" href="optional.html" title="Part III. Advanced Configuration"><link rel="previous" href="locking.html" title="Chapter 14. File and Record Locking"><link rel="next" href="InterdomainTrusts.html" title="Chapter 16. 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 15. 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 15. Securing Samba</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Andrew</span> <span class="surname">Tridgell</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><tt class="email"><<a href="mailto:tridge@samba.org">tridge@samba.org</a>></tt></p></div></div></div></div><div><div class="author"><h3 class="author"><span class="firstname">John</span> <span class="othername">H.</span> <span class="surname">Terpstra</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><tt class="email"><<a href="mailto:jht@samba.org">jht@samba.org</a>></tt></p></div></div></div></div><div><p class="pubdate">May 26, 2003</p></div></div><div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><a href="securing-samba.html#id2918114">Introduction</a></dt><dt><a href="securing-samba.html#id2918159">Features and Benefits</a></dt><dt><a href="securing-samba.html#id2918244">Technical Discussion of Protective Measures and Issues</a></dt><dd><dl><dt><a href="securing-samba.html#id2918263">Using Host-Based Protection</a></dt><dt><a href="securing-samba.html#id2918364">User-Based Protection</a></dt><dt><a href="securing-samba.html#id2918424">Using Interface Protection</a></dt><dt><a href="securing-samba.html#id2918507">Using a Firewall</a></dt><dt><a href="securing-samba.html#id2918564">Using IPC$ Share-Based Denials </a></dt><dt><a href="securing-samba.html#id2918648">NTLMv2 Security</a></dt></dl></dd><dt><a href="securing-samba.html#id2918707">Upgrading Samba</a></dt><dt><a href="securing-samba.html#id2918731">Common Errors</a></dt><dd><dl><dt><a href="securing-samba.html#id2918750">Smbclient Works on Localhost, but the Network Is Dead</a></dt><dt><a href="securing-samba.html#id2918774">Why Can Users Access Home Directories of Other Users?</a></dt></dl></dd></dl></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2918114"></a>Introduction</h2></div></div><div></div></div><p>
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.
</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="id2918159"></a>Features and Benefits</h2></div></div><div></div></div><p>
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.
</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 may be secured from connections that originate from outside the local network. This may 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 <i class="parameter"><tt>[IPC$]</tt></i>
auto-share. The <i class="parameter"><tt>[IPC$]</tt></i> 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 <link linkend="AccessControls">.
</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2918244"></a>Technical Discussion of Protective Measures and Issues</h2></div></div><div></div></div><p>
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.
</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2918263"></a>Using Host-Based Protection</h3></div></div><div></div></div><p>
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.
</p><p>
One of the simplest fixes in this case is to use the <a class="indexterm" name="id2918285"></a><i class="parameter"><tt>hosts allow</tt></i> and
<a class="indexterm" name="id2918298"></a><i class="parameter"><tt>hosts deny</tt></i> options in the Samba <tt class="filename">smb.conf</tt> configuration file to only
allow access to your server from a specific range of hosts. An example might be:
</p><table class="simplelist" border="0" summary="Simple list"><tr><td><i class="parameter"><tt>hosts allow = 127.0.0.1 192.168.2.0/24 192.168.3.0/24</tt></i></td></tr><tr><td><i class="parameter"><tt>hosts deny = 0.0.0.0/0</tt></i></td></tr></table><p>
The above will only allow SMB connections from <tt class="constant">localhost</tt> (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 <span class="errorname">not listening on called name</span> error.
</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2918364"></a>User-Based Protection</h3></div></div><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 <tt class="filename">smb.conf</tt> <i class="parameter"><tt>[global]</tt></i> section put:
</p><table class="simplelist" border="0" summary="Simple list"><tr><td><i class="parameter"><tt>valid users = @smbusers, jacko</tt></i></td></tr></table><p>
This restricts all server access to either 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="id2918424"></a>Using Interface Protection</h3></div></div><div></div></div><p>
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.
</p><p>
You can change this behavior using options like this:
</p><table class="simplelist" border="0" summary="Simple list"><tr><td><i class="parameter"><tt>interfaces = eth* lo</tt></i></td></tr><tr><td><i class="parameter"><tt>bind interfaces only = yes</tt></i></td></tr></table><p>
This tells Samba to only listen for connections on interfaces with a
name starting with <tt class="constant">eth</tt> such as <tt class="constant">eth0, eth1</tt> plus on the loopback
interface called <tt class="constant">lo</tt>. 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.
</p><p>
If you use the above and someone tries to make an SMB connection to
your host over a PPP interface called <tt class="constant">ppp0,</tt> 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.
</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2918507"></a>Using a Firewall</h3></div></div><div></div></div><p>
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.
</p><p>
If you are setting up a firewall, you need to know what TCP and
UDP ports to allow and block. Samba uses the following:
</p><table class="simplelist" border="0" summary="Simple list"><tr><td>UDP/137 - used by nmbd</td></tr><tr><td>UDP/138 - used by nmbd</td></tr><tr><td>TCP/139 - used by smbd</td></tr><tr><td>TCP/445 - used by smbd</td></tr></table><p>
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.
</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2918564"></a>Using IPC$ Share-Based Denials </h3></div></div><div></div></div><p>
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.
</p><p>
To do this you could use:
</p><table class="simplelist" border="0" summary="Simple list"><tr><td> </td></tr><tr><td><i class="parameter"><tt>[IPC$]</tt></i></td></tr><tr><td><i class="parameter"><tt>hosts allow = 192.168.115.0/24 127.0.0.1</tt></i></td></tr><tr><td><i class="parameter"><tt>hosts deny = 0.0.0.0/0</tt></i></td></tr></table><p>
This instructs Samba that IPC$ 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.
</p><p>
If you use this method, then clients will be given an <span class="errorname">`access denied'</span>
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.
</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2918648"></a>NTLMv2 Security</h3></div></div><div></div></div><p>
To configure NTLMv2 authentication, the following registry keys are worth knowing about:
</p><p>
</p><pre class="screen">
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
"lmcompatibilitylevel"=dword:00000003
</pre><p>
</p><p>
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.
</p><p>
</p><pre class="screen">
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0]
"NtlmMinClientSec"=dword:00080000
</pre><p>
</p><p>
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.
</p></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2918707"></a>Upgrading Samba</h2></div></div><div></div></div><p>
Please check regularly on <ulink 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.
</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2918731"></a>Common Errors</h2></div></div><div></div></div><p>
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.
</p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2918750"></a>Smbclient Works on Localhost, but the Network Is Dead</h3></div></div><div></div></div><p>
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.
</p><p>
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.
</p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2918774"></a>Why Can Users Access Home Directories of Other Users?</h3></div></div><div></div></div><p>
“<span class="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.
</span>”
</p><p>“<span class="quote">
User xyzzy can map his home directory. Once mapped user xyzzy can also map
anyone else's home directory.
</span>”</p><p>
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.
</p><p>
If your UNIX home directories are set up so that one user can happily <b class="command">cd</b>
into another users directory and execute <b class="command">ls</b>, the UNIX security solution is to change file
permissions on the user's home directories such that the <b class="command">cd</b> and <b class="command">ls</b> are denied.
</p><p>
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.
</p><p>
Samba allows the behavior you require. Simply put the <a class="indexterm" name="id2918859"></a><i class="parameter"><tt>only user</tt></i> = %S
option in the <i class="parameter"><tt>[homes]</tt></i> share definition.
</p><p>
The <a class="indexterm" name="id2918883"></a><i class="parameter"><tt>only user</tt></i> works in conjunction with the <a class="indexterm" name="id2918899"></a><i class="parameter"><tt>users</tt></i> = list,
so to get the behavior you require, add the line :
</p><table class="simplelist" border="0" summary="Simple list"><tr><td><i class="parameter"><tt>users = %S</tt></i></td></tr></table><p>
this is equivalent to adding
</p><table class="simplelist" border="0" summary="Simple list"><tr><td><i class="parameter"><tt>valid users = %S</tt></i></td></tr></table><p>
to the definition of the <i class="parameter"><tt>[homes]</tt></i> share, as recommended in
the <tt class="filename">smb.conf</tt> man page.
</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 14. 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 16. Interdomain Trust Relationships</td></tr></table></div></body></html>
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