1 <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter 17. Securing Samba</title><link rel="stylesheet" href="samba.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.68.1"><link rel="start" href="index.html" title="The Official Samba-3 HOWTO and Reference Guide"><link rel="up" href="optional.html" title="Part III. Advanced Configuration"><link rel="prev" href="locking.html" title="Chapter 16. File and Record Locking"><link rel="next" href="InterdomainTrusts.html" title="Chapter 18. 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 17. 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 17. 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><code class="email"><<a 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="surname">Terpstra</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><code class="email"><<a 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#id2585584">Introduction</a></span></dt><dt><span class="sect1"><a href="securing-samba.html#id2585695">Features and Benefits</a></span></dt><dt><span class="sect1"><a href="securing-samba.html#id2585840">Technical Discussion of Protective Measures and Issues</a></span></dt><dd><dl><dt><span class="sect2"><a href="securing-samba.html#id2585856">Using Host-Based Protection</a></span></dt><dt><span class="sect2"><a href="securing-samba.html#id2586002">User-Based Protection</a></span></dt><dt><span class="sect2"><a href="securing-samba.html#id2586063">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#id2586416">Using IPC$ Share-Based Denials </a></span></dt><dt><span class="sect2"><a href="securing-samba.html#id2586562">NTLMv2 Security</a></span></dt></dl></dd><dt><span class="sect1"><a href="securing-samba.html#id2586617">Upgrading Samba</a></span></dt><dt><span class="sect1"><a href="securing-samba.html#id2586661">Common Errors</a></span></dt><dd><dl><dt><span class="sect2"><a href="securing-samba.html#id2586676">Smbclient Works on Localhost, but the Network Is Dead</a></span></dt><dt><span class="sect2"><a href="securing-samba.html#id2586705">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="id2585584"></a>Introduction</h2></div></div></div><p>
2 <a class="indexterm" name="id2585592"></a>
3 <a class="indexterm" name="id2585599"></a>
4 <a class="indexterm" name="id2585606"></a>
5 <a class="indexterm" name="id2585613"></a>
6 <a class="indexterm" name="id2585620"></a>
7 <a class="indexterm" name="id2585626"></a>
8 <a class="indexterm" name="id2585633"></a>
9 The information contained in this chapter applies in general to all Samba installations. Security is
10 everyone's concern in the information technology world. A surprising number of Samba servers are being
11 installed on machines that have direct internet access, thus security is made more critical than it would have been had the
12 server been located behind a firewall and on a private network. Paranoia regarding server security is causing
13 some network administrators to insist on the installation of robust firewalls even on servers that are located
14 inside secured networks. This chapter provides information to assist the administrator who understands
15 how to create the needed barriers and deterents against “<span class="quote">the enemy</span>”, no matter where [s]he may
17 </p><div class="blockquote"><blockquote class="blockquote"><p>
18 A new apprentice reported for duty to the chief engineer of a boiler house. He said, “<span class="quote">Here I am,
19 if you will show me the boiler I'll start working on it.</span>” Then engineer replied, “<span class="quote">You're leaning
21 </p></blockquote></div><p>
22 Security concerns are just like that. You need to know a little about the subject to appreciate
23 how obvious most of it really is. The challenge for most of us is to discover that first morsel
24 of knowledge with which we may unlock the secrets of the masters.
25 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2585695"></a>Features and Benefits</h2></div></div></div><p>
26 <a class="indexterm" name="id2585703"></a>
27 <a class="indexterm" name="id2585710"></a>
28 <a class="indexterm" name="id2585716"></a>
29 <a class="indexterm" name="id2585723"></a>
30 There are three levels at which security principles must be observed in order to render a site
31 at least moderately secure. They are the perimeter firewall, the configuration of the host
32 server that is running Samba, and Samba itself.
34 Samba permits a most flexible approach to network security. As far as possible Samba implements
35 the latest protocols to permit more secure MS Windows file and print operations.
37 <a class="indexterm" name="id2585744"></a>
38 <a class="indexterm" name="id2585751"></a>
39 <a class="indexterm" name="id2585758"></a>
40 Samba can be secured from connections that originate from outside the local network. This can be done using
41 <span class="emphasis"><em>host-based protection</em></span>, using Samba's implementation of a technology known as
42 “<span class="quote">tcpwrappers,</span>” or it may be done be using <span class="emphasis"><em>interface-based exclusion</em></span> so
43 <span class="application">smbd</span> will bind only to specifically permitted interfaces. It is also possible to set specific share- or
44 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.
46 <a class="indexterm" name="id2585805"></a>
47 <a class="indexterm" name="id2585814"></a>
48 <a class="indexterm" name="id2585821"></a>
49 Another method by which Samba may be secured is by setting Access Control Entries (ACEs) in an Access
50 Control List (ACL) on the shares themselves. This is discussed in
51 <a href="AccessControls.html" title="Chapter 15. File, Directory, and Share Access Controls">File, Directory, and Share Access Controls</a>.
52 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2585840"></a>Technical Discussion of Protective Measures and Issues</h2></div></div></div><p>
53 The key challenge of security is that protective measures suffice at best
54 only to close the door on known exploits and breach techniques. Never assume that
55 because you have followed these few measures, the Samba server is now an impenetrable
56 fortress! Given the history of information systems so far, it is only a matter of time
57 before someone will find yet another vulnerability.
58 </p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2585856"></a>Using Host-Based Protection</h3></div></div></div><p>
59 <a class="indexterm" name="id2585864"></a>
60 <a class="indexterm" name="id2585871"></a>
61 <a class="indexterm" name="id2585878"></a>
62 In many installations of Samba, the greatest threat comes from outside
63 your immediate network. By default, Samba accepts connections from
64 any host, which means that if you run an insecure version of Samba on
65 a host that is directly connected to the Internet, you can be
66 especially vulnerable.
68 <a class="indexterm" name="id2585893"></a>
69 <a class="indexterm" name="id2585900"></a>
70 One of the simplest fixes in this case is to use the <a class="indexterm" name="id2585908"></a>hosts allow and
71 <a class="indexterm" name="id2585915"></a>hosts deny options in the Samba <code class="filename">smb.conf</code> configuration file to
72 allow access to your server only from a specific range of hosts. An example might be:
73 </p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2585936"></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="id2585949"></a><em class="parameter"><code>hosts deny = 0.0.0.0/0</code></em></td></tr></table><p>
75 <a class="indexterm" name="id2585965"></a>
76 <a class="indexterm" name="id2585972"></a>
77 <a class="indexterm" name="id2585979"></a>
78 The above will allow SMB connections only from <code class="constant">localhost</code> (your own
79 computer) and from the two private networks 192.168.2 and 192.168.3. All other
80 connections will be refused as soon as the client sends its first packet. The refusal
81 will be marked as <code class="literal">not listening on called name</code> error.
82 </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2586002"></a>User-Based Protection</h3></div></div></div><p>
83 If you want to restrict access to your server to valid users only, then the following
84 method may be of use. In the <code class="filename">smb.conf</code> <em class="parameter"><code>[global]</code></em> section put:
85 </p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2586030"></a><em class="parameter"><code>valid users = @smbusers, jacko</code></em></td></tr></table><p>
87 <a class="indexterm" name="id2586046"></a>
88 This restricts all server access either to the user <span class="emphasis"><em>jacko</em></span>
89 or to members of the system group <span class="emphasis"><em>smbusers</em></span>.
90 </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2586063"></a>Using Interface Protection</h3></div></div></div><p>
91 <a class="indexterm" name="id2586071"></a>
92 <a class="indexterm" name="id2586078"></a>
93 <a class="indexterm" name="id2586085"></a>
94 By default, Samba accepts connections on any network interface that
95 it finds on your system. That means if you have an ISDN line or a PPP
96 connection to the Internet then Samba will accept connections on those
97 links. This may not be what you want.
99 You can change this behavior using options like this:
100 </p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2586106"></a><em class="parameter"><code>interfaces = eth* lo</code></em></td></tr><tr><td><a class="indexterm" name="id2586119"></a><em class="parameter"><code>bind interfaces only = yes</code></em></td></tr></table><p>
102 <a class="indexterm" name="id2586135"></a>
103 <a class="indexterm" name="id2586142"></a>
104 <a class="indexterm" name="id2586149"></a>
105 <a class="indexterm" name="id2586156"></a>
106 This tells Samba to listen for connections only on interfaces with a name starting with
107 <code class="constant">eth</code> such as <code class="constant">eth0</code> or <code class="constant">eth1</code>, plus on the loopback interface called
108 <code class="constant">lo</code>. The name you will need to use depends on what OS you are using. In the above, I used
109 the common name for Ethernet adapters on Linux.
111 <a class="indexterm" name="id2586185"></a>
112 <a class="indexterm" name="id2586192"></a>
113 <a class="indexterm" name="id2586199"></a>
114 <a class="indexterm" name="id2586206"></a>
115 If you use the above and someone tries to make an SMB connection to your host over a PPP interface called
116 <code class="constant">ppp0</code>, then [s]he will get a TCP connection refused reply. In that case, no Samba code
117 is run at all, because the operating system has been told not to pass connections from that interface to any
118 Samba process. However, the refusal helps a would-be cracker by confirming that the IP address provides
119 valid active services.
121 <a class="indexterm" name="id2586227"></a>
122 <a class="indexterm" name="id2586234"></a>
123 <a class="indexterm" name="id2586240"></a>
124 <a class="indexterm" name="id2586247"></a>
125 <a class="indexterm" name="id2586254"></a>
126 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
128 probe an interface with the sole intention of finding valid IP addresses for later use in exploitation
129 or denial of service attacks. This method of dealing with potential malicious activity demands the
130 use of appropriate firewall mechanisms.
131 </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>
132 <a class="indexterm" name="id2586283"></a>
133 <a class="indexterm" name="id2586290"></a>
134 <a class="indexterm" name="id2586297"></a>
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
137 if your firewall is not active for some reason.
139 If you are setting up a firewall, you need to know what TCP and UDP ports to allow and block. Samba uses
141 <a class="indexterm" name="id2586314"></a>
142 <a class="indexterm" name="id2586320"></a>
143 <a class="indexterm" name="id2586327"></a>
144 <a class="indexterm" name="id2586334"></a>
145 <a class="indexterm" name="id2586341"></a>
146 </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 <a class="indexterm" name="id2586376"></a>
148 The last one is important because many older firewall setups may not be aware of it, given that this port
149 was only added to the protocol in recent years.
151 <a class="indexterm" name="id2586388"></a>
152 <a class="indexterm" name="id2586396"></a>
153 <a class="indexterm" name="id2586402"></a>
154 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.
157 </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2586416"></a>Using IPC$ Share-Based Denials </h3></div></div></div><p>
158 <a class="indexterm" name="id2586424"></a>
159 <a class="indexterm" name="id2586431"></a>
160 <a class="indexterm" name="id2586438"></a>
161 If the above methods are not suitable, then you could also place a more specific deny on the IPC$ share that
162 is used in the recently discovered security hole. This allows you to offer access to other shares while
163 denying access to IPC$ from potentially untrustworthy hosts.
165 To do this you could use:
166 </p><table class="simplelist" border="0" summary="Simple list"><tr><td> </td></tr><tr><td><em class="parameter"><code>[IPC$]</code></em></td></tr><tr><td><a class="indexterm" name="id2586468"></a><em class="parameter"><code>hosts allow = 192.168.115.0/24 127.0.0.1</code></em></td></tr><tr><td><a class="indexterm" name="id2586481"></a><em class="parameter"><code>hosts deny = 0.0.0.0/0</code></em></td></tr></table><p>
168 <a class="indexterm" name="id2586497"></a>
169 <a class="indexterm" name="id2586504"></a>
170 <a class="indexterm" name="id2586511"></a>
171 This instructs Samba that IPC$ connections are not allowed from anywhere except the two listed network
172 addresses (localhost and the 192.168.115 subnet). Connections to other shares are still allowed. Because the
173 IPC$ share is the only share that is always accessible anonymously, this provides some level of protection
174 against attackers who do not know a valid username/password for your host.
176 <a class="indexterm" name="id2586528"></a>
177 <a class="indexterm" name="id2586535"></a>
178 <a class="indexterm" name="id2586542"></a>
179 If you use this method, then clients will be given an <code class="literal">`access denied'</code> reply when they try
180 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
183 </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2586562"></a>NTLMv2 Security</h3></div></div></div><p>
184 <a class="indexterm" name="id2586570"></a>
185 To configure NTLMv2 authentication, the following registry keys are worth knowing about:
187 </p><pre class="screen">
188 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
189 "lmcompatibilitylevel"=dword:00000003
192 The value 0x00000003 means to send NTLMv2 response only. Clients will use NTLMv2 authentication;
193 use NTLMv2 session security if the server supports it. Domain controllers accept LM,
194 NTLM, and NTLMv2 authentication.
196 </p><pre class="screen">
197 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0]
198 "NtlmMinClientSec"=dword:00080000
201 The value 0x00080000 means permit only NTLMv2 session security. If either NtlmMinClientSec or
202 NtlmMinServerSec is set to 0x00080000, the connection will fail if NTLMv2
203 session security is negotiated.
204 </p></div></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2586617"></a>Upgrading Samba</h2></div></div></div><p>
205 <a class="indexterm" name="id2586625"></a>
206 <a class="indexterm" name="id2586632"></a>
207 <a class="indexterm" name="id2586639"></a>
208 Please check regularly on <a 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
210 upgrade Samba promptly when a security vulnerability is discovered. Check with your OS vendor for OS-specific
212 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2586661"></a>Common Errors</h2></div></div></div><p>
213 If all Samba and host platform configurations were really as intuitive as one might like them to be, this
214 chapter would not be necessary. Security issues are often vexing for a support person to resolve, not because
215 of the complexity of the problem, but because most administrators who post what turns out to be a security
216 problem request are totally convinced that the problem is with Samba.
217 </p><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2586676"></a>Smbclient Works on Localhost, but the Network Is Dead</h3></div></div></div><p>
218 This is a common problem. Linux vendors tend to install a default firewall.
219 With the default firewall in place, only traffic on the loopback adapter (IP address 127.0.0.1)
220 is allowed through the firewall.
222 The solution is either to remove the firewall (stop it) or modify the firewall script to
223 allow SMB networking traffic through. See <a href="securing-samba.html#firewallports" title="Using a Firewall">the Using a
224 Firewall</a> section.
225 </p></div><div class="sect2" lang="en"><div class="titlepage"><div><div><h3 class="title"><a name="id2586705"></a>Why Can Users Access Other Users' Home Directories?</h3></div></div></div><p>
226 “<span class="quote">
227 <a class="indexterm" name="id2586716"></a>
228 <a class="indexterm" name="id2586723"></a>
229 We are unable to keep individual users from mapping to any other user's home directory once they have
230 supplied a valid password! They only need to enter their own password. I have not found any method to
231 configure Samba so that users may map only their own home directory.
233 </p><p>“<span class="quote">
234 User xyzzy can map his home directory. Once mapped, user xyzzy can also map anyone else's home directory.
235 </span>”</p><p>
236 <a class="indexterm" name="id2586745"></a>
237 <a class="indexterm" name="id2586752"></a>
238 This is not a security flaw, it is by design. Samba allows users to have exactly the same access to the UNIX
239 file system as when they were logged on to the UNIX box, except that it only allows such views onto the file
240 system as are allowed by the defined shares.
242 <a class="indexterm" name="id2586766"></a>
243 <a class="indexterm" name="id2586773"></a>
244 If your UNIX home directories are set up so that one user can happily <span><strong class="command">cd</strong></span>
245 into another user's directory and execute <span><strong class="command">ls</strong></span>, the UNIX security solution is to change file
246 permissions on the user's home directories so that the <span><strong class="command">cd</strong></span> and <span><strong class="command">ls</strong></span> are denied.
248 <a class="indexterm" name="id2586810"></a>
249 <a class="indexterm" name="id2586817"></a>
250 Samba tries very hard not to second guess the UNIX administrator's security policies and
251 trusts the UNIX admin to set the policies and permissions he or she desires.
253 Samba allows the behavior you require. Simply put the <a class="indexterm" name="id2586831"></a>only user = %S
254 option in the <em class="parameter"><code>[homes]</code></em> share definition.
256 The <a class="indexterm" name="id2586849"></a>only user works in conjunction with the <a class="indexterm" name="id2586856"></a>users = list,
257 so to get the behavior you require, add the line:
258 </p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2586870"></a><em class="parameter"><code>users = %S</code></em></td></tr></table><p>
259 This is equivalent to adding
260 </p><table class="simplelist" border="0" summary="Simple list"><tr><td><a class="indexterm" name="id2586888"></a><em class="parameter"><code>valid users = %S</code></em></td></tr></table><p>
261 to the definition of the <em class="parameter"><code>[homes]</code></em> share, as recommended in
262 the <code class="filename">smb.conf</code> man page.
263 </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 16. 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 18. Interdomain Trust Relationships</td></tr></table></div></body></html>
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