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<chapter id="securing-samba">
<chapterinfo>
&author.tridge;
&author.jht;
<pubdate>17 March 2003</pubdate>
</chapterinfo>
<title>Securing Samba</title>
<sect1>
<title>Introduction</title>
<para>
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.
</para>
</sect1>
<sect1>
<title>Using host based protection</title>
<para>
In many installations of Samba the greatest threat comes for 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.
</para>
<para>
One of the simplest fixes in this case is to use the <command>hosts allow</command> and
<command>hosts deny</command> options in the Samba &smb.conf; configuration file to only
allow access to your server from a specific range of hosts. An example
might be:
</para>
<para><programlisting>
hosts allow = 127.0.0.1 192.168.2.0/24 192.168.3.0/24
hosts deny = 0.0.0.0/0
</programlisting></para>
<para>
The above will only allow SMB connections from 'localhost' (your own
computer) and from the two private networks 192.168.2 and
192.168.3. All other connections will be refused connections as soon
as the client sends its first packet. The refusal will be marked as a
'not listening on called name' error.
</para>
</sect1>
<sect1>
<title>Using interface protection</title>
<para>
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.
</para>
<para>
You can change this behaviour using options like the following:
</para>
<para><programlisting>
interfaces = eth* lo
bind interfaces only = yes
</programlisting></para>
<para>
This tells Samba to only listen for connections on interfaces with a
name starting with 'eth' such as eth0, eth1, plus on the loopback
interface called 'lo'. 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 a SMB connection to
your host over a PPP interface called 'ppp0' 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 process.
</para>
</sect1>
<sect1>
<title>Using a firewall</title>
<para>
Many people use a firewall to deny access to services that they don't
want exposed outside their network. This can be a very good idea,
although I would recommend using it in conjunction with the above
methods so that you are protected even if your firewall is not active
for some reason.
</para>
<para>
If you are setting up a firewall then you need to know what TCP and
UDP ports to allow and block. Samba uses the following:
</para>
<para><programlisting>
UDP/137 - used by nmbd
UDP/138 - used by nmbd
TCP/139 - used by smbd
TCP/445 - used by smbd
</programlisting></para>
<para>
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.
</para>
</sect1>
<sect1>
<title>Using a IPC$ share deny</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 that you could use:
</para>
<para><programlisting>
[ipc$]
hosts allow = 192.168.115.0/24 127.0.0.1
hosts deny = 0.0.0.0/0
</programlisting></para>
<para>
this would tell Samba that IPC$ connections are not allowed from
anywhere but the two listed places (localhost and a local
subnet). Connections to other shares would still be 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 username/password for your host.
</para>
<para>
If you use this method then clients will be given a 'access denied'
reply when they try to access the IPC$ share. That means that those
clients will not be able to browse shares, and may also be unable to
access some other resources.
</para>
<para>
This is not recommended unless you cannot use one of the other
methods listed above for some reason.
</para>
</sect1>
<sect1>
<title>NTLMv2 Security</title>
<para>
To configure NTLMv2 authentication the following registry keys are worth knowing about:
</para>
<para>
<programlisting>
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa]
"lmcompatibilitylevel"=dword:00000003
0x3 - 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.
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa\MSV1_0]
"NtlmMinClientSec"=dword:00080000
0x80000 - NTLMv2 session security. If either NtlmMinClientSec or
NtlmMinServerSec is set to 0x80000, the connection will fail if NTLMv2
session security is not negotiated.
</programlisting>
</para>
</sect1>
<sect1>
<title>Upgrading Samba</title>
<para>
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.
</para>
</sect1>
</chapter>
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