<chapter id="ServerType">
<chapterinfo>
&author.tridge;
&author.jelmer;
&author.jht;
</chapterinfo>
<title>Server Types and Security Modes</title>
<para>
This chapter provides information regarding the types of server that Samba may be
configured to be. A Microsoft network administrator who wishes to migrate to or to
use Samba will want to know what, within a Samba context, terms familiar to MS Windows
administrator mean. This means that it is essential also to define how critical security
modes function BEFORE we get into the details of how to configure the server itself.
</para>
<para>
The chapter provides an overview of the security modes of which Samba is capable
and how these relate to MS Windows servers and clients.
</para>
<para>
A question often asked is, "Why would I want to use Samba?" Most chapters contain a section
that highlights features and benefits. We hope that the information provided will help to
answer this question. Be warned though, we want to be fair and reasonable, so not all
features are positive towards Samba so the benefit may be on the side of our competition.
</para>
<sect1>
<title>Features and Benefits</title>
<para>
Two men were walking down a dusty road, when one suddenly kicked up a small red stone. It
hurt his toe and lodged in his sandal. He took the stone out and cursed it with a passion
and fury fitting his anguish. The other looked at the stone and said, that is a garnet - I
can turn that into a precious gem and some day it will make a princess very happy!
</para>
<para>
The moral of this tale: Two men, two very different perspectives regarding the same stone.
Like it or not, Samba is like that stone. Treat it the right way and it can bring great
pleasure, but if you are forced upon it and have no time for its secrets then it can be
a source of discomfort.
</para>
<para>
Samba started out as a project that sought to provide interoperability for MS Windows 3.x
clients with a UNIX server. It has grown up a lot since its humble beginnings and now provides
features and functionality fit for large scale deployment. It also has some warts. In sections
like this one we will tell of both.
</para>
<para>
So now, what are the benefits of features mentioned in this chapter?
</para>
<itemizedlist>
<listitem><para>
Samba-3 can replace an MS Windows NT4 Domain Controller
</para></listitem>
<listitem><para>
Samba-3 offers excellent interoperability with MS Windows NT4
style domains as well as natively with Microsoft Active
Directory domains.
</para></listitem>
<listitem><para>
Samba-3 permits full NT4 style Interdomain Trusts
</para></listitem>
<listitem><para>
Samba has security modes that permit more flexible
authentication than is possible with MS Windows NT4 Domain Controllers.
</para></listitem>
<listitem><para>
Samba-3 permits use of multiple account database backends
</para></listitem>
<listitem><para>
The account (password) database backends can be distributed
and replicated using multiple methods. This gives Samba-3
greater flexibility than MS Windows NT4 and in many cases a
significantly higher utility than Active Directory domains
with MS Windows 200x.
</para></listitem>
</itemizedlist>
</sect1>
<sect1>
<title>Server Types</title>
<para>Administrators of Microsoft networks often refer to three
different type of servers:</para>
<itemizedlist>
<listitem><para>Domain Controller</para>
<itemizedlist>
<listitem><para>Primary Domain Controller</para></listitem>
<listitem><para>Backup Domain Controller</para></listitem>
<listitem><para>ADS Domain Controller</para></listitem>
</itemizedlist>
</listitem>
<listitem><para>Domain Member Server</para>
<itemizedlist>
<listitem><para>Active Directory Domain Server</para></listitem>
<listitem><para>NT4 Style Domain Domain Server</para></listitem>
</itemizedlist>
</listitem>
<listitem><para>Stand Alone Server</para></listitem>
</itemizedlist>
<para>
The chapters covering Domain Control, Backup Domain Control and Domain Membership provide
pertinent information regarding Samba configuration for each of these server roles.
The reader is strongly encouraged to become intimately familiar with the information
presented.
</para>
</sect1>
<sect1>
<title>Samba Security Modes</title>
<para>
In this section the function and purpose of Samba's <smbconfoption><name>security</name></smbconfoption>
modes are described. An accurate understanding of how Samba implements each security
mode as well as how to configure MS Windows clients for each mode will significantly
reduce user complaints and administrator heartache.
</para>
<para>
In the SMB/CIFS networking world, there are only two types of security: <emphasis>USER Level</emphasis>
and <emphasis>SHARE Level</emphasis>. We refer to these collectively as <emphasis>security levels</emphasis>. In implementing these two <emphasis>security levels</emphasis> Samba provides flexibilities
that are not available with Microsoft Windows NT4 / 200x servers. Samba knows of five (5)
ways that allow the security levels to be implemented. In actual fact, Samba implements
<emphasis>SHARE Level</emphasis> security only one way, but has four ways of implementing
<emphasis>USER Level</emphasis> security. Collectively, we call the Samba implementations
<emphasis>Security Modes</emphasis>. These are: <emphasis>SHARE</emphasis>, <emphasis>USER</emphasis>, <emphasis>DOMAIN</emphasis>,
<emphasis>ADS</emphasis>, and <emphasis>SERVER</emphasis>
modes. They are documented in this chapter.
</para>
<para>
A SMB server tells the client at startup what <emphasis>security level</emphasis>
it is running. There are two options: <emphasis>share level</emphasis> and
<emphasis>user level</emphasis>. Which of these two the client receives affects
the way the client then tries to authenticate itself. It does not directly affect
(to any great extent) the way the Samba server does security. This may sound strange,
but it fits in with the client/server approach of SMB. In SMB everything is initiated
and controlled by the client, and the server can only tell the client what is
available and whether an action is allowed.
</para>
<sect2>
<title>User Level Security</title>
<para>
We will describe <emphasis>user level</emphasis> security first, as it's simpler.
In <emphasis>user level</emphasis> security, the client will send a
<emphasis>session setup</emphasis> command directly after the protocol negotiation.
This contains a username and password. The server can either accept or reject that
username/password combination. Note that at this stage the server has no idea what
share the client will eventually try to connect to, so it can't base the
<emphasis>accept/reject</emphasis> on anything other than:
</para>
<orderedlist>
<listitem><para>The username/password</para></listitem>
<listitem><para>The name of the client machine</para></listitem>
</orderedlist>
<para>
If the server accepts the username/password then the client expects to be able to
mount shares (using a <emphasis>tree connection</emphasis>) without specifying a
password. It expects that all access rights will be as the username/password
specified in the <emphasis>session setup</emphasis>.
</para>
<para>
It is also possible for a client to send multiple <emphasis>session setup</emphasis>
requests. When the server responds, it gives the client a <emphasis>uid</emphasis> to use
as an authentication tag for that username/password. The client can maintain multiple
authentication contexts in this way (WinDD is an example of an application that does this).
</para>
<sect3>
<title>Example Configuration</title>
<para>
The &smb.conf; parameter that sets <emphasis>User Level Security</emphasis> is:
</para>
<para><smbconfblock>
<smbconfoption><name>security</name><value>user</value></smbconfoption>
</smbconfblock></para>
<para>
This is the default setting since samba-2.2.x.
</para>
</sect3>
</sect2>
<sect2>
<title>Share Level Security</title>
<para>
Ok, now for share level security. In share level security, the client authenticates
itself separately for each share. It will send a password along with each
<emphasis>tree connection</emphasis> (share mount). It does not explicitly send a
username with this operation. The client expects a password to be associated
with each share, independent of the user. This means that Samba has to work out what
username the client probably wants to use. It is never explicitly sent the username.
Some commercial SMB servers such as NT actually associate passwords directly with
shares in share level security, but Samba always uses the unix authentication scheme
where it is a username/password pair that is authenticated, not a share/password pair.
</para>
<para>
To gain understanding of the MS Windows networking parallels to this, one should think
in terms of MS Windows 9x/Me where one can create a shared folder that provides read-only
or full access, with or without a password.
</para>
<para>
Many clients send a <emphasis>session setup</emphasis> even if the server is in share
level security. They normally send a valid username but no password. Samba records
this username in a list of <emphasis>possible usernames</emphasis>. When the client
then does a <emphasis>tree connection</emphasis> it also adds to this list the name
of the share they try to connect to (useful for home directories) and any users
listed in the <smbconfoption><name>user</name></smbconfoption> &smb.conf; line. The password is then checked
in turn against these <emphasis>possible usernames</emphasis>. If a match is found
then the client is authenticated as that user.
</para>
<sect3>
<title>Example Configuration</title>
<para>
The &smb.conf; parameter that sets <emphasis>Share Level Security</emphasis> is:
</para>
<para><smbconfblock>
<smbconfoption><name>security</name><value>share</value></smbconfoption>
</smbconfblock></para>
<para>
Please note that there are reports that recent MS Windows clients do not like to work
with share mode security servers. You are strongly discouraged from using share level security.
</para>
</sect3>
</sect2>
<sect2>
<title>Domain Security Mode (User Level Security)</title>
<para>
When Samba is operating in <smbconfoption><name>security</name><value>domain</value></smbconfoption> mode,
the Samba server has a domain security trust account (a machine account) and will cause
all authentication requests to be passed through to the domain controllers.
</para>
<sect3>
<title>Example Configuration</title>
<para><emphasis>
Samba as a Domain Member Server
</emphasis></para>
<para>
This method involves addition of the following parameters in the &smb.conf; file:
</para>
<para><smbconfblock>
<smbconfoption><name>security</name><value>domain</value></smbconfoption>
<smbconfoption><name>workgroup</name><value>&example.workgroup;</value></smbconfoption>
</smbconfblock></para>
<para>
In order for this method to work, the Samba server needs to join the MS Windows NT
security domain. This is done as follows:
</para>
<procedure>
<step><para>On the MS Windows NT domain controller, using
the Server Manager, add a machine account for the Samba server.
</para></step>
<step><para>Next, on the UNIX/Linux system execute:</para>
<para><screen>&rootprompt;<userinput>net rpc join -U administrator%password</userinput></screen></para>
</step>
</procedure>
<note><para>
Samba-2.2.4 and later can auto-join a Windows NT4 style Domain just by executing:
<screen>
&rootprompt;<userinput>smbpasswd -j <replaceable>DOMAIN_NAME</replaceable> -r <replaceable>PDC_NAME</replaceable> \
-U Administrator%<replaceable>password</replaceable></userinput>
</screen>
Samba-3 can do the same by executing:
<screen>
&rootprompt;<userinput>net rpc join -U Administrator%<replaceable>password</replaceable></userinput>
</screen>
It is not necessary with Samba-3 to specify the <replaceable>DOMAIN_NAME</replaceable> or the
<replaceable>PDC_NAME</replaceable> as it figures this out from the &smb.conf; file settings.
</para></note>
<para>
Use of this mode of authentication does require there to be a standard UNIX account
for each user in order to assign a UID once the account has been authenticated by
the remote Windows DC. This account can be blocked to prevent logons by clients other than
MS Windows through means such as setting an invalid shell in the
<filename>/etc/passwd</filename> entry.
</para>
<para>
An alternative to assigning UIDs to Windows users on a Samba member server is
presented in <link linkend="winbind">the chapter about winbind</link>.
</para>
<para>
For more information of being a domain member, see <link linkend="domain-member">the chapter about domain membership</link>.
</para>
</sect3>
</sect2>
<sect2>
<title>ADS Security Mode (User Level Security)</title>
<para>
Both Samba 2.2 and 3.0 can join an Active Directory domain. This is
possible if the domain is run in native mode. Active Directory in
native mode perfectly allows NT4-style domain members. This is contrary to
popular belief. The only thing that Active Directory in native mode
prohibits is Backup Domain Controllers running NT4.
</para>
<para>
If you are using Active Directory, starting with Samba-3 you can
join as a native AD member. Why would you want to do that?
Your security policy might prohibit the use of NT-compatible
authentication protocols. All your machines are running Windows 2000
and above and all use Kerberos. In this case Samba as a NT4-style
domain would still require NT-compatible authentication data. Samba in
AD-member mode can accept Kerberos tickets.
</para>
<sect3>
<title>Example Configuration</title>
<para><smbconfblock>
<smbconfoption><name>realm</name><value>your.kerberos.REALM</value></smbconfoption>
<smbconfoption><name>security</name><value>ADS</value></smbconfoption>
</smbconfblock></para>
<para>
The following parameter may be required:
</para>
<para><smbconfblock>
<smbconfoption><name>ads server</name><value>your.kerberos.server</value></smbconfoption>
</smbconfblock></para>
<para>
Please refer to <link linkend="domain-member">the chapter on domain membership</link>
for more information regarding this configuration option.
</para>
</sect3>
</sect2>
<sect2>
<title>Server Security (User Level Security)</title>
<para>
Server security mode is a left over from the time when Samba was not capable of acting
as a domain member server. It is highly recommended NOT to use this feature. Server
security mode has many draw backs. The draw backs include:
</para>
<itemizedlist>
<listitem><para>Potential Account Lockout on MS Windows NT4/200x password servers</para></listitem>
<listitem><para>Lack of assurance that the password server is the one specified</para></listitem>
<listitem><para>Does not work with Winbind, particularly needed when storing profiles remotely</para></listitem>
<listitem><para>This mode may open connections to the password server, and keep them open for extended periods.</para></listitem>
<listitem><para>Security on the Samba server breaks badly when the remote password server suddenly shuts down</para></listitem>
<listitem><para>With this mode there is NO security account in the domain that the password server belongs to for the Samba server.</para></listitem>
</itemizedlist>
<para>
In server security mode the Samba server reports to the client that it is in user level
security. The client then does a <emphasis>session setup</emphasis> as described earlier.
The Samba server takes the username/password that the client sends and attempts to login to the
<smbconfoption><name>password server</name></smbconfoption> by sending exactly the same username/password that
it got from the client. If that server is in user level security and accepts the password,
then Samba accepts the clients connection. This allows the Samba server to use another SMB
server as the <smbconfoption><name>password server</name></smbconfoption>.
</para>
<para>
You should also note that at the very start of all this, where the server tells the client
what security level it is in, it also tells the client if it supports encryption. If it
does then it supplies the client with a random cryptkey. The client will then send all
passwords in encrypted form. Samba supports this type of encryption by default.
</para>
<para>
The parameter <smbconfoption><name>security</name><value>server</value></smbconfoption> means that Samba reports to clients that
it is running in <emphasis>user mode</emphasis> but actually passes off all authentication
requests to another <emphasis>user mode</emphasis> server. This requires an additional
parameter <smbconfoption><name>password server</name></smbconfoption> that points to the real authentication server.
That real authentication server can be another Samba server or can be a Windows NT server,
the later natively capable of encrypted password support.
</para>
<note><para>
When Samba is running in <emphasis>server security mode</emphasis> it is essential that
the parameter <emphasis>password server</emphasis> is set to the precise NetBIOS machine
name of the target authentication server. Samba can NOT determine this from NetBIOS name
lookups because the choice of the target authentication server is arbitrary and can not
be determined from a domain name. In essence, a Samba server that is in
<emphasis>server security mode</emphasis> is operating in what used to be known as
workgroup mode.
</para></note>
<sect3>
<title>Example Configuration</title>
<para><emphasis>
Using MS Windows NT as an authentication server
</emphasis></para>
<para>
This method involves the additions of the following parameters in the &smb.conf; file:
</para>
<para><smbconfblock>
<smbconfoption><name>encrypt passwords</name><value>Yes</value></smbconfoption>
<smbconfoption><name>security</name><value>server</value></smbconfoption>
<smbconfoption><name>password server</name><value>"NetBIOS_name_of_a_DC"</value></smbconfoption>
</smbconfblock></para>
<para>
There are two ways of identifying whether or not a username and password pair was valid.
One uses the reply information provided as part of the authentication messaging
process, the other uses just an error code.
</para>
<para>
The down-side of this mode of configuration is the fact that for security reasons Samba
will send the password server a bogus username and a bogus password and if the remote
server fails to reject the username and password pair then an alternative mode of
identification of validation is used. Where a site uses password lock out after a
certain number of failed authentication attempts this will result in user lockouts.
</para>
<para>
Use of this mode of authentication does require there to be a standard UNIX account
for the user, though this account can be blocked to prevent logons by non-SMB/CIFS clients.
</para>
</sect3>
</sect2>
</sect1>
<sect1>
<title>Password checking</title>
<para>
MS Windows clients may use encrypted passwords as part of a challenge/response
authentication model (a.k.a. NTLMv1 and NTLMv2) or alone, or clear text strings for simple
password based authentication. It should be realized that with the SMB protocol,
the password is passed over the network either in plain text or encrypted, but
not both in the same authentication request.
</para>
<para>
When encrypted passwords are used, a password that has been entered by the user
is encrypted in two ways:
</para>
<itemizedlist>
<listitem><para>An MD4 hash of the UNICODE of the password
string. This is known as the NT hash.
</para></listitem>
<listitem><para>The password is converted to upper case,
and then padded or truncated to 14 bytes. This string is
then appended with 5 bytes of NULL characters and split to
form two 56 bit DES keys to encrypt a "magic" 8 byte value.
The resulting 16 bytes form the LanMan hash.
</para></listitem>
</itemizedlist>
<para>
MS Windows 95 pre-service pack 1, MS Windows NT versions 3.x and version 4.0
pre-service pack 3 will use either mode of password authentication. All
versions of MS Windows that follow these versions no longer support plain
text passwords by default.
</para>
<para>
MS Windows clients have a habit of dropping network mappings that have been idle
for 10 minutes or longer. When the user attempts to use the mapped drive
connection that has been dropped, the client re-establishes the connection using
a cached copy of the password.
</para>
<para>
When Microsoft changed the default password mode, support was dropped for caching
of the plain text password. This means that when the registry parameter is changed
to re-enable use of plain text passwords it appears to work, but when a dropped
service connection mapping attempts to revalidate it will fail if the remote
authentication server does not support encrypted passwords. This means that it
is definitely not a good idea to re-enable plain text password support in such clients.
</para>
<para>
The following parameters can be used to work around the issue of Windows 9x clients
upper casing usernames and password before transmitting them to the SMB server
when using clear text authentication.
</para>
<para><smbconfblock>
<smbconfoption><name>password level</name><value><replaceable>integer</replaceable></value></smbconfoption>
<smbconfoption><name>username level</name><value><replaceable>integer</replaceable></value></smbconfoption>
</smbconfblock></para>
<para>
By default Samba will lower case the username before attempting to lookup the user
in the database of local system accounts. Because UNIX usernames conventionally
only contain lower-case character, the <smbconfoption><name>username level</name></smbconfoption> parameter
is rarely needed.
</para>
<para>
However, passwords on UNIX systems often make use of mixed-case characters.
This means that in order for a user on a Windows 9x client to connect to a Samba
server using clear text authentication, the <smbconfoption><name>password level</name></smbconfoption>
must be set to the maximum number of upper case letters which <emphasis>could</emphasis>
appear in a password. Note that if the server OS uses the traditional DES version
of crypt(), a <smbconfoption><name>password level</name></smbconfoption> of 8 will result in case
insensitive passwords as seen from Windows users. This will also result in longer
login times as Samba has to compute the permutations of the password string and
try them one by one until a match is located (or all combinations fail).
</para>
<para>
The best option to adopt is to enable support for encrypted passwords wherever
Samba is used. Most attempts to apply the registry change to re-enable plain text
passwords will eventually lead to user complaints and unhappiness.
</para>
</sect1>
<sect1>
<title>Common Errors</title>
<para>
We all make mistakes. It is Ok to make mistakes, so long as they are made in the right places
and at the right time. A mistake that causes lost productivity is seldom tolerated. A mistake
made in a developmental test lab is expected.
</para>
<para>
Here we look at common mistakes and misapprehensions that have been the subject of discussions
on the Samba mailing lists. Many of these are avoidable by doing you homework before attempting
a Samba implementation. Some are the result of misunderstanding of the English language. The
English language has many turns of phrase that are potentially vague and may be highly confusing
to those for whom English is not their native tongue.
</para>
<sect2>
<title>What makes Samba a SERVER?</title>
<para>
To some the nature of the Samba <emphasis>security</emphasis> mode is very obvious, but entirely
wrong all the same. It is assumed that <smbconfoption><name>security</name><value>server</value></smbconfoption> means that Samba
will act as a server. Not so! See above - this setting means that Samba will <emphasis>try</emphasis>
to use another SMB server as its source of user authentication alone.
</para>
</sect2>
<sect2>
<title>What makes Samba a Domain Controller?</title>
<para>
The &smb.conf; parameter <smbconfoption><name>security</name><value>domain</value></smbconfoption> does NOT really make Samba behave
as a Domain Controller! This setting means we want Samba to be a domain member!
</para>
</sect2>
<sect2>
<title>What makes Samba a Domain Member?</title>
<para>
Guess! So many others do. But whatever you do, do NOT think that <smbconfoption><name>security</name><value>user</value></smbconfoption>
makes Samba act as a domain member. Read the manufacturers manual before the warranty expires! See
<link linkend="domain-member">the chapter about domain membership</link> for more information.
</para>
</sect2>
<sect2>
<title>Constantly Losing Connections to Password Server</title>
<para>
<quote>
Why does server_validate() simply give up rather than re-establishing its connection to the
password server? Though I am not fluent in the SMB protocol, perhaps the cluster server
process passes along to its client workstation the session key it receives from the password
server, which means the password hashes submitted by the client would not work on a subsequent
connection, whose session key would be different. So server_validate() must give up.</quote>
</para>
<para>
Indeed. That's why <smbconfoption><name>security</name><value>server</value></smbconfoption> is at best a nasty hack. Please use <smbconfoption><name>security</name><value>domain</value></smbconfoption>.
<smbconfoption><name>security</name><value>server</value></smbconfoption> mode is also known as pass-through authentication.
</para>
</sect2>
</sect1>
</chapter>