Docbook XML conversion: projdoc

[Samba/gebeck_regimport.git] / docs / docbook / projdoc / security_level.xml

<chapter id="securitylevels">
<chapterinfo>
        &author.tridge;
        &author.jelmer;
</chapterinfo>
<title>Samba as Stand-Alone Server</title>

<para>
In this section the function and purpose of Samba's <emphasis>security</emphasis>
modes are described.
</para>

<sect1>
<title>User and Share security level</title>

<para>
A SMB server tells the client at startup what "security level" it is
running. There are two options "share level" and "user level". 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. I know this is
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>
I'll describe user level security first, as its simpler. In user level
security the client will send a "session setup" 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 "accept/reject" on anything other than:
</para>

<orderedlist>
<listitem><para>the username/password</para></listitem>
<listitem><para>the machine that the client is coming from</para></listitem>
</orderedlist>

<para>
If the server accepts the username/password then the client expects to
be able to mount any share (using a "tree connection") without
specifying a password. It expects that all access rights will be as
the username/password specified in the "session setup". 
</para>

<para>
It is also possible for a client to send multiple "session setup"
requests. When the server responds it gives the client a "uid" 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>

</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 "tree connection" (share mount). It does not
explicitly send a username with this operation. The client is
expecting 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 that is authenticated, not a "share/password".
</para>

<para>
Many clients send a "session setup" 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 "possible
usernames". When the client then does a "tree connection" 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 <command>user =</command> &smb.conf;
line. The password is then checked in turn against these "possible
usernames". If a match is found then the client is authenticated as
that user.
</para>

</sect2>

<sect2>
<title>Server Level Security</title>

<para>
Finally "server level" security. In server level security the samba
server reports to the client that it is in user level security. The
client then does a "session setup" as described earlier. The samba
server takes the username/password that the client sends and attempts
to login to the "password server" 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 "password server". 
</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. You have to compile samba with encryption
enabled to support this feature, and you have to maintain a separate
smbpasswd file with SMB style encrypted passwords. It is
cryptographically impossible to translate from unix style encryption
to SMB style encryption, although there are some fairly simple management
schemes by which the two could be kept in sync.
</para>

<para>
"security = server" means that Samba reports to clients that
it is running in "user mode" but actually passes off all authentication
requests to another "user mode" server. This requires an additional
parameter "password server =" 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>
<emphasis>Server</emphasis> level security is incompatible with what is known 
as <emphasis>schannel</emphasis> or "sign and seal" protocols. This means that
if you want to use <emphasis>server</emphasis> level security you must disable
the use of "sign and seal" on all machines on your network.
</para></note>

<sect3>
<title>Configuring Samba for Seemless Windows Network Integration</title>

<para>
MS Windows clients may use encrypted passwords as part of a challenege/response
authentication model (a.k.a. NTLMv1) 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 trucated 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 for 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 client
upper casing usernames and password before transmitting them to the SMB server
when using clear text authentication.
</para>

<para><programlisting>
        <ulink url="smb.conf.5.html#PASSWORDLEVEL">passsword level</ulink> = <replaceable>integer</replaceable>
        <ulink url="smb.conf.5.html#USERNAMELEVEL">username level</ulink> = <replaceable>integer</replaceable>
</programlisting></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 <parameter>username level</parameter> 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 <parameter>password level</parameter>
must be set to the maximum number of upper case letter which <emphasis>could</emphasis>
appear is a password.  Note that the server OS uses the traditional DES version
of crypt(), a <parameter>password level</parameter> 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 
where ever Samba is used. There are three configuration possibilities 
for support of encrypted passwords:
</para>

</sect3>
<sect3>
<title>Use MS Windows NT as an authentication server</title>

<para>
This method involves the additions of the following parameters in the &smb.conf; file:
</para>

<para><programlisting>
        encrypt passwords = Yes
        security = server
        password server = "NetBIOS_name_of_PDC"
</programlisting></para>


<para>
There are two ways of identifying whether or not a username and 
password pair was valid or not. 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, this account can be blocked 
to prevent logons by other than MS Windows clients.
</para>

</sect3>
</sect2>

<sect2>
<title>Domain Level Security</title>

<para>
When samba is operating in <emphasis>security = domain</emphasis> mode this means that
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>Samba as a member of an MS Windows NT security domain</title>

<para>
This method involves addition of the following parameters in the &smb.conf; file:
</para>

<para><programlisting>
        encrypt passwords = Yes
        security = domain
        workgroup = "name of NT domain"
        password server = *
</programlisting></para>

<para>
The use of the "*" argument to <command>password server</command> will cause samba to locate the
domain controller in a way analogous to the way this is done within MS Windows NT.
This is the default behaviour.
</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>

<itemizedlist>
        <listitem><para>On the MS Windows NT domain controller using 
        the Server Manager add a machine account for the Samba server.
        </para></listitem>
        
        <listitem><para>Next, on the Linux system execute: 
        <command>smbpasswd -r PDC_NAME -j DOMAIN_NAME</command> (samba 2.x)

        <command>net join -U administrator%password</command>   (samba-3)
        </para></listitem>
</itemizedlist>

<para>
Use of this mode of authentication does require there to be a standard Unix account
for the 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 things 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 the <link linkend="winbind">Winbind Overview</link> chapter
in this HOWTO collection.
</para>

</sect3>
</sect2>

<sect2>
<title>ADS Level Security</title>

<para>
For information about the configuration option please refer to the entire section entitled
<emphasis>Samba as an ADS Domain Member.</emphasis>
</para>

</sect2>
</sect1>
</chapter>