source: trunk/samba/docs/htmldocs/Samba3-Developers-Guide/pwencrypt.html @ 17

Last change on this file since 17 was 1, checked in by Paul Smedley, 14 years ago

Initial code import

File size: 11.0 KB
Line 
1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter 13. LanMan and NT Password Encryption</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="SAMBA Developers Guide"><link rel="up" href="pt03.html" title="Part III. Samba Subsystems"><link rel="prev" href="wins.html" title="Chapter 12. Samba WINS Internals"><link rel="next" href="pt04.html" title="Part IV. Debugging and tracing"></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 13. LanMan and NT Password Encryption</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="wins.html">Prev</a> </td><th width="60%" align="center">Part III. Samba Subsystems</th><td width="20%" align="right"> <a accesskey="n" href="pt04.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="pwencrypt"></a>Chapter 13. LanMan and NT Password Encryption</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Jeremy</span> <span class="surname">Allison</span></h3><div class="affiliation"><span class="orgname">Samba Team<br></span><div class="address"><p><br>
2                                <code class="email">&lt;<a href="mailto:samba@samba.org">samba@samba.org</a>&gt;</code><br>
3                        </p></div></div></div></div><div><p class="pubdate">19 Apr 1999</p></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="pwencrypt.html#id295876">Introduction</a></span></dt><dt><span class="sect1"><a href="pwencrypt.html#id295893">How does it work?</a></span></dt><dt><span class="sect1"><a href="pwencrypt.html#id295954">The smbpasswd file</a></span></dt></dl></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id295876"></a>Introduction</h2></div></div></div><p>With the development of LanManager and Windows NT
4        compatible password encryption for Samba, it is now able
5        to validate user connections in exactly the same way as
6        a LanManager or Windows NT server.</p><p>This document describes how the SMB password encryption
7        algorithm works and what issues there are in choosing whether
8        you want to use it. You should read it carefully, especially
9        the part about security and the "PROS and CONS" section.</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id295893"></a>How does it work?</h2></div></div></div><p>LanManager encryption is somewhat similar to UNIX
10        password encryption. The server uses a file containing a
11        hashed value of a user's password.  This is created by taking
12        the user's plaintext password, capitalising it, and either
13        truncating to 14 bytes or padding to 14 bytes with null bytes.
14        This 14 byte value is used as two 56 bit DES keys to encrypt
15        a 'magic' eight byte value, forming a 16 byte value which is
16        stored by the server and client. Let this value be known as
17        the "hashed password".</p><p>Windows NT encryption is a higher quality mechanism,
18        consisting of doing an MD4 hash on a Unicode version of the user's
19        password. This also produces a 16 byte hash value that is
20        non-reversible.</p><p>When a client (LanManager, Windows for WorkGroups, Windows
21        95 or Windows NT) wishes to mount a Samba drive (or use a Samba
22        resource), it first requests a connection and negotiates the
23        protocol that the client and server will use. In the reply to this
24        request the Samba server generates and appends an 8 byte, random
25        value - this is stored in the Samba server after the reply is sent
26        and is known as the "challenge".  The challenge is different for
27        every client connection.</p><p>The client then uses the hashed password (16 byte values
28        described above), appended with 5 null bytes, as three 56 bit
29        DES keys, each of which is used to encrypt the challenge 8 byte
30        value, forming a 24 byte value known as the "response".</p><p>In the SMB call SMBsessionsetupX (when user level security
31        is selected) or the call SMBtconX (when share level security is
32        selected), the 24 byte response is returned by the client to the
33        Samba server.  For Windows NT protocol levels the above calculation
34        is done on both hashes of the user's password and both responses are
35        returned in the SMB call, giving two 24 byte values.</p><p>The Samba server then reproduces the above calculation, using
36        its own stored value of the 16 byte hashed password (read from the
37        <code class="filename">smbpasswd</code> file - described later) and the challenge
38        value that it kept from the negotiate protocol reply. It then checks
39        to see if the 24 byte value it calculates matches the 24 byte value
40        returned to it from the client.</p><p>If these values match exactly, then the client knew the
41        correct password (or the 16 byte hashed value - see security note
42        below) and is thus allowed access. If not, then the client did not
43        know the correct password and is denied access.</p><p>Note that the Samba server never knows or stores the cleartext
44        of the user's password - just the 16 byte hashed values derived from
45        it. Also note that the cleartext password or 16 byte hashed values
46        are never transmitted over the network - thus increasing security.</p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id295954"></a>The smbpasswd file</h2></div></div></div><a name="SMBPASSWDFILEFORMAT"></a><p>In order for Samba to participate in the above protocol
47        it must be able to look up the 16 byte hashed values given a user name.
48        Unfortunately, as the UNIX password value is also a one way hash
49        function (ie. it is impossible to retrieve the cleartext of the user's
50        password given the UNIX hash of it), a separate password file
51        containing this 16 byte value must be kept. To minimise problems with
52        these two password files, getting out of sync, the UNIX <code class="filename">
53        /etc/passwd</code> and the <code class="filename">smbpasswd</code> file,
54        a utility, <span><strong class="command">mksmbpasswd.sh</strong></span>, is provided to generate
55        a smbpasswd file from a UNIX <code class="filename">/etc/passwd</code> file.
56        </p><p>To generate the smbpasswd file from your <code class="filename">/etc/passwd
57        </code> file use the following command:</p><p><code class="prompt">$ </code><strong class="userinput"><code>cat /etc/passwd | mksmbpasswd.sh
58        &gt; /usr/local/samba/private/smbpasswd</code></strong></p><p>If you are running on a system that uses NIS, use</p><p><code class="prompt">$ </code><strong class="userinput"><code>ypcat passwd | mksmbpasswd.sh
59        &gt; /usr/local/samba/private/smbpasswd</code></strong></p><p>The <span><strong class="command">mksmbpasswd.sh</strong></span> program is found in
60        the Samba source directory. By default, the smbpasswd file is
61        stored in :</p><p><code class="filename">/usr/local/samba/private/smbpasswd</code></p><p>The owner of the <code class="filename">/usr/local/samba/private/</code> 
62        directory should be set to root, and the permissions on it should
63        be set to 0500 (<span><strong class="command">chmod 500 /usr/local/samba/private</strong></span>).
64        </p><p>Likewise, the smbpasswd file inside the private directory should
65        be owned by root and the permissions on is should be set to 0600
66        (<span><strong class="command">chmod 600 smbpasswd</strong></span>).</p><p>The format of the smbpasswd file is (The line has been
67        wrapped here. It should appear as one entry per line in
68        your smbpasswd file.)</p><pre class="programlisting">
69username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
70        [Account type]:LCT-&lt;last-change-time&gt;:Long name
71        </pre><p>Although only the <em class="replaceable"><code>username</code></em>,
72        <em class="replaceable"><code>uid</code></em>, <em class="replaceable"><code>
73        XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX</code></em>,
74        [<em class="replaceable"><code>Account type</code></em>] and <em class="replaceable"><code>
75        last-change-time</code></em> sections are significant
76        and are looked at in the Samba code.</p><p>It is <span class="emphasis"><em>VITALLY</em></span> important that there by 32
77        'X' characters between the two ':' characters in the XXX sections -
78        the smbpasswd and Samba code will fail to validate any entries that
79        do not have 32 characters  between ':' characters. The first XXX
80        section is for the Lanman password hash, the second is for the
81        Windows NT version.</p><p>When the password file is created all users have password entries
82        consisting of 32 'X' characters. By default this disallows any access
83        as this user. When a user has a password set, the 'X' characters change
84        to 32 ascii hexadecimal digits (0-9, A-F). These are an ascii
85        representation of the 16 byte hashed value of a user's password.</p><p>To set a user to have no password (not recommended), edit the file
86        using vi, and replace the first 11 characters with the ascii text
87        <code class="constant">"NO PASSWORD"</code> (minus the quotes).</p><p>For example, to clear the password for user bob, his smbpasswd file
88        entry would look like :</p><pre class="programlisting">
89bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
90        [U          ]:LCT-00000000:Bob's full name:/bobhome:/bobshell
91        </pre><p>If you are allowing users to use the smbpasswd command to set
92        their own passwords, you may want to give users NO PASSWORD initially
93        so they do not have to enter a previous password when changing to their
94        new password (not recommended). In order for you to allow this the
95        <span><strong class="command">smbpasswd</strong></span> program must be able to connect to the
96        <span><strong class="command">smbd</strong></span> daemon as that user with no password. Enable this
97        by adding the line :</p><p><span><strong class="command">null passwords = yes</strong></span></p><p>to the [global] section of the smb.conf file (this is why
98        the above scenario is not recommended). Preferably, allocate your
99        users a default password to begin with, so you do not have
100        to enable this on your server.</p><p><span class="emphasis"><em>Note : </em></span>This file should be protected very
101        carefully. Anyone with access to this file can (with enough knowledge of
102        the protocols) gain access to your SMB server. The file is thus more
103        sensitive than a normal unix <code class="filename">/etc/passwd</code> file.</p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="wins.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="pt03.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="pt04.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 12. Samba WINS Internals </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Part IV. Debugging and tracing</td></tr></table></div></body></html>
Note: See TracBrowser for help on using the repository browser.