| 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 V1.75.2"><link rel="home" 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" title="Chapter 13. LanMan and NT Password Encryption"><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>
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| 2 | <code class="email"><<a class="email" href="mailto:samba@samba.org">samba@samba.org</a>></code><br>
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| 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#id332270">Introduction</a></span></dt><dt><span class="sect1"><a href="pwencrypt.html#id332287">How does it work?</a></span></dt><dt><span class="sect1"><a href="pwencrypt.html#id332348">The smbpasswd file</a></span></dt></dl></div><div class="sect1" title="Introduction"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id332270"></a>Introduction</h2></div></div></div><p>With the development of LanManager and Windows NT
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| 4 | compatible password encryption for Samba, it is now able
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| 5 | to validate user connections in exactly the same way as
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| 6 | a LanManager or Windows NT server.</p><p>This document describes how the SMB password encryption
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| 7 | algorithm works and what issues there are in choosing whether
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| 8 | you want to use it. You should read it carefully, especially
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| 9 | the part about security and the "PROS and CONS" section.</p></div><div class="sect1" title="How does it work?"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id332287"></a>How does it work?</h2></div></div></div><p>LanManager encryption is somewhat similar to UNIX
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| 10 | password encryption. The server uses a file containing a
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| 11 | hashed value of a user's password. This is created by taking
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| 12 | the user's plaintext password, capitalising it, and either
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| 13 | truncating to 14 bytes or padding to 14 bytes with null bytes.
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| 14 | This 14 byte value is used as two 56 bit DES keys to encrypt
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| 15 | a 'magic' eight byte value, forming a 16 byte value which is
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| 16 | stored by the server and client. Let this value be known as
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| 17 | the "hashed password".</p><p>Windows NT encryption is a higher quality mechanism,
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| 18 | consisting of doing an MD4 hash on a Unicode version of the user's
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| 19 | password. This also produces a 16 byte hash value that is
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| 20 | non-reversible.</p><p>When a client (LanManager, Windows for WorkGroups, Windows
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| 21 | 95 or Windows NT) wishes to mount a Samba drive (or use a Samba
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| 22 | resource), it first requests a connection and negotiates the
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| 23 | protocol that the client and server will use. In the reply to this
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| 24 | request the Samba server generates and appends an 8 byte, random
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| 25 | value - this is stored in the Samba server after the reply is sent
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| 26 | and is known as the "challenge". The challenge is different for
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| 27 | every client connection.</p><p>The client then uses the hashed password (16 byte values
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| 28 | described above), appended with 5 null bytes, as three 56 bit
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| 29 | DES keys, each of which is used to encrypt the challenge 8 byte
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| 30 | value, forming a 24 byte value known as the "response".</p><p>In the SMB call SMBsessionsetupX (when user level security
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| 31 | is selected) or the call SMBtconX (when share level security is
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| 32 | selected), the 24 byte response is returned by the client to the
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| 33 | Samba server. For Windows NT protocol levels the above calculation
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| 34 | is done on both hashes of the user's password and both responses are
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| 35 | returned in the SMB call, giving two 24 byte values.</p><p>The Samba server then reproduces the above calculation, using
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| 36 | its own stored value of the 16 byte hashed password (read from the
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| 37 | <code class="filename">smbpasswd</code> file - described later) and the challenge
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| 38 | value that it kept from the negotiate protocol reply. It then checks
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| 39 | to see if the 24 byte value it calculates matches the 24 byte value
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| 40 | returned to it from the client.</p><p>If these values match exactly, then the client knew the
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| 41 | correct password (or the 16 byte hashed value - see security note
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| 42 | below) and is thus allowed access. If not, then the client did not
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| 43 | know the correct password and is denied access.</p><p>Note that the Samba server never knows or stores the cleartext
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| 44 | of the user's password - just the 16 byte hashed values derived from
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| 45 | it. Also note that the cleartext password or 16 byte hashed values
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| 46 | are never transmitted over the network - thus increasing security.</p></div><div class="sect1" title="The smbpasswd file"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id332348"></a>The smbpasswd file</h2></div></div></div><a name="SMBPASSWDFILEFORMAT"></a><p>In order for Samba to participate in the above protocol
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| 47 | it must be able to look up the 16 byte hashed values given a user name.
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| 48 | Unfortunately, as the UNIX password value is also a one way hash
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| 49 | function (ie. it is impossible to retrieve the cleartext of the user's
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| 50 | password given the UNIX hash of it), a separate password file
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| 51 | containing this 16 byte value must be kept. To minimise problems with
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| 52 | these two password files, getting out of sync, the UNIX <code class="filename">
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| 53 | /etc/passwd</code> and the <code class="filename">smbpasswd</code> file,
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| 54 | a utility, <code class="literal">mksmbpasswd.sh</code>, is provided to generate
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| 55 | a smbpasswd file from a UNIX <code class="filename">/etc/passwd</code> file.
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| 56 | </p><p>To generate the smbpasswd file from your <code class="filename">/etc/passwd
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| 57 | </code> file use the following command:</p><p><code class="prompt">$ </code><strong class="userinput"><code>cat /etc/passwd | mksmbpasswd.sh
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| 58 | > /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
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| 59 | > /usr/local/samba/private/smbpasswd</code></strong></p><p>The <code class="literal">mksmbpasswd.sh</code> program is found in
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| 60 | the Samba source directory. By default, the smbpasswd file is
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| 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>
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| 62 | directory should be set to root, and the permissions on it should
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| 63 | be set to 0500 (<code class="literal">chmod 500 /usr/local/samba/private</code>).
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| 64 | </p><p>Likewise, the smbpasswd file inside the private directory should
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| 65 | be owned by root and the permissions on is should be set to 0600
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| 66 | (<code class="literal">chmod 600 smbpasswd</code>).</p><p>The format of the smbpasswd file is (The line has been
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| 67 | wrapped here. It should appear as one entry per line in
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| 68 | your smbpasswd file.)</p><pre class="programlisting">
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| 69 | username:uid:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
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| 70 | [Account type]:LCT-<last-change-time>:Long name
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| 71 | </pre><p>Although only the <em class="replaceable"><code>username</code></em>,
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| 72 | <em class="replaceable"><code>uid</code></em>, <em class="replaceable"><code>
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| 73 | XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX</code></em>,
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| 74 | [<em class="replaceable"><code>Account type</code></em>] and <em class="replaceable"><code>
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| 75 | last-change-time</code></em> sections are significant
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| 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
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| 77 | 'X' characters between the two ':' characters in the XXX sections -
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| 78 | the smbpasswd and Samba code will fail to validate any entries that
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| 79 | do not have 32 characters between ':' characters. The first XXX
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| 80 | section is for the Lanman password hash, the second is for the
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| 81 | Windows NT version.</p><p>When the password file is created all users have password entries
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| 82 | consisting of 32 'X' characters. By default this disallows any access
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| 83 | as this user. When a user has a password set, the 'X' characters change
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| 84 | to 32 ascii hexadecimal digits (0-9, A-F). These are an ascii
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| 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
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| 86 | using vi, and replace the first 11 characters with the ascii text
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| 87 | <code class="constant">"NO PASSWORD"</code> (minus the quotes).</p><p>For example, to clear the password for user bob, his smbpasswd file
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| 88 | entry would look like :</p><pre class="programlisting">
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| 89 | bob:100:NO PASSWORDXXXXXXXXXXXXXXXXXXXXX:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX:
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| 90 | [U ]:LCT-00000000:Bob's full name:/bobhome:/bobshell
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| 91 | </pre><p>If you are allowing users to use the smbpasswd command to set
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| 92 | their own passwords, you may want to give users NO PASSWORD initially
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| 93 | so they do not have to enter a previous password when changing to their
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| 94 | new password (not recommended). In order for you to allow this the
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| 95 | <code class="literal">smbpasswd</code> program must be able to connect to the
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| 96 | <code class="literal">smbd</code> daemon as that user with no password. Enable this
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| 97 | by adding the line :</p><p><code class="literal">null passwords = yes</code></p><p>to the [global] section of the smb.conf file (this is why
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| 98 | the above scenario is not recommended). Preferably, allocate your
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| 99 | users a default password to begin with, so you do not have
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| 100 | to enable this on your server.</p><p><span class="emphasis"><em>Note : </em></span>This file should be protected very
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| 101 | carefully. Anyone with access to this file can (with enough knowledge of
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| 102 | the protocols) gain access to your SMB server. The file is thus more
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| 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>
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