1 <html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Chapter 1. NetBIOS in a Unix World</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="pt01.html" title="Part I. The protocol"><link rel="prev" href="pt01.html" title="Part I. The protocol"><link rel="next" href="ntdomain.html" title="Chapter 2. NT Domain RPC's"></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 1. NetBIOS in a Unix World</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="pt01.html">Prev</a> </td><th width="60%" align="center">Part I. The protocol</th><td width="20%" align="right"> <a accesskey="n" href="ntdomain.html">Next</a></td></tr></table><hr></div><div class="chapter" lang="en"><div class="titlepage"><div><div><h2 class="title"><a name="unix-smb"></a>Chapter 1. NetBIOS in a Unix World</h2></div><div><div class="author"><h3 class="author"><span class="firstname">Andrew</span> <span class="surname">Tridgell</span></h3></div></div><div><p class="pubdate">April 1995</p></div></div></div><div class="toc"><p><b>Table of Contents</b></p><dl><dt><span class="sect1"><a href="unix-smb.html#id2468238">Introduction</a></span></dt><dt><span class="sect1"><a href="unix-smb.html#id2468258">Usernames</a></span></dt><dt><span class="sect1"><a href="unix-smb.html#id2466345">File Ownership</a></span></dt><dt><span class="sect1"><a href="unix-smb.html#id2466376">Passwords</a></span></dt><dt><span class="sect1"><a href="unix-smb.html#id2466414">Locking</a></span></dt><dt><span class="sect1"><a href="unix-smb.html#id2466473">Deny Modes</a></span></dt><dt><span class="sect1"><a href="unix-smb.html#id2466499">Trapdoor UIDs</a></span></dt><dt><span class="sect1"><a href="unix-smb.html#id2466520">Port numbers</a></span></dt><dt><span class="sect1"><a href="unix-smb.html#id2466565">Protocol Complexity</a></span></dt></dl></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2468238"></a>Introduction</h2></div></div></div><p>
2 This is a short document that describes some of the issues that
3 confront a SMB implementation on unix, and how Samba copes with
4 them. They may help people who are looking at unix<->PC
7 It was written to help out a person who was writing a paper on unix to
9 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2468258"></a>Usernames</h2></div></div></div><p>
10 The SMB protocol has only a loose username concept. Early SMB
11 protocols (such as CORE and COREPLUS) have no username concept at
12 all. Even in later protocols clients often attempt operations
13 (particularly printer operations) without first validating a username
16 Unix security is based around username/password pairs. A unix box
17 should not allow clients to do any substantive operation without some
20 The problem mostly manifests itself when the unix server is in "share
21 level" security mode. This is the default mode as the alternative
22 "user level" security mode usually forces a client to connect to the
23 server as the same user for each connected share, which is
24 inconvenient in many sites.
26 In "share level" security the client normally gives a username in the
27 "session setup" protocol, but does not supply an accompanying
28 password. The client then connects to resources using the "tree
29 connect" protocol, and supplies a password. The problem is that the
30 user on the PC types the username and the password in different
31 contexts, unaware that they need to go together to give access to the
32 server. The username is normally the one the user typed in when they
33 "logged onto" the PC (this assumes Windows for Workgroups). The
34 password is the one they chose when connecting to the disk or printer.
36 The user often chooses a totally different username for their login as
37 for the drive connection. Often they also want to access different
38 drives as different usernames. The unix server needs some way of
39 divining the correct username to combine with each password.
41 Samba tries to avoid this problem using several methods. These succeed
42 in the vast majority of cases. The methods include username maps, the
43 service%user syntax, the saving of session setup usernames for later
44 validation and the derivation of the username from the service name
45 (either directly or via the user= option).
46 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2466345"></a>File Ownership</h2></div></div></div><p>
47 The commonly used SMB protocols have no way of saying "you can't do
48 that because you don't own the file". They have, in fact, no concept
49 of file ownership at all.
51 This brings up all sorts of interesting problems. For example, when
52 you copy a file to a unix drive, and the file is world writeable but
53 owned by another user the file will transfer correctly but will
54 receive the wrong date. This is because the utime() call under unix
55 only succeeds for the owner of the file, or root, even if the file is
56 world writeable. For security reasons Samba does all file operations
57 as the validated user, not root, so the utime() fails. This can stuff
58 up shared development diectories as programs like "make" will not get
59 file time comparisons right.
61 There are several possible solutions to this problem, including
62 username mapping, and forcing a specific username for particular
64 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2466376"></a>Passwords</h2></div></div></div><p>
65 Many SMB clients uppercase passwords before sending them. I have no
66 idea why they do this. Interestingly WfWg uppercases the password only
67 if the server is running a protocol greater than COREPLUS, so
68 obviously it isn't just the data entry routines that are to blame.
70 Unix passwords are case sensitive. So if users use mixed case
71 passwords they are in trouble.
73 Samba can try to cope with this by either using the "password level"
74 option which causes Samba to try the offered password with up to the
75 specified number of case changes, or by using the "password server"
76 option which allows Samba to do its validation via another machine
77 (typically a WinNT server).
79 Samba supports the password encryption method used by SMB
80 clients. Note that the use of password encryption in Microsoft
81 networking leads to password hashes that are "plain text equivalent".
82 This means that it is *VERY* important to ensure that the Samba
83 smbpasswd file containing these password hashes is only readable
84 by the root user. See the documentation ENCRYPTION.txt for more
86 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2466414"></a>Locking</h2></div></div></div><p>
87 Since samba 2.2, samba supports other types of locking as well. This
90 The locking calls available under a DOS/Windows environment are much
91 richer than those available in unix. This means a unix server (like
92 Samba) choosing to use the standard fcntl() based unix locking calls
93 to implement SMB locking has to improvise a bit.
95 One major problem is that dos locks can be in a 32 bit (unsigned)
96 range. Unix locking calls are 32 bits, but are signed, giving only a 31
97 bit range. Unfortunately OLE2 clients use the top bit to select a
98 locking range used for OLE semaphores.
100 To work around this problem Samba compresses the 32 bit range into 31
101 bits by appropriate bit shifting. This seems to work but is not
102 ideal. In a future version a separate SMB lockd may be added to cope
105 It also doesn't help that many unix lockd daemons are very buggy and
106 crash at the slightest provocation. They normally go mostly unused in
107 a unix environment because few unix programs use byte range
108 locking. The stress of huge numbers of lock requests from dos/windows
109 clients can kill the daemon on some systems.
111 The second major problem is the "opportunistic locking" requested by
112 some clients. If a client requests opportunistic locking then it is
113 asking the server to notify it if anyone else tries to do something on
114 the same file, at which time the client will say if it is willing to
115 give up its lock. Unix has no simple way of implementing
116 opportunistic locking, and currently Samba has no support for it.
117 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2466473"></a>Deny Modes</h2></div></div></div><p>
118 When a SMB client opens a file it asks for a particular "deny mode" to
119 be placed on the file. These modes (DENY_NONE, DENY_READ, DENY_WRITE,
120 DENY_ALL, DENY_FCB and DENY_DOS) specify what actions should be
121 allowed by anyone else who tries to use the file at the same time. If
122 DENY_READ is placed on the file, for example, then any attempt to open
123 the file for reading should fail.
125 Unix has no equivalent notion. To implement this Samba uses either lock
126 files based on the files inode and placed in a separate lock
127 directory or a shared memory implementation. The lock file method
128 is clumsy and consumes processing and file resources,
129 the shared memory implementation is vastly prefered and is turned on
130 by default for those systems that support it.
131 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2466499"></a>Trapdoor UIDs</h2></div></div></div><p>
132 A SMB session can run with several uids on the one socket. This
133 happens when a user connects to two shares with different
134 usernames. To cope with this the unix server needs to switch uids
135 within the one process. On some unixes (such as SCO) this is not
136 possible. This means that on those unixes the client is restricted to
139 Note that you can also get the "trapdoor uid" message for other
140 reasons. Please see the FAQ for details.
141 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2466520"></a>Port numbers</h2></div></div></div><p>
142 There is a convention that clients on sockets use high "unprivileged"
143 port numbers (>1000) and connect to servers on low "privilegedg" port
144 numbers. This is enforced in Unix as non-root users can't open a
145 socket for listening on port numbers less than 1000.
147 Most PC based SMB clients (such as WfWg and WinNT) don't follow this
148 convention completely. The main culprit is the netbios nameserving on
149 udp port 137. Name query requests come from a source port of 137. This
150 is a problem when you combine it with the common firewalling technique
151 of not allowing incoming packets on low port numbers. This means that
152 these clients can't query a netbios nameserver on the other side of a
153 low port based firewall.
155 The problem is more severe with netbios node status queries. I've
156 found that WfWg, Win95 and WinNT3.5 all respond to netbios node status
157 queries on port 137 no matter what the source port was in the
158 request. This works between machines that are both using port 137, but
159 it means it's not possible for a unix user to do a node status request
160 to any of these OSes unless they are running as root. The answer comes
161 back, but it goes to port 137 which the unix user can't listen
162 on. Interestingly WinNT3.1 got this right - it sends node status
163 responses back to the source port in the request.
164 </p></div><div class="sect1" lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a name="id2466565"></a>Protocol Complexity</h2></div></div></div><p>
165 There are many "protocol levels" in the SMB protocol. It seems that
166 each time new functionality was added to a Microsoft operating system,
167 they added the equivalent functions in a new protocol level of the SMB
168 protocol to "externalise" the new capabilities.
170 This means the protocol is very "rich", offering many ways of doing
171 each file operation. This means SMB servers need to be complex and
172 large. It also means it is very difficult to make them bug free. It is
173 not just Samba that suffers from this problem, other servers such as
174 WinNT don't support every variation of every call and it has almost
175 certainly been a headache for MS developers to support the myriad of
176 SMB calls that are available.
178 There are about 65 "top level" operations in the SMB protocol (things
179 like SMBread and SMBwrite). Some of these include hundreds of
180 sub-functions (SMBtrans has at least 120 sub-functions, like
181 DosPrintQAdd and NetSessionEnum). All of them take several options
182 that can change the way they work. Many take dozens of possible
183 "information levels" that change the structures that need to be
184 returned. Samba supports all but 2 of the "top level" functions. It
185 supports only 8 (so far) of the SMBtrans sub-functions. Even NT
186 doesn't support them all.
188 Samba currently supports up to the "NT LM 0.12" protocol, which is the
189 one preferred by Win95 and WinNT3.5. Luckily this protocol level has a
190 "capabilities" field which specifies which super-duper new-fangled
191 options the server suports. This helps to make the implementation of
192 this protocol level much easier.
194 There is also a problem with the SMB specications. SMB is a X/Open
195 spec, but the X/Open book is far from ideal, and fails to cover many
196 important issues, leaving much to the imagination. Microsoft recently
197 renamed the SMB protocol CIFS (Common Internet File System) and have
198 published new specifications. These are far superior to the old
199 X/Open documents but there are still undocumented calls and features.
200 This specification is actively being worked on by a CIFS developers
201 mailing list hosted by Microsft.
202 </p></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="pt01.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="pt01.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ntdomain.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Part I. The protocol </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 2. NT Domain RPC's</td></tr></table></div></body></html>
projects / samba / blob