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-Contributor: Andrew Tridgell <samba@samba.org>
-Date: April 1995
-
-Subject: Discussion of NetBIOS in a Unix World
-============================================================================
-
-This is a short document that describes some of the issues that
-confront a SMB implementation on unix, and how Samba copes with
-them. They may help people who are looking at unix<->PC
-interoperability.
-
-It was written to help out a person who was writing a paper on unix to
-PC connectivity.
-
-
-Usernames
-=========
-
-The SMB protocol has only a loose username concept. Early SMB
-protocols (such as CORE and COREPLUS) have no username concept at
-all. Even in later protocols clients often attempt operations
-(particularly printer operations) without first validating a username
-on the server.
-
-Unix security is based around username/password pairs. A unix box
-should not allow clients to do any substantive operation without some
-sort of validation.
-
-The problem mostly manifests itself when the unix server is in "share
-level" security mode. This is the default mode as the alternative
-"user level" security mode usually forces a client to connect to the
-server as the same user for each connected share, which is
-inconvenient in many sites.
-
-In "share level" security the client normally gives a username in the
-"session setup" protocol, but does not supply an accompanying
-password. The client then connects to resources using the "tree
-connect" protocol, and supplies a password. The problem is that the
-user on the PC types the username and the password in different
-contexts, unaware that they need to go together to give access to the
-server. The username is normally the one the user typed in when they
-"logged onto" the PC (this assumes Windows for Workgroups). The
-password is the one they chose when connecting to the disk or printer.
-
-The user often chooses a totally different username for their login as
-for the drive connection. Often they also want to access different
-drives as different usernames. The unix server needs some way of
-divining the correct username to combine with each password.
-
-Samba tries to avoid this problem using several methods. These succeed
-in the vast majority of cases. The methods include username maps, the
-service%user syntax, the saving of session setup usernames for later
-validation and the derivation of the username from the service name
-(either directly or via the user= option).
-
-File Ownership
-==============
-
-The commonly used SMB protocols have no way of saying "you can't do
-that because you don't own the file". They have, in fact, no concept
-of file ownership at all.
-
-This brings up all sorts of interesting problems. For example, when
-you copy a file to a unix drive, and the file is world writeable but
-owned by another user the file will transfer correctly but will
-receive the wrong date. This is because the utime() call under unix
-only succeeds for the owner of the file, or root, even if the file is
-world writeable. For security reasons Samba does all file operations
-as the validated user, not root, so the utime() fails. This can stuff
-up shared development diectories as programs like "make" will not get
-file time comparisons right.
-
-There are several possible solutions to this problem, including
-username mapping, and forcing a specific username for particular
-shares.
-
-Passwords
-=========
-
-Many SMB clients uppercase passwords before sending them. I have no
-idea why they do this. Interestingly WfWg uppercases the password only
-if the server is running a protocol greater than COREPLUS, so
-obviously it isn't just the data entry routines that are to blame.
-
-Unix passwords are case sensitive. So if users use mixed case
-passwords they are in trouble.
-
-Samba can try to cope with this by either using the "password level"
-option which causes Samba to try the offered password with up to the
-specified number of case changes, or by using the "password server"
-option which allows Samba to do its validation via another machine
-(typically a WinNT server).
-
-Samba supports the password encryption method used by SMB
-clients. Note that the use of password encryption in Microsoft
-networking leads to password hashes that are "plain text equivalent".
-This means that it is *VERY* important to ensure that the Samba
-smbpasswd file containing these password hashes is only readable
-by the root user. See the documentation ENCRYPTION.txt for more
-details.
-
-
-Locking
-=======
-
-The locking calls available under a DOS/Windows environment are much
-richer than those available in unix. This means a unix server (like
-Samba) choosing to use the standard fcntl() based unix locking calls
-to implement SMB locking has to improvise a bit.
-
-One major problem is that dos locks can be in a 32 bit (unsigned)
-range. Unix locking calls are 32 bits, but are signed, giving only a 31
-bit range. Unfortunately OLE2 clients use the top bit to select a
-locking range used for OLE semaphores.
-
-To work around this problem Samba compresses the 32 bit range into 31
-bits by appropriate bit shifting. This seems to work but is not
-ideal. In a future version a separate SMB lockd may be added to cope
-with the problem.
-
-It also doesn't help that many unix lockd daemons are very buggy and
-crash at the slightest provocation. They normally go mostly unused in
-a unix environment because few unix programs use byte range
-locking. The stress of huge numbers of lock requests from dos/windows
-clients can kill the daemon on some systems.
-
-The second major problem is the "opportunistic locking" requested by
-some clients. If a client requests opportunistic locking then it is
-asking the server to notify it if anyone else tries to do something on
-the same file, at which time the client will say if it is willing to
-give up its lock. Unix has no simple way of implementing
-opportunistic locking, and currently Samba has no support for it.
-
-Deny Modes
-==========
-
-When a SMB client opens a file it asks for a particular "deny mode" to
-be placed on the file. These modes (DENY_NONE, DENY_READ, DENY_WRITE,
-DENY_ALL, DENY_FCB and DENY_DOS) specify what actions should be
-allowed by anyone else who tries to use the file at the same time. If
-DENY_READ is placed on the file, for example, then any attempt to open
-the file for reading should fail.
-
-Unix has no equivalent notion. To implement this Samba uses either lock
-files based on the files inode and placed in a separate lock
-directory or a shared memory implementation. The lock file method
-is clumsy and consumes processing and file resources,
-the shared memory implementation is vastly prefered and is turned on
-by default for those systems that support it.
-
-Trapdoor UIDs
-=============
-
-A SMB session can run with several uids on the one socket. This
-happens when a user connects to two shares with different
-usernames. To cope with this the unix server needs to switch uids
-within the one process. On some unixes (such as SCO) this is not
-possible. This means that on those unixes the client is restricted to
-a single uid.
-
-Note that you can also get the "trapdoor uid" message for other
-reasons. Please see the FAQ for details.
-
-Port numbers
-============
-
-There is a convention that clients on sockets use high "unprivilaged"
-port numbers (>1000) and connect to servers on low "privilaged" port
-numbers. This is enforced in Unix as non-root users can't open a
-socket for listening on port numbers less than 1000.
-
-Most PC based SMB clients (such as WfWg and WinNT) don't follow this
-convention completely. The main culprit is the netbios nameserving on
-udp port 137. Name query requests come from a source port of 137. This
-is a problem when you combine it with the common firewalling technique
-of not allowing incoming packets on low port numbers. This means that
-these clients can't query a netbios nameserver on the other side of a
-low port based firewall.
-
-The problem is more severe with netbios node status queries. I've
-found that WfWg, Win95 and WinNT3.5 all respond to netbios node status
-queries on port 137 no matter what the source port was in the
-request. This works between machines that are both using port 137, but
-it means it's not possible for a unix user to do a node status request
-to any of these OSes unless they are running as root. The answer comes
-back, but it goes to port 137 which the unix user can't listen
-on. Interestingly WinNT3.1 got this right - it sends node status
-responses back to the source port in the request.
-
-
-Protocol Complexity
-===================
-
-There are many "protocol levels" in the SMB protocol. It seems that
-each time new functionality was added to a Microsoft operating system,
-they added the equivalent functions in a new protocol level of the SMB
-protocol to "externalise" the new capabilities.
-
-This means the protocol is very "rich", offering many ways of doing
-each file operation. This means SMB servers need to be complex and
-large. It also means it is very difficult to make them bug free. It is
-not just Samba that suffers from this problem, other servers such as
-WinNT don't support every variation of every call and it has almost
-certainly been a headache for MS developers to support the myriad of
-SMB calls that are available.
-
-There are about 65 "top level" operations in the SMB protocol (things
-like SMBread and SMBwrite). Some of these include hundreds of
-sub-functions (SMBtrans has at least 120 sub-functions, like
-DosPrintQAdd and NetSessionEnum). All of them take several options
-that can change the way they work. Many take dozens of possible
-"information levels" that change the structures that need to be
-returned. Samba supports all but 2 of the "top level" functions. It
-supports only 8 (so far) of the SMBtrans sub-functions. Even NT
-doesn't support them all.
-
-Samba currently supports up to the "NT LM 0.12" protocol, which is the
-one preferred by Win95 and WinNT3.5. Luckily this protocol level has a
-"capabilities" field which specifies which super-duper new-fangled
-options the server suports. This helps to make the implementation of
-this protocol level much easier.
-
-There is also a problem with the SMB specications. SMB is a X/Open
-spec, but the X/Open book is far from ideal, and fails to cover many
-important issues, leaving much to the imagination. Microsoft recently
-renamed the SMB protocol CIFS (Common Internet File System) and have
-published new specifications. These are far superior to the old
-X/Open documents but there are still undocumented calls and features.
-This specification is actively being worked on by a CIFS developers
-mailing list hosted by Microsft.
-