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diff --git a/docs/textdocs/UNIX-SMB.txt b/docs/textdocs/UNIX-SMB.txt deleted file mode 100644 index c3d7643cbc..0000000000 --- a/docs/textdocs/UNIX-SMB.txt +++ /dev/null @@ -1,231 +0,0 @@ -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. - |