Server Message Block Protocol (SMB)

There are very many systems which can use the NetBIOS / NetBEUI interface or make use of the NetBIOS Frames Protocol, but perhaps one of the most important is the Server Message Block Protocol (SMB). The Server Message Block Protocol (SMB), is an application level protocol used by networking systems and operating systems such as Microsoft's Windows for Workgroups, Windows 95 / 98 / ME, LAN Manager, Windows NT, Windows 2000 and IBM's OS/2 and LAN Server, NetWare 6 and the SAMBA implementation and as such deserves special attention.

SMB is described in Protocols for X/Open PC Interworking: SMB, Version 2 .

SMB History

In 1987 Microsoft announced the LAN Manager program and in 1988 IBM announced the OS/2 LAN Server, both use versions of the Server Message Block Protocol. Enhancements and changes to the protocol have been made and a history can be found at:

"http://samba.anu.edu.au/cifs/docs/smb-history.html" History of SMB

Microsoft and a number of other companies, are proposing an updated version of SMB as an internet standard The Common Internet File System (CIFS).

SMB Overview

The Server Message Block Protocol (SMB), is an application level protocol see OSI diagram .

SMB is used to implement network session control, network file and print sharing and messaging. SMB is used to provide broadly analogous functionality as the AppleTalk Session Protocol, AppleTalk Filing Protocol, Printer Access Protocol etc in the AppleTalk suite of protocols. SMB is also broadly analogous with Novell's NetWare Core Protocol (NCP). It is difficult to find a non-proprietary protocol or protocols with in the TCP/IP suite which can be compared to SMB; file sharing via FTP or NFS and network printing via LPR are examples of similar functionality.

SMB requires a transport /session protocol and the early versions of IBM's implementation were closely linked with NetBIOS. In general SMB runs either over the NetBIOS Frames Protocol (NBF), NetBIOS over TCP/IP, or NetBIOS over IPX; the most recent versions of CIFS can run directly over TCP/IP.

See: OSI diagram for details of the relationship between the various protocols.

SMB has inherited some of the advantages and disadvantages of NetBIOS, in particular, prior to the latest versions of CIFS it was directly linked with the NetBIOS addressing scheme.

SMB Dialects

The SMB protocol has been developed and enhanced since it was first introduced. The original version is known as the "core protocol" and is understood by systems implementing later versions which are supersets of the original. Systems using SMB negotiate which version i.e. dialect they will support.

The function SMBnegprot 0x72 is used at the beginning of a session to establish the dialect to be used. (See SMB Command Codes below.)

When packets are being sent to negotiate the dialect, a string is used to indicate which dialects are supported. So just as the use of the string "SMB" within SMB packets makes identifying such packets easier, the use of readable strings makes understanding which dialects are used easier. Below is a table giving some of the strings used to identify dialects and the terms commonly used to refer to the given dialect.

SMB Addressing

Prior to the latest versions of CIFS, SMB uses network names which are strings of 16 bytes. In general these names are mapped directly on to NetBIOS names (see NetBIOS names above). The traditional SMB names of systems can be up to 15 characters long and are padded with blanks if necessary. The 16th byte is used to indicate whether the name refers to a server or another function.

In Microsoft networks with NT 3.x and NT 4.0 systems some names are used with NT 3.x and NT 4.0 Domains as well as for computer names. Some examples of names and use of the 16th byte are given below:

Unique NetBIOS names will map to SMB individual system names, and NetBIOS group names will map to workgroup or domain names.

Like NetBIOS names, traditional SMB names are non hierarchical and constitute a flat non-routable name space which does not scale well.

SMB on NBF

SMB on NBF datagram frames

SMB on NBF session frames

SMB frame header

The general format of SMB frame headers is given below:

SMB is very analogous to the NetWare Core Protocol (NCF); there are numerous functions available for accomplishing various tasks. There are very many SMB frames depending upon the function, all share the same header format; the second field, command, determines the function and possibly the format of the rest of the frame following the header.

SMB Command Codes

Below is a table giving some of the Core SMB commands:

Below is a table giving some of the Core plus commands:

Below is a table giving some of the LANMAN 1.0 SMB commands:

SMB Error Class

Below is a table giving some of the SMB Error class values:

SMB Return Codes for Error class 0x00

Below is a table giving some of the SMB Return Code Values when the Error class is 0x00:

SMB Return Codes for Error class 0x02

Below is a table giving some of the SMB Return Code Values when the Error class is 0x02:

Further information

Just what is SMB? V1.0 Richard Sharpe