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Server Message Block

Server Message Block

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Encyclopedia
In computer networking, Server Message Block (SMB), also known as Common Internet File System (CIFS, ˈ) operates as an application-layer network protocol
Application layer
The Internet protocol suite and the Open Systems Interconnection model of computer networking each specify a group of protocols and methods identified by the name application layer....

 mainly used to provide shared access to files
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...

, printers
Computer printer
In computing, a printer is a peripheral which produces a text or graphics of documents stored in electronic form, usually on physical print media such as paper or transparencies. Many printers are primarily used as local peripherals, and are attached by a printer cable or, in most new printers, a...

, serial port
Serial port
In computing, a serial port is a serial communication physical interface through which information transfers in or out one bit at a time...

s, and miscellaneous communications between nodes on a network. It also provides an authenticated inter-process communication
Inter-process communication
In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...

 mechanism. Most usage of SMB involves computers running Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

, where it was known as "Microsoft Windows Network" before the subsequent introduction of Active Directory
Active Directory
Active Directory is a directory service created by Microsoft for Windows domain networks. It is included in most Windows Server operating systems. Server computers on which Active Directory is running are called domain controllers....

. Corresponding Windows service
Windows Service
On Microsoft Windows operating systems, a Windows service is a long-running executable that performs specific functions and which is designed not to require user intervention. Windows services can be configured to start when the operating system is booted and run in the background as long as...

s are the "Server Service" (for the server component) and "Workstation Service" (for the client component).

The Server Message Block protocol can run atop the Session
Session layer
The session layer is layer 5 of the seven-layer OSI model of computer networking.The session layer provides the mechanism for opening, closing and managing a session between end-user application processes, i.e., a semi-permanent dialogue. Communication sessions consist of requests and responses...

 (and lower) network layers in several ways:
  • directly over TCP
    Transmission Control Protocol
    The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...

    , port 445;
  • via the NetBIOS
    NetBIOS
    NetBIOS is an acronym for Network Basic Input/Output System. It provides services related to the session layer of the OSI model allowing applications on separate computers to communicate over a local area network. As strictly an API, NetBIOS is not a networking protocol...

     API, which in turn can run on several transports
    Transport layer
    In computer networking, the transport layer or layer 4 provides end-to-end communication services for applications within a layered architecture of network components and protocols...

    :
    • on UDP
      User Datagram Protocol
      The User Datagram Protocol is one of the core members of the Internet Protocol Suite, the set of network protocols used for the Internet. With UDP, computer applications can send messages, in this case referred to as datagrams, to other hosts on an Internet Protocol network without requiring...

       ports 137, 138 & TCP ports 137, 139 — see NetBIOS over TCP/IP;
    • on several legacy protocols such as NBF (incorrectly referred to as NetBEUI).

History


Barry Feigenbaum originally designed SMB at IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

 with the aim of turning DOS
DOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...

 "Interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....

 33" (21h) local file-access into a networked file-system. Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 has made considerable modifications to the most commonly used version. Microsoft merged the SMB protocol with the LAN Manager
LAN Manager
LAN Manager was a Network Operating System available from multiple vendors and developed by Microsoft in cooperation with 3Com Corporation. It was designed to succeed 3Com's 3+Share network server software which ran atop a heavily modified version of MS-DOS.-Development history:LAN Manager was...

 product which it had started developing for OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...

 with 3Com
3Com
3Com was a pioneering digital electronics manufacturer best known for its computer network infrastructure products. The company was co-founded in 1979 by Robert Metcalfe, Howard Charney, Bruce Borden, and Greg Shaw...

 circa 1990, and continued to add features to the protocol in Windows for Workgroups (circa 1992) and in later versions of Windows.

SMB was originally designed to run on top of the NetBIOS
NetBIOS
NetBIOS is an acronym for Network Basic Input/Output System. It provides services related to the session layer of the OSI model allowing applications on separate computers to communicate over a local area network. As strictly an API, NetBIOS is not a networking protocol...

/NetBEUI
NetBEUI
NetBIOS Frames or NBF protocol is a non-routable network- and transport-level data protocol most commonly used as one of the layers of Microsoft Windows networking in the 1990s. NBF protocol or NetBIOS over IEEE 802.2 LLC is used by a number of network operating systems released in the 1990s, such...

 API
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

 (typically implemented with NBF, NetBIOS over IPX/SPX
IPX/SPX
IPX/SPX stands for Internetwork Packet Exchange/Sequenced Packet Exchange. IPX and SPX are networking protocols used primarily on networks using the Novell NetWare operating systems.-Protocol Layers:...

, or NBT). Since Windows 2000
Windows 2000
Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...

, SMB runs, by default, with a thin layer, similar to the Session Message packet of NBT's Session Service, on top of TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...

, using TCP port 445 rather than TCP port 139 — a feature known as "direct host SMB".

At around the time when Sun Microsystems announced WebNFS
WebNFS
WebNFS is an extension to the NFS file system for allowing clients to access a file system over the internet using a simplified, firewall-friendly protocol....

, Microsoft launched an initiative in 1996 to rename SMB to Common Internet File System (CIFS), and added more features, including support for symbolic link
Symbolic link
In computing, a symbolic link is a special type of file that contains a reference to another file or directory in the form of an absolute or relative path and that affects pathname resolution. Symbolic links were already present by 1978 in mini-computer operating systems from DEC and Data...

s, hard link
Hard link
In computing, a hard link is a directory entry that associates a name with a file on a file system. . The term is used in file systems which allow multiple hard links to be created for the same file. This has the effect of creating multiple names for the same file, causing an aliasing effect: e.g...

s, larger file sizes, and an initial attempt at supporting direct connections over TCP port 445 without requiring NetBIOS
NetBIOS
NetBIOS is an acronym for Network Basic Input/Output System. It provides services related to the session layer of the OSI model allowing applications on separate computers to communicate over a local area network. As strictly an API, NetBIOS is not a networking protocol...

 as a transport (a largely experimental effort that required further refinement). Microsoft submitted some partial specifications as Internet-Drafts
Internet Draft
Internet Drafts is a series of working documents published by the IETF. Typically, they are drafts for RFCs, but may be other works in progress not intended for publication as RFCs. It is considered inappropriate to rely on Internet Drafts for reference purposes...

 to the IETF
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...

, though these submissions have expired.

The Samba
Samba (software)
Samba is a free software re-implementation, originally developed by Andrew Tridgell, of the SMB/CIFS networking protocol. As of version 3, Samba provides file and print services for various Microsoft Windows clients and can integrate with a Windows Server domain, either as a Primary Domain...

 project originated with the aim of reverse engineering
Reverse engineering
Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...

 the SMB protocol and implementing an SMB server to allow MS-DOS
MS-DOS
MS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...

 clients to use SMB to access files on Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...

 machines. Because of the importance of the SMB protocol in interacting with the widespread Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

 platform, Samba became a popular free
Free software
Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

 implementation of a compatible SMB client and server for interoperating with non-Microsoft operating systems.

Microsoft introduced SMB2 with Windows Vista in 2006, and later improved on it in Windows 7.

Client-server approach


SMB works through a client-server
Client-server
The client–server model of computing is a distributed application that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients. Often clients and servers communicate over a computer network on separate hardware, but both...

 approach, where a client
Client (computing)
A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

 makes specific requests and the server responds accordingly. One section of the SMB protocol specifically deals with access to filesystems, such that clients may make requests to a file server
File server
In computing, a file server is a computer attached to a network that has the primary purpose of providing a location for shared disk access, i.e. shared storage of computer files that can be accessed by the workstations that are attached to the computer network...

; but some other sections of the SMB protocol specialize in inter-process communication
Inter-process communication
In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...

 (IPC). The Inter-Process Communication (IPC) share or ipc$ is a network share on computers running Microsoft Windows. This virtual share is used to facilitate communication between processes and computers over SMB, often to exchange data between computers that have been authenticated.

Developers have optimized the SMB protocol for local subnet
Subnetwork
A subnetwork, or subnet, is a logically visible subdivision of an IP network. The practice of dividing a network into subnetworks is called subnetting....

 usage, but users have also put SMB to work to access different subnets across the Internet—exploit
Exploit (computer security)
An exploit is a piece of software, a chunk of data, or sequence of commands that takes advantage of a bug, glitch or vulnerability in order to cause unintended or unanticipated behavior to occur on computer software, hardware, or something electronic...

s involving file-sharing or print-sharing in MS Windows environments usually focus on such usage.

SMB (Server Message Block) servers make their file systems and other resource
Resource (computer science)
A resource, or system resource, is any physical or virtual component of limited availability within a computer system. Every device connected to a computer system is a resource. Every internal system component is a resource...

s available to clients on the network. Client computers may want access to the shared file systems and printers on the server, and in this primary functionality SMB has become best-known and most heavily used. However, the SMB file-server aspect would count for little without the NT domains suite of protocols, which provide NT-style domain-based authentication
Authentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...

 at the very least. Almost all implementations of SMB servers use NT Domain authentication to validate user-access to resources.

NetBIOS


The use of the SMB protocol has often correlated with a significant increase in broadcast traffic on a network. However the SMB itself does not use broadcasts—the broadcast problems commonly associated with SMB actually originate with the NetBIOS service location protocol
Service Location Protocol
The Service Location Protocol is a service discovery protocol that allows computers and other devices to find services in a local area network without prior configuration. SLP has been designed to scale from small, unmanaged networks to large enterprise networks...

. By default, a Microsoft Windows NT 4.0 server used NetBIOS to advertise and locate services. NetBIOS functions by broadcasting services available on a particular host at regular intervals. While this usually makes for an acceptable default in a network with a smaller number hosts, increased broadcast traffic can cause problems as the size of the network increases. The implementation of name resolution infrastructure in the form of Windows Internet Naming Service
Windows Internet Naming Service
Windows Internet Name Service is Microsoft's implementation of NetBIOS Name Service , a name server and service for NetBIOS computer names. Effectively WINS is to NetBIOS names what DNS is to domain names — a central mapping of host names to network addresses...

 (WINS) or Domain Name System
Domain name system
The Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities...

 (DNS) resolves this problem. WINS was a proprietary implementation used with Windows NT 4.0 networks, but brought about its own issues and complexities in the design and maintenance of a Microsoft network.

Since the release of Windows 2000, the use of WINS for name resolution has been deprecated by Microsoft, with hierarchical Dynamic DNS
Dynamic DNS
Dynamic DNS or DDNS is a term used for the updating in real time of Internet Domain Name System name servers to keep up to date the active DNS configuration of their configured hostnames, addresses and other information....

 now configured as the default name resolution protocol for all Windows operating systems. Resolution of (short) NETBIOS names by DNS requires that a DNS client expand short names, usually by appending a connection-specific DNS suffix to its DNS lookup queries. WINS can still be configured on clients as a secondary name resolution protocol for interoperability with legacy Windows environments and applications. Further, Microsoft DNS servers can forward name resolution requests to legacy WINS servers in order to support name resolution integration with legacy (pre-Windows 2000) environments that do not support DNS.

WAN performance issues


Network designer
Network planning and design
Network planning and design is an iterative process, encompassingtopological design, network-synthesis, and network-realization, and is aimed at ensuring that a new network or service meets the needs of the subscriber and operator....

s have found that latency
Lag
Lag is a common word meaning to fail to keep up or to fall behind. In real-time applications, the term is used when the application fails to respond in a timely fashion to inputs...

 has a significant impact on the performance of the SMB 1.0 protocol, that it performs more poorly than other protocols like FTP
File Transfer Protocol
File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server...

. Monitoring reveals a high degree of "chattiness" and a disregard of network latency between hosts. For example, a VPN
Virtual private network
A virtual private network is a network that uses primarily public telecommunication infrastructure, such as the Internet, to provide remote offices or traveling users access to a central organizational network....

 connection over the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

 will often introduce network latency. Microsoft has explained that performance issues come about primarily because SMB 1.0 is a block-level rather than a streaming
Streaming algorithm
In computer science, streaming algorithms are algorithms forprocessing data streams in which the input is presented as a sequence ofitems and can be examined in only a few passes...

 protocol, that was originally designed for small LANs
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...

; it has a block size that is limited to 64K, SMB signing creates an additional overhead and the TCP window size
TCP tuning
TCP tuning techniques adjust the network congestion avoidance parameters of TCP connections over high-bandwidth, high-latency networks. Well-tuned networks can perform up to 10 times faster in some cases.- Bandwidth-delay product :...

 is not optimized for WAN links. Solutions to this problem include the updated SMB 2.0 protocol, Offline Files, TCP window scaling
TCP window scale option
The TCP window scale option is an option to increase the TCP receive window size above its maximum value of 65,535 bytes.This TCP option, along with several others, is defined in IETF RFC 1323 which deals with Long-Fat Networks, or LFN....

 and WAN acceleration
WAN Optimization
WAN optimization is a collection of techniques for increasing data-transfer efficiencies across wide-area networks. In 2008, the WAN optimization market was estimated to be $1 billion , and it will grow to $4.4 billion according to Gartner, a technology research firm.The most common measures of...

 devices from various network vendors that cache and optimize SMB 1.0.

Microsoft's modifications


Microsoft added several extensions to its own SMB implementation. For example, it added NTLM
NTLM
In a Windows network, NTLM is a suite of Microsoft security protocols that provides authentication, integrity, and confidentiality to users....

, then NTLMv2 authentication protocols in order to address security weakness in the original LanMan authentication
LM hash
LM hash, LanMan, or LAN Manager hash was the primary hash that Microsoft LAN Manager and Microsoft Windows versions prior to Windows NT used to store user passwords...

. LanMan authentication derived from the original legacy SMB specification's requirement to use IBM "LanManager" passwords, but implemented DES
Data Encryption Standard
The Data Encryption Standard is a block cipher that uses shared secret encryption. It was selected by the National Bureau of Standards as an official Federal Information Processing Standard for the United States in 1976 and which has subsequently enjoyed widespread use internationally. It is...

 in a flawed manner that allowed passwords to be cracked. Later, Kerberos authentication was also added. The NT 4.0 Domain
Windows Server domain
A Windows domain is a collection of security principals that share a central directory database. This central database contains the user accounts and security information for...

 logon protocols initially used 40-bit encryption outside of the United States of America, because of export restrictions on stronger 128-bit encryption (subsequently lifted in 1996 when President Bill Clinton
Bill Clinton
William Jefferson "Bill" Clinton is an American politician who served as the 42nd President of the United States from 1993 to 2001. Inaugurated at age 46, he was the third-youngest president. He took office at the end of the Cold War, and was the first president of the baby boomer generation...

 signed Executive order 13026). Opportunistic locking support has changed with each server release.

Opportunistic locking



In the SMB protocol, opportunistic locking is a file locking
File locking
File locking is a mechanism that restricts access to a computer file by allowing only one user or process access at any specific time. Systems implement locking to prevent the classic interceding update scenario ....

 mechanism designed to improve performance by controlling caching
Cache
In computer engineering, a cache is a component that transparently stores data so that future requests for that data can be served faster. The data that is stored within a cache might be values that have been computed earlier or duplicates of original values that are stored elsewhere...

 of network files by the client. Contrary to the traditional lock
Lock (computer science)
In computer science, a lock is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. Locks are one way of enforcing concurrency control policies.-Types:...

s, OpLocks are not used in order to provide mutual exclusion. The main goal of OpLocks is to provide synchronization for caching. There are three types of opportunistic locks:

Batch Locks


Batch OpLocks were created originally to support a particular behavior of MS-DOS batch file execution operation in which the file is opened and closed many times in a short period, which is a performance problem. To solve this, a client may ask for a OpLock of type "batch". In this case, the client delays sending the close request and if a subsequent open request is given, the two requests cancel each other.

Exclusive Locks


When an application opens in "shared mode" a file hosted on an SMB server which is not opened by any other process (or other clients) the client receives an exclusive OpLock from the server. This means that the client may now assume that it is the only process with access to this particular file, and the client may now cache all changes to the file before committing it to the server. This is a performance improvement, since fewer round-trips are required in order to read and write to the file. If another client/process tries to open the same file, the server sends a message to the client (called a break or revocation) which invalidates the exclusive lock previously given to the client. The client then flushes all changes to the file.

Level 2 OpLocks


If an exclusive OpLock is held by a client and a locked file is opened by a third party, the client has to relinquish its exclusive OpLock to allow the other client's write/read access. A client may then receive a "Level 2 OpLock" from the server. A Level 2 OpLock allows the caching of read requests, but excludes write caching.

Breaks


In contrast with the SMB protocol's "standard" behavior, a break request may be sent from server to client. It informs the client that an OpLock is no longer valid. This happens, for example, when another client wishes to open a file in a way that invalidates the OpLock. The first client is then sent an OpLock break and required to send all its local changes (in case of batch or exclusive OpLocks), if any, and acknowledge the OpLock break. Upon this acknowledgment the server can reply to the second client in a consistent manner.

SMB2


Microsoft introduced a new version of the Server Message Block (SMB) protocol (SMB 2.0 or SMB2) with Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...

 in 2006. Although the protocol is proprietary, its specification has been published to allow other systems to interoperate with Microsoft operating systems that use the new protocol.

SMB2 reduces the 'chattiness' of the SMB 1.0 protocol by reducing the number of commands and subcommands from over a hundred to just nineteen. It has mechanisms for pipelining, that is, sending additional requests before the response to a previous request arrives, thereby improving performance over high latency
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...

 links. It adds the ability to compound multiple actions into a single request, which significantly reduces the number of round-trips
Round-trip delay time
In telecommunications, the round-trip delay time or round-trip time is the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgment of that signal to be received...

 the client needs to make to the server, improving performance as a result. SMB1 also has a compounding mechanism — known as AndX — to compound multiple actions, but Microsoft clients rarely use AndX. It also introduces the notion of "durable file handles": these allow a connection to an SMB server to survive brief network outages, as are typical in a wireless network, without having to incur the overhead of re-negotiating a new session.

SMB2 includes support for symbolic link
Symbolic link
In computing, a symbolic link is a special type of file that contains a reference to another file or directory in the form of an absolute or relative path and that affects pathname resolution. Symbolic links were already present by 1978 in mini-computer operating systems from DEC and Data...

s. Other improvements include caching of file properties, improved message signing with HMAC
HMAC
In cryptography, HMAC is a specific construction for calculating a message authentication code involving a cryptographic hash function in combination with a secret key. As with any MAC, it may be used to simultaneously verify both the data integrity and the authenticity of a message...

 SHA-256 hashing algorithm and better scalability by increasing the number of users, shares and open files per server among others.
The SMB1 protocol uses 16-bit data sizes, which amongst other things, limits the maximum block size to 64K. SMB2 uses 32 or 64-bit wide storage fields, and 128 bits in the case of file-handles, thereby removing previous constraints on block sizes, which improves performance with large file transfers over fast networks.

Windows Vista and later operating systems use SMB2 when communicating with other machines running Windows Vista or later. SMB1 continues in use for connections with older versions of Windows, as well as systems like Samba and various vendors' NAS
Network-attached storage
Network-attached storage is file-level computer data storage connected to a computer network providing data access to heterogeneous clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration of those elements...

 solutions. Samba 3.5 also includes experimental support for SMB2. Samba 3.6 fully supports SMB2, except the modification of user quotas using the Windows quota management tools.

SMB2 brings a number of benefits to third party implementers of SMB protocols. SMB1, originally designed by IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

, was reverse engineered
Reverse engineering
Reverse engineering is the process of discovering the technological principles of a device, object, or system through analysis of its structure, function, and operation...

, and later became part of a wide variety of non-Windows operating systems such as Samba, Xenix
Xenix
Xenix is a version of the Unix operating system, licensed to Microsoft from AT&T in the late 1970s. The Santa Cruz Operation later acquired exclusive rights to the software, and eventually superseded it with SCO UNIX ....

, OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...

 and VMS
OpenVMS
OpenVMS , previously known as VAX-11/VMS, VAX/VMS or VMS, is a computer server operating system that runs on VAX, Alpha and Itanium-based families of computers. Contrary to what its name suggests, OpenVMS is not open source software; however, the source listings are available for purchase...

 (Pathworks
Pathworks
Pathworks was the tradename used by Digital Equipment Corporation of Maynard, Massachusetts for a series of programs that eased the interoperation of Digital's minicomputers with personal computers....

). X/Open
X/Open
X/Open Company, Ltd. was a consortium founded by several European UNIX systems manufacturers in 1984 to identify and promote open standards in the field of information technology. More specifically, the original aim was to define a single specification for operating systems derived from UNIX, to...

 standardised it partially; it also had draft IETF
Internet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...

 standards which lapsed. (See http://ubiqx.org/cifs/Intro.html for historical detail.) SMB2 is also a relatively clean break with the past. Microsoft's SMB1 code has to work with a large variety of SMB clients and servers. SMB1 features many versions of information for commands (selecting what structure to return for a particular request) because features such as Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...

 support were retro-fitted at a later date. SMB2 involves significantly reduced compatibility-testing for implementers of the protocol. SMB2 code has considerably less complexity since far less variability exists (for example, non-Unicode code paths become redundant as SMB2 requires Unicode support).

SMB 2.1


SMB 2.1, introduced with Windows 7 and Server 2008 R2, introduced minor performance enhancements with a new opportunistic locking mechanism.

Features


The SMB "Inter-Process Communication" (IPC) system provides named pipe
Named pipe
In computing, a named pipe is an extension to the traditional pipe concept on Unix and Unix-like systems, and is one of the methods of inter-process communication. The concept is also found in Microsoft Windows, although the semantics differ substantially...

s and was one of the first inter-process mechanisms commonly available to programmers that provides a means for services to inherit the authentication carried out when a client first connected to an SMB server.

Some services that operate over named pipes, such as those which use Microsoft's own implementation of DCE/RPC
DCE/RPC
DCE/RPC, short for "Distributed Computing Environment / Remote Procedure Calls", is the remote procedure call system developed for the Distributed Computing Environment...

 over SMB, known as MSRPC
MSRPC
Microsoft RPC is a modified version of DCE/RPC. Additions include support for Unicode strings, implicit handles, inheritance of interfaces , and complex calculations in the variable-length string and structure paradigms already present in DCE/RPC.- Example :The DCE 1.0 reference implementation...

 over SMB, also allow MSRPC client programs to perform authentication, which over-rides the authorization provided by the SMB server, but only in the context of the MSRPC client program that successfully makes the additional authentication.

Since Windows domain controller
Domain controller
On Windows Server Systems, a domain controller is a server that responds to security authentication requests within the Windows Server domain...

s use SMB to transmit policies
Group Policy
Group Policy is a feature of the Microsoft Windows NT family of operating systems. Group Policy is a set of rules that control the working environment of user accounts and computer accounts. Group Policy provides the centralized management and configuration of operating systems, applications, and...

 at login, they have packet-signing
Digital signature
A digital signature or digital signature scheme is a mathematical scheme for demonstrating the authenticity of a digital message or document. A valid digital signature gives a recipient reason to believe that the message was created by a known sender, and that it was not altered in transit...

 enabled by default to prevent man-in-the-middle attack
Man-in-the-middle attack
In cryptography, the man-in-the-middle attack , bucket-brigade attack, or sometimes Janus attack, is a form of active eavesdropping in which the attacker makes independent connections with the victims and relays messages between them, making them believe that they are talking directly to each other...

s; the feature can also be turned on for any server running Windows NT 4.0 Service Pack 3 or later. The design of Server Message Block version 2 (SMB2) aims to mitigate this performance-limitation by coalescing SMB signals into single packets.

SMB supports opportunistic locking — a special type of locking-mechanism — on files in order to improve performance.

SMB serves as the basis for Microsoft's Distributed File System
Distributed File System (Microsoft)
Distributed File System is a set of client and server services that allow an organization using Microsoft Windows servers to organize many distributed SMB file shares into a distributed file system...

 implementation.

Security


Over the years, there have been many security vulnerabilities in Microsoft's implementation of the protocol or components that it directly relies on, with the most recent vulnerability involving the SMB2 implementation.

Specifications for SMB and SMB2 Protocols


The specifications for the SMB are proprietary and were originally closed, thereby forcing other vendors and projects to reverse-engineer the protocol in order to interoperate with it. The SMB 1.0 protocol was eventually published some time after it was reverse engineered, whereas the SMB 2.0 procotol was made available from Microsoft's MSDN Open Specifications Developer Center from the outset. There are a number of specifications that are relevant to the SMB protocol:

MS-CIFS is a recent replacement (2007) for the draft-leach-cifs-v1-spec-02.txt a document widely used to implement SMB clients,
but also known to have errors of omission and commission

Versions and implementations


The list below explicitly refers to "SMB" as including an SMB client or an SMB server, plus the various protocols that extend SMB, such as the Network Neighborhood suite of protocols and the NT Domains suite.
  • Microsoft Windows
    Microsoft Windows
    Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

     includes an SMB client and server in all members of the Windows NT
    Windows NT
    Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...

     family and in Windows 95
    Windows 95
    Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Windows products...

    , Windows 98
    Windows 98
    Windows 98 is a graphical operating system by Microsoft. It is the second major release in the Windows 9x line of operating systems. It was released to manufacturing on 15 May 1998 and to retail on 25 June 1998. Windows 98 is the successor to Windows 95. Like its predecessor, it is a hybrid...

    , and Windows Me
    Windows Me
    Windows Millennium Edition, or Windows Me , is a graphical operating system released on September 14, 2000 by Microsoft, and was the last operating system released in the Windows 9x series. Support for Windows Me ended on July 11, 2006....

    .
  • Samba
    Samba (software)
    Samba is a free software re-implementation, originally developed by Andrew Tridgell, of the SMB/CIFS networking protocol. As of version 3, Samba provides file and print services for various Microsoft Windows clients and can integrate with a Windows Server domain, either as a Primary Domain...

    , which re-implements the SMB protocol and the Microsoft extensions to it as free software
    Free software
    Free software, software libre or libre software is software that can be used, studied, and modified without restriction, and which can be copied and redistributed in modified or unmodified form either without restriction, or with restrictions that only ensure that further recipients can also do...

    , includes an SMB server and a command-line SMB client. Version 3.0 or later is required for NTLMv2 authentication support, while versions 3.6+ or 4.0+ are required for SMB2 interoperability.
  • Samba TNG: a fork
    Fork (software development)
    In software engineering, a project fork happens when developers take a legal copy of source code from one software package and start independent development on it, creating a distinct piece of software...

     of Samba.
  • The Linux kernel
    Linux kernel
    The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....

     includes two SMB client implementations that use the Linux VFS
    Virtual file system
    A virtual file system or virtual filesystem switch is an abstraction layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way...

    , providing access to files on an SMB server through the standard file system API
    Application programming interface
    An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...

    : smbfs and cifs. Also it is possible to mount the whole hierarchy of workgroups/servers/shares ("neighborhood") through FUSE
    Filesystem in Userspace
    Filesystem in Userspace is a loadable kernel module for Unix-like computer operating systems that lets non-privileged users create their own file systems without editing kernel code...

     kernel module and its userspace counterpart fusesmb.
  • agorum core
    Agorum core
    agorum core is a free Open-Source Enterprise Content Management system by agorum Software GmbH from Germany. One of the main features is the Document-Network-Share. With that the documents within the ECM are shown as a normal network share...

    , open source enterprise content management system
    Content management system
    A content management system is a system providing a collection of procedures used to manage work flow in a collaborative environment. These procedures can be manual or computer-based...

     with fully integrated CIFS-Server for accessing documents.
  • ONStor Inc. offers an SMB implementation that also supports NFS protocol so users can access the same data through both protocols.
  • Novell NetWare
    Novell NetWare
    NetWare is a network operating system developed by Novell, Inc. It initially used cooperative multitasking to run various services on a personal computer, with network protocols based on the archetypal Xerox Network Systems stack....

     version 6 and newer has a CIFS server implementation providing access to NetWare volumes for Microsoft Network clients.
  • FreeBSD
    FreeBSD
    FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...

     includes an SMB client implementation called smbfs that uses its VFS
    Virtual file system
    A virtual file system or virtual filesystem switch is an abstraction layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way...

    .
  • NetBSD
    NetBSD
    NetBSD is a freely available open source version of the Berkeley Software Distribution Unix operating system. It was the second open source BSD descendant to be formally released, after 386BSD, and continues to be actively developed. The NetBSD project is primarily focused on high quality design,...

     and Mac OS X
    Mac OS X
    Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

     include SMB client implementations called smbfs, originally derived from the FreeBSD smbfs; they use the NetBSD and OS X VFS.
  • Solaris has a project called CIFS client for Solaris, based on the Mac OS X smbfs.
  • OpenSolaris
    OpenSolaris
    OpenSolaris was an open source computer operating system based on Solaris created by Sun Microsystems. It was also the name of the project initiated by Sun to build a developer and user community around the software...

     added in-kernel CIFS server support in October 2007.
  • Sun Microsystems Cascade, which became known as PC-Netlink, represents a port
    Porting
    In computer science, porting is the process of adapting software so that an executable program can be created for a computing environment that is different from the one for which it was originally designed...

     of Advanced Server for Unix. Sun
    Sun Microsystems
    Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...

     took over two years making the code useful, due to the poor quality of the original port.
  • FreeNAS
    FreeNAS
    FreeNAS is a free network-attached storage server, supporting: CIFS , FTP, NFS, rsync, AFP protocols, iSCSI, S.M.A.R.T., local user authentication, and software RAID , with a web-based configuration interface. FreeNAS takes less than 64 MB once installed on CompactFlash, hard drive or USB flash...

    , a dedicated small-sized NAS
    Network-attached storage
    Network-attached storage is file-level computer data storage connected to a computer network providing data access to heterogeneous clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration of those elements...

     server, runs FreeBSD
    FreeBSD
    FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...

     for Network-attached storage
    Network-attached storage
    Network-attached storage is file-level computer data storage connected to a computer network providing data access to heterogeneous clients. NAS not only operates as a file server, but is specialized for this task either by its hardware, software, or configuration of those elements...

     (NAS) services, and supports protocols including CIFS/Samba
  • Advanced Server for Unix (AS/U) comprises a port of Windows NT 3.51's SMB server code to Unix
    Unix
    Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

    . Microsoft licensed the code to AT&T
    AT&T
    AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...

    , which then licensed it to major Unix vendors. The poor quality of the original port (allegedly carried out by Microsoft itself) has caused any vendor sub-licensing it significant grief.
  • VERITAS Software
    VERITAS Software
    Veritas Software Corp. was an international software company that was founded in 1983 as Tolerant Systems, renamed Veritas Software Corp. in 1989, and merged with Symantec in 2005. It was headquartered in Mountain View, California...

     has an implementation of SMB.
  • SCO has a port of Advanced Server for Unix.
  • SCO also has VisionFS, a Microsoft-independent re-implementation of SMB developed through reference to Samba source code.
  • EMC
    EMC Corporation
    EMC Corporation , a Financial Times Global 500, Fortune 500 and S&P 500 company, develops, delivers and supports information infrastructure and virtual infrastructure hardware, software, and services. EMC is headquartered in Hopkinton, Massachusetts, USA.Former Intel executive Richard Egan and his...

     has an SMB server with its Celerra
    Celerra
    Celerra is a discontinued NAS device produced by EMC Corporation. It was available as an integrated unit or as a NAS header which can be added to an independent EMC storage array such as a CLARiiON or a Symmetrix. It supports CIFS, NFS, FTP, NDMP, TFTP and MPFS protocols...

     platforms
  • NetApp has an SMB server implementation
  • Isilon has a cluster storage or scale-out NAS SMB server implementation from Likewise Software, although the first version of their product was based on Samba
  • Likewise Software offers Likewise-CIFS, an open source SMB/CIFS file server with support for both SMB1 and SMB2.
  • Objective Development's Sharity
    Sharity
    In computing, Sharity is a program to allow a Unix system to mount SMB fileshares. It is developed by Christian Starkjohann of Objective Development Software GmbH and is proprietary software. The current version is 3.3....

     provides an SMB file-system
    File system
    A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device which contain it. A file system organizes data in an efficient manner and is tuned to the...

     client
    Client (computing)
    A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

     for Unix.
  • The Alfresco
    Alfresco (software)
    Alfresco is a Free/Libre enterprise content management system for Microsoft Windows and Unix-like operating systems. Alfresco comes in two flavours. Alfresco Community Edition is free software, LGPL licensed open source and open standards. Alfresco Enterprise Edition is commercially & proprietary...

     content-management system
    Content management system
    A content management system is a system providing a collection of procedures used to manage work flow in a collaborative environment. These procedures can be manual or computer-based...

     includes JLAN, a Java
    Java (programming language)
    Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

     implementation of an SMB server
  • The LogicalDOC
    LogicalDOC
    LogicalDOC is an Free/Libre document management system that is designed to handle and share documents within an organization.LogicalDOC is a content repository, with Lucene indexing, jBPM workflow, and a set of automatic import procedures....

     document management system includes a client for connecting to SMB, implemented in Java
    Java (programming language)
    Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

  • JCIFS offers an implementation of an SMB client in Java
  • RTSMB, a CIFS/SMB implementation written in ANSI C. EBS designed RTSMB from scratch, independently of MS or SAMBA design reference, to run in embedded device
    Embedded system
    An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...

    s.
  • Visuality Systems NQ CIFS, a CIFS (SMB) server and client solution for embedded devices — ported to many popular real-time operating system
    Real-time operating system
    A real-time operating system is an operating system intended to serve real-time application requests.A key characteristic of a RTOS is the level of its consistency concerning the amount of time it takes to accept and complete an application's task; the variability is jitter...

    s (RTOSs)
  • Thursby Software Systems developed DAVE, the first commercial implementation of SMB/CIFS for Classic Mac OS in 1996, co-writing the Mac standards with Microsoft in 2002. It was later ported to Mac OS X. Thursby's DAVE, ADmitMac and ADmitMac PKI products include both an SMB client and an SMB server, and support Microsoft's DFS
    Distributed File System (Microsoft)
    Distributed File System is a set of client and server services that allow an organization using Microsoft Windows servers to organize many distributed SMB file shares into a distributed file system...

    .
  • An iPhone
    IPhone
    The iPhone is a line of Internet and multimedia-enabled smartphones marketed by Apple Inc. The first iPhone was unveiled by Steve Jobs, then CEO of Apple, on January 9, 2007, and released on June 29, 2007...

     application named Flash Files has an SMB server implementation.
  • Some Sony-Ericsson mobile phones have an SMB server built in.
  • The plan9 distribution contains both a CIFS/SMB server aquarela and client cifs
  • Blue Share by Blue Peach, a portable embedded CIFS Client Server stack offering both Real and Protected Mode (Multi-Process) operation, and secure authentication.
  • pysmb A python implementation of SMB/CIFS Client.

See also

  • Active Directory
    Active Directory
    Active Directory is a directory service created by Microsoft for Windows domain networks. It is included in most Windows Server operating systems. Server computers on which Active Directory is running are called domain controllers....

  • Administrative share
  • Shared file access
  • AppleTalk
    AppleTalk
    AppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...

  • File Transfer Protocol
    File Transfer Protocol
    File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server...

     (FTP)
  • Network File System (NFS)
  • Remote File System
    Remote File System
    The Remote File System was a distributed file system developed by AT&T in the 1980s. It was first delivered with UNIX System V Release 3 .Compared to NFS it made quite different design decisions...

     (RFS)
  • WebDAV
    WebDAV
    Web-based Distributed Authoring and Versioning is a set of methods based on the Hypertext Transfer Protocol that facilitates collaboration between users in editing and managing documents and files stored on World Wide Web servers...

  • Uniform Naming Convention (UNC)
  • DCE/RPC
    DCE/RPC
    DCE/RPC, short for "Distributed Computing Environment / Remote Procedure Calls", is the remote procedure call system developed for the Distributed Computing Environment...

     services
  • Network Neighborhood

External links