SMB (protocol)

SMB (protocol)
Name	SMB
Other names	Server Message Block
Developer	Microsoft, IBM
Initial release	1980s
Latest release	SMB 3.1.1 (2015)
Os	Microsoft Windows, Linux, macOS, FreeBSD
License	Proprietary, open source implementations

SMB (protocol) Server Message Block (SMB) is a network file and printer sharing protocol used for providing shared access to files, printers, and serial ports between nodes on a network. It originated in the 1980s and evolved through multiple versions to support features such as file locking, authentication, encryption, and remote procedure calls. SMB is widely implemented across operating systems and integrated into enterprise environments, cloud platforms, and embedded systems.

Overview

SMB provides file and resource sharing semantics enabling clients to access files hosted by servers over a network. It defines a client–server model with operations for file read/write, directory enumeration, file locking, and named pipe communication. SMB interacts with authentication and authorization services like Kerberos (protocol), NTLM, and integrates with directory services such as Active Directory for identity and access management. The protocol also supports features used by virtualization and storage technologies, including Hyper-V, VMware vSphere, and Microsoft Exchange for data access and backup.

History and development

SMB traces roots to work by IBM on network file systems in the 1980s and early Microsoft networking efforts. Early SMB variants were deployed with MS-DOS-based products and later incorporated into Microsoft Windows for Workgroups and Windows NT. Major milestones include wide adoption with Windows 95, the CIFS rebranding and specification discussions involving Intel and industry partners, and significant revisions in the 2000s led by Microsoft to improve performance and security. The introduction of SMB 2.0 coincided with Windows Vista and Windows Server 2008, while SMB 3.0 was released alongside Windows Server 2012 to support features for cloud and scale-out storage. Subsequent updates enhanced encryption and dialect negotiation, reflecting influence from enterprise storage vendors, virtualization providers, and standards bodies.

Protocol architecture and operation

SMB operates as an application-layer network protocol typically transported over TCP/IP using port 445, and historically over NetBIOS over TCP/IP using ports 137–139. The protocol defines message types, session setup, tree connect, create/open, read/write, and close operations, along with oplock and leasing mechanisms to coordinate caching between clients and servers. SMB messages encapsulate requests and responses and may be encrypted or signed; dialect negotiation determines capabilities between endpoints. The protocol integrates with SMB2/SMB3 semantics for compound requests and durable handles to support failover in clustered storage and Scale-Out File Server scenarios. Interactions with remote management and file service APIs relate SMB to Windows Management Instrumentation, REST APIs in cloud platforms such as Microsoft Azure, and storage protocols like NFS and iSCSI.

Implementations and extensions

Implementations include Microsoft Windows native server and client stacks, the open source Samba suite widely used on Linux and Unix systems, and embedded or networked storage appliances from vendors such as NetApp, Dell EMC, and Hewlett Packard Enterprise. Cloud providers like Amazon Web Services, Microsoft Azure, and Google Cloud Platform offer managed file services that expose SMB endpoints backed by distributed storage. Extensions and dialects introduce features for multichannel networking, RDMA acceleration via RDMA, SMB Direct, transparent failover for clustered file systems, and persistent handles for virtualization and backup solutions such as Veeam and Commvault. Interoperability work has involved projects and organizations including Open Source Initiative contributors, industry consortiums, and vendors like IBM, Intel, and Broadcom.

Security and vulnerabilities

Security in SMB encompasses authentication protocols, message signing, and encryption. Historically exploited vectors include vulnerabilities leveraged by notable malware such as WannaCry and NotPetya, which abused flaws in SMB implementations to propagate across networks. Hardening measures involve disabling unused dialects, enforcing SMB signing, requiring SMB encryption in modern dialects, and integrating with identity systems like Active Directory Federation Services for secure access. Vulnerabilities discovered in SMB stacks have led to coordinated disclosure and patching efforts involving vendors, CERT Coordination Center, and national cybersecurity agencies such as CISA. Mitigations often require updates across client, server, and intermediary devices including networked storage and routers from vendors like Cisco, Juniper Networks, and Arista Networks.

Performance and use cases

SMB is used for desktop file sharing, server-based file services, virtual machine disk sharing, backup targets, and application data repositories. Performance enhancements in SMB 2.x and 3.x—such as pipelining, large MTU support, multichannel, and SMB Direct—improve throughput and latency for workloads in datacenters and cloud environments. Use cases span small business file sharing, enterprise file servers integrated with Active Directory, branch-office synchronization, and large-scale storage for services like Microsoft SharePoint and SQL Server. Comparative deployments often consider alternatives such as NFS for Unix-centric environments and object storage APIs used by Amazon S3 and OpenStack Swift in cloud-native architectures.

Category:Network protocols