What is SMB?
Also known as: Server Message Block, CIFS
SMB (Server Message Block) is a network file sharing protocol developed by Microsoft, used by Windows and Samba on Unix to provide shared access to files, printers, and serial ports over a network.
SMB (Server Message Block) is a network protocol for providing shared access to files, printers, and other communications between nodes on a network. Originally developed by IBM in the 1980s, it was later heavily extended by Microsoft and became the primary file sharing mechanism for Windows operating systems. The protocol operates in a client-server model, where a client sends SMB requests to a server to open, read, write, and close files, as well as to enumerate directories and access printers.
SMB runs over TCP port 445 (direct hosting) or, in older implementations, over NetBIOS on TCP ports 137-139. The protocol has evolved through several major versions: SMB 1.0 (also known as CIFS, Common Internet File System), SMB 2.0 (introduced with Windows Vista), SMB 2.1 (Windows 7), SMB 3.0 (Windows 8 and Windows Server 2012), and SMB 3.1.1 (Windows 10). Each version improved performance, security, and features. SMB 3.0 added encryption, multichannel support, and SMB Direct (RDMA). SMB 1.0 is now deprecated due to security vulnerabilities and is disabled by default on modern Windows systems.
SMB is the backbone of Windows file sharing and is also implemented in open-source software like Samba, which allows Unix and Linux systems to act as SMB clients and servers. The protocol supports authentication (NTLM and Kerberos), authorization via share-level and user-level permissions, and opportunistic locking (oplocks) for caching. SMB is a core component of enterprise storage environments, often used with Windows File Servers, NAS appliances, and in hybrid cloud scenarios via Azure Files.
Key facts
- SMB 1.0 (CIFS) is deprecated; modern systems use SMB 2.0 or higher.
- Default TCP port is 445; older versions used NetBIOS over TCP ports 137-139.
- SMB 3.0 added encryption, multichannel, and SMB Direct (RDMA) support.
- Samba is the most common open-source implementation of SMB for Unix/Linux.
- SMB supports Kerberos and NTLM authentication, plus opportunistic locking.
- examples:
References
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