What are SMB's?

Server Message Blocks are a type of "messaging protocol" that LAN Manager (and NT)
clients and servers use to communicate with each other. SMB's are a higher level protocol
that can be transported over NetBEUI, NetBIOS over IPX, and NetBIOS over TCP/IP (or
NBT).
SMBs are used by Windows 3.X, Win95, WintNT and OS/2. When it comes to security
and the compromise of security on an NT network, the one thing to remember about
SMBs is that it allows for remote access to shared directories, the registry, and other
system services, making it a deadly protocol in the eyes of security conscience people.
The SMB protocol was originally developed by IBM, and then jointly developed by
Microsoft and IBM. Network requests that are sent using SMB's are encoded as Network
Control Blocks (NCB) data structures. The NCB data structures are encoded in SMB
format for transmission across the network. SMB is used in many Microsoft and IBM
networking software:
 
* MS-Net
* IBM PC Network
* IBM LAN Server
* MS LAN Manager
* LAN Manager for Unix
* DEC Pathworks
* MS Windows for Workgroups
* Ungermann-Bass Net/1
* NT Networks through support for LAN Manager
SMB Messages can be categorized into four types:
Session Control: Used to establish or discontinue Redirector connections with a remote
network resource such as a directory or printer. (The redirector is explained below)
File: Used to access and manipulate file system resources on the remote computer.
Printer: Used by the Redirector to send print data to a remote printer or queue, and to
obtain the status of remote print devices.
Message: Used by applications and system components to send unicast or broadcast
messages.