At its most basic, SMB is a protocol to allow devices to perform a number of functions on each other over a (usually local) network. SMB has been around for so long and maintains so much backwards compatibility that it contains an almost absurd amount of vestigial functionality, but its modern core use is simpler than it seems. For the most part, today SMB is used to map network drives, send data to printers, read and write remote files, perform remote administration, and access services on remote machines.
SMB runs directly over TCP (port 445) or over NetBIOS (usually port 139, rarely port 137 or 138). To begin an SMB session, the two participants agree on a dialect, authentication is performed, and the initiator connects to a ‘tree.’ For most intents and purposes, the tree can be thought of as a network share. The PCAP below, shown in Wireshark, demonstrates a simple session setup and tree connect. In this case, the machine 192.168.10.31 is connecting to the “c$” share (equivalent to the C:\ drive) on the 192.168.10.30 machine, which is called “admin-pc