NetBIOS-DGM

In short: The datagram service of NetBIOS (Network Basic Input/Output System) — connectionless message transmission in the local network, used among other things for browser/name announcements.

In more detail: NetBIOS provides Windows networks with basic services for name resolution, session management and datagram distribution. The datagram service (port 138) runs connectionlessly over UDP and is used, for example, for broadcast messages within a Windows workgroup network — today largely replaced by DNS and Active Directory.

In Depth

NetBIOS (Network Basic Input/Output System) dates from the early 1980s and was originally developed for small, local PC networks, long before TCP/IP and DNS were widespread in the PC world. The datagram service (NetBIOS-DGM, UDP port 138) is the connectionless part of NetBIOS: among other things it’s used for the “browser service”, through which Windows computers in a workgroup announce themselves to each other via broadcast and build a list of the computers visible on the network (formerly visible in “Network Neighborhood”).

Since NetBIOS-DGM works connectionlessly, there’s no delivery guarantee — for more reliable, session-based communication there’s the separate NetBIOS session service over TCP port 139. In modern networks, NetBIOS should not be reachable from the public internet, since it has historically been the target of worm attacks several times (e.g. Conficker used NetBIOS-related services, among others, to spread).

See also: NetBIOS-NS