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Monday, July 23, 2012

Broadcasting Packets to UDP Port 137 and Port 138

Port 137 and Port 138 are basic windows ports.

Here are the function of these ports:
  • Port 137 is NetBIOS name service
This is the way how NetBIOS-based services find each other. These names uniquely identify the machine and the services running on the machine (and the IP address doesn't matter) on a NetBIOS network. In a centralized NetBIOS naming server (WINS server) the machines find each other either using broadcasts or looking them up.
  • Port 138 is NetBIOS datagram service
Primarily used for broadcasting information. It is primarily used by the SMB browser service that fills the information within the "Network Neighborhood" icon.
UDP ports 137 and 138 are considered normal traffic that every computer is announcing their existence in a network environment because of WINS/NetBT name resolution.

When a machine boots up it broadcasts its NetBIOS name on the local subnet to register with the browse master, a computer with resources broadcasts an announcement every 12 minutes to refresh the browse lists.

Browser traffic is normally done by broadcast, over UDP port 137. (If there is a WINS server on the network, browser traffic will be directed and not broadcast.)


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