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This is a list of TCP and UDP port numbers used by protocols for operation of network applications. The Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) only need one port for bidirectional traffic. TCP usually uses port numbers that match the services of the corresponding UDP implementations, if they exist, and vice versa.
For example, the NetBIOS Name Service (NBNS), running over UDP or TCP port 137, allows any computer to register its hostname with other computers. An attacker could contact any host and claim that they are a particular service the host regularly contacts, such as a file server.
UDP is a connectionless protocol meaning that messages are sent without negotiating a connection and that UDP does not keep track of what it has sent. [ 1 ] [ 2 ] UDP provides checksums for data integrity , and port numbers for addressing different functions at the source and destination of the datagram.
[citation needed] In NBT, the name service operates on UDP port 137 (TCP port 137 can also be used, but rarely is). The name service primitives offered by NetBIOS are: Add name – registers a NetBIOS name. Add group name – registers a NetBIOS "group" name. Delete name – un-registers a NetBIOS name or group name.
The program decodes and provides the user with all NetBIOS name packets it receives on UDP port 137. Its many command line options can effectively disable a NetBIOS network and prevent computers from rejoining it. According to Sir Dystic, "NBName can disable entire LANs and prevent machines from rejoining them...nodes on a NetBIOS network ...
One concern about the move from TCP to UDP is that TCP is widely adopted and many of the "middleboxes" in the Internet infrastructure are tuned for TCP and rate-limit or even block UDP. Google carried out a number of exploratory experiments to characterize this and found that only a small number of connections were blocked in this manner. [3]
When you get a message from a "MAILER-DAEMON" or a "Mail Delivery Subsystem" with a subject similar to "Failed Delivery," this means that an email you sent was undeliverable and has been bounced back to you. These messages are sent automatically and often include the reason for the delivery failure.
discard stream tcp nowait root internal discard dgram udp wait root internal The Discard Protocol is the TCP/UDP equivalent of the Unix file-system node /dev/null. Such a service is guaranteed to receive what is sent to it and can be used for debugging code requiring a guaranteed reception TCP or UDP payloads.