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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.
Telnet consists of two components: (1) the protocol itself and (2) the service component. The telnet protocol is a client-server protocol, based on a reliable connection-oriented transport. [2] This protocol is used to establish a connection to Transmission Control Protocol (TCP) port number 23 or 2323, where a Telnet server application is ...
The protocol partly relies on the remote party's rlogin client to provide information honestly, including source port and source host name. A malicious client can forge this and gain access, as the rlogin protocol has no means of authenticating the client is running on a trusted machine.
netcat (often abbreviated to nc) is a computer networking utility for reading from and writing to network connections using TCP or UDP.The command is designed to be a dependable back-end that can be used directly or easily driven by other programs and scripts.
For TCP, port number 0 is reserved and cannot be used, while for UDP, the source port is optional and a value of zero means no port. A process associates its input or output channels via an internet socket , which is a type of file descriptor , associated with a transport protocol , a network address such as an IP address , and a port number.
Port numbers are categorized into three basic categories: well-known, registered, and dynamic or private. The well-known ports are assigned by the Internet Assigned Numbers Authority (IANA) and are typically used by system-level processes. Well-known applications running as servers and passively listening for connections typically use these ports.
Some examples of service ports used for banner grabbing are those used by Hyper Text Transfer Protocol , File Transfer Protocol , and Simple Mail Transfer Protocol ; ports 80, 21, and 587 respectively. Tools commonly used to perform banner grabbing are Telnet, Nmap and Netcat.
For an example, telnet can be configured as follows (line taken from a machine running AIX version 5.1): telnet stream tcp6 nowait root /usr/sbin/telnetd telnetd -a The first word, telnet, is the official name of the service. It is resolved using the system database to map port numbers and protocols to service names.