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In computer networking, port knocking is a method of externally opening ports on a firewall by generating a connection attempt on a set of prespecified closed ports. Once a correct sequence of connection attempts is received, the firewall rules are dynamically modified to allow the host which sent the connection attempts to connect over specific port(s).
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.
An ephemeral port is a communications endpoint of a transport layer protocol of the Internet protocol suite that is used for only a short period of time for the duration of a communication session. Such short-lived ports are allocated automatically within a predefined range of port numbers by the IP stack software of a computer operating system.
The firewalls also note the endpoints in order to allow responses from the server to pass back through. The server then sends each client's endpoint and session information to the other client, or peer. Each client tries to connect to its peer through the specified IP address and port that the peer's firewall has opened for the server.
A port knock setup needs a firewall that logs every port access, a daemon which has means to change the firewall rules (!), and, depending on how complex the knock sequences should be, cryptographic hash generators, handling of multiple knock attempts coming in at the same time, etc.
All TCP NAT traversal and hole punching techniques have to solve the port prediction problem. A NAT port allocation can be one of the two: predictable the gateway uses a simple algorithm to map the local port to the NAT port. Most of the time a NAT will use port preservation, which means that the local port is mapped to the same port on the NAT.
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The Linux kernel packet scheduler is an integral part of the Linux kernel's network stack and manages the transmit and receive ring buffers of all NICs. The packet scheduler is configured using the utility called tc (short for traffic control ).