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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).
When an outbound connection from a private endpoint passes through a firewall, it receives a public endpoint (public IP address and port number), and the firewall translates traffic between them. Until the connection is closed, the client and server communicate through the public endpoint, and the firewall directs traffic appropriately.
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.
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.
UDP hole punching is a method for establishing bidirectional UDP connections between Internet hosts in private networks using network address translators. The technique is not applicable in all scenarios or with all types of NATs, as NAT operating characteristics are not standardized.
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Open Network Computing (ONC) Remote Procedure Call (RPC), commonly known as Sun RPC is a remote procedure call system. ONC was originally developed by Sun Microsystems in the 1980s as part of their Network File System project. ONC is based on calling conventions used in Unix and the C programming language.
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.