Port Forwarding

Source

Port forwarding allows remote computers (for example, computers on the Internet) to connect to a specific computer or service within a private local-area network (LAN).

In a typical residential network, nodes obtain Internet access through a DSL or cable modem connected to a router or network address translator (NAT/NAPT). Hosts on the private network are connected to an Ethernet switch or communicate via a wireless LAN. The NAT device's external interface is configured with a public IP address. The computers behind the router, on the other hand, are invisible to hosts on the Internet as they each communicate only with a private IP address.

When configuring port forwarding, the network administrator sets aside one port number on the gateway for the exclusive use of communicating with a service in the private network, located on a specific host. External hosts must know this port number and the address of the gateway to communicate with the network-internal service. Often, the port numbers of well-known Internet services, such as port number 80 for web services (HTTP), are used in port forwarding, so that common Internet services may be implemented on hosts within private networks.

Typical applications include the following:

• Running a public HTTP server within a private LAN
• Permitting Secure Shell access to a host on the private LAN from the Internet
• Permitting FTP access to a host on a private LAN from the Internet

Dynamic port forwarding

Dynamic Port Forwarding (DPF) is an on-demand method of traversing a firewall/NAT through the use of firewall pinholes. The goal is to enable clients to connect securely to a trusted server that acts as an intermediary for the purpose of sending/receiving data to one or many destination servers.

DPF can be implemented by setting up a local application (such as SSH) as a SOCKS proxy server, which can be used to process data transmissions through the network or over the Internet. Programs (such as web browsers) must be configured individually to direct traffic through the proxy, which acts as a secure tunnel to another server. Once the proxy is no longer needed, the programs must be reconfigured to their original settings. Because of the manual requirements of DPF, it is not often used.

DPF is a powerful tool with many uses.

• A user connected to the Internet through a coffee shop, hotel, or otherwise minimally secure network may wish to use DPF as a way of protecting data.
• DPF can be used to bypass firewalls that restrict access to outside websites, such as in a corporate network.
• DPF can be used as a precaution against hacking.

Ref. Wikipedia