Below are the most basic terms related to firewalling. You might be already familiar with them. If so, you can skip this section and start configuring your firewall.
Rules
The Plesk Firewall module enforces user-defined rules (or custom rules) to process data packets. Each packet is first assessed and then handled depending on how this packet matches the security criteria set in the rules. The Plesk Firewall will consequently apply the rules that go first in the list of rules.
Hosts
A host is any computer that is connected to (or a part of) a network. The Firewall can be configured to prohibit access by specific hosts.
Ports
Ports are virtual connection points used by networking services (do not confuse virtual ports with physical ports like USB ports). Each port has an identification number, and common services are associated with specific ports by convention. See Appendix for the list of services and associated ports.
Packets
Communication protocols, such as TCP, divide the data flowing between hosts into chunks that are called packets. Each packet includes the data being transmitted and bears information on the type of the packet, destination address and the packet source. Using this information, the Firewall analyzes an individual packet and accepts or rejects it based on comparison with the specified rules.
Services
In simple terms, services are based on protocols that let one computer access a type of data stored on another computer. Many host computers that are connected to the Internet offer services. For example, HTTP servers use the HyperText Transfer Protocol to provide World Wide Web service, FTP servers offer File Transfer Protocol services, SMTP servers use the Simple Mail Transport Protocol to exchange e-mail, and POP servers use the Post Office Protocol to exchange e-mail.
The Plesk Firewall offers you a predefined set of services that can be controlled using the firewall rules. For the list of services and ports commonly associated with them, see Appendix.
System Policies
System policies are rules defining how the firewall will handle all incoming, outgoing and forwarded communications that do not match the explicitly defined firewall rules. In the Plesk Firewall, the system policies are displayed at the bottom of the rules list.