12.External Security

Typically, an operating system offers various services to other network computers and users. These services are usually provided
through ports or numbered access points beyond the operating systems network address. These services include offerings such as file sharing, print services, e-mail, web sites, and file transfer protocols (FTP).

At the front line of security are hardware devices known as firewalls. At the operating system level, there are a number of software firewalls available. Most modern operating systems include a software firewall which is enabled by default.

A software firewall can be configured to allow or deny network traffic to or from a service or application running on the operating system. Therefore, one can install and be running an insecure service such as Telnet or FTP and not have to be threatened by a security breach because the firewall would deny all traffic trying to connect to the service on that port.

Graphical User Interfaces
Today, most modern operating systems contain Graphical User Interfaces (GUIs). A few of the older ones tightly integrated the GUI to the kernel – one of the central components of the operating system. More modern operating systems are modular separating the graphics subsystem from the kernel.

A GUI is basically the pictures you see on the screen that help you navigate your computer. They include the icons and the menus. Many operating systems allow the user to install or create any user interface they desire.

Graphical user interfaces tend to change and evolve over time. For example, Windows has modified its user interface almost every time a new version of Windows is released. The Mac OS GUI changed dramatically with the introduction of Mac OS X in 2001.