F-SECURE LINUX SECURITY, Руководство пользователя

Страница: 53 / 219

Firewall Rules
Firewall rules define what kind of Internet traffic is allowed or blocked.
Each security level has a predefined set of firewall rules , which you cannot change. The selected
security level affects the priority which your own rules receive in relation to the predefined rules.
A firewall rule can be applied to traffic from the Internet to your computer (inbound), or from
your computer to the Internet (outbound). A rule can also be applied to both directions at the
same time.
A firewall rule consists of firewall services , which specify the type of traffic and the ports that
this type of traffic uses. For example, a rule called Web browsing has a service called HTTP ,
which uses the TCP and port number 80.
Firewall rules also define whether firewall alert pop-ups are shown to you about the traffic that
matches the firewall rules .
When do you have to add a new firewall rule?
You may have to add a new firewall rule if you want to allow traffic that is
blocked or if you want to block specific Internet traffic.
By adding all the services that the program or device needs to the same rule,
you can easily:
• turn the rule on or off later, or
• remove the rule if you uninstall the program or remove the device.
You also have to add a new rule if you have denied certain type of traffic but
you want to allow it to certain IP addresses. In this case, you already have a
general "deny"
firewall rule . To allow the traffic to certain IP addresses, you
have to create a more specific "allow" rule.
Firewall Services
Firewall services define the type of traffic to which a firewall rule applies.
Network services , such as web browsing, file sharing or remote console access , are examples
of these firewall services.
A service uses a certain protocol and port . For example, the HTTP service uses the TCP
protocol and the port number 80.
A firewall service uses two kinds of ports:
•
Initiator port : the port on the computer that starts the connection.
F-Secure Linux Security | Using the Product | 53