User's Manual

Section 3.13:Firewall Configuration 45
Medium
If you choose Medium, your firewall will not allow remote machines to have access to certain
resources on your system. By default, access to the following resources are not allowed:
• Ports lower than 1023 — the standard reserved ports, used by most system services, such as
FTP, SSH, telnet, and HTTP
• The NFS server port (2049)
• The local X Window System display for remote X clients
• The X Font server port (by default, xfs does not listen on the network, it is disabled in the
font server)
If you want to allow resources such as RealAudio™, while still blocking access to normal sys-
tem services, choose
Medium. Select Customize to allow specific services through the firewall.
No Firewall
No firewall provides complete access to your system and does no security checking. Security
checking is the disabling of access to certain services. This should only be selected if you are
running on a trusted network (not the Internet) or plan to do more firewall configuration later.
Choose
Customize to add trusted devices or to allow additional incoming services.
Trusted Devices
Selecting any of the Trusted Devices allows access to your system for all traffic from that
device; it is excluded from the firewall rules. For example, if you are running a local network,
but are connected to the Internet via a PPP dialup, you can check
eth0 and any traffic coming
from your local network will be allowed. Selecting
eth0 as trusted means all traffic over the
Ethernet is allowed, put the ppp0 interface is still firewalled. If you want to restrict traffic on an
interface, leave it unchecked.
It is not recommended that you make any device that is connected to public networks, such as
the Internet, a
Trusted Device.
Allow Incoming
Enabling these options allow the specified services to pass through the firewall. Note, during a
workstation installation, the majority of these services are not installed on the system.
DHCP
If you allow incoming DHCP queries and replies, you allow any network interface that
uses DHCP to determine its IP address. DHCP is normally enabled. If DHCP is not
enabled, your computer can no longer get an IP address.