Red Hat LINUX ADVANCED SERVER 2.1 Manual page 41

Chapter 3. Installing Red Hat Linux Advanced Server
Note
If you select a medium or high firewall to be setup during this installation, network authentication
methods (NIS and LDAP) will not work.
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
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FTP, SSH, telnet, HTTP, and NIS.
The NFS server port (2049) — NFS is disabled for both remote severs and local clients.
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The local X Window System display for remote X clients.
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The X Font server port (by default, xfs does not listen on the network; it is disabled in the font
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server).
If you want to allow resources such as RealAudio™ while still blocking access to normal system
services, choose Medium. Select Customize to allow specific services through the firewall.
Note
If you select a medium or high firewall to be setup during this installation, network authentication
methods (NIS and LDAP) will not work.
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.
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.
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