Some explanation of rules used in the firewall script files
The privileged ports, 0 through 1023, are usually referenced in total.
UNPRIVPORTS
The unprivileged ports, 1024 through 65535, are usually referenced in total. They are addresses
dynamically assigned to the client side of a connection.
Default Policy
A firewall has a default policy and a collection of actions to take in response to specific message types. This means that if a given packet has not been selected by any other rule, then the default policy rule will be applied.
NOTE: People with dynamically assigned IPs from an ISP may include the following two lines in their declarations for the firewall. The lines will determine the pppO IP address, and the network of the remote ppp server.
IPADDR=7sbin/ifconfig | grep -A 4 pppO | awk Vinet/ { print $2 }’ | sed -e s/addr://’ MY_ISP=7sbin/ifconfig | grep -A 4 pppO | awk VP-t-P/ { print $3}’ | sed -e s/P-t-P://1 cut -d ‘.’ -f 1-3\0/24
Enabling Local Traffic
Since the default policies for all example firewall rule script files in this book are to deny everything, some of these rules must be unset. Local network services do not go through the external network interface. They go through a special, private interface called the loopback interface. None of your local network programs will work until loopback traffic is allowed.
# Unlimited traffic on the loopback interface.
ipchains -A input -i $LOOPBACK_INTERFACE -j ACCEPT ipchains -A output -i $LOOPBACK_INTERFACE -j ACCEPT
Source Address Filtering