Filtering is done on those chains based on the rules you set up.
When a packet comes to the firewall, it steps through the rules in the chain until it finds a rule that
matches. A match might depend on where a packet came from or where it is going, for example.
When a match is made, the chain jumps to the action (also called a target) for that rule, which
might define that the packet should be accepted or dropped, or have some other action done on it.
Setting up a firewall can be serious business. A misconfigured firewall can reject legitimate
requests, forward packets to the wrong places, or even make your computer completely
inaccessible from the network. Be very cautious if you are trying the following procedure on a
computer that you rely on to be safe and accessible from a network.
1. From a Terminal window, become root user:
$ su -
Password: ********
#
2. Type the following to see what filtering firewall rules are set on your system:
# iptables ??“L
Chain INPUT (policy ACCEPT)
Target prot opt source destination
Chain FORWARD (policy ACCEPT)
Target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
The example output shows that no filtering rules are currently set for this Linux system,
meaning that all packets are accepted (policy ACCEPT) by default.
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