Iptables provides packet filtering, network address translation (NAT) and other packet mangling.
Two of the most common uses of iptables is to provide firewall support and NAT.
List all the rules in effect (by default, none):
# iptables --list
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
Configuring iptables manually is challenging for the uninitiated. Fortunately, there are many configuration tools (wizards) available to assist: e.g., fwbuilder, bastille, ferm.
Manual configuration:
More to come... this is just a start since I noticed there wasn't much info on iptables.
Resources:
- Gentle Introductions/Overviews
Iptables Basics: http://www.justlinux.com/nhf/Security/IPtables_Basics.html
Securing Debian: Adding firewall capabilities: http://www.debian.org/doc/manuals/securing-debian-howto/ch-sec-services.en.html#s-firewall-setup
- More Detailed Reference
iptables-tutorial: http://iptables-tutorial.frozentux.net/
Packet Filter HOWTO: http://www.netfilter.org/documentation/HOWTO//packet-filtering-HOWTO.html
Firewall and Advanced Routing Under Linux: http://ornellas.apanela.com/dokuwiki/pub:firewall_and_adv_routing
On this wiki there's the Firewalls page.