There are LinuxDistributions that exist only to provide firewalling; PerryLorier is working on a FireWall-on-a-disc system. You can technically speaking shut a Linux machine down into Kernel-only mode and still be running a FireWall.
To create a rule that will send back an ICMP message, use
iptables -A chain [...] --jump REJECT --reject-with icmp-port-unreachable
The type corresponds to an ICMP error and can be one of:
iptables -D chain [rule number] iptables -D chain [rule description]
Hint: if you want to delete a rule and you don't want to have to mess around with specifying ports etc, try iptables -L --line-numbers. Then you can just use iptables -D FORWARD 1 to remove it.
iptables [-t <table>] -F [chain]
This removes all rules from the specified table and chain, or all the chains in the table if none is specified.
Hint: It won't delete any user-defined chains, although it will remove the rules within them, nor will it set the default policy for the table. This, though, should:
iptables -t filter -F iptables -t filter -X iptables -t nat -F iptables -t nat -X iptables -t mangle -F iptables -t mangle -X iptables -t filter -P INPUT ACCEPT iptables -t filter -P FORWARD ACCEPT iptables -t filter -P OUTPUT ACCEPT iptables -t nat -P PREROUTING ACCEPT iptables -t nat -P OUTPUT ACCEPT iptables -t nat -P POSTROUTING ACCEPT iptables -t mangle -P PREROUTING ACCEPT iptables -t mangle -P INPUT ACCEPT iptables -t mangle -P FORWARD ACCEPT iptables -t mangle -P OUTPUT ACCEPT iptables -t mangle -P POSTROUTING ACCEPT
iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
If you have a FireWall running iptables, chances are you'll want to forward a port at some point (to run a P2P app, a game server etc).
Experiment with this command line, substituting the emphasized bits according to your needs:
iptables -t nat -A PREROUTING -i ppp0 -j DNAT -p tcp --to=10.69.1.200 --dport 4661
Make sure you have ECN (Explicit Congestion Notification) disabled and don't have any TypeOfService settings in your FireWall script. If you know what you're doing, try iptables -t mangle -F PREROUTING which should clean up any of them.
Alternatively, you can go with the Don't fix good science to work with a bad implementation, or manually add rules allowing access to the NZ Herald IPs.
Also, it should be noted that some home routers don't seem to like ECNs either. If you're having problems accessing the InterNet with a home ADSL router, and tcpdump(8) output is mentioning packets with SWE?, try turning ECNs off as seen in the ECN page.
If you want to be able to run a process that responds to requests on a Port below 1024 without running it as the SuperUser, a simple approach is to have it bind to some port above 1024, then configure a lower layer in the NetworkStack to do the legwork. On Linux, a convenient way to achieve this is by using iptables(8):
iptables --table nat -A PREROUTING -p tcp --dport $external_port -i eth0 -j REDIRECT --to-ports $local_port
This way, you could have a process bind to port 8080 locally, but have it appear to outsiders as though it was listening on port 80.
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