iptables port forwarding

Question

I have a linksys router that is letting traffic through just fine, but when I try to let that same traffic through to my linux box (fedora 10) it stalls. I've followed every iptable rule I can find to no avail. If I chkconfig iptables off -> reboot. Voila. But obviously I don't want to leave things like that.

Any ideas? Any "tools" to see exactly what ports are or are not open?

Answer 1

• nmap: scan for open ports.
• netcat: like telnet, but on steroids. the swiss army knife of connection debugging. great for manually connecting to ports and shoving data in.
• Wireshark: packet sniffer. can grab everything off the network if your card supports promiscuous mode (I think every ethernet card does.) great for double-checking that your traffic in and out looks how you'd expect.

I would also recommend rechecking your IPtables rules. OUTPUT can be a bit challenging.

Answer 2

nmap can check open ports. Can we have some more information? What iptables rules are you using? A little more information. What ports are you trying to block?

Answer 3

If you're looking for an easier way of writing iptables rules, you can check out Firehol which makes this very easy.

Answer 4

I won't give you a syntax for making iptables rules, since there are lots of tutorials out in the web describing this much better than I can. If you want to debug your existing rules, then

• post the output of your iptables -vL and, so that we know what rules you are using currently, and
• have a look at the output of iptables -vL yourself: The first column tells you how many packets matched the given rule. If it shows zero, it means that the packets have probably been matched by an earlier rule.