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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?

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  • 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.

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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?

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well, I want to block ALL ports, then allow only the few I need apps to get data through. For instance, the basics, 80, 21, 25, 3306.. I want those open, and say something new like 2106? How do I (syntax please) open those ports for incoming traffic to an application running on the box? – Gene R Oct 2 '09 at 14:17

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

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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.
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