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9. Troubleshooting

There is one problem that occurs often enough to mention here. If you get the following error:

/lib/modules/2.4.2-2/kernel/net/ipv4/netfilter/ip_tables.o init_modules: Device or resource busy Hints: insmod errors can be caused by incorrect module parameters; including invalid IO or IRQ parameters.

perhaps iptables or your kernel needs to be upgraded...

then you are probably running Red Hat 7.x. The folks at Red Hat, in all their wisdom, decided to load the ipchains module by default on startup. I guess this was for backwards compatibility for those who haven't learned iptables yet. However, the problem is that ipchains and iptables are mutually incompatible. Since ipchains has been secretly loaded by RH, you cannot use iptables commands. To see if this is your problem, do the command ``lsmod'' and look for the module named ``ipchains''. If you see it, that is your problem. The quick fix is to execute the command ``rmmod ipchains'' before you issue any iptables commands. To permanently remove these commands from your startup scripts, the following command should work: ``/sbin/chkconfig --level 2345 ipchains off''. (Thanks to Rasmus Glud for pointing this command out to me).
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