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Something I just stumbled upon. I was configuring Shorewall 4, and there's a flag called DISABLE_IPV6 in shorewall.conf. Oh yeah, I thought, that makes sense - I don't yes IPV6, so I'll set that to Yes. Did so, and when I ran Shorewall it showed my lots of error messages like this:
FATAL: Module ip6_tables not found. ip6tables v1.3.8: can't initialize ip6tables table `filter': iptables who? (do you need to insmod?) Perhaps ip6tables or your kernel needs to be upgraded.
Well, I don't have ip6_tables compiled, which is just the reason I liked that flag when I saw it. Read the source, and I found that the flag doesn't mean "don't use IPV6", but rather it means "use ip6tables to take some special action to disable IPV6". Well.
I searched for the piece of documentation I was missing (because I wasn't assuming there'd be anything to misinterpret about the meaning of that flag), but I still can't find anything... apart from this forum post where somebody states "... this is because in a perversely twisted form of logic the configuration file requires you to have IPV6 support to be able to disable it ..." Well put.