[CentOS] Help with iptables rule for blocking UDP port 53

[nate]

Sean Carolan wrote:

> I do have a rule for blocking TCP, forgot to mention that.  You can
> see from my tcpdump output above that the inbound packet is UDP
> though.  I wonder why iptables doesn't block it even with this rule?

Try to insert the rule (-I) instead of append (-A). I recall encountering
weirdness between using the two different methods for adding a rule.
I don't know why, but it seems to make a difference in some cases.
The man page doesn't make it clear to me what the difference is and why
it (might) cause a change of behavior.

I'm not an iptables expert, for my real firewalls I use OpenBSD.

nate