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I have two IP addresses on my physical eth interface. Lets say (eth0:0) and (eth0). So it works. But I use application, working on the only one of this addresses on port 12000. It is setup in this application internally to work on address I set app to work on address too, using iptables, in such way:

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
iptables -t nat -A PREROUTING -p tcp -d --dport 12000 -j DNAT --to

Now I want to block all incoming traffic to IP address (interface eth0:0) except port 12000. All the rest traffic must be unchanged. I am not iptables expert. Can anybody help me?

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assuming your iptables rule work on the same machine with your application, you have to define a rule to accept incoming and outgoing packet to the address and application port, then you just simply drop everything else, it seems like this :

iptables -A INPUT -p tcp -d --dport 12000 -j ACCEPT
iptables -A OUTPUT -p tcp -s --sport 12000 -j ACCEPT

iptables -A INPUT -j DROP
iptables -A OUTPUT -j DROP

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