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Will this rules make kernel to see the same packet at least twice?

iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE                                                                   
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE                                                                    

Found the stuff in a forgotten config file. A remark about option "-d 0/0" was also there, did this option makes sense when certain log targets are used in iptables - to avoid incorrect traffic calculation?

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No, packets will not be seen twice. Only one of those two iptables rules will ever match a given packet, since a packet cannot exit the system by two different network interfaces at the same time. The routing algorithm always selects (at most) one output interface.

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-o specifies that a particular packet is to be tested for its interface, it does not specify that a packet is to leave by that interface. (The latter would be routing and is controlled by ip route.)

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