conntrack, NAT and icmp echo reply

Hi all

I have NAT box, with very simple rule
iptables -t nat -I POSTROUTING -s 10.0.0.0/8 -j MASQUERADE
It can be SNAT also, and it works fine, as NAT.

When i generate icmp _reply_ packet, to some host
hping -I ppp0 -1 --icmptype 0 8.8.8.8

It will pass the box, and will exit it without NAT, e.g. with original IP 10.x.x.x
on outgoing interface, which is not expected behavior IMHO.
Is it a bug or feature?

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Denys Fedoryshchenko, Network Engineer, Virtual ISP S.A.L.