Let's assume we have a router which has internal ip(192.168.1.1) and external ip(192.0.2.5) and a computer behind the router which has internal ip(192.168.1.100). And the router has already configured to MASQUARADE source ip so user can access the internet.
And i am wandering when the internet server response user's request with a packet(SRC:18.104.22.168->DST:192.0.2.5) and this packet goes into iptables but no rule have been configured to DNAT it back to user's ip, how does the user even receive the response?
And i've google it someone said it is DNATed automatically by Netfilter, i am wandering if it true or not, and if it is true, when does this Hidden-DNAT performed, is it performed when the packet arrived FILTER table or NAT table or something else? And after it is DNATed will it goes into FILTER's INPUT table or FORWARD table?