We want to allow connections from the internet to the office server whose local IP is 10.0.0.3. Let`s take a look at the common setup where a network administrator wants to access an office server from the internet. Network address translation works by modifying network address information in the packets IP header. Packets passing through the router are not processed against the rules of the output chain. output - used to process packets that originated from the router and leave it through one of the interfaces.Packets passing through the router are not processed against the rules of the input chain. input - used to process packets entering the router through one of the interfaces with the destination IP address which is one of the router's addresses.
Since RouterOS v7 the firewall NAT has two new INPUT and OUTPUT chains which are traversed for packets delivered to and sent from applications running on the local machine: A NAT router performing dstnat replaces the destination IP address of an IP packet as it travels through the router towards a private network. It is most commonly used to make hosts on a private network to be accessible from the Internet. This type of NAT is performed on packets that are destined for the natted network. A reverse operation is applied to the reply packets traveling in the other direction. A NAT router replaces the private source address of an IP packet with a new public IP address as it travels through the router. This type of NAT is performed on packets that are originated from a natted network. Whenever NAT rules are changed or added, the connection tracking table should be cleared otherwise NAT rules may seem to be not functioning correctly until connection entry expires.
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