Outbound NAT

Back to our examples, as we discussed at the beginning of this chapter, let's use the example of a small company with 10 computers and just a single public IP address in its WAN connection. Moving on in this scenario, we have the goal to connect all those computers to the internet just using firewall capabilities. How do we achieve that? By creating an outbound NAT! Let's see how things work. The following is an example topology of outbound NAT traffic:

Figure 6.5 – Outbound NAT example

As we can see in the preceding figure, three clients are each accessing a website. Let's pick the host 192.168.10.11: it is accessing the https://cloudfence.eu website, but to the CloudFence web server, the source IP address is the public IP of OPNsense firewall 200.200.200.1 with source port 10200. So what is happening here? The outbound NAT rule is translating from the internal source IP to a public IP address, so from a TCP perspective...