NAT Security rule

L0 Member

NAT Security rule

I'm used to working on Cisco ASA and I'm having a hard time understanding why the security rule states Untrust-L3 for both the source and destination zone. Typically wouldn't that be Untrust-L3 to DMZ? Is there a specific reason for this behavior?


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Community Team Member

Re: NAT Security rule

Community Manager

Re: NAT Security rule

Hi @Glitchen


When a new packet arrives on an interface, the firewall needs to determine it's source and destination zones, these are determined by using the routing table.


So when a packet arrives from the internet, both it's source as destination IPs will be located in the untrust zone: they both follow the default gateway when performing a route lookup

(eg. packet from to, both ip will resolve via to the untrust zone)


Next, the NAT policy is checked to see if this packet matches something, and if it does, which zone the final destination is located in, this is where, post translation, it is deterined that security wise the (final) destination is dmz (eg. translates to, which is routed to the dmz interface).


The next step is to verify the security policy for matches to see if the packet is going to be allowed or not. Since we now know the final destination is DMZ, you can build a security policy to allow from untrust to dmz

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