Migrating from Juniper ScreenOS firewalls to FortiGates, there are some differences to note with static NATs, i.e., Mapped IPs (MIPs) on a Netscreen and Virtual IPs (VIPs) on a FortiGate. While the Juniper MIPs on an interface are always used by the firewall whenever a packet traverses the interface, the virtual IP objects on a FortiGate must be used at least once in the security policy before they are really used by the firewall.
On the FortiGate firewall, address objects and virtual IPs (VIPs) can be set up with an interface. For address objects this has no technical relevance – the address objects simply only appear on policies if the appropriate interface is selected. But for virtual IPs, this setting has relevance on how connections are NATed. This can be problematic.
MIP DIP VIP. I am sometimes confused with the NAT names of the Juniper ScreenOS devices. Therefore, I drew a small figure with a few basic examples for these NAT types.
I am using a Cisco router for my basic ISP connection with a NAT/PAT configuration that translates all client connections to the IPv4 address of the outside interface of the router. Furthermore, I am translating all my static public IPv4 addresses to private ones through static NAT entries. I basically thought, that only the IPv4 addresses in the mere IPv4 packet header would be translated. However, this was not true since I immediately discovered that public DNS addresses are translated to my private IPv4 addresses, too. This was a bit confusing since I have not explicitly configured an application layer gateway (ALG) on that router.
“Google is my friend” and helped me one more time to find out the appropriate solution: The “no ip nat service alg udp dns” keyword to disable the DNS rewrite. (The synonym from Cisco for DNS rewrite is: DNS doctoring.) Here comes a basic example:
During my job I am frequently discussing with people why they use NAT or why they believe that NAT adds any security to their networks, mainly some obscurity as NAT (PAT) hides the internal network structure. However, NAT does not add any real security to a network while it breaks almost any good concepts of a structured network design. To emphasize this thesis, here is a discussion: