Fortinet FortiGate Series Administration Manual page 449

Firewall Virtual IP
FortiGate Version 4.0 MR1 Administration Guide
01-410-89802-20090903
http://docs.fortinet.com/ • Feedback
Note: If the NAT check box is not selected when building the firewall policy, the resulting
policy does not perform full (source and destination) NAT; instead, it performs destination
network address translation (DNAT).
For inbound traffic, DNAT translates packets' destination address to the mapped private IP
address, but does not translate the source address. The private network is aware of the
source's public IP address. For reply traffic, the FortiGate unit translates packets' private
network source IP address to match the destination address of the originating packets,
which is maintained in the session table.
A typical example of static NAT is to allow client access from a public network to a web
server on a private network that is protected by a FortiGate unit. Reduced to its essence,
this example involves only three hosts, as shown in
private network, the client computer on another network, such as the Internet, and the
FortiGate unit connecting the two networks.
When a client computer attempts to contact the web server, it uses the virtual IP on the
FortiGate unit's external interface. The FortiGate unit receives the packets. The addresses
in the packets are translated to private network IP addresses, and the packet is forwarded
to the web server on the private network.
Figure 247: A simple static NAT virtual IP example
The packets sent from the client computer have a source IP of 192.168.37.55 and a
destination IP of 192.168.37.4. The FortiGate unit receives these packets at its external
interface, and matches them to a firewall policy for the virtual IP. The virtual IP settings
map 192.168.37.4 to 10.10.10.42, so the FortiGate unit changes the packets' addresses.
The source address is changed to 10.10.10.2 and the destination is changed to
10.10.10.42. The FortiGate unit makes a note of this translation in the firewall session
table it maintains internally. The packets are then sent on to the web server.
Figure 248: Example of packet address remapping during NAT from client to server
Note that the client computer's address does not appear in the packets the server
receives. After the FortiGate unit translates the network addresses, there is no reference
to the client computer's IP address, except in its session table. The web server has no
indication that another network exists. As far as the server can tell, all packets are sent by
the FortiGate unit.
How virtual IPs map connections through FortiGate units
Figure 247: the web server on a
449