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# Action Src Iface Src Net Dest Iface Dest Net Parameters
2 Allow any all-nets core wan_ip http
These two rules allow us to access the web server via the NetDefend Firewall's external IP address. Rule 1 states
that address translation can take place if the connection has been permitted, and rule 2 permits the connection.
Of course, we also need a rule that allows internal machines to be dynamically address translated to the Internet.
In this example, we use a rule that permits everything from the internal network to access the Internet via NAT
hide:
# Action Src Iface Src Net Dest Iface Dest Net Parameters
3 NAT lan lannet any all-nets All
The problem with this rule set is that it will not work at all for traffic from the internal network.
In order to illustrate exactly what happens, we use the following IP addresses:
• wan_ip (195.55.66.77): a public IP address
• lan_ip (10.0.0.1): the NetDefend Firewall's private internal IP address
• wwwsrv (10.0.0.2): the web servers private IP address
• PC1 (10.0.0.3): a machine with a private IP address
The order of events is as follows:
• PC1 sends a packet to wan_ip to reach www.ourcompany.com:
10.0.0.3:1038 => 195.55.66.77:80
• NetDefendOS translates the address in accordance with rule 1 and forwards the packet in accordance with
rule 2:
10.0.0.3:1038 => 10.0.0.2:80
• wwwsrv processes the packet and replies:
10.0.0.2:80 => 10.0.0.3:1038
This reply arrives directly to PC1 without passing through the NetDefend Firewall. This causes problems.
The reason this will not work is because PC1 expects a reply from 195.55.66.77:80 and not 10.0.0.2:80. The
unexpected reply is discarded and PC1 continues to wait for a response from 195.55.66.77:80 which will never
arrive.
Making a minor change to the rule set in the same way as described above, will solve the problem. In this
example, for no particular reason, we choose to use option 2:
# Action Src Iface Src Net Dest Iface Dest Net Parameters
1 SAT any all-nets core wan_ip http SETDEST wwwsrv 80
2 NAT lan lannet any all-nets All
3 Allow any all-nets core wan_ip http
• PC1 sends a packet to wan_ip to reach "www.ourcompany.com":
10.0.0.3:1038 => 195.55.66.77:80
• NetDefendOS address translates this statically in accordance with rule 1 and dynamically in accordance with
rule 2:
10.0.0.1:32789 => 10.0.0.2:80
• wwwsrv processes the packet and replies:
10.0.0.2:80 => 10.0.0.1:32789
• The reply arrives and both address translations are restored:
195.55.66.77:80 => 10.0.0.3:1038
In this way, the reply arrives at PC1 from the expected address.
7.4.1. Translation of a Single IP
Address (1:1)
Chapter 7. Address Translation
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