D-Link NetDefend DFL-1660, User Manual

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# Action Src Iface Src Net Dest Iface Dest Net Parameters
1 SAT any all-nets core wan_ip http SETDEST wwwsrv 80
2 Allow any all-nets core wan_ip http
These two rules allow us to access the web server via the firewall's external IP address. Rule 1 states that
address translation will take place if the connection is 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 core wan_ip all_services
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 IPv4 address
• lan_ip (10.0.0.1): the NetDefend Firewall's internal, private IPv4 address
• wwwsrv (10.0.0.2): the web server's private IPv4 address
• PC1 (10.0.0.3): a PC with a private IPv4 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, 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 core wan_ip all_services
3 Allow any all-nets core wan_ip http
• PC1 sends traffic to wan_ip in order 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 traffic and replies:
10.0.0.2:80 => 10.0.0.1:32789
7.4.1. Translation of a Single IP
Address (1:1) Chapter 7. Address Translation
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