D-Link DFL-2500 - Manual (Page 218)

# Action Src Iface Src Net Dest Iface Dest Net Parameters
5 NAT lan lannet any all-nets All
What happens now?
• External traffic to wan_ip:80 will match rules 1 and 3, and will be sent to wwwsrv. Correct.
• Return traffic from wwwsrv:80 will match rules 2 and 4, and will appear to be sent from
wan_ip:80. Correct.
• Internal traffic to wan_ip:80 will match rules 1 and 3, and will be sent to wwwsrv. Almost
correct; the packets will arrive at wwwsrv, but:
• Return traffic from wwwsrv:80 to internal machines will be sent directly to the machines
themselves. This will not work, as the packets will be interpreted as coming from the wrong
address.
We will now try moving the NAT rule between the SAT and FwdFast rules:
# Action Src Iface Src Net Dest Iface Dest Net Parameters
1 SAT any all-nets core wan_ip http SETDEST wwwsrv 80
2 SAT lan wwwsrv any all-nets 80 -> All SETSRC wan_ip 80
3 NAT lan lannet any all-nets All
4 FwdFast any all-nets core wan_ip http
5 FwdFast lan wwwsrv any all-nets 80 -> All
What happens now?
• External traffic to wan_ip:80 will match rules 1 and 4, and will be sent to wwwsrv. Correct.
• Return traffic from wwwsrv:80 will match rules 2 and 3. The replies will therefore be
dynamically address translated. This changes the source port to a completely different port,
which will not work.
The problem can be solved using the following rule set:
# Action Src Iface Src Net Dest Iface Dest Net Parameters
1 SAT any all-nets core wan_ip http SETDEST wwwsrv 80
2 SAT lan wwwsrv any all-nets 80 -> All SETSRC wan_ip 80
3 FwdFast lan wwwsrv any all-nets 80 -> All
4 NAT lan lannet any all-nets All
5 FwdFast lan wwwsrv any all-nets 80 -> All
• External traffic to wan_ip:80 will match rules 1 and 5, and will be sent to wwwsrv.
• Return traffic from wwwsrv:80 will match rules 2 and 3.
• Internal traffic to wan_ip:80 will match rules 1 and 4, and will be sent to wwwsrv. The sender
address will be the D-Link Firewall's internal IP address, guaranteeing that return traffic passes
through the D-Link Firewall.
• Return traffic will automatically be handled by the D-Link Firewall's stateful inspection
mechanism.
7.3.7. SAT and FwdFast Rules Chapter 7. Address Translation
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