D-Link DFL-2500 - Manual (Page 212)

# Action Src Iface Src Net Dest Iface Dest Net Parameters
3 Allow ext2 ext2net core wan_ip http
4 NAT lan lannet any all-nets All
This increases the number of rules for each interface allowed to communicate with the web server. However, the
rule ordering is unimportant, which may help avoid errors.
If option 2 was selected, the rule set must be adjusted thus:
# Action Src Iface Src Net Dest Iface Dest Net Parameters
1 SAT any all-nets core wan_ip http SETDEST
10.10.10.5 80
2 NAT lan lannet any all-nets All
3 Allow any all-nets core wan_ip http
This means that the number of rules does not need to be increased. This is good as long as all interfaces can be
entrusted to communicate with the web server. However, if, at a later point, you add an interface that cannot be
entrusted to communicate with the web server, separate Drop rules would have to be placed before the rule
granting all machines access to the web server.
Determining the best course of action must be done on a case-by-case basis, taking all circumstances into
account.
Example 7.4. Enabling Traffic to a Web Server on an Internal Network
The example we have decided to use is that of a web server with a private address located on an internal
network. From a security standpoint, this approach is wrong, as web servers are very vulnerable to attack and
should therefore be located in a DMZ. However, due to its simplicity, we have chosen to use this model in our
example.
In order for external users to access the web server, they must be able to contact it using a public address. In this
example, we have chosen to translate port 80 on the D-Link Firewall's external address to port 80 on the web
server:
# 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 D-Link 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 D-Link 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
• PC1 sends a packet to wan_ip to reach "www.ourcompany.com":
10.0.0.3:1038 => 195.55.66.77:80
7.3.1. Translation of a Single IP
Address (1:1) Chapter 7. Address Translation
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